**Introduction**

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Carbon Nanotubes and Their**

**Introductory Chapter: Carbon Nanotubes and Their** 

Carbon nanotubes (CNTs) is smart carbon materials which significantly utilized for the potential applications, large-scale synthesis, structural evaluation, and physical as well as chemical properties of carbon nanotubes and/or related conjugated carbon materials. Here, the various areas of carbon nanotubes (CNT) related topics including carbon materials for their various synthesis methodologies, total morphological, orientational, elemental, structural evaluations and characterizations as well as potential applications in different fields including bio-materials as well as nanomaterials for scaffolds as promising cell carrier for tissue engineering, designing antimicrobial polymeric bio-composite mats and natural medicinal plant polymer for cosmeceutical applications, carbon nanotubes/silicon composites to materials of Li secondary batteries, CNT composites with metal oxides as well as hierarchical crystalline nanotube on the cooperative Phenomena of Functional Molecular Group as the Target of

Expression of New Physical and chemical Properties were discussed in this chapter.

Substantial nanotube discovery which originated to existent in 1991 was CNTs. That buckyballs are spherical and round, nanotubes are tubic cylinders which are not folded round to generate to sphere [1]. CNTs are confined of C-atom connected in hexagonal shapes, with every C-atom covalently-bonded to three other C-atoms. CNTs have average dia. as lower as <1.0 nm and sizes up to several centimeters. A carbon nanotube is a long-tube-shaped carbonmaterial, made of only carbon, having a diameter measuring in nanometer scale. A nanometer

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.75738

**Applications**

**1. Introduction**

**2. Literature review**

**Applications**

Mohammed Muzibur Rahman and

Mohammed Muzibur Rahman and

http://dx.doi.org/10.5772/intechopen.75738

Additional information is available at the end of the chapter

Abdullah Mohamed AsiriAdditional information is available at the end of the chapter

Abdullah Mohamed Asiri

#### **Introductory Chapter: Carbon Nanotubes and Their Applications Introductory Chapter: Carbon Nanotubes and Their Applications**

DOI: 10.5772/intechopen.75738

Mohammed Muzibur Rahman and Abdullah Mohamed Asiri Mohammed Muzibur Rahman and

Additional information is available at the end of the chapter Abdullah Mohamed AsiriAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75738

#### **1. Introduction**

Carbon nanotubes (CNTs) is smart carbon materials which significantly utilized for the potential applications, large-scale synthesis, structural evaluation, and physical as well as chemical properties of carbon nanotubes and/or related conjugated carbon materials. Here, the various areas of carbon nanotubes (CNT) related topics including carbon materials for their various synthesis methodologies, total morphological, orientational, elemental, structural evaluations and characterizations as well as potential applications in different fields including bio-materials as well as nanomaterials for scaffolds as promising cell carrier for tissue engineering, designing antimicrobial polymeric bio-composite mats and natural medicinal plant polymer for cosmeceutical applications, carbon nanotubes/silicon composites to materials of Li secondary batteries, CNT composites with metal oxides as well as hierarchical crystalline nanotube on the cooperative Phenomena of Functional Molecular Group as the Target of Expression of New Physical and chemical Properties were discussed in this chapter.

#### **2. Literature review**

Substantial nanotube discovery which originated to existent in 1991 was CNTs. That buckyballs are spherical and round, nanotubes are tubic cylinders which are not folded round to generate to sphere [1]. CNTs are confined of C-atom connected in hexagonal shapes, with every C-atom covalently-bonded to three other C-atoms. CNTs have average dia. as lower as <1.0 nm and sizes up to several centimeters. A carbon nanotube is a long-tube-shaped carbonmaterial, made of only carbon, having a diameter measuring in nanometer scale. A nanometer

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

is one/billionth of a meter, or about 10,000 times smaller than a normal human hair. CNTs are exceptional and unique because the bonding between the carbon-atoms is very strong and the tubes can have extreme characteristic aspect ratios. Generally CNTs are allotropes of carbon with a cylindrical tubic nanostructure. These tubic-cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, nano-electronics, optics-electronics and other fields of nanomaterials science and technology. Due to the nanomaterial's extraordinary strength and stiffness, nanotubes have been created with L-to-D (Length-to-diameter) ratio [2] considerably bigger than for any other modified material. Additionally, due to CNTs exceptional characteristic property such as thermal conductivity, mechanical, and electrical, CNTs find potential applications as additives to different structural nanomaterials [3, 4]. For large number of potential applications with CNTs, the challenge lies in the alignment and ordering of CNTs to take advantages of their highly anisotropic thermal, electrical, physical characteristics. CNTs have emerged as a new category of nano-sized particles for incorporation into various liquid crystal systems, attracting favorable interest from both basic level science research and as well as industrial applications [1]. As a result of the exceptional properties of CNTs, the novel materials can be envisioned that exhibit property enhancements at lower concentration than in conventional composite technology [5]. CNTs also represent a promising material due to their unique physicochemical properties: their nanoscale needle shape, high chemical stability, thermal conductivity, and mechanical strength, which confer an advantage in the fabrication of field emitters. The utilization of CNTs relies on their electronic properties since they can be either metallic or semi-conductive, depending on the geometric configuration of a graphene sheet rolled up as a tube (i.e., diameter and chiral angle) [6–9]. Global attention on CNTs also attracted by many researchers have evaluated the extraordinary properties of CNTs toward development of nanocomposites and sheets holding highly oriented CNTs with enhanced electrical and mechanical properties. Exert of electrical field to a matrix containing CNTs can also lead to expansion of a highly oriented network from the negative electrode toward the positive electrode, which acting as a pathway for transferring current from the negative electrode toward the positive electrode [10–14]. The CNT arrays or vertically aligned CNTs itself is an extensive area of research apart from its originator CNT. As one can expect the complexity to grown long continuous CNTs [15, 16], scientist and researchers have derived out a brilliantly new idea of CNT yarn. The idea of aligning carbon nanotubes into arrays was perceived in 1994 by Ajayan et al. [17] by cutting thin slices of the nanotube-polymer composites. One of the most promising CNT applications of the field emission is an electron source. Lots of electronic devices are based on electron sources which are the most crucial component served as state-of-art vacuum nano-electronics and emerging novel devices in nowadays, such as field emission displays, emerging sensor, energy storage equipment, scanning ultrafast electron microscopes, X-ray generators, free electron lasers, Terra Hartz sources and so on [18–20]. Recently, such electron sources have intrigued a strong interest and encouraged the further studies of pulsed electron emission, opening the door to high-tech novel devices. For example, pulsed electron emission opens a way toward the time resolved electron microscopy, because electrical gating and source control enable time resolution down to picoseconds, while using optical control enables creation of electron pulses with duration down to tens of femtoseconds. Such dense and short electron bunches can become a popular platform for material and device imaging, inspection, and failure analysis. They would enable exciting technological developments like four-dimensional time resolved electron microscopy, spectroscopy, holography, single-electron sources, and carrier envelope phase detection [21, 22]. Many other investigations in terms of electrode materials are focused on CNTs and their modified or hybrid materials. Compared with aluminum foil or copper foil, metal-free current collector is a research hotspot, CNT film or cellulose papers used as negative electrode or anode current collector was applied in flexible lithium ion battery in many researches [23, 24]. Different modern and advanced techniques have been developed for exploring the mechanical properties of individual carbon materials or CNTs. One method for measuring the Young's modulus of a CNT is to fabricate a nanotube beam that is clamped at each end to a ceramic membrane (or otherwise supported) and to measure its vertical deflection versus the force applied at a point midway along its length [25]. The atomic force microscope is a natural and convenient means for studying the Young's modulus of CNTs, because it allows measurement of the deflection of a sample as a function

Introductory Chapter: Carbon Nanotubes and Their Applications

http://dx.doi.org/10.5772/intechopen.75738

5

Furthermore, CNTs, which have potential in chemical stability, thermal and electrical conductivity, mechanical strength, and flexibility, may be the best alternative materials for application in flexible transparent conductive electrodes. Actually, CNTs in some applications, which have low conductivity regardless of transmittance, are being commercialized. The rough surface of CNT films due to their tubular structures brings about a serious problem in the application of organic light-emitting diodes [27]. In CNTs, a relatively high contact resistance of the tube-tube junction may lead to insufficient sheet resistance [28]. Regarding this issue, the separation method of metallic and semiconducting components in CNTs by controlling the diameter and chirality of CNTs has been introduced in this book. CNTs have an affinity to also aggregate owing to the van der Waals attractive interaction between their sidewalls [29]. This tube-to-tube contacts results hydrophobic nature of CNTs which is responsible for their poor solubility in water, and also incompatible with a majority of solvents [30]. As a result, CNTs are precipitated in solvent, which the lack of solubility and the difficulty of manipulating them in solvents limit the development of CNT-based devices or composites of interest for new applications. In order to obtain fine dispersion in the selected solutions specially water, it is important to break the cohesion of aggregated CNTs. Development of efficient processes and chemical treatments that are able to control the quality of the CNT samples and to induce both their dispersion and partial or complete de-bundling remains highly challenging.

Finally, the growth of dense and long CNT arrays and their application in spinning threads or CNT yarns, non-homogeneous electron emitter plane with large FE current fluctuations and a short emission life-time, LC-CNT surface interaction lead to pinning orientational order uniformly along the CNT, aligned-CNT network within the matrix via electric fields, tensile strength and strength distribution of CNTs, hybrid-types of CNTs-based electronic devices, polymer wrapping methods by disperse CNT using gelatin for making decomposable biopolymer, electrochemical capacitors based on CNTs, hydrogenation and dehydrogenation

of applied force when used in contact mode [26].

**3. Conclusion**

failure analysis. They would enable exciting technological developments like four-dimensional time resolved electron microscopy, spectroscopy, holography, single-electron sources, and carrier envelope phase detection [21, 22]. Many other investigations in terms of electrode materials are focused on CNTs and their modified or hybrid materials. Compared with aluminum foil or copper foil, metal-free current collector is a research hotspot, CNT film or cellulose papers used as negative electrode or anode current collector was applied in flexible lithium ion battery in many researches [23, 24]. Different modern and advanced techniques have been developed for exploring the mechanical properties of individual carbon materials or CNTs. One method for measuring the Young's modulus of a CNT is to fabricate a nanotube beam that is clamped at each end to a ceramic membrane (or otherwise supported) and to measure its vertical deflection versus the force applied at a point midway along its length [25]. The atomic force microscope is a natural and convenient means for studying the Young's modulus of CNTs, because it allows measurement of the deflection of a sample as a function of applied force when used in contact mode [26].

Furthermore, CNTs, which have potential in chemical stability, thermal and electrical conductivity, mechanical strength, and flexibility, may be the best alternative materials for application in flexible transparent conductive electrodes. Actually, CNTs in some applications, which have low conductivity regardless of transmittance, are being commercialized. The rough surface of CNT films due to their tubular structures brings about a serious problem in the application of organic light-emitting diodes [27]. In CNTs, a relatively high contact resistance of the tube-tube junction may lead to insufficient sheet resistance [28]. Regarding this issue, the separation method of metallic and semiconducting components in CNTs by controlling the diameter and chirality of CNTs has been introduced in this book. CNTs have an affinity to also aggregate owing to the van der Waals attractive interaction between their sidewalls [29]. This tube-to-tube contacts results hydrophobic nature of CNTs which is responsible for their poor solubility in water, and also incompatible with a majority of solvents [30]. As a result, CNTs are precipitated in solvent, which the lack of solubility and the difficulty of manipulating them in solvents limit the development of CNT-based devices or composites of interest for new applications. In order to obtain fine dispersion in the selected solutions specially water, it is important to break the cohesion of aggregated CNTs. Development of efficient processes and chemical treatments that are able to control the quality of the CNT samples and to induce both their dispersion and partial or complete de-bundling remains highly challenging.

#### **3. Conclusion**

is one/billionth of a meter, or about 10,000 times smaller than a normal human hair. CNTs are exceptional and unique because the bonding between the carbon-atoms is very strong and the tubes can have extreme characteristic aspect ratios. Generally CNTs are allotropes of carbon with a cylindrical tubic nanostructure. These tubic-cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, nano-electronics, optics-electronics and other fields of nanomaterials science and technology. Due to the nanomaterial's extraordinary strength and stiffness, nanotubes have been created with L-to-D (Length-to-diameter) ratio [2] considerably bigger than for any other modified material. Additionally, due to CNTs exceptional characteristic property such as thermal conductivity, mechanical, and electrical, CNTs find potential applications as additives to different structural nanomaterials [3, 4]. For large number of potential applications with CNTs, the challenge lies in the alignment and ordering of CNTs to take advantages of their highly anisotropic thermal, electrical, physical characteristics. CNTs have emerged as a new category of nano-sized particles for incorporation into various liquid crystal systems, attracting favorable interest from both basic level science research and as well as industrial applications [1]. As a result of the exceptional properties of CNTs, the novel materials can be envisioned that exhibit property enhancements at lower concentration than in conventional composite technology [5]. CNTs also represent a promising material due to their unique physicochemical properties: their nanoscale needle shape, high chemical stability, thermal conductivity, and mechanical strength, which confer an advantage in the fabrication of field emitters. The utilization of CNTs relies on their electronic properties since they can be either metallic or semi-conductive, depending on the geometric configuration of a graphene sheet rolled up as a tube (i.e., diameter and chiral angle) [6–9]. Global attention on CNTs also attracted by many researchers have evaluated the extraordinary properties of CNTs toward development of nanocomposites and sheets holding highly oriented CNTs with enhanced electrical and mechanical properties. Exert of electrical field to a matrix containing CNTs can also lead to expansion of a highly oriented network from the negative electrode toward the positive electrode, which acting as a pathway for transferring current from the negative electrode toward the positive electrode [10–14]. The CNT arrays or vertically aligned CNTs itself is an extensive area of research apart from its originator CNT. As one can expect the complexity to grown long continuous CNTs [15, 16], scientist and researchers have derived out a brilliantly new idea of CNT yarn. The idea of aligning carbon nanotubes into arrays was perceived in 1994 by Ajayan et al. [17] by cutting thin slices of the nanotube-polymer composites. One of the most promising CNT applications of the field emission is an electron source. Lots of electronic devices are based on electron sources which are the most crucial component served as state-of-art vacuum nano-electronics and emerging novel devices in nowadays, such as field emission displays, emerging sensor, energy storage equipment, scanning ultrafast electron microscopes, X-ray generators, free electron lasers, Terra Hartz sources and so on [18–20]. Recently, such electron sources have intrigued a strong interest and encouraged the further studies of pulsed electron emission, opening the door to high-tech novel devices. For example, pulsed electron emission opens a way toward the time resolved electron microscopy, because electrical gating and source control enable time resolution down to picoseconds, while using optical control enables creation of electron pulses with duration down to tens of femtoseconds. Such dense and short electron bunches can become a popular platform for material and device imaging, inspection, and

4 Carbon Nanotubes - Recent Progress

Finally, the growth of dense and long CNT arrays and their application in spinning threads or CNT yarns, non-homogeneous electron emitter plane with large FE current fluctuations and a short emission life-time, LC-CNT surface interaction lead to pinning orientational order uniformly along the CNT, aligned-CNT network within the matrix via electric fields, tensile strength and strength distribution of CNTs, hybrid-types of CNTs-based electronic devices, polymer wrapping methods by disperse CNT using gelatin for making decomposable biopolymer, electrochemical capacitors based on CNTs, hydrogenation and dehydrogenation studies of CNTs functionalized with BH3, etc. are discussed. Here, we believe that it offers a broad field of existing developments in recent development of carbon nanotube technology research and an excellent introduction to preparation, synthesis, potential characteristic properties in field of materials and composites for their potential applications. This work aims to bridge the gap between undergraduate, graduate, and researches in carbon materials, nanomaterials, polymers, biomaterials, nanocomposites, catalysis as well as bio-medical sciences, in order to initiate scientist/researchers into different carbon nanotubes study in as straight-forward way as conceivable and also introduce the researcher to the opportunities offered by the science and technological fields. We hope, it will offer the evaluation of the state-of-the-art methods and advances of carbon researches and bio-science and nanotechnologies. Distinguished scientist were significantly contributed to present their novel ideas and recent advanced development in the field of carbon materials, carbon nanotubes, or carbon nanotube conjugated related materials in various research and practical fields. The prime goal is to introduce audience about CNTs for the students, researchers, technologists, physicists, chemists, biologists, engineers and professionals who are interested in carbon nanotubes and associated carbon related materials.

[7] Saito R, Fujita M, Dresselhaus G, Dresselhaus MS. Electronic structure of chiral gra-

Introductory Chapter: Carbon Nanotubes and Their Applications

http://dx.doi.org/10.5772/intechopen.75738

7

[8] Tanaka K, Okahara K, Okada M, Yamabe T. Electronic properties of bucky-tube model.

[9] Kim JY, Kim M, Kim HM, Joo J, Choi JH. Electrical and optical studies of organic light emitting devices using SWCNTs-polymer nanocomposites. Optical Materials. 2002;

[10] Gupta P, Rajput M, Singla N, Kumar V, Lahiri D. Electric field and current assisted alignment of CNT inside polymer matrix and its effects on electrical and mechanical proper-

[11] Khan SU, Pothnis JR, Kim JK. Effects of carbon nanotube alignment on electrical and mechanical properties of epoxy nanocomposites. Composites Part A: Applied Science

[12] Ma C, Zhang W, Zhu Y, Ji L, Zhang R, Koratkar N, Liang J. Alignment and dispersion of functionalized carbon nanotubes in polymer composites induced by an electric field.

[13] Mecklenburg M, Mizushima D, Ohtake N, Bauhofer W, Fiedler B, Schulte K. On the manufacturing and electrical and mechanical properties of ultra-high wt.% fraction aligned MWCNT and randomly oriented CNT epoxy composites. Carbon. 2015;**91**:275-290 [14] Wang Q, Dai J, Li W, Wei Z, Jiang J. The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites. Composites Science

[15] Pan ZW, Xie SS, Chang BH, Wang CY, Lu L, Liu W, Zhou WY, Li WZ, Qian LX. Very long

[16] Zhu HW, Xu CL, Wu DH, Wei BQ, Vajtai R, Ajayan PM. Direct synthesis of long single-

[17] Ajayan PM, Stephan O, Colliex C, Trauth D. Aligned carbon nanotube arrays formed by cutting a polymer resin-nanotube composite. Science. AAAS-Weekly Paper Edition.

[18] Nojeh A. Hindawi: Carbon nanotube electron sources: From electron beams to energy conversion and optophononics. ISRN Nanomaterials. 2014;**2014**(1):879827-879851 [19] Tian S, Li H, Zhang Y, Liu S, Fu Y. Elsevier: Potential field emitters: HfC nanorods

[20] Xu NS, Huq SE. Elsevier: Novel cold cathode materials and applications. Materials

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#### **Author details**

Mohammed Muzibur Rahman\* and Abdullah Mohamed Asiri

\*Address all correspondence to: mmrahmanh@gmail.com

Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

#### **References**


[7] Saito R, Fujita M, Dresselhaus G, Dresselhaus MS. Electronic structure of chiral graphene tubules. Applied Physics Letters. 1992;**60**:2204-2206

studies of CNTs functionalized with BH3, etc. are discussed. Here, we believe that it offers a broad field of existing developments in recent development of carbon nanotube technology research and an excellent introduction to preparation, synthesis, potential characteristic properties in field of materials and composites for their potential applications. This work aims to bridge the gap between undergraduate, graduate, and researches in carbon materials, nanomaterials, polymers, biomaterials, nanocomposites, catalysis as well as bio-medical sciences, in order to initiate scientist/researchers into different carbon nanotubes study in as straight-forward way as conceivable and also introduce the researcher to the opportunities offered by the science and technological fields. We hope, it will offer the evaluation of the state-of-the-art methods and advances of carbon researches and bio-science and nanotechnologies. Distinguished scientist were significantly contributed to present their novel ideas and recent advanced development in the field of carbon materials, carbon nanotubes, or carbon nanotube conjugated related materials in various research and practical fields. The prime goal is to introduce audience about CNTs for the students, researchers, technologists, physicists, chemists, biologists, engineers and professionals who are interested in carbon nanotubes and

associated carbon related materials.

6 Carbon Nanotubes - Recent Progress

Mohammed Muzibur Rahman\* and Abdullah Mohamed Asiri

tubules. Physical Review Letters. 1992;**68**:1579-1581

Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

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\*Address all correspondence to: mmrahmanh@gmail.com

**Author details**

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[22] Lyashenko DA, Svirko YP, Petrov MI, Obraztsov AN. Springer: The laser assisted field electron emission from carbon nanostructure. Journal of the European Optical Society-Rapid Publications. 2017;**13**(1):4-10

**Section 2**

**State-of-the-Art Physical**


**State-of-the-Art Physical**

[22] Lyashenko DA, Svirko YP, Petrov MI, Obraztsov AN. Springer: The laser assisted field electron emission from carbon nanostructure. Journal of the European Optical Society-

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[29] Fei B, Lu H, Hu Z, Xin JH. Solubilization, purification and functionalization of carbon nanotubes using polyoxometalate. Nanotechnology. Feb 21, 2006;**17**:1589-1593

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light-emitting diodes. Scientific Reports. 2015;**5**:9464

**Chapter 2**

**Provisional chapter**

**Rapid Growth of Dense and Long Carbon Nanotube**

Carbon nanotubes (CNTs), a dependable allotrope of carbon, are foreseen to lead technology further to its reach. Also, the most researched carbon allotrope in its form. Drawable CNTs have recently unraveled numerous possible applications utilizing vertically aligned CNTs also termed as "CNT forest." In recent years, the rapid growth of dense and long carbon nanotube arrays has succeeded in surpassing its challenges by synthesizing dense and long CNT arrays. Length, density, and drawability tuning in the synthesis of CNT arrays have always been a complex issue lately. However, numerous research techniques emerged focusing on length and density control. Hence, this book chapter aids in unveiling the current achievements in the growth of dense and long CNT arrays and their application in spinning threads or CNT yarns with numerous other possible applications.

**Keywords:** CNTs, rapid growth, CNT yarns, dry spinning, vertically aligned CNTs

Carbon nanotubes (CNTs) have not yet been into its full-fledged practical applications. However, it is prophesied to acquire the industrial and commercial market soon confidently. CNT arrays aka vertically aligned CNTs (VACNTs) itself are broad areas of research apart from its originator CNT. As one can expect the complexity to grow long continuous CNTs [1, 2], scientists and researchers have derived out a brilliantly new idea of CNT yarn. The idea of aligning carbon nanotubes into arrays was perceived in 1994 by Ajayan et al. [3] by cutting thin slices of the nanotube-polymer composite, which followed by the idea of aligned phases in applications like aligned liquid crystals [4], carbon fiber-reinforced polymer [5], and so on. Soon in 1995, Heer et al. produced CNT films from aligned CNT [6]. Later, in 1996 large-scale synthesis of aligned

**Rapid Growth of Dense and Long Carbon Nanotube** 

DOI: 10.5772/intechopen.70702

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Arrays and Its Application in Spinning Thread**

**Arrays and Its Application in Spinning Thread**

Yasuhiko Hayashi, Karthik Paneer Selvam and

Yasuhiko Hayashi, Karthik Paneer Selvam and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70702

Maik Scholz

**Abstract**

**1. Introduction**

Maik Scholz

**Provisional chapter**

#### **Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread Arrays and Its Application in Spinning Thread**

**Rapid Growth of Dense and Long Carbon Nanotube** 

DOI: 10.5772/intechopen.70702

Yasuhiko Hayashi, Karthik Paneer Selvam and Maik Scholz Maik Scholz Additional information is available at the end of the chapter

Yasuhiko Hayashi, Karthik Paneer Selvam and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70702

#### **Abstract**

Carbon nanotubes (CNTs), a dependable allotrope of carbon, are foreseen to lead technology further to its reach. Also, the most researched carbon allotrope in its form. Drawable CNTs have recently unraveled numerous possible applications utilizing vertically aligned CNTs also termed as "CNT forest." In recent years, the rapid growth of dense and long carbon nanotube arrays has succeeded in surpassing its challenges by synthesizing dense and long CNT arrays. Length, density, and drawability tuning in the synthesis of CNT arrays have always been a complex issue lately. However, numerous research techniques emerged focusing on length and density control. Hence, this book chapter aids in unveiling the current achievements in the growth of dense and long CNT arrays and their application in spinning threads or CNT yarns with numerous other possible applications.

**Keywords:** CNTs, rapid growth, CNT yarns, dry spinning, vertically aligned CNTs

#### **1. Introduction**

Carbon nanotubes (CNTs) have not yet been into its full-fledged practical applications. However, it is prophesied to acquire the industrial and commercial market soon confidently. CNT arrays aka vertically aligned CNTs (VACNTs) itself are broad areas of research apart from its originator CNT. As one can expect the complexity to grow long continuous CNTs [1, 2], scientists and researchers have derived out a brilliantly new idea of CNT yarn. The idea of aligning carbon nanotubes into arrays was perceived in 1994 by Ajayan et al. [3] by cutting thin slices of the nanotube-polymer composite, which followed by the idea of aligned phases in applications like aligned liquid crystals [4], carbon fiber-reinforced polymer [5], and so on. Soon in 1995, Heer et al. produced CNT films from aligned CNT [6]. Later, in 1996 large-scale synthesis of aligned

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

carbon nanotubes was reported by Li et al. [7] with chemical vapor deposition (CVD) growth of CNTs using iron nanoparticles catalyzed mesoporous silica, which extended the research on aligned carbon nanotubes. The urge to produce long CNTs gave rise to CNT yarns, and therefore CNT yarns were introduced as a substitute to long CNTs. On the other hand, CNT films from aligned CNT have also gained attention and encouraged research in those directions.

and sheets. The growth mechanism and types should be apparently introduced to give a

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

Many experiments performed by numerous research groups worldwide are taken into consideration in briefing all the possibilities of growth and application of CNT array and CNT yarns/films derived from them. After seeing the background of the research and how CNT array research has been disabling its limitations and improvising every year, the potential

Chemical vapor deposition is the most practiced and renowned method for growing CNT arrays. The initial steps include substrate preparation for the growth of CNT arrays, and the most common substrate used is silicon with several tens to few hundred nanometres thick

deposits Fe nanoparticles of few tens of nanometers. In a quartz tube, the Fe-coated substrate is placed. Then, this setup is put into a furnace, with one end of the quartz tube fed with reactive mixture gases (carbon precursor gas and carrier gas) and the other end to a vacuum pump. The quartz tube is then pressurized to certain total pressure say 200 Torr, and gases

H4

temperature in the furnace increased to 650 °C which allows the catalyst to decompose the carbon and grow into CNT array. The reaction time and the flow rate decide the lengths of the CNT arrays [60]. CVD technique has further progressed into water-assisted CVD, where the reaction gas mixture and water vapor are combined. This method yielded in producing

Floating catalyst chemical vapor deposition (FCCVD) method is also carried out in a quartz tube reactor and all other parameters being alike with CVD. The difference is avoiding metal catalyst deposition onto the substrate used for CNT growth. Instead, it the metal catalyst is passed into the quartz tube along with the reaction mixture gas. Hence, the name is derived as floating catalyst CVD, considering an example for the growth of CNT arrays. Ferrocene is injected into the reactor along with ethanol and thiophene at a rate

temperature being 1300 °C [10]. FCCV methods avoid the deposition of the catalyst layer onto the substrate reducing one step of processing of CNT arrays. In another experiment, a comparatively low temperature of 800 °C was used to grow a 500 μm height of CNT array. Ferrocene was dissolved in xylene and fed to a horizontal furnace containing quartz tube

Thus, it leaves us with the idea about all the parameters involved in growing a CNT array by

. To this substrate, a metal catalyst is deposited such as ferrocene which

) and carrier gas flow rate at 190 sccm. The

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13

gas mixture is fed as a carrier gas at 4000 sccm with the

bright idea about all the necessary factors employed in growing the VACNTs.

The methods that can grow CNTs or specifically VACNTs are discussed below.

applications of CNT arrays can be estimated.

**2.1. Chemical vapor deposition (CVD)**

layer of Al2

O3

or SiO2

long and aligned CNT arrays [8, 9].

of 0.15 ml min−1, and the Ar-H2

CVD technique as listed below:

with the substrate [11].

flow rates of 380 sccm for carbon precursor (C<sup>2</sup>

**2.2. Floating catalyst chemical vapor deposition (FCCVD)**

The understanding of the VACNTs and their growth parameters is a cumbersome issue, as too many factors are interlinked to each other making it a slightly challenging and complicated discussion. Nevertheless, for the easy understanding, we have mentioned subheadings in this chapter discussing each issue detailing all factors as deeply as possible to attain the VACNT growth and application in the spinning thread.

CNT forest aka aligned carbon nanotubes (ACNTs) or vertically aligned CNT (VACNT) arrays have been disabling its limitations as aggressive research is going on worldwide. Many practical applications have been demonstrated using CNT forest. CNT array research has grown widely in all directions. Not long ago, the research on CNT arrays has peaked, and relatively high number of research reports has published lately. Pushing the limit and experimenting with all possible applications.

The pathway to CNT yarns can be realized by the chart shown in **Figure 1**.

**Figure 1.** Showing all the parameters of CNT in application to CNT yarn and the factors for this growth.

### **2. Growth of CNT arrays/VACNTs**

Before proceeding to the segment of dealing with the properties and modification of VACNTs, their various applications and research background and potential applications as CNT yarns and sheets. The growth mechanism and types should be apparently introduced to give a bright idea about all the necessary factors employed in growing the VACNTs.

Many experiments performed by numerous research groups worldwide are taken into consideration in briefing all the possibilities of growth and application of CNT array and CNT yarns/films derived from them. After seeing the background of the research and how CNT array research has been disabling its limitations and improvising every year, the potential applications of CNT arrays can be estimated.

The methods that can grow CNTs or specifically VACNTs are discussed below.

#### **2.1. Chemical vapor deposition (CVD)**

carbon nanotubes was reported by Li et al. [7] with chemical vapor deposition (CVD) growth of CNTs using iron nanoparticles catalyzed mesoporous silica, which extended the research on aligned carbon nanotubes. The urge to produce long CNTs gave rise to CNT yarns, and therefore CNT yarns were introduced as a substitute to long CNTs. On the other hand, CNT films from aligned CNT have also gained attention and encouraged research in those directions.

The understanding of the VACNTs and their growth parameters is a cumbersome issue, as too many factors are interlinked to each other making it a slightly challenging and complicated discussion. Nevertheless, for the easy understanding, we have mentioned subheadings in this chapter discussing each issue detailing all factors as deeply as possible to attain the VACNT

CNT forest aka aligned carbon nanotubes (ACNTs) or vertically aligned CNT (VACNT) arrays have been disabling its limitations as aggressive research is going on worldwide. Many practical applications have been demonstrated using CNT forest. CNT array research has grown widely in all directions. Not long ago, the research on CNT arrays has peaked, and relatively high number of research reports has published lately. Pushing the limit and experimenting

Before proceeding to the segment of dealing with the properties and modification of VACNTs, their various applications and research background and potential applications as CNT yarns

**Figure 1.** Showing all the parameters of CNT in application to CNT yarn and the factors for this growth.

The pathway to CNT yarns can be realized by the chart shown in **Figure 1**.

growth and application in the spinning thread.

**2. Growth of CNT arrays/VACNTs**

with all possible applications.

12 Carbon Nanotubes - Recent Progress

Chemical vapor deposition is the most practiced and renowned method for growing CNT arrays. The initial steps include substrate preparation for the growth of CNT arrays, and the most common substrate used is silicon with several tens to few hundred nanometres thick layer of Al2 O3 or SiO2 . To this substrate, a metal catalyst is deposited such as ferrocene which deposits Fe nanoparticles of few tens of nanometers. In a quartz tube, the Fe-coated substrate is placed. Then, this setup is put into a furnace, with one end of the quartz tube fed with reactive mixture gases (carbon precursor gas and carrier gas) and the other end to a vacuum pump. The quartz tube is then pressurized to certain total pressure say 200 Torr, and gases flow rates of 380 sccm for carbon precursor (C<sup>2</sup> H4 ) and carrier gas flow rate at 190 sccm. The temperature in the furnace increased to 650 °C which allows the catalyst to decompose the carbon and grow into CNT array. The reaction time and the flow rate decide the lengths of the CNT arrays [60]. CVD technique has further progressed into water-assisted CVD, where the reaction gas mixture and water vapor are combined. This method yielded in producing long and aligned CNT arrays [8, 9].

#### **2.2. Floating catalyst chemical vapor deposition (FCCVD)**

Floating catalyst chemical vapor deposition (FCCVD) method is also carried out in a quartz tube reactor and all other parameters being alike with CVD. The difference is avoiding metal catalyst deposition onto the substrate used for CNT growth. Instead, it the metal catalyst is passed into the quartz tube along with the reaction mixture gas. Hence, the name is derived as floating catalyst CVD, considering an example for the growth of CNT arrays. Ferrocene is injected into the reactor along with ethanol and thiophene at a rate of 0.15 ml min−1, and the Ar-H2 gas mixture is fed as a carrier gas at 4000 sccm with the temperature being 1300 °C [10]. FCCV methods avoid the deposition of the catalyst layer onto the substrate reducing one step of processing of CNT arrays. In another experiment, a comparatively low temperature of 800 °C was used to grow a 500 μm height of CNT array. Ferrocene was dissolved in xylene and fed to a horizontal furnace containing quartz tube with the substrate [11].

Thus, it leaves us with the idea about all the parameters involved in growing a CNT array by CVD technique as listed below:


Now, a careful observation to make is that the parameters mentioned above are variable. Hence, it opens a broad range of experimental possibilities to grow CNT arrays.

of an MWNT forest to be drily drawable into sheets and yarns. After drawing out the CNT network from the forest in most of the cases to forming yarns, the network is twisted [14–16]. It leads to a rapid densification and increases the mechanical strength considerably. Some research groups apply additional liquid agents instead of twisting [17]. These agents help to densify the yarns leading to a functionalized surface of the yarns [18]. With this additional treatment, mechanical strength is further increased. Additional investigations were done in the direction of heating the yarns while spinning and using multiple arrays at once to form

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

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15

The understanding of aligned carbon nanotubes and its possible applications as yarns and

**4. Factors influencing drawing/spinning of CNT yarns and CNT films**

**Figure 2.** Showing a typical CNT forest observed under scanning electron microscopy produced by our group.

Residual catalyst particle Can sometimes be found No particle (bottom growth)

NT length Uses very short CNTs Uses long and aligned CNTs

Binder Required Not required

CNT dispersion Required Not required

Simplicity in process Complicated Very simple

**Table 1.** The comparison between wet and dry processes of CNT yarn spinning.

**Wet process Dry process**

4.1. Long and dense growth of Vertically Aligned/Super aligned CNTs (VACNT/SACNT)

The comparison of wet process and dry process is mentioned in **Table 1**.

yarns [19] as shown in **Figure 2**.

films can be divided into the following segments.

4.2. Drawing of CNT Yarns/fibers using ACNTs 4.3. Drawing of CNT sheets/films using ACNTs

#### **3. Drawing of yarns and sheets**

There are two methods employed commonly to spin or to draw the CNTs or CNT arrays into CNT yarn and CNT sheets/films.

#### **3.1. Wet process**

In a broad sense, wet process method to draw CNT yarns/fibers/threads does not have a direct comparison with dry-drawing method. It is naturally due to the dissimilarities between wet and dry processes. Though technically both process yield CNT yarns as the final product, the initial process is vastly different. The wet chemical process uses CNTs, whereas dry process uses VACNT. However, considering the end product's perspective, both methods can be compared.

In the wet process, CNTs are dispersed in a dispersant (acid), and then the CNTs are pumped out through a nozzle via coagulant, which aligns the CNTs into yarns/fibers. Chlorosulfonic acid is one such superacid allowing the CNTs to dissolve [12].

#### **3.2. Dry process**

Spinning CNT yarns or drawing CNT sheets from CNT arrays is among the two-step dry methods the most widely used. The synthesis mentioned above CNT forests is the first step. In the second phase of fabricating yarns or sheets, a cluster or bundle of CNT is pulled out of the assembly. The following nanotubes get attached through interconnected bundles at the top and bottom of the previous CNTs, thus forming a network of axially aligned CNTs. Kuznetsov et al. [13] show that the interconnection density is a key parameter for the ability of an MWNT forest to be drily drawable into sheets and yarns. After drawing out the CNT network from the forest in most of the cases to forming yarns, the network is twisted [14–16]. It leads to a rapid densification and increases the mechanical strength considerably. Some research groups apply additional liquid agents instead of twisting [17]. These agents help to densify the yarns leading to a functionalized surface of the yarns [18]. With this additional treatment, mechanical strength is further increased. Additional investigations were done in the direction of heating the yarns while spinning and using multiple arrays at once to form yarns [19] as shown in **Figure 2**.

The comparison of wet process and dry process is mentioned in **Table 1**.

**1.** Substrate

**2.** Oxide layer

**6.** Carrier gas

**9.** Temperature **10.**Reaction time

**3.1. Wet process**

**3.2. Dry process**

**3. Drawing of yarns and sheets**

CNT yarn and CNT sheets/films.

Now, a careful observation to make is that the parameters mentioned above are variable.

There are two methods employed commonly to spin or to draw the CNTs or CNT arrays into

In a broad sense, wet process method to draw CNT yarns/fibers/threads does not have a direct comparison with dry-drawing method. It is naturally due to the dissimilarities between wet and dry processes. Though technically both process yield CNT yarns as the final product, the initial process is vastly different. The wet chemical process uses CNTs, whereas dry process uses VACNT. However, considering the end product's perspective, both methods can be compared. In the wet process, CNTs are dispersed in a dispersant (acid), and then the CNTs are pumped out through a nozzle via coagulant, which aligns the CNTs into yarns/fibers. Chlorosulfonic

Spinning CNT yarns or drawing CNT sheets from CNT arrays is among the two-step dry methods the most widely used. The synthesis mentioned above CNT forests is the first step. In the second phase of fabricating yarns or sheets, a cluster or bundle of CNT is pulled out of the assembly. The following nanotubes get attached through interconnected bundles at the top and bottom of the previous CNTs, thus forming a network of axially aligned CNTs. Kuznetsov et al. [13] show that the interconnection density is a key parameter for the ability

Hence, it opens a broad range of experimental possibilities to grow CNT arrays.

acid is one such superacid allowing the CNTs to dissolve [12].

**7.** Pressure **8.** Flow rate

**3.** Metal catalyst

**4.** Reaction gas mixture

14 Carbon Nanotubes - Recent Progress

**5.** Carbon precursor

The understanding of aligned carbon nanotubes and its possible applications as yarns and films can be divided into the following segments.

**Figure 2.** Showing a typical CNT forest observed under scanning electron microscopy produced by our group.


**Table 1.** The comparison between wet and dry processes of CNT yarn spinning.

#### **4. Factors influencing drawing/spinning of CNT yarns and CNT films**


#### **4.1. Long and dense growth of aligned/super-aligned CNTs**

Some of the core challenges relating to CNT array's growth are:

4.1.1. Long heights of the CNT


Other parameters concerning growth of VACNTs


#### *4.1.1. Long heights/lengths of CNT forest*

Growing CNTs vertically in a long length is a critical challenge in the CNT array growth. Achieving long CNT forest can result in higher lengths of CNT yarns or films per unit area. In the application's point of view, obtaining drawable CNT forest is the real challenge. Many researchers have reported many possible lengths of drawable CNT forests. Hence, the following discussion is the current possible maximum drawable CNT forest heights which give an insight in the initial heights obtained for CNT forests. Few examples are briefed below for taming the inkling of the process for lengthy heights of CNT forest.

the process time was 20 min [25]. The method does not include pre-deposition of the catalyst layer. Instead, it uses iron chloride powder and acetylene gas because iron chloride has a high dehydrogenation activity on acetylene. They termed the process as chlorine-mediated chemi-

**Figure 3.** Depicting the lengths of VACNTs grown with growth time of 3 h, 2 h, 1 h 30 min 10 min (reprint from Ref. [22]

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

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In 1999, Shoushan et al. [26] have achieved 30–240 μm heights of MWCNT-VACNT grown by CVD reaction times of 5–60 min. The condition for growth is electrochemical etching of porous silicon of 2 in diameter, patterning with 5-nm-thick layer of Fe by electron beam evaporation with a shadow mask. The substrate was placed in the quartz tube reactor housed in a tube furnace. The operating temperature was 700 °C with Ar flow; ethylene was then flown at 1000 sccm for 15–60 min. The diameters of the MWCNT-VACNT were found to be 16 ± 2 nm. Ren et al. [27] have grown 0.1–50 μm heights of VACNTs using plasma-enhanced chemical

Lee et al. measured the possibility in factor determining the upper limit to the height of spinnable carbon nanotube forests [28]. By synthesizing CNT forests at different growth times such as 3, 6, 9, 12, 15, and 60 min. The heights were found to be 260 μm for 3 min growth time and 1.7 mm height for 60 mm growth time. An important observation was made, as the growth time increased the degree of alignment decreased. It was verified by Raman analysis

Hence a clear observation can be made by all the reports is that the process time with a certain flow rate of the precursor yields in governing the lengths of VACNT. Also, these parameters

It can be understood that the first challenge which lies in the geometry for VACNTs has a large area with long and densely aligned CNTs arrays. For large scale growth of CNT on a large area array, many researcher groups have been experimenting with it since a decade. CNT array growth area on large wafers has attained a growth area of 8-inch (18 cm) by Chen et al. [29]

as it showed G and D bands intensity ratios different at top and bottom.

have a grave effect on the degree of alignment of the VACNTs.

cal vapor deposition (CM-CVD).

with permission).

vapor deposition (PECVD).

*4.1.2. Large area growth of VACNT*

The record for the large height of VACNT array was reported in 2014 by Cho et al. [20], with a height of 21.7 mm, and not tested for drawability. Its water-assisted thermal CVD process produced such long VACNTs, and most of the CNTs were double walled. The growth rate was calculated to be 27.47 μm min−1.

Another report stated that 4.7-mm-long CNT arrays were grown with high efficiency and long-life catalyst film using Al<sup>2</sup> O3 and Fe as buffer layer and catalyst. The VACNTs were spinnable [21]. Similarly, in another event, 4-mm high-quality CNT array was grown without water assistance and showed that the catalyst lifetime lasted for 3 h [22] as illustrated in **Figure 3**.

In a recent report, aligned spinnable CNTs of heights 400 μm were synthesized by Alvarez et al. [23]; Fe and Co are used as the catalyst sources with 1.2 nm thin film. About 5 nm Al2 O3 -layered Si wafer was used to accommodate the Fe and Co catalyst. The substrate was annealed at 400 °C under Ar gas flow; then, the temperature was increased to 700 °C, and ethylene was fed at 300 sccm with 1000 sccm of Ar for 20 min. In another report of drawable VAMWCNT, the height of was 2.0 mm (2000 μm) grown in 16 min that account for 0.1 mm min−1 by Inoue et al. [24]. It was synthesized by conventional CVD system with iron chloride (FeCl2 ) as catalyst and acetylene as a carbon source. The same group has previously reported drawable VAMWCNT array measuring a height of 2.1 mm with same precursor material, but Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread http://dx.doi.org/10.5772/intechopen.70702 17

**Figure 3.** Depicting the lengths of VACNTs grown with growth time of 3 h, 2 h, 1 h 30 min 10 min (reprint from Ref. [22] with permission).

the process time was 20 min [25]. The method does not include pre-deposition of the catalyst layer. Instead, it uses iron chloride powder and acetylene gas because iron chloride has a high dehydrogenation activity on acetylene. They termed the process as chlorine-mediated chemical vapor deposition (CM-CVD).

In 1999, Shoushan et al. [26] have achieved 30–240 μm heights of MWCNT-VACNT grown by CVD reaction times of 5–60 min. The condition for growth is electrochemical etching of porous silicon of 2 in diameter, patterning with 5-nm-thick layer of Fe by electron beam evaporation with a shadow mask. The substrate was placed in the quartz tube reactor housed in a tube furnace. The operating temperature was 700 °C with Ar flow; ethylene was then flown at 1000 sccm for 15–60 min. The diameters of the MWCNT-VACNT were found to be 16 ± 2 nm. Ren et al. [27] have grown 0.1–50 μm heights of VACNTs using plasma-enhanced chemical vapor deposition (PECVD).

Lee et al. measured the possibility in factor determining the upper limit to the height of spinnable carbon nanotube forests [28]. By synthesizing CNT forests at different growth times such as 3, 6, 9, 12, 15, and 60 min. The heights were found to be 260 μm for 3 min growth time and 1.7 mm height for 60 mm growth time. An important observation was made, as the growth time increased the degree of alignment decreased. It was verified by Raman analysis as it showed G and D bands intensity ratios different at top and bottom.

Hence a clear observation can be made by all the reports is that the process time with a certain flow rate of the precursor yields in governing the lengths of VACNT. Also, these parameters have a grave effect on the degree of alignment of the VACNTs.

#### *4.1.2. Large area growth of VACNT*

**4.1. Long and dense growth of aligned/super-aligned CNTs**

4.1.1. Long heights of the CNT

4.1.3. Density of the CNTs per unit area 4.1.4. Degree of alignment of the CNTs

4.1.7. Directing purity and catalyst

*4.1.1. Long heights/lengths of CNT forest*

was calculated to be 27.47 μm min−1.

long-life catalyst film using Al<sup>2</sup>

**Figure 3**.

Al2 O3

(FeCl2

Other parameters concerning growth of VACNTs

4.1.6. Directing the chirality of the CNT in the array

4.1.2. Area of the substrate

16 Carbon Nanotubes - Recent Progress

Some of the core challenges relating to CNT array's growth are:

4.1.5. Directing number of walls and diameter of the CNT array

taming the inkling of the process for lengthy heights of CNT forest.

O3

Growing CNTs vertically in a long length is a critical challenge in the CNT array growth. Achieving long CNT forest can result in higher lengths of CNT yarns or films per unit area. In the application's point of view, obtaining drawable CNT forest is the real challenge. Many researchers have reported many possible lengths of drawable CNT forests. Hence, the following discussion is the current possible maximum drawable CNT forest heights which give an insight in the initial heights obtained for CNT forests. Few examples are briefed below for

The record for the large height of VACNT array was reported in 2014 by Cho et al. [20], with a height of 21.7 mm, and not tested for drawability. Its water-assisted thermal CVD process produced such long VACNTs, and most of the CNTs were double walled. The growth rate

Another report stated that 4.7-mm-long CNT arrays were grown with high efficiency and

spinnable [21]. Similarly, in another event, 4-mm high-quality CNT array was grown without water assistance and showed that the catalyst lifetime lasted for 3 h [22] as illustrated in

In a recent report, aligned spinnable CNTs of heights 400 μm were synthesized by Alvarez et al. [23]; Fe and Co are used as the catalyst sources with 1.2 nm thin film. About 5 nm


) as catalyst and acetylene as a carbon source. The same group has previously reported drawable VAMWCNT array measuring a height of 2.1 mm with same precursor material, but

and Fe as buffer layer and catalyst. The VACNTs were

It can be understood that the first challenge which lies in the geometry for VACNTs has a large area with long and densely aligned CNTs arrays. For large scale growth of CNT on a large area array, many researcher groups have been experimenting with it since a decade. CNT array growth area on large wafers has attained a growth area of 8-inch (18 cm) by Chen et al. [29] in 2010, and the super-aligned CNT film (SACNT) was free-standing, ultra-thin, lightweight transparent and conductive. The height of the SACNT array was 250 μm, and transmittance was measured up to 83%. This high area SACNT can be employed in many advanced applications such as transparent conducting films by replacing conventional indium tin oxide (ITO).

surface property of the CNTs. Therefore, the surface purity of the CNTs is crucial for the

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

High alignment: floating catalyst; chemical vapor deposition method, is a reliable method to synthesize highly aligned CNT sheets, as the fabrication process is easy and low cost [10].

As mentioned above the many factors govern the growth and alignment of VACNT in such regard it involves parameters like a catalyst, the thickness of the catalyst nanoparticles thin film, precursor, growth method, carrier gas, flow rate, time of the process, initialization and

Principally, a number of walls and diameter control is a significant research alongside VACNTs,

Controlling the catalyst size by sulfur is a recent report which claimed that the controlling of particle size by influencing the catalyst size by sulfur. The injecting time and temperature of sulfur resulted in controlling the majority of the CNTs walls to SWCNT, DWCNT, and MWCNT [33]. In another event, water-assisted growth CNT arrays yielded in SWCNTs with a uniform diameter. The mechanism behind the uniformity was prophesied as water contributes to uniform catalyst distribution. Hence high-density uniform SWCNT arrays were obtained [9]. By using FCCVD method DWCNTs were achieved by coating a monolayer of the

dow. Also, the buffer layer thickness regulates size and distribution of catalyst particles. In an ideal circumstance, uniformly distributed, dense and small-sized catalyst nanoparticles

grown, and spinnability was tested at different thicknesses keeping one constant at a time. It was found that at a low thickness of Fe like <0.8 nm VACNTs grown are short. However, for 1 nm Fe layer the VACNT averaged to 0.7 μm and the thickness of 1–1.2 nm resulted in highly

As it is quite familiar that CNTs can be classified into three types of chirality, that is, armchair, chiral, and zigzag, depending on the specific discrete angle rolling of the graphene. Not

O3

O3

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19

layer is beneficial for the spinnability

was thermally grown followed by Fe layer, and VACNT was

of a narrow win-

and numerous fruitful and reproducible synthesis parameters have been established.

organic polymer during the process of CVD [34]. An example is shown in **Figure 4**.

Buffer layer influence on catalyst morphology and spinnability of VACNT.

The spinnability of VACNT depends on buffer layer thickness such as Al<sup>2</sup>

O3

spinnable VACNTs. Moreover, it was found that Al2

*4.1.6. Directing the chirality of the CNT in the array*

drawability of the CNT array.

Directing purity and catalyst.

yield the excellent product. Al2

of VACNTs [35].

Other growth governing parameters are:

Directing the chirality of the CNT in the array.

Directing number of walls and diameter of the CNT array.

termination of VACNT growth process, and substrates of growth.

*4.1.5. Directing number of walls and diameter of the CNT array*

Previously, Zhang et al. [30], have grown SACNT on a 4-inch diameter wafer, using low-pressure CVD (LP-CVD) system. To guarantee the uniformity LP-CVD system was suggested. Also, SWCNT arrays were grown using water-assisted CVD method on a 100 mm (3.9 inches) substrate [9].

#### *4.1.3. Dense CNT forest (yarns/films)*

Similar to the height of the CNTs in the CNT array, CNTs density is also a key factor classifying the application of the grown CNT array. The density depends on two factors mainly the thickness of the CNTs in the array and number of walls of the CNTs in the array and to some extent on the degree of the alignment of the CNTs in the array.

Many researcher groups have succeeded to obtain persistent densities in the past decade; one such report attained a density of 22 SWNTs in 1 × 0.75 μm [31]. Nevertheless, the drawability was not determined this VACNT. Furthermore, SWCNT array is grown by a water-assisted report a high density of 6.4 × 10<sup>11</sup> cm−2. High-density growth was possible due to the petite catalyst size of 0.5 nm [9].

In a recent report, density of CNT arrays in CNT films was improved by eliminating the pores and gaps in the thin film resulting in density improvement by 109% [32], which also, in turn, improved the mechanical strength of 765 ± 15 and 184 ± 58 MPa and electrical conductivity to (1.65 ± 0.15) × 10<sup>5</sup> and (1.04 ± 0.10) × 10<sup>5</sup> S m−1. The pores and gaps were eliminated by synthesizing continuous CNT aerogel by FCCVD method. With precursor containing ethanol as the carbon source with ferrocene, thiophene, and deionized water carrier gas was a mixture of H2 and Ar which was fed to the furnace at 1200–1300 °C. Sock-like aerogel was formed by millions of CNTs which was densified by spraying ethanol. After that the film was passed through smooth plates at 100 N pressure, which further densified the film, again the film was rolled by two oppositely rolling rollers to improve the packing density.

As noticed, two approaches can be established to obtain dense CNT arrays, that is, by growing dense CNT arrays in the initial process, or densifying the CNT yarns after drawing/spinning (post-treatment) of CNT arrays.

#### *4.1.4. Alignment of carbon nanotubes*

Ordinary CNT array and super-aligned CNT array are presently known classifications of CNT arrays concerning the degree of alignment of CNTs.

The degree of alignment is a deciding factor for the successful drawing of the CNT yarn or CNT films. The degree of alignment is a subject depending upon the van der Waals forces between two consecutive CNTs in the array. Hence van der Waals forces are responsible for holding the successive CNTs together. These, van der Waals forces are dependent on the surface property of the CNTs. Therefore, the surface purity of the CNTs is crucial for the drawability of the CNT array.

High alignment: floating catalyst; chemical vapor deposition method, is a reliable method to synthesize highly aligned CNT sheets, as the fabrication process is easy and low cost [10].

Other growth governing parameters are:

Directing number of walls and diameter of the CNT array.

Directing the chirality of the CNT in the array.

Directing purity and catalyst.

in 2010, and the super-aligned CNT film (SACNT) was free-standing, ultra-thin, lightweight transparent and conductive. The height of the SACNT array was 250 μm, and transmittance was measured up to 83%. This high area SACNT can be employed in many advanced applications such as transparent conducting films by replacing conventional indium tin oxide (ITO). Previously, Zhang et al. [30], have grown SACNT on a 4-inch diameter wafer, using low-pressure CVD (LP-CVD) system. To guarantee the uniformity LP-CVD system was suggested. Also, SWCNT arrays were grown using water-assisted CVD method on a 100 mm (3.9 inches)

Similar to the height of the CNTs in the CNT array, CNTs density is also a key factor classifying the application of the grown CNT array. The density depends on two factors mainly the thickness of the CNTs in the array and number of walls of the CNTs in the array and to some

Many researcher groups have succeeded to obtain persistent densities in the past decade; one such report attained a density of 22 SWNTs in 1 × 0.75 μm [31]. Nevertheless, the drawability was not determined this VACNT. Furthermore, SWCNT array is grown by a water-assisted report a high density of 6.4 × 10<sup>11</sup> cm−2. High-density growth was possible due to the petite

In a recent report, density of CNT arrays in CNT films was improved by eliminating the pores and gaps in the thin film resulting in density improvement by 109% [32], which also, in turn, improved the mechanical strength of 765 ± 15 and 184 ± 58 MPa and electrical conductivity

thesizing continuous CNT aerogel by FCCVD method. With precursor containing ethanol as the carbon source with ferrocene, thiophene, and deionized water carrier gas was a mixture

As noticed, two approaches can be established to obtain dense CNT arrays, that is, by growing dense CNT arrays in the initial process, or densifying the CNT yarns after drawing/spin-

Ordinary CNT array and super-aligned CNT array are presently known classifications of

The degree of alignment is a deciding factor for the successful drawing of the CNT yarn or CNT films. The degree of alignment is a subject depending upon the van der Waals forces between two consecutive CNTs in the array. Hence van der Waals forces are responsible for holding the successive CNTs together. These, van der Waals forces are dependent on the

rolled by two oppositely rolling rollers to improve the packing density.

CNT arrays concerning the degree of alignment of CNTs.

 and Ar which was fed to the furnace at 1200–1300 °C. Sock-like aerogel was formed by millions of CNTs which was densified by spraying ethanol. After that the film was passed through smooth plates at 100 N pressure, which further densified the film, again the film was

and (1.04 ± 0.10) × 10<sup>5</sup> S m−1. The pores and gaps were eliminated by syn-

extent on the degree of the alignment of the CNTs in the array.

substrate [9].

18 Carbon Nanotubes - Recent Progress

*4.1.3. Dense CNT forest (yarns/films)*

catalyst size of 0.5 nm [9].

ning (post-treatment) of CNT arrays.

*4.1.4. Alignment of carbon nanotubes*

to (1.65 ± 0.15) × 10<sup>5</sup>

of H2

As mentioned above the many factors govern the growth and alignment of VACNT in such regard it involves parameters like a catalyst, the thickness of the catalyst nanoparticles thin film, precursor, growth method, carrier gas, flow rate, time of the process, initialization and termination of VACNT growth process, and substrates of growth.

#### *4.1.5. Directing number of walls and diameter of the CNT array*

Principally, a number of walls and diameter control is a significant research alongside VACNTs, and numerous fruitful and reproducible synthesis parameters have been established.

Controlling the catalyst size by sulfur is a recent report which claimed that the controlling of particle size by influencing the catalyst size by sulfur. The injecting time and temperature of sulfur resulted in controlling the majority of the CNTs walls to SWCNT, DWCNT, and MWCNT [33]. In another event, water-assisted growth CNT arrays yielded in SWCNTs with a uniform diameter. The mechanism behind the uniformity was prophesied as water contributes to uniform catalyst distribution. Hence high-density uniform SWCNT arrays were obtained [9]. By using FCCVD method DWCNTs were achieved by coating a monolayer of the organic polymer during the process of CVD [34]. An example is shown in **Figure 4**.

Buffer layer influence on catalyst morphology and spinnability of VACNT.

The spinnability of VACNT depends on buffer layer thickness such as Al<sup>2</sup> O3 of a narrow window. Also, the buffer layer thickness regulates size and distribution of catalyst particles. In an ideal circumstance, uniformly distributed, dense and small-sized catalyst nanoparticles yield the excellent product. Al2 O3 was thermally grown followed by Fe layer, and VACNT was grown, and spinnability was tested at different thicknesses keeping one constant at a time. It was found that at a low thickness of Fe like <0.8 nm VACNTs grown are short. However, for 1 nm Fe layer the VACNT averaged to 0.7 μm and the thickness of 1–1.2 nm resulted in highly spinnable VACNTs. Moreover, it was found that Al2 O3 layer is beneficial for the spinnability of VACNTs [35].

#### *4.1.6. Directing the chirality of the CNT in the array*

As it is quite familiar that CNTs can be classified into three types of chirality, that is, armchair, chiral, and zigzag, depending on the specific discrete angle rolling of the graphene. Not

CNT yarn drawing speed and spinning rates.

**Figure 5.** A simple dry drawing of CNT forest into yarns by twisting.

316 m length of CNT yarn [37].

**5.1. Electrical properties**

**4.3. Drawing of CNT sheets/films using ACNTs**

**5. Properties of CNT yarns and CNT films**

possible technique to improve the electrical conductivity.

role.

Apart from the challenges mentioned above in growing VACNTs, regarding industrial application and mass production of CNTs, drawing speed and spinning rates play a prominent

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

http://dx.doi.org/10.5772/intechopen.70702

21

Alvarez et al. developed a dry spinning process which has improved drawing rates at about 15.93 m s−1 [23]. Improved drawing rates were achieved by using spinnable CNT arrays which get entirely separated from the growth substrate and the growing nanoparticle. Due to the nature of separating from the growth substrate, the CNT threads have higher diameter uniformity. The substrates can be reused for CNT forest growth as the CNT is wholly detached from the substrate and the nanoparticle. It was also possible to produce long CNT fibers by joining CNT ribbons from different batches. A typical 8-inch diameter substrate can spin

Like the drawing of CNT yarns, CNT sheets can be drawn using ACNTs produced by CVD process or rolled out from when produced by FCCVD process, or to deposit on to a surface while synthesizing CNTs. The CNT sheet drawing process is slightly different from CNT yarn. In a recent report CNTs synthesized by FCCVD method could deposit on an ethanol pre-wetted paper [38]. Xu et al. reported continuous production of CNT cylinder film. Moreover, the film had the strength of 9.6 GPa. FCCVD method was used to synthesize continuous produc-

Electrical properties of CNT yarns can be stated as the most important property of CNT yarns, as the CNT yarns are highly suitable for electrical and electronic applications. Many researchers suggest numerous methods to improve the conductivity of the CNT yarns. Ideally, yarns drawn from metallic SWCNT would constitute for maximum electrical conductivity. However, the growth of metallic SWCNT is a great challenge. The electrical conductivity of CNT yarns is dependent on the intertube spacing, purity, and as mentioned before chirality of the CNT. Therefore, densification, eliminating impurities, and controlling chirality are a

tion of CNT cylinder which was rolled onto a roller with densifying by ethanol [39].

**Figure 4.** Showing the growth of CNTs at various temperatures (reprinted from Ref. [33] with permission).

considerable research has driven researchers in atomic level structural control for directing the chirality of the CNT in the array.

#### *4.1.7. Directing the purity and catalyst*

The most commonly used catalyst for synthesizing ACNTs is ferrocene; the catalyst plays a crucial role in guiding the number of walls, size, and layers of CNTs in the ACNTs. This catalyst can also be a cause for impurity as there is a possibility of the catalyst embeds into the CNTs while growing. Mostly the concentration of catalyst particle depends on the type of synthesis. If the synthesis is CVD method, the impurity is relatively less than that of FCCVD method.

#### **4.2. Drawing of CNT yarns and fibers using VACNTs**

It is challenging to grow long CNTs which lead to a spinning method to obtain long CNT spinning threads. A typical CNT yarn drawing is shown in **Figure 5**.

The model anticipated for drawing of VACNTs into yarns is realized by Zhang et al. [30]. It is recorded that pulling of yarns while observing under scanning electron microscope and optical microscope revealed that super-aligned CNTs (SACNTs) hold strong van der Waal forces which keep the CNTs aligned. Also, the SACNTs bundles are joined end to end resulting in a continuous yarn, which was not for the case of normally aligned CNTs. Hence SACNTs possess van der Waals forces which can be used to spinning CNT yarn. It was also stated that the difference between normally aligned CNTs and SACNTs was that the SACNTs had very clean surfaces.

An optimum condition for the growth and direct drawing of, number of walls controlled VACNTs was performed by design of experiment method. All factors responsible for the direct spinning process such as regulating the flow rates of carbon source, catalyst source, sulfur, water, hydrogen, and reaction temperature were evaluated [36].

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread http://dx.doi.org/10.5772/intechopen.70702 21

**Figure 5.** A simple dry drawing of CNT forest into yarns by twisting.

CNT yarn drawing speed and spinning rates.

Apart from the challenges mentioned above in growing VACNTs, regarding industrial application and mass production of CNTs, drawing speed and spinning rates play a prominent role.

Alvarez et al. developed a dry spinning process which has improved drawing rates at about 15.93 m s−1 [23]. Improved drawing rates were achieved by using spinnable CNT arrays which get entirely separated from the growth substrate and the growing nanoparticle. Due to the nature of separating from the growth substrate, the CNT threads have higher diameter uniformity. The substrates can be reused for CNT forest growth as the CNT is wholly detached from the substrate and the nanoparticle. It was also possible to produce long CNT fibers by joining CNT ribbons from different batches. A typical 8-inch diameter substrate can spin 316 m length of CNT yarn [37].

#### **4.3. Drawing of CNT sheets/films using ACNTs**

Like the drawing of CNT yarns, CNT sheets can be drawn using ACNTs produced by CVD process or rolled out from when produced by FCCVD process, or to deposit on to a surface while synthesizing CNTs. The CNT sheet drawing process is slightly different from CNT yarn. In a recent report CNTs synthesized by FCCVD method could deposit on an ethanol pre-wetted paper [38]. Xu et al. reported continuous production of CNT cylinder film. Moreover, the film had the strength of 9.6 GPa. FCCVD method was used to synthesize continuous production of CNT cylinder which was rolled onto a roller with densifying by ethanol [39].

#### **5. Properties of CNT yarns and CNT films**

#### **5.1. Electrical properties**

considerable research has driven researchers in atomic level structural control for directing

**Figure 4.** Showing the growth of CNTs at various temperatures (reprinted from Ref. [33] with permission).

The most commonly used catalyst for synthesizing ACNTs is ferrocene; the catalyst plays a crucial role in guiding the number of walls, size, and layers of CNTs in the ACNTs. This catalyst can also be a cause for impurity as there is a possibility of the catalyst embeds into the CNTs while growing. Mostly the concentration of catalyst particle depends on the type of synthesis. If the synthesis is CVD method, the impurity is relatively less than that of FCCVD

It is challenging to grow long CNTs which lead to a spinning method to obtain long CNT

The model anticipated for drawing of VACNTs into yarns is realized by Zhang et al. [30]. It is recorded that pulling of yarns while observing under scanning electron microscope and optical microscope revealed that super-aligned CNTs (SACNTs) hold strong van der Waal forces which keep the CNTs aligned. Also, the SACNTs bundles are joined end to end resulting in a continuous yarn, which was not for the case of normally aligned CNTs. Hence SACNTs possess van der Waals forces which can be used to spinning CNT yarn. It was also stated that the difference between normally aligned CNTs and SACNTs was that the SACNTs had very

An optimum condition for the growth and direct drawing of, number of walls controlled VACNTs was performed by design of experiment method. All factors responsible for the direct spinning process such as regulating the flow rates of carbon source, catalyst source,

the chirality of the CNT in the array.

20 Carbon Nanotubes - Recent Progress

*4.1.7. Directing the purity and catalyst*

**4.2. Drawing of CNT yarns and fibers using VACNTs**

spinning threads. A typical CNT yarn drawing is shown in **Figure 5**.

sulfur, water, hydrogen, and reaction temperature were evaluated [36].

method.

clean surfaces.

Electrical properties of CNT yarns can be stated as the most important property of CNT yarns, as the CNT yarns are highly suitable for electrical and electronic applications. Many researchers suggest numerous methods to improve the conductivity of the CNT yarns. Ideally, yarns drawn from metallic SWCNT would constitute for maximum electrical conductivity. However, the growth of metallic SWCNT is a great challenge. The electrical conductivity of CNT yarns is dependent on the intertube spacing, purity, and as mentioned before chirality of the CNT. Therefore, densification, eliminating impurities, and controlling chirality are a possible technique to improve the electrical conductivity.

For CNT yarns spun from DWCNT with diameters of 5–20 μm, the electrical conductivity and maximum current carrying, the capability was calculated to be 5.9 × 10<sup>5</sup> S m−1 and 1 × 10<sup>5</sup> A cm−1 [40] by high current-induced thermal burnout analysis in the air. Doping is also a possible approach to improve the electrical conductivity of CNT yarns, in such an event iodine doped CNT cables have resulted in specific conductivity greater than that of metals [41]. With an electrical resistivity of 10–7 ohm, for DWCNT drew cables.

the thermal damping mechanism in SACNT Bucky papers. It is found that the heat transfer speed is different for different densities of SACNT Bucky papers. It was about 562.2 6

s−1 for parallel SACNT Bucky papers [49]. Concerning with the electrochemical

and HNO3

Rapid Growth of Dense and Long Carbon Nanotube Arrays and Its Application in Spinning Thread

to CNT film was observed to

http://dx.doi.org/10.5772/intechopen.70702

23

SO4

and H2

O2

and HNO3

treat-

O2

. This soak-

O2

have a high cycle performance rate of 364 and 391 mA h g−1 [50]. FCCVD grown CNTA electrode in an organic electrolyte had a specific capacitance of 24.5 Fg−1 higher than that of

Replacing electrical wires with CNT yarns is the primary application of CNT yarns, in such an

CNT fibre microelectrodes as dopamine sensor [52], as special oxygen-containing functional

Weavable non-volatile memory devices by carbon fibers [53] using MWCNT fibers for an electrode with graphene oxide as the active material. In which MWCNT array is wound over a glass rod and densifying with ethanol and coating with graphene oxide of 10–20 nm. MWCNT and graphene oxide are joined non-covalently. With further processing, it works as a fiber based memory cell which exhibits write once and read many times type memory effect. Pure 3D CNT aerogels were produced by the spontaneous expansion of SACNT films, the

ing resulted in producing aerogels with a tremendously low apparent density of 0.12 mg cm−3 and a porosity of 99.95% [54], which leads to applications such as energy storage, catalysis as

Water transport phenomena through VADWCNT array was reported by Matsumoto et al. [55]. By Parylene-C coating one side of the VADWCNT array and then peeling off the other end of VADWCNT array from the Si substrate. Finally, the Parylene-C was finely milled to expose the CNT array which was a flexible and durable membrane. Then it was used to be

Wire-shaped micro-supercapacitors with aligned titania wire were wounded by CNT yarn

ment and increasing the cycle rate performance, CNT yarns can be used as electrodes for Li-ion batteries [50]. Neurotransmitter dynamic detection by CNT yarn electrodes [58].

Platinum-coated CNT films were demonstrated to have used in fuel cells and solar cells [38].

attempt insulation of CNT yarns, was made to replace existing copper wiring [51].

expansion of SACNT films was formed by soaking in a mixture of H<sup>2</sup>

tested for water transport by dead-end filtration apparatus.

[56]. Field emission applications [26, 57]. By functionalizing walls by H2

CNT sheets are produced by SWCNTs as electrochemical actuators [59].

55.4 mm2

aqueous electrolyte [11].

**6.1. CNT fiber/yarn/thread**

**6. Applications**

property of SACNTs a surface oxidized by H2

groups, were observed on the surface of carbon.

they show high adsorption abilities.

**6.2. Applications of CNT films/sheets**

Improving the electrical conductivity methods include mechanical condensation and acid treatment of CNT yarns, which increased the conductivity of the CNT yarns by nine times [42]. Which leads to a maximum current density of 66,000 A cm−2. It was also observed that mechanical condensation reduced resistance only a few ohms but increased the electrical conductivity by four times. Further acid (HNO3 ) treatment drastically reduced resistance by 50% and improving the conductivity. Acid treatment helps in eliminating amorphous carbon and catalyst particle on the surface of the CNTs hence improving the conductivity. Metal deposition on CNT yarn has also been produced by depositing Cu and Au, on CNT yarns by electroless deposition, the metal-doped CNT yarns had metal like conductivities but the strength reduced by 30–50% [43].

#### **5.2. Mechanical properties**

The mechanical strength of CNT yarn relies upon all the major components of CNT yarn synthesis. Mechanical strength also varies with the length of the CNT yarn. Hence comparing all the reports about mechanical strength with constant length is a difficult issue. Researchers mostly pick the most suitable lengths to determine the mechanical properties and many methods to improve the mechanical strength. Hence it takes the sharp eye in concluding the bestobtained result till date, however, it is very beneficial to know all the approaches taken in improving the mechanical strength.

Oxygen plasma-treated CNT yarn improved the tensile strength and Young's moduli to 2.2 GPa and 200 GPa, respectively [44]. Crosslinking technique is a new technology established in CNT yarns and found to improve the strength from 0.2 to 1.4 GPa [45]. Shrinking effect caused by passing freshly produced CNT yarn into volatile solvents (water, ethanol, and acetone) has found to increase the tensile strength to 1 GPa, in which acetone is resulting in the best tensile strength [46]. Small-molecule functionalized with electron beam irradiation improved the tensile strength by 25% upon functionalization, and a total increase of 88% tensile strength was observed when irradiated with the electron beam after functionalization [47]. In a report, recently it was found that mechanical strength of CNT yarns can improve by acid treatment followed by epoxy treatment [48]. Moreover, a maximum stiffness of 62.0 GPa was recorded. Also, CNT films have strengths up to 9.6 GPa produced by FCCVD method [39].

#### **5.3. Other properties**

Apart from the mechanical and electrical properties of VACNT, thermal properties of VACNTs has also been a locus point in determining the underlying properties of VACNTs. In a study, thermal transfer speed in highly aligned CNTs was investigated to determine the thermal damping mechanism in SACNT Bucky papers. It is found that the heat transfer speed is different for different densities of SACNT Bucky papers. It was about 562.2 6 55.4 mm2 s−1 for parallel SACNT Bucky papers [49]. Concerning with the electrochemical property of SACNTs a surface oxidized by H2 O2 and HNO3 to CNT film was observed to have a high cycle performance rate of 364 and 391 mA h g−1 [50]. FCCVD grown CNTA electrode in an organic electrolyte had a specific capacitance of 24.5 Fg−1 higher than that of aqueous electrolyte [11].

### **6. Applications**

For CNT yarns spun from DWCNT with diameters of 5–20 μm, the electrical conductivity and maximum current carrying, the capability was calculated to be 5.9 × 10<sup>5</sup> S m−1 and 1 × 10<sup>5</sup> A cm−1 [40] by high current-induced thermal burnout analysis in the air. Doping is also a possible approach to improve the electrical conductivity of CNT yarns, in such an event iodine doped CNT cables have resulted in specific conductivity greater than that of metals [41]. With

Improving the electrical conductivity methods include mechanical condensation and acid treatment of CNT yarns, which increased the conductivity of the CNT yarns by nine times [42]. Which leads to a maximum current density of 66,000 A cm−2. It was also observed that mechanical condensation reduced resistance only a few ohms but increased the electrical con-

and improving the conductivity. Acid treatment helps in eliminating amorphous carbon and catalyst particle on the surface of the CNTs hence improving the conductivity. Metal deposition on CNT yarn has also been produced by depositing Cu and Au, on CNT yarns by electroless deposition, the metal-doped CNT yarns had metal like conductivities but the strength

The mechanical strength of CNT yarn relies upon all the major components of CNT yarn synthesis. Mechanical strength also varies with the length of the CNT yarn. Hence comparing all the reports about mechanical strength with constant length is a difficult issue. Researchers mostly pick the most suitable lengths to determine the mechanical properties and many methods to improve the mechanical strength. Hence it takes the sharp eye in concluding the bestobtained result till date, however, it is very beneficial to know all the approaches taken in

Oxygen plasma-treated CNT yarn improved the tensile strength and Young's moduli to 2.2 GPa and 200 GPa, respectively [44]. Crosslinking technique is a new technology established in CNT yarns and found to improve the strength from 0.2 to 1.4 GPa [45]. Shrinking effect caused by passing freshly produced CNT yarn into volatile solvents (water, ethanol, and acetone) has found to increase the tensile strength to 1 GPa, in which acetone is resulting in the best tensile strength [46]. Small-molecule functionalized with electron beam irradiation improved the tensile strength by 25% upon functionalization, and a total increase of 88% tensile strength was observed when irradiated with the electron beam after functionalization [47]. In a report, recently it was found that mechanical strength of CNT yarns can improve by acid treatment followed by epoxy treatment [48]. Moreover, a maximum stiffness of 62.0 GPa was recorded.

Apart from the mechanical and electrical properties of VACNT, thermal properties of VACNTs has also been a locus point in determining the underlying properties of VACNTs. In a study, thermal transfer speed in highly aligned CNTs was investigated to determine

Also, CNT films have strengths up to 9.6 GPa produced by FCCVD method [39].

) treatment drastically reduced resistance by 50%

an electrical resistivity of 10–7 ohm, for DWCNT drew cables.

ductivity by four times. Further acid (HNO3

reduced by 30–50% [43].

22 Carbon Nanotubes - Recent Progress

**5.2. Mechanical properties**

improving the mechanical strength.

**5.3. Other properties**

#### **6.1. CNT fiber/yarn/thread**

Replacing electrical wires with CNT yarns is the primary application of CNT yarns, in such an attempt insulation of CNT yarns, was made to replace existing copper wiring [51].

CNT fibre microelectrodes as dopamine sensor [52], as special oxygen-containing functional groups, were observed on the surface of carbon.

Weavable non-volatile memory devices by carbon fibers [53] using MWCNT fibers for an electrode with graphene oxide as the active material. In which MWCNT array is wound over a glass rod and densifying with ethanol and coating with graphene oxide of 10–20 nm. MWCNT and graphene oxide are joined non-covalently. With further processing, it works as a fiber based memory cell which exhibits write once and read many times type memory effect.

Pure 3D CNT aerogels were produced by the spontaneous expansion of SACNT films, the expansion of SACNT films was formed by soaking in a mixture of H<sup>2</sup> SO4 and H2 O2 . This soaking resulted in producing aerogels with a tremendously low apparent density of 0.12 mg cm−3 and a porosity of 99.95% [54], which leads to applications such as energy storage, catalysis as they show high adsorption abilities.

Water transport phenomena through VADWCNT array was reported by Matsumoto et al. [55]. By Parylene-C coating one side of the VADWCNT array and then peeling off the other end of VADWCNT array from the Si substrate. Finally, the Parylene-C was finely milled to expose the CNT array which was a flexible and durable membrane. Then it was used to be tested for water transport by dead-end filtration apparatus.

Wire-shaped micro-supercapacitors with aligned titania wire were wounded by CNT yarn [56]. Field emission applications [26, 57]. By functionalizing walls by H2 O2 and HNO3 treatment and increasing the cycle rate performance, CNT yarns can be used as electrodes for Li-ion batteries [50]. Neurotransmitter dynamic detection by CNT yarn electrodes [58].

#### **6.2. Applications of CNT films/sheets**

Platinum-coated CNT films were demonstrated to have used in fuel cells and solar cells [38]. CNT sheets are produced by SWCNTs as electrochemical actuators [59].

#### **Acknowledgements**

The authors thank SGU-MEXT for their kind support for providing funds for PhD degree students in Japan.

[9] Wyss R, Klare J, Park H, Noy A, Bakajin O, Lulevich V. Water-assisted growth of uniform 100 mm diameter SWCNT arrays. ACS Applied Materials & Interfaces. 2014;**6**:

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#### **Author details**

Yasuhiko Hayashi1 \*, Karthik Paneer Selvam1 and Maik Scholz2

\*Address all correspondence to: hayashi.yasuhiko@okayama-u.ac.jp

1 Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan

2 Leibniz Institute for Solid State and Material Research Dresden, Dresden, Germany

#### **References**


[9] Wyss R, Klare J, Park H, Noy A, Bakajin O, Lulevich V. Water-assisted growth of uniform 100 mm diameter SWCNT arrays. ACS Applied Materials & Interfaces. 2014;**6**: 21019-21025

**Acknowledgements**

24 Carbon Nanotubes - Recent Progress

students in Japan.

**Author details**

Yasuhiko Hayashi1

Japan

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The authors thank SGU-MEXT for their kind support for providing funds for PhD degree

1 Graduate School of Natural Science and Technology, Okayama University, Okayama,

2 Leibniz Institute for Solid State and Material Research Dresden, Dresden, Germany

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**Chapter 3**

**Provisional chapter**

**Stability and Reliability of an Electrical Device**

**Stability and Reliability of an Electrical Device** 

**Nanotubes as a Field Emitter**

**Nanotubes as a Field Emitter**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72581

Norihiro Shimoi

**Abstract**

**1. Introduction**

Norihiro Shimoi

**Employing Highly Crystalline Single-Walled Carbon**

Carbon nanomaterial is drawing keen interest from researchers as well as materials scientists. Carbon nanotubes (CNTs)—and their nanoscale needle shape—offering chemical stability, thermal conductivity, and mechanical strength exhibit unique properties as a quasi-one-dimensional material. Among the expected applications, field emission electron sources appear the most promising industrially and are approaching practical utilization. However, efforts to construct a field emission (FE) cathode with single-walled carbon nanotubes (SWCNTs) have so far only helped average out a non-homogeneous electron emitter plane with large FE current fluctuations and a short emission life-time because they failed to realize a stable emission current owing to crystal defects of the carbon network in CNTs. The utilization of CNTs to obtain an effective cathode, one with a stable emission and low FE current fluctuation, relies on the ability to disperse CNTs uniformly in liquid media. In particular, highly crystalline SWCNTs hold promise to obtain good stability and reliability. The author successfully manufactured highly crystalline SWCNTs-based FE lighting elements that exhibit stable electron emission, a long emission life-time, and low power consumption for electron emitters. This FE device employing highly crystalline SWCNTs has the potential for conserving energy through low power consumption in our habitats.

**Employing Highly Crystalline Single-Walled Carbon** 

DOI: 10.5772/intechopen.72581

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

The further development of electronic systems necessitates the production of efficient devices employing carbon nano-materials. Carbon nanotubes (CNTs) represent a promising material due to their unique physicochemical properties: their nanoscale needle shape, high chemical

**Keywords:** single-walled carbon nanotube, high crystallization, field emission, wet coating process, thin film, scratch, planar light source, cathode luminescence


**Provisional chapter**

#### **Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled Carbon Nanotubes as a Field Emitter Employing Highly Crystalline Single-Walled Carbon Nanotubes as a Field Emitter**

**Stability and Reliability of an Electrical Device** 

DOI: 10.5772/intechopen.72581

Norihiro Shimoi Norihiro Shimoi Additional information is available at the end of the chapter

[53] Sun G, Liu J, Zheng L, Huang W, Zhang H. Preparation of Weavable, All-Carbon Fibers for Non-Volatile Memory Devices. Angew Chem-ger Edit. 2013;**125**:13593-13597

[54] Luo Y, Luo S, Wu H, Li M, Wang K, Yan L, Jiang K, Li Q, Fan S, Wang J. Self-expansion construction of ultralight carbon nanotube aerogels with a 3D and hierarchical cellular

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[56] Chen T, Dai L. Flexible and wearable wire-shaped micro supercapacitors based on highly aligned titania and carbon nanotubes. Energy Storage Materials. 2016;**2**:21-26 [57] Wei Y, Jiang J, Liu L, Chen Z, Fan F. Vacuum-breakdown-induced needle-shaped ends of multiwalled carbon nanotube yarns and their field emission applications. Nano Letters.

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structure. Small. 2017:1700966

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2007;**7**:3792-3797

28 Carbon Nanotubes - Recent Progress

2822-2832

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72581

#### **Abstract**

Carbon nanomaterial is drawing keen interest from researchers as well as materials scientists. Carbon nanotubes (CNTs)—and their nanoscale needle shape—offering chemical stability, thermal conductivity, and mechanical strength exhibit unique properties as a quasi-one-dimensional material. Among the expected applications, field emission electron sources appear the most promising industrially and are approaching practical utilization. However, efforts to construct a field emission (FE) cathode with single-walled carbon nanotubes (SWCNTs) have so far only helped average out a non-homogeneous electron emitter plane with large FE current fluctuations and a short emission life-time because they failed to realize a stable emission current owing to crystal defects of the carbon network in CNTs. The utilization of CNTs to obtain an effective cathode, one with a stable emission and low FE current fluctuation, relies on the ability to disperse CNTs uniformly in liquid media. In particular, highly crystalline SWCNTs hold promise to obtain good stability and reliability. The author successfully manufactured highly crystalline SWCNTs-based FE lighting elements that exhibit stable electron emission, a long emission life-time, and low power consumption for electron emitters. This FE device employing highly crystalline SWCNTs has the potential for conserving energy through low power consumption in our habitats.

**Keywords:** single-walled carbon nanotube, high crystallization, field emission, wet coating process, thin film, scratch, planar light source, cathode luminescence

#### **1. Introduction**

The further development of electronic systems necessitates the production of efficient devices employing carbon nano-materials. Carbon nanotubes (CNTs) represent a promising material due to their unique physicochemical properties: their nanoscale needle shape, high chemical

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

stability, thermal conductivity, and mechanical strength. These properties confer an advantage in the fabrication of field emitters. The utilization of single-walled carbon nanotubes (SWCNTs) relies on their electronic properties since they can be either metallic or semiconductive, depending on the geometric configuration of a graphene sheet rolled up as a tube (i.e., diameter and chiral angle) [1–4].

of FE from the arc-produced nanotubes, concluding with a description of the application of the nanotube emitters in a flat-panel lighting device. Here, the author successfully manufactured SWCNT-based field emission lighting elements exhibiting stable electron emission, adequate

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled…

http://dx.doi.org/10.5772/intechopen.72581

31

Purified and highly crystalline SWCNTs synthesized by arcing and sintered at high temperature in a vacuum have been expected to serve as an electrical source material for a field emitter though the homogeneous dispersion of highly crystalline SWCNTs has yet to be reported. This is because highly crystalline SWCNTs have almost no defects in the carbon network on their surface, nor do they combine with another functional group of dispersants. It is difficult for SWCNTs aggregated as a secondary particle to separate uniformly into single nanotubes. Furthermore, despite the necessity of a thin film homogeneously dispersing highly crystalline SWCNTs for a field emission cathode plate, as yet no method for synthesizing a thin film with SWCNTs for a field emitter

In this chapter, commercial arc-SWCNTs (ASP-100F, Hanwha Chemicals Co. Ltd., Korea) were used. The SWCNTs were annealed at a high temperature (around 1000 K) in a high vacuum (pressure, >10−5 Pa) to obtain highly crystalline SWCNTs. **Figure 1** shows transmission electron microscopy (TEM, HR-3000, Hitachi High-Technologies Corporation, Japan) images of the SWCNTs (a) after and (b) before annealing as a reference. As shown in **Figure 1(a)**, the crystallinity of the SWCNTs was significantly improved after annealing at a high temperature in a tight vacuum.

Here, analytical grade reagents were used in the experiments. The annealed arc-SWCNTs checked for high crystallization were measured by Raman-shift. The Raman spectrum measured by the blue laser beam (Blue) at a 473 nm wavelength is shown in **Figure 2**. Raman peaks from the G+ band mode (1590 cm−1) and from the G\_ band mode (1565 cm−1) presumed to be nanotube structures. A Raman peak intensity of near 1350 cm−1 from the D band mode indicated lacks of carbon network on a nanotube. The intensity ratio between the G+ band and the D band was 0.0096; this value confirms that the SWCNT has a carbon network with

**Figure 1.** TEM images with annealed and unannealed SWCNTs [24]. (a) Highly crystalline SWCNTs with annealing

treatment. (b) SWCNTs with crystal defects without annealing treatment.

luminance, and low power consumption for the electron emitters.

**2. Experimental crystallization of annealed SWCNTs**

exhibiting a good homogeneity of plane-light emission has been developed.

high crystallization.

CNTs express one-dimensional circumscription effects and have characteristics as quantum wires coherently [5–7], and CNTs have the highest Young's modulus of all known materials mechanically [8]. Owing to these outstanding properties, wide-ranging applications for nanotubes are currently under investigation, including their use as electron field emitters [9], probes in scanning-type microscopes [10], gas (e.g. hydrogen) storage materials [11], and as electrode materials for secondary batteries as well as in capacitors [12]. Among these proposed applications, field emission electron sources appear the most promising industrially and in fact are approaching practical utilization. When a highly electrical field in the order of 103 V/μm is applied onto a surface of an electron emitter, electrons emit from inside the emitter's solid to a vacuum atmosphere by the quantum-mechanical tunneling effect. This phenomenon is called the field emission. Such an extremely high field can be obtained on the sharp tip having a thin needle because electric fields concentrate at the top of the sharp tip. It is said that the carbon nanotubes possess the following physical or chemical properties for field emitters: 1) a high aspect ratio with a sharp tip, 2) high mechanical strength, and 3) high chemical stability. Field emission (FE) phenomenon from an isolated single multiwalled carbon nanotube (MWCNT) was first reported by Rinzler et al. [13] in 1995, and FE from a MWCNT film was reported by de Heer et al. that same year [14]. Subsequently, many experimental studies on FE from MWCNTs [15–18] and SWCNTs [19] have appeared. Field emission microscopy (FEM) has also been used to clarify the geometric structures of the nanotube tips [16, 19]. Moreover, many studies have attempted to employ CNTs as field emitters in a display cathode with a lower driving voltage and stable electron emission. These trials mostly involve the vertical alignment of the CNTs in their fabrication that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) or laser abrasion fabrication; for example, screen printing with high-viscosity submicron to micron scale metal-particle paste to fabricate patterned field emitters has been proposed [20–22]. However, these approaches could not obtain the homogeneous electron emitter plane with small FE current fluctuations as they failed to construct a uniform thin film employing a homogeneous dispersion of CNTs.

The utilization of CNTs to obtain an effective cathode—with both a stable field emission and low FE current fluctuation—hinges on the ability to disperse them uniformly in liquid media. In particular, highly crystalline SWCNTs can be expected to emit electrons stably with a low turn-on and driving voltage, yet their homogeneous dispersion has yet to be reported. For this study, we selected a low-viscosity solvent with an In<sup>2</sup> O3 -SnO2 (tin-doped indium oxide; ITO) precursor solution as the conductive matrix material being dispersed highly crystalline SWCNTs. A dispersant was added to obtain the well-dispersed highly crystalline SWCNTs, and the mixture with the ITO solvent, highly crystalline SWCNTs, and the dispersant was agitated by an ultra-sonic homogenizer.

This chapter first briefly reviews the synthesis of highly crystalline SWCNTs by arc discharge and their structural characterization [23], followed by a discussion of the characteristic properties of FE from the arc-produced nanotubes, concluding with a description of the application of the nanotube emitters in a flat-panel lighting device. Here, the author successfully manufactured SWCNT-based field emission lighting elements exhibiting stable electron emission, adequate luminance, and low power consumption for the electron emitters.

#### **2. Experimental crystallization of annealed SWCNTs**

stability, thermal conductivity, and mechanical strength. These properties confer an advantage in the fabrication of field emitters. The utilization of single-walled carbon nanotubes (SWCNTs) relies on their electronic properties since they can be either metallic or semiconductive, depending on the geometric configuration of a graphene sheet rolled up as a tube

CNTs express one-dimensional circumscription effects and have characteristics as quantum wires coherently [5–7], and CNTs have the highest Young's modulus of all known materials mechanically [8]. Owing to these outstanding properties, wide-ranging applications for nanotubes are currently under investigation, including their use as electron field emitters [9], probes in scanning-type microscopes [10], gas (e.g. hydrogen) storage materials [11], and as electrode materials for secondary batteries as well as in capacitors [12]. Among these proposed applications, field emission electron sources appear the most promising industrially and in fact are approaching practical utilization. When a highly electrical field in the order of 103 V/μm is applied onto a surface of an electron emitter, electrons emit from inside the emitter's solid to a vacuum atmosphere by the quantum-mechanical tunneling effect. This phenomenon is called the field emission. Such an extremely high field can be obtained on the sharp tip having a thin needle because electric fields concentrate at the top of the sharp tip. It is said that the carbon nanotubes possess the following physical or chemical properties for field emitters: 1) a high aspect ratio with a sharp tip, 2) high mechanical strength, and 3) high chemical stability. Field emission (FE) phenomenon from an isolated single multiwalled carbon nanotube (MWCNT) was first reported by Rinzler et al. [13] in 1995, and FE from a MWCNT film was reported by de Heer et al. that same year [14]. Subsequently, many experimental studies on FE from MWCNTs [15–18] and SWCNTs [19] have appeared. Field emission microscopy (FEM) has also been used to clarify the geometric structures of the nanotube tips [16, 19]. Moreover, many studies have attempted to employ CNTs as field emitters in a display cathode with a lower driving voltage and stable electron emission. These trials mostly involve the vertical alignment of the CNTs in their fabrication that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) or laser abrasion fabrication; for example, screen printing with high-viscosity submicron to micron scale metal-particle paste to fabricate patterned field emitters has been proposed [20–22]. However, these approaches could not obtain the homogeneous electron emitter plane with small FE current fluctuations as they failed to construct a uniform thin film employing a homogeneous dispersion of CNTs.

The utilization of CNTs to obtain an effective cathode—with both a stable field emission and low FE current fluctuation—hinges on the ability to disperse them uniformly in liquid media. In particular, highly crystalline SWCNTs can be expected to emit electrons stably with a low turn-on and driving voltage, yet their homogeneous dispersion has yet to be

indium oxide; ITO) precursor solution as the conductive matrix material being dispersed highly crystalline SWCNTs. A dispersant was added to obtain the well-dispersed highly crystalline SWCNTs, and the mixture with the ITO solvent, highly crystalline SWCNTs, and

This chapter first briefly reviews the synthesis of highly crystalline SWCNTs by arc discharge and their structural characterization [23], followed by a discussion of the characteristic properties

O3 -SnO2

(tin-doped

reported. For this study, we selected a low-viscosity solvent with an In<sup>2</sup>

the dispersant was agitated by an ultra-sonic homogenizer.

(i.e., diameter and chiral angle) [1–4].

30 Carbon Nanotubes - Recent Progress

Purified and highly crystalline SWCNTs synthesized by arcing and sintered at high temperature in a vacuum have been expected to serve as an electrical source material for a field emitter though the homogeneous dispersion of highly crystalline SWCNTs has yet to be reported. This is because highly crystalline SWCNTs have almost no defects in the carbon network on their surface, nor do they combine with another functional group of dispersants. It is difficult for SWCNTs aggregated as a secondary particle to separate uniformly into single nanotubes. Furthermore, despite the necessity of a thin film homogeneously dispersing highly crystalline SWCNTs for a field emission cathode plate, as yet no method for synthesizing a thin film with SWCNTs for a field emitter exhibiting a good homogeneity of plane-light emission has been developed.

In this chapter, commercial arc-SWCNTs (ASP-100F, Hanwha Chemicals Co. Ltd., Korea) were used. The SWCNTs were annealed at a high temperature (around 1000 K) in a high vacuum (pressure, >10−5 Pa) to obtain highly crystalline SWCNTs. **Figure 1** shows transmission electron microscopy (TEM, HR-3000, Hitachi High-Technologies Corporation, Japan) images of the SWCNTs (a) after and (b) before annealing as a reference. As shown in **Figure 1(a)**, the crystallinity of the SWCNTs was significantly improved after annealing at a high temperature in a tight vacuum.

Here, analytical grade reagents were used in the experiments. The annealed arc-SWCNTs checked for high crystallization were measured by Raman-shift. The Raman spectrum measured by the blue laser beam (Blue) at a 473 nm wavelength is shown in **Figure 2**. Raman peaks from the G+ band mode (1590 cm−1) and from the G\_ band mode (1565 cm−1) presumed to be nanotube structures. A Raman peak intensity of near 1350 cm−1 from the D band mode indicated lacks of carbon network on a nanotube. The intensity ratio between the G+ band and the D band was 0.0096; this value confirms that the SWCNT has a carbon network with high crystallization.

**Figure 1.** TEM images with annealed and unannealed SWCNTs [24]. (a) Highly crystalline SWCNTs with annealing treatment. (b) SWCNTs with crystal defects without annealing treatment.

In order to confirm the crystallinity of SWCNTs, the TG first derivative (dTG) curves for both of the annealed SWCNTs and non-annealed samples presented in **Figure 4** indicate the weight fraction of tubes at each burning temperature. It is generally known that the burning temperature of SWCNTs is inversely proportional to the amount of crystal defects in the SWCNTs; hence, the SWCNT crystallinity can be inferred from the shape of the dTG curves. There are a few peaks in the non-annealed SWCNT dTG curve, indicating a range of different crystallinities; the burning temperature of the SWCNTs increased after annealing, signaling an increase in the degree of crystallinity of the SWCNTs. However, the non-annealed SWCNT dTG curve also indicated the

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled…

http://dx.doi.org/10.5772/intechopen.72581

33

**Figure 3.** Dispersed SWCNTs mixture and TEM observation [25].

measured under an air atmosphere.

presence of some lower crystallinity tubes that might affect the field-emission stability.

The relationship between dispersion concentration and crystallinity retention rate for highly crystalline SWCNT dispersions prepared by ultra-sonication and agitation with ceramic beads is shown in **Figure 4**. The dispersion concentration in **Figure 5** was calculated from the ratio of dispersion transparency before and after centrifugation of the solution, as determined by UV–vis measurements, and includes highly crystalline SWCNT aggregates. The crystallinity retention was calculated from the dot product of dTG measurement curves obtained before and after dispersion of the highly crystalline SWCNTs and is presented as cosθ. A reduction in cosθ correlates to a deterioration in the crystallinity of the dispersed highly crystalline SWCNTs. Thus, **Figure 5** indicates that the SWCNT crystallinity deteriorates with

**Figure 4.** First derivative thermogravimetric curves of the commercial and annealed single-walled carbon nanotubes

**Figure 2.** Raman spectrum with G and D band modes of annealed SWCNTs. [25].

#### **3. Construction of a thin film including highly crystalline SWCNTs for field emission**

A wet-coating process was adopted to fabricate a cathode with stable FE at a low driving voltage using SWCNTs. The homogeneous dispersion of SWCNTs in a liquid medium was the key. Highly crystalline SWCNTs are expected to lower the threshold field and emit electrons stably at a low driving voltage, but no homogeneous dispersion of SWCNTs in liquids has ever been reported. In general, a dispersant is used to facilitate the homogeneous dispersion of nano-carbon materials in liquids; however, highly crystalline SWCNTs cannot bind to the functional groups in a dispersant because there are very few defective binding sites in their carbon network that can be used to bind to these in the dispersant. We attempted to separate highly crystalline SWCNT aggregates physically into uniform single nanotubes in a low-viscosity solvent containing an In<sup>2</sup> O3 -SnO2 (tin-doped indium oxide; ITO) precursor solution. A nonionic dispersant was added to the solvent to facilitate the dispersion of SWCNTs, and the mixture was agitated with ceramic beads in a shaking machine.

The physical properties for producing a mixture including highly dispersant SWCNTs are as follows: Butyl acetate 99%, ethyl cellulose (EC) (abt. 49% ethoxy 100 cP), and sodium *linear*alkyl-benzenesulfonate 95% (DBS) were obtained from Wako Co. Ltd. Japan. The initial mixture was prepared by mixing the SWCNTs powder with the ITO precursor from Kojundo-Kagaku Co. Ltd. Japan and the DBS and the EC in a proportion of about 1: 600: 1: 4. The last component, EC, was tried in different proportions. The mixture was later agitated with ZrO<sup>2</sup> beads of different diameters in various concentrations from 4 g to 8 g in a shaking machine at periods ranging from 6 h up to 12 h. In all cases, the resulting mixture solution shown in **Figure 3** had low viscosity. The TEMs of the annealed SWCNTs are shown in **Figure 3**; almost only carbon nanotubes can be seen in the micrographs, with no other carbon contamination present.

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 33

**Figure 3.** Dispersed SWCNTs mixture and TEM observation [25].

**3. Construction of a thin film including highly crystalline SWCNTs** 

A wet-coating process was adopted to fabricate a cathode with stable FE at a low driving voltage using SWCNTs. The homogeneous dispersion of SWCNTs in a liquid medium was the key. Highly crystalline SWCNTs are expected to lower the threshold field and emit electrons stably at a low driving voltage, but no homogeneous dispersion of SWCNTs in liquids has ever been reported. In general, a dispersant is used to facilitate the homogeneous dispersion of nano-carbon materials in liquids; however, highly crystalline SWCNTs cannot bind to the functional groups in a dispersant because there are very few defective binding sites in their carbon network that can be used to bind to these in the dispersant. We attempted to separate highly crystalline SWCNT aggregates physically into uniform single nanotubes in a low-vis-

nonionic dispersant was added to the solvent to facilitate the dispersion of SWCNTs, and the

The physical properties for producing a mixture including highly dispersant SWCNTs are as follows: Butyl acetate 99%, ethyl cellulose (EC) (abt. 49% ethoxy 100 cP), and sodium *linear*alkyl-benzenesulfonate 95% (DBS) were obtained from Wako Co. Ltd. Japan. The initial mixture was prepared by mixing the SWCNTs powder with the ITO precursor from Kojundo-Kagaku Co. Ltd. Japan and the DBS and the EC in a proportion of about 1: 600: 1: 4. The last compo-

different diameters in various concentrations from 4 g to 8 g in a shaking machine at periods ranging from 6 h up to 12 h. In all cases, the resulting mixture solution shown in **Figure 3** had low viscosity. The TEMs of the annealed SWCNTs are shown in **Figure 3**; almost only carbon nanotubes can be seen in the micrographs, with no other carbon contamination present.

nent, EC, was tried in different proportions. The mixture was later agitated with ZrO<sup>2</sup>

(tin-doped indium oxide; ITO) precursor solution. A

beads of

**for field emission**

32 Carbon Nanotubes - Recent Progress

cosity solvent containing an In<sup>2</sup>

O3 -SnO2

**Figure 2.** Raman spectrum with G and D band modes of annealed SWCNTs. [25].

mixture was agitated with ceramic beads in a shaking machine.

In order to confirm the crystallinity of SWCNTs, the TG first derivative (dTG) curves for both of the annealed SWCNTs and non-annealed samples presented in **Figure 4** indicate the weight fraction of tubes at each burning temperature. It is generally known that the burning temperature of SWCNTs is inversely proportional to the amount of crystal defects in the SWCNTs; hence, the SWCNT crystallinity can be inferred from the shape of the dTG curves. There are a few peaks in the non-annealed SWCNT dTG curve, indicating a range of different crystallinities; the burning temperature of the SWCNTs increased after annealing, signaling an increase in the degree of crystallinity of the SWCNTs. However, the non-annealed SWCNT dTG curve also indicated the presence of some lower crystallinity tubes that might affect the field-emission stability.

The relationship between dispersion concentration and crystallinity retention rate for highly crystalline SWCNT dispersions prepared by ultra-sonication and agitation with ceramic beads is shown in **Figure 4**. The dispersion concentration in **Figure 5** was calculated from the ratio of dispersion transparency before and after centrifugation of the solution, as determined by UV–vis measurements, and includes highly crystalline SWCNT aggregates. The crystallinity retention was calculated from the dot product of dTG measurement curves obtained before and after dispersion of the highly crystalline SWCNTs and is presented as cosθ. A reduction in cosθ correlates to a deterioration in the crystallinity of the dispersed highly crystalline SWCNTs. Thus, **Figure 5** indicates that the SWCNT crystallinity deteriorates with

**Figure 4.** First derivative thermogravimetric curves of the commercial and annealed single-walled carbon nanotubes measured under an air atmosphere.

**Figure 5.** Dispersion concentration vs. crystallinity retention rate for highly crystalline single-walled carbon nanotubes dispersed by ultra-sonication and agitation. Ultra-sonication at 40 kHz was performed for 0, 5, and 10 hours. Agitation was carried out under different conditions and repeated several times.

In this chapter, the SWCNTs coating weights per unit area were controlled by the numbers of spray cycles to verify both the field emission property and brightness efficiency of a flat-plane light emission panel. The sintered film was fabricated using an activation process without the standing or aligning treatment of the CNTs in order to emit electrons easily at low voltage.

**Figure 6.** SEM images of ITO film with dispersant SWCNTs after sintering [25]. (a) Overview of a scratched ITO film. White circles indicate points of SWCNT bundles protruding from the ITO film as measured by SEM. (b) and (c) enlarged

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images arrowed in the image of (a). The white arrows indicates SWCNTs from the grooved face.

Then the SWCNTs for the field emitter protruding from the grooved face were aligned in a direction parallel to the ITO film plane and substrate per the schematic drawing in **Figure 7**, which shows schematic drawings of the models with random directions for the SWCNTs. The dotted square boxes indicate scratched areas, and the tops of SWCNTs that possibly emit electrons are highlighted with red circles in the area. These depend on the density of the SWCNTs in a coated film. The direction of the SWCNTs exposed by scratching aligns in a direction parallel to the substrate, so electrical fields are yielded at the top of each SWCNT uniformly without the fluctuation of the protruding SWCNT length. We thus obtained high homogeneity for the electron plane-emission with the dispersion and density of SWCNTs in the coated film.

**Figure 7.** Images of frequency in the exposure of SWCNTs in a scratched area in cases of both low and high SWCNT

density. Red circles indicate the exposed tops of SWCNTs. [25].

increasing sonication time; we presume this to be an effect of the uncontrollable cavitation that occurs during ultra-sonication. The agitation method was the most successful method for highly crystalline SWCNT dispersion, with a concentration of more than 90 wt% and minimal crystallinity deterioration achieved. However, it should be noted that the crystallinity of the dispersed highly crystalline SWCNTs also deteriorates with repeated milling and at higher operation pressures. This is likely owing to a difference in atom binding energy and cohesion energy depending on the higher-order structure of SWCNTs [25, 26].

In this chapter attempts were made to disperse SWCNTs with a low-viscosity butyl acetate solvent including an organic In<sup>2</sup> O3 -SnO2 (ITO) precursor and non-ionic dispersant. The author successfully employed a dispersion method for highly crystalline SWCNTs to obtain a mixture of them dispersed as thin bundles, aggregating with a few nanotubes. The agitated mixture was sprayed onto a metal-coated glass substrate heated to around 400 K, and the coated film was sintered at 900 K in a vacuum to remove any organic components and then form bridges in order to produce a conductive ITO film. After a film coated with the mixture was sintered in a vacuum, it was activated by scratching it appropriately to obtain field emission properties with a low turn-on field. **Figure 6** shows SEM images of an ITO film with dispersed SWCNTs after vacuum sintering. The ITO film was scratched by a thin metal rod; this revealed SWCNTs or their bundles protruding from both sides of the edge of the ITO film. As shown in **Figure 6(a)**, highly crystalline SWCNT bundles were dispersed. As shown in **Figures 6(b)** and **6(c)**, the SWCNT bundles were exposed in the scratched grooves on the ITO film subjected to activation treatment. The white circles in the SEM image of **Figure 6(a)** indicate the exposed SWCNT bundles that were exposed by scratching the ITO film in which SWCNTs were dispersed. White circles show the distances between their points to be almost identically aligned. SWCNTs in the ITO film were dispersed homogeneously and lay in random directions. Thus, SWCNTs would appear in same direction in the ITO film with the same probability, and homogeneously dispersed SWCNTs protruding from the grooved face of the ITO film could be observed.

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 35

**Figure 6.** SEM images of ITO film with dispersant SWCNTs after sintering [25]. (a) Overview of a scratched ITO film. White circles indicate points of SWCNT bundles protruding from the ITO film as measured by SEM. (b) and (c) enlarged images arrowed in the image of (a). The white arrows indicates SWCNTs from the grooved face.

In this chapter, the SWCNTs coating weights per unit area were controlled by the numbers of spray cycles to verify both the field emission property and brightness efficiency of a flat-plane light emission panel. The sintered film was fabricated using an activation process without the standing or aligning treatment of the CNTs in order to emit electrons easily at low voltage.

increasing sonication time; we presume this to be an effect of the uncontrollable cavitation that occurs during ultra-sonication. The agitation method was the most successful method for highly crystalline SWCNT dispersion, with a concentration of more than 90 wt% and minimal crystallinity deterioration achieved. However, it should be noted that the crystallinity of the dispersed highly crystalline SWCNTs also deteriorates with repeated milling and at higher operation pressures. This is likely owing to a difference in atom binding energy and cohesion

**Figure 5.** Dispersion concentration vs. crystallinity retention rate for highly crystalline single-walled carbon nanotubes dispersed by ultra-sonication and agitation. Ultra-sonication at 40 kHz was performed for 0, 5, and 10 hours. Agitation

In this chapter attempts were made to disperse SWCNTs with a low-viscosity butyl acetate

successfully employed a dispersion method for highly crystalline SWCNTs to obtain a mixture of them dispersed as thin bundles, aggregating with a few nanotubes. The agitated mixture was sprayed onto a metal-coated glass substrate heated to around 400 K, and the coated film was sintered at 900 K in a vacuum to remove any organic components and then form bridges in order to produce a conductive ITO film. After a film coated with the mixture was sintered in a vacuum, it was activated by scratching it appropriately to obtain field emission properties with a low turn-on field. **Figure 6** shows SEM images of an ITO film with dispersed SWCNTs after vacuum sintering. The ITO film was scratched by a thin metal rod; this revealed SWCNTs or their bundles protruding from both sides of the edge of the ITO film. As shown in **Figure 6(a)**, highly crystalline SWCNT bundles were dispersed. As shown in **Figures 6(b)** and **6(c)**, the SWCNT bundles were exposed in the scratched grooves on the ITO film subjected to activation treatment. The white circles in the SEM image of **Figure 6(a)** indicate the exposed SWCNT bundles that were exposed by scratching the ITO film in which SWCNTs were dispersed. White circles show the distances between their points to be almost identically aligned. SWCNTs in the ITO film were dispersed homogeneously and lay in random directions. Thus, SWCNTs would appear in same direction in the ITO film with the same probability, and homogeneously dispersed SWCNTs protruding from the grooved face of the ITO film could be observed.

(ITO) precursor and non-ionic dispersant. The author

energy depending on the higher-order structure of SWCNTs [25, 26].

O3 -SnO2

was carried out under different conditions and repeated several times.

solvent including an organic In<sup>2</sup>

34 Carbon Nanotubes - Recent Progress

Then the SWCNTs for the field emitter protruding from the grooved face were aligned in a direction parallel to the ITO film plane and substrate per the schematic drawing in **Figure 7**, which shows schematic drawings of the models with random directions for the SWCNTs. The dotted square boxes indicate scratched areas, and the tops of SWCNTs that possibly emit electrons are highlighted with red circles in the area. These depend on the density of the SWCNTs in a coated film. The direction of the SWCNTs exposed by scratching aligns in a direction parallel to the substrate, so electrical fields are yielded at the top of each SWCNT uniformly without the fluctuation of the protruding SWCNT length. We thus obtained high homogeneity for the electron plane-emission with the dispersion and density of SWCNTs in the coated film.

**Figure 7.** Images of frequency in the exposure of SWCNTs in a scratched area in cases of both low and high SWCNT density. Red circles indicate the exposed tops of SWCNTs. [25].

#### **4. Field emission properties**

This activated field emission (FE) cathode employing highly crystalline SWCNTs has the potential to yield a new flat-plane light emission device and so provide a novel approach to daily lighting needs while also contributing to energy-saving through its low power consumption. FE properties were measured with a phosphor anode plate on a sputtered ITO pattern film and 1.0 millimeter glass plate as a spacer as shown in **Figure 8**.

The sample shown in **Figure 8** was set into the electrical circuit in **Figure 9** to measure field emission and brightness properties. This measurement system was constructed with a power supply unit having an amplifier, a function generator, an oscilloscope as a field emission current monitor, and a PC for data storage. The voltage for the sample was designed for a periodic voltage at 60 Hz of the triangle wave for field emission measurements. The field emission current was converted to bias voltage data by a resistor at around 10 to 1 MΩ for the purpose of preventing signal noise from any leaking current appearing in measurement applications.

The author attempted to control field emission characteristics by the weight of the SWCNTs included in a coated ITO film with activation by a simple scratching. **Figure 10** shows the current density-electrical field characteristics of the cathode with a 20 x 20 mm diode depending on the content of the SWCNTs in the coating film. For example, the contents for measurements were chosen at 1.07, 0.69, 0.35, and 0.09 mg/cm2 . The turn-on field increased as the SWCNT density rose to around 0.8 to 1.7 V/μm. The high SWCNT density obtained a low driving supply because of the increase in the field emission site available to emit electrons from the tip of each SWCNT.

The F-N electron tunneling function has been proposed as follows [27]:

$$I = \ a \ast V^2 \ast \exp\left(-\frac{b}{V}\right) \tag{1}$$

**Figure 8.** Schematic diagram of the system for FE measurement having a diode structure [25].

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**Figure 9.** Schematic diagram of the electrical circuit system for FE measurement with a diode structure [25].

**Figure 10.** Current density-electrical field characteristics of certain conditions: 0.09, 0.35, 0.69, and 1.07 mg/cm<sup>2</sup>

[25].

where

$$a = a \ast A \ast \beta^2 \ast \exp\left(\frac{B \ast 1.44E - 7}{\phi^{0\delta}}\right) / (1.1 \ast \phi) \tag{2}$$

$$b = \frac{0.95 \ast B \ast \phi^{\wedge 2}}{\beta} \tag{3}$$

$$A = 1.54E-6, B = 6.87E+7\tag{4}$$

From Eq. (1), *V* means a voltage added between two electrodes; this is converted to an electrical field by the gap of the measurement sample in **Figure 8**. *I* indicates a current flow passing through the phosphor plate in the anode area at 20 × 20 mm2 . *α* is equivalent to the electron emission site area for the field emission based on Fowler-Nordheim tunneling, *β* is the field enhancement factor of the electrical field localized at the top of a SWCNT against a macroscopic applied electric field, and *φ* of 4.7 eV is the work function in the bulk region of carbon materials. **Figure 11** shows the distributions of *α* and *β* converted from the current densityelectrical field characteristics by Eqs. (1), (2), (3) and (4) against the SWCNT coating density.

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 37

**Figure 8.** Schematic diagram of the system for FE measurement having a diode structure [25].

**4. Field emission properties**

36 Carbon Nanotubes - Recent Progress

This activated field emission (FE) cathode employing highly crystalline SWCNTs has the potential to yield a new flat-plane light emission device and so provide a novel approach to daily lighting needs while also contributing to energy-saving through its low power consumption. FE properties were measured with a phosphor anode plate on a sputtered ITO

The sample shown in **Figure 8** was set into the electrical circuit in **Figure 9** to measure field emission and brightness properties. This measurement system was constructed with a power supply unit having an amplifier, a function generator, an oscilloscope as a field emission current monitor, and a PC for data storage. The voltage for the sample was designed for a periodic voltage at 60 Hz of the triangle wave for field emission measurements. The field emission current was converted to bias voltage data by a resistor at around 10 to 1 MΩ for the purpose of preventing signal noise from any leaking current appearing in measurement applications. The author attempted to control field emission characteristics by the weight of the SWCNTs included in a coated ITO film with activation by a simple scratching. **Figure 10** shows the current density-electrical field characteristics of the cathode with a 20 x 20 mm diode depending on the content of the SWCNTs in the coating film. For example, the contents for measure-

SWCNT density rose to around 0.8 to 1.7 V/μm. The high SWCNT density obtained a low driving supply because of the increase in the field emission site available to emit electrons

> \_\_\_\_\_\_\_\_\_ *B* ∗ 1.44*E* − 7

*A* = 1.54*E* − 6, *B* = 6.87*E* + 7 (4)

From Eq. (1), *V* means a voltage added between two electrodes; this is converted to an electrical field by the gap of the measurement sample in **Figure 8**. *I* indicates a current flow passing

emission site area for the field emission based on Fowler-Nordheim tunneling, *β* is the field enhancement factor of the electrical field localized at the top of a SWCNT against a macroscopic applied electric field, and *φ* of 4.7 eV is the work function in the bulk region of carbon materials. **Figure 11** shows the distributions of *α* and *β* converted from the current densityelectrical field characteristics by Eqs. (1), (2), (3) and (4) against the SWCNT coating density.

. The turn-on field increased as the

*<sup>V</sup>*) (1)

*<sup>φ</sup>*0.5 )/(1.1 <sup>∗</sup> *<sup>φ</sup>*) (2)

. *α* is equivalent to the electron

\_\_\_\_\_\_\_\_\_\_\_ *<sup>β</sup>* (3)

pattern film and 1.0 millimeter glass plate as a spacer as shown in **Figure 8**.

ments were chosen at 1.07, 0.69, 0.35, and 0.09 mg/cm2

*<sup>I</sup>* <sup>=</sup> *<sup>a</sup>* <sup>∗</sup> *<sup>V</sup>*<sup>2</sup> <sup>∗</sup> exp(−\_\_*<sup>b</sup>*

*<sup>b</sup>* <sup>=</sup> 0.95 <sup>∗</sup> *<sup>B</sup>* <sup>∗</sup> *<sup>φ</sup>*3/2

through the phosphor plate in the anode area at 20 × 20 mm2

*<sup>a</sup>* <sup>=</sup> *<sup>α</sup>* <sup>∗</sup> *<sup>A</sup>* <sup>∗</sup> *<sup>β</sup>*<sup>2</sup> <sup>∗</sup> exp(

The F-N electron tunneling function has been proposed as follows [27]:

from the tip of each SWCNT.

where

**Figure 9.** Schematic diagram of the electrical circuit system for FE measurement with a diode structure [25].

**Figure 10.** Current density-electrical field characteristics of certain conditions: 0.09, 0.35, 0.69, and 1.07 mg/cm<sup>2</sup> [25].

The flat emission-plane—especially for the content of SWCNTs at 0.69 mg/cm<sup>2</sup> —from the diode generated a stable, homogeneous brightness of over 2000 cd/m2 in the sampling area of 400 mm2 without any brightness fluctuation. The brightness of the sample with the content of SWCNTs at 0.69 mg/cm2 was measured at nine locations in the emission area, as captured by a charge coupled device (CCD) camera in **Figure 12**. Furthermore, the differences between each brightness result of the measurement points in the plane shown in **Figure 8** were within 5% of those shown in **Figure 13**; accordingly, stable brightness uniformity was achieved in the plane-emission area. *Brightness Efficiency*(*lm*/*W*) <sup>=</sup> *Brightness*(*cd*/*m*2) <sup>×</sup> *Lighting area*(*m*2) <sup>×</sup> 3.14 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *Voltage* <sup>×</sup> 0.5 <sup>×</sup> *FE Current* <sup>×</sup> exp(−2.4) (5)

$$\text{Brightness} \quad \text{Efficiency} \{ \text{lm/N} \} = \frac{\text{Brightness} \{ \text{cd/} \text{m}^2 \} \times \text{Lights} \{ \text{m}^2 \} \times 3.14}{\text{Voltage} \times 0.5 \times \text{FE} \text{ Current} \times \exp(-2.4)} \tag{5}$$

**Figure 11.** The dependence between the emission site area, field enhancement factor, and SWCNT density [25].

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through a ND filter. The added voltage between the anode and cathode is 3.0 kV with a peak-to-peak range of periodic

among nine areas captured by CCD

**Figure 12.** Light emission brightness image of the SWCNT content 0.69 mg/cm<sup>2</sup>

voltage in the triangle wave; the flow current is 250 μA as a peak currentw [25].

**Figure 13.** Brightness results at the points of flat light emission in **Figure 12** [25].

As calculated by the luminance measured by a luminance meter and the load power compensated as a direct current (DC) load power for the effective voltage of the triangle wave—indicated as *Voltage* × *0.5* in Eq. (5)—as a bias voltage and effective field emission current, which was indicated as *FE Current* × *exp.(−2.4)* in Eq. (5), the author was able to achieve a brightness efficiency calculated by Eq. (5) of more than 70 lm/W within an area of approximately 25 cd/m<sup>2</sup> of brightness standard deviation by controlling the contents of SWCNTs in the coated film. Phosphor plates for brightness efficiency measurements were chosen for their green lighting from Nichia-Kagaku Co. Japan. The green circular points in **Figure 14** indicate the brightness efficiency with a green phosphor emitting light. The square blue points represent the deviation of brightness employing the green light in the plane-emission area shown in **Figure 12**. The brightness properties from the developed cathode depended on the content of the SWCNTs and could be obtained with high brightness homogeneity and high driving efficiency by controlling the SWCNT density in the coated ITO film. However, the brightness efficiency in case of a SWCNT density of more than 0.69 mg/cm2 decreased with an increase in the SWCNT density from **Figure 14**. This is probably owing to the electrical field strength for field emission focused on each top of the SWCNTs being weaker than 0.69 mg/cm2 ; though SWCNTs as electron emission sites are sufficient for obtaining a high field emission current. This can be explained by the tendency of the field enhancement factor in **Figure 11**, which causes the distance between neighboring SWCNTs as field emitters to narrow with the increase in SWCNTs density. Moreover, the tendency of the brightness deviation is for a stability of more than around 0.69 mg/cm2 . Most likely, this is because enough SWCNTs for field emitters exist in the ITO film for a high SWCNT density. From **Figure 14**, it is apparent that a high brightness efficiency could be obtained by controlling the SWCNTs density in the ITO film. To our knowledge, this is the first time a field emitter plane device has achieved a high brightness efficiency of over 70 lm/W.

**Figure 15** shows the field emission life time up to 100 hours at SWCNT contents of 0.09, 0.35, and 0.69 mg/cm2 . FE-current was measured as a peak of field emission current with a periodic voltage of the rectangular wave with a frequency of 60 Hz and duty ratio of 10%, and initial FE-current density was adjusted approximately to 45 mA/ cm<sup>2</sup> . This result implies that the radioactive half-time will be over 5000 hours at 0.69 mg/cm2 , and other samples with a SWCNT density of more than 0.69 mg/cm2 have the same emission-life tendency as at 0.69 mg/cm2 . Attenuation of the loading field emission current depends on the uniformity of the loading power, equal to the electrical field strength, given to each SWCNT for field emitters. A field emitter-plane employing SWCNTs will have the high stability with a high crystallization of Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 39

The flat emission-plane—especially for the content of SWCNTs at 0.69 mg/cm<sup>2</sup>

without any brightness fluctuation. The brightness of the sample with the content

by a charge coupled device (CCD) camera in **Figure 12**. Furthermore, the differences between each brightness result of the measurement points in the plane shown in **Figure 8** were within 5% of those shown in **Figure 13**; accordingly, stable brightness uniformity was achieved in the

*Brightness Efficiency*(*lm*/*W*) <sup>=</sup> *Brightness*(*cd*/*m*2) <sup>×</sup> *Lighting area*(*m*2) <sup>×</sup> 3.14 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *Voltage* <sup>×</sup> 0.5 <sup>×</sup> *FE Current* <sup>×</sup> exp(−2.4) (5)

As calculated by the luminance measured by a luminance meter and the load power compensated as a direct current (DC) load power for the effective voltage of the triangle wave—indicated as *Voltage* × *0.5* in Eq. (5)—as a bias voltage and effective field emission current, which was indicated as *FE Current* × *exp.(−2.4)* in Eq. (5), the author was able to achieve a brightness efficiency

ness standard deviation by controlling the contents of SWCNTs in the coated film. Phosphor plates for brightness efficiency measurements were chosen for their green lighting from Nichia-Kagaku Co. Japan. The green circular points in **Figure 14** indicate the brightness efficiency with a green phosphor emitting light. The square blue points represent the deviation of brightness employing the green light in the plane-emission area shown in **Figure 12**. The brightness properties from the developed cathode depended on the content of the SWCNTs and could be obtained with high brightness homogeneity and high driving efficiency by controlling the SWCNT density in the coated ITO film. However, the brightness efficiency in case of a SWCNT density of

is probably owing to the electrical field strength for field emission focused on each top of the

ficient for obtaining a high field emission current. This can be explained by the tendency of the field enhancement factor in **Figure 11**, which causes the distance between neighboring SWCNTs as field emitters to narrow with the increase in SWCNTs density. Moreover, the tendency of

is because enough SWCNTs for field emitters exist in the ITO film for a high SWCNT density. From **Figure 14**, it is apparent that a high brightness efficiency could be obtained by controlling the SWCNTs density in the ITO film. To our knowledge, this is the first time a field emitter plane

**Figure 15** shows the field emission life time up to 100 hours at SWCNT contents of 0.09, 0.35,

voltage of the rectangular wave with a frequency of 60 Hz and duty ratio of 10%, and initial

Attenuation of the loading field emission current depends on the uniformity of the loading power, equal to the electrical field strength, given to each SWCNT for field emitters. A field emitter-plane employing SWCNTs will have the high stability with a high crystallization of

. FE-current was measured as a peak of field emission current with a periodic

have the same emission-life tendency as at 0.69 mg/cm2

the brightness deviation is for a stability of more than around 0.69 mg/cm2

device has achieved a high brightness efficiency of over 70 lm/W.

FE-current density was adjusted approximately to 45 mA/ cm<sup>2</sup>

radioactive half-time will be over 5000 hours at 0.69 mg/cm2

calculated by Eq. (5) of more than 70 lm/W within an area of approximately 25 cd/m<sup>2</sup>

was measured at nine locations in the emission area, as captured

decreased with an increase in the SWCNT density from **Figure 14**. This

; though SWCNTs as electron emission sites are suf-

diode generated a stable, homogeneous brightness of over 2000 cd/m2

400 mm2

of SWCNTs at 0.69 mg/cm2

38 Carbon Nanotubes - Recent Progress

plane-emission area.

more than 0.69 mg/cm2

and 0.69 mg/cm2

density of more than 0.69 mg/cm2

SWCNTs being weaker than 0.69 mg/cm2

—from the

of bright-

. Most likely, this

.

. This result implies that the

, and other samples with a SWCNT

in the sampling area of

**Figure 11.** The dependence between the emission site area, field enhancement factor, and SWCNT density [25].

**Figure 12.** Light emission brightness image of the SWCNT content 0.69 mg/cm<sup>2</sup> among nine areas captured by CCD through a ND filter. The added voltage between the anode and cathode is 3.0 kV with a peak-to-peak range of periodic voltage in the triangle wave; the flow current is 250 μA as a peak currentw [25].

**Figure 13.** Brightness results at the points of flat light emission in **Figure 12** [25].

**Figure 14.** SWCNT content density with the dependence of brightness efficiency indicated on the left vertical axis and brightness homogeneity on the right vertical axis. The peak-to-peak voltage condition of the triangle wave is fixed at 5 kV [25].

the SWCNTs and high emission site homogeneity from **Figures 11** and **14**, and it will obtain a long emission life-time to withstand practical use as a plane-light emitting device. The coated, highly crystalline SWCNT cathode has potential for realizing a visible ray flat plane-emission device with low power consumption and a long product life cycle.

the SWCNTs density; it averages approximately 10 μm at 0.69 mg/cm<sup>2</sup>

ness of over 2000 cd/m2

SWCNT density [25].

dispersed in the ITO film and align randomly, their protrusions aligning along a direction in the groove made by a voluntary scratching process appear uniform. The protruding SWCNT length measured by SEM was generally 4–6 μm. The ratio of the average distance and length of SWCNTs via SEM images was nearly 2, and it is believed that the electrical field to obtain a high

**Figure 16.** The dependence on the distance between neighboring SWCNTs protruding from the edges of ITO film and

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the SWCNTs for the field emitter protruding from the grooved face were aligned in a direction parallel to the ITO film plane and substrate as shown in **Figure 6** and the schematic drawing in **Figure 7**, which shows such drawings of the models with random directions for the SWCNTs. The dotted square boxes indicate scratched areas, and the tops of SWCNTs that possibly emit electrons are highlighted with red circles in them. These depend on the density of the SWCNTs in a coated film. The direction of the SWCNTs exposed by scratching aligns in a direction parallel to the substrate, thereby uniformly yielding electrical fields at the top of each SWCNT without any fluctuation of the protruding SWCNT length. We thus obtained high homogeneity of the electron plane-emission with the dispersion and density of SWCNTs in the coated film. The flat plane-emission from the simple diode panel generated a stable and homogeneous bright-

the plane. The current fluctuation of emissions from the flat plane panel with an added voltage of 4 kV DC was stable over a period of 150 minutes as shown in **Figure 17**. This degree of electron emission stability is excellent for highly purified and crystallized SWCNTs in a diode panel. In particular, highly crystallized SWCNTs have no defects in their carbon network [23, 29], and such perfect crystallization prevents SWCNTs from breaking down with a large field emission current. Previously, it was necessary for a stable field emission current by CNTs with low power consumption to construct the triode structure, to synthesize carbon nanotubes designed with vertical alignment and standing height uniformly on an electrode by CVD with high temperature, and to obtain a high vacuum atmosphere around over 10−6 Pa in a panel [30]. In this study, we were able to obtain a stable large field emission current in a simple diode panel for the first time ever.

in the sampling area as shown in **Figure 12** without current fluctuation in

field emission current is added at the top of each protruding SWCNT of 0.69 mg/cm<sup>2</sup>

. If SWCNTs are well

[28]. Then

**Figure 16** shows the distance distribution between neighboring SWCNTs depending on the SWCNT density. The results were measured from the total number of SWCNTs protruding from the groove face of an arbitrarily scratched ITO film by visual observation with an SEM image, as that in **Figure 7**. SWCNTs were exposed from the side wall of the grooved face into the ITO film. The distance between SWCNTs protruding from the grooved face depends on

**Figure 15.** The lifetime of field emission current density up to 100 hours. The half time against the initial field emission current density of 0.69 mg/cm2 will be over 5000 hours [25].

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 41

**Figure 16.** The dependence on the distance between neighboring SWCNTs protruding from the edges of ITO film and SWCNT density [25].

the SWCNTs density; it averages approximately 10 μm at 0.69 mg/cm<sup>2</sup> . If SWCNTs are well dispersed in the ITO film and align randomly, their protrusions aligning along a direction in the groove made by a voluntary scratching process appear uniform. The protruding SWCNT length measured by SEM was generally 4–6 μm. The ratio of the average distance and length of SWCNTs via SEM images was nearly 2, and it is believed that the electrical field to obtain a high field emission current is added at the top of each protruding SWCNT of 0.69 mg/cm<sup>2</sup> [28]. Then the SWCNTs for the field emitter protruding from the grooved face were aligned in a direction parallel to the ITO film plane and substrate as shown in **Figure 6** and the schematic drawing in **Figure 7**, which shows such drawings of the models with random directions for the SWCNTs. The dotted square boxes indicate scratched areas, and the tops of SWCNTs that possibly emit electrons are highlighted with red circles in them. These depend on the density of the SWCNTs in a coated film. The direction of the SWCNTs exposed by scratching aligns in a direction parallel to the substrate, thereby uniformly yielding electrical fields at the top of each SWCNT without any fluctuation of the protruding SWCNT length. We thus obtained high homogeneity of the electron plane-emission with the dispersion and density of SWCNTs in the coated film.

The flat plane-emission from the simple diode panel generated a stable and homogeneous brightness of over 2000 cd/m2 in the sampling area as shown in **Figure 12** without current fluctuation in the plane. The current fluctuation of emissions from the flat plane panel with an added voltage of 4 kV DC was stable over a period of 150 minutes as shown in **Figure 17**. This degree of electron emission stability is excellent for highly purified and crystallized SWCNTs in a diode panel. In particular, highly crystallized SWCNTs have no defects in their carbon network [23, 29], and such perfect crystallization prevents SWCNTs from breaking down with a large field emission current. Previously, it was necessary for a stable field emission current by CNTs with low power consumption to construct the triode structure, to synthesize carbon nanotubes designed with vertical alignment and standing height uniformly on an electrode by CVD with high temperature, and to obtain a high vacuum atmosphere around over 10−6 Pa in a panel [30]. In this study, we were able to obtain a stable large field emission current in a simple diode panel for the first time ever.

**Figure 15.** The lifetime of field emission current density up to 100 hours. The half time against the initial field emission

the SWCNTs and high emission site homogeneity from **Figures 11** and **14**, and it will obtain a long emission life-time to withstand practical use as a plane-light emitting device. The coated, highly crystalline SWCNT cathode has potential for realizing a visible ray flat plane-emission

**Figure 14.** SWCNT content density with the dependence of brightness efficiency indicated on the left vertical axis and brightness homogeneity on the right vertical axis. The peak-to-peak voltage condition of the triangle wave is fixed at

**Figure 16** shows the distance distribution between neighboring SWCNTs depending on the SWCNT density. The results were measured from the total number of SWCNTs protruding from the groove face of an arbitrarily scratched ITO film by visual observation with an SEM image, as that in **Figure 7**. SWCNTs were exposed from the side wall of the grooved face into the ITO film. The distance between SWCNTs protruding from the grooved face depends on

device with low power consumption and a long product life cycle.

will be over 5000 hours [25].

current density of 0.69 mg/cm2

5 kV [25].

40 Carbon Nanotubes - Recent Progress

the tops of the SWCNTs, exposed on the both sides of the nicks scraped on the coating by a picking tool [25]. Thus, no exposed SWCNT was raised for field emission of electrons. The edges of the SWCNTs, as well as the width of the stripe patterned gate electrodes referred to **Figure 18**, were designed in an attempt to minimize the driving voltage for on/off switching of the FE electrons. In relation to the field intensity at the edges of the tops of the SWCNTs exposed on the both sides of the nick in **Figure 18**, the electrical potential distribution in the space, along with the electrode width dependence of electrical potential distribution at the edges of the SWCNTs, were simulated according to the surface charge method [33–36].

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The cathode electrodes were grounded stationary while the anode electrode applied a directcurrent (DC) voltage. The acceleration conditions for the electrons irradiated on the phosphor were taken into consideration to obtain a high brightness efficiency within a few kilo-volts. Conditions to apply electrical potential to emit FE electrons were controlled for the gate elec-

The length of the SWCNTs protruded from the wall, the width of the nicks made using a picking tool and the aperture of the stripe patterned electrodes of the gate electrodes were designed at 1 μm, 30 μm, and 40 μm, respectively. This was done to change the voltage applied to the gate electrodes to the negative direction in order to transit of electrical potential. A schematic diagram of the calculation model, as well as the calculation results, is shown

The electrical potential along the z-axis in cases where the voltage applied to the gate electrodes set at 0, −1, −3, and −5 V was calculated. The z-axis was defined as the direction extended from the origin located above the edge of the SWCNT where electrons are emitted. The potential distribution depended on the voltage applied to the gate electrode. It was determined that the threshold voltage of the FE electron emissions was reached at a voltage of approximately −3 V applied to the gates of the corresponding model. We attempted to fabricate a prototype

**Figure 19.** Schematic diagram of (left) calculation model and (right) calculation results [32]. Copyright (2017) The Japan

device of a triode planar light-emitting device based on the above calculation results.

trodes because the FE electrons were emitted at constant field intensity.

in **Figure 19**.

Society of Applied Physics.

**Figure 17.** Current fluctuation with 4kVDC loading for 150 minutes [31].

#### **5. Basic design of the triode structure**

The following time, a triode structure employing highly crystalline SWCNTs was attempted to serve as a prototype device of a flat-panel illumination shown in **Figure 18**. This device had a cathodoluminescence structure and had three electrodes for the FE electron emissions as a cathode, the switching of FE electron emissions on and off as a gate, and the radiation of visible rays by electron impact as an anode electrode. The cathode and the gate electrode, which apply voltage sequentially (called as line sequential scanning method) for electron emissions from the cathodes respectively, were patterned along the same line referred to **Figure 18**. Furthermore, a mixture of the long afterglow phosphor (P-1 from Mitsubishi Chemical) material with fluorescent material (P-22 from Nichia) that conducts a high luminance output was used for the anode electrode.

A stripe patterned structure was adopted for the gate electrodes used for on/off switching of the FE electrons, as shown in **Figure 18**. An FE electron emission site existed at the edges of

**Figure 18.** Basic structure of the triode flat-panel light-emission device [32]. Copyright (2017) The Japan Society of Applied Physics.

the tops of the SWCNTs, exposed on the both sides of the nicks scraped on the coating by a picking tool [25]. Thus, no exposed SWCNT was raised for field emission of electrons. The edges of the SWCNTs, as well as the width of the stripe patterned gate electrodes referred to **Figure 18**, were designed in an attempt to minimize the driving voltage for on/off switching of the FE electrons. In relation to the field intensity at the edges of the tops of the SWCNTs exposed on the both sides of the nick in **Figure 18**, the electrical potential distribution in the space, along with the electrode width dependence of electrical potential distribution at the edges of the SWCNTs, were simulated according to the surface charge method [33–36].

The cathode electrodes were grounded stationary while the anode electrode applied a directcurrent (DC) voltage. The acceleration conditions for the electrons irradiated on the phosphor were taken into consideration to obtain a high brightness efficiency within a few kilo-volts. Conditions to apply electrical potential to emit FE electrons were controlled for the gate electrodes because the FE electrons were emitted at constant field intensity.

The length of the SWCNTs protruded from the wall, the width of the nicks made using a picking tool and the aperture of the stripe patterned electrodes of the gate electrodes were designed at 1 μm, 30 μm, and 40 μm, respectively. This was done to change the voltage applied to the gate electrodes to the negative direction in order to transit of electrical potential. A schematic diagram of the calculation model, as well as the calculation results, is shown in **Figure 19**.

**5. Basic design of the triode structure**

**Figure 17.** Current fluctuation with 4kVDC loading for 150 minutes [31].

used for the anode electrode.

42 Carbon Nanotubes - Recent Progress

Applied Physics.

The following time, a triode structure employing highly crystalline SWCNTs was attempted to serve as a prototype device of a flat-panel illumination shown in **Figure 18**. This device had a cathodoluminescence structure and had three electrodes for the FE electron emissions as a cathode, the switching of FE electron emissions on and off as a gate, and the radiation of visible rays by electron impact as an anode electrode. The cathode and the gate electrode, which apply voltage sequentially (called as line sequential scanning method) for electron emissions from the cathodes respectively, were patterned along the same line referred to **Figure 18**. Furthermore, a mixture of the long afterglow phosphor (P-1 from Mitsubishi Chemical) material with fluorescent material (P-22 from Nichia) that conducts a high luminance output was

A stripe patterned structure was adopted for the gate electrodes used for on/off switching of the FE electrons, as shown in **Figure 18**. An FE electron emission site existed at the edges of

**Figure 18.** Basic structure of the triode flat-panel light-emission device [32]. Copyright (2017) The Japan Society of

The electrical potential along the z-axis in cases where the voltage applied to the gate electrodes set at 0, −1, −3, and −5 V was calculated. The z-axis was defined as the direction extended from the origin located above the edge of the SWCNT where electrons are emitted. The potential distribution depended on the voltage applied to the gate electrode. It was determined that the threshold voltage of the FE electron emissions was reached at a voltage of approximately −3 V applied to the gates of the corresponding model. We attempted to fabricate a prototype device of a triode planar light-emitting device based on the above calculation results.

**Figure 19.** Schematic diagram of (left) calculation model and (right) calculation results [32]. Copyright (2017) The Japan Society of Applied Physics.

### **6. Correlation between driving power and light-emitting characteristics of a light-emission device**

The prototype trial pattern for a triode-structure planar light-emission device is shown in **Figure 20**. Thin silicon oxidation film as an insulation layer was formed on a silicon wafer by using thermal oxidation treatment to assemble cathode, insulator and gate electrodes. The electrodes were laminated outward nearest the silicon oxide film by using the SWCNTs within an ITO film for the cathode electrode, a silicon oxide film of 1-μm thickness (formed using the TEOS-CVD method) as an insulation film, and a thin chrome film as a gate electrode. Gate electrodes, insulation films and cathodic electrodes installed SWCNTs were designed in a stripe pattern at the FE electron emission area as shown in **Figure 20**. The cathode electrodes between the gate electrodes were scratched in a line using a picking tool to expose and protrude the SWCNTs as FE emitters. The light-emitting area was designed to have an area of 15 × 15 mm<sup>2</sup> .

Electron leakage at the gate electrodes was verified when electrons were emitted from the SWCNTs. The cathode and gate electrodes were installed on the ground. The spectra detected at the both electrodes being applied a pulse voltage is expressed in **Figure 21**. The voltage applied to the anode electrode by blue solid line and the current flowing through the cathode and gate electrodes are respectively represented by solid lines of red and gray. Almost no current was flowing through the gate electrode, while the FE current loading between the cathode and anode electrodes was confirmed.

A mixed powder with the long-afterglow-type (P-1) and the high-luminance-output-type (P-22) phosphors was employed as a fluorescent material for the line scanning sequential test. The weight ratio of the mixture was designed to obtain the highest brightness efficiency. The frame scanning frequency was 60 Hz. A sequential timing chart to apply voltage to the gate electrodes on the six electrode lines of the prototype device was adopted for our development. We developed a scanning algorithm to amplify the light-emission luminance by overlapping the afterglow component of the long-afterglow-type phosphor with the high-luminance-out-

**Figure 21.** Cathode and gate-anode electrodes: Synchrony of a leaked current flowing through cathode and gate electrodes with signals of voltage applied to the anode, and depiction of a leaked current flowing through gate electrode

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The FE current value flowing throughout the device and the luminance output from the anode were controlled by the voltage applied to the gate electrodes. The correlation between the energy power consumption and the luminance output by driving the device is shown in **Figure 24** from the results of light-emission evaluations. We could succeed to obtain a high luminance output of

the test panel. This outcome indicates that we were able to derive the higher performance in the world for a flat-panel light-emission device by achieving a brightness efficiency of 87 lm/W.

**Figure 22.** Schematic diagram of electrode wire connections and electrical circuit for drive [32]. Copyright (2017) The

and higher with a power consumption of approximately 0.1 W during driving

put fluorescent material component of the subsequent frame scanning.

[32]. Copyright (2017) The Japan Society of Applied Physics.

over 12,000 cd/m2

Japan Society of Applied Physics.

A pulsed voltage was applied to the gate electrode, and a waveform was prepared to apply negative voltage by adding an offset voltage as a pulse signal. The system was driven by an operation mechanism to cut off the FE current, and it timed with the application of a few volts at the gate in the negative direction. A schematic diagram of the loading system for the prototype trial pattern for a triode-structure planar light-emission device is shown in **Figure 22**.

Conditions of the light emission, when switching on/off of the gate electrode, are shown in **Figure 23**. A stable FE current flowed between the cathode and anode electrodes when the voltage on the gate was 0 V. However, when a voltage of −3 V was applied to the gate electrode, we could find no current was flowing between the cathode and anode electrodes. These results of lighting emission indicated a successful control of the on-and-off switching of the current by switching gate electrodes.

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled… http://dx.doi.org/10.5772/intechopen.72581 45

**6. Correlation between driving power and light-emitting** 

The prototype trial pattern for a triode-structure planar light-emission device is shown in **Figure 20**. Thin silicon oxidation film as an insulation layer was formed on a silicon wafer by using thermal oxidation treatment to assemble cathode, insulator and gate electrodes. The electrodes were laminated outward nearest the silicon oxide film by using the SWCNTs within an ITO film for the cathode electrode, a silicon oxide film of 1-μm thickness (formed using the TEOS-CVD method) as an insulation film, and a thin chrome film as a gate electrode. Gate electrodes, insulation films and cathodic electrodes installed SWCNTs were designed in a stripe pattern at the FE electron emission area as shown in **Figure 20**. The cathode electrodes between the gate electrodes were scratched in a line using a picking tool to expose and protrude the SWCNTs as FE emitters. The light-emitting area was designed to have an area of 15 × 15 mm<sup>2</sup>

Electron leakage at the gate electrodes was verified when electrons were emitted from the SWCNTs. The cathode and gate electrodes were installed on the ground. The spectra detected at the both electrodes being applied a pulse voltage is expressed in **Figure 21**. The voltage applied to the anode electrode by blue solid line and the current flowing through the cathode and gate electrodes are respectively represented by solid lines of red and gray. Almost no current was flowing through the gate electrode, while the FE current loading between the

A pulsed voltage was applied to the gate electrode, and a waveform was prepared to apply negative voltage by adding an offset voltage as a pulse signal. The system was driven by an operation mechanism to cut off the FE current, and it timed with the application of a few volts at the gate in the negative direction. A schematic diagram of the loading system for the prototype trial pattern for a triode-structure planar light-emission device is shown in **Figure 22**. Conditions of the light emission, when switching on/off of the gate electrode, are shown in **Figure 23**. A stable FE current flowed between the cathode and anode electrodes when the voltage on the gate was 0 V. However, when a voltage of −3 V was applied to the gate electrode, we could find no current was flowing between the cathode and anode electrodes. These results of lighting emission indicated a successful control of the on-and-off switching of the

**Figure 20.** Prototype trial pattern of cathode gate electrode structure [32]. Copyright (2017) The Japan Society of Applied

.

**characteristics of a light-emission device**

44 Carbon Nanotubes - Recent Progress

cathode and anode electrodes was confirmed.

current by switching gate electrodes.

Physics.

**Figure 21.** Cathode and gate-anode electrodes: Synchrony of a leaked current flowing through cathode and gate electrodes with signals of voltage applied to the anode, and depiction of a leaked current flowing through gate electrode [32]. Copyright (2017) The Japan Society of Applied Physics.

A mixed powder with the long-afterglow-type (P-1) and the high-luminance-output-type (P-22) phosphors was employed as a fluorescent material for the line scanning sequential test. The weight ratio of the mixture was designed to obtain the highest brightness efficiency. The frame scanning frequency was 60 Hz. A sequential timing chart to apply voltage to the gate electrodes on the six electrode lines of the prototype device was adopted for our development. We developed a scanning algorithm to amplify the light-emission luminance by overlapping the afterglow component of the long-afterglow-type phosphor with the high-luminance-output fluorescent material component of the subsequent frame scanning.

The FE current value flowing throughout the device and the luminance output from the anode were controlled by the voltage applied to the gate electrodes. The correlation between the energy power consumption and the luminance output by driving the device is shown in **Figure 24** from the results of light-emission evaluations. We could succeed to obtain a high luminance output of over 12,000 cd/m2 and higher with a power consumption of approximately 0.1 W during driving the test panel. This outcome indicates that we were able to derive the higher performance in the world for a flat-panel light-emission device by achieving a brightness efficiency of 87 lm/W.

**Figure 22.** Schematic diagram of electrode wire connections and electrical circuit for drive [32]. Copyright (2017) The Japan Society of Applied Physics.

**7. Conclusion**

**Acknowledgements**

**Author details**

Norihiro Shimoi

**References**

**21**:147-151

The attempt to apply highly crystalline SWCNTs as electron emission source was successfully achieved to obtain a planar light-emission device with low power consumption. Highly crystalline SWCNTs are an electrical element that can make a significant improvement in FE characteristics. A thin cathodic electrode film assembled via a wet process employing a highly crystalline SWCNT is expected to provide energy conservation as an FE electron emission source. By combining such elemental technologies both of the control the on-and-off switching of electron emissions in an arbitrary manner and the amplification of the luminance output with the persistence characteristics of a fluorescent screen, a flat-panel light-emission device with high brightness efficiency with an energy conserve driving method was assembled for the first time in the world. The application of FE electron sources employing highly crystalline SWCNTs was determined to be effective for conserving energy based on such results, and is expected to establish other

Stability and Reliability of an Electrical Device Employing Highly Crystalline Single-Walled…

http://dx.doi.org/10.5772/intechopen.72581

47

This work was partially supported by DOWA Holdings Co. Ltd. The author thanks the co-

[1] Hamada N, Sawada S, Oshiyama A. New one-dimensional conductors: Graphitic micro-

[2] Saito R, Fujita M, Dresselhaus G, Dresselhaus MS. Electronic structure of chiral gra-

[3] Tanaka K, Okahara K, Okada M, Yamabe T. Electronic properties of bucky-tube model.

[4] Kim J-Y, Kim M, Kim HM, Joo J, Choi J-H. Electrical and optical studies of organic light emitting devices using SWCNTs-polymer nanocomposites. Optical Materials. 2002;

devices that are driven with low-power consumption in the future.

researchers from DOWA for their discussions and suggestions.

Address all correspondence to: norihiro.shimoi.c8@tohoku.ac.jp

tubules. Physical Review Letters. 1992;**68**:1579-1581

Chemical Physics Letters. 1992;**191**(5):469-472

phene tubules. Applied Physics Letters. 1992;**60**:2204-2206

Graduate School of Environmental Studies, Tohoku University, Sendai, Japan

**Figure 23.** (a) Flat-panel light emission driven by line-sequential scanning (shaded by ND filter) [32]. (b) On-and-off chart for voltage applied to gate electrode and current flowing through cathode [32]. Copyright (2017) The Japan Society of Applied Physics.

We succeeded in assembling an FE planer electron-emission source pattern via a line-sequential-scanning method with an optimized design that the individual gate electrodes driven with an on/off FE current flowing. This was accomplished while conserving energy in cathode electrodes activated by making up scratching to emit electrons more effectively from the cathodic electrode employing a highly crystalline SWCNT. We also succeeded in amplifying the luminance-output by combining long-afterglow and high-luminance phosphors for an anode electrode to overlap the afterglow component of fluorescent materials to establish a basic design for assembling a triode flat-panel light-emission device with a line scanning sequential method. To the best of our knowledge, the creation of a flat-panel light-emission device that could achieve to obtain a high brightness efficiency of 87 lm/W was a remarkable outcome for the first time ever. Improvements to the driving performance for electron emissions at FE sources largely depend on the application of the highly crystalline SWCNTs. We believe this achievement assumes a leading role in electronic device creation through the effective use of a highly crystalline SWCNT, and we confidently need to promote its industrial design for practical use.

**Figure 24.** Relationship between power consumption and output luminance of triode flat-panel light-emission device [32]. Copyright (2017) The Japan Society of Applied Physics.

### **7. Conclusion**

The attempt to apply highly crystalline SWCNTs as electron emission source was successfully achieved to obtain a planar light-emission device with low power consumption. Highly crystalline SWCNTs are an electrical element that can make a significant improvement in FE characteristics. A thin cathodic electrode film assembled via a wet process employing a highly crystalline SWCNT is expected to provide energy conservation as an FE electron emission source. By combining such elemental technologies both of the control the on-and-off switching of electron emissions in an arbitrary manner and the amplification of the luminance output with the persistence characteristics of a fluorescent screen, a flat-panel light-emission device with high brightness efficiency with an energy conserve driving method was assembled for the first time in the world.

The application of FE electron sources employing highly crystalline SWCNTs was determined to be effective for conserving energy based on such results, and is expected to establish other devices that are driven with low-power consumption in the future.

### **Acknowledgements**

We succeeded in assembling an FE planer electron-emission source pattern via a line-sequential-scanning method with an optimized design that the individual gate electrodes driven with an on/off FE current flowing. This was accomplished while conserving energy in cathode electrodes activated by making up scratching to emit electrons more effectively from the cathodic electrode employing a highly crystalline SWCNT. We also succeeded in amplifying the luminance-output by combining long-afterglow and high-luminance phosphors for an anode electrode to overlap the afterglow component of fluorescent materials to establish a basic design for assembling a triode flat-panel light-emission device with a line scanning sequential method. To the best of our knowledge, the creation of a flat-panel light-emission device that could achieve to obtain a high brightness efficiency of 87 lm/W was a remarkable outcome for the first time ever. Improvements to the driving performance for electron emissions at FE sources largely depend on the application of the highly crystalline SWCNTs. We believe this achievement assumes a leading role in electronic device creation through the effective use of a highly crystalline SWCNT, and we confidently need to promote its industrial design for practical use.

**Figure 24.** Relationship between power consumption and output luminance of triode flat-panel light-emission device [32].

Copyright (2017) The Japan Society of Applied Physics.

**Figure 23.** (a) Flat-panel light emission driven by line-sequential scanning (shaded by ND filter) [32]. (b) On-and-off chart for voltage applied to gate electrode and current flowing through cathode [32]. Copyright (2017) The Japan Society

of Applied Physics.

46 Carbon Nanotubes - Recent Progress

This work was partially supported by DOWA Holdings Co. Ltd. The author thanks the coresearchers from DOWA for their discussions and suggestions.

#### **Author details**

Norihiro Shimoi

Address all correspondence to: norihiro.shimoi.c8@tohoku.ac.jp

Graduate School of Environmental Studies, Tohoku University, Sendai, Japan

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**Chapter 4**

**Provisional chapter**

**Carbon Nanotubes as a Pulsed Electron Sources**

**Carbon Nanotubes as a Pulsed Electron Sources**

DOI: 10.5772/intechopen.70528

One of the most promising applications of carbon nanotubes (CNTs) is the emission electronics, in which CNTs are used as a field emission electron source. Lots of electronic devices are based on electron sources, which are the most important components served as state-of-art nanoelectronics devices in nowadays, such as field emission displays, miniature mass spectrometers, scanning ultrafast electron microscopes, X-ray generators, free electron lasers, THz sources, and so on. Field electron emission is based on the physical phenomenon of quantum tunneling, in which electrons are emitted from the surface of materials into vacuum on the condition of applied voltage. CNTs have many advantages as field emitters comparing with conventional metallic emitters like familiar examples of tungsten, gold, copper, and molybdenum. Electron emission from cold cathodes of CNTs has attracted attentions of numerous scientists, and substantial researches have been conducted on their properties and applications. In this chapter, we will focus on the field emission of CNT cold cathodes as an electron source, including of how to synthesize it by chemical vapor deposition (CVD) method and how to realize its electron emission. In addition, we will report pulsed electron emission of CNT cathodes. The combination of the laser pulse and the cold cathode will offer the possibility of pulsed field emission. Our approach demonstrates the growth mechanism and the emission mechanism of CNTs, which is beneficial for controlling the performance of its

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Electron field emission (or field emission for short), is a tunnel effect of electron induced by high electric field at the surface of a solid material. A distinct advantage is that no extra energy is needed for the electron tunneling process. One of the most promising applications of the field emission is an electron source. Lots of electronic devices are based on electron

**Keywords:** carbon nanotubes (CNTs), cathodes, field emission, laser, pulse

Additional information is available at the end of the chapter

fascinating application on emerging fields.

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70528

Xianqi Wei

**Abstract**

**1. Introduction**

Xianqi Wei

**Provisional chapter**

## **Carbon Nanotubes as a Pulsed Electron Sources**

**Carbon Nanotubes as a Pulsed Electron Sources**

DOI: 10.5772/intechopen.70528

#### Xianqi Wei Xianqi Wei Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70528

#### **Abstract**

One of the most promising applications of carbon nanotubes (CNTs) is the emission electronics, in which CNTs are used as a field emission electron source. Lots of electronic devices are based on electron sources, which are the most important components served as state-of-art nanoelectronics devices in nowadays, such as field emission displays, miniature mass spectrometers, scanning ultrafast electron microscopes, X-ray generators, free electron lasers, THz sources, and so on. Field electron emission is based on the physical phenomenon of quantum tunneling, in which electrons are emitted from the surface of materials into vacuum on the condition of applied voltage. CNTs have many advantages as field emitters comparing with conventional metallic emitters like familiar examples of tungsten, gold, copper, and molybdenum. Electron emission from cold cathodes of CNTs has attracted attentions of numerous scientists, and substantial researches have been conducted on their properties and applications. In this chapter, we will focus on the field emission of CNT cold cathodes as an electron source, including of how to synthesize it by chemical vapor deposition (CVD) method and how to realize its electron emission. In addition, we will report pulsed electron emission of CNT cathodes. The combination of the laser pulse and the cold cathode will offer the possibility of pulsed field emission. Our approach demonstrates the growth mechanism and the emission mechanism of CNTs, which is beneficial for controlling the performance of its fascinating application on emerging fields.

**Keywords:** carbon nanotubes (CNTs), cathodes, field emission, laser, pulse

#### **1. Introduction**

Electron field emission (or field emission for short), is a tunnel effect of electron induced by high electric field at the surface of a solid material. A distinct advantage is that no extra energy is needed for the electron tunneling process. One of the most promising applications of the field emission is an electron source. Lots of electronic devices are based on electron

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

sources, which are the most crucial component served as state-of-art vacuum nanoelectronics and emerging novel devices in nowadays, such as field emission displays, emerging sensor, energy storage equipment, scanning ultrafast electron microscopes, X-ray generators, free electron lasers (FELs), THz sources, and so on [1–3].

**2. Growth of CNTs**

**2.1. Carbon nanotube arrays (CNT arrays)**

phonon emissions, appears at 2700 cm−1 [4, 14].

**Figure 1.** CNT arrays. (a) Top view, (b) sectional view, (c) TEM, (d) Raman spectrum.

tion (for short CVD) with iron phthalocyanine (FeC32N8

The growth of CNT arrays follows a high temperature catalytic chemical vapor deposi-

phthalocyanines act as not only a carbon source but also as a metallic catalyst. The growth temperature is 800–900°C for 15 min. The flow rate of hydrogen and argon are 25 and 60 sccm, respectively. The prepared CNT arrays are multi-walled nanotubes with the height of 6–10 μm and the diameter of 20–100 nm, as shown in **Figure 1**. The densely packed CNTs grow on the surface of the Si substrate. The roots stand vertically on the substrate surface and the tips entangles with each other. Detailed structures of CNT arrays are characterized by TEM as shown in **Figure 1(c)**. The synthesized CNT arrays are multi-walled nanotubes, and the interlayer spacing is about 0.34 nm, which is close to the (002) crystal plane of graphite. Besides, Raman Spectroscopy is employed to perform the Raman pattern of CNT arrays as shown in **Figure 1(d)**. Two of the sharp peaks are present at 1350 and 1350 cm−1, respectively, corresponding to the disorder-induced D band and the tangential mode G band of the main Raman feature of multi-walled CNTs. Moreover, the ratio of intensities of the D and G bands, which is less than one indicates a high quantity of structure defects. And, the second-order 2D band, which is caused by the double resonant Raman scattering with two-

H16). In the growth process, the

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 53

With the advent of electron beam from nanometer-scale field emitters induced by femtosecond laser, electron sources are gradually replaced by ultrafast pulsed electron sources. These sources could potentially output extremely high emission current in a very short time, providing sufficient brightness, high-energy density, high frequency (up to optical frequency), and ultrashort coherent electron bunches [4–6]. Recently, such electron sources have intrigued a strong interest and encouraged the further studies of pulsed electron emission, opening the door to high-tech novel devices. For example, pulsed electron emission opens a way toward the time-resolved electron microscopy, because electrical gating and source control enable time resolution down to picoseconds, while using optical control enables creation of electron pulses with duration down to tens of femtoseconds. Such dense and short electron bunches can become a popular platform for material and device imaging, inspection, and failure analysis. They would enable exciting technological developments like four-dimensional (4D) time resolved electron microscopy, spectroscopy, holography, single-electron sources, and carrier envelope phase detection [7, 8]. Besides, pulsed electron sources are also becoming a hot topic in fields of medicine, physics, chemistry, industry, and communication, both in military and domestic industries.

In the past few decades, metallic nanotips have been considered an attractive and popular candidate as emitters of field emission cold cathodes, such as W, Mo, Au, Cu, Co, and so on. However, it is certainly worth considering that the complicated process of manufacture and thermal ablation of metallic nanotips result in poor thermal stability and mechanical properties for practical application. Besides, metallic tips suffer from serious thermal effect in pulsed field emission, because of illuminating metallic emitters for a long time by induced laser. Moreover, with the development of technology, the main trend of new generation electronic devices faces a great challenge on limitations of high power, high frequency, and compact small size. Conventional metallic emitters cannot satisfy the higher requirements of emerging devices, which have to provide high-energy density and pulsed emission electric current [9]. Highperformance field emitters with adequate excellent emission properties are highly desired. To extend the studies on conventional metallic nanotips, this paper studies the pulsed electron emitters in a popular material of CNTs.

Since the discovery by Iijima in 1991, CNTs have received more and more research interests. One of the most popular studies is about the intriguing explorations of the high-performance field emitter as an emission electron source due to the CNTs' extraordinary instinct properties of high aspect ratio, good electrical conductivity, excellent thermal conductivity, robust chemical and mechanical properties. CNTs have been considered promising field emission candidates since they were first demonstrated as electron field emitters in 1995 [10–12]. These years, considerable researches have been conducted on their useful properties, as well as synthesis methods and various applications [13]. Here, we mainly introduce the property and application about the field emission of CNTs, which are synthesized by high temperature catalytic chemical vapor deposition (CVD) method.

#### **2. Growth of CNTs**

sources, which are the most crucial component served as state-of-art vacuum nanoelectronics and emerging novel devices in nowadays, such as field emission displays, emerging sensor, energy storage equipment, scanning ultrafast electron microscopes, X-ray generators, free

With the advent of electron beam from nanometer-scale field emitters induced by femtosecond laser, electron sources are gradually replaced by ultrafast pulsed electron sources. These sources could potentially output extremely high emission current in a very short time, providing sufficient brightness, high-energy density, high frequency (up to optical frequency), and ultrashort coherent electron bunches [4–6]. Recently, such electron sources have intrigued a strong interest and encouraged the further studies of pulsed electron emission, opening the door to high-tech novel devices. For example, pulsed electron emission opens a way toward the time-resolved electron microscopy, because electrical gating and source control enable time resolution down to picoseconds, while using optical control enables creation of electron pulses with duration down to tens of femtoseconds. Such dense and short electron bunches can become a popular platform for material and device imaging, inspection, and failure analysis. They would enable exciting technological developments like four-dimensional (4D) time resolved electron microscopy, spectroscopy, holography, single-electron sources, and carrier envelope phase detection [7, 8]. Besides, pulsed electron sources are also becoming a hot topic in fields of medicine, phys-

ics, chemistry, industry, and communication, both in military and domestic industries.

In the past few decades, metallic nanotips have been considered an attractive and popular candidate as emitters of field emission cold cathodes, such as W, Mo, Au, Cu, Co, and so on. However, it is certainly worth considering that the complicated process of manufacture and thermal ablation of metallic nanotips result in poor thermal stability and mechanical properties for practical application. Besides, metallic tips suffer from serious thermal effect in pulsed field emission, because of illuminating metallic emitters for a long time by induced laser. Moreover, with the development of technology, the main trend of new generation electronic devices faces a great challenge on limitations of high power, high frequency, and compact small size. Conventional metallic emitters cannot satisfy the higher requirements of emerging devices, which have to provide high-energy density and pulsed emission electric current [9]. Highperformance field emitters with adequate excellent emission properties are highly desired. To extend the studies on conventional metallic nanotips, this paper studies the pulsed electron

Since the discovery by Iijima in 1991, CNTs have received more and more research interests. One of the most popular studies is about the intriguing explorations of the high-performance field emitter as an emission electron source due to the CNTs' extraordinary instinct properties of high aspect ratio, good electrical conductivity, excellent thermal conductivity, robust chemical and mechanical properties. CNTs have been considered promising field emission candidates since they were first demonstrated as electron field emitters in 1995 [10–12]. These years, considerable researches have been conducted on their useful properties, as well as synthesis methods and various applications [13]. Here, we mainly introduce the property and application about the field emission of CNTs, which are synthesized by high temperature

electron lasers (FELs), THz sources, and so on [1–3].

52 Carbon Nanotubes - Recent Progress

emitters in a popular material of CNTs.

catalytic chemical vapor deposition (CVD) method.

#### **2.1. Carbon nanotube arrays (CNT arrays)**

The growth of CNT arrays follows a high temperature catalytic chemical vapor deposition (for short CVD) with iron phthalocyanine (FeC32N8 H16). In the growth process, the phthalocyanines act as not only a carbon source but also as a metallic catalyst. The growth temperature is 800–900°C for 15 min. The flow rate of hydrogen and argon are 25 and 60 sccm, respectively. The prepared CNT arrays are multi-walled nanotubes with the height of 6–10 μm and the diameter of 20–100 nm, as shown in **Figure 1**. The densely packed CNTs grow on the surface of the Si substrate. The roots stand vertically on the substrate surface and the tips entangles with each other. Detailed structures of CNT arrays are characterized by TEM as shown in **Figure 1(c)**. The synthesized CNT arrays are multi-walled nanotubes, and the interlayer spacing is about 0.34 nm, which is close to the (002) crystal plane of graphite. Besides, Raman Spectroscopy is employed to perform the Raman pattern of CNT arrays as shown in **Figure 1(d)**. Two of the sharp peaks are present at 1350 and 1350 cm−1, respectively, corresponding to the disorder-induced D band and the tangential mode G band of the main Raman feature of multi-walled CNTs. Moreover, the ratio of intensities of the D and G bands, which is less than one indicates a high quantity of structure defects. And, the second-order 2D band, which is caused by the double resonant Raman scattering with twophonon emissions, appears at 2700 cm−1 [4, 14].

**Figure 1.** CNT arrays. (a) Top view, (b) sectional view, (c) TEM, (d) Raman spectrum.

#### **2.2. Carbon nanotubes arrays-graphene composite (CNTs-G)**

CNTs have been considered as a promising emitter because of their unique properties. For example, the electrical conductivity is about 10<sup>5</sup> S/cm and thermal conductivity is 3500 W/mK, which is better than that of many metallic emitters [15, 16]. Graphene, a desired electrical and thermal transport medium with two-dimensional sp2 hybridized carbon atoms, is potential for field emission because of its remarkable electronic, mechanical, and thermal properties. However, the CNTs emitters are commonly attached onto the electrode surface to form the CNTs cathodes, resulting in a large contact resistance and unstable emission current. Graphene, a transparent cathode, have to design a special structure for field emission. The field emission still faces great challenges on stability and thermal effect, especially in pulsed field emission. In this part, we report the composite of CNTs-G to improve the electric and thermal transport as an emitter.

CNTs/graphene composite fabricated by transfer technology. The interaction between CNTs and graphene is mainly due to the Van der Waals force for the CNTs/graphene composite. Here, the 3D CNTs-G composite is directly synthesized, which is quite different from physical contact of the CNTs/graphene composite between the interface of CNTs and graphene. The 3D composite, having the vertically CNT arrays and the outstanding graphene, could provide a good electrical and thermal contact and exhibits a high performance as a thermal

The graphene sheets on the top surface of the CNTs-G composite are characterized by TEM, as shown in **Figure 2(c)**, and the composite shows a typical peak of 2D band at 2700 cm−1 in comparison with CNT arrays, as shown in **Figure 2(d)**. To obtain more detailed information on the chemical composition and surface electronic state of the CNTs-G composite, the X-ray photoelectron spectroscopy (XPS) examination is implemented, as shown in **Figure 3**. It can be clearly observed that Fe, O, N, and C elements exist in the spectrum. The elements of Fe

for C1s at 284.8 eV is observed to evaluate the binding behavior of the elements in the CNTs-G

The principle of field emission is based on the application of a very high electric field to extract electrons from a metal or a highly doped semiconducting surface. However, if the field emission cathode surface has a high point or a protrusion, electrons may be extracted at

H16). And, a sharp peak

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 55

and N arise from the carbon source of iron phthalocyanine (FeC32N8

composite, which is ascribed to the graphitic carbon [17, 18].

**Figure 3.** The full survey spectrum XPS spectra of the CNTs-G composite.

*2.2.2. Top-down structure of the CNTs-G composite*

interface material.

#### *2.2.1. Bottom-up structure of the CNTs-G composite*

CNTs-G composite grows by the same method as CNT arrays but the different parameters of gas flow rate. The reducing gas of hydrogen is 10 sccm and protective gas of argon is 60 sccm, respectively. The composite of CNTs-G consists of graphene sheets and CNT arrays. Graphene sheets is on the top surface and CNT arrays is on the bottom attached on the substrate surface, as shown in **Figure 2**. Graphene sheets and CNT arrays combine together tightly and form a three-dimensional (3D) cathode, which is completely different from the

**Figure 2.** The composite of carbon nanotube arrays-graphene (CNTs-G). (a) Top view, (b) sectional view, (c) TEM, (d) Raman spectrum.

CNTs/graphene composite fabricated by transfer technology. The interaction between CNTs and graphene is mainly due to the Van der Waals force for the CNTs/graphene composite. Here, the 3D CNTs-G composite is directly synthesized, which is quite different from physical contact of the CNTs/graphene composite between the interface of CNTs and graphene. The 3D composite, having the vertically CNT arrays and the outstanding graphene, could provide a good electrical and thermal contact and exhibits a high performance as a thermal interface material.

The graphene sheets on the top surface of the CNTs-G composite are characterized by TEM, as shown in **Figure 2(c)**, and the composite shows a typical peak of 2D band at 2700 cm−1 in comparison with CNT arrays, as shown in **Figure 2(d)**. To obtain more detailed information on the chemical composition and surface electronic state of the CNTs-G composite, the X-ray photoelectron spectroscopy (XPS) examination is implemented, as shown in **Figure 3**. It can be clearly observed that Fe, O, N, and C elements exist in the spectrum. The elements of Fe and N arise from the carbon source of iron phthalocyanine (FeC32N8 H16). And, a sharp peak for C1s at 284.8 eV is observed to evaluate the binding behavior of the elements in the CNTs-G composite, which is ascribed to the graphitic carbon [17, 18].

#### *2.2.2. Top-down structure of the CNTs-G composite*

**2.2. Carbon nanotubes arrays-graphene composite (CNTs-G)**

mal transport medium with two-dimensional sp2

54 Carbon Nanotubes - Recent Progress

*2.2.1. Bottom-up structure of the CNTs-G composite*

(d) Raman spectrum.

CNTs have been considered as a promising emitter because of their unique properties. For example, the electrical conductivity is about 10<sup>5</sup> S/cm and thermal conductivity is 3500 W/mK, which is better than that of many metallic emitters [15, 16]. Graphene, a desired electrical and ther-

emission because of its remarkable electronic, mechanical, and thermal properties. However, the CNTs emitters are commonly attached onto the electrode surface to form the CNTs cathodes, resulting in a large contact resistance and unstable emission current. Graphene, a transparent cathode, have to design a special structure for field emission. The field emission still faces great challenges on stability and thermal effect, especially in pulsed field emission. In this part, we report the composite of CNTs-G to improve the electric and thermal transport as an emitter.

CNTs-G composite grows by the same method as CNT arrays but the different parameters of gas flow rate. The reducing gas of hydrogen is 10 sccm and protective gas of argon is 60 sccm, respectively. The composite of CNTs-G consists of graphene sheets and CNT arrays. Graphene sheets is on the top surface and CNT arrays is on the bottom attached on the substrate surface, as shown in **Figure 2**. Graphene sheets and CNT arrays combine together tightly and form a three-dimensional (3D) cathode, which is completely different from the

**Figure 2.** The composite of carbon nanotube arrays-graphene (CNTs-G). (a) Top view, (b) sectional view, (c) TEM,

hybridized carbon atoms, is potential for field

The principle of field emission is based on the application of a very high electric field to extract electrons from a metal or a highly doped semiconducting surface. However, if the field emission cathode surface has a high point or a protrusion, electrons may be extracted at

**Figure 3.** The full survey spectrum XPS spectra of the CNTs-G composite.

a considerably lower applied gap field. This is because the lines of force converge at the sharp point and the physical geometry of the tip provides a field enhancement [3]. So, it is desired a composite of CNTs-G, which is a top-down 3D structure. That is, CNT arrays are on the top surface of graphene, and it is tightly attached on the substrate surface as an electrode. This 3D CNTs-G composite is different from the above in the part (i). Graphene films are on the bottom grown on the substrate and CNT arrays are on the top surface, as shown in **Figure 4**. This composite was found by accident, when grew patterned CNT arrays.

the multilayer graphene films. Due to the CNT arrays as field emitters and high electrical and thermal conductive of graphene films as electrode, the composite could provide a good elec-

The phenomenon of the field emission is associated with a quantum mechanical tunneling process whereby electrons tunnel through a potential barrier under the influence of a high electric field. The field emission characteristic is usually described by F-N theory, given by:

*<sup>V</sup>*<sup>2</sup>) <sup>=</sup> ln*<sup>A</sup>* <sup>−</sup> \_\_*<sup>B</sup>*

enhancement factor, *φ* is the work function of field emitters, *d* is the distance of anode–cathode [3, 19]. Generally, it was used to improve the field emission properties by enhancing *β*. The sharp tips or small radius curvature of the emitter apex gives a high local electric field enhancement. In the past few years, to improve *β* of CNTs, novel composites of CNTs/graphene hybrids, three-dimensional CNTs and plasma treatment of CNTs have been reported to explore optimized CNTs emitters. In this part, we will introduce a cylindrical structure of CNT arrays cathodes. The morphology of CNT arrays on cylindrical substrate can be controlled by tuning the size of the substrate diameter. The work function *ϕ* of CNT arrays is supposed to 4.89 eV

The area of CNT arrays grown on the flat substrate of Si is about 1.7 cm × 1.8 cm. The field emission measurement is carried out using diode parallel plate in vacuum 5 × 10−5 Pa. The anode–cathode spacing is 400 μm. The experimental set-up is shown in **Figure 5(a)**. **Figure 5(b)** gives the current density versus voltage (*J*-*V*) curves. The turn-on voltage is about 700 V (defined as the applied voltage required to generate an emission current den-

corresponding to *J*-*V* curves, and the bottom right is the picture of current emission on 1500 V. The F-N curve follows the linear relationship, confirming that the detected current

Nevertheless, we present a method to synthesize CNT arrays on cylindrical waveguide of the optical fiber substrate. CNT arrays are synthesized on the fiber surface by CVD using a same method as reported the above. The substrate is commercial quartz optical fiber with a diameter of 220 μm. To prepare fiber substrate before CNTs growth, the polymer coating

In the previous research, CNTs were synthesized on a flat substrate such as Si, SiO<sup>2</sup>

). The insert of top left in **Figure 5(b)** is Fowler-Nordheim (F-N) plots

*<sup>V</sup>* (1)

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 57

, and Au.

*<sup>β</sup>* , *S* is the effective area of field emitters, *β* is the field

*J*\_\_

*d* \_\_\_\_\_\_\_\_\_\_\_

trical and thermal contact and exhibits a high-performance field emission.

**3. Field emission**

where *A* = 1.54 × 10<sup>−</sup><sup>6</sup> *S*

in this paper [20].

**3.1. Planar structure**

sity of 1 mA/cm<sup>2</sup>

**3.2. Cylindrical structure**

ln(

*β*2 \_\_

is in agreement with field emission mechanism.

*<sup>φ</sup>* and *<sup>B</sup>* <sup>=</sup> −6.83 <sup>×</sup> <sup>10</sup><sup>7</sup> *<sup>φ</sup>*<sup>3</sup>⁄<sup>2</sup>

From the EDX analysis shown in **Figure 4(d)**, it indicates that the composite mainly consists of C, Fe, and Si elements. The element of Fe results from the iron phthalocyanine (FeC32N8 H16), and the Si results from the substrate. Raman spectrum is carried out to further determine the structure information, as shown in **Figure 4(c)**. In comparison with CNT arrays, the Raman spectrum of films on the substrate have a typical peak at 2700 cm−1, which is in accord with the 2D peak of graphene. The ratio of 2D peak height/G peak height indicates the films are

**Figure 4.** The top-down structure of CNTs-G composite. (a) Top view of a ribbon of CNT arrays; (b) cross sectional view; (c) Raman spectrum; (d) EDX analysis.

the multilayer graphene films. Due to the CNT arrays as field emitters and high electrical and thermal conductive of graphene films as electrode, the composite could provide a good electrical and thermal contact and exhibits a high-performance field emission.

#### **3. Field emission**

H16),

a considerably lower applied gap field. This is because the lines of force converge at the sharp point and the physical geometry of the tip provides a field enhancement [3]. So, it is desired a composite of CNTs-G, which is a top-down 3D structure. That is, CNT arrays are on the top surface of graphene, and it is tightly attached on the substrate surface as an electrode. This 3D CNTs-G composite is different from the above in the part (i). Graphene films are on the bottom grown on the substrate and CNT arrays are on the top surface, as shown in **Figure 4**.

From the EDX analysis shown in **Figure 4(d)**, it indicates that the composite mainly consists of C, Fe, and Si elements. The element of Fe results from the iron phthalocyanine (FeC32N8

and the Si results from the substrate. Raman spectrum is carried out to further determine the structure information, as shown in **Figure 4(c)**. In comparison with CNT arrays, the Raman spectrum of films on the substrate have a typical peak at 2700 cm−1, which is in accord with the 2D peak of graphene. The ratio of 2D peak height/G peak height indicates the films are

**Figure 4.** The top-down structure of CNTs-G composite. (a) Top view of a ribbon of CNT arrays; (b) cross sectional view;

(c) Raman spectrum; (d) EDX analysis.

This composite was found by accident, when grew patterned CNT arrays.

56 Carbon Nanotubes - Recent Progress

The phenomenon of the field emission is associated with a quantum mechanical tunneling process whereby electrons tunnel through a potential barrier under the influence of a high electric field. The field emission characteristic is usually described by F-N theory, given by:

$$\ln\left(\frac{J}{V^2}\right) = \ln A - \frac{B}{V} \tag{1}$$

where *A* = 1.54 × 10<sup>−</sup><sup>6</sup> *S β*2 \_\_ *<sup>φ</sup>* and *<sup>B</sup>* <sup>=</sup> −6.83 <sup>×</sup> <sup>10</sup><sup>7</sup> *<sup>φ</sup>*<sup>3</sup>⁄<sup>2</sup> *d* \_\_\_\_\_\_\_\_\_\_\_ *<sup>β</sup>* , *S* is the effective area of field emitters, *β* is the field enhancement factor, *φ* is the work function of field emitters, *d* is the distance of anode–cathode [3, 19]. Generally, it was used to improve the field emission properties by enhancing *β*. The sharp tips or small radius curvature of the emitter apex gives a high local electric field enhancement. In the past few years, to improve *β* of CNTs, novel composites of CNTs/graphene hybrids, three-dimensional CNTs and plasma treatment of CNTs have been reported to explore optimized CNTs emitters. In this part, we will introduce a cylindrical structure of CNT arrays cathodes. The morphology of CNT arrays on cylindrical substrate can be controlled by tuning the size of the substrate diameter. The work function *ϕ* of CNT arrays is supposed to 4.89 eV in this paper [20].

#### **3.1. Planar structure**

The area of CNT arrays grown on the flat substrate of Si is about 1.7 cm × 1.8 cm. The field emission measurement is carried out using diode parallel plate in vacuum 5 × 10−5 Pa. The anode–cathode spacing is 400 μm. The experimental set-up is shown in **Figure 5(a)**. **Figure 5(b)** gives the current density versus voltage (*J*-*V*) curves. The turn-on voltage is about 700 V (defined as the applied voltage required to generate an emission current density of 1 mA/cm<sup>2</sup> ). The insert of top left in **Figure 5(b)** is Fowler-Nordheim (F-N) plots corresponding to *J*-*V* curves, and the bottom right is the picture of current emission on 1500 V. The F-N curve follows the linear relationship, confirming that the detected current is in agreement with field emission mechanism.

#### **3.2. Cylindrical structure**

In the previous research, CNTs were synthesized on a flat substrate such as Si, SiO<sup>2</sup> , and Au. Nevertheless, we present a method to synthesize CNT arrays on cylindrical waveguide of the optical fiber substrate. CNT arrays are synthesized on the fiber surface by CVD using a same method as reported the above. The substrate is commercial quartz optical fiber with a diameter of 220 μm. To prepare fiber substrate before CNTs growth, the polymer coating

**Figure 5.** The field emission of the CNT arrays on Si substrate. (a) Structure diagram of experimental set-up, (b) measurement results of the field emission.

at 1585 cm−1, and 2D band appears at 2700 cm−1, which is in agreement with the typical Raman feature of multi-walled CNTs [4, 14]. To further characterize the detailed information of the CNT arrays covered on the surface of fiber substrate, the X-ray diffractometer is employed to analyze. As shown in Figure 7, the XRD pattern of CNT arrays on fiber substrate exhibits a sharp (002) Bragg reflection at about 2θ = 26.2°, which is derived from the ordered arrangement of the con-

from the quartz fiber substrate. **Figure 8(a)** is a cross-section SEM image of CNT arrays covered on the fiber substrate. CNT arrays vertically stand on the fiber surface with the uniform length and roots density. **Figure 8(c)** is a zoom in image of the CNTs roots marked in **Figure 8(a)**. The dense roots are highly aligned and vertically stand on fiber surface. The surface morphology of the CNTs

**Figure 8.** The SEM images of CNT arrays synthesized on the fiber substrate with the diameter of 200 μm, (a) the crosssection SEM image, (b) the top view SEM image of the surface morphology, (c) a zoomed in view of the roots, (d) a zoomed

in top view of the tips.

resulting

59

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528

centric cylinders of graphitic carbon. And the peak centered on 2θ = 21.9° shows the SiO<sup>2</sup>

**Figure 7.** The X-ray diffraction pattern of CNT arrays covered on the quartz fiber surface.

layer should be removed. Then, quartz optical fiber is corroded in dilute hydrofluoric acid (~13 wt.% HF) to the desired size by controlling the corrosion time. After treatment, the prepared fiber is used as substrate for the growth of CNT arrays.

The microstructures and morphologies of the products are characterized by SEM, TEM, and Raman spectroscopy. **Figure 6(a)** is an overall view, showing the densely packed CNT arrays covered on the fiber surface. Detailed structures of CNT arrays are characterized by TEM. **Figure 6(b)** and **(c)** are the low-magnification and high-resolution TEM image. The synthesized CNT arrays are multi-walled nanotubes with the diameters of 20–100 nm. As shown in **Figure 7(c)**, the interlayer spacing is about 0.34 nm, which is close to the (002) crystal plane of graphite. Besides, Raman pattern of CNT arrays is showed in **Figure 6(d)**. The D band appears at 1352 cm−1, G band appears

**Figure 6.** The scanning electron micrograph (SEM) image of CNT arrays covered on the surface of fiber substrate. (a) a overall view, (b) a low-magnification TEM image of CNTs, and (c) a high-resolution TEM image of a CNT. (d) the Raman spectrum of CNT arrays.

**Figure 7.** The X-ray diffraction pattern of CNT arrays covered on the quartz fiber surface.

layer should be removed. Then, quartz optical fiber is corroded in dilute hydrofluoric acid (~13 wt.% HF) to the desired size by controlling the corrosion time. After treatment, the pre-

**Figure 5.** The field emission of the CNT arrays on Si substrate. (a) Structure diagram of experimental set-up, (b) measurement

The microstructures and morphologies of the products are characterized by SEM, TEM, and Raman spectroscopy. **Figure 6(a)** is an overall view, showing the densely packed CNT arrays covered on the fiber surface. Detailed structures of CNT arrays are characterized by TEM. **Figure 6(b)** and **(c)** are the low-magnification and high-resolution TEM image. The synthesized CNT arrays are multi-walled nanotubes with the diameters of 20–100 nm. As shown in **Figure 7(c)**, the interlayer spacing is about 0.34 nm, which is close to the (002) crystal plane of graphite. Besides, Raman pattern of CNT arrays is showed in **Figure 6(d)**. The D band appears at 1352 cm−1, G band appears

**Figure 6.** The scanning electron micrograph (SEM) image of CNT arrays covered on the surface of fiber substrate. (a) a overall view, (b) a low-magnification TEM image of CNTs, and (c) a high-resolution TEM image of a CNT. (d) the Raman

pared fiber is used as substrate for the growth of CNT arrays.

spectrum of CNT arrays.

results of the field emission.

58 Carbon Nanotubes - Recent Progress

at 1585 cm−1, and 2D band appears at 2700 cm−1, which is in agreement with the typical Raman feature of multi-walled CNTs [4, 14]. To further characterize the detailed information of the CNT arrays covered on the surface of fiber substrate, the X-ray diffractometer is employed to analyze. As shown in Figure 7, the XRD pattern of CNT arrays on fiber substrate exhibits a sharp (002) Bragg reflection at about 2θ = 26.2°, which is derived from the ordered arrangement of the concentric cylinders of graphitic carbon. And the peak centered on 2θ = 21.9° shows the SiO<sup>2</sup> resulting from the quartz fiber substrate. **Figure 8(a)** is a cross-section SEM image of CNT arrays covered on the fiber substrate. CNT arrays vertically stand on the fiber surface with the uniform length and roots density. **Figure 8(c)** is a zoom in image of the CNTs roots marked in **Figure 8(a)**. The dense roots are highly aligned and vertically stand on fiber surface. The surface morphology of the CNTs

**Figure 8.** The SEM images of CNT arrays synthesized on the fiber substrate with the diameter of 200 μm, (a) the crosssection SEM image, (b) the top view SEM image of the surface morphology, (c) a zoomed in view of the roots, (d) a zoomed in top view of the tips.

tips are showed in **Figure 8(b)**, and a magnified top view image shows in **Figure 8(d)**. Though the dense roots of CNT arrays vertically attach on the fiber surface, the tips radiate outward dispersedly along the radial direction of fiber substrate [14]. With the fiber diameter decreased, the gaps between the CNT arrays tips are obviously distinct. And for that, the change of fiber diameter will greatly affect the tips morphology of CNT arrays, which cover on the cylindrical surface of optical fiber substrate. Decreasing the fiber diameter, the gaps between CNT tips will enlarge. The distance between the neighboring CNT arrays tips is greatly dependent on the fiber substrate, which could be changed by simply tuning the fiber diameter, as shown in **Figure 9**. **Figure 9(a)**, **(b)** is the overall view of CNT arrays covered on the surface of fibers with diameters of 200 and 20 μm. It is noticeable that remarkable cracks on fiber substrate with diameter of 20 μm, because the tips gap of nanotubes is large on a smaller diameter curve surface. **Figure 9(c)** is a cross-section image of CNT arrays covered on the cylindrical surface of 200 μm diameter fiber, and **Figure 9(d)** is CNT arrays on the surface of fiber with a smaller diameter of 20 μm [14]. **Figure 10** gives the field emission measurement results of CNT arrays covered on fiber substrates with diameters of 200, 100, 40, 30, and 20 μm. **Figure 10(a)** is the *J*-*V* curves. For the fiber substrate with the diameter of 20 μm, the CNTs cathode performs the minimum turn-on voltage *V*to (the applied voltage requires to generate an emission current density of 1 mA/cm<sup>2</sup> ), yielding the maximum emission current at a low applied voltage than the others. When the diameter of fiber substrate decreases from 200 to 20 μm, the turn-on voltage *V*to obviously decreases and the emission current density increased at the same applied voltage. *β* of the CNT arrays increases from 7823 to 11,631, and the turn-on voltage *V*to decreases from 1290 to 109 V, corresponding to the data as shown in **Table 1**.

It is a well-known fact that *β* of nano cathodes has a significant effect on field emission properties. This is because the local electric field on the sharp emitters resulting in a field enhancement, which is expressed as *β*. However, there is a screening effect on the top tips of emitters, because of close spacing on dense individual tips. Screening effect weaken *β* of the emitter and result in a high *V*to and small emission current, which is a defect for field emission*.* Thus, *β* is greatly dependent on the surface morphology of cathodes. The method of growing CNT arrays on fiber surface is efficient and feasible to tuning *β* by fiber diameter. With the fiber diameter decreasing, the gaps of tips enlarge and *β* increase subsequently because of weakening screening effect of CNT emitters. [14]. To illustrate the screening effect on the tips of CNT arrays, a simple electrostatic simulation is carried out by commercial software (Comsol 3.5a, emes mode of AC/DC Module). **Figure 11** shows the simulation results, performing equipotential lines of the electrostatic field near CNT

**Figure 11.** Simulation potential lines of the electric field near CNTs tips on a surface with diameter (Comsol 3.5a, emes mode of AC/DC module). (a) 200 μm, and (b) 20 μm, respectively. The unit in color bar is volt. The color bar ranges from

**Table 1.** The turn-on voltage (*V*to) and the field enhancement factor (*β*) of CNTs cathodes covered on fibers surfaces with

**Figure 10.** Field emission results of CNT arrays covered on fibers surfaces with diameters of 20, 30, 40, 100, and 200 μm.

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 61

**Diameter (μm) 20 30 40 100 200** *V*to 109 190 270 730 1290 *β* 11.631 11.338 10.562 9247 7823

(a) *J*-*V* curve of emission current, (b) the F-N plots.

diameters of 20, 30, 40, 100, and 200 μm.

−1500 to −1465 V.

**Figure 9.** (a) The overall view of CNT arrays on fiber surfaces with diameters of 200 μm; (b) overall view of CNT arrays on fiber surface with diameter of 20 μm; (c) and (d) are cross-section images of CNT arrays on the surface of fiber substrate with the diameter of 200 and 20 μm, respectively.

tips are showed in **Figure 8(b)**, and a magnified top view image shows in **Figure 8(d)**. Though the dense roots of CNT arrays vertically attach on the fiber surface, the tips radiate outward dispersedly along the radial direction of fiber substrate [14]. With the fiber diameter decreased, the gaps between the CNT arrays tips are obviously distinct. And for that, the change of fiber diameter will greatly affect the tips morphology of CNT arrays, which cover on the cylindrical surface of optical fiber substrate. Decreasing the fiber diameter, the gaps between CNT tips will enlarge. The distance between the neighboring CNT arrays tips is greatly dependent on the fiber substrate, which could be changed by simply tuning the fiber diameter, as shown in **Figure 9**. **Figure 9(a)**, **(b)** is the overall view of CNT arrays covered on the surface of fibers with diameters of 200 and 20 μm. It is noticeable that remarkable cracks on fiber substrate with diameter of 20 μm, because the tips gap of nanotubes is large on a smaller diameter curve surface. **Figure 9(c)** is a cross-section image of CNT arrays covered on the cylindrical surface of 200 μm diameter fiber, and **Figure 9(d)** is CNT arrays on the surface of fiber with a smaller diameter of 20 μm [14]. **Figure 10** gives the field emission measurement results of CNT arrays covered on fiber substrates with diameters of 200, 100, 40, 30, and 20 μm. **Figure 10(a)** is the *J*-*V* curves. For the fiber substrate with the diameter of 20 μm, the CNTs cathode performs the minimum turn-on voltage *V*to (the applied voltage requires to gener-

applied voltage than the others. When the diameter of fiber substrate decreases from 200 to 20 μm, the turn-on voltage *V*to obviously decreases and the emission current density increased at the same applied voltage. *β* of the CNT arrays increases from 7823 to 11,631, and the turn-on voltage *V*to

It is a well-known fact that *β* of nano cathodes has a significant effect on field emission properties. This is because the local electric field on the sharp emitters resulting in a field enhancement, which is expressed as *β*. However, there is a screening effect on the top tips of emitters, because of close spacing on dense individual tips. Screening effect weaken

**Figure 9.** (a) The overall view of CNT arrays on fiber surfaces with diameters of 200 μm; (b) overall view of CNT arrays on fiber surface with diameter of 20 μm; (c) and (d) are cross-section images of CNT arrays on the surface of fiber

decreases from 1290 to 109 V, corresponding to the data as shown in **Table 1**.

), yielding the maximum emission current at a low

ate an emission current density of 1 mA/cm<sup>2</sup>

60 Carbon Nanotubes - Recent Progress

substrate with the diameter of 200 and 20 μm, respectively.

**Figure 10.** Field emission results of CNT arrays covered on fibers surfaces with diameters of 20, 30, 40, 100, and 200 μm. (a) *J*-*V* curve of emission current, (b) the F-N plots.


**Table 1.** The turn-on voltage (*V*to) and the field enhancement factor (*β*) of CNTs cathodes covered on fibers surfaces with diameters of 20, 30, 40, 100, and 200 μm.

*β* of the emitter and result in a high *V*to and small emission current, which is a defect for field emission*.* Thus, *β* is greatly dependent on the surface morphology of cathodes. The method of growing CNT arrays on fiber surface is efficient and feasible to tuning *β* by fiber diameter. With the fiber diameter decreasing, the gaps of tips enlarge and *β* increase subsequently because of weakening screening effect of CNT emitters. [14]. To illustrate the screening effect on the tips of CNT arrays, a simple electrostatic simulation is carried out by commercial software (Comsol 3.5a, emes mode of AC/DC Module). **Figure 11** shows the simulation results, performing equipotential lines of the electrostatic field near CNT

**Figure 11.** Simulation potential lines of the electric field near CNTs tips on a surface with diameter (Comsol 3.5a, emes mode of AC/DC module). (a) 200 μm, and (b) 20 μm, respectively. The unit in color bar is volt. The color bar ranges from −1500 to −1465 V.

tips on fiber substrates, a circular curving surface with curvature radius 200 and 20 μm, as shown in **Figure 11(a)** and **(b)**, respectively. With the curvature radius of substrate reduce, the spacing and the density of the equipotential lines on tips surface increase. This result demonstrates the decreased screening effect of the CNT arrays that lies on a smaller diameter fiber substrate.

### **4. Laser-induced field emission**

Laser-induced field emission could offer opportunities for dramatic improvements in performance of electronic devices due to the possibility of manipulation and control of coherent electron motion in ultrafast spatiotemporal scales [8, 21]. As one of the most important cold cathode materials, CNTs have been widely studied. Synthesis of well-aligned CNTs on substrates of various geometries is also of special interest to the exploration of laser-induced field emission. Here, CNT arrays on fiber substrate serve as an emitter to explore the mechanism of laser-induced field emission.

From Eq.(1), we know that enhancing *β* or decreasing *ϕ* can improve field emission properties. As for a given material for cathode, *ϕ* is a constant. Generally, enhancing *β* by changing the tip morphology of the emitters is one of the main strategies. Herein, the improvement of field emission can be attributed to decreasing *ϕ* of CNT arrays because of the photon absorption. In this part, it will take advantage of laser leakage from an optical fiber to illuminate the cathode for realizing laser-induced field emission.

**Figure 13(a)**, when the fiber diameter fixed at 30 μm, *β* of the CNT arrays cathode is a constant of 4.89 eV. However, the turn-on voltage and emission current density are modulated by introducing laser into the optical fiber. The enhanced property is evaluated by measuring

and listed in **Table 2**. *β* of the cathode on the optical fiber with diameter of 30 μm is 4672, while that of the cathode on the fiber with diameter of 40 and 140 μm is 2804 and 2526. The different *β* is attributed to the different curvature of the cathode on fibers with different diameters, agreeing with the above discussion. What we are concerned here is the change of the work function **Δ***ϕ*of the cathode causing by the laser field. From **Table 2**, the work function significantly reduces to 4.14 eV when the fiber substrate diameter is 30 μm on the laser-on state. While the cathodes on fibers with diameters of 40 and 140 μm, the work func-

**Figure 13.** Laser-induced field emission measurement results. (a) The emission current density versus applied field voltage (*J*-*V*) curves of cathode of CNT arrays on optic fiber with the diameter of 30 μm on laser-on and laser-off cases.

Inset is the corresponding F-N plot. (b) Fiber diameter is 40 μm. (c) Fiber diameter is 140 μm.

and *β* are deduced

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 63

**Δ***ϕ*. According to the F-N plot and Eq.(1), the value of the effective*ϕ*, **Δ***ϕ*,

**Figure 12.** Schematic of laser-induced field emission measurement set-up.

tion reduce to 4.28 and 4.76 eV, respectively.

The cathode of laser-induced field emission is the CNT arrays emitters, which cover on the fiber substrate, as reported in **Figure 6**. **Figure 12** shows the schematic diagram of laserinduced field emission measurement set-up for the cathode of CNT arrays on an optical fiber.

The field emission properties of the cathode are measured in two cases: with laser-on and laser-off. **Figure 13(a)** shows the measurement results of the CNT arrays cathodes on optical fiber with the diameter of 30 μm. The turn-on voltage (a voltage requires an emission current density reaching to 1 mA·cm−2) of laser-on state is 0.89 kV (*V*to-on), and 1.01 kV (*V*to-off) on laseroff state. *V*to-on is much lower than *V*to-off. Besides, emission current on laser-on state is enhanced comparing with emission current on laser-off state. At 1 kV bias voltage, the emission current density on laser-on state (*J* E-on) is 3.78 mA·cm−2 while 1.10 mA·cm−2 on laser-off state (*J* E-off). The field emission properties on laser-on state are better than that of properties on laser-off state, as shown in **Figure 13(b)** and **(c)**. **Figure 13(b)** shows the emission current of CNT arrays on fiber diameter of 40 μm on laser-on and laser-off state. **Figure 13(c)** is the field emission results on fiber diameter of 140 μm.

As mentioned above, *β* and *ϕ* of the cathode are two critical factors for the field emission properties. *β* strongly depends on the cathode morphology. For a given optical fiber substrate, *β* keeps constant whenever CNT is on laser-on or laser-off states. Thus, the enhanced emission property is attributed to effective *ϕ* decreased by photon absorption. As shown in

**Figure 12.** Schematic of laser-induced field emission measurement set-up.

tips on fiber substrates, a circular curving surface with curvature radius 200 and 20 μm, as shown in **Figure 11(a)** and **(b)**, respectively. With the curvature radius of substrate reduce, the spacing and the density of the equipotential lines on tips surface increase. This result demonstrates the decreased screening effect of the CNT arrays that lies on a smaller diam-

Laser-induced field emission could offer opportunities for dramatic improvements in performance of electronic devices due to the possibility of manipulation and control of coherent electron motion in ultrafast spatiotemporal scales [8, 21]. As one of the most important cold cathode materials, CNTs have been widely studied. Synthesis of well-aligned CNTs on substrates of various geometries is also of special interest to the exploration of laser-induced field emission. Here, CNT arrays on fiber substrate serve as an emitter to explore the mechanism

From Eq.(1), we know that enhancing *β* or decreasing *ϕ* can improve field emission properties. As for a given material for cathode, *ϕ* is a constant. Generally, enhancing *β* by changing the tip morphology of the emitters is one of the main strategies. Herein, the improvement of field emission can be attributed to decreasing *ϕ* of CNT arrays because of the photon absorption. In this part, it will take advantage of laser leakage from an optical fiber to illuminate the cathode

The cathode of laser-induced field emission is the CNT arrays emitters, which cover on the fiber substrate, as reported in **Figure 6**. **Figure 12** shows the schematic diagram of laserinduced field emission measurement set-up for the cathode of CNT arrays on an optical fiber.

The field emission properties of the cathode are measured in two cases: with laser-on and laser-off. **Figure 13(a)** shows the measurement results of the CNT arrays cathodes on optical fiber with the diameter of 30 μm. The turn-on voltage (a voltage requires an emission current density reaching to 1 mA·cm−2) of laser-on state is 0.89 kV (*V*to-on), and 1.01 kV (*V*to-off) on laseroff state. *V*to-on is much lower than *V*to-off. Besides, emission current on laser-on state is enhanced comparing with emission current on laser-off state. At 1 kV bias voltage, the emission current

field emission properties on laser-on state are better than that of properties on laser-off state, as shown in **Figure 13(b)** and **(c)**. **Figure 13(b)** shows the emission current of CNT arrays on fiber diameter of 40 μm on laser-on and laser-off state. **Figure 13(c)** is the field emission results

As mentioned above, *β* and *ϕ* of the cathode are two critical factors for the field emission properties. *β* strongly depends on the cathode morphology. For a given optical fiber substrate, *β* keeps constant whenever CNT is on laser-on or laser-off states. Thus, the enhanced emission property is attributed to effective *ϕ* decreased by photon absorption. As shown in

E-on) is 3.78 mA·cm−2 while 1.10 mA·cm−2 on laser-off state (*J*

E-off). The

eter fiber substrate.

62 Carbon Nanotubes - Recent Progress

**4. Laser-induced field emission**

of laser-induced field emission.

density on laser-on state (*J*

on fiber diameter of 140 μm.

for realizing laser-induced field emission.

**Figure 13(a)**, when the fiber diameter fixed at 30 μm, *β* of the CNT arrays cathode is a constant of 4.89 eV. However, the turn-on voltage and emission current density are modulated by introducing laser into the optical fiber. The enhanced property is evaluated by measuring **Δ***ϕ*. According to the F-N plot and Eq.(1), the value of the effective*ϕ*, **Δ***ϕ*, and *β* are deduced and listed in **Table 2**. *β* of the cathode on the optical fiber with diameter of 30 μm is 4672, while that of the cathode on the fiber with diameter of 40 and 140 μm is 2804 and 2526. The different *β* is attributed to the different curvature of the cathode on fibers with different diameters, agreeing with the above discussion. What we are concerned here is the change of the work function **Δ***ϕ*of the cathode causing by the laser field. From **Table 2**, the work function significantly reduces to 4.14 eV when the fiber substrate diameter is 30 μm on the laser-on state. While the cathodes on fibers with diameters of 40 and 140 μm, the work function reduce to 4.28 and 4.76 eV, respectively.

**Figure 13.** Laser-induced field emission measurement results. (a) The emission current density versus applied field voltage (*J*-*V*) curves of cathode of CNT arrays on optic fiber with the diameter of 30 μm on laser-on and laser-off cases. Inset is the corresponding F-N plot. (b) Fiber diameter is 40 μm. (c) Fiber diameter is 140 μm.


pulsed emission current from CNT cathodes, when the pulse width (Pulse-on denotes the pulse duration expressing by T1) and the trough width (Pulse-off denotes time interval of two pulses expressing by T2) are both 10,000 ms. Importantly, the emission electronic pulses is almost synchronous with laser pulses with the electronic pulse duration of 10,000 ms or so, proving that electron emissions are intensively depended on the condition of laser pulse-on

tion on applied voltage of 600 V. The results demonstrate the realization of emission pulses from CNT cathodes. Besides, the field emission current density is about 0.0023 mA/cm<sup>2</sup>

laser pulse-off, which is much less than the emission current on laser pulse-on. It can be said

**Figure 15** shows emission pulses of pulsed field emission from CNT emitters illuminated by laser pulses, on the applied voltage of 100, 300, 500, 700, 900, and 1000 V with the same laser work condition. Increasing applied voltage, the peak value of emission pulses markedly increases on low applied voltage, as shown in **Figure 15(a)**–**(c)**. Although the peak value of emission pulses increase with the continually increasing applied voltage, the emission pulses trend to decrease on high applied voltage, as shown in **Figure 15(d)**–**(f)**. **Table 3** demonstrates values of the ratio of ɳ = Ion/Ioff on different applied voltage. Ion is the average peak value of emission pulses density on pulse-on, and Ioff is the average trough value of emission current density on pulse-off. The emission pulses are visible on low applied voltages, but the pulses

**Figure 15.** Electronic pulses of pulsed field emission from CNT cathodes illuminated by the same pulsed laser, but on the different applied voltage. Laser pulse-on (T1) and laser pulse-off (T2) are both 10000 ms. (a) on applied voltage of

with a little fluctua-

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528

on

65

(T1). The average peak value of emission pulses density is 0.0068 mA/cm<sup>2</sup>

cannot be distinguished on the high voltage of 1000 V (ɳ ≈ 1, that is Ion ≈ Ioff).

the emission current can be enhanced by laser field.

500 V, (b) 600 V, (c) 700 V, and (d) 800 V.

**Table 2.** The effective work function *ϕ*, Δ*ϕ*=|*ϕ*laser-on\_*ϕ*laser-off|, and the field enhancement factor β of CNT arrays on fibers substrate with the diameter of 30, 40, and 140 μm on both laser on and off state.

The key factor of influencing *ϕ* is the laser field. On the laser-on state, the leakage of laser from the fiber surface is absorbed by CNT cathodes, when laser propagate through the fiber substrate. The electrons are excited to high-energy states and then emit to vacuum on applied voltages. So, the effective work function *ϕ* of CNT cathodes should subtract **Δ***ϕ* (arise from absorbing photons) from 4.89 eV for laser-induced field emission. These results open a new way to improve field emission properties by the work function of cathodes and provide a preparation for realizing pulsed field emission.

#### **5. Pulsed field emission**

Illuminating a sharp cathode tip with laser pulses leads to pulsed field emission. A number of studies clarified the intriguing characteristics of the electron emission processes. This part explores pulsed field emission from CNT cathodes.

As mentioned above, the field emission property was modulated by optical field using a continuous wave laser (CW laser) in the laser-induced field emission. Electrons pump up a high-energy state by laser illumination and then tunnel through barrier to emit into vacuum. The emission current is closely related to CW laser field. So, the CW laser is replaced by pulsed laser to desire a discontinuous modulated signal of pulsed emission current. Herein, the pulsed field emission is based on laser-induced field emission. Illuminating CNT arrays with infrared laser pulses leads to pulsed field emission. **Figure 14(a)** shows the experimental set-up of pulsed field emission from CNT cathodes on Si substrate. **Figure 14(b)** shows the

**Figure 14.** (a) Experimental set-up of pulsed field emission. (b) Electronic pulses from CNT cathodes illuminated by pulsed laser, on applied voltage of 600 V. Laser pulse-on and laser pulse-off are both 10000 ms.

pulsed emission current from CNT cathodes, when the pulse width (Pulse-on denotes the pulse duration expressing by T1) and the trough width (Pulse-off denotes time interval of two pulses expressing by T2) are both 10,000 ms. Importantly, the emission electronic pulses is almost synchronous with laser pulses with the electronic pulse duration of 10,000 ms or so, proving that electron emissions are intensively depended on the condition of laser pulse-on (T1). The average peak value of emission pulses density is 0.0068 mA/cm<sup>2</sup> with a little fluctuation on applied voltage of 600 V. The results demonstrate the realization of emission pulses from CNT cathodes. Besides, the field emission current density is about 0.0023 mA/cm<sup>2</sup> on laser pulse-off, which is much less than the emission current on laser pulse-on. It can be said the emission current can be enhanced by laser field.

The key factor of influencing *ϕ* is the laser field. On the laser-on state, the leakage of laser from the fiber surface is absorbed by CNT cathodes, when laser propagate through the fiber substrate. The electrons are excited to high-energy states and then emit to vacuum on applied voltages. So, the effective work function *ϕ* of CNT cathodes should subtract **Δ***ϕ* (arise from absorbing photons) from 4.89 eV for laser-induced field emission. These results open a new way to improve field emission properties by the work function of cathodes and provide a

**Table 2.** The effective work function *ϕ*, Δ*ϕ*=|*ϕ*laser-on\_*ϕ*laser-off|, and the field enhancement factor β of CNT arrays on fibers

**Diameter 30 μm 40 μm 140 μm** *ϕ*laser-on 4.14 eV 4.28 eV 4.76 eV **Δ***ϕ* 0.74 0.49 0.12 **β** 4672 2804 2526

substrate with the diameter of 30, 40, and 140 μm on both laser on and off state.

Illuminating a sharp cathode tip with laser pulses leads to pulsed field emission. A number of studies clarified the intriguing characteristics of the electron emission processes. This part

As mentioned above, the field emission property was modulated by optical field using a continuous wave laser (CW laser) in the laser-induced field emission. Electrons pump up a high-energy state by laser illumination and then tunnel through barrier to emit into vacuum. The emission current is closely related to CW laser field. So, the CW laser is replaced by pulsed laser to desire a discontinuous modulated signal of pulsed emission current. Herein, the pulsed field emission is based on laser-induced field emission. Illuminating CNT arrays with infrared laser pulses leads to pulsed field emission. **Figure 14(a)** shows the experimental set-up of pulsed field emission from CNT cathodes on Si substrate. **Figure 14(b)** shows the

**Figure 14.** (a) Experimental set-up of pulsed field emission. (b) Electronic pulses from CNT cathodes illuminated by

pulsed laser, on applied voltage of 600 V. Laser pulse-on and laser pulse-off are both 10000 ms.

preparation for realizing pulsed field emission.

explores pulsed field emission from CNT cathodes.

**5. Pulsed field emission**

64 Carbon Nanotubes - Recent Progress

**Figure 15** shows emission pulses of pulsed field emission from CNT emitters illuminated by laser pulses, on the applied voltage of 100, 300, 500, 700, 900, and 1000 V with the same laser work condition. Increasing applied voltage, the peak value of emission pulses markedly increases on low applied voltage, as shown in **Figure 15(a)**–**(c)**. Although the peak value of emission pulses increase with the continually increasing applied voltage, the emission pulses trend to decrease on high applied voltage, as shown in **Figure 15(d)**–**(f)**. **Table 3** demonstrates values of the ratio of ɳ = Ion/Ioff on different applied voltage. Ion is the average peak value of emission pulses density on pulse-on, and Ioff is the average trough value of emission current density on pulse-off. The emission pulses are visible on low applied voltages, but the pulses cannot be distinguished on the high voltage of 1000 V (ɳ ≈ 1, that is Ion ≈ Ioff).

**Figure 15.** Electronic pulses of pulsed field emission from CNT cathodes illuminated by the same pulsed laser, but on the different applied voltage. Laser pulse-on (T1) and laser pulse-off (T2) are both 10000 ms. (a) on applied voltage of 500 V, (b) 600 V, (c) 700 V, and (d) 800 V.


Ion is the average peak value of emission electronic pulses density on pulse-on, and Ioff is the average trough value of emission current density on electron pulse-off.

**Table 3.** The ratio values of ɳ = Ion/Ioff on different applied voltage of 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 V.

the high frequency and short pulse because of the thermal effect, as shown in **Figure 17(b)**. Nevertheless, there is an advantage of enhancing emission current for thermal effect in the pulsed field emission. As shown in **Figure 17**, the peak value of the electronic pulses is quite different on the same work condition but different T1. For **Figure 17(b)**, the emission current

**Figure 17.** Electronic pulses with width from CNT arrays cathode on applied voltage of 500 V. (a) T1 = 50 ms, T2 = 1000 ms.

**Figure 17(a)**. With the T1 increase, the emission current is enhanced resulting from thermal

These results show that thermal effect enhances the pulsed field emission of CNT cathodes illuminating by pulsed laser, but emitting short electronic pulses are strongly influenced by thermal effect resulting in pulse broadening. So, the optimum work condition of laser pulse is T2 ≥ T1 for pulsed field emission, especially in realizing ultrashort electronic pulses. It provides

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61,172,040) and Science and Technology Program of Zhejiang Province, China

2 Department of Microelectronics, School of Electronics and Information Engineering, Xi'an

3 Research Institute of Xi'an Jiaotong University, Hangzhou City, Zhejiang Province, China

1 Science School, HuaiHai Institute of Technology, Lianyungang, Jiangsu, China

comparing with

Carbon Nanotubes as a Pulsed Electron Sources http://dx.doi.org/10.5772/intechopen.70528 67

is compelled drop down before reach to the peak value of 0.006 mA/cm<sup>2</sup>

effect, which is beneficial for the pulsed field emission.

a preparation for us studying ultrafast electron source.

\*Address all correspondence to: wei.wxq@163.com

Jiaotong University, Xi'an, Shaanxi, China

**Acknowledgements**

(b) T1 = 400 ms, T2 = 1000 ms.

(Grant No. 2017C31087).

**Author details**

Xianqi Wei1,2,3\*

**Figure 16.** Electronic pulses with width of 50 ms from CNT arrays cathode on applied voltage of 450 V. (a) T1 = 50 ms, T2 = 1000 ms. (b) the detailed information of an electronic pulse.

In fact, emission mechanism is dominated by the electric field on high voltages, while laser field does not play an important role in the emission process. As the applied voltage continuously increases, emission electrons induced by laser pulses can be ignored due to it is too weak comparing with emission current of the field emission, just leading to a fluctuation for emission current. So, the pulsed field emission happens on the low applied voltage, and is greatly depending on the laser pulses. The results explain the emission mechanism and pave a way for realizing pulsed field emission from CNT cathodes.

Shortening the pulse cycle is a conventional method in order to increase the frequency of emission pulses for pulsed field emission. We try to decrease the pulse width to study short electronic pulses from pulsed field emission of CNT cathodes. **Figure 16** shows emission pulses of 50 ms from CNT emitters, on the laser work condition of T1 = 50 ms and T2 = 1000 ms. The results show the width and the time pulse-off of emission pulse is 50 and 1000 ms, synchronously with the laser pulse train. **Figure 16(b)** is the detailed information about an electronic pulse of 50 ms.

However, there is an inevitable problem of thermal effect existing in the pulsed field emission. Laser illumination makes CNT emitters heat up and causes a thermal effect during the emission process. For the further information of the thermal effect, we detect the response time of electronic pulses by oscilloscope with sampling rate of 2.5 GS/s. The results show that the electron emission is not instantaneous with the laser field. The rise and fall time spend more than 2 s on laser pulse-on and pulse-off, as shown in **Figure 17**. Generally, the thermal effect will lead to a response time, but the field emission is instantaneous. For the pulsed field emission, the response time and pulse broadening are the weaknesses causing by thermal effect. Especially, in the short emission electronic pulses, pulse broadening limits the possibility of

**Figure 17.** Electronic pulses with width from CNT arrays cathode on applied voltage of 500 V. (a) T1 = 50 ms, T2 = 1000 ms. (b) T1 = 400 ms, T2 = 1000 ms.

the high frequency and short pulse because of the thermal effect, as shown in **Figure 17(b)**. Nevertheless, there is an advantage of enhancing emission current for thermal effect in the pulsed field emission. As shown in **Figure 17**, the peak value of the electronic pulses is quite different on the same work condition but different T1. For **Figure 17(b)**, the emission current is compelled drop down before reach to the peak value of 0.006 mA/cm<sup>2</sup> comparing with **Figure 17(a)**. With the T1 increase, the emission current is enhanced resulting from thermal effect, which is beneficial for the pulsed field emission.

These results show that thermal effect enhances the pulsed field emission of CNT cathodes illuminating by pulsed laser, but emitting short electronic pulses are strongly influenced by thermal effect resulting in pulse broadening. So, the optimum work condition of laser pulse is T2 ≥ T1 for pulsed field emission, especially in realizing ultrashort electronic pulses. It provides a preparation for us studying ultrafast electron source.

#### **Acknowledgements**

In fact, emission mechanism is dominated by the electric field on high voltages, while laser field does not play an important role in the emission process. As the applied voltage continuously increases, emission electrons induced by laser pulses can be ignored due to it is too weak comparing with emission current of the field emission, just leading to a fluctuation for emission current. So, the pulsed field emission happens on the low applied voltage, and is greatly depending on the laser pulses. The results explain the emission mechanism and pave

**Figure 16.** Electronic pulses with width of 50 ms from CNT arrays cathode on applied voltage of 450 V. (a) T1 = 50 ms,

**Voltage (V) 100 200 300 400 500 600 700 800 900 1000**

) Ion/Ioff 2.09 2.48 3.53 4.07 5.07 2.09 1.98 1.55 1.39 ≈1

Ion is the average peak value of emission electronic pulses density on pulse-on, and Ioff is the average trough value of

**Table 3.** The ratio values of ɳ = Ion/Ioff on different applied voltage of 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 V.

Shortening the pulse cycle is a conventional method in order to increase the frequency of emission pulses for pulsed field emission. We try to decrease the pulse width to study short electronic pulses from pulsed field emission of CNT cathodes. **Figure 16** shows emission pulses of 50 ms from CNT emitters, on the laser work condition of T1 = 50 ms and T2 = 1000 ms. The results show the width and the time pulse-off of emission pulse is 50 and 1000 ms, synchronously with the laser pulse train. **Figure 16(b)** is the detailed information about an electronic pulse of 50 ms. However, there is an inevitable problem of thermal effect existing in the pulsed field emission. Laser illumination makes CNT emitters heat up and causes a thermal effect during the emission process. For the further information of the thermal effect, we detect the response time of electronic pulses by oscilloscope with sampling rate of 2.5 GS/s. The results show that the electron emission is not instantaneous with the laser field. The rise and fall time spend more than 2 s on laser pulse-on and pulse-off, as shown in **Figure 17**. Generally, the thermal effect will lead to a response time, but the field emission is instantaneous. For the pulsed field emission, the response time and pulse broadening are the weaknesses causing by thermal effect. Especially, in the short emission electronic pulses, pulse broadening limits the possibility of

a way for realizing pulsed field emission from CNT cathodes.

T2 = 1000 ms. (b) the detailed information of an electronic pulse.

Ratio (mA/cm<sup>2</sup>

66 Carbon Nanotubes - Recent Progress

emission current density on electron pulse-off.

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61,172,040) and Science and Technology Program of Zhejiang Province, China (Grant No. 2017C31087).

#### **Author details**

Xianqi Wei1,2,3\*

\*Address all correspondence to: wei.wxq@163.com

1 Science School, HuaiHai Institute of Technology, Lianyungang, Jiangsu, China

2 Department of Microelectronics, School of Electronics and Information Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, China

3 Research Institute of Xi'an Jiaotong University, Hangzhou City, Zhejiang Province, China

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**Chapter 5**

**Provisional chapter**

**Electric Field Induced Alignment of Carbon Nanotubes:**

In the current chapter, achievement of aligned carbon nanotube (CNT) network within the matrix via various kinds of electric fields (AC and DC) was evaluated. In this case, alignment mechanism of CNTs within the matrix and two useful techniques for justification of CNT alignment throughout the matrix were examined and presented, respectively. Afterward, effective factors in matter of CNT alignment and applicable procedures for fabrication of nanocomposites containing aligned CNTs were studied and presented, respectively. At the end, significant effects of CNT alignment on overall properties of nanocomposites that include electrical and mechanical properties were evaluated. Achieved results revealed that alignment of CNTs within the matrix can lead to significant improvement in the electrical and mechanical properties of nanocomposites at the same filler loading compared with randomly distribution of CNTs within the matrix,

while production steps and conditions can also highly affect the outcome data.

**Keywords:** carbon nanotube, nanocomposite, electric field, alignment, electrophoresis

Carbon nanotube (CNT) is a unique material with fantastic electrical, thermal, and mechanical properties, and since its discovery by Iijima in 1991 [1], global attention attracted toward it, and many researchers have evaluated the extraordinary properties of CNTs toward development of nanocomposites and sheets holding highly oriented CNTs with enhanced electrical

**Electric Field Induced Alignment of Carbon Nanotubes:** 

DOI: 10.5772/intechopen.70481

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Methodology and Outcomes**

**Methodology and Outcomes**

Seyyed Mojtaba Mousavi,

and Arash Vojood

**Abstract**

**1. Introduction**

and mechanical properties [2–7].

Ali Mohammad Amani, Seyyed Alireza Hashemi,

Ali Mohammad Amani, Seyyed Alireza Hashemi, Seyyed Mojtaba Mousavi, Seyyed Milad Abrishamifar

Seyyed Milad Abrishamifar and Arash Vojood

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70481

**Provisional chapter**

#### **Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes Methodology and Outcomes**

**Electric Field Induced Alignment of Carbon Nanotubes:** 

DOI: 10.5772/intechopen.70481

Ali Mohammad Amani, Seyyed Alireza Hashemi, Seyyed Mojtaba Mousavi, Seyyed Milad Abrishamifar and Arash Vojood Seyyed Mojtaba Mousavi, Seyyed Milad Abrishamifar and Arash Vojood

Ali Mohammad Amani, Seyyed Alireza Hashemi,

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70481

#### **Abstract**

In the current chapter, achievement of aligned carbon nanotube (CNT) network within the matrix via various kinds of electric fields (AC and DC) was evaluated. In this case, alignment mechanism of CNTs within the matrix and two useful techniques for justification of CNT alignment throughout the matrix were examined and presented, respectively. Afterward, effective factors in matter of CNT alignment and applicable procedures for fabrication of nanocomposites containing aligned CNTs were studied and presented, respectively. At the end, significant effects of CNT alignment on overall properties of nanocomposites that include electrical and mechanical properties were evaluated. Achieved results revealed that alignment of CNTs within the matrix can lead to significant improvement in the electrical and mechanical properties of nanocomposites at the same filler loading compared with randomly distribution of CNTs within the matrix, while production steps and conditions can also highly affect the outcome data.

**Keywords:** carbon nanotube, nanocomposite, electric field, alignment, electrophoresis

#### **1. Introduction**

Carbon nanotube (CNT) is a unique material with fantastic electrical, thermal, and mechanical properties, and since its discovery by Iijima in 1991 [1], global attention attracted toward it, and many researchers have evaluated the extraordinary properties of CNTs toward development of nanocomposites and sheets holding highly oriented CNTs with enhanced electrical and mechanical properties [2–7].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Alignment of CNTs within the matrix can boost their properties in various fields include electrical and mechanical properties [2–7]. What is more, alignment of CNTs can be achieved via various methods such as electrical field [2–4, 7, 8], magnetic field [9], shear flow [10, 11], mechanical force [12, 13], and electrospinning [14, 15], which electrical field– induced alignment is the simplest and most manageable method for alignment of CNTs within the matrix.

Besides, exert of electrical field to a matrix containing CNTs can lead to expansion of a highly oriented network from the negative electrode toward the positive electrode, which acting as a pathway for transferring current from the negative electrode toward the positive electrode. In this case, interruption of applied current can lead to collapse of developed network which known as relaxation mechanism [16].

In this chapter, first the alignment mechanism of CNTs within the matrix was evaluated, and thence, useful techniques for justification of CNT alignment in the content of matrix were presented. Afterward, effective techniques for fabrication of nanocomposites containing highly and randomly oriented CNTs were discussed, and effect of alignment on various properties of nanocomposites was examined.

**3. Justification of CNT alignment**

to derive from the stretching of sp2

(G<sup>∥</sup>

direction are denoted as D<sup>∥</sup>

There are several effective techniques for evaluation of CNT alignment within the matrix. In this case, the most common technique is via micro Raman spectroscopy. Raman spectrum of CNT/polymer nanocomposites features two characteristic peaks, namely D-band and G-band. The band at 1350 cm−1 is known as D-band and corresponds to overall amount of defects in carbon atom bonds in curved graphene sheets. Therefore, the structural defects including heteroatoms, vacancies, impurities, as well as pentagon-heptagon pairs in helical structures of CNTs can activate the D-band feature. The other strong band observed at 1590 cm−1, called G-band, corresponds to the graphitization/crystallization degree of CNTs. G-band is believed

**Figure 1.** Alignment behavior of CNTs under application of electric field, (1) rotation, (2) head-to-head contact,

Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes

http://dx.doi.org/10.5772/intechopen.70481

73

(3) migration toward the negative electrode and (4) formation of aligned network between electrodes.

phitic CNTs [19–21]. Alignment of CNTs within the polymer matrix has significant effect on both D-band and G-band peaks [22–24]; this introduces Raman spectroscopy as a comple-

For this purpose, D (G) band intensities obtained parallel and perpendicular to CNT alignment

ues corresponds to improvement of CNT alignment within the matrix, due to higher surface area of aligned CNT projected to applied laser light in parallel direction than perpendicular direction. Abbasi et al. [19] and Arjmand et al. [20, 25, 26] used this technique in order to examine the

) and D⊥ (G⊥), respectively. Enhancement of D<sup>∥</sup>

mentary technique to verify the alignment of SWCNTs within the polymer matrix.


/D⊥ and G<sup>∥</sup>

/G⊥ val-

### **2. Alignment mechanism of CNTs within the matrix**

Alignment mechanism of CNTs consisted from four stages [16]. In the first stage, due to the application of electric field, a dipole moment induced at the edges of CNTs resulted in their rotation to a certain angle and thence aligned in the direction of electric field. In the second stage, polarized CNTs attract each other, and head-to-head contact occurs, forming an aggregating in a chain-like structure. In the third stage, CNTs migrate toward and attach to the negative electrode, respectively. When CNTs are close enough to the electrode in order to transfer the charge, they discharge and aggregate onto the electrodes. Attached CNTs to the electrode become sources of high field strength and primary locations for absorption of other CNTs. In the fourth stage, following the connection of the first CNT bundle to the negative electrode, other CNT bundles attached the first bundle, and thence aligned CNT network spans negative-positive electrode spacing. In fact, CNTs form a pathway for transferring the current from the negative electrode toward the positive electrode and if the electric field interrupted during the alignment process, highly oriented network breaks down and CNTs return to their primary locations, this phenomenon is known as relaxation mechanism [16, 17]. Likewise, the translational motion of CNTs toward the negative electrode, justifying the presence of a negative surface charge, is governed by the electrophoretic mobility of charged CNTs. In case of relaxation mechanism, the Brownain diffusion is relevant to the motion and tends to fade of the aligned network, till it returns to the primary randomly distribution situation [18]. In **Figure 1**, alignment process of CNTs within matrix can be seen.

**Figure 1.** Alignment behavior of CNTs under application of electric field, (1) rotation, (2) head-to-head contact, (3) migration toward the negative electrode and (4) formation of aligned network between electrodes.

#### **3. Justification of CNT alignment**

Alignment of CNTs within the matrix can boost their properties in various fields include electrical and mechanical properties [2–7]. What is more, alignment of CNTs can be achieved via various methods such as electrical field [2–4, 7, 8], magnetic field [9], shear flow [10, 11], mechanical force [12, 13], and electrospinning [14, 15], which electrical field– induced alignment is the simplest and most manageable method for alignment of CNTs

Besides, exert of electrical field to a matrix containing CNTs can lead to expansion of a highly oriented network from the negative electrode toward the positive electrode, which acting as a pathway for transferring current from the negative electrode toward the positive electrode. In this case, interruption of applied current can lead to collapse of developed network which

In this chapter, first the alignment mechanism of CNTs within the matrix was evaluated, and thence, useful techniques for justification of CNT alignment in the content of matrix were presented. Afterward, effective techniques for fabrication of nanocomposites containing highly and randomly oriented CNTs were discussed, and effect of alignment on various properties

Alignment mechanism of CNTs consisted from four stages [16]. In the first stage, due to the application of electric field, a dipole moment induced at the edges of CNTs resulted in their rotation to a certain angle and thence aligned in the direction of electric field. In the second stage, polarized CNTs attract each other, and head-to-head contact occurs, forming an aggregating in a chain-like structure. In the third stage, CNTs migrate toward and attach to the negative electrode, respectively. When CNTs are close enough to the electrode in order to transfer the charge, they discharge and aggregate onto the electrodes. Attached CNTs to the electrode become sources of high field strength and primary locations for absorption of other CNTs. In the fourth stage, following the connection of the first CNT bundle to the negative electrode, other CNT bundles attached the first bundle, and thence aligned CNT network spans negative-positive electrode spacing. In fact, CNTs form a pathway for transferring the current from the negative electrode toward the positive electrode and if the electric field interrupted during the alignment process, highly oriented network breaks down and CNTs return to their primary locations, this phenomenon is known as relaxation mechanism [16, 17]. Likewise, the translational motion of CNTs toward the negative electrode, justifying the presence of a negative surface charge, is governed by the electrophoretic mobility of charged CNTs. In case of relaxation mechanism, the Brownain diffusion is relevant to the motion and tends to fade of the aligned network, till it returns to the primary randomly distribution situation [18]. In **Figure 1**, alignment

within the matrix.

72 Carbon Nanotubes - Recent Progress

known as relaxation mechanism [16].

of nanocomposites was examined.

process of CNTs within matrix can be seen.

**2. Alignment mechanism of CNTs within the matrix**

There are several effective techniques for evaluation of CNT alignment within the matrix. In this case, the most common technique is via micro Raman spectroscopy. Raman spectrum of CNT/polymer nanocomposites features two characteristic peaks, namely D-band and G-band. The band at 1350 cm−1 is known as D-band and corresponds to overall amount of defects in carbon atom bonds in curved graphene sheets. Therefore, the structural defects including heteroatoms, vacancies, impurities, as well as pentagon-heptagon pairs in helical structures of CNTs can activate the D-band feature. The other strong band observed at 1590 cm−1, called G-band, corresponds to the graphitization/crystallization degree of CNTs. G-band is believed to derive from the stretching of sp2 -hybridized carbon atom bonds along the surface of graphitic CNTs [19–21]. Alignment of CNTs within the polymer matrix has significant effect on both D-band and G-band peaks [22–24]; this introduces Raman spectroscopy as a complementary technique to verify the alignment of SWCNTs within the polymer matrix.

For this purpose, D (G) band intensities obtained parallel and perpendicular to CNT alignment direction are denoted as D<sup>∥</sup> (G<sup>∥</sup> ) and D⊥ (G⊥), respectively. Enhancement of D<sup>∥</sup> /D⊥ and G<sup>∥</sup> /G⊥ values corresponds to improvement of CNT alignment within the matrix, due to higher surface area of aligned CNT projected to applied laser light in parallel direction than perpendicular direction. Abbasi et al. [19] and Arjmand et al. [20, 25, 26] used this technique in order to examine the alignment degree for compression-molded and injection-molded multi-walled carbon nanotube (MWCNT)/polymer nanocomposites. Their obtained results revealed that alignment of MWCNT via injection molding method had higher D<sup>∥</sup> /D⊥ and G<sup>∥</sup> /G⊥ values compared to the compressionmolded samples, which contain randomly distributed CNTs. They also found out that D-band is more sensitive to MWCNT alignment than G-band; thus, higher values of D<sup>∥</sup> /D⊥ than G<sup>∥</sup> /G⊥ are expected for the aligned nanocomposites compared to unaligned nanocomposites.

The higher the ratio of G<sup>∥</sup> /G⊥ the better the quality of aligned CNT network [27]. In a study by Chapkin et al. [28], they have developed a technique, which has utilized polarized Raman spectroscopy for evaluation of CNT alignment within the polymer matrix under applied electric field. In this case, the effect of electric field strength between range 200 and 1100 V/cm on the degree of alignment and required time for achieving highly oriented CNT network was examined. The Raman scattering intensity is proportional to falling CNTs within the excitation volume of the laser. Increase in the concentration of CNTs within the matrix not only can increase the likelihood of multiple scattering events but also can increase the opacity of the mixture, which can lead to decrease in the penetration depth of the laser. Comparison between intensities of Raman spectra provided by orthogonally polarized incident light in nanocomposites containing wellaligned CNTs is an evaluation method for alignment characterization. Electrostatically aligned network of CNTs can lead to increase in the G-band intensity for parallel polarization direction and decrease for perpendicular direction in comparison to unaligned network. The G<sup>∥</sup> /G<sup>⊥</sup> ratio indicating the degree of CNT alignment, which increases in this ratio, is the sign of better alignment within the matrix. In an unaligned specimen, this ratio is approximately 1. By applying the electric field between electrodes and alignment of CNTs between them, the intensities related to Raman peaks in the CNT spectrum begin to rise. In this case, the intensity of G-band was increased for the parallel polarization and decreased for the perpendicular polarization than unaligned specimens. Besides, by interruption of electric field, a considerable drop in the normalized G-band intensity was observed which is known as relaxation mechanism [16]. Decrease in the G-band intensity due to interruption of electric field indicating the loss of CNT alignment and collapse of aligned network. Furthermore, viscosity of matrix can highly affect the alignment behavior of CNTs. In this regard, a rapid loss in CNT alignment on the order of tens of milliseconds observed for low viscosity systems by interruption of the electric field [29, 30]. However, it has been observed that the alignment degree of CNTs in a matrix with high viscosity is higher than in a matrix with lower viscosity [31]. Increase in the electric field strength can either improve the G<sup>∥</sup> /G⊥ ratio or degree of alignment within the matrix. On the other hand, the polarization of a CNT highly depends on its conductivity [32, 33] and aspect ratio [32]. Conductivity of a CNT is based on its charity and behavior, which could be metallic or semiconducting. It has been proved that metallic CNTs presenting a greater degree of alignment via applied electric field than semiconducting CNTs [28]. Khan et al. [3] and Chen et al [4] have used micro Raman spectroscopy for determining the degree of CNT alignment via applied DC and AC electric fields, respectively.

path for transferring the current toward the positive electrode under electrophoresis, which verifying the presence of negative surface charge on CNT surfaces [17]. In **Figure 2**, a view of

**Figure 2.** (a–c) Justification of CNT alignment via micro Raman spectroscopy [3, 19, 20], (d) Justification of CNT

Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes

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75

Many effective factors involved in case of CNT alignment, which evaluation of their effects on aligned network is very essential. Homogenous dispersion of pristine CNT in the matrix is difficult to achieve, which is due to the poor interfacial interaction between CNTs and matrix [34, 35]. What is more, functionalization of CNTs can improve their dispersion within the matrix and enhance the align network. The alignment of CNTs within the matrix has significant effect on overall properties of developed nanocomposites, which are mainly dependent on the degree of CNT dispersion, type of matrix, and interfacial bonds between the matrix and fillers [3]. On the other hand, functionalization of CNTs can improve their dispersion and thus alignment of CNTs within the matrix due to the better interaction with matrix. In fact, functionalization of CNT can change its nature from hydrophobic to hydrophilic [36]. But functionalization of CNTs can increase their resistivity and examination of nanocomposites containing aligned CNTs revealed that highly oriented network consisted form pristine CNTs

various justification methods for CNT alignment can be seen.

**4. Effective factors on CNT alignment**

alignment via constant current on suspension surface [16].

In addition, Monti et al. [16] and Martin et al. [17] have measured the electrical current during the exertion of electric field as an indicator for formation of a highly oriented network in the area between two opposite electrodes. In fact, when highly oriented CNT network completed, the negative surface charge on the surface of the suspension has become constant, which shows highly oriented network has completed. Moreover, this network is a conductive Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes http://dx.doi.org/10.5772/intechopen.70481 75

**Figure 2.** (a–c) Justification of CNT alignment via micro Raman spectroscopy [3, 19, 20], (d) Justification of CNT alignment via constant current on suspension surface [16].

path for transferring the current toward the positive electrode under electrophoresis, which verifying the presence of negative surface charge on CNT surfaces [17]. In **Figure 2**, a view of various justification methods for CNT alignment can be seen.

#### **4. Effective factors on CNT alignment**

alignment degree for compression-molded and injection-molded multi-walled carbon nanotube (MWCNT)/polymer nanocomposites. Their obtained results revealed that alignment of MWCNT

/D⊥ and G<sup>∥</sup>

/G⊥ the better the quality of aligned CNT network [27]. In a study by

molded samples, which contain randomly distributed CNTs. They also found out that D-band is

Chapkin et al. [28], they have developed a technique, which has utilized polarized Raman spectroscopy for evaluation of CNT alignment within the polymer matrix under applied electric field. In this case, the effect of electric field strength between range 200 and 1100 V/cm on the degree of alignment and required time for achieving highly oriented CNT network was examined. The Raman scattering intensity is proportional to falling CNTs within the excitation volume of the laser. Increase in the concentration of CNTs within the matrix not only can increase the likelihood of multiple scattering events but also can increase the opacity of the mixture, which can lead to decrease in the penetration depth of the laser. Comparison between intensities of Raman spectra provided by orthogonally polarized incident light in nanocomposites containing wellaligned CNTs is an evaluation method for alignment characterization. Electrostatically aligned network of CNTs can lead to increase in the G-band intensity for parallel polarization direction and decrease for perpendicular direction in comparison to unaligned network. The G<sup>∥</sup>

ratio indicating the degree of CNT alignment, which increases in this ratio, is the sign of better alignment within the matrix. In an unaligned specimen, this ratio is approximately 1. By applying the electric field between electrodes and alignment of CNTs between them, the intensities related to Raman peaks in the CNT spectrum begin to rise. In this case, the intensity of G-band was increased for the parallel polarization and decreased for the perpendicular polarization than unaligned specimens. Besides, by interruption of electric field, a considerable drop in the normalized G-band intensity was observed which is known as relaxation mechanism [16]. Decrease in the G-band intensity due to interruption of electric field indicating the loss of CNT alignment and collapse of aligned network. Furthermore, viscosity of matrix can highly affect the alignment behavior of CNTs. In this regard, a rapid loss in CNT alignment on the order of tens of milliseconds observed for low viscosity systems by interruption of the electric field [29, 30]. However, it has been observed that the alignment degree of CNTs in a matrix with high viscosity is higher than in a matrix with lower viscosity [31]. Increase in the electric field strength can either improve

/G⊥ ratio or degree of alignment within the matrix. On the other hand, the polarization of a CNT highly depends on its conductivity [32, 33] and aspect ratio [32]. Conductivity of a CNT is based on its charity and behavior, which could be metallic or semiconducting. It has been proved that metallic CNTs presenting a greater degree of alignment via applied electric field than semiconducting CNTs [28]. Khan et al. [3] and Chen et al [4] have used micro Raman spectroscopy for determining the degree of CNT alignment via applied DC and AC electric fields, respectively.

In addition, Monti et al. [16] and Martin et al. [17] have measured the electrical current during the exertion of electric field as an indicator for formation of a highly oriented network in the area between two opposite electrodes. In fact, when highly oriented CNT network completed, the negative surface charge on the surface of the suspension has become constant, which shows highly oriented network has completed. Moreover, this network is a conductive

more sensitive to MWCNT alignment than G-band; thus, higher values of D<sup>∥</sup>

expected for the aligned nanocomposites compared to unaligned nanocomposites.

/G⊥ values compared to the compression-

/D⊥ than G<sup>∥</sup>

/G⊥ are

/G<sup>⊥</sup>

via injection molding method had higher D<sup>∥</sup>

The higher the ratio of G<sup>∥</sup>

74 Carbon Nanotubes - Recent Progress

the G<sup>∥</sup>

Many effective factors involved in case of CNT alignment, which evaluation of their effects on aligned network is very essential. Homogenous dispersion of pristine CNT in the matrix is difficult to achieve, which is due to the poor interfacial interaction between CNTs and matrix [34, 35]. What is more, functionalization of CNTs can improve their dispersion within the matrix and enhance the align network. The alignment of CNTs within the matrix has significant effect on overall properties of developed nanocomposites, which are mainly dependent on the degree of CNT dispersion, type of matrix, and interfacial bonds between the matrix and fillers [3]. On the other hand, functionalization of CNTs can improve their dispersion and thus alignment of CNTs within the matrix due to the better interaction with matrix. In fact, functionalization of CNT can change its nature from hydrophobic to hydrophilic [36]. But functionalization of CNTs can increase their resistivity and examination of nanocomposites containing aligned CNTs revealed that highly oriented network consisted form pristine CNTs

**Figure 3.** Comparison between performance of pristine and functionalized CNTs for formation of aligned network under application of AC electric field [4].

presenting higher electrical conductivity than functionalized CNTs [8]. A view of aligned CNTs networks containing pristine and functionalized CNTs can be seen in **Figure 3**. As can be seen in this figure, pristine CNTs have formed agglomerated bundles, while functionalized CNTs formed approximately homogenous aligned network.

Furthermore, alignment of CNTs within the matrix can be achieved via both AC and DC electric fields, while behavior of CNTs highly depends on the type of employed electric field and surface charge on the surface of suspension [17, 37]. In regard of DC electric field, migration of CNTs toward the negative electrode is governed by their electrophoretic mobility, which highly depends on the surface charge, while the electrophoretic mobility for AC electric field is zero. This specification of AC electric field can lead to creation of homogenous aligned network within the matrix due to their dielectrophoretic behavior. Likewise, usage of AC electric field found to be more effective compared with DC electric field [4, 17]. Moreover, increase in the electric field voltage can enhance the degree of CNT alignment, while for both AC and DC electric fields, increase in the electric field voltage can increase the aggregation rate, which highlights the requirement for optimization of voltage level [3, 8]. It was revealed that increase in the frequency of AC electric field can enhance the alignment degree and increase the transvers connections between aligned CNT bundles in the direction of electric field [8]. On the other hand, AC electric field presented both better dispersion and alignment within the matrix compared with DC electric field [4]. In **Figure 4**, effect of CNT weight percentage and DC electric field exerting time and strength on the quality of final aligned network can be seen. As can be seen in this figure, increase in CNT weight percentage and voltage can lead to increase in agglomeration rate and creation of robe-like CNT bundles which can be seen in **Figure 4(g)**. Moreover, a view of aligned CNT network in fully cured samples can be seen in **Figure 5**.

**Figure 4.** Optical images from aligned CNT networks with respect to their concentration, size and duration which DC

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**Figure 5.** Optical micrographs of suspension containing (a) randomly distributed SWCNTs before the field application, (b) alignment of SWCNTs after 180 s of field application, (c) alignment of SWCNTs after 15 min of field application, (d) a view of relaxation mechanism 10 min after switching the electric field off [16], (e and g) random distribution of MWCNTs

within the matrix and (f and h) alignment of MWCNTs within the matrix via DC electric field [3].

electric field was applied between electrodes, (a–f) 100 V/cm for 30 min and (g) 200 V/cm for 30 min [3].

In addition, applied voltage and viscosity of matrix can highly affect the alignment process. Increase in the voltage and decrease in the viscosity of matrix can decrease the alignment time. However, unnecessary increase in the electric field voltage can improve the aggregation rate of CNTs and lead to creation of rope-like thick CNT bundles. On the other hand, due to high aspect ratio of CNTs, longitudinal polarization overcomes the transverse polarization. It is worth noting that after completion of the longitudinal polarization, CNTs start to attach each other in the transverse direction, forming transverse crosslinking [3, 16]. In **Figure 6**(**a**–**c**), effect of viscosity Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes http://dx.doi.org/10.5772/intechopen.70481 77

presenting higher electrical conductivity than functionalized CNTs [8]. A view of aligned CNTs networks containing pristine and functionalized CNTs can be seen in **Figure 3**. As can be seen in this figure, pristine CNTs have formed agglomerated bundles, while functionalized

**Figure 3.** Comparison between performance of pristine and functionalized CNTs for formation of aligned network

Furthermore, alignment of CNTs within the matrix can be achieved via both AC and DC electric fields, while behavior of CNTs highly depends on the type of employed electric field and surface charge on the surface of suspension [17, 37]. In regard of DC electric field, migration of CNTs toward the negative electrode is governed by their electrophoretic mobility, which highly depends on the surface charge, while the electrophoretic mobility for AC electric field is zero. This specification of AC electric field can lead to creation of homogenous aligned network within the matrix due to their dielectrophoretic behavior. Likewise, usage of AC electric field found to be more effective compared with DC electric field [4, 17]. Moreover, increase in the electric field voltage can enhance the degree of CNT alignment, while for both AC and DC electric fields, increase in the electric field voltage can increase the aggregation rate, which highlights the requirement for optimization of voltage level [3, 8]. It was revealed that increase in the frequency of AC electric field can enhance the alignment degree and increase the transvers connections between aligned CNT bundles in the direction of electric field [8]. On the other hand, AC electric field presented both better dispersion and alignment within the matrix compared with DC electric field [4]. In **Figure 4**, effect of CNT weight percentage and DC electric field exerting time and strength on the quality of final aligned network can be seen. As can be seen in this figure, increase in CNT weight percentage and voltage can lead to increase in agglomeration rate and creation of robe-like CNT bundles which can be seen in **Figure 4(g)**. Moreover, a view of aligned CNT network in fully cured samples can be seen in **Figure 5**.

In addition, applied voltage and viscosity of matrix can highly affect the alignment process. Increase in the voltage and decrease in the viscosity of matrix can decrease the alignment time. However, unnecessary increase in the electric field voltage can improve the aggregation rate of CNTs and lead to creation of rope-like thick CNT bundles. On the other hand, due to high aspect ratio of CNTs, longitudinal polarization overcomes the transverse polarization. It is worth noting that after completion of the longitudinal polarization, CNTs start to attach each other in the transverse direction, forming transverse crosslinking [3, 16]. In **Figure 6**(**a**–**c**), effect of viscosity

CNTs formed approximately homogenous aligned network.

under application of AC electric field [4].

76 Carbon Nanotubes - Recent Progress

**Figure 4.** Optical images from aligned CNT networks with respect to their concentration, size and duration which DC electric field was applied between electrodes, (a–f) 100 V/cm for 30 min and (g) 200 V/cm for 30 min [3].

**Figure 5.** Optical micrographs of suspension containing (a) randomly distributed SWCNTs before the field application, (b) alignment of SWCNTs after 180 s of field application, (c) alignment of SWCNTs after 15 min of field application, (d) a view of relaxation mechanism 10 min after switching the electric field off [16], (e and g) random distribution of MWCNTs within the matrix and (f and h) alignment of MWCNTs within the matrix via DC electric field [3].

can improve their dispersion within the matrix. Thence, MWCNT/poly methyl meth acrylate (PMMA) composites fabricated using in situ polymerization technique. In this regard, 0.1 wt% 2,2-azobisisobutylonitrile (AIBN) was dissolved in liquid methyl methacrylate (MMA) monomer. Then, desirable weight percentage of MWCNTs added to the MMA/AIBN solution and ultrasonicated for 15 min. In the next step, in order to align MWCNTs in the matrix, AC voltage of 300 Vp-p (peak to peak) at 500 Hz was maintained across the copper electrodes. The resulting nanocomposite was allowed to cure under the AC electric field for 2 h at 70°C.

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Zhu et al [7] developed nanocomposites containing aligned MWCNTs within the epoxy resin. In this regard, MWCNTs were synthesized with mean diameter and average length of 20 nm and 10 μm, respectively. Afterward, they have developed two different kinds of MWCNT. For the first MWCNT type, MWCNTs were purified with hydrofluoric acid for 24 h in order to remove impurities and residual catalyst from synthesized MWCNTs. Thence, purified MWCNTs were placed in an oven at 100°C for 24 h, thereby resulting in production of pristine MWCNT. For development of second type of MWCNTs, pristine MWCNTs were

up to 150°C for 30 min. Thereon, MWCNTs were filtered and rinsed continuously with deionized water in order to make it chemically neutral. This procedure can lead to production of functionalized MWCNTs with carboxyl (–COOH) and hydroxyl (–OH) functional groups. the oxidized MWCNTs were thence refluxed in thionyl chloride at 80°C for 1 h, and residual suspension was distilled and added into a mixture of methylene dichloride, 1,2-ethylenediamine, and triethylamine. Resulting suspension was stirred for 1 h at room temperature. Eventually, MWCNTs were filtrated out, rinsed with deionized water till become chemically neutral, and then dried at 100°C for 24 h to gain amine functionalized MWCNT. In case of matrix, they have used an epoxy resin which could be cured via emission of UV beams and while irradiated by 265-nm UV light, it was fast polymerized within several minutes. In the next stage, specific amount of prepared MWCNTs was poured in the epoxy resin and ultrasonicated for 20 min. Then, 2 mL of the resulting suspension poured into the related mold, and thence, an AC voltage of 2000 Vp-p (peak to peak) at 200 Hz was applied to the suspension for approximately 10 min. Then, UV light was emitted to the suspension for 10 min, which resulted in fast polymerize (cure) of the suspension. A view of their fabrication procedure can be seen in

Khan et al. [3] used CVD fabricated MWCNTs for development of nanocomposites containing highly oriented CNTs. For this regard, first MWCNTs were subjected to ultra-violet/ozone

from it. This step can change the nature of CNTs from hydrophobic to hydrophilic due to the creation of oxygen-based functional groups on CNT surface, thereby enhancing the interaction between matrix and CNTs. Functionalized MWCNTs sonicated in acetone and Tritone x-100. Thence, epoxy resin was added to the previous suspension and sonicated for further 1 h. Thereon, CNT/epoxy mixture was placed in the vacuum oven for 2 h in order to evaporate acetone from the suspension. For further degassing, suspension was passed through a threeroller mill for six times. Resulting mixture was then degassed for further 20 min to eliminate trapped air bubbles, and thereupon, curing agent triethylenetetramine was added to the epoxy

) treatment for 30 min used equipment consisted from a low-pressure mercury vapor

(volume ratio 1:1) and resulting suspension heated

to samples placed 20 mm far

added into a solution of H<sup>2</sup>

**Figure 7(a)**.

(UV-O<sup>3</sup>

SO<sup>4</sup>

grid lamp, which could irradiate UV radiation of 28 W/cm<sup>2</sup>

and HNO<sup>3</sup>

**Figure 6.** Effect of electric field strength and viscosity of matrix on the required time for (a) rotation, (b) head to head contact and (c) migration, (d) shows comparison between required time for alignment of CNTs within the matrix and migration of CNTs toward negative electrode [16].

and electric field strength on required times for rotation, formation of chain-like structure, and migration can be seen, respectively. Besides, in **Figure 6(d)**, comparison between total alignment time and required time for CNT migration toward negative electrode can be seen.

#### **5. Methodology**

In order to achieve a nanocomposite containing highly oriented CNT network with homogenous distribution, development of a practical procedure which has considered all of effective factors in matter of CNT alignment and nanocomposites preparation is essential. In this case, Ma et al. [4] synthesized MWCNTs using chemical vapor deposition (CVD) technique and thence functionalized fabricated MWCNTs using acidic method. In this regard, a mixture of H<sup>2</sup> SO<sup>4</sup> and HNO<sup>3</sup> acid (1:1 volume ratio) was prepared, and thereon, MWCNTs were added to the previous solution and heated up to 150°C for 30 min. Then, MWCNTs were rinsed with deionized water till they become chemically neutral. FTIR examination revealed that these MWCNTs have hydroxyl (─OH) and carboxyl (─COOH) functional groups, which can improve their dispersion within the matrix. Thence, MWCNT/poly methyl meth acrylate (PMMA) composites fabricated using in situ polymerization technique. In this regard, 0.1 wt% 2,2-azobisisobutylonitrile (AIBN) was dissolved in liquid methyl methacrylate (MMA) monomer. Then, desirable weight percentage of MWCNTs added to the MMA/AIBN solution and ultrasonicated for 15 min. In the next step, in order to align MWCNTs in the matrix, AC voltage of 300 Vp-p (peak to peak) at 500 Hz was maintained across the copper electrodes. The resulting nanocomposite was allowed to cure under the AC electric field for 2 h at 70°C.

Zhu et al [7] developed nanocomposites containing aligned MWCNTs within the epoxy resin. In this regard, MWCNTs were synthesized with mean diameter and average length of 20 nm and 10 μm, respectively. Afterward, they have developed two different kinds of MWCNT. For the first MWCNT type, MWCNTs were purified with hydrofluoric acid for 24 h in order to remove impurities and residual catalyst from synthesized MWCNTs. Thence, purified MWCNTs were placed in an oven at 100°C for 24 h, thereby resulting in production of pristine MWCNT. For development of second type of MWCNTs, pristine MWCNTs were added into a solution of H<sup>2</sup> SO<sup>4</sup> and HNO<sup>3</sup> (volume ratio 1:1) and resulting suspension heated up to 150°C for 30 min. Thereon, MWCNTs were filtered and rinsed continuously with deionized water in order to make it chemically neutral. This procedure can lead to production of functionalized MWCNTs with carboxyl (–COOH) and hydroxyl (–OH) functional groups. the oxidized MWCNTs were thence refluxed in thionyl chloride at 80°C for 1 h, and residual suspension was distilled and added into a mixture of methylene dichloride, 1,2-ethylenediamine, and triethylamine. Resulting suspension was stirred for 1 h at room temperature. Eventually, MWCNTs were filtrated out, rinsed with deionized water till become chemically neutral, and then dried at 100°C for 24 h to gain amine functionalized MWCNT. In case of matrix, they have used an epoxy resin which could be cured via emission of UV beams and while irradiated by 265-nm UV light, it was fast polymerized within several minutes. In the next stage, specific amount of prepared MWCNTs was poured in the epoxy resin and ultrasonicated for 20 min. Then, 2 mL of the resulting suspension poured into the related mold, and thence, an AC voltage of 2000 Vp-p (peak to peak) at 200 Hz was applied to the suspension for approximately 10 min. Then, UV light was emitted to the suspension for 10 min, which resulted in fast polymerize (cure) of the suspension. A view of their fabrication procedure can be seen in **Figure 7(a)**.

and electric field strength on required times for rotation, formation of chain-like structure, and migration can be seen, respectively. Besides, in **Figure 6(d)**, comparison between total alignment time and required time for CNT migration toward negative electrode can be seen.

**Figure 6.** Effect of electric field strength and viscosity of matrix on the required time for (a) rotation, (b) head to head contact and (c) migration, (d) shows comparison between required time for alignment of CNTs within the matrix and

In order to achieve a nanocomposite containing highly oriented CNT network with homogenous distribution, development of a practical procedure which has considered all of effective factors in matter of CNT alignment and nanocomposites preparation is essential. In this case, Ma et al. [4] synthesized MWCNTs using chemical vapor deposition (CVD) technique and thence functionalized fabricated MWCNTs using acidic method. In this regard, a mix-

added to the previous solution and heated up to 150°C for 30 min. Then, MWCNTs were rinsed with deionized water till they become chemically neutral. FTIR examination revealed that these MWCNTs have hydroxyl (─OH) and carboxyl (─COOH) functional groups, which

acid (1:1 volume ratio) was prepared, and thereon, MWCNTs were

**5. Methodology**

78 Carbon Nanotubes - Recent Progress

SO<sup>4</sup>

and HNO<sup>3</sup>

migration of CNTs toward negative electrode [16].

ture of H<sup>2</sup>

Khan et al. [3] used CVD fabricated MWCNTs for development of nanocomposites containing highly oriented CNTs. For this regard, first MWCNTs were subjected to ultra-violet/ozone (UV-O<sup>3</sup> ) treatment for 30 min used equipment consisted from a low-pressure mercury vapor grid lamp, which could irradiate UV radiation of 28 W/cm<sup>2</sup> to samples placed 20 mm far from it. This step can change the nature of CNTs from hydrophobic to hydrophilic due to the creation of oxygen-based functional groups on CNT surface, thereby enhancing the interaction between matrix and CNTs. Functionalized MWCNTs sonicated in acetone and Tritone x-100. Thence, epoxy resin was added to the previous suspension and sonicated for further 1 h. Thereon, CNT/epoxy mixture was placed in the vacuum oven for 2 h in order to evaporate acetone from the suspension. For further degassing, suspension was passed through a threeroller mill for six times. Resulting mixture was then degassed for further 20 min to eliminate trapped air bubbles, and thereupon, curing agent triethylenetetramine was added to the epoxy resin with ratio 12:100. MWCNTs aligned due to application of DC electric field in a place between two aluminum electrodes with 50 mm distance. In this case, 100 or 200 V/cm electric field was applied to the mixture depend on the concentration of MWCNTs. An elevated temperature of 100°C was applied to the mixture after completion of alignment, which resulted in the rapid cure of the mixture. Primary cured composite was then postcured at 120°C for 2 h. A view of their production technique can be seen in **Figure 7(b)**.

and lead creation of stress concentration areas. Bubbles are created due to various factors; diffusion of air into the polymer matrix and amalgamation of small bubbles have the main role in the creation of large bubbles. After completion of matrix-curing steps, the bubbles transform to voids and due to higher internal pressure than pressure in border with matrix and matrix itself, thereby resulting in growth of cracks in radial direction around the void [39, 42, 44]. For this regard, researchers have developed various kind of techniques to minimize the overall amount of bubble based voids, including molecular dispersion of bubbles in the polymer matrix [39], simultaneous usage of vacuum and vibration [45], and vacuum

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Moreover, addition of curing agent to the epoxy resin can lead to generation of a great amount of bubbles throughout the matrix, which is due to the reaction between the resin and the curing agent and can be removed via vacuum shock technique before completion of the first curing step. For instance, the presence of bubble-based voids throughout the epoxy resin with 1.0 wt% randomly oriented single-walled carbon nanotube (SWCNTS) can lead to 63% decrease in EMI shielding (X-band) and significant reduction in the electrical conductivity compared with nanocomposites holding small amount of bubble-based voids (less than 2%) [42]. What is more, creation of moisture throughout the dielectric nanocomposites can lead to significant decrease in both dielectric constant and dielectric loss [46]. Absorbed moisture not only can lead to decrease in the electrical performance but also can deteriorate the reliability of the dielectric materials [47]. Thus, removal of moisture and bubbles from nanocomposites has

Alignment of CNTs within the matrix can lead to obvious anisotropy in different directions. In this regard, aligned network presented significant enhancement in mechanical and electrical properties for parallel direction compared to perpendicular direction and randomly distribution of CNTs within the matrix [2–4, 6–8, 13, 20, 35]. Khan et al. [3] achieved remarkably low percolation threshold and significant improvement in mechanical properties of nanocomposites containing aligned MWCNTs. In this case, low percolation threshold of 0.0031 vol% was obtained for parallel direction of CNT alignment, which is one order higher than 0.034 vol% corresponding to the randomly oriented CNTs or that measured in transverse direction of CNT alignment. It is worth noting that the conductivity of developed nanocomposites increased by about four orders of magnitude due to increase in CNT weight percentage from 0.001 to 0.01 wt%, which shows a percolation behavior at even very low weight percentage of CNTs. **Figure 8(a)** shows the effect of CNT alignment on the electrical conductivity of developed nanocomposites containing aligned MWCNTs. As can be seen, alignment of CNTs within the matrix can lead to significant enhancement in the electrical conductivity, which is due to the increase in the interface of CNT bundles that enhanced the contact resistance [27, 48]. This trend for parallel direction is significantly higher than perpendicular direction and randomly distributed CNTs. On application of electric field between electrodes, CNTs immediately align in the direction of electric field to form interconnected bundles even at a very low CNT content, which can lead to achievement of percolation in the direction of CNT alignment, while

shock technique [42].

high level of importance.

**6. Outcomes**

On the other hand, despite the fabrication process, which is very important in development of nanocomposites containing aligned CNTs in the direction of electric field, destructive factors such as bubble based voids can also significantly affect the overall properties of nanocomposites includes mechanical and electrical properties [38–43]. Formation of bubble-based voids in the thermoset resins, such as epoxy resin, can highly affect their structural properties

**Figure 7.** Production procedures presented by (a) Zhu et al. [7] and (b) Khan et al. [3].

and lead creation of stress concentration areas. Bubbles are created due to various factors; diffusion of air into the polymer matrix and amalgamation of small bubbles have the main role in the creation of large bubbles. After completion of matrix-curing steps, the bubbles transform to voids and due to higher internal pressure than pressure in border with matrix and matrix itself, thereby resulting in growth of cracks in radial direction around the void [39, 42, 44]. For this regard, researchers have developed various kind of techniques to minimize the overall amount of bubble based voids, including molecular dispersion of bubbles in the polymer matrix [39], simultaneous usage of vacuum and vibration [45], and vacuum shock technique [42].

Moreover, addition of curing agent to the epoxy resin can lead to generation of a great amount of bubbles throughout the matrix, which is due to the reaction between the resin and the curing agent and can be removed via vacuum shock technique before completion of the first curing step. For instance, the presence of bubble-based voids throughout the epoxy resin with 1.0 wt% randomly oriented single-walled carbon nanotube (SWCNTS) can lead to 63% decrease in EMI shielding (X-band) and significant reduction in the electrical conductivity compared with nanocomposites holding small amount of bubble-based voids (less than 2%) [42]. What is more, creation of moisture throughout the dielectric nanocomposites can lead to significant decrease in both dielectric constant and dielectric loss [46]. Absorbed moisture not only can lead to decrease in the electrical performance but also can deteriorate the reliability of the dielectric materials [47]. Thus, removal of moisture and bubbles from nanocomposites has high level of importance.

#### **6. Outcomes**

resin with ratio 12:100. MWCNTs aligned due to application of DC electric field in a place between two aluminum electrodes with 50 mm distance. In this case, 100 or 200 V/cm electric field was applied to the mixture depend on the concentration of MWCNTs. An elevated temperature of 100°C was applied to the mixture after completion of alignment, which resulted in the rapid cure of the mixture. Primary cured composite was then postcured at 120°C for 2 h. A

On the other hand, despite the fabrication process, which is very important in development of nanocomposites containing aligned CNTs in the direction of electric field, destructive factors such as bubble based voids can also significantly affect the overall properties of nanocomposites includes mechanical and electrical properties [38–43]. Formation of bubble-based voids in the thermoset resins, such as epoxy resin, can highly affect their structural properties

view of their production technique can be seen in **Figure 7(b)**.

80 Carbon Nanotubes - Recent Progress

**Figure 7.** Production procedures presented by (a) Zhu et al. [7] and (b) Khan et al. [3].

Alignment of CNTs within the matrix can lead to obvious anisotropy in different directions. In this regard, aligned network presented significant enhancement in mechanical and electrical properties for parallel direction compared to perpendicular direction and randomly distribution of CNTs within the matrix [2–4, 6–8, 13, 20, 35]. Khan et al. [3] achieved remarkably low percolation threshold and significant improvement in mechanical properties of nanocomposites containing aligned MWCNTs. In this case, low percolation threshold of 0.0031 vol% was obtained for parallel direction of CNT alignment, which is one order higher than 0.034 vol% corresponding to the randomly oriented CNTs or that measured in transverse direction of CNT alignment. It is worth noting that the conductivity of developed nanocomposites increased by about four orders of magnitude due to increase in CNT weight percentage from 0.001 to 0.01 wt%, which shows a percolation behavior at even very low weight percentage of CNTs. **Figure 8(a)** shows the effect of CNT alignment on the electrical conductivity of developed nanocomposites containing aligned MWCNTs. As can be seen, alignment of CNTs within the matrix can lead to significant enhancement in the electrical conductivity, which is due to the increase in the interface of CNT bundles that enhanced the contact resistance [27, 48]. This trend for parallel direction is significantly higher than perpendicular direction and randomly distributed CNTs. On application of electric field between electrodes, CNTs immediately align in the direction of electric field to form interconnected bundles even at a very low CNT content, which can lead to achievement of percolation in the direction of CNT alignment, while the similar trend is not occurred for perpendicular direction of CNT alignment at the same content [3]. Despite CNT content, there are some other effective factors which are also responsible for the anisotropy in percolation include applied voltage, polymerization time (cure time), applied temperature, and viscosity of the matrix [8]. Moreover, it should be mentioned that transverse migration and connection starts once the alignment in the longitudinal direction (parallel direction) completed. In this case, increase in CNT weight percentage can enhance the rate of transverse direction and thus minimize the differences between parallel and perpendicular directions. On the other hand, the same as electrical conductivity, alignment of CNTs within the matrix shows significant improvement in the mechanical properties of nanocomposites include tensile strength, storage modulus, and quasi-static fracture toughness, which indicating the string reinforcing effect of high-modulus CNTs. In this regard, alignment of MWCNTs within the matrix led to 40% and 15% improvement in the Young's modulus for nanocomposites containing 0.3 wt% aligned and unaligned MWCNT, respectively. However, it was observed that by further increase in the CNT concentration beyond 0.3 wt%, overall mechanical properties of developed nanocomposites containing aligned MWCNTs decreased, which is due to the reduction in the degree of alignment at high CNT concentration. In fact, increase in the CNT concentration can lead to significant increase in the viscosity of matrix as well as fillers packing, which can lead to higher resistance and less available free spaces for CNTs in order to move and align in the direction of electric field. In **Figure 8**(**b**−**d**), effect of CNT alignment on storage modulus, Young's modulus, and quasi-static fracture toughness of nanocomposites can be seen [3].

On the other hand, present gaps and pores inside the aligned network due to low-weight percentage of CNTs can increase the contact resistance and thus reduce the electrical conductivity [49, 50]. Alignment of CNTs can lead to significant drop in obtained resistivity for a percolation threshold [51]. Moreover, alignment of CNTs can avoid the entanglement among CNTs in a certain degree, which can avoid creation of the conductive path [31]. It was also reported that alignment of CNTs within the matrix can lead increase in the volume resistivity and thus decrease in the conductivity of matrix [52]. In a work by Gupta et al. [2], they have aligned CNTs with current-assisted technique, which was led to the formation of shortest, continuous path for the flow of electrons, and thus formation of a highly anisotropic conductive path. Obtained results showed that current passage assisted alignment of CNTs can lead to significant 360% improvement in the conductivity for parallel direction than random distribution. However, lower conductivity in transverse direction than randomly distributed structure has been seen, which shows the efficiency of alignment. Despite of that, CNTs are forming a liner path with minimal distance between the electrodes in order to assisting current directly pass through. Electric field–induced alignment recorded 28% increase in the conductivity for parallel direction than randomly distribution, while the transverse direction showed 58% decrease in the electrical conductivity. In fact, current-induced alignment resulted in creation of continuous channels of aligned CNT, where CNTs are contacted end to end with each other and formed and uninterrupted conductive pathway. In this case, electrons will get direct path of flow, which can lead to fantastic enhancement in the electrical conductivity.

**Figure 8.** Effect of MWCNT alignment within the epoxy resin on (a) electrical conductivity, (b) storage modulus,

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**Figure 9.** Effect of MWCNT alignment on (a) storage modulus and (b) electrical conductivity of developed nanocomposites [7].

(c) Young's modulus, and (d) quasi-static fracture toughness of developed nanocomposites [3].

Zhu et al. [7] reported that alignment of MWCNTs via AC electric field can lead to significant improvement in the storage modulus and electrical conductivity of developed nanocomposites. Their obtained results revealed that increase in concentration of MWCNTs as well as their alignment can lead significant enhancement in the storage modulus of developed nanocomposites. Electric Field Induced Alignment of Carbon Nanotubes: Methodology and Outcomes http://dx.doi.org/10.5772/intechopen.70481 83

the similar trend is not occurred for perpendicular direction of CNT alignment at the same content [3]. Despite CNT content, there are some other effective factors which are also responsible for the anisotropy in percolation include applied voltage, polymerization time (cure time), applied temperature, and viscosity of the matrix [8]. Moreover, it should be mentioned that transverse migration and connection starts once the alignment in the longitudinal direction (parallel direction) completed. In this case, increase in CNT weight percentage can enhance the rate of transverse direction and thus minimize the differences between parallel and perpendicular directions. On the other hand, the same as electrical conductivity, alignment of CNTs within the matrix shows significant improvement in the mechanical properties of nanocomposites include tensile strength, storage modulus, and quasi-static fracture toughness, which indicating the string reinforcing effect of high-modulus CNTs. In this regard, alignment of MWCNTs within the matrix led to 40% and 15% improvement in the Young's modulus for nanocomposites containing 0.3 wt% aligned and unaligned MWCNT, respectively. However, it was observed that by further increase in the CNT concentration beyond 0.3 wt%, overall mechanical properties of developed nanocomposites containing aligned MWCNTs decreased, which is due to the reduction in the degree of alignment at high CNT concentration. In fact, increase in the CNT concentration can lead to significant increase in the viscosity of matrix as well as fillers packing, which can lead to higher resistance and less available free spaces for CNTs in order to move and align in the direction of electric field. In **Figure 8**(**b**−**d**), effect of CNT alignment on storage modulus, Young's modulus,

On the other hand, present gaps and pores inside the aligned network due to low-weight percentage of CNTs can increase the contact resistance and thus reduce the electrical conductivity [49, 50]. Alignment of CNTs can lead to significant drop in obtained resistivity for a percolation threshold [51]. Moreover, alignment of CNTs can avoid the entanglement among CNTs in a certain degree, which can avoid creation of the conductive path [31]. It was also reported that alignment of CNTs within the matrix can lead increase in the volume resistivity and thus decrease in the conductivity of matrix [52]. In a work by Gupta et al. [2], they have aligned CNTs with current-assisted technique, which was led to the formation of shortest, continuous path for the flow of electrons, and thus formation of a highly anisotropic conductive path. Obtained results showed that current passage assisted alignment of CNTs can lead to significant 360% improvement in the conductivity for parallel direction than random distribution. However, lower conductivity in transverse direction than randomly distributed structure has been seen, which shows the efficiency of alignment. Despite of that, CNTs are forming a liner path with minimal distance between the electrodes in order to assisting current directly pass through. Electric field–induced alignment recorded 28% increase in the conductivity for parallel direction than randomly distribution, while the transverse direction showed 58% decrease in the electrical conductivity. In fact, current-induced alignment resulted in creation of continuous channels of aligned CNT, where CNTs are contacted end to end with each other and formed and uninterrupted conductive pathway. In this case, electrons will get direct path of flow, which can lead to

Zhu et al. [7] reported that alignment of MWCNTs via AC electric field can lead to significant improvement in the storage modulus and electrical conductivity of developed nanocomposites. Their obtained results revealed that increase in concentration of MWCNTs as well as their alignment can lead significant enhancement in the storage modulus of developed nanocomposites.

and quasi-static fracture toughness of nanocomposites can be seen [3].

82 Carbon Nanotubes - Recent Progress

fantastic enhancement in the electrical conductivity.

**Figure 8.** Effect of MWCNT alignment within the epoxy resin on (a) electrical conductivity, (b) storage modulus, (c) Young's modulus, and (d) quasi-static fracture toughness of developed nanocomposites [3].

**Figure 9.** Effect of MWCNT alignment on (a) storage modulus and (b) electrical conductivity of developed nanocomposites [7].

Furthermore, nanocomposites containing amine functionalized MWCNTs (a-MWCNT) have shown higher storage modulus compared to pristine MWCNTs (p-MWCNT), which is due to the better compatibility and interaction between the matrix and a-MWCNTs. Moreover, measured storage modulus of aligned CNTs in parallel direction was greater than perpendicular direction and their randomly distribution within the matrix. On the other hand, nanocomposites containing p-MWCNT were presented higher electrical conductivity compared to a-MWCNT by about two orders of magnitude, which could be due to the decrease in length of a-MWCNT during chemical functionalization and increase in the a-MWCNT resistance. **Figure 9(a)** and **(b)** illustrated the effect of MWCNT alignment within the epoxy resin on storage modulus and electrical conductivity, respectively.

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#### **7. Conclusions**

CNTs presenting extraordinary mechanical, thermal, and electrical properties, which have reinforcement of nanocomposites with various kinds of CNT, can lead to significant improvement in their overall properties. In this case, application of electric field to the suspension containing CNT at different filler loadings can lead their alignment within the matrix and obvious anisotropy in different directions. In fact, CNTs act as path for transferring current from negative to positive electrode. This phenomenon can boost both electrical and mechanical properties of developed nanocomposites at same filler content. On the other hand, achieved results revealed that the overall properties of nanocomposites include mechanical and electrical properties are higher for parallel direction than perpendicular direction and random distribution of CNTs within the matrix, which is due to the desire of CNTs in formation of longitudinal connections than transverse connections.

Eventually, by alignment of CNTs within the matrix, significant improvement in overall properties of nanocomposites at same filler loadings compared with randomly distribution can be achieved, which is very essential for aerospace and aviation industries that encounter with serious limits in matter of structures' weight.

#### **Author details**

Ali Mohammad Amani1,2, Seyyed Alireza Hashemi1,2\*, Seyyed Mojtaba Mousavi1,2, Seyyed Milad Abrishamifar<sup>3</sup> and Arash Vojood<sup>4</sup>

\*Address all correspondence to: s.a.hashemi0@gmail.com

1 Department of Medical Nanotechnology, School of Advanced Medical Sciences and Technologies, Shiraz University of Medical Sciences, Shiraz, Iran

2 Pharmaceutical Sciences Research Center, Shiraz University of Medical Sciences, Shiraz, Iran

3 Department of Chemical Engineering, New York International University of Technology and Management, New York, USA

4 Young Researchers and Elite Club, Ardabil Branch, Islamic Azad University, Ardabil, Iran

#### **References**

Furthermore, nanocomposites containing amine functionalized MWCNTs (a-MWCNT) have shown higher storage modulus compared to pristine MWCNTs (p-MWCNT), which is due to the better compatibility and interaction between the matrix and a-MWCNTs. Moreover, measured storage modulus of aligned CNTs in parallel direction was greater than perpendicular direction and their randomly distribution within the matrix. On the other hand, nanocomposites containing p-MWCNT were presented higher electrical conductivity compared to a-MWCNT by about two orders of magnitude, which could be due to the decrease in length of a-MWCNT during chemical functionalization and increase in the a-MWCNT resistance. **Figure 9(a)** and **(b)** illustrated the effect of MWCNT alignment within the epoxy resin on storage modulus and electrical

CNTs presenting extraordinary mechanical, thermal, and electrical properties, which have reinforcement of nanocomposites with various kinds of CNT, can lead to significant improvement in their overall properties. In this case, application of electric field to the suspension containing CNT at different filler loadings can lead their alignment within the matrix and obvious anisotropy in different directions. In fact, CNTs act as path for transferring current from negative to positive electrode. This phenomenon can boost both electrical and mechanical properties of developed nanocomposites at same filler content. On the other hand, achieved results revealed that the overall properties of nanocomposites include mechanical and electrical properties are higher for parallel direction than perpendicular direction and random distribution of CNTs within the matrix, which is due to the desire of CNTs in formation of longitudinal connections than trans-

Eventually, by alignment of CNTs within the matrix, significant improvement in overall properties of nanocomposites at same filler loadings compared with randomly distribution can be achieved, which is very essential for aerospace and aviation industries that encounter with

Ali Mohammad Amani1,2, Seyyed Alireza Hashemi1,2\*, Seyyed Mojtaba Mousavi1,2,

1 Department of Medical Nanotechnology, School of Advanced Medical Sciences and

2 Pharmaceutical Sciences Research Center, Shiraz University of Medical Sciences, Shiraz,

3 Department of Chemical Engineering, New York International University of Technology and

4 Young Researchers and Elite Club, Ardabil Branch, Islamic Azad University, Ardabil, Iran

and Arash Vojood<sup>4</sup>

\*Address all correspondence to: s.a.hashemi0@gmail.com

Technologies, Shiraz University of Medical Sciences, Shiraz, Iran

conductivity, respectively.

84 Carbon Nanotubes - Recent Progress

**7. Conclusions**

verse connections.

**Author details**

Iran

Seyyed Milad Abrishamifar<sup>3</sup>

Management, New York, USA

serious limits in matter of structures' weight.


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**Chapter 6**

**Provisional chapter**

**Mechanical and Fracture Properties of Carbon**

**Mechanical and Fracture Properties of Carbon** 

DOI: 10.5772/intechopen.70578

Carbon nanotubes (CNTs) have attracted much interest because of their superior electrical, thermal, and mechanical properties. These unique properties of CNTs have come to the attention of many scientists and engineers worldwide, eager to incorporate these novel materials into composites and electronic devices. However, before the utilization of these materials becomes mainstream, it is necessary to develop protocols for tailoring the material properties, so that composites and devices can be engineered to given specifications. In this chapter, we review our recent studies, in which we investigate the nominal tensile strength and strength distribution of multi-walled CNTs (MWCNTs) synthesized by the catalytic chemical vapor deposition (CVD) method, followed by a series of high-temperature annealing steps that culminate with annealing at 2900°C. The structural-mechanical relationships of such MWCNTs are investigated through tensile-loading experiments with individual MWCNTs, Weibull-Poisson statistics, transmission electron microscope (TEM)

**Keywords:** carbon nanotubes, tensile strength, Weibull-Poisson statistics, structural defects,

Carbon nanotubes (CNTs) have attracted much interest because of their potential application as next-generation electronic and structural materials. In particular, their superior electrical, thermal, and mechanical properties, including high electrical and thermal conductivity [1, 2], negative thermal expansion coefficient [3–10], and high mechanical strength, exceeding 100 GPa [11, 12], make them a candidate material for nano- and microscale composites,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Keiichi Shirasu, Go Yamamoto, Daniel Nelias and

Additional information is available at the end of the chapter

observation, and Raman spectroscopy analysis.

Additional information is available at the end of the chapter

Keiichi Shirasu, Go Yamamoto, Daniel Nelias

http://dx.doi.org/10.5772/intechopen.70578

**Nanotubes**

**Nanotubes**

Toshiyuki Hashida

**Abstract**

heat treatment

**1. Introduction**

and Toshiyuki Hashida


**Provisional chapter**

#### **Mechanical and Fracture Properties of Carbon Nanotubes Nanotubes**

**Mechanical and Fracture Properties of Carbon** 

DOI: 10.5772/intechopen.70578

Keiichi Shirasu, Go Yamamoto, Daniel Nelias

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88 Carbon Nanotubes - Recent Progress

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Keiichi Shirasu, Go Yamamoto, Daniel Nelias and Toshiyuki Hashida and Toshiyuki Hashida Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70578

#### **Abstract**

Carbon nanotubes (CNTs) have attracted much interest because of their superior electrical, thermal, and mechanical properties. These unique properties of CNTs have come to the attention of many scientists and engineers worldwide, eager to incorporate these novel materials into composites and electronic devices. However, before the utilization of these materials becomes mainstream, it is necessary to develop protocols for tailoring the material properties, so that composites and devices can be engineered to given specifications. In this chapter, we review our recent studies, in which we investigate the nominal tensile strength and strength distribution of multi-walled CNTs (MWCNTs) synthesized by the catalytic chemical vapor deposition (CVD) method, followed by a series of high-temperature annealing steps that culminate with annealing at 2900°C. The structural-mechanical relationships of such MWCNTs are investigated through tensile-loading experiments with individual MWCNTs, Weibull-Poisson statistics, transmission electron microscope (TEM) observation, and Raman spectroscopy analysis.

**Keywords:** carbon nanotubes, tensile strength, Weibull-Poisson statistics, structural defects, heat treatment

#### **1. Introduction**

Carbon nanotubes (CNTs) have attracted much interest because of their potential application as next-generation electronic and structural materials. In particular, their superior electrical, thermal, and mechanical properties, including high electrical and thermal conductivity [1, 2], negative thermal expansion coefficient [3–10], and high mechanical strength, exceeding 100 GPa [11, 12], make them a candidate material for nano- and microscale composites,

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

sensors, actuators, and other electronic devices. Additionally, continuous multi-walled CNT (MWCNT) yarns and sheets, which are prepared by directly drawing MWCNTs from spinnable MWCNT arrays, have been developed [13, 14], and new processing methods utilizing MWCNT yarns and sheets have emerged as means of producing preforms and composites with higher MWCNT volume fractions [15–25]. The most recent reviews on these topics were reported by Di et al. [26] and Goh et al. [27].

that involves wavelike distortions in the nanotube. Gaillard et al. [48] also used a similar experimental setup, but their technique is relatively simpler: the resonance frequency of an electrostatically driven MWCNT is determined using a dark-field optical microscope. They found that there was a correlation between the defect density and the bending modulus, which suggests that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter. The other method for evaluation of the tensile strength and Young's modulus of CNTs is the tensile testing method [11, 12, 37–41, 49–51]. Yu et al. [37] measured the stress-strain response and strength at failure of individual arc discharge-grown MWCNTs (~30 μm long) using a manipulator tool operated inside a scanning electron microscope (SEM). They reported measured tensile strengths and Young's moduli of MWCNTs ranging from ~11 to ~63 GPa and from 270 to 950 GPa, respectively. Peng et al. [11] reported that defect-free individual MWCNTs were shown to possess a mechanical strength equivalent to the theoretical value (100 GPa) using a precise in situ transmission electron microscopy (TEM) method with a micro-electromechanical system (MEMS) material testing system. They also performed a study on the effect of electron irradiation parameters on the resulting MWCNT strength. They found that as the irradiation-induced defect density increased, the tensile strength decreased, with that of three nonirradiated samples and a sample irradiated at a higher dose being ~100 GPa on average and 35 GPa, respectively. Yamamoto et al. [29] performed a study of the effect of acid treatment on the tensile strength of CVD-grown MWCNTs (~9 μm long), using a piezoactuated nanomanipulator. The acid treatment introduced deep nanoscale defects as well as negatively charged functional groups onto the surface of the MWCNTs. The defects in these acid-treated MWCNTs had a channel-like structure, as if a ring of material was cut away from the MWCNT around its circumference [29]. By comparing the SEM images of MWCNTs acquired before and after fracture, it was found that all the nanotubes tested fractured in the so-called *sword-in-sheath failure* mode; the fracture of the acid-treated MWCNTs mostly occurred at the nanodefects. Tensile-loading experiments revealed that the tensile strengths of pristine MWCNTs were in the range of 2–48 GPa (mean, 20 GPa). However, the acid-treated MWCNTs with nanoscale defects possessed a tensile strength of 1–18 GPa (mean, 6 GPa), which is approximately 70% lower than that of the pristine MWCNTs. These results indicated that the channel-like defects associated with the acid etching were typically the weakest points in the acid-treated MWCNT structure and that stress concentration was present at the defect region. In these studies, the tensile strength was calculated from either the fractured cross-sectional area (effective strength) or the cross-sectional area of the outermost layer of the MWCNT. Few research groups have examined the nominal (or *engineering*) strength and its Weibull distribution, which are required for investigations of crack bridging characteristics and mechanical properties of composites reinforced with MWCNTs (a detailed discussion is given in [52]). Yu et al. [37] calculated the nominal tensile strengths to be in the range of 1.4–2.9 GPa, which is much

Mechanical and Fracture Properties of Carbon Nanotubes

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91

lower than the effective tensile strength (11–63 GPa).

Here, we review our recent studies in which the strength properties of individual MWCNTs synthesized by a CVD method, followed by a series of high-temperature annealing steps that culminate with annealing at 2900°C, are investigated by a manipulator tool operated inside

The Young's modulus and strength of CNTs are well known to depend critically on the structure (e.g., geometry, crystallinity, and defect type and density), which in turn depends on the manufacturing route and subsequent treatment [28–31]. Quantum mechanics calculations [32–36] predict that defect-free single-walled CNTs possess Young's modulus values of ~1 TPa, tensile strengths >100 GPa, and failure strains of ~15–30%, depending on the chirality. However, experimental measurements [29, 37–41], which have all involved MWCNTs, have reported markedly lower values for fracture strengths and failure strains. For example, Ding et al. [40] showed that unpurified arc discharge-grown MWCNTs yielded a mean modulus value of 955 GPa, in good agreement with theory, but mean fracture strengths and failure strains that were only 24 GPa and 2.6%, respectively. Calculations [35] have suggested that defects introduced by oxidative pitting during nanotube purification can markedly reduce fracture strength. Therefore, for the development of basic design concepts for the use of CNTs in nanocomposites that require high strength, experimental evaluation of the mechanical properties of CNTs is crucial.

Several techniques have been developed for exploring the mechanical properties of individual CNTs. One method for measuring the Young's modulus of a CNT is to fabricate a nanotube beam that is clamped at each end to a ceramic membrane (or otherwise supported) and to measure its vertical deflection versus the force applied at a point midway along its length [28]. The atomic force microscope (AFM) is a natural and convenient means for studying the Young's modulus of CNTs, because it allows measurement of the deflection of a sample as a function of applied force when used in contact mode [28, 30, 42–46]. Salvetat et al. [28] deposited a droplet of a MWCNT suspension on a wellpolished alumina ultrafiltration membrane and evaluated the Young's modulus using the abovementioned method. They found that MWCNTs grown by catalytic chemical vapor deposition (CVD) have Young's moduli in the range of 12–50 GPa (mean: 27 GPa). These values are considerably lower than the moduli of arc discharge-grown MWCNTs (600–1100 GPa). Recently, Elumeeva et al. [30] investigated the Young's modulus of four types of MWCNTs synthesized by the CVD method followed by a series of high-temperature annealing steps at 2200, 2600, and 2800°C using a method similar to that of Salvetat et al.'s study [28]. The experimental results showed that the Young's modulus increased for the annealed MWCNTs with respect to the as-grown ones. Poncharal et al. presented a vibrating reed technique for testing the bending modulus of MWCNTs [47]. The elastic bending modulus as a function of diameter was found to decrease sharply (from approximately 1 TPa to 100 GPa) with increasing diameter (from 8 to 40 nm), which was attributed to the crossover from a uniform elastic mode to an elastic mode that involves wavelike distortions in the nanotube. Gaillard et al. [48] also used a similar experimental setup, but their technique is relatively simpler: the resonance frequency of an electrostatically driven MWCNT is determined using a dark-field optical microscope. They found that there was a correlation between the defect density and the bending modulus, which suggests that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter. The other method for evaluation of the tensile strength and Young's modulus of CNTs is the tensile testing method [11, 12, 37–41, 49–51]. Yu et al. [37] measured the stress-strain response and strength at failure of individual arc discharge-grown MWCNTs (~30 μm long) using a manipulator tool operated inside a scanning electron microscope (SEM). They reported measured tensile strengths and Young's moduli of MWCNTs ranging from ~11 to ~63 GPa and from 270 to 950 GPa, respectively. Peng et al. [11] reported that defect-free individual MWCNTs were shown to possess a mechanical strength equivalent to the theoretical value (100 GPa) using a precise in situ transmission electron microscopy (TEM) method with a micro-electromechanical system (MEMS) material testing system. They also performed a study on the effect of electron irradiation parameters on the resulting MWCNT strength. They found that as the irradiation-induced defect density increased, the tensile strength decreased, with that of three nonirradiated samples and a sample irradiated at a higher dose being ~100 GPa on average and 35 GPa, respectively. Yamamoto et al. [29] performed a study of the effect of acid treatment on the tensile strength of CVD-grown MWCNTs (~9 μm long), using a piezoactuated nanomanipulator. The acid treatment introduced deep nanoscale defects as well as negatively charged functional groups onto the surface of the MWCNTs. The defects in these acid-treated MWCNTs had a channel-like structure, as if a ring of material was cut away from the MWCNT around its circumference [29]. By comparing the SEM images of MWCNTs acquired before and after fracture, it was found that all the nanotubes tested fractured in the so-called *sword-in-sheath failure* mode; the fracture of the acid-treated MWCNTs mostly occurred at the nanodefects. Tensile-loading experiments revealed that the tensile strengths of pristine MWCNTs were in the range of 2–48 GPa (mean, 20 GPa). However, the acid-treated MWCNTs with nanoscale defects possessed a tensile strength of 1–18 GPa (mean, 6 GPa), which is approximately 70% lower than that of the pristine MWCNTs. These results indicated that the channel-like defects associated with the acid etching were typically the weakest points in the acid-treated MWCNT structure and that stress concentration was present at the defect region. In these studies, the tensile strength was calculated from either the fractured cross-sectional area (effective strength) or the cross-sectional area of the outermost layer of the MWCNT. Few research groups have examined the nominal (or *engineering*) strength and its Weibull distribution, which are required for investigations of crack bridging characteristics and mechanical properties of composites reinforced with MWCNTs (a detailed discussion is given in [52]). Yu et al. [37] calculated the nominal tensile strengths to be in the range of 1.4–2.9 GPa, which is much lower than the effective tensile strength (11–63 GPa).

sensors, actuators, and other electronic devices. Additionally, continuous multi-walled CNT (MWCNT) yarns and sheets, which are prepared by directly drawing MWCNTs from spinnable MWCNT arrays, have been developed [13, 14], and new processing methods utilizing MWCNT yarns and sheets have emerged as means of producing preforms and composites with higher MWCNT volume fractions [15–25]. The most recent reviews on

The Young's modulus and strength of CNTs are well known to depend critically on the structure (e.g., geometry, crystallinity, and defect type and density), which in turn depends on the manufacturing route and subsequent treatment [28–31]. Quantum mechanics calculations [32–36] predict that defect-free single-walled CNTs possess Young's modulus values of ~1 TPa, tensile strengths >100 GPa, and failure strains of ~15–30%, depending on the chirality. However, experimental measurements [29, 37–41], which have all involved MWCNTs, have reported markedly lower values for fracture strengths and failure strains. For example, Ding et al. [40] showed that unpurified arc discharge-grown MWCNTs yielded a mean modulus value of 955 GPa, in good agreement with theory, but mean fracture strengths and failure strains that were only 24 GPa and 2.6%, respectively. Calculations [35] have suggested that defects introduced by oxidative pitting during nanotube purification can markedly reduce fracture strength. Therefore, for the development of basic design concepts for the use of CNTs in nanocomposites that require high strength, experimental evaluation of the mechanical properties

Several techniques have been developed for exploring the mechanical properties of individual CNTs. One method for measuring the Young's modulus of a CNT is to fabricate a nanotube beam that is clamped at each end to a ceramic membrane (or otherwise supported) and to measure its vertical deflection versus the force applied at a point midway along its length [28]. The atomic force microscope (AFM) is a natural and convenient means for studying the Young's modulus of CNTs, because it allows measurement of the deflection of a sample as a function of applied force when used in contact mode [28, 30, 42–46]. Salvetat et al. [28] deposited a droplet of a MWCNT suspension on a wellpolished alumina ultrafiltration membrane and evaluated the Young's modulus using the abovementioned method. They found that MWCNTs grown by catalytic chemical vapor deposition (CVD) have Young's moduli in the range of 12–50 GPa (mean: 27 GPa). These values are considerably lower than the moduli of arc discharge-grown MWCNTs (600–1100 GPa). Recently, Elumeeva et al. [30] investigated the Young's modulus of four types of MWCNTs synthesized by the CVD method followed by a series of high-temperature annealing steps at 2200, 2600, and 2800°C using a method similar to that of Salvetat et al.'s study [28]. The experimental results showed that the Young's modulus increased for the annealed MWCNTs with respect to the as-grown ones. Poncharal et al. presented a vibrating reed technique for testing the bending modulus of MWCNTs [47]. The elastic bending modulus as a function of diameter was found to decrease sharply (from approximately 1 TPa to 100 GPa) with increasing diameter (from 8 to 40 nm), which was attributed to the crossover from a uniform elastic mode to an elastic mode

these topics were reported by Di et al. [26] and Goh et al. [27].

of CNTs is crucial.

90 Carbon Nanotubes - Recent Progress

Here, we review our recent studies in which the strength properties of individual MWCNTs synthesized by a CVD method, followed by a series of high-temperature annealing steps that culminate with annealing at 2900°C, are investigated by a manipulator tool operated inside an SEM [52–54]. The relationship between the MWCNT structure and strength properties of MWCNTs with a significantly different nanostructure is investigated through tensile tests of individual MWCNTs, transmission electron microscope (TEM) observations, and Raman spectroscopy analysis.

#### **2. Structural characterization of carbon nanotubes**

MWCNT materials (acquired from Hodogaya Chemical, Japan) synthesized by a catalytic CVD process were thermally annealed in a graphite crucible using a resistance-heated graphite element furnace at 1200, 1800, 2200, and 2600°C under an argon atmosphere [53]. The temperature was raised at a heating rate of approximately 60°C/min to the predetermined temperature and held there for 1 h before cooling to ambient temperature. The average outer diameter, inner diameter, and length of the MWCNTs are approximately 70, 7 nm and 7.8 μm, respectively. **Figure 1** shows typical TEM images of the four types of MWCNTs. At an annealing temperature of 1200°C (**Figure 1a**), the sample consists of turbostratic elementary domains 2–3 graphene layers thick. Each elementary domain is tilted at an angle with respect to the nanotube axis, forming larger wrinkled layers. When the annealing temperature increased to 1800°C (**Figure 1b**), the turbostratic structure disappears, and instead undulated fringes are formed by hooking the adjacent elementary domains together, i.e., both the in-plane and c-axis crystallite sizes appear to increase in this temperature range. For the samples annealed at 2200°C (**Figure 1c**), even though the undulated structure seems to remain unchanged, the graphitic planes become aligned, and the crystallite sizes increase further. With thermal annealing at 2600°C (**Figure 1d**), the undulating structure disappears, and the MWCNTs consist of nested graphitic cylinders that are almost perfectly aligned with the nanotube axis. However, these MWCNTs are observed to possess structural defects such as abrupt structural changes from constant-diameter cylinders and unevenly spaced lattice fringes. Hereafter, these kinds of MWCNTs are referred to as H-MWCNTs.

Spinnable MWCNT arrays were obtained by a thermal CVD method using C2 H2 and FeCl2 as the base material and the catalyst, respectively. The procedure for the fabrication of the MWCNT arrays follows [55]. The average outer diameter and inner diameter were approximately 40 and 7 nm, respectively, and the length of the MWCNTs was ~700 μm. The MWCNTs were thermally annealed in a graphite crucible (Kurata Giken SCC-U-80/150) using a resistance-heated graphite element furnace at 2000°C in a vacuum, followed by heat treatment at 2400 and 2900°C under an argon atmosphere. **Figure 2** shows TEM images of some of the MWCNTs. The as-grown MWCNTs consist of slightly undulating graphitic cylinders with respect to the nanotube axis (**Figure 2a**). Additionally, these MWCNTs possess several types of structural defects, such as *kinks and bends*, *discontinuous flaws*, and *remnant catalysts* (**Figure 2c–f**). For the samples annealed at 2400°C, the degree of waviness of the nanotube walls seems to decrease. Following thermal annealing at 2900°C (**Figure 2b**), the undulated structure disappears, and the MWCNT consists of nested graphitic cylinders that are almost perfectly aligned with the nanotube axis. However, structural defects such as *discontinuous flaws* and *kinks and bends* are still observed for a subset of the samples. The structural defect densities for the MWCNTs prepared in this study are summarized in **Table 1**. The thermally

annealed MWCNTs possessed a smaller amount of structural defects, characterized by *discontinuous flaws* and *kinks and bends* and no *remnant catalyst* compared with the as-grown

**Figure 1.** TEM images showing structural evolution of H-MWCNTs [53]. Annealing temperatures are (a) 1200°C, (b)

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Next, we used Raman spectroscopy to evaluate whether any structural evolution occurs during thermal annealing. The Raman scattering spectrum of the MWCNTs shows a pair of bands around 1360 cm−1 (D-band) and 1590 cm−1 (G-band) [56]. Thus, the relative intensity ratio of the G-band to D-band peak, i.e., *R* = *I*G/*I*D, is known to depend on the number of defects in the nanotubes [57]. The Raman intensity ratios (*R*) of the H-MWCNTs and S-MWCNTs are shown in **Figure 3**. The Raman spectra of the MWCNTs showed a pair of bands near 1360 and 1590 cm−1. The *R* values of the H-MWCNTs increased from 1.0 to 10.1 with increasing annealing temperature. For the S-MWCNTs, there is no clear difference in the *R* values between the as-grown MWCNTs and those annealed at 2000°C (*R* = 3.2 and 3.8), suggesting that both types of MWCNTs have the same degree of crystallinity. On the other hand, the *R* values increase with increasing annealing temperature over the temperature range 2000–2900°C

MWCNTs. In this chapter, we call these nanotubes S-MWCNTs.

1800°C, (c) 2200°C, and (d) 2600°C.

an SEM [52–54]. The relationship between the MWCNT structure and strength properties of MWCNTs with a significantly different nanostructure is investigated through tensile tests of individual MWCNTs, transmission electron microscope (TEM) observations, and Raman

MWCNT materials (acquired from Hodogaya Chemical, Japan) synthesized by a catalytic CVD process were thermally annealed in a graphite crucible using a resistance-heated graphite element furnace at 1200, 1800, 2200, and 2600°C under an argon atmosphere [53]. The temperature was raised at a heating rate of approximately 60°C/min to the predetermined temperature and held there for 1 h before cooling to ambient temperature. The average outer diameter, inner diameter, and length of the MWCNTs are approximately 70, 7 nm and 7.8 μm, respectively. **Figure 1** shows typical TEM images of the four types of MWCNTs. At an annealing temperature of 1200°C (**Figure 1a**), the sample consists of turbostratic elementary domains 2–3 graphene layers thick. Each elementary domain is tilted at an angle with respect to the nanotube axis, forming larger wrinkled layers. When the annealing temperature increased to 1800°C (**Figure 1b**), the turbostratic structure disappears, and instead undulated fringes are formed by hooking the adjacent elementary domains together, i.e., both the in-plane and c-axis crystallite sizes appear to increase in this temperature range. For the samples annealed at 2200°C (**Figure 1c**), even though the undulated structure seems to remain unchanged, the graphitic planes become aligned, and the crystallite sizes increase further. With thermal annealing at 2600°C (**Figure 1d**), the undulating structure disappears, and the MWCNTs consist of nested graphitic cylinders that are almost perfectly aligned with the nanotube axis. However, these MWCNTs are observed to possess structural defects such as abrupt structural changes from constant-diameter cylinders and unevenly spaced lattice fringes. Hereafter, these kinds of MWCNTs are referred to as H-MWCNTs.

Spinnable MWCNT arrays were obtained by a thermal CVD method using C2

as the base material and the catalyst, respectively. The procedure for the fabrication of the MWCNT arrays follows [55]. The average outer diameter and inner diameter were approximately 40 and 7 nm, respectively, and the length of the MWCNTs was ~700 μm. The MWCNTs were thermally annealed in a graphite crucible (Kurata Giken SCC-U-80/150) using a resistance-heated graphite element furnace at 2000°C in a vacuum, followed by heat treatment at 2400 and 2900°C under an argon atmosphere. **Figure 2** shows TEM images of some of the MWCNTs. The as-grown MWCNTs consist of slightly undulating graphitic cylinders with respect to the nanotube axis (**Figure 2a**). Additionally, these MWCNTs possess several types of structural defects, such as *kinks and bends*, *discontinuous flaws*, and *remnant catalysts* (**Figure 2c–f**). For the samples annealed at 2400°C, the degree of waviness of the nanotube walls seems to decrease. Following thermal annealing at 2900°C (**Figure 2b**), the undulated structure disappears, and the MWCNT consists of nested graphitic cylinders that are almost perfectly aligned with the nanotube axis. However, structural defects such as *discontinuous flaws* and *kinks and bends* are still observed for a subset of the samples. The structural defect densities for the MWCNTs prepared in this study are summarized in **Table 1**. The thermally

H2

and FeCl2

**2. Structural characterization of carbon nanotubes**

spectroscopy analysis.

92 Carbon Nanotubes - Recent Progress

**Figure 1.** TEM images showing structural evolution of H-MWCNTs [53]. Annealing temperatures are (a) 1200°C, (b) 1800°C, (c) 2200°C, and (d) 2600°C.

annealed MWCNTs possessed a smaller amount of structural defects, characterized by *discontinuous flaws* and *kinks and bends* and no *remnant catalyst* compared with the as-grown MWCNTs. In this chapter, we call these nanotubes S-MWCNTs.

Next, we used Raman spectroscopy to evaluate whether any structural evolution occurs during thermal annealing. The Raman scattering spectrum of the MWCNTs shows a pair of bands around 1360 cm−1 (D-band) and 1590 cm−1 (G-band) [56]. Thus, the relative intensity ratio of the G-band to D-band peak, i.e., *R* = *I*G/*I*D, is known to depend on the number of defects in the nanotubes [57]. The Raman intensity ratios (*R*) of the H-MWCNTs and S-MWCNTs are shown in **Figure 3**. The Raman spectra of the MWCNTs showed a pair of bands near 1360 and 1590 cm−1. The *R* values of the H-MWCNTs increased from 1.0 to 10.1 with increasing annealing temperature. For the S-MWCNTs, there is no clear difference in the *R* values between the as-grown MWCNTs and those annealed at 2000°C (*R* = 3.2 and 3.8), suggesting that both types of MWCNTs have the same degree of crystallinity. On the other hand, the *R* values increase with increasing annealing temperature over the temperature range 2000–2900°C

**Figure 2.** TEM images of the (a, c-f) as-grown S-MWCNTs and (b) MWCNTs thermally annealed at 2900 °C [52]. Arrows indicate the position of structural defects such as (c) *kinks and bends*, (d, e) *discontinuous flaws* (i.e., discontinuity in nanotube layers and *voids and holes*), and (f) *remnant catalyst*.

force-sensing elements, and the force constants of each were obtained in situ prior to the tensile tests using the resonance method developed by Sader et al. [58]. An individual MWCNT was clamped onto the cantilever tip by local electron-beam-induced deposition (EBID) of a carbonaceous material [59]. The applied force can be calculated as follows (**Figure 1d**):

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**Figure 3.** Raman intensity ratio as a function of MWCNT annealing temperature [52, 53].

*F* = *k*(Δ*x* − Δ*L*) (1)

where *k* is the force constant, Δ*x* is the displacement of the cantilever, and Δ*L* is the nanotube elongation. The nanotube elongation was determined by counting the number of pixels in the acquired SEM images. The movement rate of the XY motion stage of the manipulator for the tensile tests was approximately 0.1 μm/s. After the MWCNT broke, both cantilevers with attached MWCNT fragments were transferred to a TEM sample stage and examined in the TEM to determine the outer diameters. We measured the full cross-sectional area, including the inner hole of each of the broken MWCNTs, using TEM and used the measured values to

The fracture morphology of MWCNTs is divided into two groups: the complete fracture of nanotube walls (*clean break* or *sword and sheath failure* mode) and *sword-in-sheath failure* mode, which depends on their crystallinity and existence of structural defects. Two series of SEM and TEM images for each of the two individual H-MWCNTs, captured before and after breaking, are shown in **Figure 5**. In the first series (**Figure 5A1**–**A5**), a H-MWCNT annealed at 1800°C with a gauge length of 13.1 μm was clamped onto the cantilever and tungsten wire tips by local EBID and then loaded in increments until failure. After breaking, the fragment of the same MWCNT attached to the cantilever tip had a length of 13.3 μm (**Figure 5A2** and **A4**). The other fragment of the same MWCNT attached to the tungsten wire had a length of at least 0.2 μm (**Figure 5A3**

calculate the tensile strength.


**Table 1.** Structural defect densities for three types of S-MWCNTs [52].

(*R* = 23.3). This result suggests that defects in the structure of the MWCNTs were removed by annealing to produce a more stable graphite planar structure.

#### **3. Tensile properties**

#### **3.1. Fracture behavior**

Uniaxial tensile tests on individual MWCNTs were carried out with a manipulator inside the vacuum chamber of a SEM (JEOL JSM6510), as shown in **Figure 4**. Further details of the experimental procedure are described elsewhere [29]. In brief, AFM cantilevers served as

**Figure 3.** Raman intensity ratio as a function of MWCNT annealing temperature [52, 53].

(*R* = 23.3). This result suggests that defects in the structure of the MWCNTs were removed by

**Figure 2.** TEM images of the (a, c-f) as-grown S-MWCNTs and (b) MWCNTs thermally annealed at 2900 °C [52]. Arrows indicate the position of structural defects such as (c) *kinks and bends*, (d, e) *discontinuous flaws* (i.e., discontinuity in

**Annealing temperature (°C) Kinks and bends (/μm) Discontinuous flaws (/μm) Remnant catalysts (/μm)**

As-grown 1.9 6.4 1.2 2400 1.1 2.5 0 2900 1.6 1.7 0

Uniaxial tensile tests on individual MWCNTs were carried out with a manipulator inside the vacuum chamber of a SEM (JEOL JSM6510), as shown in **Figure 4**. Further details of the experimental procedure are described elsewhere [29]. In brief, AFM cantilevers served as

annealing to produce a more stable graphite planar structure.

**Table 1.** Structural defect densities for three types of S-MWCNTs [52].

nanotube layers and *voids and holes*), and (f) *remnant catalyst*.

**3. Tensile properties**

94 Carbon Nanotubes - Recent Progress

**3.1. Fracture behavior**

force-sensing elements, and the force constants of each were obtained in situ prior to the tensile tests using the resonance method developed by Sader et al. [58]. An individual MWCNT was clamped onto the cantilever tip by local electron-beam-induced deposition (EBID) of a carbonaceous material [59]. The applied force can be calculated as follows (**Figure 1d**):

$$F = k(\Delta \mathbf{x} - \Delta L) \tag{1}$$

where *k* is the force constant, Δ*x* is the displacement of the cantilever, and Δ*L* is the nanotube elongation. The nanotube elongation was determined by counting the number of pixels in the acquired SEM images. The movement rate of the XY motion stage of the manipulator for the tensile tests was approximately 0.1 μm/s. After the MWCNT broke, both cantilevers with attached MWCNT fragments were transferred to a TEM sample stage and examined in the TEM to determine the outer diameters. We measured the full cross-sectional area, including the inner hole of each of the broken MWCNTs, using TEM and used the measured values to calculate the tensile strength.

The fracture morphology of MWCNTs is divided into two groups: the complete fracture of nanotube walls (*clean break* or *sword and sheath failure* mode) and *sword-in-sheath failure* mode, which depends on their crystallinity and existence of structural defects. Two series of SEM and TEM images for each of the two individual H-MWCNTs, captured before and after breaking, are shown in **Figure 5**. In the first series (**Figure 5A1**–**A5**), a H-MWCNT annealed at 1800°C with a gauge length of 13.1 μm was clamped onto the cantilever and tungsten wire tips by local EBID and then loaded in increments until failure. After breaking, the fragment of the same MWCNT attached to the cantilever tip had a length of 13.3 μm (**Figure 5A2** and **A4**). The other fragment of the same MWCNT attached to the tungsten wire had a length of at least 0.2 μm (**Figure 5A3**

**Figure 4.** (a, b) Nanomanipulator system used for tensile tests on individual MWCNTs. (c) A SEM image of two AFM cantilever tips holding a MWCNT, which is attached at both ends to the AFM silicon tip surface by electron beam deposition of carbonaceous material. (d) Schematic description of cantilever displacement during the tensile test.

and **A5**), indicating that the MWCNT underwent failure in the so-called clean break manner (**Figure 6A1**–**A3**), as observed for CVD-grown MWCNTs under tensile loading [38]. Similar *clean break*-type failure was observed for H-MWCNTs annealed at 1200 and 2200°C. In contrast, the H-MWCNTs annealed at 2600°C broke in the outer walls, and the inner core was pulled away from the outer walls (**Figure 5B1**–**B3**), i.e., they underwent failure in a *sword-in-sheath failure* mode (**Figure 6B1**–**B3**), as observed for arc discharge-grown MWCNTs under tensile loading [11, 37]. An 11.1-μm-long section of this MWCNT was loaded and fractured in the middle of the gauge length. The resulting fragment attached to the cantilever tip had a length of at least 2.4 μm (**Figure 5B2**), whereas the other fragment on the tungsten wire had a length of at least 10.6 μm (**Figure 5B3**). Thus, the sum of the fragment lengths far exceeded the original section length. This apparent discrepancy can be explained as resulting from a *sword-in-sheath*-type failure.

the other fragment of the same MWCNT attached to the force-sensing cantilever had a length of approximately 3.8 μm (**Figure 7b2**). Thus, the sum of the fragment lengths (4.3 μm) exceeded the original section length. However, the length of the sword part of the nanotube (1.1 μm, **Figure 7c**) was shorter than that of the MWCNT attached to the high-force constant cantilever tip (1.9 μm). This suggests that the inner walls may break at positions that are far away from the outer walls, as shown in **Figure 7d**. This behavior can explain the *sword and sheath failure*. Of the 23 tested as-grown S-MWCNTs, 9 MWCNTs fractured as a *clean break*,

**Figure 6.** Schematic description of possible fracture mechanisms of MWCNTs annealed at different temperatures [53].

**Figure 5.** Two series of SEM and TEM images for each of two individual H-MWCNTs, captured before and after their

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breaking [53]. Annealing temperatures are (A1–A5) 1800°C and (B1–B3) 2600°C.

and the remaining 14 MWCNTs failed leaving a *sword and sheath failure*.

Shown are examples of (A1–A3) *clean break*-type failure and (B1–B3) *sword-in-sheath*-type failure.

Next, SEM and TEM images of an individual as-grown S-MWCNT linked between two opposing AFM cantilever tips before and after tensile loading are shown in **Figure 7**. An as-grown S-MWCNT having a gauge length of 3.2 μm was clamped onto the cantilevers and then loaded in increments until failure. After breaking, the fragment of the same MWCNT attached to the high-force constant cantilever tip had a length of 0.5 μm (**Figure 7b1**), while

**Figure 5.** Two series of SEM and TEM images for each of two individual H-MWCNTs, captured before and after their breaking [53]. Annealing temperatures are (A1–A5) 1800°C and (B1–B3) 2600°C.

and **A5**), indicating that the MWCNT underwent failure in the so-called clean break manner (**Figure 6A1**–**A3**), as observed for CVD-grown MWCNTs under tensile loading [38]. Similar *clean break*-type failure was observed for H-MWCNTs annealed at 1200 and 2200°C. In contrast, the H-MWCNTs annealed at 2600°C broke in the outer walls, and the inner core was pulled away from the outer walls (**Figure 5B1**–**B3**), i.e., they underwent failure in a *sword-in-sheath failure* mode (**Figure 6B1**–**B3**), as observed for arc discharge-grown MWCNTs under tensile loading [11, 37]. An 11.1-μm-long section of this MWCNT was loaded and fractured in the middle of the gauge length. The resulting fragment attached to the cantilever tip had a length of at least 2.4 μm (**Figure 5B2**), whereas the other fragment on the tungsten wire had a length of at least 10.6 μm (**Figure 5B3**). Thus, the sum of the fragment lengths far exceeded the original section length. This

**Figure 4.** (a, b) Nanomanipulator system used for tensile tests on individual MWCNTs. (c) A SEM image of two AFM cantilever tips holding a MWCNT, which is attached at both ends to the AFM silicon tip surface by electron beam deposition of carbonaceous material. (d) Schematic description of cantilever displacement during the tensile test.

96 Carbon Nanotubes - Recent Progress

apparent discrepancy can be explained as resulting from a *sword-in-sheath*-type failure.

Next, SEM and TEM images of an individual as-grown S-MWCNT linked between two opposing AFM cantilever tips before and after tensile loading are shown in **Figure 7**. An as-grown S-MWCNT having a gauge length of 3.2 μm was clamped onto the cantilevers and then loaded in increments until failure. After breaking, the fragment of the same MWCNT attached to the high-force constant cantilever tip had a length of 0.5 μm (**Figure 7b1**), while

**Figure 6.** Schematic description of possible fracture mechanisms of MWCNTs annealed at different temperatures [53]. Shown are examples of (A1–A3) *clean break*-type failure and (B1–B3) *sword-in-sheath*-type failure.

the other fragment of the same MWCNT attached to the force-sensing cantilever had a length of approximately 3.8 μm (**Figure 7b2**). Thus, the sum of the fragment lengths (4.3 μm) exceeded the original section length. However, the length of the sword part of the nanotube (1.1 μm, **Figure 7c**) was shorter than that of the MWCNT attached to the high-force constant cantilever tip (1.9 μm). This suggests that the inner walls may break at positions that are far away from the outer walls, as shown in **Figure 7d**. This behavior can explain the *sword and sheath failure*. Of the 23 tested as-grown S-MWCNTs, 9 MWCNTs fractured as a *clean break*, and the remaining 14 MWCNTs failed leaving a *sword and sheath failure*.

**Figure 7.** SEM images of S-MWCNT (a) before and (b) after the tensile test [54]. TEM image of the broken MWCNT is indicated in (c). TEM image in (c) shows the tips of the MWCNT that failed leaving the sword and sheath failure. (d) Schematic description of sword and sheath failure.

In addition to the evaluation of the fracture behavior of the MWCNTs mentioned above, we identified the fracture locations of individual S-MWCNTs broken in the uniaxial tensile tests using a piezo manipulator inside the vacuum chamber of the SEM and TEM. Of the five tested MWCNTs, three MWCNTs underwent failure at a *discontinuous flaw*, and the remaining two MWCNTs fractured at a *kink and bend*. A series of TEM images for an individual as-grown S-MWCNT, captured before and after breaking, are shown in **Figure 8**. We found that the fracture of this MWCNT occurred at a *kink and bend* structure and occurred as a *clean break*. **Figure 9** shows the characteristic features observed in a MWCNT, which fractured at a *discontinuous flaw*. The MWCNT fractured leaving a *sword and sheath failure* (**Figure 7d**). Furthermore, this MWCNT featured *hole defects* on the surface of its outer wall and the fracture occurred at a *hole defect.* This finding suggests that the fracture properties of such MWCNTs are dominated by the aforementioned structural defects.

#### **3.2. Mechanical properties**

The dependences of the nominal tensile strength upon the fracture cross section ratio and Raman intensity ratio are shown in **Figure 10**. The fracture cross section ratio was calculated by dividing the fractured cross-sectional area by the full cross-sectional area of the MWCNTs, including the inner hole. A higher fracture cross section ratio (for a given outer diameter) means a larger number of fractured walls in the MWCNT. It can

be seen from **Figure 10a** that the H-MWCNTs annealed at 1800 and 2200°C and the as-grown S-MWCNTs had an intermediate level of crystallinity, as measured from the Raman intensity ratio (*R* = 1.4–4.9), compared with that of the H-MWCNTs thermally

**Figure 9.** TEM image of S-MWCNT fractured at the *discontinuous flaw* [52].

**Figure 8.** TEM images of S-MWCNT fractured at the *kink and bend* (a, b) before and (c, d) after the tensile test [52].

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**Figure 8.** TEM images of S-MWCNT fractured at the *kink and bend* (a, b) before and (c, d) after the tensile test [52].

**Figure 9.** TEM image of S-MWCNT fractured at the *discontinuous flaw* [52].

In addition to the evaluation of the fracture behavior of the MWCNTs mentioned above, we identified the fracture locations of individual S-MWCNTs broken in the uniaxial tensile tests using a piezo manipulator inside the vacuum chamber of the SEM and TEM. Of the five tested MWCNTs, three MWCNTs underwent failure at a *discontinuous flaw*, and the remaining two MWCNTs fractured at a *kink and bend*. A series of TEM images for an individual as-grown S-MWCNT, captured before and after breaking, are shown in **Figure 8**. We found that the fracture of this MWCNT occurred at a *kink and bend* structure and occurred as a *clean break*. **Figure 9** shows the characteristic features observed in a MWCNT, which fractured at a *discontinuous flaw*. The MWCNT fractured leaving a *sword and sheath failure* (**Figure 7d**). Furthermore, this MWCNT featured *hole defects* on the surface of its outer wall and the fracture occurred at a *hole defect.* This finding suggests that the fracture properties of such MWCNTs are dominated by the aforementioned

**Figure 7.** SEM images of S-MWCNT (a) before and (b) after the tensile test [54]. TEM image of the broken MWCNT is indicated in (c). TEM image in (c) shows the tips of the MWCNT that failed leaving the sword and sheath failure. (d)

The dependences of the nominal tensile strength upon the fracture cross section ratio and Raman intensity ratio are shown in **Figure 10**. The fracture cross section ratio was calculated by dividing the fractured cross-sectional area by the full cross-sectional area of the MWCNTs, including the inner hole. A higher fracture cross section ratio (for a given outer diameter) means a larger number of fractured walls in the MWCNT. It can

structural defects.

**3.2. Mechanical properties**

Schematic description of sword and sheath failure.

98 Carbon Nanotubes - Recent Progress

be seen from **Figure 10a** that the H-MWCNTs annealed at 1800 and 2200°C and the as-grown S-MWCNTs had an intermediate level of crystallinity, as measured from the Raman intensity ratio (*R* = 1.4–4.9), compared with that of the H-MWCNTs thermally annealed at 1200 and 2600°C (*R* values of 1.0 and 10.1, respectively). **Figure 10b** shows that the MWCNTs with an intermediate level of crystallinity produced complete fracture of the nanotube walls (*clean break* or *sword and sheath failure*) and exhibited higher nominal tensile strength. The slightly lower value of the fracture cross section ratio of the as-grown S-MWCNTs compared with that of the H-MWCNTs thermally annealed at 1800 and 2200°C was caused by the larger hollow core. Although the H-MWCNTs annealed at 1200°C failed in a *clean break* manner (**Figure 10b**), their nominal tensile strength was observed to be small (~1 GPa) because of their relatively low crystallinity (*R* = 1.0) which reflects their amorphous graphite structure and many defects. The amorphous structure might explain the low load-bearing ability of these MWCNTs. When the annealing temperature was 2600°C, an abrupt decrease in both the nominal tensile strength and the fracture cross section ratio was observed, although the crystallinity considerably increased (*R* = 10.1). This result can be attributed to a decrease in the number of cross-linking defects, i.e., interwall sp<sup>3</sup> bonding, owing to the high-temperature thermal annealing. The removal of cross-linking defects decreases the load transfer from the outer walls to the inner ones, resulting in multiwall-type *sword-in-sheath failure* (i.e., decrease of the fracture cross section ratio) and degradation of the load-bearing ability. These results suggest that improvements to the nominal tensile strength of MWCNTs might be achieved by inducing appropriate interactions between adjacent nanotube walls to enable sufficient load transfer to the MWCNT inner layers. This effect should be balanced to permit an adequate load transfer between the inner and outer walls to give *clean break* fractures. On the other hand, the nominal tensile strength for the thermally annealed S-MWCNTs at 2900°C shows nearly the same value as that of the as-grown S-MWCNTs, even though a slight decrease in the strength is observed for the thermally annealed S-MWCNTs. This result suggests that the structural changes observed over this annealing temperature range may not have a significant impact on the nominal tensile strength of the S-MWCNTs. The H-MWCNTs had small defects, such as vacancies, Stone-Wales defects, or cross-linking defects, but no *discontinuous flaws* and *kinks and bends*, as shown in the S-MWCNTs. As a result, thermal annealing led to a decrease in the small defects with concomitant increases in the nominal tensile strength (0.9 to 5.9 GPa). On the other hand, although the as-grown S-MWCNTs exhibit an intermediate level of crystallinity (*R* = 3.2), they include structural defects such as *discontinuous flaws* and *kinks and bends*. Such structural defects and *hole defects* were still observed in the thermally annealed S-MWCNTs (**Table 1**). Thus, the annealing treatments led to no major changes in the controlling defects and had no major effects on the fracture morphology and nominal tensile strength of the S-MWCNTs, despite their

A Weibull plot of the nominal tensile strength of the MWCNTs is shown in **Figure 11**. Supplementing the experimental results of this study, **Figure 11** also gives some results evaluated using data from the literature for previously reported CVD-grown MWCNTs and arc discharge-grown MWCNTs [37, 38, 40, 52, 53]. These nominal values were calculated based on the literature values. **Table 2** shows the outer diameter, nominal tensile strength, and Weibull scale and shape parameters of the MWCNTs. As with other brittle materials, the strength distribution of CNTs does not follow a Gaussian distribution, and failure of nanotubes is described by Weibull-Poisson statistics. If *σ*nom is the failure strength of a nanotube, the cumulative distribution function *f*(*σ*nom) for the two-parameter Weibull distribution

(*σ*nom) = 1 − exp[−(

where *f*(*σ*nom) is the probability of failure, *α* is the scale parameter, and *β* is the shape parameter. The *β* values of the MWCNTs are calculated to be in the range of 1.6–3.0. The comparatively low value of the shape parameter for MWCNTs indicates a wide variability in their tensile strength, more so than that of carbon fibers [60] and SiC fibers [61] (*β* = 15 and 7–11, respectively). This may result from the irregular nanotube structure, which reflects a larger

In this chapter, we reviewed the nominal tensile strength and Weibull scale and shape parameters of the nominal tensile strength distribution of MWCNTs based on our recent previous studies. The comparatively low value of the shape parameter for MWCNTs resulted

*σ* \_\_\_\_ nom *α* ) *β*

] (2)

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high crystallinity.

is defined as [39]:

**4. Conclusions**

*f*

tube defect density relative to carbon fibers and SiC fibers.

**3.3. Weibull distribution**

**Figure 10.** Nominal tensile strength of the MWCNTs as a function of (a) Raman intensity ratio and (b) fracture cross section ratio [52].

of the as-grown S-MWCNTs, even though a slight decrease in the strength is observed for the thermally annealed S-MWCNTs. This result suggests that the structural changes observed over this annealing temperature range may not have a significant impact on the nominal tensile strength of the S-MWCNTs. The H-MWCNTs had small defects, such as vacancies, Stone-Wales defects, or cross-linking defects, but no *discontinuous flaws* and *kinks and bends*, as shown in the S-MWCNTs. As a result, thermal annealing led to a decrease in the small defects with concomitant increases in the nominal tensile strength (0.9 to 5.9 GPa). On the other hand, although the as-grown S-MWCNTs exhibit an intermediate level of crystallinity (*R* = 3.2), they include structural defects such as *discontinuous flaws* and *kinks and bends*. Such structural defects and *hole defects* were still observed in the thermally annealed S-MWCNTs (**Table 1**). Thus, the annealing treatments led to no major changes in the controlling defects and had no major effects on the fracture morphology and nominal tensile strength of the S-MWCNTs, despite their high crystallinity.

#### **3.3. Weibull distribution**

annealed at 1200 and 2600°C (*R* values of 1.0 and 10.1, respectively). **Figure 10b** shows that the MWCNTs with an intermediate level of crystallinity produced complete fracture of the nanotube walls (*clean break* or *sword and sheath failure*) and exhibited higher nominal tensile strength. The slightly lower value of the fracture cross section ratio of the as-grown S-MWCNTs compared with that of the H-MWCNTs thermally annealed at 1800 and 2200°C was caused by the larger hollow core. Although the H-MWCNTs annealed at 1200°C failed in a *clean break* manner (**Figure 10b**), their nominal tensile strength was observed to be small (~1 GPa) because of their relatively low crystallinity (*R* = 1.0) which reflects their amorphous graphite structure and many defects. The amorphous structure might explain the low load-bearing ability of these MWCNTs. When the annealing temperature was 2600°C, an abrupt decrease in both the nominal tensile strength and the fracture cross section ratio was observed, although the crystallinity considerably increased (*R* = 10.1). This result can be attributed to a decrease in the

ture thermal annealing. The removal of cross-linking defects decreases the load transfer from the outer walls to the inner ones, resulting in multiwall-type *sword-in-sheath failure* (i.e., decrease of the fracture cross section ratio) and degradation of the load-bearing ability. These results suggest that improvements to the nominal tensile strength of MWCNTs might be achieved by inducing appropriate interactions between adjacent nanotube walls to enable sufficient load transfer to the MWCNT inner layers. This effect should be balanced to permit an adequate load transfer between the inner and outer walls to give *clean break* fractures. On the other hand, the nominal tensile strength for the thermally annealed S-MWCNTs at 2900°C shows nearly the same value as that

0 2.

Sword-in-sheath failure

0

**Figure 10.** Nominal tensile strength of the MWCNTs as a function of (a) Raman intensity ratio and (b) fracture cross

2

4

6

Nominal tensile strength (GPa)

8

10

S-MWCNT As-grown 2900*䉝* H-MWCNT 1200*䉝* 1800*䉝* 2200*䉝* 2600*䉝*

0.4 . 0 6 0.8

Fracture cross section ratio (-)

*R*=10.1

1.0

*R*=4.9

*R*=1.4

*R*=1.0

*R*=23.3

*R*=3.2

Clean break

bonding, owing to the high-tempera-

number of cross-linking defects, i.e., interwall sp<sup>3</sup>

100 Carbon Nanotubes - Recent Progress

S-MWCNT

a b

 As-grown 2900*䉝*

H-MWCNT 1200*䉝* 1800*䉝* 2200*䉝* 2600*䉝*

1 10

*R 㻔I* <sup>G</sup> value /*I*

D*㻕*

Sword-in-sheath failure

0

section ratio [52].

2

4

6

Nominal tensile strength (GPa)

8

10

A Weibull plot of the nominal tensile strength of the MWCNTs is shown in **Figure 11**. Supplementing the experimental results of this study, **Figure 11** also gives some results evaluated using data from the literature for previously reported CVD-grown MWCNTs and arc discharge-grown MWCNTs [37, 38, 40, 52, 53]. These nominal values were calculated based on the literature values. **Table 2** shows the outer diameter, nominal tensile strength, and Weibull scale and shape parameters of the MWCNTs. As with other brittle materials, the strength distribution of CNTs does not follow a Gaussian distribution, and failure of nanotubes is described by Weibull-Poisson statistics. If *σ*nom is the failure strength of a nanotube, the cumulative distribution function *f*(*σ*nom) for the two-parameter Weibull distribution is defined as [39]:

$$f\_{\left(\vartheta\_{\text{un}}\right)} = 1 - \exp\left[-\left(\frac{\sigma\_{\text{nom}}}{\alpha}\right)^{\beta}\right] \tag{2}$$

where *f*(*σ*nom) is the probability of failure, *α* is the scale parameter, and *β* is the shape parameter. The *β* values of the MWCNTs are calculated to be in the range of 1.6–3.0. The comparatively low value of the shape parameter for MWCNTs indicates a wide variability in their tensile strength, more so than that of carbon fibers [60] and SiC fibers [61] (*β* = 15 and 7–11, respectively). This may result from the irregular nanotube structure, which reflects a larger tube defect density relative to carbon fibers and SiC fibers.

#### **4. Conclusions**

In this chapter, we reviewed the nominal tensile strength and Weibull scale and shape parameters of the nominal tensile strength distribution of MWCNTs based on our recent previous studies. The comparatively low value of the shape parameter for MWCNTs resulted

tensile strength, suggesting that there is an optimal nanotube defect density for increasing the nominal tensile strength, not too low but also not too high, so as to permit an adequate load transfer between the nanotube walls. To improve the properties of macroscopic CNT composite performance, the structure and properties of MWCNT yarns and sheets must be optimized at all hierarchical levels: from individual MWCNTs to MWCNT bundles, MWCNT networks, and MWCNT yarns and sheets. Future research efforts aimed at each of the following levels should be pursued to improve mechanical properties, particularly the nominal tensile strength of CVD-grown MWCNTs: (1) improved synthesis methods should be developed to reduce structural defects such as *discontinuous flaws* and *kinks and bends*; and (2) the degree of inter-wall cross-linking and load transfer between adjacent nanotube walls should be optimized by posttreatments, such as thermal annealing and electron irradiation. We believe that the above improvements might enable the realization of higher nominal tensile strength. More well-defined CNT architectures should contribute to enhanced mechanical properties as well as improved the electrical and thermal properties of MWCNT yarns

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The authors thank Toyo Tanso Co., Ltd. for its technical assistance in thermal annealing of the MWCNTs. The authors acknowledge Prof. Y. Inoue and Prof. Y. Shimamura of Shizuoka University for their useful guidance; Dr. T. Miyazaki of the Technical Division, School of Engineering, Tohoku University, for technical assistance in the TEM analysis; Prof. K. Hirahara of Osaka University and R. Bekarevich of the National Institute for Material Science for technical assistance in the tensile testing; and our colleague, Mr. I. Tamaki of the Fracture and Reliability Research Institute (FRRI), Tohoku University, for helpful discussions. We thank Joshua Green, MS, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. This research was supported in part by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program. This research was partially supported by a Grant-in-Aid for Young Scientists (B) 16 K20904 and a Grant-in-Aid for Young

and composites.

**Acknowledgements**

Scientists (A) 15H05502.

\*, Go Yamamoto2

3 Univ Lyon, INSA-Lyon, Villeurbanne, France

, Daniel Nelias<sup>3</sup>

1 Fracture and Reliability Research Institute, Tohoku University, Sendai, Japan 2 Department of Aerospace Engineering, Tohoku University, Sendai, Japan

\*Address all correspondence to: keiichi.shirasu@rift.mech.tohoku.ac.jp

and Toshiyuki Hashida<sup>1</sup>

**Author details**

Keiichi Shirasu<sup>1</sup>

**Figure 11.** Weibull plot for the nominal tensile strength of MWCNTs [52].


**Table 2.** Measured properties for MWCNTs. Shown are the outer diameter, nominal tensile strength, and Weibull scale and shape parameters. The outer diameter and nominal tensile strength data are presented as average values ± standard deviations.

from the irregular nanotube structure, which reflects a larger tube defect density relative to conventional fiber materials. Nonetheless, the MWCNTs with an intermediate level of crystallinity produced complete fracture of nanotube walls and exhibited higher nominal tensile strength, suggesting that there is an optimal nanotube defect density for increasing the nominal tensile strength, not too low but also not too high, so as to permit an adequate load transfer between the nanotube walls. To improve the properties of macroscopic CNT composite performance, the structure and properties of MWCNT yarns and sheets must be optimized at all hierarchical levels: from individual MWCNTs to MWCNT bundles, MWCNT networks, and MWCNT yarns and sheets. Future research efforts aimed at each of the following levels should be pursued to improve mechanical properties, particularly the nominal tensile strength of CVD-grown MWCNTs: (1) improved synthesis methods should be developed to reduce structural defects such as *discontinuous flaws* and *kinks and bends*; and (2) the degree of inter-wall cross-linking and load transfer between adjacent nanotube walls should be optimized by posttreatments, such as thermal annealing and electron irradiation. We believe that the above improvements might enable the realization of higher nominal tensile strength. More well-defined CNT architectures should contribute to enhanced mechanical properties as well as improved the electrical and thermal properties of MWCNT yarns and composites.

#### **Acknowledgements**

The authors thank Toyo Tanso Co., Ltd. for its technical assistance in thermal annealing of the MWCNTs. The authors acknowledge Prof. Y. Inoue and Prof. Y. Shimamura of Shizuoka University for their useful guidance; Dr. T. Miyazaki of the Technical Division, School of Engineering, Tohoku University, for technical assistance in the TEM analysis; Prof. K. Hirahara of Osaka University and R. Bekarevich of the National Institute for Material Science for technical assistance in the tensile testing; and our colleague, Mr. I. Tamaki of the Fracture and Reliability Research Institute (FRRI), Tohoku University, for helpful discussions. We thank Joshua Green, MS, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. This research was supported in part by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program. This research was partially supported by a Grant-in-Aid for Young Scientists (B) 16 K20904 and a Grant-in-Aid for Young Scientists (A) 15H05502.

#### **Author details**

from the irregular nanotube structure, which reflects a larger tube defect density relative to conventional fiber materials. Nonetheless, the MWCNTs with an intermediate level of crystallinity produced complete fracture of nanotube walls and exhibited higher nominal

**Table 2.** Measured properties for MWCNTs. Shown are the outer diameter, nominal tensile strength, and Weibull scale and shape parameters. The outer diameter and nominal tensile strength data are presented as average values ± standard


ln nom, GPa

**Nominal tensile strength (GPa)**

Arc-discharge-grown 25 ± 7 1.6 ± 1.0 1.9 2.0 [37] CVD-grown 97 ± 25 1.5 ± 1.1 1.7 1.6 [38] Arc-discharge-grown 11 ± 3 3.4 ± 2.6 3.8 1.6 [40] S-MWCNT (CVD, as-grown) 36 ± 7 5.2 ± 2.1 5.9 2.7 [52] S-MWCNT (CVD, 2900°C) 35 ± 6 3.8 ± 1.6 4.4 2.4 [52] H-MWCNT (CVD, 1200°C) 90 ± 40 0.9 ± 0.5 1.1 1.6 [53] H-MWCNT (CVD, 1800°C) 72 ± 21 6.0 ± 2.7 6.8 2.1 [53] H-MWCNT (CVD, 2200°C) 69 ± 16 5.8 ± 2.0 6.6 3.0 [53] H-MWCNT (CVD, 2600°C) 67 ± 12 2.0 ± 1.1 2.3 2.0 [53]

**Scale parameter,**  *α* **(GPa)**

**Figure 11.** Weibull plot for the nominal tensile strength of MWCNTs [52].

**(nm)**

**MWCNT type Outer diameter** 

CVD-grown MWCNTs

**Shape parameter,** 

**Ref.**

*β* **(−)**

 Barber et al., 2005 [38] S-MWCNT (As-grown) [52] S-MWCNT 2900 [52] H-MWCNT 1200 [53] H-MWCNT 1800 [53] H-MWCNT 2200 [53] H-MWCNT 2600 [53] Arc discharge-grown MWCNTs Yu et al., 2000 [37] Ding et al., 2007 [40]


deviations.



ln -ln 1-*f* 


0

nom

1

2

102 Carbon Nanotubes - Recent Progress

Keiichi Shirasu<sup>1</sup> \*, Go Yamamoto2 , Daniel Nelias<sup>3</sup> and Toshiyuki Hashida<sup>1</sup>


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**Chapter 7**

**Provisional chapter**

**Charged Carbon Nanotubes**

**Charged Carbon Nanotubes**

Devashree Atre and Shashank Pant

Devashree Atre and Shashank Pant

http://dx.doi.org/10.5772/intechopen.70480

confined water, density fluctuations

**Abstract**

CNTs.

**1. Introduction**

car parts etc. [2, 3]

**1.1. What are carbon nanotubes?**

broadly divided into two types:

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

DOI: 10.5772/intechopen.70480

As the degree of functionalization on CNTs greatly affects its properties, the structure and dynamics of water confined inside pristine and functionalized/charged carbon nanotubes (CNTs) is of prime importance. The presence of charges on the surface of CNTs results in hydrophobic to hydrophilic transitions which increase its occupancy of the water molecules thereby breaking down 1D water wires, as seen in pristine

Carbon nanotubes (CNTs) are carbon allotropes having a cylindrical nanostructure. This nanostructure has unusual properties which increases its applications in the field of biology, electronics, water desalination, material science and optics. This material has significantly higher stiffness and exceptional strength. CNTs have been constructed in a wide range of diameter to length ratio [1]. In addition to the mechanical strength and stiffness, it has high thermal conductivity and wide range of electrical properties. Owing to this extremely high strength, CNTs have found a place in the materials used for designing bats,

**Keywords:** carbon nanotubes, breakdown of 1D water wires, functionalization,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

CNTs belong to the fullerene family and the name is derived from their long and hollow structure where walls are formed by carbon atoms sheets called graphene [4]. These graphene sheets are rolled at a specific angle, which decides the properties of CNTs. Nanotubes can be


#### **Chapter 7**

**Provisional chapter**

## **Charged Carbon Nanotubes**

**Charged Carbon Nanotubes**

#### Devashree Atre and Shashank Pant Devashree Atre and Shashank Pant Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70480

[55] Inoue Y, Kakihata K, Hirono Y, Horie T, Ishida A, Mimura H. One-step grown aligned bulk carbon nanotubes by chloride mediated chemical vapor deposition. Applied

[56] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Raman spectroscopy of carbon nano-

[57] Kataura H, Kumazawa Y, Maniwa Y, Ohtsuka Y, Sen R, Suzuki S, Achiba Y. Diameter

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[60] Okabe T, Takeda N. Elastoplastic shear-lag analysis of single-fiber composites and strength prediction of unidirectional multi-fiber composites. Composites Part A: Applied

[61] Curtin WA, Netravali AN, Park JM. Strength distribution of Carborundum polycrystalline SiC fibres as derived from the single-fibre-composite test. Journal of Materials

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paraffin precursor. Journal of Applied Physics. 2005;**98**:014905

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Science. 1994;**29**:4718-4728

Physics Letters. 2008;**92**:213113

108 Carbon Nanotubes - Recent Progress

tubes. Physics Reports. 2005;**409**:47-99

#### **Abstract**

As the degree of functionalization on CNTs greatly affects its properties, the structure and dynamics of water confined inside pristine and functionalized/charged carbon nanotubes (CNTs) is of prime importance. The presence of charges on the surface of CNTs results in hydrophobic to hydrophilic transitions which increase its occupancy of the water molecules thereby breaking down 1D water wires, as seen in pristine CNTs.

DOI: 10.5772/intechopen.70480

**Keywords:** carbon nanotubes, breakdown of 1D water wires, functionalization, confined water, density fluctuations

#### **1. Introduction**

#### **1.1. What are carbon nanotubes?**

Carbon nanotubes (CNTs) are carbon allotropes having a cylindrical nanostructure. This nanostructure has unusual properties which increases its applications in the field of biology, electronics, water desalination, material science and optics. This material has significantly higher stiffness and exceptional strength. CNTs have been constructed in a wide range of diameter to length ratio [1]. In addition to the mechanical strength and stiffness, it has high thermal conductivity and wide range of electrical properties. Owing to this extremely high strength, CNTs have found a place in the materials used for designing bats, car parts etc. [2, 3]

CNTs belong to the fullerene family and the name is derived from their long and hollow structure where walls are formed by carbon atoms sheets called graphene [4]. These graphene sheets are rolled at a specific angle, which decides the properties of CNTs. Nanotubes can be broadly divided into two types:

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons


Individual CNTs can align themselves by pi-pi stacking and Van der Waals interactions. In more chemical terms, carbon nanotubes bond due to sp2 hybrid carbon atoms. These bonds are stronger than the alkanes and provide CNTs which extremely high in strength. A typical carbon nanotube is show in **Figure 1**.

The wrapping of the graphene sheets is represented by a pair of indices (n,m) where n and m denote the number of unit vector in the crystal structure of graphene as shown in **Figure 2**.

MWNTs can consist of many rolled up graphene sheets and the process can be done either by using concentric single walled nanotubes or by rolling up a single graphene sheet around itself. Recently, many modifications of CNTs morphology have been reported [5–7].

#### **1.2. Application of carbon nanotubes**

Owning to its stiffness, mechanical strength, conductivity and structure carbon nanotubes have widespread applications in the area of sports, transistors, material science, biology etc. [8–14]. The Easton-Bell Sports Inc. has been using CNT, in partnership with Zyvex Performance Materials, in designing bicycle components like handlebars, seatposts, crank etc. Owning to the nanometer sized diameter and tunable length which can reach up to a few centimeters CNTs can be a drug vehicle for therapy purposes [15]. The sp2 hybridized orbitals and a large porous surface area can be easily exploited to load large quantity of drugs inside the tube. Not only drug but specific molecules which can increases the targeting ability can be easily loaded in the nanotubes. Drugs like doxorubicin (DOX) can be absorbed on the CNT surface by pi-pi stacking [16], this type of loading is usually pH dependent thus proving a handle to release drugs.

In addition to the drug delivery the optical properties of CNTs provide NIR (Non Infrared Radiation) region of absorption [17, 18]. In contrast, biological tissues and cells provide maximum transmission in this range, thus making it a suitable candidate for bio-imaging, tracing, diagnosis etc. If chemotherapeutic drugs are combined with CNT optical properties it might increase the treatment effectiveness. The low toxicity of CNTs makes it an utmost important candidate for biomedical applications. We can tune the toxicity of the CNTs by varying the length, aggregation, surface functionalization etc. thus, care must be taken before using them. But the usual MTT assays cannot be used for testing the cytotoxicity of CNTs because MTTformazan crystals might stick to the surface of the CNTs thereby making it less soluble in

**Figure 2.** The (n,m) naming scheme of nanotubes can be thought of as a vector C in an infinite graphene sheet; this will describe how to roll up the sheet to form the nanotube. Also, a1 and a2 denotes the unit vector for the graphene.

Charged Carbon Nanotubes

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http://dx.doi.org/10.5772/intechopen.70480

Carbon nanotubes serve as a robust material for water desalination and in water decontamination. The CNT hollow pores provide frictionless transport of water molecules and the tunable diameter can be used for removing unwanted particles. The functionalized CNTs can also be employed to selectively remove particular particles by specific or non-specific interactions. Thus, CNTs can be used as a "Gate-Keeper" molecule [20–27]. The smooth hydrophobic walls and the pore diameter of CNTs allow ultra-efficient water transport. A prototype of

The chemical modifications of carbon nanotubes (CNTs) are an emerging area of research as it provides a handle to tune the solubilization [28] of CNTs. The reaction for the modification

isopropanol and DMSO [19].

CNT membrane is shown in **Figure 3**.

**1.3. Functionalization of carbon nanotubes**

of CNTs can be broadly divided into two areas:

**Figure 1.** A typical carbon nanotube (a) side view and (b) showing the carbon nanotube pore.

**Figure 2.** The (n,m) naming scheme of nanotubes can be thought of as a vector C in an infinite graphene sheet; this will describe how to roll up the sheet to form the nanotube. Also, a1 and a2 denotes the unit vector for the graphene.

In addition to the drug delivery the optical properties of CNTs provide NIR (Non Infrared Radiation) region of absorption [17, 18]. In contrast, biological tissues and cells provide maximum transmission in this range, thus making it a suitable candidate for bio-imaging, tracing, diagnosis etc. If chemotherapeutic drugs are combined with CNT optical properties it might increase the treatment effectiveness. The low toxicity of CNTs makes it an utmost important candidate for biomedical applications. We can tune the toxicity of the CNTs by varying the length, aggregation, surface functionalization etc. thus, care must be taken before using them. But the usual MTT assays cannot be used for testing the cytotoxicity of CNTs because MTTformazan crystals might stick to the surface of the CNTs thereby making it less soluble in isopropanol and DMSO [19].

Carbon nanotubes serve as a robust material for water desalination and in water decontamination. The CNT hollow pores provide frictionless transport of water molecules and the tunable diameter can be used for removing unwanted particles. The functionalized CNTs can also be employed to selectively remove particular particles by specific or non-specific interactions. Thus, CNTs can be used as a "Gate-Keeper" molecule [20–27]. The smooth hydrophobic walls and the pore diameter of CNTs allow ultra-efficient water transport. A prototype of CNT membrane is shown in **Figure 3**.

#### **1.3. Functionalization of carbon nanotubes**

**1.** Single wall carbon nanotubes (SWNTs). **2.** Multi wall carbon nanotubes (MWNTs).

110 Carbon Nanotubes - Recent Progress

carbon nanotube is show in **Figure 1**.

**1.2. Application of carbon nanotubes**

handle to release drugs.

Individual CNTs can align themselves by pi-pi stacking and Van der Waals interactions. In more chemical terms, carbon nanotubes bond due to sp2 hybrid carbon atoms. These bonds are stronger than the alkanes and provide CNTs which extremely high in strength. A typical

The wrapping of the graphene sheets is represented by a pair of indices (n,m) where n and m denote the number of unit vector in the crystal structure of graphene as shown in **Figure 2**.

MWNTs can consist of many rolled up graphene sheets and the process can be done either by using concentric single walled nanotubes or by rolling up a single graphene sheet around

Owning to its stiffness, mechanical strength, conductivity and structure carbon nanotubes have widespread applications in the area of sports, transistors, material science, biology etc. [8–14]. The Easton-Bell Sports Inc. has been using CNT, in partnership with Zyvex Performance Materials, in designing bicycle components like handlebars, seatposts, crank etc. Owning to the nanometer sized diameter and tunable length which can reach up to a few centimeters CNTs can be a drug vehicle for therapy purposes [15]. The sp2 hybridized orbitals and a large porous surface area can be easily exploited to load large quantity of drugs inside the tube. Not only drug but specific molecules which can increases the targeting ability can be easily loaded in the nanotubes. Drugs like doxorubicin (DOX) can be absorbed on the CNT surface by pi-pi stacking [16], this type of loading is usually pH dependent thus proving a

itself. Recently, many modifications of CNTs morphology have been reported [5–7].

**Figure 1.** A typical carbon nanotube (a) side view and (b) showing the carbon nanotube pore.

The chemical modifications of carbon nanotubes (CNTs) are an emerging area of research as it provides a handle to tune the solubilization [28] of CNTs. The reaction for the modification of CNTs can be broadly divided into two areas:

In order, to solubilize CNTs attachment of large chemical group is needed. These larger chemical groups can be certain chemical compounds or can also be certain polymers. These chemical compounds wrap around the CNTs and hinder the formation of CNT bundles in the solvent, thereby solubilizing it in the solvent. The solubility of CNT can be easily modulated by varying the nature of the functional groups. As, the chemical modification [30–35] of CNT greatly alters the chemical and the electronic structure of the nanotubes, good attention has been paid to the non-covalent and non-specific modifications. This type of modification results in almost the same effect without altering the chemical and electronic nature of the

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Water is ubiquitous yet the most anomalous substance on earth, the origin of its anomalous properties is yet to be fully understood [36–42]. Water is tetrahedral in structure and it forms 4 hydrogen bonds in the fluid state and 6 hydrogen bonds in its solid state i.e. ICE. It is formed by 2 hydrogen and 1 oxygen molecule with the presence of two lone pairs on the oxygen. Due to polarization effect there is a presence of partial positive charge on the hydrogen atoms and partial negative charge on the oxygen atom. Due to its wide range of applications, water is a prime candidate under study for almost all the systems. Its structure along with its dynamic and thermodynamic properties of water confined under nanopores of carbon nanotubes has been widely studied. It has been observed that the properties of confined water are distinctly different from that of bulk or non-confined water. Despite of the hydrophobic nature of carbon nanotubes, it is occupied by 4–5 water molecules. The most interesting feature of water inside the 6,6 pristine CNT is the presence of 1 D water wires (1 dimensional) [43]. The water molecules entering the nanotube channel lose on average two hydrogen bonds and only a fraction of lost energy can be recovered by the VdW interactions between water and the nanotube. The presence of 1D water wire inside pristine

Further, extensive amount of work has been put in to study the structure of water confined inside carbon nanotubes. Molecular dynamics simulations suggested the presence of ice like structural ordering of water at room temperature [44–46]. The presence of ice like structures has been quantified by various matrices like the calculation of order parameter, radial distribution curves and snapshots of system. Before moving further, I would like to explain how one of the most important parameter, radial distribution works and what type of information we get from it. Radial distribution function or RDF gives us the structure of the system under study and it works under the assumption that the system should be radially symmetric. It gives us an idea about the variation of the density with respect to the distance. Although, it will not gives us the idea about the instantaneous density of the system but averaged density. In much coarse terms it gives us the probability of finding a particle at a distance of "r" from

nanotube. A typical scheme for modification is shown in **Figure 4**.

**2. Properties of solvent inside carbon nanotubes**

**2.1. Structure of water**

6,6 CNT is shown in **Figure 5**.

the central particle.

**Figure 3.** A water desalination setup where 4 CNTs form the membrane, yellow balls represents impurity and red-white molecules are the water. Movement of water through the CNT membrane is seen while impurity does not move through the membrane.


The fluorination [29] of CNTs has been extensively studied and this modification has resulted in the solvated individual CNTs in alcohol type solvents, while the second modification results in changes in the CNT properties as it might lead to the shortening of the nanotubes. Further different methods have been incorporated especially ester modification at the terminal carbon of CNTs. This modification can be easily removed by acid-base chemistry, thus enabling us to get back to the initial state of CNTs.

In order, to solubilize CNTs attachment of large chemical group is needed. These larger chemical groups can be certain chemical compounds or can also be certain polymers. These chemical compounds wrap around the CNTs and hinder the formation of CNT bundles in the solvent, thereby solubilizing it in the solvent. The solubility of CNT can be easily modulated by varying the nature of the functional groups. As, the chemical modification [30–35] of CNT greatly alters the chemical and the electronic structure of the nanotubes, good attention has been paid to the non-covalent and non-specific modifications. This type of modification results in almost the same effect without altering the chemical and electronic nature of the nanotube. A typical scheme for modification is shown in **Figure 4**.

#### **2. Properties of solvent inside carbon nanotubes**

#### **2.1. Structure of water**

**1.** Direct attachment of groups on the graphitic carbon atoms.

The fluorination [29] of CNTs has been extensively studied and this modification has resulted in the solvated individual CNTs in alcohol type solvents, while the second modification results in changes in the CNT properties as it might lead to the shortening of the nanotubes. Further different methods have been incorporated especially ester modification at the terminal carbon of CNTs. This modification can be easily removed by acid-base chemistry, thus enabling us to

**Figure 3.** A water desalination setup where 4 CNTs form the membrane, yellow balls represents impurity and red-white molecules are the water. Movement of water through the CNT membrane is seen while impurity does not move through

**2.** Modifications on the CNT bound- carboxylic acids.

get back to the initial state of CNTs.

the membrane.

112 Carbon Nanotubes - Recent Progress

**Figure 4.** Carbon nanotube modification scheme.

Water is ubiquitous yet the most anomalous substance on earth, the origin of its anomalous properties is yet to be fully understood [36–42]. Water is tetrahedral in structure and it forms 4 hydrogen bonds in the fluid state and 6 hydrogen bonds in its solid state i.e. ICE. It is formed by 2 hydrogen and 1 oxygen molecule with the presence of two lone pairs on the oxygen. Due to polarization effect there is a presence of partial positive charge on the hydrogen atoms and partial negative charge on the oxygen atom. Due to its wide range of applications, water is a prime candidate under study for almost all the systems. Its structure along with its dynamic and thermodynamic properties of water confined under nanopores of carbon nanotubes has been widely studied. It has been observed that the properties of confined water are distinctly different from that of bulk or non-confined water. Despite of the hydrophobic nature of carbon nanotubes, it is occupied by 4–5 water molecules. The most interesting feature of water inside the 6,6 pristine CNT is the presence of 1 D water wires (1 dimensional) [43]. The water molecules entering the nanotube channel lose on average two hydrogen bonds and only a fraction of lost energy can be recovered by the VdW interactions between water and the nanotube. The presence of 1D water wire inside pristine 6,6 CNT is shown in **Figure 5**.

Further, extensive amount of work has been put in to study the structure of water confined inside carbon nanotubes. Molecular dynamics simulations suggested the presence of ice like structural ordering of water at room temperature [44–46]. The presence of ice like structures has been quantified by various matrices like the calculation of order parameter, radial distribution curves and snapshots of system. Before moving further, I would like to explain how one of the most important parameter, radial distribution works and what type of information we get from it. Radial distribution function or RDF gives us the structure of the system under study and it works under the assumption that the system should be radially symmetric. It gives us an idea about the variation of the density with respect to the distance. Although, it will not gives us the idea about the instantaneous density of the system but averaged density. In much coarse terms it gives us the probability of finding a particle at a distance of "r" from the central particle.

**Figure 5.** Presence of 1 D water wires inside 6,6 pristine CNT. Hydrogen bonds are shown in black.

The structure (RDF) of confined water is shown in **Figure 6**. The figure clearly shows the presence of peaks at regular intervals which suggest a ordered structure. No such peak was observed in the case of bulk water. A typical snapshot of the simulations performed by Koga et al. is shown in **Figure 6(b)**. The presence of ordered water nanotubes was seen in the simulation at room temperature but under axial pressure, the presence of square, pentagonal and hexagonal water nanotubes was observed in (14, 14), (15,15) and (16,16) nanotubes.

and the absence of equally spaced peaks. Thus the presence of charge on the CNT surface although increases its solubility in water but greatly affects the properties of confined water. Now, an interesting question arises on the presence of 1D water wires inside functionalized CNTs. The very first study by Pant et al. clearly shows the breakdown of the 1D water wires rather the presence of 3 linear water wires inside such a confined space was observed as shown in **Figure 9**. The presence of 3 water wires was surprising because earlier the presence

axial pressure of 50 MPa in molecular dynamics simulations. Figure taken from Koga et al. [44].

**Figure 7.** Distribution of charges on CNT. The blue area shows the presence of charged carbon atoms.

**Figure 6.** (a) Radial distribution curve along the axis of 6,6 pristine CNT. It shows the presence of regular spaced peaks, (b) snapshots of quenched water co-ordinates in (a) square, (b) pentagonal, (c) hexagonal ice nanotubes in (14,14), (15,15) and (16,16) SWNTs, (d–f) shows the corresponding liquid phase. The ice nanotubes were formed upon cooling under

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These structural studies suggests the presence of well ordered ice like structure of water inside CNTs at room temperature. Further, this ice-like ordered structures were seen even at a high temperatures like 320 K. Very recently, there has been an effort to study the properties of confined water inside functionalized CNTs. The functionalization was done by incorporating charges on the surface of CNTs i.e. by placing charge on the carbon atoms as shown in **Figure 7**. The presence of charge results in decreasing the ordered ice like structure of water. The main reason for such disordering or absence of an ice-like ordered structure is the hydrophobic to hydrophilic transition in the CNT. Due to the presence of charges on the CNT surface, it does not behave like a hydrophobic wall and there is a loss in the ordering [47–52] as shown in **Figure 8**. It clearly shows the decrease in the peak intensity

**Figure 6.** (a) Radial distribution curve along the axis of 6,6 pristine CNT. It shows the presence of regular spaced peaks, (b) snapshots of quenched water co-ordinates in (a) square, (b) pentagonal, (c) hexagonal ice nanotubes in (14,14), (15,15) and (16,16) SWNTs, (d–f) shows the corresponding liquid phase. The ice nanotubes were formed upon cooling under axial pressure of 50 MPa in molecular dynamics simulations. Figure taken from Koga et al. [44].

and the absence of equally spaced peaks. Thus the presence of charge on the CNT surface although increases its solubility in water but greatly affects the properties of confined water.

Now, an interesting question arises on the presence of 1D water wires inside functionalized CNTs. The very first study by Pant et al. clearly shows the breakdown of the 1D water wires rather the presence of 3 linear water wires inside such a confined space was observed as shown in **Figure 9**. The presence of 3 water wires was surprising because earlier the presence

The structure (RDF) of confined water is shown in **Figure 6**. The figure clearly shows the presence of peaks at regular intervals which suggest a ordered structure. No such peak was observed in the case of bulk water. A typical snapshot of the simulations performed by Koga et al. is shown in **Figure 6(b)**. The presence of ordered water nanotubes was seen in the simulation at room temperature but under axial pressure, the presence of square, pentagonal and

These structural studies suggests the presence of well ordered ice like structure of water inside CNTs at room temperature. Further, this ice-like ordered structures were seen even at a high temperatures like 320 K. Very recently, there has been an effort to study the properties of confined water inside functionalized CNTs. The functionalization was done by incorporating charges on the surface of CNTs i.e. by placing charge on the carbon atoms as shown in **Figure 7**. The presence of charge results in decreasing the ordered ice like structure of water. The main reason for such disordering or absence of an ice-like ordered structure is the hydrophobic to hydrophilic transition in the CNT. Due to the presence of charges on the CNT surface, it does not behave like a hydrophobic wall and there is a loss in the ordering [47–52] as shown in **Figure 8**. It clearly shows the decrease in the peak intensity

hexagonal water nanotubes was observed in (14, 14), (15,15) and (16,16) nanotubes.

**Figure 5.** Presence of 1 D water wires inside 6,6 pristine CNT. Hydrogen bonds are shown in black.

114 Carbon Nanotubes - Recent Progress

**Figure 7.** Distribution of charges on CNT. The blue area shows the presence of charged carbon atoms.

Now, a question arises on how the degree of functionalization affects the confined water? In-order to answer this question we can increase the functionalization and also distribute the functional groups on the CNT surface in the form of various patterns. This can provide us with an easy handle to change the properties of the tube and it can even be use to mimic the

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A single permeation event is considered when a water molecule enters from one of CNT and leaves from the other end. The permeation or flow of water from the CNT has been extensively studied and enhanced water flow was observed. This can be related to the breaking of the hydrogen bonds i.e. in the pristine 6,6 CNT presence of 1D water wires was observed where each water molecule on average forms 1–2 hydrogen bonds which is in contrast with bulk water as it forms 4 hydrogen bonds. Thus the movement of water is less hindered inside the tube and it experiences less resistance from the neighboring water molecules. On, the other hand in the presence of functionalization there is a significant amount of decrease in the water flow. Due to the effect of functionalization, the nature of the tube changes from hydrophobic to hydrophilic and this transitions breaks the formation of 1D water wires, also presence of functional groups on the CNT surfaces results in the formation of hydrophilichydrophilic interactions between water and functional groups. These forces results in the

**Figure 11** clearly shows the decrease in the flow rate with the increase in the charge % on the surface of CNT. This can be further explained by the variation of hydrogen bonds between the confined water molecules because in the 6,6 pristine CNT, one water molecule on an average

properties of complex biological systems like aquaporins [53].

**2.2. Permeation, diffusion of water through carbon nanotubes**

decrease of water flow as shown in **Figure 10**.

**Figure 10.** Variation of flow with the charge %.

**Figure 8.** Decrease in the ordering of confined water inside CNT.

**Figure 9.** Presence of 3 water wires in functionalized CNT.

of 1D water wire in 6,6 pristine CNT was correlated to the diameter of the CNT and its hydrophobic nature of CNT. Although the functionalization of CNT does not change its diameter but it does changed its hydrophilic nature, thus it suggests the presence of 1D wires in pristine CNT is only dependent on its hydrophobic nature and not on its diameter.

Now, a question arises on how the degree of functionalization affects the confined water? In-order to answer this question we can increase the functionalization and also distribute the functional groups on the CNT surface in the form of various patterns. This can provide us with an easy handle to change the properties of the tube and it can even be use to mimic the properties of complex biological systems like aquaporins [53].

#### **2.2. Permeation, diffusion of water through carbon nanotubes**

A single permeation event is considered when a water molecule enters from one of CNT and leaves from the other end. The permeation or flow of water from the CNT has been extensively studied and enhanced water flow was observed. This can be related to the breaking of the hydrogen bonds i.e. in the pristine 6,6 CNT presence of 1D water wires was observed where each water molecule on average forms 1–2 hydrogen bonds which is in contrast with bulk water as it forms 4 hydrogen bonds. Thus the movement of water is less hindered inside the tube and it experiences less resistance from the neighboring water molecules. On, the other hand in the presence of functionalization there is a significant amount of decrease in the water flow. Due to the effect of functionalization, the nature of the tube changes from hydrophobic to hydrophilic and this transitions breaks the formation of 1D water wires, also presence of functional groups on the CNT surfaces results in the formation of hydrophilichydrophilic interactions between water and functional groups. These forces results in the decrease of water flow as shown in **Figure 10**.

**Figure 11** clearly shows the decrease in the flow rate with the increase in the charge % on the surface of CNT. This can be further explained by the variation of hydrogen bonds between the confined water molecules because in the 6,6 pristine CNT, one water molecule on an average

**Figure 10.** Variation of flow with the charge %.

of 1D water wire in 6,6 pristine CNT was correlated to the diameter of the CNT and its hydrophobic nature of CNT. Although the functionalization of CNT does not change its diameter but it does changed its hydrophilic nature, thus it suggests the presence of 1D wires in pristine

CNT is only dependent on its hydrophobic nature and not on its diameter.

**Figure 8.** Decrease in the ordering of confined water inside CNT.

116 Carbon Nanotubes - Recent Progress

**Figure 9.** Presence of 3 water wires in functionalized CNT.

**Figure 11.** (a) Shows the variation of the hydrogen bonds with the charge %. (b–d) Shows the behavior of the water dipole moment and the decrease in the number of flipping events with the increase in the charge %.

forms 1–2 hydrogen bonds but in the presence of functionalized groups on CNT surface the number of hydrogen bonds almost linearly increases due to the breakdown of 1D water wire as shown in **Figure 11(a)**. This linear increase in the number of hydrogen bonds also prevents the dipole moment of water from flipping inside the CNT as shown in **Figure 11(b)**. A flip event for water is defined if Ɵ passes through 90°, thus the increase in the number of hydrogen bonds results in a potential barrier for the water's dipole moment to flip. This behavior is only affected by the presence of charges and not on the pattern of charge distribution.

As, we can observe from **Figure 11(a)**, the average number of hydrogen bonds for the highest charged system almost reached 3.5, which matches well with the number of hydrogen bonds in the bulk solvent, thus suggesting that degree of functionalization provides us with a good handle to tune the properties of the system under study.

Now, I would to talk about the macroscopic details of the confined spaces, like pores of carbon nanotubes which can be understood by studying the water density fluctuations. Water density fluctuations gives us the information about the nature of the confined space i.e. it help us to understand if the pore is hydrophobic or hydrophilic in nature. The water density fluctuation can be calculated by the following formula:

$$\kappa = \frac{\left( \langle N^{\natural} \rangle - \langle N \rangle \right)}{\langle N \rangle} \tag{1}$$

where Ƙ is the variance in the variance in the occupancy of water molecules normalized by its mean value. The ratio of variance and the mean value approaches ƿkbTXt, where Xt is the isothermal compressibility, in macroscopic limits. **Figure 12**, shows fluctuations in the density of confined water inside pristine and functionalized carbon nanotubes. The two horizontal lines indicate two limiting cases for fluctuations in bulk water and in ideal gas. The relative fluctuations in the highest charged CNTs increases by almost 5 times compared to that of pristine CNTs. This suggests that the compressibility of water near the hydrophobic CNTs is

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In conclusion charged or functionalized carbon nanotubes behave quite differently compared to pristine CNTs. Functionalization not only affects properties like band gap, conductivity and metallic nature of CNTs but it also greatly affects the properties of confined fluids. The functionalization of CNTs changes the overall nature of the CNTs and increases the hydrophilicity, which varies almost linearly with the degree of functionalization. As the degree of functionalization provides us with a handle on the properties of confined water, it might be interesting to see if we can use carbon nanotubes as a prototype for studying complex biologi-

more owing to the wet-dry transitions [45, 54].

**Figure 12.** Normalized variance as the function of degree of hydrophobicity.

**3. Conclusion**

cal system like aquaporins.

**Figure 12.** Normalized variance as the function of degree of hydrophobicity.

where Ƙ is the variance in the variance in the occupancy of water molecules normalized by its mean value. The ratio of variance and the mean value approaches ƿkbTXt, where Xt is the isothermal compressibility, in macroscopic limits. **Figure 12**, shows fluctuations in the density of confined water inside pristine and functionalized carbon nanotubes. The two horizontal lines indicate two limiting cases for fluctuations in bulk water and in ideal gas. The relative fluctuations in the highest charged CNTs increases by almost 5 times compared to that of pristine CNTs. This suggests that the compressibility of water near the hydrophobic CNTs is more owing to the wet-dry transitions [45, 54].

#### **3. Conclusion**

forms 1–2 hydrogen bonds but in the presence of functionalized groups on CNT surface the number of hydrogen bonds almost linearly increases due to the breakdown of 1D water wire as shown in **Figure 11(a)**. This linear increase in the number of hydrogen bonds also prevents the dipole moment of water from flipping inside the CNT as shown in **Figure 11(b)**. A flip event for water is defined if Ɵ passes through 90°, thus the increase in the number of hydrogen bonds results in a potential barrier for the water's dipole moment to flip. This behavior is

**Figure 11.** (a) Shows the variation of the hydrogen bonds with the charge %. (b–d) Shows the behavior of the water

dipole moment and the decrease in the number of flipping events with the increase in the charge %.

As, we can observe from **Figure 11(a)**, the average number of hydrogen bonds for the highest charged system almost reached 3.5, which matches well with the number of hydrogen bonds in the bulk solvent, thus suggesting that degree of functionalization provides us with a good

Now, I would to talk about the macroscopic details of the confined spaces, like pores of carbon nanotubes which can be understood by studying the water density fluctuations. Water density fluctuations gives us the information about the nature of the confined space i.e. it help us to understand if the pore is hydrophobic or hydrophilic in nature. The water density fluctuation can

〉 − 〈*N*〉) \_\_\_\_\_\_\_\_\_\_

〈*N*〉 (1)

only affected by the presence of charges and not on the pattern of charge distribution.

handle to tune the properties of the system under study.

be calculated by the following formula:

118 Carbon Nanotubes - Recent Progress

*<sup>κ</sup>* <sup>=</sup> (〈*N*<sup>2</sup>

In conclusion charged or functionalized carbon nanotubes behave quite differently compared to pristine CNTs. Functionalization not only affects properties like band gap, conductivity and metallic nature of CNTs but it also greatly affects the properties of confined fluids. The functionalization of CNTs changes the overall nature of the CNTs and increases the hydrophilicity, which varies almost linearly with the degree of functionalization. As the degree of functionalization provides us with a handle on the properties of confined water, it might be interesting to see if we can use carbon nanotubes as a prototype for studying complex biological system like aquaporins.

#### **Author details**

Devashree Atre1 and Shashank Pant<sup>2</sup> \*

\*Address all correspondence to: pant5@illinois.edu

1 Civil and Environmental Engineering, University of Illinois, Urbana-Champaign, IL, USA

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**Section 3**

**State-of-the-Art Nanocomposites**

**State-of-the-Art Nanocomposites**

**Chapter 8**

**Provisional chapter**

**Highly Anisotropic Polymer Composites Based on**

**Highly Anisotropic Polymer Composites Based on** 

DOI: 10.5772/intechopen.71533

Properties of polymers can be optimized through processing methods or by the addition of nanofillers. Carbon nanotubes (CNTs) have gained attention due to their promising behavior. Carbon nanotubes are essentially a sheet of graphene wrapped into a cylindrical shape with either a single or multiple walls. Tube diameters are approximately nanometres, but they can be micrometres in length. Due to the unique properties of nanotubes, they offer promise in composite materials with current research dedicated to embedding them in a polymer matrix. If improvements are to be made , the nanotubes need to be in an aligned configuration. This presents challenges due to the strong intermolecular forces that cause nanotube agglomeration leading to poor quality dispersion and random CNT orientations. Thus, there are two particular challenges to the formation of polymer composites based on CNTs: dispersion and alignment, and these are the focus of this chapter. We consider recent developments in the synthesis of carbon nanotubes and their properties. Next, we detail the challenges of dispersion and alignment that are presented in the preparation of polymer/CNT composites. Finally, we review the literature to identify progress made in preparing high performance polymer/CNT composites

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

The properties of polymer-based materials can be substantially modified and enhanced through the inclusion of filler particles. Carbon nanotubes (CNTs) exhibit high flexibility, low mass density and high values of the aspect ratio in excess of 1000. Some types of carbon nanotubes can exhibit metallic or semi-conducting behaviour and coupled to the high

**Carbon Nanotubes**

Meruyert Nazhipkyzy

**Abstract**

**1. Introduction**

**Carbon Nanotubes**

Geoffrey R. Mitchell, Fred J. Davis,

http://dx.doi.org/10.5772/intechopen.71533

Saeed Mohan and Meruyert Nazhipkyzy

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Geoffrey R. Mitchell, Fred J. Davis, Saeed Mohan and

and their properties and present one particular solution.

**Keywords:** composites, alignment, polymer, carbon nanotube

**Provisional chapter**

#### **Highly Anisotropic Polymer Composites Based on Carbon Nanotubes Carbon Nanotubes**

**Highly Anisotropic Polymer Composites Based on** 

DOI: 10.5772/intechopen.71533

Geoffrey R. Mitchell, Fred J. Davis, Saeed Mohan and Meruyert Nazhipkyzy Meruyert Nazhipkyzy Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Geoffrey R. Mitchell, Fred J. Davis, Saeed Mohan and

http://dx.doi.org/10.5772/intechopen.71533

#### **Abstract**

Properties of polymers can be optimized through processing methods or by the addition of nanofillers. Carbon nanotubes (CNTs) have gained attention due to their promising behavior. Carbon nanotubes are essentially a sheet of graphene wrapped into a cylindrical shape with either a single or multiple walls. Tube diameters are approximately nanometres, but they can be micrometres in length. Due to the unique properties of nanotubes, they offer promise in composite materials with current research dedicated to embedding them in a polymer matrix. If improvements are to be made , the nanotubes need to be in an aligned configuration. This presents challenges due to the strong intermolecular forces that cause nanotube agglomeration leading to poor quality dispersion and random CNT orientations. Thus, there are two particular challenges to the formation of polymer composites based on CNTs: dispersion and alignment, and these are the focus of this chapter. We consider recent developments in the synthesis of carbon nanotubes and their properties. Next, we detail the challenges of dispersion and alignment that are presented in the preparation of polymer/CNT composites. Finally, we review the literature to identify progress made in preparing high performance polymer/CNT composites and their properties and present one particular solution.

**Keywords:** composites, alignment, polymer, carbon nanotube

#### **1. Introduction**

The properties of polymer-based materials can be substantially modified and enhanced through the inclusion of filler particles. Carbon nanotubes (CNTs) exhibit high flexibility, low mass density and high values of the aspect ratio in excess of 1000. Some types of carbon nanotubes can exhibit metallic or semi-conducting behaviour and coupled to the high

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

tensile strength and modulus; this combination is ideally suited to their use in composites. The first polymer nanocomposites using CNTs were reported in the 1990s [1] and since then there has been worldwide activity on the methodology to optimise the preparation of such nanocomposites to fully deliver on their promise. Major problems that have been encountered are aggregation of the carbon nanotubes, thereby limiting the full dispersion of the nanoparticles in the polymer matrix, which has a deleterious impact on the properties. Equally the image of a stiff rod embedded in a polymer matrix is far from the reality, and hence carbon nanotubes are often coiled within the polymer matrix and the lack of macroscopic alignment of the tubes severely limits their properties. A wide variety of approaches have been explored including in-situ polymerisation of the matrix, functionalisation of the carbon nanotubes and the use of electric fields to align the carbon nanotubes. This chapter reviews these challenges of dispersion and alignment in the light of recent work both by the authors and that reported in the literature. We identify an approach most likely to succeed.

CNT [22]. Recently, CNT has also been produced from precious novel metal [23–27], catalysts based on ceramic nanoparticles [28, 29] and semiconductor nanostructures [30–33]. This work has led to a new interpretation of the role of the catalyst in growth of nanotubes in which sites of adsorption of carbon are necessary for fixing the nanodimensional curvature. For the growth of CNTs, the formation of nanocrystals of metallic catalysts via CCVD as well as the formation of non-conducting substrates on the surface is necessary. Typical substrates include aluminium oxide, silicon and magnesium oxide. Nevertheless, in spite of the advantages of the CVD method, still there are many problems connected with the growth of high-density CNTs with high purity. Even though many years have passed since the initial discovery of CNTs, it remains challenging to synthesise good quality and high length CNTs because of the

Highly Anisotropic Polymer Composites Based on Carbon Nanotubes

http://dx.doi.org/10.5772/intechopen.71533

129

There are various ways to use other CVD methods, such as water-assisted CVD [34, 35], oxygen-assisted CVD [36], hot filament CVD [37, 38], microwave plasma CVD [39] or radiofrequency CVD [40]. It is known that the catalysed decomposition of hydrocarbons form carbon fibres as fibrous carbon has been received by a cyanogen transmission on the heated porcelain in 1890 [41]. In 1980, Endo [42] developed the floating catalytic reactor with the use of particles of the catalyst with a diameter of 10 nanometres. This method is a predecessor of the production of CNT widely used today involving an aerosol [43–49] where pyrolysis of hydrocarbons takes place in the presence of a catalyst (Fe, Ni, Co, etc.) generating both MWCNTs and SWCNTs. Key parameters of the growth of CNTs using CVD are the atmosphere, a carbon source, the catalyst and temperature of growth. Low temperatures (300–800°C) lead to the formation of MWCNTs, whereas reaction at high temperatures (600–1150°C) promotes the

The choice of catalyst and catalyst support are very important aspects which affect CNT expansion. Therefore, the preparation stage represents a decisive step in synthesis of CNTs. The influence of the structure and the morphology of the nanoparticles of the catalyst on the growth of CNTs by means of CVD are summarised in the review of Szabo et al. [52]. Flahaut et al. have reported the influence of the conditions of preparation of the catalyst for synthesis of CNTs by means of CCVD using as fuel, urea or citric acid. They have found that the more moderate conditions of burning achieved in the case of citric acid limit the formation of carbon nanofibres or increase the selectivity of the synthesis of CCVD towards CNT with a lower

In order to solve these problems and to improve the main characteristics of CNTs, many miscellaneous fabrication approaches for carbon nanotubes have been considered. For example, Xiang et al. prepared CNT through CCVD using acetylene with a series of catalysts prepared from two-layer double Co/Fe/Al (LDHs) hydroxides. They have noted that the Co content in precursors exerted a noticeable impact on the growth of CNTs. An increase in Co content led

Lyu et al. produced high-quality and high-purity double-walled carbon nanotubes (DWCNTs)

observed DWNT bunches without any amorphous carbon and a low level of defects [54].

O3

as the catalyst at 900°C. They

to the formation of CNTs with smaller diameters and lower structural frustration [53]**.**

many factors that influence the growth process.

number of walls depending on the catalytic composition.

by catalytic decomposition of benzene and Fe-Mo/Al<sup>2</sup>

growth of SWCNTs [50, 51]**.**

#### **2. Synthesis-property relationships for carbon nanotubes**

The ideal carbon nanotube is a nanoscaled graphene (one leaf of graphite) that is rolled up into a cylindrical shape; structures containing only one wall of cylinder are called single walled carbon nanotubes (SWCNTs), whereas structures with two or more concentric graphene cylinders are termed multi-walled carbon nanotubes (MWCNTs) [2].

Since their discovery in 1991 by Iijima [3], carbon nanotubes have been the subject of a worldwide attention because of their unique properties and associated wide range of applications [4–15]. Over the past two decades, nanotubes have found applications in various areas, such as sources of field emissions, photodetectors, non-volatile memory, ultra-sensitive chemical and transparent conducting membranes, biosensing devices, double layer-type capacitors, transparent electrode films, radio wave absorbents and even in modern electronics.

The most common and widely used nanotube synthesis techniques are arc discharge, chemical vapour deposition and laser ablation [16–21]. CNTs were first produced by arc discharge process or by laser ablation methods; however, recently there has been a significant increase in the synthesis of CNTs by catalytic chemical vapour deposition (CCVD). The catalytic CVD method is an easy and cheap way to produce CNTs at low temperatures and pressures. Compared to the other methods, catalytic CVD provides a higher yield and higher purity of the grown nanotubes and allows control of the parameters of the growth and structure of CNTs.

Hydrocarbons obtained from fossil fuels and the hydrocarbons from plants are the two main sources of carbon for the synthesis of CNTs. There are natural precursors and products containing carboniferous materials. Common fossil fuel CNT precursors are methane, ethylene, acetylene, benzene, xylene and carbon monoxide. There are several reports on the use of natural precursors for synthesis of CNT, such as camphor, turpentine oil, palm oil, etc. Nanoparticles of transition metals (Fe, Ni and Co) are usually used for the catalytic growth of CNT [22]. Recently, CNT has also been produced from precious novel metal [23–27], catalysts based on ceramic nanoparticles [28, 29] and semiconductor nanostructures [30–33]. This work has led to a new interpretation of the role of the catalyst in growth of nanotubes in which sites of adsorption of carbon are necessary for fixing the nanodimensional curvature. For the growth of CNTs, the formation of nanocrystals of metallic catalysts via CCVD as well as the formation of non-conducting substrates on the surface is necessary. Typical substrates include aluminium oxide, silicon and magnesium oxide. Nevertheless, in spite of the advantages of the CVD method, still there are many problems connected with the growth of high-density CNTs with high purity. Even though many years have passed since the initial discovery of CNTs, it remains challenging to synthesise good quality and high length CNTs because of the many factors that influence the growth process.

tensile strength and modulus; this combination is ideally suited to their use in composites. The first polymer nanocomposites using CNTs were reported in the 1990s [1] and since then there has been worldwide activity on the methodology to optimise the preparation of such nanocomposites to fully deliver on their promise. Major problems that have been encountered are aggregation of the carbon nanotubes, thereby limiting the full dispersion of the nanoparticles in the polymer matrix, which has a deleterious impact on the properties. Equally the image of a stiff rod embedded in a polymer matrix is far from the reality, and hence carbon nanotubes are often coiled within the polymer matrix and the lack of macroscopic alignment of the tubes severely limits their properties. A wide variety of approaches have been explored including in-situ polymerisation of the matrix, functionalisation of the carbon nanotubes and the use of electric fields to align the carbon nanotubes. This chapter reviews these challenges of dispersion and alignment in the light of recent work both by the authors and that reported in the literature. We identify an approach most

The ideal carbon nanotube is a nanoscaled graphene (one leaf of graphite) that is rolled up into a cylindrical shape; structures containing only one wall of cylinder are called single walled carbon nanotubes (SWCNTs), whereas structures with two or more concentric graphene cyl-

Since their discovery in 1991 by Iijima [3], carbon nanotubes have been the subject of a worldwide attention because of their unique properties and associated wide range of applications [4–15]. Over the past two decades, nanotubes have found applications in various areas, such as sources of field emissions, photodetectors, non-volatile memory, ultra-sensitive chemical and transparent conducting membranes, biosensing devices, double layer-type capacitors,

The most common and widely used nanotube synthesis techniques are arc discharge, chemical vapour deposition and laser ablation [16–21]. CNTs were first produced by arc discharge process or by laser ablation methods; however, recently there has been a significant increase in the synthesis of CNTs by catalytic chemical vapour deposition (CCVD). The catalytic CVD method is an easy and cheap way to produce CNTs at low temperatures and pressures. Compared to the other methods, catalytic CVD provides a higher yield and higher purity of the grown nanotubes and allows control of the parameters of the growth and structure of

Hydrocarbons obtained from fossil fuels and the hydrocarbons from plants are the two main sources of carbon for the synthesis of CNTs. There are natural precursors and products containing carboniferous materials. Common fossil fuel CNT precursors are methane, ethylene, acetylene, benzene, xylene and carbon monoxide. There are several reports on the use of natural precursors for synthesis of CNT, such as camphor, turpentine oil, palm oil, etc. Nanoparticles of transition metals (Fe, Ni and Co) are usually used for the catalytic growth of

transparent electrode films, radio wave absorbents and even in modern electronics.

**2. Synthesis-property relationships for carbon nanotubes**

inders are termed multi-walled carbon nanotubes (MWCNTs) [2].

likely to succeed.

128 Carbon Nanotubes - Recent Progress

CNTs.

There are various ways to use other CVD methods, such as water-assisted CVD [34, 35], oxygen-assisted CVD [36], hot filament CVD [37, 38], microwave plasma CVD [39] or radiofrequency CVD [40]. It is known that the catalysed decomposition of hydrocarbons form carbon fibres as fibrous carbon has been received by a cyanogen transmission on the heated porcelain in 1890 [41]. In 1980, Endo [42] developed the floating catalytic reactor with the use of particles of the catalyst with a diameter of 10 nanometres. This method is a predecessor of the production of CNT widely used today involving an aerosol [43–49] where pyrolysis of hydrocarbons takes place in the presence of a catalyst (Fe, Ni, Co, etc.) generating both MWCNTs and SWCNTs. Key parameters of the growth of CNTs using CVD are the atmosphere, a carbon source, the catalyst and temperature of growth. Low temperatures (300–800°C) lead to the formation of MWCNTs, whereas reaction at high temperatures (600–1150°C) promotes the growth of SWCNTs [50, 51]**.**

The choice of catalyst and catalyst support are very important aspects which affect CNT expansion. Therefore, the preparation stage represents a decisive step in synthesis of CNTs. The influence of the structure and the morphology of the nanoparticles of the catalyst on the growth of CNTs by means of CVD are summarised in the review of Szabo et al. [52]. Flahaut et al. have reported the influence of the conditions of preparation of the catalyst for synthesis of CNTs by means of CCVD using as fuel, urea or citric acid. They have found that the more moderate conditions of burning achieved in the case of citric acid limit the formation of carbon nanofibres or increase the selectivity of the synthesis of CCVD towards CNT with a lower number of walls depending on the catalytic composition.

In order to solve these problems and to improve the main characteristics of CNTs, many miscellaneous fabrication approaches for carbon nanotubes have been considered. For example, Xiang et al. prepared CNT through CCVD using acetylene with a series of catalysts prepared from two-layer double Co/Fe/Al (LDHs) hydroxides. They have noted that the Co content in precursors exerted a noticeable impact on the growth of CNTs. An increase in Co content led to the formation of CNTs with smaller diameters and lower structural frustration [53]**.**

Lyu et al. produced high-quality and high-purity double-walled carbon nanotubes (DWCNTs) by catalytic decomposition of benzene and Fe-Mo/Al<sup>2</sup> O3 as the catalyst at 900°C. They observed DWNT bunches without any amorphous carbon and a low level of defects [54]. Zhang et al. produced MWCNTs with a diameter of 40–60 nanometres by catalytic decomposition of methane at 680°C within 120 minutes together with the use of binary aerogels with nickel oxide as the catalyst [55, 56]. Sano and his colleagues have identified two systems of metal catalyst/source of carbon for growth of CNTs: Ethanol/Co and benzene/Fe. In addition, they investigated two different reactors (the reactor with a gas stream and a liquid reactor) on quality of CNT [57]**.**

method with diatomaceous earth as the substrate and nickel salt as a catalyst. In fact, the catalytic activity of Co and Ni is supported on diatomite and its role in the production of multiwalled carbon nanotubes has been investigated. In this specific example, the diatomite support was prepared by an impregnation method and the CNTs were synthesised on the

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Diatomaceous earth, known as a diatomite, is a soft, powdery, porous and silica-rich mineral, which is found in fossilised deposits near dried up bodies of water. This mineral is the skeletal remains and the result of the accumulation of dead diatoms is found in marine sediments. Diatoms are microscopic single-celled algae that usually have shells that are divided into two halves. The shells of diatoms are hard due to the presence of silica in their cell walls. These silica-rich hard shells are usually found crushed up into tiny, sharp shards, which is the main characteristic of fossilised diatomite. Thanks to having properties, such as low density, high porosity, high surface area, abrasiveness, insulating properties, inertness, absorptive capacity, brightness and high silica content, diatomite has a wide variety of uses, and it is a component in many products. Many different methods of obtaining nanotubes from a variety of materials

have been proposed up to this time, but to our knowledge, few have used diatomite.

**Figure 1** shows spectra for samples obtained on diatomite at temperatures of 650 and 700°C,

surface of this as a substrate.

peak G is located at 1605 cm−1.

**Figure 1.** Raman spectra of CNT samples on a diatomite.

Jiang et al. investigated growth of CNT in situ on previously processed graphite electrode (GE) through CCVD with the use of Ni (NO<sup>3</sup> ) 2 as the catalyst [58]. The CNTs prepared with this method had external and internal diameters of 80 and 20, respectively. The length of the CNTs was observed as 200–1000 nanometres; this was attributed to the shorter time of growth. These authors were interested in the concentration of carboxyl and hydroxyl groups on the surface of carbon nanotubes as they are essential features for applications in many fields of science, such as nanomedicine, biosensors or polymeric nanocomposites [58]**.**

A promising one-step method of fabricating carbon nanotubes is flame synthesis. It involves the direct combustion of hydrocarbons in the presence of an oxidiser. The CNTs are normally grown directly on an alloy substrate. The use of metal-organics such as iron pentacarbonyl, ferrocene or metal nitrate dissolved in fuel has been reported. To date, CNTs have only been reported to grow on alloy substrates.

Synthesis using a flame is a very energy efficient process because the fuel is a source of both heat and carbon. The temperatures can reach 1600 K, which is difficult to achieve by means of CVD in a typical furnace. For the synthesis of CNTs over a large area, it is more economic to use a rastering system and a repeated flame for the control of both time and the area of a flame [59]. The higher production efficiency and a lower energy consumption compared to CVD make this method suitable for industrial production. Synthesis using a flame represents a simple single-step method without the need of a substrate. The mechanism of the growth of CNTs in the synthesis of a flame of hydrocarbons can be divided into three main stages. In the first stage of this, hydrocarbon fuel pyrolysis in a previously heated zone results in the formation of hydrocarbon particles which will be a carbon source for CNTs; in second, particles of metal are formed on the surface of the alloy and in final stage, hydrocarbons are absorbed by the catalyst resulting in the growth of CNTs.

Institute of Combustion Problems (ICP), conducts research work on "Synthesis of carbon nanomaterials in flames." We include here one example of report which has been performed in ICP. In this work, oil deposit soot, prepared when burning a propane-butane mixture with the use of electric field, has been chosen among various types of natural materials for preparing porous carbon material (PCM). The PCM is used as the catalyst carrier. We have prepared multiwalled nanotubes (MWNT). We are searching for new methods of creation of catalytic systems (catalyst/carrier), which allow for the control of the structure of carbon particles. This is an important problem, the solution to which can lead to the development of approaches to the synthesis of carbon nanotubes (CNT) with specific functional properties [60]. In addition, ICP has developed a new approach for CNTs synthesis from diatomite. In this work [61], multiwalled nanotubes were obtained using a chemical vapour deposition method with diatomaceous earth as the substrate and nickel salt as a catalyst. In fact, the catalytic activity of Co and Ni is supported on diatomite and its role in the production of multiwalled carbon nanotubes has been investigated. In this specific example, the diatomite support was prepared by an impregnation method and the CNTs were synthesised on the surface of this as a substrate.

Zhang et al. produced MWCNTs with a diameter of 40–60 nanometres by catalytic decomposition of methane at 680°C within 120 minutes together with the use of binary aerogels with nickel oxide as the catalyst [55, 56]. Sano and his colleagues have identified two systems of metal catalyst/source of carbon for growth of CNTs: Ethanol/Co and benzene/Fe. In addition, they investigated two different reactors (the reactor with a gas stream and a liquid reactor)

Jiang et al. investigated growth of CNT in situ on previously processed graphite electrode

this method had external and internal diameters of 80 and 20, respectively. The length of the CNTs was observed as 200–1000 nanometres; this was attributed to the shorter time of growth. These authors were interested in the concentration of carboxyl and hydroxyl groups on the surface of carbon nanotubes as they are essential features for applications in many fields of

A promising one-step method of fabricating carbon nanotubes is flame synthesis. It involves the direct combustion of hydrocarbons in the presence of an oxidiser. The CNTs are normally grown directly on an alloy substrate. The use of metal-organics such as iron pentacarbonyl, ferrocene or metal nitrate dissolved in fuel has been reported. To date, CNTs have only been

Synthesis using a flame is a very energy efficient process because the fuel is a source of both heat and carbon. The temperatures can reach 1600 K, which is difficult to achieve by means of CVD in a typical furnace. For the synthesis of CNTs over a large area, it is more economic to use a rastering system and a repeated flame for the control of both time and the area of a flame [59]. The higher production efficiency and a lower energy consumption compared to CVD make this method suitable for industrial production. Synthesis using a flame represents a simple single-step method without the need of a substrate. The mechanism of the growth of CNTs in the synthesis of a flame of hydrocarbons can be divided into three main stages. In the first stage of this, hydrocarbon fuel pyrolysis in a previously heated zone results in the formation of hydrocarbon particles which will be a carbon source for CNTs; in second, particles of metal are formed on the surface of the alloy and in final stage, hydrocarbons are absorbed by

Institute of Combustion Problems (ICP), conducts research work on "Synthesis of carbon nanomaterials in flames." We include here one example of report which has been performed in ICP. In this work, oil deposit soot, prepared when burning a propane-butane mixture with the use of electric field, has been chosen among various types of natural materials for preparing porous carbon material (PCM). The PCM is used as the catalyst carrier. We have prepared multiwalled nanotubes (MWNT). We are searching for new methods of creation of catalytic systems (catalyst/carrier), which allow for the control of the structure of carbon particles. This is an important problem, the solution to which can lead to the development of approaches to the synthesis of carbon nanotubes (CNT) with specific functional properties [60]. In addition, ICP has developed a new approach for CNTs synthesis from diatomite. In this work [61], multiwalled nanotubes were obtained using a chemical vapour deposition

as the catalyst [58]. The CNTs prepared with

) 2

science, such as nanomedicine, biosensors or polymeric nanocomposites [58]**.**

on quality of CNT [57]**.**

130 Carbon Nanotubes - Recent Progress

(GE) through CCVD with the use of Ni (NO<sup>3</sup>

reported to grow on alloy substrates.

the catalyst resulting in the growth of CNTs.

Diatomaceous earth, known as a diatomite, is a soft, powdery, porous and silica-rich mineral, which is found in fossilised deposits near dried up bodies of water. This mineral is the skeletal remains and the result of the accumulation of dead diatoms is found in marine sediments. Diatoms are microscopic single-celled algae that usually have shells that are divided into two halves. The shells of diatoms are hard due to the presence of silica in their cell walls. These silica-rich hard shells are usually found crushed up into tiny, sharp shards, which is the main characteristic of fossilised diatomite. Thanks to having properties, such as low density, high porosity, high surface area, abrasiveness, insulating properties, inertness, absorptive capacity, brightness and high silica content, diatomite has a wide variety of uses, and it is a component in many products. Many different methods of obtaining nanotubes from a variety of materials have been proposed up to this time, but to our knowledge, few have used diatomite.

**Figure 1** shows spectra for samples obtained on diatomite at temperatures of 650 and 700°C, peak G is located at 1605 cm−1.

**Figure 1.** Raman spectra of CNT samples on a diatomite.

The peaks D and G have a large width, which is characteristic of amorphous carbon. There is no clearly expressed second-order peak. These parameters, together with the analysis of SEM images, suggest that at these temperatures, the formation of CNTs on the diatomite does not occur or is weakly expressed [62].

polymer melt are often chosen. However, such treatments may lead to damage or breaking the CNTs into smaller units. These may be advantageous for dispersion but it may at the same time increase the threshold concentration for percolation as the aspect ratio of the tube is lowered. It is common to add surfactants as used in electrospinning of polyethylene oxide with CNTs [70] or to functionalise the CNTs to increase compatibility with the polymer matrix without destroying the electronic properties of the CNTs [71]. Huang and Terentjev [72] studied in detail the use of sonication and melt mixing to disperse CNTs in polymers and they found a critical mixing time, although they indicate that they could not exclude the possibility

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An alternative approach to that described above is to avoid the problems with the polymer by mixing with a monomer and after mixing carry out the polymerisation process. This is the only route for preparing thermosets containing CNTs, but has also been used to prepare systems with intrinsically conductive polymers such as polyaniline, for example the work of Cochet et al*.* [73]. Of course, a suitable solvent can be used to generate a uniform dispersion of the CNTs in the solvent and then introduce the polymer. This has been successfully used by a number of authors but of course the removal of the solvent becomes the challenge especially

A good dispersion of the CNTs is the first step to realise the full potential of CNT-based polymer composites. The second parallel requirement is to fully utilise the anisotropic properties of the CNTs by preparing a common alignment for the CNTs, the alignment depending on the application of the product. A variety of methods have been explored and these have been reviewed by Goh et al. [74], and it is fair to say that many are particularly suited to electronic and other thin film applications. **Figure 2** shows schematically the methodologies for producing a common alignment of CNTs. The introduction of preferred alignment into a polymeric system is usually achieved during processing, for example with fibre spinning [75]. In the case of injection, moulding or other processing, the flow of the polymer melt can be used to induce alignment, and of course in principle, CNTs are straightforward to align due to the high aspect ratio. Zhou et al. [76] have used dissipative particle dynamics to simulate these processes. They find that increasing the volume fraction and the length of CNTs leads to a significant increase in the alignment of the CNTs. The challenge is that the alignment follows the flow in the mould and it requires careful design to achieve the

Typically, the CNTs will align parallel to the flow direction, and hence an alignment in a sheet which is normal to the sheet plane is more or less impossible to achieve. The generation of alignment raises the question whether it is possible to prepare patterns of alignment. A number of reports have addressed this area. Goldberg-Oppenheimer et al*.* [78] used electrohydrodynamic patterning to produce defined patterns on a micrometre scale with the alignment of CNTs within those patterns. Dai has reviewed the use of pyrolytic procedures for large scale production of oriented and micropatterned carbon nanotube arrays normal to a substrate surface which can be transferred onto various substrates of particular interest in either a patterned or non-patterned fashion [79]. The perpendicularly aligned structure is

of small tube clusters persisting after this time.

in the industrial production of objects.

required alignment pattern.

particularly advantageous for surface functionalisation.

When the synthesis temperature is raised to 750 and 800°C, the width of the peaks is significantly reduced, the position of the main peak G is shifted to the low-frequency region and in the case of 800°C is 1583 cm−1, which is a characteristic value for CNT, graphite and graphene.

There is a manifestation of the 2D peak, which indicates an increase in the degree of crystallinity of the structure and an increase in long-range order and, consequently, an improvement in the quality of CNTs [63].

The peak D arises as a result of defects in graphite structures, and the intensity ratio of the peaks D and G is usually considered an indicator of the quality of CNTs. In the case of CNT synthesised on diatomite, we observe an increase in the intensity of the defect peak with an increase in temperature. This is because an increase in the synthesis temperature makes it possible to reduce the volume of amorphous carbon, but increases the probability of oxidation and the formation of structural defects. In addition, at high synthesis temperatures, nanotubes with a large diameter predominate, which increase the probability of forming knots and strong bends and are manifested in an increase in the D peak intensity in the Raman spectrum.

### **3. The challenge with polymer nanocomposites: dispersion, alignment and compatibility**

To achieve the maximum advantage of carbon nanotubes, they need to be well dispersed, that is not form aggregates. Dispersion is a spatial property. Good dispersion requires the average number density of individual carbon nanotubes to be uniform through the object. Of course, one challenge in this respect is the experimental method requires high magnification and large spatial coverage as highlighted by Li et al. [64], both demanding requirements, moreover there are no standards or commonly accepted procedures to follow although transmission electron micrographs is generally accepted as the gold standard [64].

Achieving good dispersion is experimentally challenging and there is a considerable body of literature on the problem; the reviews of Xie et al. [65], Spitalsky et al. [66], Gkourmpis [67] and Huang and Terentjev [68] cover the key areas. There are many routes to solve basically the same problem. If we start with the situation of traditional thermoplastic polymer, then the basic requirements have been identified and further some simulation of the process achieved [69].

CNTs made by CVD techniques are usually entangled and the first stage of dispersion is to break up these aggregates. Sonication and high shear flow with a high molecular weight polymer melt are often chosen. However, such treatments may lead to damage or breaking the CNTs into smaller units. These may be advantageous for dispersion but it may at the same time increase the threshold concentration for percolation as the aspect ratio of the tube is lowered. It is common to add surfactants as used in electrospinning of polyethylene oxide with CNTs [70] or to functionalise the CNTs to increase compatibility with the polymer matrix without destroying the electronic properties of the CNTs [71]. Huang and Terentjev [72] studied in detail the use of sonication and melt mixing to disperse CNTs in polymers and they found a critical mixing time, although they indicate that they could not exclude the possibility of small tube clusters persisting after this time.

The peaks D and G have a large width, which is characteristic of amorphous carbon. There is no clearly expressed second-order peak. These parameters, together with the analysis of SEM images, suggest that at these temperatures, the formation of CNTs on the diatomite does not

When the synthesis temperature is raised to 750 and 800°C, the width of the peaks is significantly reduced, the position of the main peak G is shifted to the low-frequency region and in the case of 800°C is 1583 cm−1, which is a characteristic value for CNT, graphite and

There is a manifestation of the 2D peak, which indicates an increase in the degree of crystallinity of the structure and an increase in long-range order and, consequently, an improvement

The peak D arises as a result of defects in graphite structures, and the intensity ratio of the peaks D and G is usually considered an indicator of the quality of CNTs. In the case of CNT synthesised on diatomite, we observe an increase in the intensity of the defect peak with an increase in temperature. This is because an increase in the synthesis temperature makes it possible to reduce the volume of amorphous carbon, but increases the probability of oxidation and the formation of structural defects. In addition, at high synthesis temperatures, nanotubes with a large diameter predominate, which increase the probability of forming knots and strong bends and are manifested in an increase in the D peak intensity in the Raman

**3. The challenge with polymer nanocomposites: dispersion, alignment** 

To achieve the maximum advantage of carbon nanotubes, they need to be well dispersed, that is not form aggregates. Dispersion is a spatial property. Good dispersion requires the average number density of individual carbon nanotubes to be uniform through the object. Of course, one challenge in this respect is the experimental method requires high magnification and large spatial coverage as highlighted by Li et al. [64], both demanding requirements, moreover there are no standards or commonly accepted procedures to follow although transmis-

Achieving good dispersion is experimentally challenging and there is a considerable body of literature on the problem; the reviews of Xie et al. [65], Spitalsky et al. [66], Gkourmpis [67] and Huang and Terentjev [68] cover the key areas. There are many routes to solve basically the same problem. If we start with the situation of traditional thermoplastic polymer, then the basic requirements have been identified and further some simulation of the process achieved

CNTs made by CVD techniques are usually entangled and the first stage of dispersion is to break up these aggregates. Sonication and high shear flow with a high molecular weight

sion electron micrographs is generally accepted as the gold standard [64].

occur or is weakly expressed [62].

132 Carbon Nanotubes - Recent Progress

in the quality of CNTs [63].

graphene.

spectrum.

[69].

**and compatibility**

An alternative approach to that described above is to avoid the problems with the polymer by mixing with a monomer and after mixing carry out the polymerisation process. This is the only route for preparing thermosets containing CNTs, but has also been used to prepare systems with intrinsically conductive polymers such as polyaniline, for example the work of Cochet et al*.* [73]. Of course, a suitable solvent can be used to generate a uniform dispersion of the CNTs in the solvent and then introduce the polymer. This has been successfully used by a number of authors but of course the removal of the solvent becomes the challenge especially in the industrial production of objects.

A good dispersion of the CNTs is the first step to realise the full potential of CNT-based polymer composites. The second parallel requirement is to fully utilise the anisotropic properties of the CNTs by preparing a common alignment for the CNTs, the alignment depending on the application of the product. A variety of methods have been explored and these have been reviewed by Goh et al. [74], and it is fair to say that many are particularly suited to electronic and other thin film applications. **Figure 2** shows schematically the methodologies for producing a common alignment of CNTs. The introduction of preferred alignment into a polymeric system is usually achieved during processing, for example with fibre spinning [75]. In the case of injection, moulding or other processing, the flow of the polymer melt can be used to induce alignment, and of course in principle, CNTs are straightforward to align due to the high aspect ratio. Zhou et al. [76] have used dissipative particle dynamics to simulate these processes. They find that increasing the volume fraction and the length of CNTs leads to a significant increase in the alignment of the CNTs. The challenge is that the alignment follows the flow in the mould and it requires careful design to achieve the required alignment pattern.

Typically, the CNTs will align parallel to the flow direction, and hence an alignment in a sheet which is normal to the sheet plane is more or less impossible to achieve. The generation of alignment raises the question whether it is possible to prepare patterns of alignment. A number of reports have addressed this area. Goldberg-Oppenheimer et al*.* [78] used electrohydrodynamic patterning to produce defined patterns on a micrometre scale with the alignment of CNTs within those patterns. Dai has reviewed the use of pyrolytic procedures for large scale production of oriented and micropatterned carbon nanotube arrays normal to a substrate surface which can be transferred onto various substrates of particular interest in either a patterned or non-patterned fashion [79]. The perpendicularly aligned structure is particularly advantageous for surface functionalisation.

**4. Some selected recent examples of polymer/CNT composites**

Interest in polymer/CNT composites has been particularly inspired by the success of carbon fibre reinforced composites, and as with these, more developed advanced materials with improved mechanical properties are an attractive benefit, but these are potentially combined with advantages of nanocomposites such as a large interfacial area per volume [86]. A review by Coleman et al*.* neatly sums up the mechanical advantages and point out there is an order of magnitude improvement over carbon nanofibres in terms of modulus [87]. These authors describe the crucial importance of even dispersion; aggregation (for example at high loading) results in a decrease in mechanical properties. Alignment in contrast improves mechanical strength but is less crucial and results in highly anisotropic properties which might be a disadvantage in bulk samples. That being said, in fibres, alignment is easy to achieve and anisotropic properties are unlikely to be a problem. Interestingly, recently Guzman de Villoria et al. have shown that CNTs [88] can be successfully used to bind together weak interfacial layer of reinforced laminate materials; they describe these reinforcing components as nanostitches. Micron diameter pinning techniques reduce some of the advantageous mechanical properties of the composite; in this case, a carbon fibre epoxy prepreg is arranged in layers. Thus, in addition to providing the advantages of highly anisotropic orientation, CNTs can be used to overcome the difficulties associated with this highly directional mechanical behaviour.

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Thostenson and Chou [89] examined the benefits of aligned nanofillers on mechanical properties by preparing composites composed of randomly orientated and aligned CNTs in a polystyrene matrix. Samples were prepared at a loading of 5 wt% CNT in the polystyrene matrix. Films of the polystyrene/CNT composite were prepared from solution casting while sonicating. Once dry, the material was heated to 155° C in a microscale compounder with mixing performed at a screw speed of 100 rpm for 3 minutes. The molten material was extruded through a rectangular die and drawn down and passed over a chill roller to solidify, resulting in film thicknesses between 80 and 120 μm depending on the draw ratio. Samples of drawn and nondrawn polystyrene material were also prepared to differentiate between any potential impact on the properties arising from alignment of the polymer chains or the CNT nanofillers. The films produced showed good dispersion due to the mixing process in the compounder and the drawing of the material resulted in an overall alignment of the tubes about a common axis. The sample that had aligned CNTs within the matrix possessed a ~50% improvement to the storage modulus as compared to the bulk material, whereas the sample with randomly orientated CNT filler particles only exhibited a 10% increase. In addition to this, the sample containing the orientated filler showed improvements to the yield and ultimate strengths of the material along the common axis of alignment of the nanotubes, suggesting a load transfer

Many studies of CNT composites focus on the mechanical advantages but their inclusion of these materials may also benefit the thermal, optical and electrical properties of the nanocomposite material. In a recent review [90], Liu and Kumar have collated the properties of a number of composites. It is clear that both thermal and electrical conductivity depend substantially on the alignment of the CNTs but it does seem that this can vary from one composite

between the polystyrene matrix and the MWCNTs.

**Figure 2.** Methodologies for producing a common alignment of CNTs (a) ex situ and (b) in situ (reproduced from Matthew et al. [77], published by The Royal Society of Chemistry).

Alternative methodologies involved the application of electric or magnetic fields. Arguin et al. [80] have used electric fields of 40 kV/m, which can lead to the alignment of MWCNTs in an epoxy resin system. Gupta et al. [81] employed electric field and current-assisted alignment of CNTs in a polyvinylidene fluoride matrix. A promising methodology [82], which is discussed in more detail in Section 5, involves the use of an intermediate state specifically electrospinning. Electrospinning is an old technology which has seen resurgence in the last 10 years. Electrospinning enables the production of nanoscale and microscale fibres from a charged melt of solution [83]. The methodology is to encapsulate the CNTs in polymer nanofibres and deposit this in the sample with a common alignment [70, 82, 84, 85]. The electrospinning process naturally aligns the CNTs within the polymer fibre.

#### **4. Some selected recent examples of polymer/CNT composites**

Interest in polymer/CNT composites has been particularly inspired by the success of carbon fibre reinforced composites, and as with these, more developed advanced materials with improved mechanical properties are an attractive benefit, but these are potentially combined with advantages of nanocomposites such as a large interfacial area per volume [86]. A review by Coleman et al*.* neatly sums up the mechanical advantages and point out there is an order of magnitude improvement over carbon nanofibres in terms of modulus [87]. These authors describe the crucial importance of even dispersion; aggregation (for example at high loading) results in a decrease in mechanical properties. Alignment in contrast improves mechanical strength but is less crucial and results in highly anisotropic properties which might be a disadvantage in bulk samples. That being said, in fibres, alignment is easy to achieve and anisotropic properties are unlikely to be a problem. Interestingly, recently Guzman de Villoria et al. have shown that CNTs [88] can be successfully used to bind together weak interfacial layer of reinforced laminate materials; they describe these reinforcing components as nanostitches. Micron diameter pinning techniques reduce some of the advantageous mechanical properties of the composite; in this case, a carbon fibre epoxy prepreg is arranged in layers. Thus, in addition to providing the advantages of highly anisotropic orientation, CNTs can be used to overcome the difficulties associated with this highly directional mechanical behaviour.

Thostenson and Chou [89] examined the benefits of aligned nanofillers on mechanical properties by preparing composites composed of randomly orientated and aligned CNTs in a polystyrene matrix. Samples were prepared at a loading of 5 wt% CNT in the polystyrene matrix. Films of the polystyrene/CNT composite were prepared from solution casting while sonicating. Once dry, the material was heated to 155° C in a microscale compounder with mixing performed at a screw speed of 100 rpm for 3 minutes. The molten material was extruded through a rectangular die and drawn down and passed over a chill roller to solidify, resulting in film thicknesses between 80 and 120 μm depending on the draw ratio. Samples of drawn and nondrawn polystyrene material were also prepared to differentiate between any potential impact on the properties arising from alignment of the polymer chains or the CNT nanofillers. The films produced showed good dispersion due to the mixing process in the compounder and the drawing of the material resulted in an overall alignment of the tubes about a common axis. The sample that had aligned CNTs within the matrix possessed a ~50% improvement to the storage modulus as compared to the bulk material, whereas the sample with randomly orientated CNT filler particles only exhibited a 10% increase. In addition to this, the sample containing the orientated filler showed improvements to the yield and ultimate strengths of the material along the common axis of alignment of the nanotubes, suggesting a load transfer between the polystyrene matrix and the MWCNTs.

Alternative methodologies involved the application of electric or magnetic fields. Arguin et al. [80] have used electric fields of 40 kV/m, which can lead to the alignment of MWCNTs in an epoxy resin system. Gupta et al. [81] employed electric field and current-assisted alignment of CNTs in a polyvinylidene fluoride matrix. A promising methodology [82], which is discussed in more detail in Section 5, involves the use of an intermediate state specifically electrospinning. Electrospinning is an old technology which has seen resurgence in the last 10 years. Electrospinning enables the production of nanoscale and microscale fibres from a charged melt of solution [83]. The methodology is to encapsulate the CNTs in polymer nanofibres and deposit this in the sample with a common alignment [70, 82, 84, 85]. The electrospinning

**Figure 2.** Methodologies for producing a common alignment of CNTs (a) ex situ and (b) in situ (reproduced from

process naturally aligns the CNTs within the polymer fibre.

Matthew et al. [77], published by The Royal Society of Chemistry).

134 Carbon Nanotubes - Recent Progress

Many studies of CNT composites focus on the mechanical advantages but their inclusion of these materials may also benefit the thermal, optical and electrical properties of the nanocomposite material. In a recent review [90], Liu and Kumar have collated the properties of a number of composites. It is clear that both thermal and electrical conductivity depend substantially on the alignment of the CNTs but it does seem that this can vary from one composite to another. For example, a polypropylene/polyethylene/CNT composite has been found to exhibit a dramatic decrease in conductivity when drawn; in contrast, electrical conductivity of a PVA/SWNT composite showed a fourfold increase when stretched. This is a consequence of the relationship between the electrical conductivity and the percolation network; this network is likely to be distorted by drawing and the consequential change in properties will depend largely upon the loading and dispersion of the CNTs in the original sample.

loadings of the filler were prepared and observations on the conductivity and mechanical properties were made. It was observed that the conductivity was significantly better along the direction of common alignment of the tube axis. In this study, composite films that did not undergo stretching were also prepared for comparison purposes. The results showed that the alignment resulted in higher conductivity levels along the stretching direction, for example the 3 wt% loading of SWCNT that had undergone the stretching process had a conductivity of ~1 × 10−6 S/cm along the stretching direction (2.8 × 10−9 S/cm perpendicular), whereas the unstretched sample had a conductivity of 5 × 10−8 S/cm. The addition of 3 wt% of the SWCNTs to the epoxy resin improved the Young's modulus by ~2% and the tensile strength by ~2.6% with respect to the CNTs common axis of alignment; however, at loadings greater than 6 wt%, it was observed that the mechanical properties perpendicular to the tube orientation were lower than the epoxy resin on its own, possibly related to flocculation of the SWCNTs. When compared to the composite samples that had not undergone the stretching procedure, the samples with the aligned filler showed superior mechanical properties along the stretching direction when comparing the values of the Young's modulus and tensile strength to the unstretched samples, however perpendicular to the stretching direction

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Exposure to an electric field has shown to provide an effective method for the alignment of the carbon nanotubes. Martin et al*.* have examined the use of both AC and DC fields to aid in the alignment of carbon nanotubes [94]. Low concentrations of CVD-grown MWCNTs were dispersed in an epoxy resin based on bisphenol-A. In the field, it was observed that a fraction of the nanotubes move towards the anode during the epoxy drying stage. These align with the tubes long axis perpendicular to the anode. The tubes themselves then act as a very high field strength at its tip attracting more tubes which form a connecting network between the anode and cathode. The AC field provided a more uniform connecting pathway between the anode and the cathode. Both fields exhibited a concentration of tubes

Measurements made by Gupta et al*.* [81] on the electrical conductivity of polyvinylidene fluoride (PVDF), MWCNT composite materials exhibited improved electrical conductivity with improved alignment of the MWCNT filler particles. Five films of the composite materials were prepared with a solution cast method. During casting, two samples were exposed to an alternating voltage at 220 V and 500 V, whereas two others were exposed to alternating pulsed currents from a 220 V and 500 V source. The fifth sample was not exposed to any field resulting in a film that possessed a randomly orientated filler. It was observed that both the alternating voltage and alternating pulsed currents resulted in alignment of the nanotubes within the matrix (with the exception of the sample prepared with an alternating voltage at 220 V). However, differences between the two alignment techniques were observed; the sample exposed to an alternating pulsed current developed conducting channels across the matrix material, where the CNTs joined up in an endto-end fashion. Meanwhile the films exposed to the alternating voltage resulted in alignment of the CNTs along a common axis, while maintaining their uniform dispersion throughout the matrix. The alternating pulsed current method provided the necessary channels of CNTs for conduction across the matrix, where measurements of the conductivity show a 360% improvement to the electrical conductivity as compared to the sample possessing randomly orientated

the mechanical properties are not as good as the unstretched samples.

highest at the anode.

Dai et al*.* [91] examined quasi-straight SWCNTs dispersed in a polymethyl methacrylate (PMMA) matrix material. Samples were prepared from solution by taking the SWCNT powder and dispersing it in dimethylformamide (DMF) followed by dissolution of PMMA. Sonication was used for 1 hour before dissolving the PMMA and for another hour after once the PMMA was fully dissolved in order to aid in the nanotube dispersion. The suspension was then air-dried in a fume cupboard until semi-dry, then subjected to a draw ratio of 50 along one direction, folded over and the process was repeated 100 times. The samples were then compressed under 10 MPa at 200°C for 5 minutes with a hydraulic press, transferred to a mould and allowed to dry and cool overnight in a fumehood at 20°C. The drawing method resulted in highly orientated nanotubes within the PMMA matrix. Samples of up to 10 wt% of nanotube loadings were prepared. Strips were cut and heated to 120, 5°C above the polymers glass transition, then a weight applied to act as the deforming force. Mechanical improvements were measured with increases to the draw ratio of the material with increase in loading of the filler particle; however, this was limited to the direction of the tubes long axis alignment. Perpendicular to the common alignment of the tubes long axis, the material exhibited worse draw ratios (weaker mechanical properties), similar to that observed with the SWCNT-epoxy resin composites looked at by Wang et al. [92]. The induced aligned arrays of SWCNTs in the PMMA matrix with the electrical conductivity significantly improved along the draw direction as compared to the perpendicular direction. Electrical conduction of the polymer/CNT nancomposite is possible through the development of an interconnecting CNT network structure within the polymer matrix. Tube contact is a necessary for the transport of electrons across the material, with adequate dispersion of the tubes and a sufficient quantity of CNT within the matrix material being required to make the material conducting throughout. The electrical conductivity of carbon nanotubes displays anisotropic properties, with electron transport more favourable along the tubes axis [93]. Due to this anisotropy in the electrical properties of the CNTs, any composite materials prepared would benefit from alignment of the tubes along a common axis as was demonstrated in this particular study. Dai et al. also measured improved thermal stability of the material with higher loading percentage of the nanofiller, increasing the maximum mass loss temperature of the material compared to pure PMMA alone [91].

Similar electrical and mechanical improvements to a matrix material along the tube orientation direction have been observed by Wang et al. who prepared composite samples of epoxy resin/SWCNTs [92]. The orientation of the SWCNTs was induced through manual stretching of semi-dried composite films with a draw ratio of 50, followed by folding of the sample and repeating the process 100 times. This drawing process resulted in a high degree of orientation of the CNT filler particles. Samples containing different weight percentage loadings of the filler were prepared and observations on the conductivity and mechanical properties were made. It was observed that the conductivity was significantly better along the direction of common alignment of the tube axis. In this study, composite films that did not undergo stretching were also prepared for comparison purposes. The results showed that the alignment resulted in higher conductivity levels along the stretching direction, for example the 3 wt% loading of SWCNT that had undergone the stretching process had a conductivity of ~1 × 10−6 S/cm along the stretching direction (2.8 × 10−9 S/cm perpendicular), whereas the unstretched sample had a conductivity of 5 × 10−8 S/cm. The addition of 3 wt% of the SWCNTs to the epoxy resin improved the Young's modulus by ~2% and the tensile strength by ~2.6% with respect to the CNTs common axis of alignment; however, at loadings greater than 6 wt%, it was observed that the mechanical properties perpendicular to the tube orientation were lower than the epoxy resin on its own, possibly related to flocculation of the SWCNTs. When compared to the composite samples that had not undergone the stretching procedure, the samples with the aligned filler showed superior mechanical properties along the stretching direction when comparing the values of the Young's modulus and tensile strength to the unstretched samples, however perpendicular to the stretching direction the mechanical properties are not as good as the unstretched samples.

to another. For example, a polypropylene/polyethylene/CNT composite has been found to exhibit a dramatic decrease in conductivity when drawn; in contrast, electrical conductivity of a PVA/SWNT composite showed a fourfold increase when stretched. This is a consequence of the relationship between the electrical conductivity and the percolation network; this network is likely to be distorted by drawing and the consequential change in properties will depend

Dai et al*.* [91] examined quasi-straight SWCNTs dispersed in a polymethyl methacrylate (PMMA) matrix material. Samples were prepared from solution by taking the SWCNT powder and dispersing it in dimethylformamide (DMF) followed by dissolution of PMMA. Sonication was used for 1 hour before dissolving the PMMA and for another hour after once the PMMA was fully dissolved in order to aid in the nanotube dispersion. The suspension was then air-dried in a fume cupboard until semi-dry, then subjected to a draw ratio of 50 along one direction, folded over and the process was repeated 100 times. The samples were then compressed under 10 MPa at 200°C for 5 minutes with a hydraulic press, transferred to a mould and allowed to dry and cool overnight in a fumehood at 20°C. The drawing method resulted in highly orientated nanotubes within the PMMA matrix. Samples of up to 10 wt% of nanotube loadings were prepared. Strips were cut and heated to 120, 5°C above the polymers glass transition, then a weight applied to act as the deforming force. Mechanical improvements were measured with increases to the draw ratio of the material with increase in loading of the filler particle; however, this was limited to the direction of the tubes long axis alignment. Perpendicular to the common alignment of the tubes long axis, the material exhibited worse draw ratios (weaker mechanical properties), similar to that observed with the SWCNT-epoxy resin composites looked at by Wang et al. [92]. The induced aligned arrays of SWCNTs in the PMMA matrix with the electrical conductivity significantly improved along the draw direction as compared to the perpendicular direction. Electrical conduction of the polymer/CNT nancomposite is possible through the development of an interconnecting CNT network structure within the polymer matrix. Tube contact is a necessary for the transport of electrons across the material, with adequate dispersion of the tubes and a sufficient quantity of CNT within the matrix material being required to make the material conducting throughout. The electrical conductivity of carbon nanotubes displays anisotropic properties, with electron transport more favourable along the tubes axis [93]. Due to this anisotropy in the electrical properties of the CNTs, any composite materials prepared would benefit from alignment of the tubes along a common axis as was demonstrated in this particular study. Dai et al. also measured improved thermal stability of the material with higher loading percentage of the nanofiller, increasing the maximum mass loss temperature of the material compared to

Similar electrical and mechanical improvements to a matrix material along the tube orientation direction have been observed by Wang et al. who prepared composite samples of epoxy resin/SWCNTs [92]. The orientation of the SWCNTs was induced through manual stretching of semi-dried composite films with a draw ratio of 50, followed by folding of the sample and repeating the process 100 times. This drawing process resulted in a high degree of orientation of the CNT filler particles. Samples containing different weight percentage

largely upon the loading and dispersion of the CNTs in the original sample.

pure PMMA alone [91].

136 Carbon Nanotubes - Recent Progress

Exposure to an electric field has shown to provide an effective method for the alignment of the carbon nanotubes. Martin et al*.* have examined the use of both AC and DC fields to aid in the alignment of carbon nanotubes [94]. Low concentrations of CVD-grown MWCNTs were dispersed in an epoxy resin based on bisphenol-A. In the field, it was observed that a fraction of the nanotubes move towards the anode during the epoxy drying stage. These align with the tubes long axis perpendicular to the anode. The tubes themselves then act as a very high field strength at its tip attracting more tubes which form a connecting network between the anode and cathode. The AC field provided a more uniform connecting pathway between the anode and the cathode. Both fields exhibited a concentration of tubes highest at the anode.

Measurements made by Gupta et al*.* [81] on the electrical conductivity of polyvinylidene fluoride (PVDF), MWCNT composite materials exhibited improved electrical conductivity with improved alignment of the MWCNT filler particles. Five films of the composite materials were prepared with a solution cast method. During casting, two samples were exposed to an alternating voltage at 220 V and 500 V, whereas two others were exposed to alternating pulsed currents from a 220 V and 500 V source. The fifth sample was not exposed to any field resulting in a film that possessed a randomly orientated filler. It was observed that both the alternating voltage and alternating pulsed currents resulted in alignment of the nanotubes within the matrix (with the exception of the sample prepared with an alternating voltage at 220 V). However, differences between the two alignment techniques were observed; the sample exposed to an alternating pulsed current developed conducting channels across the matrix material, where the CNTs joined up in an endto-end fashion. Meanwhile the films exposed to the alternating voltage resulted in alignment of the CNTs along a common axis, while maintaining their uniform dispersion throughout the matrix. The alternating pulsed current method provided the necessary channels of CNTs for conduction across the matrix, where measurements of the conductivity show a 360% improvement to the electrical conductivity as compared to the sample possessing randomly orientated CNTs. It was observed that for the sample prepared at 500 V with an alternating pulsed current, the film only exhibited 60% increase in conductivity compared to a randomly orientated sample. Observations of this film during its preparation showed electrical sparking and therefore higher temperatures in areas causing a partial melting and distortion of the film, explaining the drop in conductivity of the film prepared at 500 V compared to the one at 220 V. The conductivity of the nanocomposites was observed to be significantly higher in the direction of the tube alignment compared to perpendicular to the alignment, as would be expected given the anisotropic electrical properties of the CNT filler particles and the channels formed in the material. In addition to the electrical properties, the mechanical properties of the films were measured and showed significant improvements to the Young's modulus and the ultimate tensile strength (180% and 150% respectively) compared to the pure PVDF. However, only slight improvements were measured (26% to the Young's modulus and 30% to the ultimate tensile strength) for the samples possessing the aligned CNT filler when measured along the CNT alignment direction as compared to the samples containing the randomly orientated CNT filler.

More recent developments in alignment of carbon nanotubes involve encapsulation into a narrow fibre using techniques such as electrospinning. Nazhipkyzy et al*.* have examined [70, 82] electrospun fibres as a potential medium for the encapsulation and alignment of carbon nanotubes. Polyethylene oxide fibres were electrospun from aqueous solution containing different concentrations of nanotubes. To aid in the spinning process, a surfactant, Triton X-100, was added to the solution that greatly aided in the spinning process. Fibres were spun onto a rotating collector that resulted in aligned arrays of parallel fibres. The fibre diameters varied from 100 to 400 nm and TEM images revealed encapsulation of the tubes with the alignment of the CNTS along the electrospun fibres long axis [82].

the encapsulation of the CNTs within a fibrillar structure, where the process of drawing out and enclosing into a narrow diameter fibre can result in the nanoparticles alignment. We use the electrospinning technique to achieve this. During the spinning process, the jet undergoes a shearing force and a narrowing in which the nanoparticulates can be aligned and encapsulated within the fibre as it forms. These fibres themselves have the potential to be used in composite materials to provide mechanical reinforcement; for example, Shinde et al. were able to incorporate electrospun fibres into a resin to improve interlaminar shear stresses of

**Figure 3.** Methodologies for creating fibres from nanoparticles (dark grey). (a) Particles aligned with fibre axis and stabilised in this arrangement by a solid polymer matrix (light grey); (b) particles aligned with fibre axis and stabilised in this arrangement by cross-linking chains which link the particles together; (c) particles aligned with fibre axis and joined end-to-end by short cross-linking chains, which link particles together (reproduced with permission from Mohan

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The use of layer by layer technology selected by Mohan et al. [82] makes it straightforward to organise the deposition of electrospun fibres, followed by an embedding layer of photocurable resin. The process is completed by photocuring using both UV-visible and IR light [71]. The cycle is then repeated multiple times until the thickness of the object is achieved. Since each layer is deposited sequentially, it is possible to prepare composites with 0 and 90 degree alignment of fibres to prevent defibrillation as might take place in the single orientation case. The use of a low molar mass resin which is then cross-linked means that the resin can flow and wet all fibre surfaces. In the work reported by Mohan et al*.* [70], the electrospun mean fibre diameter was 360 nm. As the cross-linkable resin is deposited at room temperature, it means that the polymer encapsulation of the CNTs is unaffected by the resin and the spatial distribution remains intact. Thus, this multistep process overcomes both challenges of disper-

glass fibre composites [95]**.**

et al. [82]).

sion and alignment.

#### **5. The way forward**

The earlier parts of this chapter have identified the critical need for good dispersion and control of alignment of the CNTs. In Section 3, we identified a number of possible routes to achieving a high level of common alignment. However, many of the approaches reviewed were most suited for application with thin films. The authors have recently reported a methodology for preparing CNT-based composites using additive manufacturing technology, and we review this approach here as it underlines the challenges in preparing highly anisotropic CNT-based composites for bulk samples (**Figure 3**).

There has been a huge surge of research in ᵗhe area of additive manufacturing (AM) over the last few years. The methods used in AM range from thermal extrusion, where a polymer is melted, extruded and deposited in a layer by layer process; granular processing, through melting powder layers using, for example, a laser, and photopolymerisation, where selective areas of a liquid, photocurable polymer are exposed to electromagnetic radiation to induce polymerisation and solidification [71]. Through the use of computer-aided design (CAD), the additive manufacturing technique allows for the manufacturing of complex shaped parts. The methodology which we believe [70, 82] is the best way forward involves

CNTs. It was observed that for the sample prepared at 500 V with an alternating pulsed current, the film only exhibited 60% increase in conductivity compared to a randomly orientated sample. Observations of this film during its preparation showed electrical sparking and therefore higher temperatures in areas causing a partial melting and distortion of the film, explaining the drop in conductivity of the film prepared at 500 V compared to the one at 220 V. The conductivity of the nanocomposites was observed to be significantly higher in the direction of the tube alignment compared to perpendicular to the alignment, as would be expected given the anisotropic electrical properties of the CNT filler particles and the channels formed in the material. In addition to the electrical properties, the mechanical properties of the films were measured and showed significant improvements to the Young's modulus and the ultimate tensile strength (180% and 150% respectively) compared to the pure PVDF. However, only slight improvements were measured (26% to the Young's modulus and 30% to the ultimate tensile strength) for the samples possessing the aligned CNT filler when measured along the CNT alignment direction as compared

More recent developments in alignment of carbon nanotubes involve encapsulation into a narrow fibre using techniques such as electrospinning. Nazhipkyzy et al*.* have examined [70, 82] electrospun fibres as a potential medium for the encapsulation and alignment of carbon nanotubes. Polyethylene oxide fibres were electrospun from aqueous solution containing different concentrations of nanotubes. To aid in the spinning process, a surfactant, Triton X-100, was added to the solution that greatly aided in the spinning process. Fibres were spun onto a rotating collector that resulted in aligned arrays of parallel fibres. The fibre diameters varied from 100 to 400 nm and TEM images revealed encapsulation of the tubes with the alignment of the

The earlier parts of this chapter have identified the critical need for good dispersion and control of alignment of the CNTs. In Section 3, we identified a number of possible routes to achieving a high level of common alignment. However, many of the approaches reviewed were most suited for application with thin films. The authors have recently reported a methodology for preparing CNT-based composites using additive manufacturing technology, and we review this approach here as it underlines the challenges in preparing highly anisotropic

There has been a huge surge of research in ᵗhe area of additive manufacturing (AM) over the last few years. The methods used in AM range from thermal extrusion, where a polymer is melted, extruded and deposited in a layer by layer process; granular processing, through melting powder layers using, for example, a laser, and photopolymerisation, where selective areas of a liquid, photocurable polymer are exposed to electromagnetic radiation to induce polymerisation and solidification [71]. Through the use of computer-aided design (CAD), the additive manufacturing technique allows for the manufacturing of complex shaped parts. The methodology which we believe [70, 82] is the best way forward involves

to the samples containing the randomly orientated CNT filler.

CNTS along the electrospun fibres long axis [82].

CNT-based composites for bulk samples (**Figure 3**).

**5. The way forward**

138 Carbon Nanotubes - Recent Progress

**Figure 3.** Methodologies for creating fibres from nanoparticles (dark grey). (a) Particles aligned with fibre axis and stabilised in this arrangement by a solid polymer matrix (light grey); (b) particles aligned with fibre axis and stabilised in this arrangement by cross-linking chains which link the particles together; (c) particles aligned with fibre axis and joined end-to-end by short cross-linking chains, which link particles together (reproduced with permission from Mohan et al. [82]).

the encapsulation of the CNTs within a fibrillar structure, where the process of drawing out and enclosing into a narrow diameter fibre can result in the nanoparticles alignment. We use the electrospinning technique to achieve this. During the spinning process, the jet undergoes a shearing force and a narrowing in which the nanoparticulates can be aligned and encapsulated within the fibre as it forms. These fibres themselves have the potential to be used in composite materials to provide mechanical reinforcement; for example, Shinde et al. were able to incorporate electrospun fibres into a resin to improve interlaminar shear stresses of glass fibre composites [95]**.**

The use of layer by layer technology selected by Mohan et al. [82] makes it straightforward to organise the deposition of electrospun fibres, followed by an embedding layer of photocurable resin. The process is completed by photocuring using both UV-visible and IR light [71]. The cycle is then repeated multiple times until the thickness of the object is achieved. Since each layer is deposited sequentially, it is possible to prepare composites with 0 and 90 degree alignment of fibres to prevent defibrillation as might take place in the single orientation case. The use of a low molar mass resin which is then cross-linked means that the resin can flow and wet all fibre surfaces. In the work reported by Mohan et al*.* [70], the electrospun mean fibre diameter was 360 nm. As the cross-linkable resin is deposited at room temperature, it means that the polymer encapsulation of the CNTs is unaffected by the resin and the spatial distribution remains intact. Thus, this multistep process overcomes both challenges of dispersion and alignment.

#### **6. Summary**

In this chapter, we show that CNTs offer some exceptional properties for the design of multiscale composite materials. In terms of production in bulk, the technology is not quite routine, but clearly promise exists. We have laid out an approach, which we have shown, overcomes the challenges of both dispersion and alignment and deserves wider application.

[6] Shao D et al. Polyaniline Multiwalled Carbon Nanotube Magnetic Composite Prepared by Plasma-Induced Graft Technique and Its Application for Removal of Aniline and

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141

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There are great opportunities for the inclusion of CNTs in the emerging technology of additive or direct digital manufacturing. The future is especially promising in this area.

#### **Acknowledgements**

The research work was funded by the Fundação para a Ciencia e a Technologia (Portugal), through the Project reference UID/Multi/04,044/2013 and by the National Agency of Innovation (ANI) (Portugal) through project i.FILM – Multifunctional Films for Intelligent and Active Applications Project Number: 17921.

#### **Author details**

Geoffrey R. Mitchell1 \*, Fred J. Davis<sup>2</sup> , Saeed Mohan<sup>2</sup> and Meruyert Nazhipkyzy<sup>3</sup>

\*Address all correspondence to: geoffrey.mitchell@ipleiria.pt

1 Centre for Rapid and Sustainable Product Development, Institute Polytechnic of Leiria, Rua de Portugal, Marinha Grande, Portugal


#### **References**


[6] Shao D et al. Polyaniline Multiwalled Carbon Nanotube Magnetic Composite Prepared by Plasma-Induced Graft Technique and Its Application for Removal of Aniline and Phenol. Journal of Physical Chemistry C. 2010;**114**:21524-21530

**6. Summary**

140 Carbon Nanotubes - Recent Progress

**Acknowledgements**

**Author details**

Geoffrey R. Mitchell1

**References**

Applications Project Number: 17921.

\*, Fred J. Davis<sup>2</sup>

3 Institute of Combustion Problems, Almaty, Kazakhstan

Rua de Portugal, Marinha Grande, Portugal

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\*Address all correspondence to: geoffrey.mitchell@ipleiria.pt

2 Department of Chemistry, University of Reading, Whiteknights, UK

In this chapter, we show that CNTs offer some exceptional properties for the design of multiscale composite materials. In terms of production in bulk, the technology is not quite routine, but clearly promise exists. We have laid out an approach, which we have shown, overcomes

There are great opportunities for the inclusion of CNTs in the emerging technology of addi-

The research work was funded by the Fundação para a Ciencia e a Technologia (Portugal), through the Project reference UID/Multi/04,044/2013 and by the National Agency of Innovation (ANI) (Portugal) through project i.FILM – Multifunctional Films for Intelligent and Active

, Saeed Mohan<sup>2</sup>

1 Centre for Rapid and Sustainable Product Development, Institute Polytechnic of Leiria,

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**Chapter 9**

Provisional chapter

**Recent Progress on Electrochemical Capacitors Based**

This review is focused on the theoretical and practical aspects of electrochemical capacitors based on carbon nanotubes. In particular, recent improvements in the capacitance properties of the systems are discussed. In the first part, the charge storage mechanisms of the electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine single- and multi-walled carbon nanotubes. The major portion of the review is focused on the capacitance properties of modified carbon nanotubes. The electrochemical properties of nanotubes with boron, nitrogen, and other atoms incorporated into the carbon network structure as well as nanotubes modified with different functional groups are discussed. Special attention is paid to the composites of carbon nanotubes and conducting polymers, transition metal oxides, carbon nanostructures, and carbon gels. In all cases, the influences of different parameters such as porosity, structure of the electroactive layer, conductivity of the layer, nature of the heteroatoms, solvent and supporting electrolyte on the capacitance performance of hybrid materials are discussed. Finally, the capacitance properties of different systems containing carbon

Keywords: carbon nanotubes, carbon nanoparticles, conducting polymers, composites,

The last two decades can be considered a nanotechnology revolution. With each passing day, increasing attention is paid to the discovery of new nanoscale materials due to the miniaturization of devices in many areas. Nanomaterials are everywhere, from cosmetics and clothes to medicine and electronic devices. At the same time, the increased development of electronic devices requiring energy storage systems, such as batteries and supercapacitors, is starting to play a crucial role in everyday life. Supercapacitors, also called electrochemical capacitors, are

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

Recent Progress on Electrochemical Capacitors Based on

DOI: 10.5772/intechopen.71687

**on Carbon Nanotubes**

Carbon Nanotubes

Abstract

1. Introduction

Emilia Grądzka and Krzysztof Winkler

Emilia Grądzka and Krzysztof Winkler

http://dx.doi.org/10.5772/intechopen.71687

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

nanotubes are compared and summarized.

electrochemical capacitors, charge storage materials


#### **Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes** Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

DOI: 10.5772/intechopen.71687

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[94] Martin CA, Sandler JKW, Windle AH, Schwarz M-K, Bauhofer W, Schulte K, Shaffer MSP. Electric field-induced aligned multi-wall carbon nanotube networks in epoxy com-

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collector plates. Polymer Letters. 2015;**9**:105-118

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146 Carbon Nanotubes - Recent Progress

2002;**35**:L77-L80

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posites. Polymer. 2005;**46**:877-886

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Science. 1995;**268**:845-847

2015

Emilia Grądzka and Krzysztof Winkler Emilia Grądzka and Krzysztof Winkler

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71687

#### Abstract

This review is focused on the theoretical and practical aspects of electrochemical capacitors based on carbon nanotubes. In particular, recent improvements in the capacitance properties of the systems are discussed. In the first part, the charge storage mechanisms of the electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine single- and multi-walled carbon nanotubes. The major portion of the review is focused on the capacitance properties of modified carbon nanotubes. The electrochemical properties of nanotubes with boron, nitrogen, and other atoms incorporated into the carbon network structure as well as nanotubes modified with different functional groups are discussed. Special attention is paid to the composites of carbon nanotubes and conducting polymers, transition metal oxides, carbon nanostructures, and carbon gels. In all cases, the influences of different parameters such as porosity, structure of the electroactive layer, conductivity of the layer, nature of the heteroatoms, solvent and supporting electrolyte on the capacitance performance of hybrid materials are discussed. Finally, the capacitance properties of different systems containing carbon nanotubes are compared and summarized.

Keywords: carbon nanotubes, carbon nanoparticles, conducting polymers, composites, electrochemical capacitors, charge storage materials

#### 1. Introduction

The last two decades can be considered a nanotechnology revolution. With each passing day, increasing attention is paid to the discovery of new nanoscale materials due to the miniaturization of devices in many areas. Nanomaterials are everywhere, from cosmetics and clothes to medicine and electronic devices. At the same time, the increased development of electronic devices requiring energy storage systems, such as batteries and supercapacitors, is starting to play a crucial role in everyday life. Supercapacitors, also called electrochemical capacitors, are

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

energy saving units that can provide a huge amount of energy in a short time. Compared with batteries, electrochemical capacitors offer great advantages of high power capability, high rates of charge and discharge, high cycle life, flexible packaging and low weight [1–4].

electrode/electrolyte interface defined as electric-double layer capacitance (EDLC) (Figure 1a) and (ii) an electrochemical oxidation/reduction reaction in electroactive materials accompanied

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

Typical supercapacitors consist of two electrodes separated by a dielectric material or electrolyte solution. In the case of EDLC, where the capacitance comes from the electrostatic charge accumulated at the electrode/electrolyte interface and strongly depends on the surface area of the electrode accessible to the electrolyte ions, mostly porous carbons are used as electrode materials due to their low price, facile synthesis, and sustainability. They are prepared by heat treatment and subsequent chemical activation with organic materials or, in the case of "carbide-derived carbons" (CDCs), by extracting metal atoms in metal carbides. More exotic materials, such as carbon nanotubes (CNTs) and graphene, are also being developed for supercapacitor applications. The operating voltage of supercapacitors depends on the solvent and electrolytes separating both electrodes. Tetraethylammonium tetrafluoroborate is often used as the supporting electrolyte in organic solvents. This salt offers a relatively high operating voltage of approxi-

potential of 1.2 V can be obtained. Moreover, room-temperature ionic liquids with an operating voltage as high as 4 V could also be used as electrolyte systems. However, supercapacitors based on ionic liquids have poor device power performance due to their low ionic conductivity, i.e.,

The specific capacitance for EDLC is assumed to follow that of a parallel-plate capacitor:

<sup>C</sup> <sup>¼</sup> <sup>ε</sup>rε<sup>0</sup>

where ε<sup>r</sup> is the electrolyte dielectric constant, ε<sup>0</sup> is the permittivity of vacuum, A is the specific surface area of the electrode, and d is the effective thickness of the EDLC. Depending on the pore size of the electrode material, two models are proposed: an electric double cylinder capacitor (EDCC) model that describes mesoporous carbon electrodes (Figure 2a) and an electric

Figure 1. Schematic diagram of supercapacitors: (a) EDLC and (b) pseudocapacitor (M represents the metal atom).

Reproduced with permission from Ref. [32]. Copyright 2014 The Royal Society of Chemistry.

. In aqueous solutions, a maximum

http://dx.doi.org/10.5772/intechopen.71687

149

dA (1)

with Faradaic charge transfer named pseudocapacitance (Figure 1b) [4, 29–32].

mately 2.5 V and a high ionic conductivity of 20–60 mS cm�<sup>1</sup>

below 20 mS cm�<sup>1</sup> [23, 30, 33].

An interesting class of nanomaterial for storage devices is carbon nanotubes due to their large open surface that is completely exposed to electrolyte ions, good electrical conductivity, high surface area, mechanical strength, good corrosion resistance, chemical stability and low mass density [4–8]. However, so far, pristine CNTs have not met the commercial requirements for energy storage devices applications due to their poor dispersion in solvents, low electrochemical characteristics in the electrolytes, and low specific capacitance [7, 8]. The capacitance properties of carbon nanotubes may be improved by surface functionalization or by the formation of composites with redox-active systems such as conducting polymers or metal oxides. On the other hand, carbon nanotubes are excellent conducting supports to improve the properties of materials with poor conductivity, stability, and capacitance performance. Hence, carbon nanotubes-based nanocomposites or nanohybrids have recently been intensively developed because of their superior properties compared to the individual component alone. Much research on supercapacitors has aimed at increasing power and energy densities as well as lowering fabrication cost [8–18].

The objective of this review is to highlight theoretical and practical aspects of the capacitance properties of carbon nanotubes and their composites. In particular, recent progress in improving the capacitance properties of systems based on carbon nanotubes is discussed. In the first part, the charge storage mechanisms in electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine carbon nanotubes and carbon nanotubes modified by heteroatoms or functional groups. Special attention is paid to the composites of carbon nanotubes with conducting polymers, carbon nanostructures, transition metal oxides, and carbon gels.

#### 2. Principles and mechanism of charge storage

Electrochemical energy storage may be classified as encompassing either batteries or electrochemical capacitors (ECs), which are also known as supercapacitors, ultracapacitors, electrical double-layer capacitors, pseudocapacitors, gold capacitors or power coaches [19–21]. Supercapacitors fill the gap between batteries that exhibit high energy density and high power density conventional capacitors, covering several orders of magnitude both in energy and in power density. Batteries compared to supercapacitors store and deliver more energy with slower charge and discharge times. In the case of conventional capacitors, they have very high energy density in comparison to supercapacitors. Supercapacitors also exhibit low heating level, safety, long-term operation stability, low weight, and flexible packaging [4, 22, 23]. Due to these properties of semiconductors, they can be applied in many fields of electrotechnology. They can be used in electric and hybrid vehicles [3, 24, 25], high-energy pulsed lasers [26], mobile phones [26–28], laptops [27], and cameras [27, 28].

The charge-storage mechanism of electrochemical capacitors is controlled by two principal mechanisms: (i) a non-Faradaic electrostatic interaction resulting from ion adsorption at the electrode/electrolyte interface defined as electric-double layer capacitance (EDLC) (Figure 1a) and (ii) an electrochemical oxidation/reduction reaction in electroactive materials accompanied with Faradaic charge transfer named pseudocapacitance (Figure 1b) [4, 29–32].

energy saving units that can provide a huge amount of energy in a short time. Compared with batteries, electrochemical capacitors offer great advantages of high power capability, high rates

An interesting class of nanomaterial for storage devices is carbon nanotubes due to their large open surface that is completely exposed to electrolyte ions, good electrical conductivity, high surface area, mechanical strength, good corrosion resistance, chemical stability and low mass density [4–8]. However, so far, pristine CNTs have not met the commercial requirements for energy storage devices applications due to their poor dispersion in solvents, low electrochemical characteristics in the electrolytes, and low specific capacitance [7, 8]. The capacitance properties of carbon nanotubes may be improved by surface functionalization or by the formation of composites with redox-active systems such as conducting polymers or metal oxides. On the other hand, carbon nanotubes are excellent conducting supports to improve the properties of materials with poor conductivity, stability, and capacitance performance. Hence, carbon nanotubes-based nanocomposites or nanohybrids have recently been intensively developed because of their superior properties compared to the individual component alone. Much research on supercapacitors has aimed at increasing power and energy densities

The objective of this review is to highlight theoretical and practical aspects of the capacitance properties of carbon nanotubes and their composites. In particular, recent progress in improving the capacitance properties of systems based on carbon nanotubes is discussed. In the first part, the charge storage mechanisms in electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine carbon nanotubes and carbon nanotubes modified by heteroatoms or functional groups. Special attention is paid to the composites of carbon nanotubes with conducting polymers, carbon nanostructures,

Electrochemical energy storage may be classified as encompassing either batteries or electrochemical capacitors (ECs), which are also known as supercapacitors, ultracapacitors, electrical double-layer capacitors, pseudocapacitors, gold capacitors or power coaches [19–21]. Supercapacitors fill the gap between batteries that exhibit high energy density and high power density conventional capacitors, covering several orders of magnitude both in energy and in power density. Batteries compared to supercapacitors store and deliver more energy with slower charge and discharge times. In the case of conventional capacitors, they have very high energy density in comparison to supercapacitors. Supercapacitors also exhibit low heating level, safety, long-term operation stability, low weight, and flexible packaging [4, 22, 23]. Due to these properties of semiconductors, they can be applied in many fields of electrotechnology. They can be used in electric and hybrid vehicles [3, 24, 25], high-energy pulsed lasers [26],

The charge-storage mechanism of electrochemical capacitors is controlled by two principal mechanisms: (i) a non-Faradaic electrostatic interaction resulting from ion adsorption at the

of charge and discharge, high cycle life, flexible packaging and low weight [1–4].

as well as lowering fabrication cost [8–18].

148 Carbon Nanotubes - Recent Progress

transition metal oxides, and carbon gels.

2. Principles and mechanism of charge storage

mobile phones [26–28], laptops [27], and cameras [27, 28].

Typical supercapacitors consist of two electrodes separated by a dielectric material or electrolyte solution. In the case of EDLC, where the capacitance comes from the electrostatic charge accumulated at the electrode/electrolyte interface and strongly depends on the surface area of the electrode accessible to the electrolyte ions, mostly porous carbons are used as electrode materials due to their low price, facile synthesis, and sustainability. They are prepared by heat treatment and subsequent chemical activation with organic materials or, in the case of "carbide-derived carbons" (CDCs), by extracting metal atoms in metal carbides. More exotic materials, such as carbon nanotubes (CNTs) and graphene, are also being developed for supercapacitor applications. The operating voltage of supercapacitors depends on the solvent and electrolytes separating both electrodes. Tetraethylammonium tetrafluoroborate is often used as the supporting electrolyte in organic solvents. This salt offers a relatively high operating voltage of approximately 2.5 V and a high ionic conductivity of 20–60 mS cm�<sup>1</sup> . In aqueous solutions, a maximum potential of 1.2 V can be obtained. Moreover, room-temperature ionic liquids with an operating voltage as high as 4 V could also be used as electrolyte systems. However, supercapacitors based on ionic liquids have poor device power performance due to their low ionic conductivity, i.e., below 20 mS cm�<sup>1</sup> [23, 30, 33].

The specific capacitance for EDLC is assumed to follow that of a parallel-plate capacitor:

$$\mathbb{C} = \frac{\varepsilon\_r \varepsilon\_0}{dA} \tag{1}$$

where ε<sup>r</sup> is the electrolyte dielectric constant, ε<sup>0</sup> is the permittivity of vacuum, A is the specific surface area of the electrode, and d is the effective thickness of the EDLC. Depending on the pore size of the electrode material, two models are proposed: an electric double cylinder capacitor (EDCC) model that describes mesoporous carbon electrodes (Figure 2a) and an electric

Figure 1. Schematic diagram of supercapacitors: (a) EDLC and (b) pseudocapacitor (M represents the metal atom). Reproduced with permission from Ref. [32]. Copyright 2014 The Royal Society of Chemistry.

Figure 2. Schematic diagrams (top views) of (a) a negatively charged mesopore with solvated cations approaching the pore wall to form an electric double-cylinder capacitor and (b) a negatively charged micropore of radius b with cations lining up along the pore axis to form an electric wire-in-cylinder capacitor. Reproduced with permission from Ref. [34]. Copyright 2008 Wiley-VCH Verlag.

wire-in-cylinder capacitor (EWCC) model corresponding to microporous carbon electrodes (Figure 2b). The equation describing the capacitance in these two models can be written as:

$$C = \frac{\varepsilon\_r \varepsilon\_0}{b \ln\left[\frac{b}{b-d}\right]} A \tag{2}$$

where C is the capacitance and V is the operating voltage limited by the thermodynamic stability of the electrolyte solution. The total power of a supercapacitor is described by the

<sup>P</sup> <sup>¼</sup> <sup>V</sup><sup>2</sup>

where R is the internal resistance of the capacitor coming from the various types of resistances associated with the intrinsic electronic properties of the electrode material and electrolyte solution, mass transfer resistance of the ions in the electrode material, and contact resistance

The electrode is one of the most important components for charge storage and plays a crucial role in determining the energy and power density of supercapacitors [29]. In EDLC, the high surface area resulting from the highly microporous structure of carbon electrode materials such as carbon aerogels, carbon black or carbon cloth is unfavorable for electrolyte wetting and rapid ionic motions, especially at high current loads [22]. The most frequently used activated carbon electrodes exhibit low electrolyte accessibility and poor electrical conductivity. These two effects are responsible for limited energy density and high internal resistance. For this reason, carbon nanotubes (CNTs) with nanoscale size, controllable size distribution, large surface area, high mesoporosity, electrolyte accessibility, and good electrical properties are very promising candidates for replacing carbon materials as the electrode materials in high-performance capacitors. Because of this fact, an extended part of this work will focus on

3. Capacitance properties of single- and multi-walled carbon nanotubes

Carbon nanotubes exhibit large open surface area, excellent mechanical strength, chemical stability, low mass density, and relatively good electrical conductivity. All these properties make them a very good candidate for the electroactive material in charge storage devices. In these capacitors, the ions of electrolyte are adsorbed on the charged surface of carbon nanotubes, producing a Helmholtz layer. The capacitance properties of these systems depend on the number of graphene walls (single-, double-, or multi-wall), the nature of the electrode material, the composition of the electrolyte solution, and the structure of the carbon nanotube layer. The specific capacitance of pristine CNTs is relatively low and ranges from 2 to 45 F g�<sup>1</sup> for SWCNTs [35–42] and 3 to 80 F g�<sup>1</sup> for MWCNTs [36–38, 42–44]. The capacitance properties of thin films of CNTs are collected in

Such a large scattering of reported specific capacitance values is mainly related to the different procedures of CNT deposition on the conducting electrode surface and, therefore, different structures of CNTs films. Carbon nanotubes can be attached to electrodes by direct growth [45], manual manipulation [46], random spreading [47], deposition in a dc current electric field [48], or gas flow [49]. Specially designed substrates are used for film formation of vertically oriented CNTs [50–53]. Two of the most common orientations of CNTs are displayed schematically in Figure 3. In general, films of vertically oriented carbon nanotubes exhibit much better

between the current collector and the electrode [22, 23, 30, 31].

supercapacitors based on carbon nanotubes [4, 29].

<sup>4</sup><sup>R</sup> (6)

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Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

following equation:

Table 1.

$$C = \frac{\varepsilon\_r \varepsilon\_0}{b \ln\left(\frac{b}{a\_0}\right)} A \tag{3}$$

where b is the pore radius, d is the distance of approaching ions to the surface of the carbon electrode, and a<sup>0</sup> is the effective size of counterions. When the pores are large enough and the pore curvature is no longer significant, the EDCC model is reduced to the traditional planar EDLC model described by Eq. (1) [30, 34].

In contrast to EDLC, pseudocapacitance results from faradaic fast reversible redox reactions involving electrode material. During electrochemical reactions, the electrode material is reduced or oxidized and doped with counterions from the electrolyte solution. In this case, conducting polymers and metal oxides or hydroxides as electrode materials are used [30]. High-area carbon electrodes also exhibit a small pseudocapacitance component due to electrochemically active redox functionalities [22]. Pseudocapacitance is described by the following equation:

$$C = \frac{d(\Delta q)}{d(\Delta V)}\tag{4}$$

where Δq is the charge related to the faradaic process in the potential range ΔV. The pseudocapacitance can reach higher values than the EDLC capacitance. However, redox capacitors exhibit lower power density due to poor electrical conductivity and lack of stability during the cycle.

The maximum energy stored in supercapacitors is given by

$$E = \frac{1}{2}CV^2\tag{5}$$

where C is the capacitance and V is the operating voltage limited by the thermodynamic stability of the electrolyte solution. The total power of a supercapacitor is described by the following equation:

$$P = \frac{V^2}{4R} \tag{6}$$

where R is the internal resistance of the capacitor coming from the various types of resistances associated with the intrinsic electronic properties of the electrode material and electrolyte solution, mass transfer resistance of the ions in the electrode material, and contact resistance between the current collector and the electrode [22, 23, 30, 31].

The electrode is one of the most important components for charge storage and plays a crucial role in determining the energy and power density of supercapacitors [29]. In EDLC, the high surface area resulting from the highly microporous structure of carbon electrode materials such as carbon aerogels, carbon black or carbon cloth is unfavorable for electrolyte wetting and rapid ionic motions, especially at high current loads [22]. The most frequently used activated carbon electrodes exhibit low electrolyte accessibility and poor electrical conductivity. These two effects are responsible for limited energy density and high internal resistance. For this reason, carbon nanotubes (CNTs) with nanoscale size, controllable size distribution, large surface area, high mesoporosity, electrolyte accessibility, and good electrical properties are very promising candidates for replacing carbon materials as the electrode materials in high-performance capacitors. Because of this fact, an extended part of this work will focus on supercapacitors based on carbon nanotubes [4, 29].

wire-in-cylinder capacitor (EWCC) model corresponding to microporous carbon electrodes (Figure 2b). The equation describing the capacitance in these two models can be written as:

Figure 2. Schematic diagrams (top views) of (a) a negatively charged mesopore with solvated cations approaching the pore wall to form an electric double-cylinder capacitor and (b) a negatively charged micropore of radius b with cations lining up along the pore axis to form an electric wire-in-cylinder capacitor. Reproduced with permission from Ref. [34].

<sup>A</sup> (2)

<sup>A</sup> (3)

<sup>d</sup>ð Þ <sup>Δ</sup><sup>V</sup> (4)

CV<sup>2</sup> (5)

<sup>C</sup> <sup>¼</sup> <sup>ε</sup>rε<sup>0</sup> bln <sup>b</sup> b�d

<sup>C</sup> <sup>¼</sup> <sup>ε</sup>rε<sup>0</sup> bln <sup>b</sup> a0

where b is the pore radius, d is the distance of approaching ions to the surface of the carbon electrode, and a<sup>0</sup> is the effective size of counterions. When the pores are large enough and the pore curvature is no longer significant, the EDCC model is reduced to the traditional planar

In contrast to EDLC, pseudocapacitance results from faradaic fast reversible redox reactions involving electrode material. During electrochemical reactions, the electrode material is reduced or oxidized and doped with counterions from the electrolyte solution. In this case, conducting polymers and metal oxides or hydroxides as electrode materials are used [30]. High-area carbon electrodes also exhibit a small pseudocapacitance component due to electrochemically active

<sup>C</sup> <sup>¼</sup> <sup>d</sup>ð Þ <sup>Δ</sup><sup>q</sup>

where Δq is the charge related to the faradaic process in the potential range ΔV. The pseudocapacitance can reach higher values than the EDLC capacitance. However, redox capacitors exhibit lower power density due to poor electrical conductivity and lack of stability during the cycle.

> <sup>E</sup> <sup>¼</sup> <sup>1</sup> 2

redox functionalities [22]. Pseudocapacitance is described by the following equation:

The maximum energy stored in supercapacitors is given by

EDLC model described by Eq. (1) [30, 34].

Copyright 2008 Wiley-VCH Verlag.

150 Carbon Nanotubes - Recent Progress

#### 3. Capacitance properties of single- and multi-walled carbon nanotubes

Carbon nanotubes exhibit large open surface area, excellent mechanical strength, chemical stability, low mass density, and relatively good electrical conductivity. All these properties make them a very good candidate for the electroactive material in charge storage devices. In these capacitors, the ions of electrolyte are adsorbed on the charged surface of carbon nanotubes, producing a Helmholtz layer. The capacitance properties of these systems depend on the number of graphene walls (single-, double-, or multi-wall), the nature of the electrode material, the composition of the electrolyte solution, and the structure of the carbon nanotube layer. The specific capacitance of pristine CNTs is relatively low and ranges from 2 to 45 F g�<sup>1</sup> for SWCNTs [35–42] and 3 to 80 F g�<sup>1</sup> for MWCNTs [36–38, 42–44]. The capacitance properties of thin films of CNTs are collected in Table 1.

Such a large scattering of reported specific capacitance values is mainly related to the different procedures of CNT deposition on the conducting electrode surface and, therefore, different structures of CNTs films. Carbon nanotubes can be attached to electrodes by direct growth [45], manual manipulation [46], random spreading [47], deposition in a dc current electric field [48], or gas flow [49]. Specially designed substrates are used for film formation of vertically oriented CNTs [50–53]. Two of the most common orientations of CNTs are displayed schematically in Figure 3. In general, films of vertically oriented carbon nanotubes exhibit much better


4. Covalent modification of carbon nanotubes and their capacitance

activity resulting in a pseudo-Faradaic capacitance effect.

Ref. [6]. Copyright 2011 American Chemical Society.

The covalent functionalization of carbon nanotubes provides modification of their physicochemical properties. Insertion of functional groups to the carbon network solves the problem of their poor dispersion capability. Stable dispersions of modified carbon nanotubes are particularly important in the formation of thin films of capacitors or capacitive devices. Moreover, tethering electron-donating or electron-withdrawing groups on CNT surfaces changes their electronic properties by the doping effect [22, 58–62]. Such groups exhibit electrochemical

Figure 4. SEM image of the aligned carbon nanotubes formed under CVD procedure. Reproduced with permission from

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

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153

Park and co-workers showed the relationship between the electrochemical activity of functional groups attached to the CNT surface and their capacitive characteristics [58]. They found that MWCNTs covalently functionalized by carboxylic, sulfonic, and amine groups (Figure 5) showed a two- to fourfold increase in capacitance over that of pristine MWCNTs due to pseudocapacitive charging-discharging arising from the presence of functional groups. Functionalized CNTs also form more stable dispersions in deionized water and polar organic solvents. Such dispersions can be used in the formation of mechanically stable and uniform capacitor films. Functionalized CNTs, however, exhibit a lower surface area due to the reduction in the average pore size as a consequence of the presence of surface functional groups [58]. Figure 6 shows the effect of the addition of oxygen redox-active molecules on the electrochemical performance of CNTs. Nanotubular materials can be treated (CNTs-T) chemically [62–70],

properties

PC, propylene carbonate.

Table 1. Selected values of specific capacitances obtained for thin films of carbon nanotubes.

Figure 3. Schematic representation of nanotubes orientation at the electrode surface.

electrical and capacitance properties in comparison to films formed from randomly oriented nanotubes [6, 50, 54–57]. Figure 4 shows the structure of the SWCNT layer formed under CVD growth procedure. The highest specific capacitance of 52 F g�<sup>1</sup> and excellent electrochemical stability were reported for such vertical structures of SWCNTs [55].

Figure 4. SEM image of the aligned carbon nanotubes formed under CVD procedure. Reproduced with permission from Ref. [6]. Copyright 2011 American Chemical Society.

### 4. Covalent modification of carbon nanotubes and their capacitance properties

The covalent functionalization of carbon nanotubes provides modification of their physicochemical properties. Insertion of functional groups to the carbon network solves the problem of their poor dispersion capability. Stable dispersions of modified carbon nanotubes are particularly important in the formation of thin films of capacitors or capacitive devices. Moreover, tethering electron-donating or electron-withdrawing groups on CNT surfaces changes their electronic properties by the doping effect [22, 58–62]. Such groups exhibit electrochemical activity resulting in a pseudo-Faradaic capacitance effect.

Park and co-workers showed the relationship between the electrochemical activity of functional groups attached to the CNT surface and their capacitive characteristics [58]. They found that MWCNTs covalently functionalized by carboxylic, sulfonic, and amine groups (Figure 5) showed a two- to fourfold increase in capacitance over that of pristine MWCNTs due to pseudocapacitive charging-discharging arising from the presence of functional groups. Functionalized CNTs also form more stable dispersions in deionized water and polar organic solvents. Such dispersions can be used in the formation of mechanically stable and uniform capacitor films. Functionalized CNTs, however, exhibit a lower surface area due to the reduction in the average pore size as a consequence of the presence of surface functional groups [58].

Figure 6 shows the effect of the addition of oxygen redox-active molecules on the electrochemical performance of CNTs. Nanotubular materials can be treated (CNTs-T) chemically [62–70],

electrical and capacitance properties in comparison to films formed from randomly oriented nanotubes [6, 50, 54–57]. Figure 4 shows the structure of the SWCNT layer formed under CVD growth procedure. The highest specific capacitance of 52 F g�<sup>1</sup> and excellent electrochemical

stability were reported for such vertical structures of SWCNTs [55].

Figure 3. Schematic representation of nanotubes orientation at the electrode surface.

Table 1. Selected values of specific capacitances obtained for thin films of carbon nanotubes.

Carbon nanostructure Specific capacitance (F g�<sup>1</sup>

152 Carbon Nanotubes - Recent Progress

PC, propylene carbonate.

SWCNTs 2 0.1 M Na2SO4 [39]

MWCNTs 3 7 M KOH [43]

 1 M NaCl [40] 1 M NaCl [35] 5 M KOH [35] 6 M KOH [41] 1 M NaCl [35] 7 M H2SO4 [35] 6 M KOH [38] 1 M H2SO4 [42]

 7 M KOH [43] 1 M H2SO4 [42] 6 M KOH [38] 4 M H2SO4 [43] 4 M H2SO4 [43] 1 M H2SO4 [44] 6 M KOH [38] 1 M Na2SO4 [44] 6 M KOH [38] 1 M H2SO4 [38] 6 M KOH [38]

) Experimental conditions Reference

ð7Þ

155

ð8Þ

ð9Þ

A higher current observed for CNTs-T suggests that their surface area is larger compared to that of raw CNTs. The surface of oxygen-containing functional groups also decreases surface resistivity and enhances surface wettability, offering more accessible sites for the physisorption of electrolyte ions and increasing the ionic conductivity at the electrode/electrolyte interface. In the procedure proposed by Wang and co-workers [62], the specific capacitance was increased from 28 F g�<sup>1</sup> for raw CNTs to 85 F g�<sup>1</sup> for CNTs-T in H2SO4 solution. Changing the electrolyte to H2SO4/hydroquinone mixture provides a drastic improvement of the specific capacitance to

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

http://dx.doi.org/10.5772/intechopen.71687

The distribution and type of oxygen-containing functional groups depend on the type (Figure 7a)

The oxidation of CNTs is a principle reaction for further functionalization of the carbon network. The combination of CNTs with graphene oxide in a lamellar graphene-CNT structure, r[GO/CNT], shown in Figure 8 causes an increase in the electrolyte-accessible surface area due to the intercalation of CNTs between the stacked GO sheets with associated large electrochemical active sites, thus improving conductivity through the formation of a 3D network aided by CNTs. Such a covalently linked CNTs-graphene system exhibits capacitance

The high specific capacitance of CNTs could also be obtained with the incorporation of heteroatoms such as N [61, 78–87], B [86–89], P [86, 90], and F [91]. The type of defect created by the heteroatom influences the kind of conduction generated ranging from n-type transport

Figure 7. (a) Influence of the oxidant on the distribution of oxygen-containing functional groups, measured on MWCNTs. (b) Influence of % w/w HNO3 on the level of oxidation, as well as the distribution of oxygen-containing functional groups.

performance much superior than that of other carbon-based electrodes [77].

508 and 3199 F g�<sup>1</sup> for CNTs and CNTs-T, respectively [62].

and concentration (Figure 7b) of the oxidizing agent [66, 67].

Reproduced with permission from Ref. [66]. Copyright 2011 Elsevier.

Figure 5. Schematic illustration of the covalent functionalization of CNTs. Reproduced with permission from Ref. [58]. Copyright 2013 The Royal Society of Chemistry.

Figure 6. Cyclic voltammograms of (a) raw CNTs and (b) CNTs-T electrodes in 1 M H2SO4. Reproduced with permission from Ref. [62]. Copyright 2014 Elsevier.

electrochemically [59, 71, 72], photochemically [73, 74], and using plasma-induced techniques [75, 76]. The chemical modifications are usually performed in concentrated nitric acid or in a mixture of nitric and sulfuric acids. Cyclic voltammograms recorded for raw CNTs exhibit pseudorectangular cathodic and anodic profiles, which are the characteristics of ideal capacitors. In the case of CNTs-T, a pair of voltammetric peaks is observed. They are related to the redox reactions of functional groups on the CNTs-T surface:

$$\circ \text{C-OH} \rightleftharpoons \text{\raisebox{0.0pt}{\raisebox{0.0pt}{1}}} \circ \text{C-O} + \text{H}^{\cdot} + \text{e.} \tag{7}$$

$$
\therefore \text{C=O} + \text{z} \rightleftharpoons \text{z} \cdot \text{C-O} \tag{8}
$$

$$\cdot \text{COOH} \quad \bullet \text{CO}\_2 + \text{H}^+ + \text{c.} \tag{9}$$

A higher current observed for CNTs-T suggests that their surface area is larger compared to that of raw CNTs. The surface of oxygen-containing functional groups also decreases surface resistivity and enhances surface wettability, offering more accessible sites for the physisorption of electrolyte ions and increasing the ionic conductivity at the electrode/electrolyte interface. In the procedure proposed by Wang and co-workers [62], the specific capacitance was increased from 28 F g�<sup>1</sup> for raw CNTs to 85 F g�<sup>1</sup> for CNTs-T in H2SO4 solution. Changing the electrolyte to H2SO4/hydroquinone mixture provides a drastic improvement of the specific capacitance to 508 and 3199 F g�<sup>1</sup> for CNTs and CNTs-T, respectively [62].

The distribution and type of oxygen-containing functional groups depend on the type (Figure 7a) and concentration (Figure 7b) of the oxidizing agent [66, 67].

The oxidation of CNTs is a principle reaction for further functionalization of the carbon network. The combination of CNTs with graphene oxide in a lamellar graphene-CNT structure, r[GO/CNT], shown in Figure 8 causes an increase in the electrolyte-accessible surface area due to the intercalation of CNTs between the stacked GO sheets with associated large electrochemical active sites, thus improving conductivity through the formation of a 3D network aided by CNTs. Such a covalently linked CNTs-graphene system exhibits capacitance performance much superior than that of other carbon-based electrodes [77].

The high specific capacitance of CNTs could also be obtained with the incorporation of heteroatoms such as N [61, 78–87], B [86–89], P [86, 90], and F [91]. The type of defect created by the heteroatom influences the kind of conduction generated ranging from n-type transport

electrochemically [59, 71, 72], photochemically [73, 74], and using plasma-induced techniques [75, 76]. The chemical modifications are usually performed in concentrated nitric acid or in a mixture of nitric and sulfuric acids. Cyclic voltammograms recorded for raw CNTs exhibit pseudorectangular cathodic and anodic profiles, which are the characteristics of ideal capacitors. In the case of CNTs-T, a pair of voltammetric peaks is observed. They are related to the

Figure 6. Cyclic voltammograms of (a) raw CNTs and (b) CNTs-T electrodes in 1 M H2SO4. Reproduced with permission

Figure 5. Schematic illustration of the covalent functionalization of CNTs. Reproduced with permission from Ref. [58].

redox reactions of functional groups on the CNTs-T surface:

Copyright 2013 The Royal Society of Chemistry.

154 Carbon Nanotubes - Recent Progress

from Ref. [62]. Copyright 2014 Elsevier.

Figure 7. (a) Influence of the oxidant on the distribution of oxygen-containing functional groups, measured on MWCNTs. (b) Influence of % w/w HNO3 on the level of oxidation, as well as the distribution of oxygen-containing functional groups. Reproduced with permission from Ref. [66]. Copyright 2011 Elsevier.

capacitance can be explained by modification of the space charge layer in carbon. The presence of boron leads to the enhancement of the space-charge layer capacitance through increasing the

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

Thermal oxidation [88] or acid treatment [81] of doped CNTs improves the specific surface area and, as a consequence, the capacitance due to the tube end opening process. Coaxial supercapacitors based on hollow carbon nanotubes synthesized by chemical vapor deposition

The combination of synergetic effects in boron carbonitride nanotubes (BCNs) resulting from the combined co-doping of B and N in CNTs and their well-aligned vertical structure (VA) also provides a significant enhancement of specific capacitance performance. Vertically aligned boron carbonitride nanotubes (VA-BCNs) were synthesized on a Ni-Fe-coated SiO2/Si substrate using the chemical vapor deposition method with a melamine diborate precursor. In this case, nitrogen atoms are bonded to carbons in both graphitic and pyridinic forms, and the

Carbon nanotubes are combined with carbon nanostructures, conducting polymers or metal oxides to form nanocomposite materials that display favorable electronic and mechanical properties [95]. Composites containing carbon nanotubes and an electroactive phase exhibit pseudocapacitive properties like metal oxide or conducting polymers and represent an important breakthrough for developing a new generation of supercapacitors based on three reasons: (i) the percolation of the electroactive particles is more efficient with nanotubes than with other carbon materials; (ii) the open mesoporous network formed by the entanglement of nanotubes allows ions to diffuse easily to the active surface of the composite components; and (iii) since the nanotubular materials are characterized by a high resiliency, the composite electrodes can easily adapt to the volumetric changes during the charging/discharging process, which improves drastically the cycling performance [22]. However, capacitors fabricated from inorganic pseudocapacitive materials typically suffer from higher internal resistance and lower lifetimes. The combination of carbon nanotubes with other carbon nanostructures results in an enhancement of

) with an excellent rate capability

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157

) and undoped multi-walled

number of holes as a charge carrier or the DOS change at the Fermi level [88].

(CVD) exhibit high rate capacitance, long cycle life, and good flexibility [93].

resultant VA-BCNs show high specific capacitance (321 F g�<sup>1</sup>

carbon nanotubes (117 F g�<sup>1</sup>

and high durability compared to nonaligned BCNs (167 F g�<sup>1</sup>

5. Hybrid materials of carbon nanotubes for energy storage

) [94].

energy density and lowering of the internal resistance [96].

5.1. Capacitors based on carbon nanotubes and different carbon nanostructures

The continued technological progress has led to the miniaturization of electronic devices with large volumetric energy densities. Most fabricated micro-supercapacitors based on carbon nanostructures exhibit excellent rate capabilities and stability, but low volumetric energy density. In general, increased volumetric energy density is obtained by the application of porous conductive electrode materials with sufficiently high packing density. The most popular electrode material is graphene sheets because of their ultrahigh surface area and excellent

Figure 8. Synthesis scheme depicting the reaction between a GO layer and a CNT and a view of the fabricated r[GO-CNT] film. Reproduced with permission from Ref. [77]. Copyright 2013 Wiley-VCH Verlag.

Figure 9. Schematic representation of N doping in the carbon nanomaterial. Reproduced with permission from Ref. [92]. Copyright 2013 De Gruyter.

(N substitution doping) to p-type conduction (B substitution of boron in lattice) [92]. The existence of heteroatoms significantly enhances the specific capacitance values of carbon materials by the pseudocapacitance effect [83, 88]. Gueon and co-workers found that nitrogen-doped carbon nanotubes have a specific capacitance three times higher compared to undoped carbon nanomaterial [79]. Nitrogen-containing pentagons (Figure 9) can induce strong bending of the nanotubes and affect the alignment of the CNT lattice, resulting in the creation of donor states near the Fermi level and hence the specific capacitance [79, 80]. The effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon. The presence of boron leads to the enhancement of the space-charge layer capacitance through increasing the number of holes as a charge carrier or the DOS change at the Fermi level [88].

Thermal oxidation [88] or acid treatment [81] of doped CNTs improves the specific surface area and, as a consequence, the capacitance due to the tube end opening process. Coaxial supercapacitors based on hollow carbon nanotubes synthesized by chemical vapor deposition (CVD) exhibit high rate capacitance, long cycle life, and good flexibility [93].

The combination of synergetic effects in boron carbonitride nanotubes (BCNs) resulting from the combined co-doping of B and N in CNTs and their well-aligned vertical structure (VA) also provides a significant enhancement of specific capacitance performance. Vertically aligned boron carbonitride nanotubes (VA-BCNs) were synthesized on a Ni-Fe-coated SiO2/Si substrate using the chemical vapor deposition method with a melamine diborate precursor. In this case, nitrogen atoms are bonded to carbons in both graphitic and pyridinic forms, and the resultant VA-BCNs show high specific capacitance (321 F g�<sup>1</sup> ) with an excellent rate capability and high durability compared to nonaligned BCNs (167 F g�<sup>1</sup> ) and undoped multi-walled carbon nanotubes (117 F g�<sup>1</sup> ) [94].

#### 5. Hybrid materials of carbon nanotubes for energy storage

Carbon nanotubes are combined with carbon nanostructures, conducting polymers or metal oxides to form nanocomposite materials that display favorable electronic and mechanical properties [95]. Composites containing carbon nanotubes and an electroactive phase exhibit pseudocapacitive properties like metal oxide or conducting polymers and represent an important breakthrough for developing a new generation of supercapacitors based on three reasons: (i) the percolation of the electroactive particles is more efficient with nanotubes than with other carbon materials; (ii) the open mesoporous network formed by the entanglement of nanotubes allows ions to diffuse easily to the active surface of the composite components; and (iii) since the nanotubular materials are characterized by a high resiliency, the composite electrodes can easily adapt to the volumetric changes during the charging/discharging process, which improves drastically the cycling performance [22]. However, capacitors fabricated from inorganic pseudocapacitive materials typically suffer from higher internal resistance and lower lifetimes. The combination of carbon nanotubes with other carbon nanostructures results in an enhancement of energy density and lowering of the internal resistance [96].

#### 5.1. Capacitors based on carbon nanotubes and different carbon nanostructures

(N substitution doping) to p-type conduction (B substitution of boron in lattice) [92]. The existence of heteroatoms significantly enhances the specific capacitance values of carbon materials by the pseudocapacitance effect [83, 88]. Gueon and co-workers found that nitrogen-doped carbon nanotubes have a specific capacitance three times higher compared to undoped carbon nanomaterial [79]. Nitrogen-containing pentagons (Figure 9) can induce strong bending of the nanotubes and affect the alignment of the CNT lattice, resulting in the creation of donor states near the Fermi level and hence the specific capacitance [79, 80]. The effect of B-doping on the

Figure 9. Schematic representation of N doping in the carbon nanomaterial. Reproduced with permission from Ref. [92].

Copyright 2013 De Gruyter.

156 Carbon Nanotubes - Recent Progress

Figure 8. Synthesis scheme depicting the reaction between a GO layer and a CNT and a view of the fabricated

r[GO-CNT] film. Reproduced with permission from Ref. [77]. Copyright 2013 Wiley-VCH Verlag.

The continued technological progress has led to the miniaturization of electronic devices with large volumetric energy densities. Most fabricated micro-supercapacitors based on carbon nanostructures exhibit excellent rate capabilities and stability, but low volumetric energy density. In general, increased volumetric energy density is obtained by the application of porous conductive electrode materials with sufficiently high packing density. The most popular electrode material is graphene sheets because of their ultrahigh surface area and excellent conductivity as well as high mechanical and chemical stability. However, various interesting structures of graphene, such as one-dimensional fibers, two-dimensional films, and threedimensional foams, in spite of high gravimetric capacitance exhibit poor volumetric performance due to strong intersheet π-π interactions, which, while increasing the packing density, do not allow high ion accessibility. To solve this problem, the synergistic effect of graphene and carbon nanotubes is utilized by the preparation of graphene-carbon nanotubes composites. In most cases, graphene oxide is used due to its hydrophilic character. However, its conductivity is low, and hence, it is doped by carbon nanotubes with very high conductivity [97]. Numerous synthesis methods have been used and a large family of hybrid composites based on graphene oxide and carbon nanotubes with the specific capacitance in the range of 120–222 F g�<sup>1</sup> depending on the preparation procedure has been discovered [77, 98–104].

Ternary carbon composites containing carbon nanotubes, graphene, and activated carbon exhibit much better capacitance performance. Such systems have specific capacitances several times higher than those of their components (Figure 10a). The excellent electrochemical properties of ternary composites can be attributed to the high surface area and low equivalent series resistance, demonstrating that they improve the electrochemical performance for supercapacitor applications [97]. Among the multicomponent composites, a novel type of highly flexible and all-solid state supercapacitor utilizing hybrid aerogels exhibits promising properties (Figure 10b) [105].

Recently, much attention has been paid to 3D pillared vertically aligned carbon nanotubes (VACNTs)—graphene architectures with a controllable nanotube length (PL)/intertube distance (MIPD) as electrode materials for energy-related devices (Figure 11). Theoretical studies have indicated that 3D pillared architectures, consisting of parallel graphene layers supported by vertically aligned carbon nanotubes (VACNTs) in between, possess desirable out-of-plane transport and mechanical properties while maintaining the excellent proper-

Figure 11. (a) Schematic diagram of a 3D pillared VACNT graphene nanostructure. (b) SEM images of the 3D pillared VACNT graphene architectures. Reproduced with permission from Ref. [102]. Copyright 2011 American Chemical

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An ultrafast compact capacitor based on free-standing, flexible, and highly conducting films consisting of stacked nanoporous graphene layers pillared with single-walled carbon nanotubes (SWCNTs) was obtained by Pham and co-workers [109]. Figure 12 shows the carbon nanotube (CNT)-bridged graphene 3D building blocks via the Coulombic interaction between positively charged CNTs grafted by cationic surfactants and negatively charged graphene oxide sheets, followed by KOH activation. Such a structure enhances the accessible surface area and allows for fast ion diffusion. Due to this unique 3D porous structure, a remarkable electrochemical perfor-

The electrodeposition of nickel hydroxide on such hybrid nanostructures results in pseudocapacitance due to the Faradaic reaction associated with the Ni(OH)2 coating. The specific capacitance is increased from 110 F g�<sup>1</sup> obtained for VACNT-graphene architectures to 1384 F g�<sup>1</sup> for

Pristine carbon nanotubes usually provide unsatisfactory specific capacitance due to their relatively low surface area, which still needs to be enhanced. Composites containing hollow carbon nanospheres anchored to the surface of carbon nanotubes, CNT-HCS, (Figure 13) are synthesized via the hard template method following this trend. Disordered pores (~2 nm) observed in the shells of the carbon spheres facilitate the penetration of electrolyte ions and favor the rapid charge propagation during the charge/discharge process [106]. The electrochemical performance of capacitors based on the composites of carbon nanotubes and different

mance with a maximum capacitance as high as 199 F g�<sup>1</sup> was achieved [99].

ties of their building blocks [102].

Society.

Ni(OH)2-coated VACNT graphene electrode [102].

carbon nanostructures are collected in Table 2.

Figure 10. (a) Cyclic voltammograms of different carbon particles (GO, CNT, AC) and their composite recorded in 1 M H2SO4. Reproduced with permission from Ref. [97]. Copyright 2017 Elsevier. (b) Schematic diagram of the all-solid-state supercapacitor where the polymer-gel electrolyte serves as the electrolyte and the separator. Insets show the flexibility of the device and SEM image of aerogel hybrid film. Reproduced with permission from Ref. [105]. Copyright 2015 American Chemical Society.

conductivity as well as high mechanical and chemical stability. However, various interesting structures of graphene, such as one-dimensional fibers, two-dimensional films, and threedimensional foams, in spite of high gravimetric capacitance exhibit poor volumetric performance due to strong intersheet π-π interactions, which, while increasing the packing density, do not allow high ion accessibility. To solve this problem, the synergistic effect of graphene and carbon nanotubes is utilized by the preparation of graphene-carbon nanotubes composites. In most cases, graphene oxide is used due to its hydrophilic character. However, its conductivity is low, and hence, it is doped by carbon nanotubes with very high conductivity [97]. Numerous synthesis methods have been used and a large family of hybrid composites based on graphene oxide and carbon nanotubes with the specific capacitance in the range of 120–222 F g�<sup>1</sup>

Ternary carbon composites containing carbon nanotubes, graphene, and activated carbon exhibit much better capacitance performance. Such systems have specific capacitances several times higher than those of their components (Figure 10a). The excellent electrochemical properties of ternary composites can be attributed to the high surface area and low equivalent series resistance, demonstrating that they improve the electrochemical performance for supercapacitor applications [97]. Among the multicomponent composites, a novel type of highly flexible and all-solid state

Recently, much attention has been paid to 3D pillared vertically aligned carbon nanotubes (VACNTs)—graphene architectures with a controllable nanotube length (PL)/intertube distance (MIPD) as electrode materials for energy-related devices (Figure 11). Theoretical

Figure 10. (a) Cyclic voltammograms of different carbon particles (GO, CNT, AC) and their composite recorded in 1 M H2SO4. Reproduced with permission from Ref. [97]. Copyright 2017 Elsevier. (b) Schematic diagram of the all-solid-state supercapacitor where the polymer-gel electrolyte serves as the electrolyte and the separator. Insets show the flexibility of the device and SEM image of aerogel hybrid film. Reproduced with permission from Ref. [105]. Copyright 2015 American

Chemical Society.

supercapacitor utilizing hybrid aerogels exhibits promising properties (Figure 10b) [105].

depending on the preparation procedure has been discovered [77, 98–104].

158 Carbon Nanotubes - Recent Progress

Figure 11. (a) Schematic diagram of a 3D pillared VACNT graphene nanostructure. (b) SEM images of the 3D pillared VACNT graphene architectures. Reproduced with permission from Ref. [102]. Copyright 2011 American Chemical Society.

studies have indicated that 3D pillared architectures, consisting of parallel graphene layers supported by vertically aligned carbon nanotubes (VACNTs) in between, possess desirable out-of-plane transport and mechanical properties while maintaining the excellent properties of their building blocks [102].

An ultrafast compact capacitor based on free-standing, flexible, and highly conducting films consisting of stacked nanoporous graphene layers pillared with single-walled carbon nanotubes (SWCNTs) was obtained by Pham and co-workers [109]. Figure 12 shows the carbon nanotube (CNT)-bridged graphene 3D building blocks via the Coulombic interaction between positively charged CNTs grafted by cationic surfactants and negatively charged graphene oxide sheets, followed by KOH activation. Such a structure enhances the accessible surface area and allows for fast ion diffusion. Due to this unique 3D porous structure, a remarkable electrochemical performance with a maximum capacitance as high as 199 F g�<sup>1</sup> was achieved [99].

The electrodeposition of nickel hydroxide on such hybrid nanostructures results in pseudocapacitance due to the Faradaic reaction associated with the Ni(OH)2 coating. The specific capacitance is increased from 110 F g�<sup>1</sup> obtained for VACNT-graphene architectures to 1384 F g�<sup>1</sup> for Ni(OH)2-coated VACNT graphene electrode [102].

Pristine carbon nanotubes usually provide unsatisfactory specific capacitance due to their relatively low surface area, which still needs to be enhanced. Composites containing hollow carbon nanospheres anchored to the surface of carbon nanotubes, CNT-HCS, (Figure 13) are synthesized via the hard template method following this trend. Disordered pores (~2 nm) observed in the shells of the carbon spheres facilitate the penetration of electrolyte ions and favor the rapid charge propagation during the charge/discharge process [106]. The electrochemical performance of capacitors based on the composites of carbon nanotubes and different carbon nanostructures are collected in Table 2.

5.2. Capacitors based on carbon nanotubes and conducting polymers

Composite Specific capacitance (F g�<sup>1</sup> )

nanohorns.

different carbon nanostructures.

conducting polymers, and the composite materials are compared.

generates polymers with a more porous morphology [118, 127].

Composites of conducting polymers with carbon nanotubes are promising electrode materials as supercapacitors because of their good conductivity, high surface area, and excellent ability to store energy [7, 108–117]. The composites combine the large pseudocapacitance of conducting polymers with the fast charging/discharging double-layer capacitance and excellent mechanical properties of carbon nanotubes [118]. In Scheme 1, the properties of carbon nanostructures,

CNF, carbon nanofibril; rGO, reduced graphene oxide; HCS, hollow carbon spheres; AC, activated carbon; CNH, carbon

Table 2. Selected values of electrochemical performance of capacitors based on composites of carbon nanotubes and

CNF/rGO/CNT 252 216 mF cm�<sup>2</sup> 9.5 mW cm�<sup>2</sup> 28.4 mW cm�<sup>2</sup> 99.5% [105] HCS/CNT 201.5 30.1 F cm�<sup>3</sup> 11.8 kW kg�<sup>1</sup> 11.3 Wh kg�<sup>1</sup> 90% [106] SWCNT/GO – 305 F cm�<sup>3</sup> 1.085 mW cm�<sup>3</sup> 6.3 mWh cm�<sup>3</sup> 93% [98] SWCNT/GO 199 211 mF cm�<sup>3</sup> 400 kW kg�<sup>1</sup> 110.6 Wh kg�<sup>1</sup> 98.2% [99] GO/CNT/AC 636 – 550 W kg�<sup>1</sup> 16 Wh kg�<sup>1</sup> 99.8% [97] Graphene/CNT – 3.93 mF cm�<sup>3</sup> 115 W cm�<sup>3</sup> 2.42 mWh cm�<sup>3</sup> 86% [107] SWCNT/CNH 43 – 3.50 kW kg�<sup>1</sup> 6.03 Wh kg�<sup>1</sup> – [96]

Capacitance Power density Energy density Capacitance

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161

CNTs/conducting polymer composites can be prepared by chemical [7, 13, 108, 113, 114, 118–121] or electrochemical [14, 16, 95, 110–112, 115, 118, 119, 121] polymerization. This process can be carried out on pristine CNTs or CNTs modified with functional groups or heteroatoms as a noncovalent deposition polymeric layer onto the nanotubular surface or covalent functionalization of carbon walls by polymeric chains. In the case of covalent functionalization, two approaches can be used: first, when the monomer is attached to CNTs and, in the next step, when it is polymerized via chemical or electrochemical methods [122, 123], and second, in situ chemical polymerization in the presence of dopant and self-organizing agent [7, 124]. For noncovalently modified CNTs with polymers, several strategies have been developed. Currently, they involve physical mixing in solution, in situ polymerization of monomers in the presence of CNTs, surfactant- and templateassisted processing of composites, chemical functionalization of the incorporated nanotubular material, and electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures or electrochemical co-polymerization [118, 125, 126]. In Figure 14, exemplary procedures of nanocomposites formation are schematically presented. In most cases, electrochemical synthesis provides homogenous films [16, 118]. However, chemical polymerization

Figure 15 shows the SEM images of SWCNTs/PPy composites prepared by deposition of PPy on pristine SWCNTs (Figure 15a) and on functionalized SWCNTs (Figure 15b). In the case of

functionalized SWCNTs, an incomplete coverage of SWCNTs by PPy is observed [14].

Figure 12. Schematic for fabricating the ac-Gr/SWCNT hybrid nanostructure. (a) The CTAB-grafted SWCNTs are positively charged, and the GO layers are negatively charged due to their functional groups. (b) Schematic of the 3D SWCNTbridged graphene block. Reproduced with permission from Ref. [99]. Copyright 2015 American Chemical Society.

Figure 13. TEM images of CNT-HCS composite with different magnifications. Reproduced with permission from Ref. [106]. Copyright 2013 Elsevier.


CNF, carbon nanofibril; rGO, reduced graphene oxide; HCS, hollow carbon spheres; AC, activated carbon; CNH, carbon nanohorns.

Table 2. Selected values of electrochemical performance of capacitors based on composites of carbon nanotubes and different carbon nanostructures.

#### 5.2. Capacitors based on carbon nanotubes and conducting polymers

Figure 12. Schematic for fabricating the ac-Gr/SWCNT hybrid nanostructure. (a) The CTAB-grafted SWCNTs are positively charged, and the GO layers are negatively charged due to their functional groups. (b) Schematic of the 3D SWCNTbridged graphene block. Reproduced with permission from Ref. [99]. Copyright 2015 American Chemical Society.

Figure 13. TEM images of CNT-HCS composite with different magnifications. Reproduced with permission from Ref.

[106]. Copyright 2013 Elsevier.

160 Carbon Nanotubes - Recent Progress

Composites of conducting polymers with carbon nanotubes are promising electrode materials as supercapacitors because of their good conductivity, high surface area, and excellent ability to store energy [7, 108–117]. The composites combine the large pseudocapacitance of conducting polymers with the fast charging/discharging double-layer capacitance and excellent mechanical properties of carbon nanotubes [118]. In Scheme 1, the properties of carbon nanostructures, conducting polymers, and the composite materials are compared.

CNTs/conducting polymer composites can be prepared by chemical [7, 13, 108, 113, 114, 118–121] or electrochemical [14, 16, 95, 110–112, 115, 118, 119, 121] polymerization. This process can be carried out on pristine CNTs or CNTs modified with functional groups or heteroatoms as a noncovalent deposition polymeric layer onto the nanotubular surface or covalent functionalization of carbon walls by polymeric chains. In the case of covalent functionalization, two approaches can be used: first, when the monomer is attached to CNTs and, in the next step, when it is polymerized via chemical or electrochemical methods [122, 123], and second, in situ chemical polymerization in the presence of dopant and self-organizing agent [7, 124]. For noncovalently modified CNTs with polymers, several strategies have been developed. Currently, they involve physical mixing in solution, in situ polymerization of monomers in the presence of CNTs, surfactant- and templateassisted processing of composites, chemical functionalization of the incorporated nanotubular material, and electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures or electrochemical co-polymerization [118, 125, 126]. In Figure 14, exemplary procedures of nanocomposites formation are schematically presented. In most cases, electrochemical synthesis provides homogenous films [16, 118]. However, chemical polymerization generates polymers with a more porous morphology [118, 127].

Figure 15 shows the SEM images of SWCNTs/PPy composites prepared by deposition of PPy on pristine SWCNTs (Figure 15a) and on functionalized SWCNTs (Figure 15b). In the case of functionalized SWCNTs, an incomplete coverage of SWCNTs by PPy is observed [14].

Scheme 1. Properties of carbon nanostructures, conducting polymers and the composite materials.

A comparison of the electrochemical behavior of PPy, SWCNTs/PPy, and PPy/functionalized SWCNTs composites is shown in Figure 16. PPy films at a low scan rate exhibits a rectangular shape, which indicates an ideal capacitance behavior, while for higher scan rates, the curves are not rectangle-shaped, which indicates resistance-like electrochemical behavior due to the slow doping/dedoping process of the compact PPy layer. However, a pseudorectangular shape of the recorded cyclic voltammograms in all of the scan rates (up to 200 mVs -1) is observed for the composites based on pristine SWCNTs (Figure 16b) and functionalized SWCNTs (Figure 16c). The PPy/functionalized SWCNTs composite exhibits also better capacitance properties compared to PPy/SWCNTs and pristine PPy (Figure 17).

The presence of CNT components results in an increase in the practical range of electrical conductivity of the material. For example, C60-Pd polymer demonstrates pseudocapacitance behavior due to faradaic reduction of C60 in the negative potential range (Figure 19a); at potentials lower than this threshold, the material exhibits very high resistance. In the CNTs/ C60Pd composite, the potential range of electrochemical activity increases. At negative potentials, the pseudocapacitive behavior of the polymeric component is still observed. However, the material also shows double-layer capacitance, mainly attributed to the conducting CNTs, in a less negative potential range (Figure 19b). Composites based on C60-Pd polymers and SWCNTs or MWCNTs exhibit specific capacitance equal to 994 F g�<sup>1</sup> or 758 F g�<sup>1</sup> for SWCNTs/

Figure 14. A schematic representation of exemplary procedures of nanocomposites formation for electrochemical measurements. (A) in situ chemical polymerization, (B) chemical polymerization of monomers attached to carbon nanostructure, (C) electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures.

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A very promising energy storage behavior was reported for 3D structure composite based on aligned carbon nanotubes (ACNTs) offering a large specific surface area, superior electronic transfer ability through individual nanotubes, and chemical inertness [95]. Three-dimensional

C60-Pd and MWCNTs/C60-Pd composites, respectively [119].

Another way to form composites based on CNTs and conducting polymers is covalent functionalization of CNTs with monomeric units. In the next step, the composite is obtained by copolymerization of the monomer with monomer units grafted on the CNTs surface using controlled potential electrolysis. Figure 18a shows the electrochemical behavior of polypyrrole/ CNTs (PPY/CNTs) composites obtained at different charge densities. The broad peaks observed in the voltammograms are related to the redox process within the polypyrrole component. The composites exhibit higher currents than the pure polymeric material due to higher porosity (Figure 18c) compared to pristine PPy (Figure 18b) and as a consequence of the higher capacitance of PPy/CNTs [122].

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Figure 14. A schematic representation of exemplary procedures of nanocomposites formation for electrochemical measurements. (A) in situ chemical polymerization, (B) chemical polymerization of monomers attached to carbon nanostructure, (C) electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures.

A comparison of the electrochemical behavior of PPy, SWCNTs/PPy, and PPy/functionalized SWCNTs composites is shown in Figure 16. PPy films at a low scan rate exhibits a rectangular shape, which indicates an ideal capacitance behavior, while for higher scan rates, the curves are not rectangle-shaped, which indicates resistance-like electrochemical behavior due to the slow doping/dedoping process of the compact PPy layer. However, a pseudorectangular shape

composites based on pristine SWCNTs (Figure 16b) and functionalized SWCNTs (Figure 16c). The PPy/functionalized SWCNTs composite exhibits also better capacitance properties compared

Another way to form composites based on CNTs and conducting polymers is covalent functionalization of CNTs with monomeric units. In the next step, the composite is obtained by copolymerization of the monomer with monomer units grafted on the CNTs surface using controlled potential electrolysis. Figure 18a shows the electrochemical behavior of polypyrrole/ CNTs (PPY/CNTs) composites obtained at different charge densities. The broad peaks observed in the voltammograms are related to the redox process within the polypyrrole component. The composites exhibit higher currents than the pure polymeric material due to higher porosity (Figure 18c) compared to pristine PPy (Figure 18b) and as a consequence of the higher capaci-


of the recorded cyclic voltammograms in all of the scan rates (up to 200 mVs

Scheme 1. Properties of carbon nanostructures, conducting polymers and the composite materials.

to PPy/SWCNTs and pristine PPy (Figure 17).

tance of PPy/CNTs [122].

162 Carbon Nanotubes - Recent Progress

The presence of CNT components results in an increase in the practical range of electrical conductivity of the material. For example, C60-Pd polymer demonstrates pseudocapacitance behavior due to faradaic reduction of C60 in the negative potential range (Figure 19a); at potentials lower than this threshold, the material exhibits very high resistance. In the CNTs/ C60Pd composite, the potential range of electrochemical activity increases. At negative potentials, the pseudocapacitive behavior of the polymeric component is still observed. However, the material also shows double-layer capacitance, mainly attributed to the conducting CNTs, in a less negative potential range (Figure 19b). Composites based on C60-Pd polymers and SWCNTs or MWCNTs exhibit specific capacitance equal to 994 F g�<sup>1</sup> or 758 F g�<sup>1</sup> for SWCNTs/ C60-Pd and MWCNTs/C60-Pd composites, respectively [119].

A very promising energy storage behavior was reported for 3D structure composite based on aligned carbon nanotubes (ACNTs) offering a large specific surface area, superior electronic transfer ability through individual nanotubes, and chemical inertness [95]. Three-dimensional

Figure 15. SEM images of (a) SWCNTs/PPy and (b) functionalized SWCNTs/PPy. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

Figure 16. Cyclic voltammograms recorded for pristine (a) PPy, (b) SWCNTs/PPy and (c) functionalized SWCNTs/PPy composite films at different scan rates: (1) 10, (2) 20, (3) 50, (4) 100 (5) 200 mV s�<sup>1</sup> in 1M KCl solution. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

hybrid composite material composed of 2D fish scale-like polyaniline nanosheet arrays on graphene oxide sheets and carbon nanotubes exhibiting high specific capacitance of 589 F g�<sup>1</sup> compared to 397 F g�<sup>1</sup> for pristine PANI was also investigated [18]. Moreover, ternary composite systems containing carbon nanotubes, graphene, and conducting polymers were also

Figure 18. (a) Cyclic voltammograms recorded in propylene carbonate containing 0.1 M LiClO4 for the PPY/CNTs composite films and pure polypyrrole obtained in galvanostatic conditions at different charge densities. SEM images of

(b) PPY and (c) PPY/CNTs composite. Reproduced with permission from Ref. [122]. Copyright 2013 Hindawi.

Figure 17. The specific capacitance of (1) pristine PPy film, (2) PPy/SWCNTs composite film and (3) PPy/functionalized SWCNTs composite film at various scanning rates solution. Reproduced with permission from Ref. [14]. Copyright 2007

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Elsevier.

Figure 17. The specific capacitance of (1) pristine PPy film, (2) PPy/SWCNTs composite film and (3) PPy/functionalized SWCNTs composite film at various scanning rates solution. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

Figure 15. SEM images of (a) SWCNTs/PPy and (b) functionalized SWCNTs/PPy. Reproduced with permission from Ref.

Figure 16. Cyclic voltammograms recorded for pristine (a) PPy, (b) SWCNTs/PPy and (c) functionalized SWCNTs/PPy composite films at different scan rates: (1) 10, (2) 20, (3) 50, (4) 100 (5) 200 mV s�<sup>1</sup> in 1M KCl solution. Reproduced with

[14]. Copyright 2007 Elsevier.

164 Carbon Nanotubes - Recent Progress

permission from Ref. [14]. Copyright 2007 Elsevier.

Figure 18. (a) Cyclic voltammograms recorded in propylene carbonate containing 0.1 M LiClO4 for the PPY/CNTs composite films and pure polypyrrole obtained in galvanostatic conditions at different charge densities. SEM images of (b) PPY and (c) PPY/CNTs composite. Reproduced with permission from Ref. [122]. Copyright 2013 Hindawi.

hybrid composite material composed of 2D fish scale-like polyaniline nanosheet arrays on graphene oxide sheets and carbon nanotubes exhibiting high specific capacitance of 589 F g�<sup>1</sup> compared to 397 F g�<sup>1</sup> for pristine PANI was also investigated [18]. Moreover, ternary composite systems containing carbon nanotubes, graphene, and conducting polymers were also

investigated [128, 131]. The capacitance properties of selected composites of carbon nanotubes

To overcome the low energy density of supercapacitors, pseudocapacitors based on transition metal oxides have been developed [135]. The most promising among them is manganese dioxide MnO2 because of its low cost, environmental compatibility, natural abundance, high energy density, and excellent capacitive performance in aqueous electrolytes [32, 135–143]. In aqueous electrolytes, the charging mechanism of MnO2 may be described by the following reaction:

) and/or alkali cations such as K+

storage is based either on the adsorption of cations at the surface of the electrode material or on the intercalation of cations in the bulk of the electrode material. However, the reported specific capacitance values for the various structures of MnO2 electrodes are still far from the theoretical one [144], which may be attributed to the intrinsically poor electronic conductivity of MnO2. To improve the capacitive performance of MnO2, composites with carbon nanotubes characterized by high conductivity and high surface area are formed [143]. The surface morphology of CNTs/MnO2 composite and its components is shown in Figure 20. Figure 21a

Figure 20. SEM images of (a) pristine CNTs, (b) flower-like MnO2, (c and d) MnO2/CNT nanocomposite. Reproduced

with permission from Ref. [143]. Copyright 2012 Springer.

, Na+

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ð10Þ

167

. The charge

and conducting polymers are reported in Table 3.

where M represents protons (H<sup>+</sup>

5.3. Composites containing carbon nanotubes and metal oxides

Figure 19. Cyclic voltammogram of (a) C60-Pd polymer and (b) MWCNT/C60-Pd composite at 100 mV/s in acetonitrile containing 0.1 M (n-C4H9)4NClO4. Reproduced with permission from Ref. [119]. Copyright 2013 Elsevier.


Table 3. Capacitance properties of selected composites containing carbon nanostructure and conducting polymer.

investigated [128, 131]. The capacitance properties of selected composites of carbon nanotubes and conducting polymers are reported in Table 3.

#### 5.3. Composites containing carbon nanotubes and metal oxides

Figure 19. Cyclic voltammogram of (a) C60-Pd polymer and (b) MWCNT/C60-Pd composite at 100 mV/s in acetonitrile

SWCNT/PPy 305 [132] SWCNT/PANI 247 [129] SWCNT/PANI 485 [111] SWCNT/PANI 707 [133] SWCNT/PANI 1000 [134] SWCNT/C60-Pd 994 [119]

MWCNT/PANI 500 [112] MWCNT/PANI 670 [130] MWCNT/PPy 70 [120] MWCNT/PPy 268 [7] MWCNT/PPy 243 [16] MWCNT/PPy 506 [130] MWCNT/PPy 554 [126] MWCNT/C60-Pd 758 [119] MWCNT/PEDOT:PSS 30 [120] MWCNT/PEDOT 237 [126]

) Reference

containing 0.1 M (n-C4H9)4NClO4. Reproduced with permission from Ref. [119]. Copyright 2013 Elsevier.

SWCNT SWCNT/PPy 200 [14]

MWCNT MWCNT/PANI 50 [120]

Graphene/CNT Graphene/CNT/PPy 453 [132] GO/CNT GO/CNT/PANI 413 [18]

Table 3. Capacitance properties of selected composites containing carbon nanostructure and conducting polymer.

Carbon nanostructure Composite Specific capacitance (F g�<sup>1</sup>

166 Carbon Nanotubes - Recent Progress

To overcome the low energy density of supercapacitors, pseudocapacitors based on transition metal oxides have been developed [135]. The most promising among them is manganese dioxide MnO2 because of its low cost, environmental compatibility, natural abundance, high energy density, and excellent capacitive performance in aqueous electrolytes [32, 135–143]. In aqueous electrolytes, the charging mechanism of MnO2 may be described by the following reaction:

$$\mathrm{MnO}\_{2} + \mathrm{M}^{+} + \mathrm{e} \rightleftharpoons \mathrm{MnOOM} \tag{10}$$

where M represents protons (H<sup>+</sup> ) and/or alkali cations such as K+ , Na+ , and Li<sup>+</sup> . The charge storage is based either on the adsorption of cations at the surface of the electrode material or on the intercalation of cations in the bulk of the electrode material. However, the reported specific capacitance values for the various structures of MnO2 electrodes are still far from the theoretical one [144], which may be attributed to the intrinsically poor electronic conductivity of MnO2. To improve the capacitive performance of MnO2, composites with carbon nanotubes characterized by high conductivity and high surface area are formed [143]. The surface morphology of CNTs/MnO2 composite and its components is shown in Figure 20. Figure 21a

Figure 20. SEM images of (a) pristine CNTs, (b) flower-like MnO2, (c and d) MnO2/CNT nanocomposite. Reproduced with permission from Ref. [143]. Copyright 2012 Springer.

shows the comparison of the specific capacitances of pristine CNTs, pure MnO2 and a composite where CNTs are decorated by MnO2 nanoflakes. CNTs exhibit the best rate capability, but their capacitance is the lowest due to the charge storage mechanism typical for double layer capacitors. The low rate capability of MnO2 is associated with its poor electronic conductivity and low specific surface area. The combination of MnO2 and CNTs provides the formation of a MnO2/CNT nanocomposite exhibiting good rate capability and high specific capacitance. The results obtained by electrochemical impedance spectroscopy (Figure 21b) show that the MnO2/CNT composite has a much lower diffusive resistance compared to pure MnO2 because the slope of the low-frequency straight line representing the diffusive resistance of electrolyte in the electrode pores and cation diffusion in the host materials is similar to the line obtained for the CNTs but much larger than that of the pure MnO2. Additionally, the charge transfer resistance, Rct, localized between Rct observed for CNTs and MnO2 suggests high specific surface area for the unique hierarchy architecture of the CNTs/MnO2 composite where the 3D electron path network constructed by the CNT cores and the nanoporous sheath composed of tiny MnO2 nanoflakes facilitate fast electron and ion transport [143].

shown in Figure 22. In such a system, the former stores charge electrostatically, and the latter stores charge in virtue of highly reversible redox reactions of MnO2. The broad anodic peak at 0.72–0.84 and 0.59–0.80 V for the samples MnO2-in-CNT and MnO2-out-CNT, respectively, is

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The oxidation potential of MnO2-in-CNT is shifted to higher values compared to that of MnO2 out-CNT because CNT-confined manganese oxide particles can easily form lower oxidationstate species compared to exterior particles. The double-layer capacitance of MnO2-in-CNT is increased due to both the formation of Mn(IV)/Mn(III) redox couple and the desolvation of

ð11Þ

169

) Reference

associated with the oxidation process of Mn(III) to Mn(IV) according to the reaction:

electrolyte ions inside the channels of CNTs blocked by MnO2 nanoparticles [9].

SWCNT SWCNT/In2O3 201 [149]

MWCNT MWCNT/MoO3 70 [159]

CNT MnO2-out-CNT 790 [9]

Table 4. Capacitance properties of selected composites containing carbon nanostructure and metal oxide.

MC, mesoporous carbon.

SWCNT/MnO2 253 [149] SWCNT/NiC2O4 1642 [162]

MWCNT/MoO3 178 [10] MWCNT/MnO2 250 [147] MWCNT/MnO2 944 [145] MWCNT/RuO2 138 [153] MWCNT/RuO2 953 [151] MWCNT/RuO2 1050 [154] MWCNT/SnO2 93 [153] MWCNT/NiO 160 [155] MWCNT/NiO 523 [156] MWCNT/TiO2 160 [153] MWCNT/Co3O4 201 [158] MWCNT/(Sn+Mn)Ox 337 [163] MWCNT/MC/MnO2 351 [165] MWCNT@MnO2@PPy 273 [12]

MnO2-in-CNT 1250 [9] CNT/MnO2 199 [150] CNT/MnO2 214 [148]

Carbon nanostructure Composite Specific capacitance (F g�<sup>1</sup>

Apart from MnO2 [9, 17, 135, 136, 143, 145–150], other metal oxides such as hydrous RuO2 [151–154], NiO [155, 156], Fe2O3 [157], Co3O4 [158], MoO3 [10, 135, 159], V2O5 [160], CeO2 [161], and NiCo2O4 [162], In2O3 [149], TiO2 [153], SnO2 [153], and (Sn + Mn)Ox [163] are also utilized for the formation of composite electrodes. In the case of the expensive hydrous ruthenium oxide (RuO2xH2O) exhibiting excellent capacitance performance [164], relatively high conductivity and exceptional electrochemical reversibility, composites based on carbon nanotubes are formed to reduce its cost and enhance its capacitance properties [151–154].

So far, most studies have been devoted to the deposition of the pseudocapacitance phase onto carbon nanostructures. However, it is also possible to encapsulate metal oxides with CNTs. The MnO2@CNTs material exhibits a significantly higher specific capacitance compared to MnO2 outside of carbon nanotubes (Table 4). The difference in the electrochemical behaviors of MnO2 enclosed in CNTs (MnO2-in-CNT) and MnO2 deposited on CNTs surface (MnO2-out-CNT) is

Figure 21. (a) Dependence of specific capacitance on the sweep rate and (b) Nyquist plots for pristine CNTs, pure MnO2, and MnO2/CNT nanocomposite electrodes. Reproduced with permission from Ref. [143]. Copyright 2012 Springer.

shown in Figure 22. In such a system, the former stores charge electrostatically, and the latter stores charge in virtue of highly reversible redox reactions of MnO2. The broad anodic peak at 0.72–0.84 and 0.59–0.80 V for the samples MnO2-in-CNT and MnO2-out-CNT, respectively, is associated with the oxidation process of Mn(III) to Mn(IV) according to the reaction:

shows the comparison of the specific capacitances of pristine CNTs, pure MnO2 and a composite where CNTs are decorated by MnO2 nanoflakes. CNTs exhibit the best rate capability, but their capacitance is the lowest due to the charge storage mechanism typical for double layer capacitors. The low rate capability of MnO2 is associated with its poor electronic conductivity and low specific surface area. The combination of MnO2 and CNTs provides the formation of a MnO2/CNT nanocomposite exhibiting good rate capability and high specific capacitance. The results obtained by electrochemical impedance spectroscopy (Figure 21b) show that the MnO2/CNT composite has a much lower diffusive resistance compared to pure MnO2 because the slope of the low-frequency straight line representing the diffusive resistance of electrolyte in the electrode pores and cation diffusion in the host materials is similar to the line obtained for the CNTs but much larger than that of the pure MnO2. Additionally, the charge transfer resistance, Rct, localized between Rct observed for CNTs and MnO2 suggests high specific surface area for the unique hierarchy architecture of the CNTs/MnO2 composite where the 3D electron path network constructed by the CNT cores and the nanoporous sheath

168 Carbon Nanotubes - Recent Progress

composed of tiny MnO2 nanoflakes facilitate fast electron and ion transport [143].

Apart from MnO2 [9, 17, 135, 136, 143, 145–150], other metal oxides such as hydrous RuO2 [151–154], NiO [155, 156], Fe2O3 [157], Co3O4 [158], MoO3 [10, 135, 159], V2O5 [160], CeO2 [161], and NiCo2O4 [162], In2O3 [149], TiO2 [153], SnO2 [153], and (Sn + Mn)Ox [163] are also utilized for the formation of composite electrodes. In the case of the expensive hydrous ruthenium oxide (RuO2xH2O) exhibiting excellent capacitance performance [164], relatively high conductivity and exceptional electrochemical reversibility, composites based on carbon nanotubes are formed to reduce its cost and enhance its capacitance properties [151–154].

So far, most studies have been devoted to the deposition of the pseudocapacitance phase onto carbon nanostructures. However, it is also possible to encapsulate metal oxides with CNTs. The MnO2@CNTs material exhibits a significantly higher specific capacitance compared to MnO2 outside of carbon nanotubes (Table 4). The difference in the electrochemical behaviors of MnO2 enclosed in CNTs (MnO2-in-CNT) and MnO2 deposited on CNTs surface (MnO2-out-CNT) is

Figure 21. (a) Dependence of specific capacitance on the sweep rate and (b) Nyquist plots for pristine CNTs, pure MnO2, and MnO2/CNT nanocomposite electrodes. Reproduced with permission from Ref. [143]. Copyright 2012 Springer.

$$\mathrm{MnO\_2 + 4H^+ + e} \rightleftharpoons \mathrm{Mn^{3+} + 2H\_2O} \tag{11}$$

The oxidation potential of MnO2-in-CNT is shifted to higher values compared to that of MnO2 out-CNT because CNT-confined manganese oxide particles can easily form lower oxidationstate species compared to exterior particles. The double-layer capacitance of MnO2-in-CNT is increased due to both the formation of Mn(IV)/Mn(III) redox couple and the desolvation of electrolyte ions inside the channels of CNTs blocked by MnO2 nanoparticles [9].


Table 4. Capacitance properties of selected composites containing carbon nanostructure and metal oxide.

Figure 22. Cyclic voltammogram of blank cCNTs (CNTS with closed tips), oCNTs (CNTs with open tips), MnO2-out-CNT and MnO2-in-CNT in 1 M Na2SO4. Reproduced with permission from Ref. [9]. Copyright 2010 The Royal Society of Chemistry.

Figure 23. (a) Representation of the carbon aerogel internal structure and (b) targeted carbon nanotube aerogel compos-

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Figure 24. Cycle life of CAG, CAG-3%MWNT and CNAG composites in 5 M KOH. Reproduced with permission from

ite. Reproduced with permission from Ref. [166]. Copyright 2011 Springer.

Ref. [166]. Copyright 2011 Springer.

Moreover, hybrid materials such as carbon nanotubes/mesoporous carbon/MnO2 [165], carbon nanotubes/3D graphene/MnO2 [11], and carbon nanotube@MnO2@polypyrrole [12] have been developed. Table 4 shows the capacitance of exemplary composites based on carbon nanostructures and metal oxides.

#### 5.4. Capacitors based on carbon nanotubes and carbon gels

Recently, much attention has been paid to novel ultralight mesoporous carbon materials named carbon aerogels (CAGs) [15, 166–173] or carbon xerogels [8, 174–178]. Carbon aerogels exhibit many interesting properties such as low mass density, continuous porosity, high surface area, and high electrical conductivity. These properties are characteristic of the aerogel microstructure, which is a network of interconnected primary particles with diameters between 3 and 25 nm (Figure 23a). However, aerogels consist of agglomerate particles linked by covalent bridges with a ladder structure. The contact between these particles and the space due to the pores unfortunately introduces a high internal resistance within the aerogel which limits their practical applications. To overcome this problem, composites of carbon aerogels with carbon nanotubes are formed (Figure 23b). CNTs with their high electrical conductivity work as nanopathways for charges and thus improve the intrinsic conductivity of the CAGs and their mechanical integrity as well [166].

The main problem in the case of composite formation is the agglomeration of carbon nanotubes within the CAG's pore network. A very useful approach for the formation of binderless carbon nanotube aerogel (CNAG) composites proposed by Bordjiba and co-workers [166, 172] provides a significant improvement in the dispersion of carbon nanotubes within the aerogel matrix and

Figure 23. (a) Representation of the carbon aerogel internal structure and (b) targeted carbon nanotube aerogel composite. Reproduced with permission from Ref. [166]. Copyright 2011 Springer.

Moreover, hybrid materials such as carbon nanotubes/mesoporous carbon/MnO2 [165], carbon nanotubes/3D graphene/MnO2 [11], and carbon nanotube@MnO2@polypyrrole [12] have been developed. Table 4 shows the capacitance of exemplary composites based on carbon

Figure 22. Cyclic voltammogram of blank cCNTs (CNTS with closed tips), oCNTs (CNTs with open tips), MnO2-out-CNT and MnO2-in-CNT in 1 M Na2SO4. Reproduced with permission from Ref. [9]. Copyright 2010 The Royal Society of

Recently, much attention has been paid to novel ultralight mesoporous carbon materials named carbon aerogels (CAGs) [15, 166–173] or carbon xerogels [8, 174–178]. Carbon aerogels exhibit many interesting properties such as low mass density, continuous porosity, high surface area, and high electrical conductivity. These properties are characteristic of the aerogel microstructure, which is a network of interconnected primary particles with diameters between 3 and 25 nm (Figure 23a). However, aerogels consist of agglomerate particles linked by covalent bridges with a ladder structure. The contact between these particles and the space due to the pores unfortunately introduces a high internal resistance within the aerogel which limits their practical applications. To overcome this problem, composites of carbon aerogels with carbon nanotubes are formed (Figure 23b). CNTs with their high electrical conductivity work as nanopathways for charges and thus improve the intrinsic conductivity of the CAGs

The main problem in the case of composite formation is the agglomeration of carbon nanotubes within the CAG's pore network. A very useful approach for the formation of binderless carbon nanotube aerogel (CNAG) composites proposed by Bordjiba and co-workers [166, 172] provides a significant improvement in the dispersion of carbon nanotubes within the aerogel matrix and

nanostructures and metal oxides.

Chemistry.

170 Carbon Nanotubes - Recent Progress

and their mechanical integrity as well [166].

5.4. Capacitors based on carbon nanotubes and carbon gels

Figure 24. Cycle life of CAG, CAG-3%MWNT and CNAG composites in 5 M KOH. Reproduced with permission from Ref. [166]. Copyright 2011 Springer.

better interfacial coating. The CNAG material was prepared by a molding procedure, i.e., synthesis by a chemical vapor deposition method to grow carbon nanotubes directly onto a microfibrous carbon paper substrate. In the next step, the carbon aerogel was synthesized on the carbon nanotubes. The key feature of this method is eliminating the need to control the carbon nanotube concentration, which permits optimized dispersion processes to reinforce the aerogel's networks. The CNAG electrode delivered very high specific capacitances equal to 524 F g�<sup>1</sup> . Furthermore, better integration of carbon nanotubes in the matrix of carbon aerogel improves its resistance to attack by the electrolyte and assures excellent cycle life. Such a system exhibits significantly higher capacitance and stability compared to the composite processed from the dispersion of its components in liquid phase, CAG-3%MWNT (Figure 24). Apart from the capacitance performance, binderless nanostructured electrodes also reduce the cost of manufacturing and avoid complicated interferences of the binders and conductivity enhancers used in practical electrodes [166, 172].

The research on CNTs and carbon aerogel composites is relatively new because their very promising properties are still being developed. The composites that have been prepared so far exhibit excellent capacitance properties [8, 166, 170–172, 177, 178]. A large progress in this area of study can be expected in the near future.

> attention is needed to solve this problem because this system could be the future for storage devices. A very promising system seems to be the attachment of redox-active nanoparticles to carbon nanotubes. Compared to bulk materials, they exhibit unique properties arising from their nanoscale sizes, such as high electrical conductivity, large surface area, short path lengths for the transport of ions and high electrochemical activity. Ultrafast compact capacitors based on 3D hybrid structures that increase the accessible surface area and allow fast ion diffusion are introducing a new class of electrode materials for storage devices. A comparison of the properties of composites based on carbon nanotubes and their compo-

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173

Scheme 2. Comparison of properties of composites based on carbon nanotubes and their components.

The authors acknowledge the National Center of Science (project No. 2016/21/B/ST5/02496 to

nents is summarized in Scheme 2.

Acknowledgements

KW) for a financial support.

Emilia Grądzka\* and Krzysztof Winkler

\*Address all correspondence to: emilia@uwb.edu.pl

Institute of Chemistry, University of Bialystok, Poland

Author details

#### 6. Conclusion and outlook

Supercapacitors have been developed to close the gap between conventional capacitors and batteries because of their high energy density and power density. In the development of electrochemical capacitors, carbon nanotubes and their composites have been widely used as electrode materials. The specific capacitance of pristine carbon nanotubes is relatively low and depends on many factors such as the kind of carbon nanotubular material, i.e., single- or multi-walled, its orientation, i.e., open or closed tips, surface area, synthesis method, solvent and supporting electrolyte. Compared with pristine carbon nanotubes, functionalized carbon nanotubes by heteroatoms or functional groups attached to nanotubes walls are expected to display improved capacitance performance. The formation of composites based on carbon nanotubes provides especially high surface area due to the presence of CNTs which is very important in the case of storage systems. Moreover, it enhances the properties of both the carbon nanotubes and the second component. Apart from the improvement in capacitance performance, the addition of CNTs reduces cost compared to metal oxide, improves stability compared to conducting polymers which exhibit rapid degradation in performance after repetitive cycles because of their swelling and shrinking, and improves the poor volumetric performance of supercapacitors based on other carbon nanomaterials. The capacitance properties strongly depend on the localization of the redox system. It was found that encapsulation of the redox phase inside a nanotubular material provides higher specific capacitance compared to a redox system situated outside of carbon nanotubes. Recently, a new generation of cheap storage systems based on mesoporous carbon aerogels was discovered. However, in this case, there is a problem with the non-homogeneous spread of carbon nanotubes within the whole network of carbon aerogel. Hence, increasing

Scheme 2. Comparison of properties of composites based on carbon nanotubes and their components.

attention is needed to solve this problem because this system could be the future for storage devices. A very promising system seems to be the attachment of redox-active nanoparticles to carbon nanotubes. Compared to bulk materials, they exhibit unique properties arising from their nanoscale sizes, such as high electrical conductivity, large surface area, short path lengths for the transport of ions and high electrochemical activity. Ultrafast compact capacitors based on 3D hybrid structures that increase the accessible surface area and allow fast ion diffusion are introducing a new class of electrode materials for storage devices. A comparison of the properties of composites based on carbon nanotubes and their components is summarized in Scheme 2.

#### Acknowledgements

better interfacial coating. The CNAG material was prepared by a molding procedure, i.e., synthesis by a chemical vapor deposition method to grow carbon nanotubes directly onto a microfibrous carbon paper substrate. In the next step, the carbon aerogel was synthesized on the carbon nanotubes. The key feature of this method is eliminating the need to control the carbon nanotube concentration, which permits optimized dispersion processes to reinforce the aerogel's networks. The CNAG electrode delivered very high specific capacitances equal to 524 F

. Furthermore, better integration of carbon nanotubes in the matrix of carbon aerogel improves its resistance to attack by the electrolyte and assures excellent cycle life. Such a system exhibits significantly higher capacitance and stability compared to the composite processed from the dispersion of its components in liquid phase, CAG-3%MWNT (Figure 24). Apart from the capacitance performance, binderless nanostructured electrodes also reduce the cost of manufacturing and avoid complicated interferences of the binders and conductivity enhancers

The research on CNTs and carbon aerogel composites is relatively new because their very promising properties are still being developed. The composites that have been prepared so far exhibit excellent capacitance properties [8, 166, 170–172, 177, 178]. A large progress in this

Supercapacitors have been developed to close the gap between conventional capacitors and batteries because of their high energy density and power density. In the development of electrochemical capacitors, carbon nanotubes and their composites have been widely used as electrode materials. The specific capacitance of pristine carbon nanotubes is relatively low and depends on many factors such as the kind of carbon nanotubular material, i.e., single- or multi-walled, its orientation, i.e., open or closed tips, surface area, synthesis method, solvent and supporting electrolyte. Compared with pristine carbon nanotubes, functionalized carbon nanotubes by heteroatoms or functional groups attached to nanotubes walls are expected to display improved capacitance performance. The formation of composites based on carbon nanotubes provides especially high surface area due to the presence of CNTs which is very important in the case of storage systems. Moreover, it enhances the properties of both the carbon nanotubes and the second component. Apart from the improvement in capacitance performance, the addition of CNTs reduces cost compared to metal oxide, improves stability compared to conducting polymers which exhibit rapid degradation in performance after repetitive cycles because of their swelling and shrinking, and improves the poor volumetric performance of supercapacitors based on other carbon nanomaterials. The capacitance properties strongly depend on the localization of the redox system. It was found that encapsulation of the redox phase inside a nanotubular material provides higher specific capacitance compared to a redox system situated outside of carbon nanotubes. Recently, a new generation of cheap storage systems based on mesoporous carbon aerogels was discovered. However, in this case, there is a problem with the non-homogeneous spread of carbon nanotubes within the whole network of carbon aerogel. Hence, increasing

g�<sup>1</sup>

172 Carbon Nanotubes - Recent Progress

used in practical electrodes [166, 172].

6. Conclusion and outlook

area of study can be expected in the near future.

The authors acknowledge the National Center of Science (project No. 2016/21/B/ST5/02496 to KW) for a financial support.

#### Author details

Emilia Grądzka\* and Krzysztof Winkler

\*Address all correspondence to: emilia@uwb.edu.pl

Institute of Chemistry, University of Bialystok, Poland

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**Chapter 10**

**Provisional chapter**

**Studies of Nanocomposites of Carbon Nanotubes and a**

The complex specific heat is reported over a wide temperature range for a negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and carbon nanotube (CNT) composites as a function of carbon nanotube concentration. It has been observed that the combination of nanotubes (CNT) and liquid crystal (LC) provides a very useful way to align CNTs and also dramatically increases the order in the liquid crystal performance, which is useful in liquid display technology (LCD). The calorimetric scans were performed between 25 and 95°C temperatures, first allowed cooling and then heating for CNT concentration ranging from ϕw = 0 to 0.2 wt%. All 9OO4/CNT composite mesophases have transition temperatures about 1 K higher and a crystallization temperature 4 K higher as compared to the pure 9OO4 liquid crystal. A strongly first-order specific heat feature is observed, which is 0.5 K higher than in the pure 9OO4. The transition enthalpy for the composite mesophases is observed 10% lower than the pure liquid crystal. We interpret that these results arising from the LC-CNT surface interaction lead to pinning orientational order uniformly along the CNT, without pinning the position of the 9OO4 molecule. These effects of incorporating CNTs with LC are likely due to elastic coupling between CNT and LC. These effects of incorporating CNTs into LCs are likely due to an "anisotropic orientational" coupling between CNT and LC, the change in the elastic properties of

**Studies of Nanocomposites of Carbon Nanotubes and a** 

DOI: 10.5772/intechopen.73391

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Composites of nanoparticles with liquid crystal represent a unique physical system, where properties of the constituents fully mix and may lead to show new behavior. As of now,

**Negative Dielectric Anisotropy Liquid Crystal**

**Negative Dielectric Anisotropy Liquid Crystal**

Parvathalu Kalakonda and Germano S Iannacchione

composites and thermal anisotropic properties of the CNTs.

**Keywords:** carbon nanotubes, liquid crystal, chirality

Parvathalu Kalakonda and Germano S Iannacchione

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73391

**Abstract**

**1. Introduction**

**Provisional chapter**

#### **Studies of Nanocomposites of Carbon Nanotubes and a Negative Dielectric Anisotropy Liquid Crystal Negative Dielectric Anisotropy Liquid Crystal**

**Studies of Nanocomposites of Carbon Nanotubes and a** 

DOI: 10.5772/intechopen.73391

Parvathalu Kalakonda and Germano S Iannacchione Parvathalu Kalakonda and Germano S Iannacchione Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73391

#### **Abstract**

The complex specific heat is reported over a wide temperature range for a negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and carbon nanotube (CNT) composites as a function of carbon nanotube concentration. It has been observed that the combination of nanotubes (CNT) and liquid crystal (LC) provides a very useful way to align CNTs and also dramatically increases the order in the liquid crystal performance, which is useful in liquid display technology (LCD). The calorimetric scans were performed between 25 and 95°C temperatures, first allowed cooling and then heating for CNT concentration ranging from ϕw = 0 to 0.2 wt%. All 9OO4/CNT composite mesophases have transition temperatures about 1 K higher and a crystallization temperature 4 K higher as compared to the pure 9OO4 liquid crystal. A strongly first-order specific heat feature is observed, which is 0.5 K higher than in the pure 9OO4. The transition enthalpy for the composite mesophases is observed 10% lower than the pure liquid crystal. We interpret that these results arising from the LC-CNT surface interaction lead to pinning orientational order uniformly along the CNT, without pinning the position of the 9OO4 molecule. These effects of incorporating CNTs with LC are likely due to elastic coupling between CNT and LC. These effects of incorporating CNTs into LCs are likely due to an "anisotropic orientational" coupling between CNT and LC, the change in the elastic properties of composites and thermal anisotropic properties of the CNTs.

**Keywords:** carbon nanotubes, liquid crystal, chirality

#### **1. Introduction**

Composites of nanoparticles with liquid crystal represent a unique physical system, where properties of the constituents fully mix and may lead to show new behavior. As of now,

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

traditional composites are well understood, the superposition of component of the composites interfacial interactions play a very important role in holding the composite together. As the filler size shrink, the surface area begins to dominate, leading to unique behavior of the composites. Carbon nanotubes (CNT) and liquid crystals (LC) are good examples of such components. For large number of applications, the challenge lies in the alignment and ordering of CNTs to take advantages of their highly anisotropic thermal characteristics. Carbon nanotubes have emerged as a new category of nanosized particles for incorporation into different liquid crystal systems, attracting favorable interest from both basic level science research and industrial applications [1]. As a result of the exceptional properties of carbon nanotubes, the novel materials can be envisioned that exhibit property enhancements at lower concentration than in conventional composite technology [2]. In the research area of polymer nanocomposites, reported property enhancements include enhanced mechanical performance [3, 4], higher thermal and electrical conductivity [5–7], and increased property of crystallization rate [8–12]. In addition to CNT's thermal, mechanical, and magnetic properties [13–22], their unique electrical properties make them potentially very useful materials for making nanoflexible electronic devices [2, 23–26]. Liquid crystals [27–29] are anisotropic fluids that exhibit thermodynamically stable phases between an isotropic liquid and a full, three-dimensionally ordered solid. In the nematic (N) phase, LCs show orientational order, and in the smectic-A (SmA) phase, the rod-like molecules are arranged in layers with their long axes on average, normal to the layer planes. They show both orientational and partial translational order characterized by a quasi-1-d density wave. The smectic phases incorporate the structures of materials with diverse symmetry groups [30], such as that of smectic-C (SmC), which has the layer tilting away from the director. In smectic-B (SmB) phase, the molecules show short-range hexagonal ordering within the layers but not from layer to layer, and it shows a short or longrange translational order. Higher order, lower symmetry, liquid crystalline materials have been studied because three of their industrial applications as well as important physical models of self-assembly [29, 30]. When CNTs are dispersed in a liquid crystal, they can modify the physical properties and hence the phase behavior of the nanocomposite. Due to the specific surface anchoring between nanoparticle and LC, the nanoparticles can act either as nucleation sites for a given type of order or as disordering sites that stabilize the isotropic phase [31, 32]. However, if the local ordering effects of CNT surfaces are randomly arranged, this can lead to a random-field effect [33] and an overall disordering of the composites. Investigations have been made on liquid crystal nanocomposites using optical microscopy and differential scanning calorimetry (DSC), finding an enhancement of the isotropic to nematic phase transition temperature and revealing a "chimney type" phase diagram over a narrow range of CNT weight percent between 0.001 and 0.002 wt% [34]. In fact, researchers found that the addition of CNT into LC increases the clearing point of LCs because of the strong attraction of CNT and LC [35]. However, other studies have found disordering effects of CNT on LC phase transitions [31]. Other work has shown improved electro-optical switching properties of nanocomposites in thermotropic or lyotropic liquid crystals and CNTs [36–41]. In liquid crystals, the effect of carbon nanotubes on the phase ordering of LC/CNT composites depends on the surface coupling of the molecule and graphene surface, as well as the distribution of those surfaces. In fact, it is also observed that aligned CNTs can cause an increase of the orientational order in the LC [42, 43]. Such composites have been proposed as memory devices

by exploiting their nanoelectromechanical properties [44, 45]. Recently, investigations on LC/ CNT composites have shown that the CNTs can induce chirality in the bulk LC [46–48]. Most of the recent studies were focused on the nematic ordering and the isotropic (I) to nematic (N) phase transition behavior in LC/CNT composite scaffolds. In this study, we focus the phase transition behavior of the liquid crystal alkoxyphenylbenzoate (9OO4) doped with multiwall carbon nanotubes as a function of CNT weight percent. The incorporation of nanotubes in pure 9OO4 reveals that all mesophase transition temperatures and the crystallization transition temperatures shift upward. These results suggest that the interactions between molecular structure, dipole moment of liquid crystal, and graphene-like surface can allow the random dispersion of CNTs to promote both orientational and positional order. We interpret this effect in terms of pinning the director at the CNT surface with bulk-like scalar order parallel to the CNT long axis over distances that span multiple nematic domains while allowing the LC molecule to slide along the surface, accommodating four various positional orders. Our presentation is organized as follows: Following this introduction, Section 2 describes the preparation of the sample and modulated differential scanning calorimetry procedure, as well as the electroclinic procedure. Section 3 describes the calorimetric and electroclinic results of all phase transitions in the 9OO4/CNT system, Section 4 provides a discussion, and

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The liquid crystal 9OO4 is a phenyl benzoate containing an oxoester linkage group with two alkoxy end groups (see **Figure 1**). The molecular mass for 9OO4 is Mw = 424.66 g/mol with an extended molecular structure approximately 4 nm long and 0.8 nm wide. For 9OO4, modeling suggests that due to the carbon = oxygen double bond in the oxoester linking group, the effective dipole moment of 9OO4 is approximately perpendicular to the long axis and average plane of the phenyl rings. The multiwall CNTs were obtained from Nanostructured and Amorphous Materials, Inc., and have an average outer diameter of 8–15 nm, an inner diameter 3–5 nm, a length of 500–2000 nm, and a distribution of different chiral structures [46]. To reduce aggregation, a small amount of CNTs were dispersed first in acetone and shaken using a mixer for 30 min, followed by sonication for 3 hours. The 9OO4 was added to the acetone + CNT mixture to

**Figure 1.** The member of the 4-n-alkoxy phenyl-40-m-alkoxy benzoate homolog series denoted nOOm. In this study, n = 9 and m = 4 (9OO4). Note that the negative dielectric anisotropy is due to the O-C=O group in the benzoate group linking the two phenyl rings. Molecular modeling suggests that the effective dipole moment of about 2.5 is pointing

approximately perpendicular to the twisted phenyl rings, essentially out of the page [58].

Section 5 provides conclusions of our work and future directions.

**2. Methodology**

**2.1. Material and sample preparation**

by exploiting their nanoelectromechanical properties [44, 45]. Recently, investigations on LC/ CNT composites have shown that the CNTs can induce chirality in the bulk LC [46–48]. Most of the recent studies were focused on the nematic ordering and the isotropic (I) to nematic (N) phase transition behavior in LC/CNT composite scaffolds. In this study, we focus the phase transition behavior of the liquid crystal alkoxyphenylbenzoate (9OO4) doped with multiwall carbon nanotubes as a function of CNT weight percent. The incorporation of nanotubes in pure 9OO4 reveals that all mesophase transition temperatures and the crystallization transition temperatures shift upward. These results suggest that the interactions between molecular structure, dipole moment of liquid crystal, and graphene-like surface can allow the random dispersion of CNTs to promote both orientational and positional order. We interpret this effect in terms of pinning the director at the CNT surface with bulk-like scalar order parallel to the CNT long axis over distances that span multiple nematic domains while allowing the LC molecule to slide along the surface, accommodating four various positional orders. Our presentation is organized as follows: Following this introduction, Section 2 describes the preparation of the sample and modulated differential scanning calorimetry procedure, as well as the electroclinic procedure. Section 3 describes the calorimetric and electroclinic results of all phase transitions in the 9OO4/CNT system, Section 4 provides a discussion, and Section 5 provides conclusions of our work and future directions.

#### **2. Methodology**

traditional composites are well understood, the superposition of component of the composites interfacial interactions play a very important role in holding the composite together. As the filler size shrink, the surface area begins to dominate, leading to unique behavior of the composites. Carbon nanotubes (CNT) and liquid crystals (LC) are good examples of such components. For large number of applications, the challenge lies in the alignment and ordering of CNTs to take advantages of their highly anisotropic thermal characteristics. Carbon nanotubes have emerged as a new category of nanosized particles for incorporation into different liquid crystal systems, attracting favorable interest from both basic level science research and industrial applications [1]. As a result of the exceptional properties of carbon nanotubes, the novel materials can be envisioned that exhibit property enhancements at lower concentration than in conventional composite technology [2]. In the research area of polymer nanocomposites, reported property enhancements include enhanced mechanical performance [3, 4], higher thermal and electrical conductivity [5–7], and increased property of crystallization rate [8–12]. In addition to CNT's thermal, mechanical, and magnetic properties [13–22], their unique electrical properties make them potentially very useful materials for making nanoflexible electronic devices [2, 23–26]. Liquid crystals [27–29] are anisotropic fluids that exhibit thermodynamically stable phases between an isotropic liquid and a full, three-dimensionally ordered solid. In the nematic (N) phase, LCs show orientational order, and in the smectic-A (SmA) phase, the rod-like molecules are arranged in layers with their long axes on average, normal to the layer planes. They show both orientational and partial translational order characterized by a quasi-1-d density wave. The smectic phases incorporate the structures of materials with diverse symmetry groups [30], such as that of smectic-C (SmC), which has the layer tilting away from the director. In smectic-B (SmB) phase, the molecules show short-range hexagonal ordering within the layers but not from layer to layer, and it shows a short or longrange translational order. Higher order, lower symmetry, liquid crystalline materials have been studied because three of their industrial applications as well as important physical models of self-assembly [29, 30]. When CNTs are dispersed in a liquid crystal, they can modify the physical properties and hence the phase behavior of the nanocomposite. Due to the specific surface anchoring between nanoparticle and LC, the nanoparticles can act either as nucleation sites for a given type of order or as disordering sites that stabilize the isotropic phase [31, 32]. However, if the local ordering effects of CNT surfaces are randomly arranged, this can lead to a random-field effect [33] and an overall disordering of the composites. Investigations have been made on liquid crystal nanocomposites using optical microscopy and differential scanning calorimetry (DSC), finding an enhancement of the isotropic to nematic phase transition temperature and revealing a "chimney type" phase diagram over a narrow range of CNT weight percent between 0.001 and 0.002 wt% [34]. In fact, researchers found that the addition of CNT into LC increases the clearing point of LCs because of the strong attraction of CNT and LC [35]. However, other studies have found disordering effects of CNT on LC phase transitions [31]. Other work has shown improved electro-optical switching properties of nanocomposites in thermotropic or lyotropic liquid crystals and CNTs [36–41]. In liquid crystals, the effect of carbon nanotubes on the phase ordering of LC/CNT composites depends on the surface coupling of the molecule and graphene surface, as well as the distribution of those surfaces. In fact, it is also observed that aligned CNTs can cause an increase of the orientational order in the LC [42, 43]. Such composites have been proposed as memory devices

190 Carbon Nanotubes - Recent Progress

#### **2.1. Material and sample preparation**

The liquid crystal 9OO4 is a phenyl benzoate containing an oxoester linkage group with two alkoxy end groups (see **Figure 1**). The molecular mass for 9OO4 is Mw = 424.66 g/mol with an extended molecular structure approximately 4 nm long and 0.8 nm wide. For 9OO4, modeling suggests that due to the carbon = oxygen double bond in the oxoester linking group, the effective dipole moment of 9OO4 is approximately perpendicular to the long axis and average plane of the phenyl rings. The multiwall CNTs were obtained from Nanostructured and Amorphous Materials, Inc., and have an average outer diameter of 8–15 nm, an inner diameter 3–5 nm, a length of 500–2000 nm, and a distribution of different chiral structures [46]. To reduce aggregation, a small amount of CNTs were dispersed first in acetone and shaken using a mixer for 30 min, followed by sonication for 3 hours. The 9OO4 was added to the acetone + CNT mixture to

**Figure 1.** The member of the 4-n-alkoxy phenyl-40-m-alkoxy benzoate homolog series denoted nOOm. In this study, n = 9 and m = 4 (9OO4). Note that the negative dielectric anisotropy is due to the O-C=O group in the benzoate group linking the two phenyl rings. Molecular modeling suggests that the effective dipole moment of about 2.5 is pointing approximately perpendicular to the twisted phenyl rings, essentially out of the page [58].

achieve the desired final weight percent ϕw of CNTs. The mixture was then sonicated for 3 hours to facilitate good dispersion. Finally, the acetone was evaporated slowly and then allowed to degass under a modest vacuum in the isotropic phase of 9OO4 at ∼364 K (or 90°C) for about 2 hours. Microbalance is used to mass the mixture of composites to ensure complete removal of acetone before sealed in the experimental sample cell. This process was continuously repeated to prepare 0 (pure 9OO4), 0.008, 0.010, 0.025, and 0.200 wt% CNT samples and tested uniform dispersion with the help of microscopy. On cooling, pure 9OO4 exhibits the phase sequence I - 87 - N - 73 - SmA - 62 - SmC - 50 - SmB - 35 - K (crystal), where temperatures are in Celsius; while on heating, melting occurs nearly 25 K higher, which is followed by a specific heat peak almost 2 K higher than the melting peak before entering the SmA phase. Further heating yields the N and I phases. Based on the imaginary specific heat ∆C″ p behavior, the I-N transition is weakly first order, the N-SmA is continuous, the SmA-SmC is continuous, the SmC-SmB is first order, and the SmB-K is strongly first order. The initial melting from crystal is strongly first order followed by a second, weakly first order, Cp feature that indicates the presence of a narrow temperature range having a smectic-like texture, as seen under polarizing microscopy (labeled SmX) between the K and SmA phase on heating [49]. Unfortunately, the polarizing microscope images could not distinguish which type of smectic is the SmX region.

a peak in the real part of the specific heat indicates that this feature is a first-order transition and involves a latent heat. The excess specific heats were determined in order to isolate the contribution from the various transitions. A linear baseline was used over the entire temperature scan range in order to determine ∆Cp = C<sup>p</sup> − Cbaseline for both the real and imaginary parts of specific heat components, though C″p always exhibited a very shallow linear baseline that was very close to zero, indicating near equilibrium conditions for the experimental parameters used in this work. For specific heat features that are close in temperature, the wing of one peak is subtracted from the lower specific heat peak in order to isolate the excess specific heat of the lower temperature transition, denoted as δCp = ∆Cp − Cwing, where Cwing is a mimic function (polynomial) of the underlying wing. This calculation was only applied to the real component of the specific heat. The particular transition enthalpy component is simply the integration of the excess specific heat component over a consistent temperature range. Finally, for first-order transitions, the transition temperature (TIN, TCB, TBK, TKX, and TXA) is determined as the highest temperature of the two-phase coexistence region indicated by the onset of nonzero values of ∆C″ p. For continuous transitions, the transition temperature is taken as the ∆C′ p peak temperature. We have tested quasi-equilibrium parameter such as scan rate, temperature amplitude, and modulation time period by varying different values above and below specified values of nearly equilibrium.

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Details of the electroclinic effect experimental set up are described elsewhere [59]. Briefly, a cell of thickness d = 7.7 ± 0.1 μm was constructed from a pair of indium-tin-oxide–coated glass slides covered with a rubbed polyimide alignment layer. The cell was filled with 0.05 wt% CNT in 9OO4, which aligned in the planar direction. Light polarized at 22.5° relative to the director orientation was incident on the cell, subsequently passing through the cell, an analyzer, and into a detector. On applying an ac-electric field E at frequency f = 25 Hz across the cell, the director rotated in the cell's plane by an angle θ [∝ E], resulting in an ac intensity component Iac at frequency f, as measured by a lock-in amplifier. It can be shown [59] that the ratio Iac/4Idc corresponds to the spatially averaged rms director rotation θ, where Idc is the dc intensity measured at the detector. At each temperature, the electric field was ramped from 0 to 2.6 × 106 V/m (rms) over a time 450 s, and the results were fitted to a straight line.

For the pure 9OO4, calorimetry shows that the I-N phase transition occurs at TIN = 86.23°C, the N-SmA phase transition at TNA = 71.49°C, the SmA-SmC phase transition at TAC = 61.5°C, the SmC-SmB phase transition at TCB = 49.38°C, and SmB-Cry phase transition at TBK = 35.3°C in good agreement with literature values [49]. All the phase transitions are characterized by a distinct Cp peaks at a nearly equilibrium scan rate of ±0.3 K/min (**Figures 2**–**6**). The strongly first-order melting followed by a second first-order Cp feature indicates the presence of an intermediate phase (labeled SmX) between K and SmA on heating. The transition temperatures and enthalpies of pure 9OO4 on cooling and heating are presented in **Tables 1** and **2**.

**2.3. Electroclinic measurements**

**3. Results**

The electroclinic coefficient is defined as ec ≡ dθ/dE.

#### **2.2. Modulated differential scanning calorimetry (MDSC)**

Modulated differential scanning calorimetry (MTDSC/MDSC) allows for the simultaneous measurement of the evolution of both the heat flow and heat capacity. It is essentially the combination of traditional ac-calorimetry and DSC. This method allows for measuring the total heat flow of material, as well as its nonreversible and reversible heat capacities. A detailed description of the MDSC method can be found elsewhere [50–57]. MDSC experiments were performed using a Model Q200 from TA Instruments, USA. Prior to all measurements, temperature calibration was done with a sapphire disc, under the same experimental conditions used for all 9OO4/CNT samples. The analysis method used to extract the complex specific heat is based on linear response theory [50, 58]. In general, a temperature oscillation is described as T(t) = T<sup>0</sup> + T˙<sup>0</sup> t + A<sup>T</sup> sin(ωt), where T<sup>0</sup> is the initial temperature at time t = 0, T indicates the absolute temperature at time t, T˙<sup>0</sup> represents the baseline temperature scan rate, AT is the temperature amplitude, and ω (ω = 2πf) is the angular frequency of the temperature modulation. The rate of temperature is also time dependent and is given by equation T˙(t) = dT/dt = T˙<sup>0</sup> + Aqcos(ωt), where Aq represents the amplitude of temperature modulation r (Aq = ωAT). Since the applied temperature rate consists of two components, T<sup>0</sup> the underlying rate and Aqcos(ωt) the periodic rate. The measured heat flow HF can also be separated into two components in response to these temperature rates. The periodic component can be described by HFq = AHF cos(ωt − φ), where AHF represents the amplitude of the heat flow and φ is the phase angle between heat flow and temperature rate. The absolute value of the complex specific heat is written as C ∗ p = AHF/mAq, where 'm' is the mass of the sample. The phase angle φ requires a small calibration to account for finite six thermal conductivities of the sample and cell, see Ref. [57]. The real part of specific C′ p and imaginary part of specific heat C″p are then given by C′ p = C ∗ p cos(φ) and C″p = C ∗ p sin(φ), which allows for a consistent definition for the complex specific heat. Typically, under equilibrium conditions, C″ p = 0 after φ calibration. The appearance of a peak-like nonzero C″p feature commensurate with a peak in the real part of the specific heat indicates that this feature is a first-order transition and involves a latent heat. The excess specific heats were determined in order to isolate the contribution from the various transitions. A linear baseline was used over the entire temperature scan range in order to determine ∆Cp = C<sup>p</sup> − Cbaseline for both the real and imaginary parts of specific heat components, though C″p always exhibited a very shallow linear baseline that was very close to zero, indicating near equilibrium conditions for the experimental parameters used in this work. For specific heat features that are close in temperature, the wing of one peak is subtracted from the lower specific heat peak in order to isolate the excess specific heat of the lower temperature transition, denoted as δCp = ∆Cp − Cwing, where Cwing is a mimic function (polynomial) of the underlying wing. This calculation was only applied to the real component of the specific heat. The particular transition enthalpy component is simply the integration of the excess specific heat component over a consistent temperature range. Finally, for first-order transitions, the transition temperature (TIN, TCB, TBK, TKX, and TXA) is determined as the highest temperature of the two-phase coexistence region indicated by the onset of nonzero values of ∆C″ p. For continuous transitions, the transition temperature is taken as the ∆C′ p peak temperature. We have tested quasi-equilibrium parameter such as scan rate, temperature amplitude, and modulation time period by varying different values above and below specified values of nearly equilibrium.

#### **2.3. Electroclinic measurements**

Details of the electroclinic effect experimental set up are described elsewhere [59]. Briefly, a cell of thickness d = 7.7 ± 0.1 μm was constructed from a pair of indium-tin-oxide–coated glass slides covered with a rubbed polyimide alignment layer. The cell was filled with 0.05 wt% CNT in 9OO4, which aligned in the planar direction. Light polarized at 22.5° relative to the director orientation was incident on the cell, subsequently passing through the cell, an analyzer, and into a detector. On applying an ac-electric field E at frequency f = 25 Hz across the cell, the director rotated in the cell's plane by an angle θ [∝ E], resulting in an ac intensity component Iac at frequency f, as measured by a lock-in amplifier. It can be shown [59] that the ratio Iac/4Idc corresponds to the spatially averaged rms director rotation θ, where Idc is the dc intensity measured at the detector. At each temperature, the electric field was ramped from 0 to 2.6 × 106 V/m (rms) over a time 450 s, and the results were fitted to a straight line. The electroclinic coefficient is defined as ec ≡ dθ/dE.

#### **3. Results**

achieve the desired final weight percent ϕw of CNTs. The mixture was then sonicated for 3 hours to facilitate good dispersion. Finally, the acetone was evaporated slowly and then allowed to degass under a modest vacuum in the isotropic phase of 9OO4 at ∼364 K (or 90°C) for about 2 hours. Microbalance is used to mass the mixture of composites to ensure complete removal of acetone before sealed in the experimental sample cell. This process was continuously repeated to prepare 0 (pure 9OO4), 0.008, 0.010, 0.025, and 0.200 wt% CNT samples and tested uniform dispersion with the help of microscopy. On cooling, pure 9OO4 exhibits the phase sequence I - 87 - N - 73 - SmA - 62 - SmC - 50 - SmB - 35 - K (crystal), where temperatures are in Celsius; while on heating, melting occurs nearly 25 K higher, which is followed by a specific heat peak almost 2 K higher than the melting peak before entering the SmA phase. Further heating yields the N and

order, the N-SmA is continuous, the SmA-SmC is continuous, the SmC-SmB is first order, and the SmB-K is strongly first order. The initial melting from crystal is strongly first order followed by a second, weakly first order, Cp feature that indicates the presence of a narrow temperature range having a smectic-like texture, as seen under polarizing microscopy (labeled SmX) between the K and SmA phase on heating [49]. Unfortunately, the polarizing microscope images could

Modulated differential scanning calorimetry (MTDSC/MDSC) allows for the simultaneous measurement of the evolution of both the heat flow and heat capacity. It is essentially the combination of traditional ac-calorimetry and DSC. This method allows for measuring the total heat flow of material, as well as its nonreversible and reversible heat capacities. A detailed description of the MDSC method can be found elsewhere [50–57]. MDSC experiments were performed using a Model Q200 from TA Instruments, USA. Prior to all measurements, temperature calibration was done with a sapphire disc, under the same experimental conditions used for all 9OO4/CNT samples. The analysis method used to extract the complex specific heat is based on linear response theory [50, 58]. In general, a temperature oscillation is described as T(t) = T<sup>0</sup> + T˙<sup>0</sup> t + A<sup>T</sup>

is the initial temperature at time t = 0, T indicates the absolute temperature at

the underlying rate and Aqcos(ωt) the periodic rate. The measured

p and imaginary part of

represents the baseline temperature scan rate, AT is the temperature amplitude, and

can also be separated into two components in response to these temperature rates.

ω (ω = 2πf) is the angular frequency of the temperature modulation. The rate of temperature is also time dependent and is given by equation T˙(t) = dT/dt = T˙<sup>0</sup> + Aqcos(ωt), where Aq represents the amplitude of temperature modulation r (Aq = ωAT). Since the applied temperature rate con-

The periodic component can be described by HFq = AHF cos(ωt − φ), where AHF represents the amplitude of the heat flow and φ is the phase angle between heat flow and temperature rate. The absolute value of the complex specific heat is written as C ∗ p = AHF/mAq, where 'm' is the mass of the sample. The phase angle φ requires a small calibration to account for finite six thermal con-

specific heat C″p are then given by C′ p = C ∗ p cos(φ) and C″p = C ∗ p sin(φ), which allows for a consistent definition for the complex specific heat. Typically, under equilibrium conditions, C″ p = 0 after φ calibration. The appearance of a peak-like nonzero C″p feature commensurate with

ductivities of the sample and cell, see Ref. [57]. The real part of specific C′

p behavior, the I-N transition is weakly first

I phases. Based on the imaginary specific heat ∆C″

192 Carbon Nanotubes - Recent Progress

not distinguish which type of smectic is the SmX region.

sin(ωt), where T<sup>0</sup>

sists of two components, T<sup>0</sup>

time t, T˙<sup>0</sup>

heat flow HF

**2.2. Modulated differential scanning calorimetry (MDSC)**

For the pure 9OO4, calorimetry shows that the I-N phase transition occurs at TIN = 86.23°C, the N-SmA phase transition at TNA = 71.49°C, the SmA-SmC phase transition at TAC = 61.5°C, the SmC-SmB phase transition at TCB = 49.38°C, and SmB-Cry phase transition at TBK = 35.3°C in good agreement with literature values [49]. All the phase transitions are characterized by a distinct Cp peaks at a nearly equilibrium scan rate of ±0.3 K/min (**Figures 2**–**6**). The strongly first-order melting followed by a second first-order Cp feature indicates the presence of an intermediate phase (labeled SmX) between K and SmA on heating. The transition temperatures and enthalpies of pure 9OO4 on cooling and heating are presented in **Tables 1** and **2**.

**Figure 2.** (a) Excess real part specific heat associated with the I-N phase transition as function of temperature about TIN on cooling. The definitions of the symbols are indicated in the inset. (b) Excess real part of specific heat associated with the I-N phase transition as function of temperature about TIN on heating.

samples overlay each other for all CNT concentrations ϕw and for bulk 9OO4 on the SmA side while are systematically below bulk 9OO4 on the nematic side of the transition. The δC′p behavior does not show any features as a function of ϕw, and no power-law fits were attempted.

**Figure 3.** (a) The excess real part of specific heat associated with the N-SmA phase transition as function of temperature about TNA on cooling. (b) The excess real part specific heat associated with the N-SmA phase transition as function of

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For the SmA-SmC phase transition, the excess specific δC′p as the function of temperature for pure 9OO4 and 9OO4/CNT composite samples is shown in **Figure 4**. The observed shape and continuous nature of the δC′p peak for pure 9OO4 are consistent with a Landau (mean-field) second-order transition [30] with no observed signature in C″p. The δC′p (AC) for the 9OO4/ CNT samples, heights are the same as pure 9OO4 up to 0.025 wt% and then decrease for the 0.05 and 0.20 wt% samples. The step in δC′p (AC) on the SmC-side for below TAC is 0.025 J/gK

The tilt susceptibility at the SmA-SmC transition was examined by measuring the electroclinic effect [59] in the LC/CNT mixtures. In the past, it has been shown that there is a nonzero enantiomeric excess for these CNTs, as well as a net chirality for CNTs from four other manufacturers [43]. When dissolved in a liquid crystal, the CNT imparts a net chiral environment to the

for pure 9OO4 and all 9OO4/CNT samples, independent of ϕw.

temperature about TNA on heating. The definitions of the symbols are indicated in the inset.

For the I-N phase transition, the excess real specific heat signatures as a function of temperature about TIN for pure 9OO4 and 9OO4/CNT composite samples are shown in **Figure 2**. The ∆C′p of 9OO4/CNT for the I-N transition phase is steeper and narrower than in the pure 9OO4, with the peak maximum nearer the high-temperature side of the coexistence region. The ∆C′p and C″p behavior are consistent on heating and cooling, as well as being reproducible after multiple thermal cycles. The ∆C′p wings above and below the I-N transition match each other and that in pure 9OO4 on heating and cooling. The ∆C′p peak height for cooling and heating scans within the two-phase I + N coexistence region is about the same as that in pure 9OO4 up to ϕw = 0.010, decreasing markedly for the 0.025 sample and then rising with increasing ϕw up to the highest CNT content sample of 0.20 wt% studied.

For the N-SmA phase transition, the specific heat δC′p on cooling and heating as a function of temperature about TNA is shown in **Figure 3** for bulk 9OO4 and 9OO4/CNT composite samples. For all samples, the N-SmA phase transition does not exhibit any special feature in the case of imaginary specific heat, indicating an apparent absence of latent heat, and indicates the continuous nature of the transition. The δC′p of the N-SmA transition for the composite 9OO4/CNT

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**Figure 3.** (a) The excess real part of specific heat associated with the N-SmA phase transition as function of temperature about TNA on cooling. (b) The excess real part specific heat associated with the N-SmA phase transition as function of temperature about TNA on heating. The definitions of the symbols are indicated in the inset.

samples overlay each other for all CNT concentrations ϕw and for bulk 9OO4 on the SmA side while are systematically below bulk 9OO4 on the nematic side of the transition. The δC′p behavior does not show any features as a function of ϕw, and no power-law fits were attempted.

For the I-N phase transition, the excess real specific heat signatures as a function of temperature about TIN for pure 9OO4 and 9OO4/CNT composite samples are shown in **Figure 2**. The ∆C′p of 9OO4/CNT for the I-N transition phase is steeper and narrower than in the pure 9OO4, with the peak maximum nearer the high-temperature side of the coexistence region. The ∆C′p and C″p behavior are consistent on heating and cooling, as well as being reproducible after multiple thermal cycles. The ∆C′p wings above and below the I-N transition match each other and that in pure 9OO4 on heating and cooling. The ∆C′p peak height for cooling and heating scans within the two-phase I + N coexistence region is about the same as that in pure 9OO4 up to ϕw = 0.010, decreasing markedly for the 0.025 sample and then rising with

**Figure 2.** (a) Excess real part specific heat associated with the I-N phase transition as function of temperature about TIN on cooling. The definitions of the symbols are indicated in the inset. (b) Excess real part of specific heat associated with

For the N-SmA phase transition, the specific heat δC′p on cooling and heating as a function of temperature about TNA is shown in **Figure 3** for bulk 9OO4 and 9OO4/CNT composite samples. For all samples, the N-SmA phase transition does not exhibit any special feature in the case of imaginary specific heat, indicating an apparent absence of latent heat, and indicates the continuous nature of the transition. The δC′p of the N-SmA transition for the composite 9OO4/CNT

increasing ϕw up to the highest CNT content sample of 0.20 wt% studied.

the I-N phase transition as function of temperature about TIN on heating.

194 Carbon Nanotubes - Recent Progress

For the SmA-SmC phase transition, the excess specific δC′p as the function of temperature for pure 9OO4 and 9OO4/CNT composite samples is shown in **Figure 4**. The observed shape and continuous nature of the δC′p peak for pure 9OO4 are consistent with a Landau (mean-field) second-order transition [30] with no observed signature in C″p. The δC′p (AC) for the 9OO4/ CNT samples, heights are the same as pure 9OO4 up to 0.025 wt% and then decrease for the 0.05 and 0.20 wt% samples. The step in δC′p (AC) on the SmC-side for below TAC is 0.025 J/gK for pure 9OO4 and all 9OO4/CNT samples, independent of ϕw.

The tilt susceptibility at the SmA-SmC transition was examined by measuring the electroclinic effect [59] in the LC/CNT mixtures. In the past, it has been shown that there is a nonzero enantiomeric excess for these CNTs, as well as a net chirality for CNTs from four other manufacturers [43]. When dissolved in a liquid crystal, the CNT imparts a net chiral environment to the

**Figure 4.** Excess real specific heat associated with the SmA-SmC phase transition as function of temperature about TAC on cooling for different CNT content sample. The definition of the symbols is given in the inset.

**Figure 5.** Electroclinic response is a function of electric applied field (E). Data collected at temperature 63.2°C, approximately 1.2°C above SmA*-*SmC transition. Field was ramped over approximately 5 min.

director rotation extends a length scale ξ, comparable to the SmC correlation length, into the bulk liquid crystal from the narrow chiral region immediately surrounding the CNTs, which is the region that responds directly to the applied field. As an aside, we note that although TAC was obtained for this concentration of nanotubes, it is not possible to compare it with the transition temperature in the absence of nanotubes, for which chirality would be absent, and

**Table 1.** Summary of the transition temperatures for pure 9OO4 and all 9OO4/CNT samples on cooling (C) and heating (H).

temperature *TKX*, the Sm*X*-Sm*A* transition temperature *TXA*, the Sm*A*-Sm*C* transition temperature *TAC*, the Sm*C*-Sm*B* transition temperature *TCB*, and the Sm*B*-*K* transition temperature *TBK* are shown. All temperatures are given in Celsius.

the *N*-Sm*A* transition temperature *TNA*, the *K*-Sm*X* transition

**Figure 6.** The inverse electroclinic coefficient *e* − 1 *c* is shown as a function of temperature for an applied electric field at frequency 25 Hz across the cell containing a 0.05 wt% 9OO4/CNT sample. The results were fitted a 3-parameter power law and resulted in a susceptibility exponent *γ* = 0.99 ± 0.06. Inset shows an expanded view of the inverse electroclinic

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**TIN(C) TNA(C) TAC(C) TCB(C) TBK(C) TKX(H) TKA(H) TNA(H) TIN(H)**

0.000 86.19 71.27 61.10 49.38 35.00 60.96 63.55 71.46 86.48 0.008 87.13 72.68 62.49 50.33 39.16 61.36 64.24 72.72 87.27 0.010 87.14 72.69 62.50 50.26 39.34 61.45 64.34 72.85 87.27 0.025 87.37 72.52 62.53 50.25 39.66 61.49 64.06 72.45 87.10 0.050 86.92 72.50 62.28 50.17 39.85 61.42 64.00 72.62 87.35 0.200 86.88 72.48 62.27 50.13 39.56 61.37 63.98 72.51 87.27

For the SmC-SmB-K phase transition sequence on cooling, the ∆C′p and C″ p signatures for pure 9OO4 and 9OO4/CNT composite samples over a range from +4 to −10 K about TCB are shown in **Figure 7**. All peak signatures remain sharp, while the peak height of the SmC-SmB phase transition decreases with increasing ϕw. The SmC-SmB transition is marked by a strongly first-order specific heat signature at TCB = 49.38°C, with the imaginary part being much larger than the real part. The total transition enthalpy ∆HCB decreases slightly, while ∆HBK increases

no electroclinic effect would be present.

CNT weight percent *ϕw*, the *I*-*N* transition temperature *TIN*,

coefficient for low temperature range.

liquid crystal in the vicinity of the CNT surface. On application of an electric field, a surface electroclinic effect obtains, which requires the presence of a chiral symmetry environment and in which the director rotates by an angle θ ∝ E. Because of the sufficiently high concentration of CNTs, the polarized light "sees" an average rotation of the liquid crystal director for the entire sample. **Figure 5** shows the director rotation at temperature 63°C as the function applied filed.

The inverse electroclinic coefficient e<sup>c</sup> −1 is shown as a function of temperature for the 0.05 wt% 9OO4/CNT sample in **Figure 6**, approaching the SmC phase from the SmA. A three parameter (amplitude, TAC, and susceptibility exponent) power-law fit of e<sup>c</sup> −1 versus temperature resulted in γ = 0.99 ± 0.06. Despite the localization of the chirality induced in the liquid crystal to within a few nanometers of the CNT surface [58], the susceptibility exponent clearly is mean-fieldlike and apparently unaffected by the presence of the nanotubes. This result suggests that the

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**Figure 6.** The inverse electroclinic coefficient *e* − 1 *c* is shown as a function of temperature for an applied electric field at frequency 25 Hz across the cell containing a 0.05 wt% 9OO4/CNT sample. The results were fitted a 3-parameter power law and resulted in a susceptibility exponent *γ* = 0.99 ± 0.06. Inset shows an expanded view of the inverse electroclinic coefficient for low temperature range.


CNT weight percent *ϕw*, the *I*-*N* transition temperature *TIN*, the *N*-Sm*A* transition temperature *TNA*, the *K*-Sm*X* transition temperature *TKX*, the Sm*X*-Sm*A* transition temperature *TXA*, the Sm*A*-Sm*C* transition temperature *TAC*, the Sm*C*-Sm*B* transition temperature *TCB*, and the Sm*B*-*K* transition temperature *TBK* are shown. All temperatures are given in Celsius.

**Table 1.** Summary of the transition temperatures for pure 9OO4 and all 9OO4/CNT samples on cooling (C) and heating (H).

director rotation extends a length scale ξ, comparable to the SmC correlation length, into the bulk liquid crystal from the narrow chiral region immediately surrounding the CNTs, which is the region that responds directly to the applied field. As an aside, we note that although TAC was obtained for this concentration of nanotubes, it is not possible to compare it with the transition temperature in the absence of nanotubes, for which chirality would be absent, and no electroclinic effect would be present.

liquid crystal in the vicinity of the CNT surface. On application of an electric field, a surface electroclinic effect obtains, which requires the presence of a chiral symmetry environment and in which the director rotates by an angle θ ∝ E. Because of the sufficiently high concentration of CNTs, the polarized light "sees" an average rotation of the liquid crystal director for the entire sample. **Figure 5** shows the director rotation at temperature 63°C as the function applied filed.

**Figure 5.** Electroclinic response is a function of electric applied field (E). Data collected at temperature 63.2°C,

**Figure 4.** Excess real specific heat associated with the SmA-SmC phase transition as function of temperature about TAC

on cooling for different CNT content sample. The definition of the symbols is given in the inset.

approximately 1.2°C above SmA*-*SmC transition. Field was ramped over approximately 5 min.

9OO4/CNT sample in **Figure 6**, approaching the SmC phase from the SmA. A three parameter

in γ = 0.99 ± 0.06. Despite the localization of the chirality induced in the liquid crystal to within a few nanometers of the CNT surface [58], the susceptibility exponent clearly is mean-fieldlike and apparently unaffected by the presence of the nanotubes. This result suggests that the

−1 is shown as a function of temperature for the 0.05 wt%

−1 versus temperature resulted

The inverse electroclinic coefficient e<sup>c</sup>

196 Carbon Nanotubes - Recent Progress

(amplitude, TAC, and susceptibility exponent) power-law fit of e<sup>c</sup>

For the SmC-SmB-K phase transition sequence on cooling, the ∆C′p and C″ p signatures for pure 9OO4 and 9OO4/CNT composite samples over a range from +4 to −10 K about TCB are shown in **Figure 7**. All peak signatures remain sharp, while the peak height of the SmC-SmB phase transition decreases with increasing ϕw. The SmC-SmB transition is marked by a strongly first-order specific heat signature at TCB = 49.38°C, with the imaginary part being much larger than the real part. The total transition enthalpy ∆HCB decreases slightly, while ∆HBK increases


CNT weight percent *ϕw*, the *I*-*N* transition *δH*\**IN* real enthalpy, imaginary enthalpy *δH*″*IN*, the *N*-Sm*A* transition enthalpy *δHNA*, the *K*-Sm*X* total transition enthalpy ∆*HKX*, the Sm*X*-Sm*A* total transition enthalpy ∆*HXA*, the Sm*C*-Sm*B* total transition enthalpy ∆*HCB*, and the Sm*B*-*K* total transition enthalpy ∆*HBK* are shown. All enthalpies are given in joules per gram. The cumulative sum of all total transition enthalpies ∆*HT* on cooling (C) and heating (H) is also shown.

**Table 2.** Summary of the transition enthalpies for pure 9OO4 and all 9OO4/CNT samples on cooling and heating.

slightly with increasing ϕw. Similarly, the imaginary part is much larger than the nearly nonexistent ∆C′p for the SmB-K transition. This is consistent with both the SmC-SmB and SmB-K phase transitions being strongly first order, as expected. Note that the C″p behavior observed

**Figure 8.** (a) Excess real-specific heat associated with the *K*-SmX-SmC phase transition as function of temperature about *TXA* on heating. The definition of the symbols is given in the inset. (b) Imaginary specific heat as function of temperature

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For all samples, upon heating under continuous quasi-equilibrium conditions, the crystal phase superheats slightly until a strongly first-order specific heat feature is observed. Upon further heating, a second first-order feature is seen before the SmA phase appears. **Figure 8** shows pure 9OO4 and 9OO4/CNT composite samples over a range from −6 to +6 K about TXA. The K-SmX ∆Cp peak is narrower for composite samples, except for the ϕw = 0.025 wt% sample, as compared to the pure 9OO4. The melting ∆C′p peak generally decreases in amplitude with an increase in the C″p peak. The total transition enthalpy of ∆HKX = 72 J/g for pure 9OO4 decreases slightly for the 9OO4/CNT samples with increasing ϕw. The ∆HXA = 32 J/g and strongly decreases with ϕw. Interestingly, the second feature specific heat peak height decreases with increasing ϕw and suggests that this feature is not due to residual crystal melting. Because

in the SmB phase are due to a very slow phase conversion to the SmB from the SmC.

about *TXA* on heating.

**Figure 7.** (a) Excess real specific heat associated with the SmC-SmB phase transition as function of temperature about *TCB* on cooling. The definition of the symbols is given in the inset. (b) Imaginary specific heat as function of temperature about *TCB* on cooling.

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**Figure 8.** (a) Excess real-specific heat associated with the *K*-SmX-SmC phase transition as function of temperature about *TXA* on heating. The definition of the symbols is given in the inset. (b) Imaginary specific heat as function of temperature about *TXA* on heating.

slightly with increasing ϕw. Similarly, the imaginary part is much larger than the nearly nonexistent ∆C′p for the SmB-K transition. This is consistent with both the SmC-SmB and SmB-K phase transitions being strongly first order, as expected. Note that the C″p behavior observed in the SmB phase are due to a very slow phase conversion to the SmB from the SmC.

For all samples, upon heating under continuous quasi-equilibrium conditions, the crystal phase superheats slightly until a strongly first-order specific heat feature is observed. Upon further heating, a second first-order feature is seen before the SmA phase appears. **Figure 8** shows pure 9OO4 and 9OO4/CNT composite samples over a range from −6 to +6 K about TXA. The K-SmX ∆Cp peak is narrower for composite samples, except for the ϕw = 0.025 wt% sample, as compared to the pure 9OO4. The melting ∆C′p peak generally decreases in amplitude with an increase in the C″p peak. The total transition enthalpy of ∆HKX = 72 J/g for pure 9OO4 decreases slightly for the 9OO4/CNT samples with increasing ϕw. The ∆HXA = 32 J/g and strongly decreases with ϕw. Interestingly, the second feature specific heat peak height decreases with increasing ϕw and suggests that this feature is not due to residual crystal melting. Because

**Figure 7.** (a) Excess real specific heat associated with the SmC-SmB phase transition as function of temperature about *TCB* on cooling. The definition of the symbols is given in the inset. (b) Imaginary specific heat as function of temperature

**<sup>N</sup>C)** *δH***NA(C) ∆***H***CB(C) ∆***H***BK(C) ∆***H***<sup>T</sup> ∆***H***KX(H) ∆***H***KA(H) ∆***H***NA(H) ∆***H***INH) ∆***H***<sup>T</sup>**

0.000 3.9 0.95 3.1 35.9 35.9 80.2 72.0 32.0 3.0 4.9 83.4 0.008 3.8 0.99 2.6 38.9 38.9 84.1 69.2 17.1 2.9 5.1 85.7 0.010 4.3 0.96 2.6 39.7 39.7 84.4 67.4 13.6 2.9 4.8 83.9 0.025 3.2 0.69 2.6 29.1 29.1 70.1 67.8 10.5 2.9 4.2 75.0 0.050 3.4 1.01 2.8 31.2 31.2 72.4 61.9 10.3 2.8 4.4 85.3 0.200 3.6 1.03 2.7 27.4 27.4 70.0 62.0 12.7 2.9 5.0 74.6 CNT weight percent *ϕw*, the *I*-*N* transition *δH*\**IN* real enthalpy, imaginary enthalpy *δH*″*IN*, the *N*-Sm*A* transition enthalpy *δHNA*, the *K*-Sm*X* total transition enthalpy ∆*HKX*, the Sm*X*-Sm*A* total transition enthalpy ∆*HXA*, the Sm*C*-Sm*B* total transition enthalpy ∆*HCB*, and the Sm*B*-*K* total transition enthalpy ∆*HBK* are shown. All enthalpies are given in joules per

gram. The cumulative sum of all total transition enthalpies ∆*HT* on cooling (C) and heating (H) is also shown.

**Table 2.** Summary of the transition enthalpies for pure 9OO4 and all 9OO4/CNT samples on cooling and heating.

about *TCB* on cooling.

**ϕ<sup>w</sup>** *δH\****IN(C)** *δ***H″**

198 Carbon Nanotubes - Recent Progress

of the magnitude of the enthalpy involved, it is possible that this feature is a transition into an intermediate smectic phase before heating into the SmA phase. We denote this phase as SmX and are unclear from polarizing microscopy image as to whether this phase is SmB or SmClike, since the melting occurs very near TAC on cooling.

∼1.5 K in 9OO4/CNT over this range of ϕw. For the N-SmA, transition temperature in 9OO4/ CNT shifts upward by +1.31 K compared to pure 9OO4, shrinking the nematic temperature range ∆Tnem = TIN − TNA by +0.3 K for all ϕw. The SmA-SmC transition temperature in 9OO4/ CNT shifts upward by about +1.4 K, widening the SmA range by +0.4 K for all ϕw. The SmC-SmB transition temperature shifts upward by +1.01 K, widening the SmC range slightly for all ϕw studied. Finally on cooling, the SmB-K crystallization shifted upward by the largest, +4 K for all ϕw, narrowing the SmB range by 3 K. On heating, the K-SmX melting superheats an additional +0.4 K, similarly for the SmX-SmA boundary compared to pure 9OO4 for all ϕw samples. The TNA on heating is +1.2 K, and the TIN is +0.8 K higher than in pure 9OO4, nar-

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It is useful to compare these results with another high-resolution phase transition study on 8CB/CNT nanocomposites, which also used near equilibrium calorimetric conditions and identical mixing method of CNTs from the same source [31]. The 8CB is a typical rod-like molecule, with biphenyl core, to which an aliphatic tail and a polar cyano head group are attached, whereas 9OO4 has a benzoate group, linking the two phenyl rings and having alkoxy end tails (see **Figure 1**). The transition temperatures and enthalpies of the isotropic to nematic and nematic to smectic-A phase transitions in 9OO4/CNT and 8CB/CNT systems [31] are shown in **Figures 9** and **10**. In the 8CB/CNT system, both transition temperatures shift downward by ∼1.5 K with increasing ϕw, with a nonlinear ϕw dependence, while ∆Tnem remains constant as seen by the dashed and dash-dotted lines in panel (a) of **Figure 9**. Clearly, the same surface and distribution of CNTs for 8CB produce disordering effects on orientational order shifting both the I-N and N-SmA transitions. The transition enthalpies of the I-N and N-SmA phase transitions in the 8CB/CNT system increase over a broad range of ϕw and then remain constant for higher ϕw. The total transition enthalpy of 8CB/CNT system has been interpreted as the sum of the pure transition contribution and a random field induced distortion energy. Apparently, given the near constant decrease of the enthalpy of the 9OO4/CNT, the ordering is bulk like with a reduction perhaps due to suppression of long-wavelength director fluctuations. If the enthalpy suppression of the I-N and N-SmA in 9OO4/CNT was due to surface pinning, it should scale with CNT surface area, approximately linear in ϕw for these low concentrations, which is not observed. This suggests a different surface interaction for the 9OO4 than 8CB molecule with the CNT. The electroclinic effect result suggests that the director rotation θ extends a length scale ξ, comparable to the SmC correlation length, into the bulk liquid crystal forming a chiral region immediately surrounding the CNTs. This chiral region responds to the applied field, resulting in a tilt over the entire SmC correlation region. On cooling, the 9OO4/CNT SmB-K phase transition temperature increases 4 K, and K-SmX phase transition temperature increases 0.5 K on heating for all ϕw, while the total transition enthalpy remains constant on heating and cooling (see **Figures 9** and **10**). It is important to note that all experimental results

rowing the nematic range ∆Tnem by 0.4 K, similar to that seen on cooling.

presented here are reproducible over repeated scans.

**4. Discussion**

While the calorimetric and electroclinic behavior of the phases and phase transitions for the 9OO4/CNT samples are clearly retaining the character found in pure 9OO4, the phase boundaries have more shrinking changes due to the CNT. The transition temperature changes from pure 9OO4 for all 9OO4/CNT samples are shown in **Figure 9**. On cooling, the I-N transition temperature in the 9OO4/CNT samples shift upward by nearly a constant +1.18 K compared to that in pure 9OO4, while the two-phase I + N coexistence width shrink with a constant

**Figure 9.** (a) The phase transition temperature shifts for the *I*-*N* (▪), *N*-SmA (▫), SmA-SmC (•), and SmC-SmB (°) in 9OO4/CNT samples and the *I*-*N* (−). (b) *N*-SmA (− − −) transition temperature shift for 8CB/CNT as a function of *ϕw*. (c) The phase transition temperature shifts of the SmB-*K* (∆), K-SmX (▫), and SmX-SmA (°) for the 9OO4/CNT samples as a function of *ϕw*.

∼1.5 K in 9OO4/CNT over this range of ϕw. For the N-SmA, transition temperature in 9OO4/ CNT shifts upward by +1.31 K compared to pure 9OO4, shrinking the nematic temperature range ∆Tnem = TIN − TNA by +0.3 K for all ϕw. The SmA-SmC transition temperature in 9OO4/ CNT shifts upward by about +1.4 K, widening the SmA range by +0.4 K for all ϕw. The SmC-SmB transition temperature shifts upward by +1.01 K, widening the SmC range slightly for all ϕw studied. Finally on cooling, the SmB-K crystallization shifted upward by the largest, +4 K for all ϕw, narrowing the SmB range by 3 K. On heating, the K-SmX melting superheats an additional +0.4 K, similarly for the SmX-SmA boundary compared to pure 9OO4 for all ϕw samples. The TNA on heating is +1.2 K, and the TIN is +0.8 K higher than in pure 9OO4, narrowing the nematic range ∆Tnem by 0.4 K, similar to that seen on cooling.

#### **4. Discussion**

of the magnitude of the enthalpy involved, it is possible that this feature is a transition into an intermediate smectic phase before heating into the SmA phase. We denote this phase as SmX and are unclear from polarizing microscopy image as to whether this phase is SmB or SmC-

While the calorimetric and electroclinic behavior of the phases and phase transitions for the 9OO4/CNT samples are clearly retaining the character found in pure 9OO4, the phase boundaries have more shrinking changes due to the CNT. The transition temperature changes from pure 9OO4 for all 9OO4/CNT samples are shown in **Figure 9**. On cooling, the I-N transition temperature in the 9OO4/CNT samples shift upward by nearly a constant +1.18 K compared to that in pure 9OO4, while the two-phase I + N coexistence width shrink with a constant

**Figure 9.** (a) The phase transition temperature shifts for the *I*-*N* (▪), *N*-SmA (▫), SmA-SmC (•), and SmC-SmB (°) in 9OO4/CNT samples and the *I*-*N* (−). (b) *N*-SmA (− − −) transition temperature shift for 8CB/CNT as a function of *ϕw*. (c) The phase transition temperature shifts of the SmB-*K* (∆), K-SmX (▫), and SmX-SmA (°) for the 9OO4/CNT samples as a

function of *ϕw*.

like, since the melting occurs very near TAC on cooling.

200 Carbon Nanotubes - Recent Progress

It is useful to compare these results with another high-resolution phase transition study on 8CB/CNT nanocomposites, which also used near equilibrium calorimetric conditions and identical mixing method of CNTs from the same source [31]. The 8CB is a typical rod-like molecule, with biphenyl core, to which an aliphatic tail and a polar cyano head group are attached, whereas 9OO4 has a benzoate group, linking the two phenyl rings and having alkoxy end tails (see **Figure 1**). The transition temperatures and enthalpies of the isotropic to nematic and nematic to smectic-A phase transitions in 9OO4/CNT and 8CB/CNT systems [31] are shown in **Figures 9** and **10**. In the 8CB/CNT system, both transition temperatures shift downward by ∼1.5 K with increasing ϕw, with a nonlinear ϕw dependence, while ∆Tnem remains constant as seen by the dashed and dash-dotted lines in panel (a) of **Figure 9**. Clearly, the same surface and distribution of CNTs for 8CB produce disordering effects on orientational order shifting both the I-N and N-SmA transitions. The transition enthalpies of the I-N and N-SmA phase transitions in the 8CB/CNT system increase over a broad range of ϕw and then remain constant for higher ϕw. The total transition enthalpy of 8CB/CNT system has been interpreted as the sum of the pure transition contribution and a random field induced distortion energy. Apparently, given the near constant decrease of the enthalpy of the 9OO4/CNT, the ordering is bulk like with a reduction perhaps due to suppression of long-wavelength director fluctuations. If the enthalpy suppression of the I-N and N-SmA in 9OO4/CNT was due to surface pinning, it should scale with CNT surface area, approximately linear in ϕw for these low concentrations, which is not observed. This suggests a different surface interaction for the 9OO4 than 8CB molecule with the CNT. The electroclinic effect result suggests that the director rotation θ extends a length scale ξ, comparable to the SmC correlation length, into the bulk liquid crystal forming a chiral region immediately surrounding the CNTs. This chiral region responds to the applied field, resulting in a tilt over the entire SmC correlation region. On cooling, the 9OO4/CNT SmB-K phase transition temperature increases 4 K, and K-SmX phase transition temperature increases 0.5 K on heating for all ϕw, while the total transition enthalpy remains constant on heating and cooling (see **Figures 9** and **10**). It is important to note that all experimental results presented here are reproducible over repeated scans.

of the local anchoring field. For 8CB/CNT system, the nucleated local director for 8CB varies in orientation along the long-axis of the CNT creating a surface paranematic layer while further away from the CNT surface, ˆn becomes aligned parallel to the CNT strands [31]. This would mean that the local anchoring field direction varies along the CNT strands over a distance comparable to or shorter than the ˆn correlation length, leading to quenched random-field disordering effects [33]. For 9OO4/CNT system, the surface anchoring orientation correlation length must be larger than the ˆn correlation length of the LC, likely parallel to the CNT long axis with essentially no distortion (elastic strain) of ˆn further away from the strand surface. This would yield a quenched locally nonrandom field that spans more than one nematic domain, while still possessing a global (macroscopic) random distribution. This is consistent with the observed ECE in 9OO4/CNT and not in 8CB/CNT [46, 47], which would also account for the suppression of smectic order in the 8CB/CNT system as the pinned surface para-nematic layer would further disorder any positional ordering. In 9OO4/CNT, the local field spans multiple nematic domains

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and would actually suppress director fluctuations and so promote smectic ordering.

onto the graphene surface by these two LCs.

**5. Conclusions**

However, the observed enhancement of the higher order smectic phases as well as the crystal phase implies that the uniform surface orientational anchoring is not accompanied by positional pinning along the CNT for the 9OO4 molecule. This degree of freedom of the LC molecule to essentially slide along the surface and each other parallel to the CNT long-axis is needed to accommodate all the higher positional ordering (broken symmetries) in pure 9OO4, yielding the observed bulk-like behavior and phase boundary enhancement in this work. The origin of these two different behaviors between 9OO4 and 8CB with CNT is not immediately known but may be due to either the negative dielectric anisotropy of 9OO4 as opposed to that in 8CB or the different commensurate surface packing of the phenyl rings

We have presented a detailed calorimetric study on the effect of carbon nanotubes on phase transitions of the 9OO4/CNT nanocomposites as a function of CNT concentration. The complex specific heat was measured over a wide range of temperature for negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4)/carbon nanotube (CNT) composites as a function of CNT concentration. The thermal scans were performed between 25 and 95°C, first cooling followed by heating scans, as the function of CNT concentration ranging from 0 to 0.2 wt%. All mesophases have transition temperatures 1 K higher and a crystallization temperature 4 K higher than that of the pure 9OO4. The crystal phase superheats until a strongly first-order specific heat feature is observed, indicating melting 0.5 K higher than in the pure 9OO4. The enthalpy change associated with I-N and N-SmA phase transitions is only slightly changed with increasing ϕw but are generally lower than pure 9OO4. The total transition enthalpy associated with the all transitions is independent on the CNT concentration and thermal treatment. The bulk or pure-like behavior of the phase transitions is supported by the bulk-like 9OO4 ECE behavior of the SmA-SmC for the 0.05 wt% sample. We interpret that these results as arising from the LC-CNT surface interaction breaking orientational symmetry uniformly over a distance along the CNT greater than the nematic correlation length but allowing the LC to slide essentially freely on the CNT surface to accommodate various

**Figure 10.** (a) The *I*-*N* average fractional effective transition enthalpy for 9OO4/CNT (°) and 8CB/CNT (− − −) as a function of *ϕw*. (b) The *N*SmA average fractional effective transition enthalpy for 9OO4 (▫) and 8CB (− − −) nanocomposites as a function of *ϕw*. (c) The effective total transition enthalpy on heating (°) and cooling (•) for the 9OO4/CNT samples as the function of *ϕw*.

From the calorimetric observations presented here, the effect of CNTs on the phase transitions of 9OO4 apparently enhances orientational order and is compatible with various positional orders of all the higher order phases (broken symmetries) up to and including the crystal phase. This is despite the globally random dispersion of CNTs in these nanocomposites and in contrast to the disordering effects observed in the 8CB/CNT system [31]. Clearly, at such low concentrations with random CNT distribution, the specific CNT- LC boundary condition must play an important role along with the ultra-high aspect ratio (string-like structure) of the CNT nanoparticles.

To account for the opposing the behavior of the I-N transition between 9OO4/CNT and 8CB/ CNT given the similar chemical nature of both liquid crystals and common graphene surface, the CNT may promote nematic order in both liquid crystals but with different correlation lengths of the local anchoring field. For 8CB/CNT system, the nucleated local director for 8CB varies in orientation along the long-axis of the CNT creating a surface paranematic layer while further away from the CNT surface, ˆn becomes aligned parallel to the CNT strands [31]. This would mean that the local anchoring field direction varies along the CNT strands over a distance comparable to or shorter than the ˆn correlation length, leading to quenched random-field disordering effects [33]. For 9OO4/CNT system, the surface anchoring orientation correlation length must be larger than the ˆn correlation length of the LC, likely parallel to the CNT long axis with essentially no distortion (elastic strain) of ˆn further away from the strand surface. This would yield a quenched locally nonrandom field that spans more than one nematic domain, while still possessing a global (macroscopic) random distribution. This is consistent with the observed ECE in 9OO4/CNT and not in 8CB/CNT [46, 47], which would also account for the suppression of smectic order in the 8CB/CNT system as the pinned surface para-nematic layer would further disorder any positional ordering. In 9OO4/CNT, the local field spans multiple nematic domains and would actually suppress director fluctuations and so promote smectic ordering.

However, the observed enhancement of the higher order smectic phases as well as the crystal phase implies that the uniform surface orientational anchoring is not accompanied by positional pinning along the CNT for the 9OO4 molecule. This degree of freedom of the LC molecule to essentially slide along the surface and each other parallel to the CNT long-axis is needed to accommodate all the higher positional ordering (broken symmetries) in pure 9OO4, yielding the observed bulk-like behavior and phase boundary enhancement in this work. The origin of these two different behaviors between 9OO4 and 8CB with CNT is not immediately known but may be due to either the negative dielectric anisotropy of 9OO4 as opposed to that in 8CB or the different commensurate surface packing of the phenyl rings onto the graphene surface by these two LCs.

#### **5. Conclusions**

From the calorimetric observations presented here, the effect of CNTs on the phase transitions of 9OO4 apparently enhances orientational order and is compatible with various positional orders of all the higher order phases (broken symmetries) up to and including the crystal phase. This is despite the globally random dispersion of CNTs in these nanocomposites and in contrast to the disordering effects observed in the 8CB/CNT system [31]. Clearly, at such low concentrations with random CNT distribution, the specific CNT- LC boundary condition must play an important role along with the ultra-high aspect ratio (string-like structure) of

**Figure 10.** (a) The *I*-*N* average fractional effective transition enthalpy for 9OO4/CNT (°) and 8CB/CNT (− − −) as a function of *ϕw*. (b) The *N*SmA average fractional effective transition enthalpy for 9OO4 (▫) and 8CB (− − −) nanocomposites as a function of *ϕw*. (c) The effective total transition enthalpy on heating (°) and cooling (•) for the 9OO4/CNT samples as

To account for the opposing the behavior of the I-N transition between 9OO4/CNT and 8CB/ CNT given the similar chemical nature of both liquid crystals and common graphene surface, the CNT may promote nematic order in both liquid crystals but with different correlation lengths

the CNT nanoparticles.

the function of *ϕw*.

202 Carbon Nanotubes - Recent Progress

We have presented a detailed calorimetric study on the effect of carbon nanotubes on phase transitions of the 9OO4/CNT nanocomposites as a function of CNT concentration. The complex specific heat was measured over a wide range of temperature for negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4)/carbon nanotube (CNT) composites as a function of CNT concentration. The thermal scans were performed between 25 and 95°C, first cooling followed by heating scans, as the function of CNT concentration ranging from 0 to 0.2 wt%. All mesophases have transition temperatures 1 K higher and a crystallization temperature 4 K higher than that of the pure 9OO4. The crystal phase superheats until a strongly first-order specific heat feature is observed, indicating melting 0.5 K higher than in the pure 9OO4. The enthalpy change associated with I-N and N-SmA phase transitions is only slightly changed with increasing ϕw but are generally lower than pure 9OO4. The total transition enthalpy associated with the all transitions is independent on the CNT concentration and thermal treatment. The bulk or pure-like behavior of the phase transitions is supported by the bulk-like 9OO4 ECE behavior of the SmA-SmC for the 0.05 wt% sample. We interpret that these results as arising from the LC-CNT surface interaction breaking orientational symmetry uniformly over a distance along the CNT greater than the nematic correlation length but allowing the LC to slide essentially freely on the CNT surface to accommodate various translational symmetries, leading to a net ordering effect for all transitions. These results suggest that the interactions between molecular structure, dipole moment of liquid crystal, and graphene-like surface can allow a random dispersion of CNT to promote both orientational and positional order depending on the length scale of the local order and the degree of surface freedom. The observed effects are incorporating CNTs with LC likely due to elastic coupling between CNT and LC and this leads to change in the elastic properties of composites. Continued experimental efforts probing the homogeneity of the sample, frequencydependent dynamics, smectic structures via X-ray scattering, and elastic behavior via light scattering of the homogeneous sample as a function of CNT concentration and temperature would be particularly important and interesting.

[8] Bhattacharyya AR, Sreekumar TV, Liu T, Kumar S, Ericson LM, Hauge RH, Smalley RE.

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#### **Acknowledgements**

The authors are grateful to Robert P. Lemieux for providing the 9OO4 liquid crystal. This work was supported at WPI by the Department of Physics and a grant from the NSF award DMR-0821292(MRI). At CWRU, this work was supported by the National Science Foundation's Condensed Matter Physics and Solid State and Materials Chemistry Programs under grant DMR-1065491.

#### **Author details**

Parvathalu Kalakonda\* and Germano S Iannacchione

\*Address all correspondence to: parvathalu.k@gmail.com

Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts, USA

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The authors are grateful to Robert P. Lemieux for providing the 9OO4 liquid crystal. This work was supported at WPI by the Department of Physics and a grant from the NSF award DMR-0821292(MRI). At CWRU, this work was supported by the National Science Foundation's Condensed Matter Physics and Solid State and Materials Chemistry Programs under grant

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**Chapter 11**

**Provisional chapter**

**Applications of Carbon Nanotubes to Flexible**

**Applications of Carbon Nanotubes to Flexible** 

DOI: 10.5772/intechopen.72002

Transparent conductive electrodes (TCEs) have attracted great interest because of their wide range of applications in solar cells, liquid crystal displays (LCDs), organic lightemitting diodes (OLEDs), and touch screen panels (TSPs). Indium-tin-oxide (ITO) thin films as TCEs possess exceptional optoelectronic properties, but they have several disadvantages such as a brittle nature due to their low fracture strain and lack of flexibility, a high processing temperature that damages the flexible substrates, low adhesion to polymeric materials, and relative rarity on Earth, which makes their price unstable. This has motivated several research studies of late for developing alternative materials to replace ITO such as metal meshes, metal nanowires, conductive polymers, graphene, and carbon nanotubes (CNTs). Out of the abovementioned candidates, CNTs have advantages in chemical stability, thermal conductivity, mechanical strength, and flexibility. However, there are still several problems yet to be solved for achieving CNT-based flexible TCEs with excellent characteristics and high stability. In this chapter, the properties of CNTs and their applications especially for flexible TCEs are presented, including the preparation details of CNTs based on solution processes, the surface modification of flexible

substrates, and the various types of hybrid TCEs based on CNTs.

**Keywords:** carbon nanotubes, flexible transparent conductive electrodes, solution processes, hybrid-type electrodes, PEDOT:PSS, metal meshes, transmittance,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Transparent conductive electrodes (TCEs) are thin films of optically transparent and electrically conductive materials. Indium tin oxide (ITO) has been the most widely used TCEs in rigid electronics due to its exceptional electrical and optical properties. Recently, flexible

**Transparent Conductive Electrodes**

**Transparent Conductive Electrodes**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Bu-Jong Kim and Jin-Seok Park

Bu-Jong Kim and Jin-Seok Park

http://dx.doi.org/10.5772/intechopen.72002

reflectance, color properties

**1. Introduction**

**Abstract**

**Provisional chapter**

#### **Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes Transparent Conductive Electrodes**

**Applications of Carbon Nanotubes to Flexible** 

DOI: 10.5772/intechopen.72002

Bu-Jong Kim and Jin-Seok Park Bu-Jong Kim and Jin-Seok Park Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72002

#### **Abstract**

Transparent conductive electrodes (TCEs) have attracted great interest because of their wide range of applications in solar cells, liquid crystal displays (LCDs), organic lightemitting diodes (OLEDs), and touch screen panels (TSPs). Indium-tin-oxide (ITO) thin films as TCEs possess exceptional optoelectronic properties, but they have several disadvantages such as a brittle nature due to their low fracture strain and lack of flexibility, a high processing temperature that damages the flexible substrates, low adhesion to polymeric materials, and relative rarity on Earth, which makes their price unstable. This has motivated several research studies of late for developing alternative materials to replace ITO such as metal meshes, metal nanowires, conductive polymers, graphene, and carbon nanotubes (CNTs). Out of the abovementioned candidates, CNTs have advantages in chemical stability, thermal conductivity, mechanical strength, and flexibility. However, there are still several problems yet to be solved for achieving CNT-based flexible TCEs with excellent characteristics and high stability. In this chapter, the properties of CNTs and their applications especially for flexible TCEs are presented, including the preparation details of CNTs based on solution processes, the surface modification of flexible substrates, and the various types of hybrid TCEs based on CNTs.

**Keywords:** carbon nanotubes, flexible transparent conductive electrodes, solution processes, hybrid-type electrodes, PEDOT:PSS, metal meshes, transmittance, reflectance, color properties

#### **1. Introduction**

Transparent conductive electrodes (TCEs) are thin films of optically transparent and electrically conductive materials. Indium tin oxide (ITO) has been the most widely used TCEs in rigid electronics due to its exceptional electrical and optical properties. Recently, flexible

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

TCEs fabricated by solution processes, which have some advantages such as simple and continuous process and relatively low costs, have attracted enormous interest because of their wide range of applications in flexible devices such as displays, touch screen panels (TSPs), sensors, film heaters that can attach to the skin, fabrics, and papers [1, 2]. However, ITO has disadvantages in the application fields where flexibility is ensured due to its brittle nature because of its low fracture strain and absence of flexibility, a high processing temperature that damages the flexible substrates, low adhesion to polymeric materials, and their relative rarity on Earth, which makes their price unstable [3]. Therefore, this has recently motivated various researches to discover alternative materials to replace ITO films, which include metal meshes, silver nanowires, conductive polymers, graphene, and carbon nanotubes (CNTs) [4, 5]. Also, for the flexible applications, the TCEs are required to be deposited on polymer substrates such as polyethylene terephthalate (PET), polypropylene (PP), and polydimethylsiloxane (PDMS).

and posttreatment of CNTs. Then, the surface modification of flexible substrates for enhancing the adhesion between CNTs and flexible substrates, which can eventually improve the flexibility of CNT-based TCEs, is discussed. Also, the effects of PEDOT:PSS coating on the electrical characteristics and color properties (e.g., redness, yellowness) of CNT-based TCEs are described, where the various methods for PEDOT:PSS coating, such as spin coating and electrophoretic deposition (EPD), are introduced and compared. Finally, the hybrid-type TCEs, which are fabricated by coating metal meshes with CNTs, are presented along with

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211

**Figure 1(a)** shows the procedures for production of CNT suspension, including purification and dispersion processes. The synthesized CNTs necessarily contain metal catalyst and carbonaceous substances, including amorphous carbon, fullerenes, and graphite particles [15]. Therefore, to obtain high purity CNTs, the purification processes have to be conducted prior to the deposition of CNTs. Here, the chemical method induces the oxygenated functional group on CNTs, which causes damage to the surface of CNTs and eventually leads to an increase in the sheet resistance of CNTs. This indicates that it is important to determine the

After purification, the CNTs have to be properly dispersed since the van der Waals attraction forces between CNTs and the poor solubility of CNTs result in the formation of bundles and agglomerates [16]. There are several approaches for the dispersing of CNTs such as chemical modification (i.e., covalent approach), mechanical treatment, using of surfactants (i.e., noncovalent approach), and so on [17]. The chemical modification improves the solubility of CNTs

**Figure 1.** (a) The procedures for production of CNT suspension, including purification and dispersion processes and

their characteristics as flexible TCEs.

**2.1. Purification and dispersion of CNT suspension**

(b) the zeta potential measured from the prepared CNT suspension.

appropriate purification time when the chemical method is used.

**2. Preparation of CNTs**

Metal meshes and nanowires have excellent electrical properties, but they also have some problems such as visibility due to the reflectivity of metals and reliability for oxidation of metals [6]. Additionally, for metal nanowires, high contact resistances between individual nanowires have critical issues [7]. The conductive polymers, graphene, and CNTs have great advantages such as high mechanical flexibility and low processing costs because they can utilize solution processes [8]. Conductive polymers, however, are generally known to be not good enough at stability in an ambient atmosphere. Also, the poly(3, 4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which may be the most widely used conductive polymers, is tinged with the color blue, which may cause the problem of patterns of electrodes being revealed in TSPs [9]. Graphene has also been reported to be synthesized using solution processes such as the chemical reduction of exfoliated graphite oxide [10]. However, a thermal treatment at high temperature is needed for achieving high transparency and conductivity of graphene, which may restrict the use of flexible substrates.

CNTs, which have potential in chemical stability, thermal and electrical conductivity, mechanical strength, and flexibility, may be the best alternative material for applications in flexible TCEs. Actually, CNTs in some applications, which have low conductivity regardless of transmittance, are being commercialized. The rough surface of CNT films due to their tubular structures brings about a serious problem in the application of organic light-emitting diodes (OLEDs) [11]. In CNTs, furthermore, a relatively high contact resistance of the tube-tube junction may lead to insufficient sheet resistance [12]. Regarding this issue, the separation method of metallic and semiconducting components in CNTs by controlling the diameter and chirality of CNTs has been introduced in the literature [13]. The perfect separation technology, however, has not been realized yet as such a process is very complex. In addition, the chemical doping technology that lowers the sheet resistance has been introduced [14], but effectively controlling the defects in the CNTs is still difficult. As another method of improving the electric characteristics of CNTs, studies on the manufacturing of hybrid-type transparent electrodes have been attempted by several research groups.

In this chapter, the properties of CNTs and their applications especially for flexible TCEs are presented. First, the preparation details of CNTs based on solution processes are introduced, including purification and dispersion of CNT suspension, spray coating of CNTs, and posttreatment of CNTs. Then, the surface modification of flexible substrates for enhancing the adhesion between CNTs and flexible substrates, which can eventually improve the flexibility of CNT-based TCEs, is discussed. Also, the effects of PEDOT:PSS coating on the electrical characteristics and color properties (e.g., redness, yellowness) of CNT-based TCEs are described, where the various methods for PEDOT:PSS coating, such as spin coating and electrophoretic deposition (EPD), are introduced and compared. Finally, the hybrid-type TCEs, which are fabricated by coating metal meshes with CNTs, are presented along with their characteristics as flexible TCEs.

#### **2. Preparation of CNTs**

TCEs fabricated by solution processes, which have some advantages such as simple and continuous process and relatively low costs, have attracted enormous interest because of their wide range of applications in flexible devices such as displays, touch screen panels (TSPs), sensors, film heaters that can attach to the skin, fabrics, and papers [1, 2]. However, ITO has disadvantages in the application fields where flexibility is ensured due to its brittle nature because of its low fracture strain and absence of flexibility, a high processing temperature that damages the flexible substrates, low adhesion to polymeric materials, and their relative rarity on Earth, which makes their price unstable [3]. Therefore, this has recently motivated various researches to discover alternative materials to replace ITO films, which include metal meshes, silver nanowires, conductive polymers, graphene, and carbon nanotubes (CNTs) [4, 5]. Also, for the flexible applications, the TCEs are required to be deposited on polymer substrates such as polyethylene terephthalate (PET), polypropylene (PP), and polydimethylsiloxane (PDMS). Metal meshes and nanowires have excellent electrical properties, but they also have some problems such as visibility due to the reflectivity of metals and reliability for oxidation of metals [6]. Additionally, for metal nanowires, high contact resistances between individual nanowires have critical issues [7]. The conductive polymers, graphene, and CNTs have great advantages such as high mechanical flexibility and low processing costs because they can utilize solution processes [8]. Conductive polymers, however, are generally known to be not good enough at stability in an ambient atmosphere. Also, the poly(3, 4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which may be the most widely used conductive polymers, is tinged with the color blue, which may cause the problem of patterns of electrodes being revealed in TSPs [9]. Graphene has also been reported to be synthesized using solution processes such as the chemical reduction of exfoliated graphite oxide [10]. However, a thermal treatment at high temperature is needed for achieving high transparency and conductiv-

210 Carbon Nanotubes - Recent Progress

ity of graphene, which may restrict the use of flexible substrates.

electrodes have been attempted by several research groups.

CNTs, which have potential in chemical stability, thermal and electrical conductivity, mechanical strength, and flexibility, may be the best alternative material for applications in flexible TCEs. Actually, CNTs in some applications, which have low conductivity regardless of transmittance, are being commercialized. The rough surface of CNT films due to their tubular structures brings about a serious problem in the application of organic light-emitting diodes (OLEDs) [11]. In CNTs, furthermore, a relatively high contact resistance of the tube-tube junction may lead to insufficient sheet resistance [12]. Regarding this issue, the separation method of metallic and semiconducting components in CNTs by controlling the diameter and chirality of CNTs has been introduced in the literature [13]. The perfect separation technology, however, has not been realized yet as such a process is very complex. In addition, the chemical doping technology that lowers the sheet resistance has been introduced [14], but effectively controlling the defects in the CNTs is still difficult. As another method of improving the electric characteristics of CNTs, studies on the manufacturing of hybrid-type transparent

In this chapter, the properties of CNTs and their applications especially for flexible TCEs are presented. First, the preparation details of CNTs based on solution processes are introduced, including purification and dispersion of CNT suspension, spray coating of CNTs,

#### **2.1. Purification and dispersion of CNT suspension**

**Figure 1(a)** shows the procedures for production of CNT suspension, including purification and dispersion processes. The synthesized CNTs necessarily contain metal catalyst and carbonaceous substances, including amorphous carbon, fullerenes, and graphite particles [15]. Therefore, to obtain high purity CNTs, the purification processes have to be conducted prior to the deposition of CNTs. Here, the chemical method induces the oxygenated functional group on CNTs, which causes damage to the surface of CNTs and eventually leads to an increase in the sheet resistance of CNTs. This indicates that it is important to determine the appropriate purification time when the chemical method is used.

After purification, the CNTs have to be properly dispersed since the van der Waals attraction forces between CNTs and the poor solubility of CNTs result in the formation of bundles and agglomerates [16]. There are several approaches for the dispersing of CNTs such as chemical modification (i.e., covalent approach), mechanical treatment, using of surfactants (i.e., noncovalent approach), and so on [17]. The chemical modification improves the solubility of CNTs

**Figure 1.** (a) The procedures for production of CNT suspension, including purification and dispersion processes and (b) the zeta potential measured from the prepared CNT suspension.

in solvent by attaching the various moieties and functional groups. However, the chemical method may also affect the electrical properties of CNTs. For the mechanical treatment, the applied high power can effectively separate CNTs, but it can also cause shortening of the CNTs.

In contrast to the chemical and mechanical approaches, the use of surfactants is based on the adsorption of chemical moieties onto the CNTs' surface without damaging the inherent properties of CNTs. Here, the dispersion method for achieving the stable CNT suspension using surfactant is introduced. The purified CNTs are filtered out using the vacuum filtering system, and the dispersion process is carried out for about 30 min in the ultrasonic generator after adding the anionic surfactant of sodium dodecyl sulfate (SDS, NaC12H25SO<sup>4</sup> , 40 mg) together with deionized water (50 ml). Finally, only the dispersed supernatant is extracted by using the centrifugal separator with the speed of 4000 rpm for 30 min. The actual photo of the CNT suspension produced in this manner is also shown in **Figure 1(a)**. To confirm that the CNT suspension is well dispersed, the zeta potential of the CNT suspension is measured. Generally, the stable state of CNT suspension has a zeta potential of ±40 mV or higher. **Figure 1(b)** shows the zeta potential measured with the CNT suspension of **Figure 1(a)** and it was about 47.7 mV, indicating that the CNT suspension was well dispersed.

#### **2.2. Solution-based deposition of CNTs**

Recently, solution-based processes have been increasingly adopted in the deposition of thin films since they have several advantages over the vacuum deposition system such as low fabrication cost, simple process, and massive scalability. To deposit CNTs on the flexible substrates, various methods have been developed, including spray coating, dip coating, bar coating, and inkjet printing [18, 19]. Among them, the spray coating method has the advantage that the thickness of deposited CNTs can be controlled precisely. Here, the CNT-TCEs, fabricated by depositing the CNTs on the polyethylene terephthalate (PET) substrates via spray coating, are presented. The process conditions are controlled by varying the spraying time from 10 to 80 s and by fixing the other process variables such as the injection pressure of 0.2 MPa, the injection amount of 2 ml/min, and the hotplate temperature of 100°C. **Figure 2(a)** indicates the schematic of the spray coating system used in this study and **Figure 2(b)** shows the photo of the surface morphology of CNTs, deposited on the PET substrate via spray coating, obtained using the field-emission scanning electron microscope (FE-SEM).

by using the energy dispersive X-ray spectroscopy (EDXS). The amount of sulfur (S) ele-

**Figure 2.** (a) The schematic of the spray coating system and (b) the FE-SEM image indicating the surface morphology of

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213

removal of the surfactant. Meanwhile, the transmittance of the CNTs slightly increased after

For depositing CNTs on flexible substrates, various solution-based methods have been utilized. However, these methods have common problems regarding the weak adhesion between the flexible substrates and the CNTs. Generally, the most widely used method is surface modification of substrate to improve the adhesion by using plasma treatment [21, 22]. This method

posttreatment, as also shown in **Figure 3(a)**. This was because some of the CNTs

posttreatment, which was resulted from the

posttreatment and (b) the EDS

posttreatment.

posttreatment.

ment in the CNTs was decreased after the HNO<sup>3</sup>

CNTs deposited on the PET substrate via spray coating.

together with the surfactant were peeled off during the HNO<sup>3</sup>

**Figure 3.** (a) The sheet resistances and transmittances of CNTs before and after HNO<sup>3</sup>

results indicating the change of the chemical components incorporated in the CNTs due to the HNO<sup>3</sup>

**3. CNT-TCEs fabricated on corona-treated flexible substrates**

the HNO<sup>3</sup>

#### **2.3. Posttreatment of CNTs**

After the deposition of CNTs, the surfactant (e.g., SDS) may still remain in the CNTs and serves as an insulating layer, which may disturb the effective electrical contact of the CNTs. Therefore, the posttreatment for removing the remaining surfactant has to be conducted. Several posttreatment methods have been adopted to remove surfactants [20]. Here, the posttreatment of CNTs using nitric acid (HNO<sup>3</sup> ) is introduced. **Figure 3(a)** compares the sheet resistances and transmittances of CNTs before and after the HNO<sup>3</sup> posttreatment. After the HNO<sup>3</sup> posttreatment, the sheet resistance of CNTs was decreased. This was because the surfactant was removed. **Figure 3(b)** shows the change in the relative percentage of the chemical components incorporated in the CNTs due to the HNO<sup>3</sup> posttreatment, which was estimated

in solvent by attaching the various moieties and functional groups. However, the chemical method may also affect the electrical properties of CNTs. For the mechanical treatment, the applied high power can effectively separate CNTs, but it can also cause shortening of the CNTs. In contrast to the chemical and mechanical approaches, the use of surfactants is based on the adsorption of chemical moieties onto the CNTs' surface without damaging the inherent properties of CNTs. Here, the dispersion method for achieving the stable CNT suspension using surfactant is introduced. The purified CNTs are filtered out using the vacuum filtering system, and the dispersion process is carried out for about 30 min in the ultrasonic generator after

with deionized water (50 ml). Finally, only the dispersed supernatant is extracted by using the centrifugal separator with the speed of 4000 rpm for 30 min. The actual photo of the CNT suspension produced in this manner is also shown in **Figure 1(a)**. To confirm that the CNT suspension is well dispersed, the zeta potential of the CNT suspension is measured. Generally, the stable state of CNT suspension has a zeta potential of ±40 mV or higher. **Figure 1(b)** shows the zeta potential measured with the CNT suspension of **Figure 1(a)** and it was about 47.7 mV,

Recently, solution-based processes have been increasingly adopted in the deposition of thin films since they have several advantages over the vacuum deposition system such as low fabrication cost, simple process, and massive scalability. To deposit CNTs on the flexible substrates, various methods have been developed, including spray coating, dip coating, bar coating, and inkjet printing [18, 19]. Among them, the spray coating method has the advantage that the thickness of deposited CNTs can be controlled precisely. Here, the CNT-TCEs, fabricated by depositing the CNTs on the polyethylene terephthalate (PET) substrates via spray coating, are presented. The process conditions are controlled by varying the spraying time from 10 to 80 s and by fixing the other process variables such as the injection pressure of 0.2 MPa, the injection amount of 2 ml/min, and the hotplate temperature of 100°C. **Figure 2(a)** indicates the schematic of the spray coating system used in this study and **Figure 2(b)** shows the photo of the surface morphology of CNTs, deposited on the PET substrate via spray coat-

ing, obtained using the field-emission scanning electron microscope (FE-SEM).

resistances and transmittances of CNTs before and after the HNO<sup>3</sup>

components incorporated in the CNTs due to the HNO<sup>3</sup>

After the deposition of CNTs, the surfactant (e.g., SDS) may still remain in the CNTs and serves as an insulating layer, which may disturb the effective electrical contact of the CNTs. Therefore, the posttreatment for removing the remaining surfactant has to be conducted. Several posttreatment methods have been adopted to remove surfactants [20]. Here, the post-

 posttreatment, the sheet resistance of CNTs was decreased. This was because the surfactant was removed. **Figure 3(b)** shows the change in the relative percentage of the chemical

) is introduced. **Figure 3(a)** compares the sheet

posttreatment. After the

posttreatment, which was estimated

, 40 mg) together

adding the anionic surfactant of sodium dodecyl sulfate (SDS, NaC12H25SO<sup>4</sup>

indicating that the CNT suspension was well dispersed.

**2.2. Solution-based deposition of CNTs**

212 Carbon Nanotubes - Recent Progress

**2.3. Posttreatment of CNTs**

HNO<sup>3</sup>

treatment of CNTs using nitric acid (HNO<sup>3</sup>

**Figure 2.** (a) The schematic of the spray coating system and (b) the FE-SEM image indicating the surface morphology of CNTs deposited on the PET substrate via spray coating.

**Figure 3.** (a) The sheet resistances and transmittances of CNTs before and after HNO<sup>3</sup> posttreatment and (b) the EDS results indicating the change of the chemical components incorporated in the CNTs due to the HNO<sup>3</sup> posttreatment.

by using the energy dispersive X-ray spectroscopy (EDXS). The amount of sulfur (S) element in the CNTs was decreased after the HNO<sup>3</sup> posttreatment, which was resulted from the removal of the surfactant. Meanwhile, the transmittance of the CNTs slightly increased after the HNO<sup>3</sup> posttreatment, as also shown in **Figure 3(a)**. This was because some of the CNTs together with the surfactant were peeled off during the HNO<sup>3</sup> posttreatment.

#### **3. CNT-TCEs fabricated on corona-treated flexible substrates**

For depositing CNTs on flexible substrates, various solution-based methods have been utilized. However, these methods have common problems regarding the weak adhesion between the flexible substrates and the CNTs. Generally, the most widely used method is surface modification of substrate to improve the adhesion by using plasma treatment [21, 22]. This method is a chemical modification that can be formed on polar groups on the surface of substrate before film deposition and also includes the physical changes, which can increase the surface area by generating nanoscratches. However, most of these methods have several problems such as low working pressures, long treatment times, and high processing temperatures. Therefore, corona pretreatment has also been suggested because it can be carried out at atmospheric pressure and room temperature. During the corona treatment, corona energy breaks molecular bonds on the surface of nonpolar substrates, and the broken bonds then recombine with free radicals in the corona environment to form polar groups on the film surface [23]. Namely, the surface of flexible substrate can be changed from hydrophobic to hydrophilic through the corona treatment.

In Section 3, the experimental results that regard the surface modification of PET substrates through corona treatment before the deposition of CNTs and the effects of such surface modification on the properties of the CNTs as flexible TCEs are presented. The changes in the surface roughness, contact angle, and surface energy of the PET substrates due to the corona treatment are characterized in terms of the applied corona energies, PET feeding directions, and treatment times.

> root-mean-square surface roughness (σrms) was estimated, which is plotted in **Figure 5(d)**, in terms of the substrate feeding directions and the numbers of corona treatment. When the

> **Figure 5.** The AFM measurement data such as surface images and roughness for the various PET substrates: (a) nontreated, (b) 4-times corona treated, and (c) [2 + 2]-times corona treated. (d) The surface roughness indicated for

three different corona energy densities in terms of feeding directions and the numbers of treatment.

substrates appeared to have been negligibly small, regardless of the treatment manner. In contrast, when the PET substrates were treated at the higher Ecor (2195 or 5263 kJ/m<sup>2</sup>

σrms values significantly increased as the treatment number increased. Also, the degree of the increase in σrms value due to the corona treatment was seen to have been bigger in the cases of the bidirectional treatment than in the cases of the single-directional treatment. For instance, the PET substrate that was bidirectionally [2 + 2]-times treated showed the σrms value of approximately 8.82 nm, whereas the PET substrate that was single-directionally treated 4-times yielded the σrms value of approximately 5.58 nm. This indicates that the bidirectionally

compared in **Figure 6**, according to feeding directions and the numbers of treatment. In the θ<sup>c</sup> measurement, two kinds of liquids were used such as water as a polar liquid (**Figure 6(a)**–**(d)**) and diiodomethane as a nonpolar or dispersive liquid (**Figure 6(e)**–**(h)**). As the number of

using the formula in the Owens-Wendt model [24]. **Figure 6(i)** shows the polar components

treatment increased in the single-direction (i.e., once and 4-times corona treated). Also, the

with the PET substrate that was 4-times treated. These results indicate that the changes in the

values, and the total surface energy (ES

value was observed for the PET substrate that was [2 + 2]-times treated, compared

) of the PET substrates were measured from liquid drop images and

Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes

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results, the surface energies of the PET substrates were also calculated

value was observed to have increased as the number of the corona

values based on water monotonically decreased, but the θ<sup>c</sup>

values, the dispersive components (ED) obtained from

) obtained from the sum

), the change in σrms of the PET

), the

215

surface modification was performed at the low Ecor (149 kJ/m<sup>2</sup>

treated method can effectively increase the surface area.

values based on diiodomethane continuously increased.

The contact angles (θ<sup>c</sup>

From the estimated θ<sup>c</sup>

the diiodomethane-based θ<sup>c</sup>

of the EP and ED. The ES

higher ES

corona treatment increased, the θ<sup>c</sup>

(EP) calculated from the water-based θ<sup>c</sup>

#### **3.1. Surface modification of PET substrates via corona treatment**

For surface modification, the PET substrates were treated using corona discharge prior to the deposition of CNTs. **Figure 4(a)** shows the schematic of the corona discharge system. **Figure 4(b)** and **(c)** illustrates the corona treatment methods, including the feeding directions of the PET substrates and the numbers of treatment (e.g., 2-times and 4-times for the single-directional treatment and [1 + 1]-times and [2 + 2]-times for the mutually vertical bidirectional treatment). The corona discharge was generated by applying 8 kV voltage to the corona electrode. The PET substrate was fed between the two electrodes (i.e., the corona electrode and the grounded electrode) at the various feeding speeds. The corona energy densities (Ecor) were controlled to be increased from 149 to 5263 kJ/m<sup>2</sup> by decreasing the PET feeding speed from 17 to 0.5 m/min.

The surface images and roughness of PET substrates were measured by atomic force microscope (AFM) as shown in **Figures 5(a)**, **(b)**, and **(c)** for nontreated, 4-times single-directionally corona treated, and [2 + 2]-times bidirectionally corona treated, respectively. The

**Figure 4.** (a) The schematic diagram of the corona discharge system used in this study. The corona treatment methods such as (b) 4-times single-directionally corona treated and (c) [2 + 2]-times bidirectionally corona treated.

Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes http://dx.doi.org/10.5772/intechopen.72002 215

is a chemical modification that can be formed on polar groups on the surface of substrate before film deposition and also includes the physical changes, which can increase the surface area by generating nanoscratches. However, most of these methods have several problems such as low working pressures, long treatment times, and high processing temperatures. Therefore, corona pretreatment has also been suggested because it can be carried out at atmospheric pressure and room temperature. During the corona treatment, corona energy breaks molecular bonds on the surface of nonpolar substrates, and the broken bonds then recombine with free radicals in the corona environment to form polar groups on the film surface [23]. Namely, the surface of flexible substrate can be changed from hydrophobic to hydrophilic

In Section 3, the experimental results that regard the surface modification of PET substrates through corona treatment before the deposition of CNTs and the effects of such surface modification on the properties of the CNTs as flexible TCEs are presented. The changes in the surface roughness, contact angle, and surface energy of the PET substrates due to the corona treatment are characterized in terms of the applied corona energies, PET feeding directions,

For surface modification, the PET substrates were treated using corona discharge prior to the deposition of CNTs. **Figure 4(a)** shows the schematic of the corona discharge system. **Figure 4(b)** and **(c)** illustrates the corona treatment methods, including the feeding directions of the PET substrates and the numbers of treatment (e.g., 2-times and 4-times for the single-directional treatment and [1 + 1]-times and [2 + 2]-times for the mutually vertical bidirectional treatment). The corona discharge was generated by applying 8 kV voltage to the corona electrode. The PET substrate was fed between the two electrodes (i.e., the corona electrode and the grounded electrode) at the various feeding speeds. The corona energy densities (Ecor) were controlled to be increased from 149 to

The surface images and roughness of PET substrates were measured by atomic force microscope (AFM) as shown in **Figures 5(a)**, **(b)**, and **(c)** for nontreated, 4-times single-directionally corona treated, and [2 + 2]-times bidirectionally corona treated, respectively. The

**Figure 4.** (a) The schematic diagram of the corona discharge system used in this study. The corona treatment methods

such as (b) 4-times single-directionally corona treated and (c) [2 + 2]-times bidirectionally corona treated.

**3.1. Surface modification of PET substrates via corona treatment**

by decreasing the PET feeding speed from 17 to 0.5 m/min.

through the corona treatment.

214 Carbon Nanotubes - Recent Progress

and treatment times.

5263 kJ/m<sup>2</sup>

**Figure 5.** The AFM measurement data such as surface images and roughness for the various PET substrates: (a) nontreated, (b) 4-times corona treated, and (c) [2 + 2]-times corona treated. (d) The surface roughness indicated for three different corona energy densities in terms of feeding directions and the numbers of treatment.

root-mean-square surface roughness (σrms) was estimated, which is plotted in **Figure 5(d)**, in terms of the substrate feeding directions and the numbers of corona treatment. When the surface modification was performed at the low Ecor (149 kJ/m<sup>2</sup> ), the change in σrms of the PET substrates appeared to have been negligibly small, regardless of the treatment manner. In contrast, when the PET substrates were treated at the higher Ecor (2195 or 5263 kJ/m<sup>2</sup> ), the σrms values significantly increased as the treatment number increased. Also, the degree of the increase in σrms value due to the corona treatment was seen to have been bigger in the cases of the bidirectional treatment than in the cases of the single-directional treatment. For instance, the PET substrate that was bidirectionally [2 + 2]-times treated showed the σrms value of approximately 8.82 nm, whereas the PET substrate that was single-directionally treated 4-times yielded the σrms value of approximately 5.58 nm. This indicates that the bidirectionally treated method can effectively increase the surface area.

The contact angles (θ<sup>c</sup> ) of the PET substrates were measured from liquid drop images and compared in **Figure 6**, according to feeding directions and the numbers of treatment. In the θ<sup>c</sup> measurement, two kinds of liquids were used such as water as a polar liquid (**Figure 6(a)**–**(d)**) and diiodomethane as a nonpolar or dispersive liquid (**Figure 6(e)**–**(h)**). As the number of corona treatment increased, the θ<sup>c</sup> values based on water monotonically decreased, but the θ<sup>c</sup> values based on diiodomethane continuously increased.

From the estimated θ<sup>c</sup> results, the surface energies of the PET substrates were also calculated using the formula in the Owens-Wendt model [24]. **Figure 6(i)** shows the polar components (EP) calculated from the water-based θ<sup>c</sup> values, the dispersive components (ED) obtained from the diiodomethane-based θ<sup>c</sup> values, and the total surface energy (ES ) obtained from the sum of the EP and ED. The ES value was observed to have increased as the number of the corona treatment increased in the single-direction (i.e., once and 4-times corona treated). Also, the higher ES value was observed for the PET substrate that was [2 + 2]-times treated, compared with the PET substrate that was 4-times treated. These results indicate that the changes in the

**Figure 6.** [(a)–(h)] The liquid drop images pictured from the PET substrates: (a) and (e) for nontreated, (b) and (f) for once corona treated, (c) and (g) for 4-times corona treated, (d) and (h) for [2 + 2]-times corona treated, and (i) the estimated ES along with EP and ED components in terms of the PET feeding directions and the numbers of treatment.

ES values of the PET substrates due to the corona treatment may cause the change of surface chemistries. Also, the results may confirm that the ES of the PET substrates is closely related to the surface roughness.

It is known that the electrons, ions, and activated neutrons and protons produced from corona discharge react with the oxygen in the air to produce functional groups that contain oxygen, such as the COH and C═O groups, on the PET substrate surface [25]. The X-ray photoelectron spectroscopy (XPS) spectra of the carbon (C) 1*s*–state were measured for both nontreated and corona-treated PET substrates to investigate the changes in the surface chemistries of the PET substrates due to the corona treatment. For the nontreated PET substrate, three distinguishable peaks were observed and assigned to the aromatic hydrocarbons (284.7 eV), aliphatic esters (CH<sup>2</sup> O, 286.3 eV), and carboxyl component (C–O–C═O, 288.7 eV) [26], as shown in **Figure 7**. In contrast, additional oxygen polar groups [27], such as phenolic-OH (286.5 eV), C═O (287.5 eV), and carboxyl acid (COOH, 289.2 eV), were observed for the corona-treated PET substrates. The relative contributions of each chemical group in the XPS spectra are summarized in **Table 1**, according to the corona treatment conditions. The relative contents of the oxygen polar groups increased by increasing the number of repetitions of the corona treatment and were larger especially for the bidirectionally corona-treated PET substrates.

**3.2. The effect of corona treatment on the flexibility of CNT-TCEs**

**Assignment Binding energy (eV) Content (%)**

CH<sup>2</sup>

and corona-treated PET substrates.

The effect of corona treatment of PET substrates on the flexibility CNT-TCEs has been investigated by measuring the changes in the CNTs' sheet resistance (R□) due to repetitive bending. Two bending tests such as outer bending and inner bending were carried out with a bending radius of 12 mm and a bending speed of 40 mm/s at an ambient temperature and pressure, as displayed in the insets of **Figure 8(a)** and **(b)**, respectively. For all of the PET substrates that were nontreated or corona treated, the average R□ value of the CNTs was observed to have been approximately 350 ± 10 Ω/□ before the bending test was conducted. As shown in **Figure 8(a)**, the R□ value of the CNTs deposited on the nontreated PET substrate significantly increased as the bending times increased, such that the R□ value after

**Table 1.** The binding energies and relative contents of the chemical bonds observed in the XPS spectra for the nontreated

**Figure 7.** The XPS spectra for carbon (C 1s-state) measured for the nontreated and corona-treated PET substrates.

Aromatic C–C 284.70 53.98 12.31 7.72

O 285.82–286.68 24.65 18.53 18.71 Phenolic-OH 288.34–288.93 21.37 19.59 18.11 C═O 286.50 — 11.23 10.64 C–O–C═O 287.51–287.67 — 20.11 22.10 COOH 288.88–289.03 — 18.23 22.72

**Nontreated Corona treated**

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**4-times [2 + 2]-times**

It was also discovered that the PET's surface chemistry was changed from a hydrophobic nature to a hydrophilic nature after the corona treatment. It has been known that CNTs refined using acids such as HNO<sup>3</sup> may possess oxygen-related functional groups (e.g., carboxyl groups) on their outer walls [28], and also on the corona-treated PET substrate via a hydrogen bond [29]. This indicates that the adhesive force between the PET substrate and the CNTs can be enhanced by corona treatment of the PET substrate.

**Figure 7.** The XPS spectra for carbon (C 1s-state) measured for the nontreated and corona-treated PET substrates.


**Table 1.** The binding energies and relative contents of the chemical bonds observed in the XPS spectra for the nontreated and corona-treated PET substrates.

#### **3.2. The effect of corona treatment on the flexibility of CNT-TCEs**

ES

to the surface roughness.

216 Carbon Nanotubes - Recent Progress

(284.7 eV), aliphatic esters (CH<sup>2</sup>

refined using acids such as HNO<sup>3</sup>

substrates.

values of the PET substrates due to the corona treatment may cause the change of surface

**Figure 6.** [(a)–(h)] The liquid drop images pictured from the PET substrates: (a) and (e) for nontreated, (b) and (f) for once corona treated, (c) and (g) for 4-times corona treated, (d) and (h) for [2 + 2]-times corona treated, and (i) the estimated ES

along with EP and ED components in terms of the PET feeding directions and the numbers of treatment.

It is known that the electrons, ions, and activated neutrons and protons produced from corona discharge react with the oxygen in the air to produce functional groups that contain oxygen, such as the COH and C═O groups, on the PET substrate surface [25]. The X-ray photoelectron spectroscopy (XPS) spectra of the carbon (C) 1*s*–state were measured for both nontreated and corona-treated PET substrates to investigate the changes in the surface chemistries of the PET substrates due to the corona treatment. For the nontreated PET substrate, three distinguishable peaks were observed and assigned to the aromatic hydrocarbons

[26], as shown in **Figure 7**. In contrast, additional oxygen polar groups [27], such as phenolic-OH (286.5 eV), C═O (287.5 eV), and carboxyl acid (COOH, 289.2 eV), were observed for the corona-treated PET substrates. The relative contributions of each chemical group in the XPS spectra are summarized in **Table 1**, according to the corona treatment conditions. The relative contents of the oxygen polar groups increased by increasing the number of repetitions of the corona treatment and were larger especially for the bidirectionally corona-treated PET

It was also discovered that the PET's surface chemistry was changed from a hydrophobic nature to a hydrophilic nature after the corona treatment. It has been known that CNTs

boxyl groups) on their outer walls [28], and also on the corona-treated PET substrate via a hydrogen bond [29]. This indicates that the adhesive force between the PET substrate and the

CNTs can be enhanced by corona treatment of the PET substrate.

of the PET substrates is closely related

O, 286.3 eV), and carboxyl component (C–O–C═O, 288.7 eV)

may possess oxygen-related functional groups (e.g., car-

chemistries. Also, the results may confirm that the ES

The effect of corona treatment of PET substrates on the flexibility CNT-TCEs has been investigated by measuring the changes in the CNTs' sheet resistance (R□) due to repetitive bending. Two bending tests such as outer bending and inner bending were carried out with a bending radius of 12 mm and a bending speed of 40 mm/s at an ambient temperature and pressure, as displayed in the insets of **Figure 8(a)** and **(b)**, respectively. For all of the PET substrates that were nontreated or corona treated, the average R□ value of the CNTs was observed to have been approximately 350 ± 10 Ω/□ before the bending test was conducted. As shown in **Figure 8(a)**, the R□ value of the CNTs deposited on the nontreated PET substrate significantly increased as the bending times increased, such that the R□ value after

**4. CNT-TCEs coated with PEDOT:PSS**

measured from the CNT-TCEs.

tances (Tvis) for CNTs (Cx

for PEDOT:PSS films (Py

(P7

**4.1. PEDOT:PSS coating via spin coating**

CNTs) and the spin speed (for PEDOT:PSS).

Of the aforementioned in Section 1, CNTs have potential advantages such as chemical stability, thermal and electrical conductivity, mechanical strength, flexibility, solution processability, and low cost production. CNTs, however, still do not satisfy the electrical characteristics required for TCEs due to the uniform coating caused by their tubular structure and the large contact resistance at their tube-tube junction. To improve the electrical characteristics of CNTs, it was recently reported [30] that hybrid-type electrodes were introduced by combining CNTs with polymers via solution processes such as spray coating and spin coating. Adding the conductive polymers to the CNTs led to the decrease in the sheet resistance of CNTs; however, the electrical properties of the hybrid-type electrodes were sensitive to the deposition methods and combination conditions used to produce the electrodes. Furthermore, for the application of some devices such as display and TSPs, a neutral color of electrodes is required to prevent the distortion of the image color. However, the conductive polymer material is generally known to be tinged with the color blue, which may cause the problem that patterns of electrodes are revealed. Therefore, there is a necessity of studying how the chromaticity characteristic of CNTs changes or can be effectively adjusted according to the mixture with

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219

other materials (e.g., conductive polymers) for producing the hybrid-type electrodes.

In this section, hybrid-type transparent electrodes with characteristics desirable for flexible TCEs are introduced by coating CNTs with PEDOT:PSS via various methods such as spin coating, electrophoretic deposition (EPD), and electropolymerization. Their surface morphologies, sheet resistances, visible transmittances, and color properties have been characterized as functions of their preparation conditions. Also, to identify the flexible capabilities of the PEDOT:PSS-coated hybrid-type CNT-TCEs, the changes in their sheet resistances due to repetitive bending (up to 30,000 times) have been measured and compared with the results

For manufacturing hybrid-type CNT-TCEs, the CNTs were deposited on the PET substrates via spray coating by varying the spraying time from 10 to 80 s. Then, PEDOT:PSS layers were deposited on the CNTs via spin coating (for 1 min) by changing the spin speed within the range of 200–1500 rpm. The PEDOT:PSS-coated CNTs were dried in the oven for 5 min at 100°C and atmospheric pressure. The hybrid-type CNT-TCEs with various thickness combinations of CNTs and PEDOT:PSS layers were manufactured by controlling the spray time (for

**Figure 9(a)** and **(c)** displays the electrical sheet resistances (R□) and visible range transmit-

), respectively. For the CNTs, both R□ and Tvis monotonically decreased (e.g., R□ from 3035 to 379 Ω/□ and Tvis from 93.2 to 80.4%) as the spray time increased. Therefore, there is a tradeoff between R□ and Tvis. For the PEDOT:PSS films, the similar trends in R□ and Tvis appeared

) according to the increased spray time from 20 (C<sup>1</sup>

) according to the decreased spin speed from 1500 (P<sup>1</sup>

) to 80 s (C7

) and

) to 200 rpm

**Figure 8.** The changes in R□ of CNTs during bending: (a) for outer bending and (b) for inner bending. [(c)–(f)] The comparison of FE-SEM images for the CNTs before and after bending. The photos (c) and (d) indicate the CNTs deposited on the nontreated PET substrates. The photos (e) and (f) indicate the CNTs deposited on the [2+2]-times corona-treated PET substrates.

16,000 times bending was almost more than 300% larger than the initial R□ value before the bending. The increase in the CNTs' R□ value due to bending may be attributed to the weak adhesion between the CNTs and the PET substrate. In contrast, the increase in the R□ due to bending was observed to have been substantially alleviated for the CNTs that were deposited on the corona-treated PET substrates. As shown in **Figure 8(b)**, the similar results were observed in the case of the inner bending test. For both bending tests, the increase in R□ after 16,000 bending repetitions was observed to have been minimal when the CNTs were deposited on the PET substrates that were [2 + 2]-times corona treated. Accordingly, the results shown in **Figure 8** confirm that the corona pretreatment improved the adhesion of the CNTs to the PET substrate.

**Figure 8(c)** and **(e)** shows the FE-SEM surface morphologies of the CNTs before they were bent, and **Figure 8(d)** and **(f)** displays the surface morphologies of the CNTs after they underwent bending. As shown in **Figure 8(d)**, the repetitive bending caused the marked reduction in the CNTs' density for the CNTs deposited on the untreated PET substrate. In contrast, as shown in **Figure 8(f)**, the CNTs deposited on the [2 + 2]-times corona-treated PET substrate revealed insignificant changes in the surface morphology even after the bending of 16,000 times. As a result, the higher surface roughness, the greater contact angles, and the higher surface energies, which the corona-treated PET substrates revealed, indicated that the surfaces of the PET substrates became hydrophilic from hydrophobic and thereby led to the improvement of adhesion between the CNTs and the PET substrates.

### **4. CNT-TCEs coated with PEDOT:PSS**

Of the aforementioned in Section 1, CNTs have potential advantages such as chemical stability, thermal and electrical conductivity, mechanical strength, flexibility, solution processability, and low cost production. CNTs, however, still do not satisfy the electrical characteristics required for TCEs due to the uniform coating caused by their tubular structure and the large contact resistance at their tube-tube junction. To improve the electrical characteristics of CNTs, it was recently reported [30] that hybrid-type electrodes were introduced by combining CNTs with polymers via solution processes such as spray coating and spin coating. Adding the conductive polymers to the CNTs led to the decrease in the sheet resistance of CNTs; however, the electrical properties of the hybrid-type electrodes were sensitive to the deposition methods and combination conditions used to produce the electrodes. Furthermore, for the application of some devices such as display and TSPs, a neutral color of electrodes is required to prevent the distortion of the image color. However, the conductive polymer material is generally known to be tinged with the color blue, which may cause the problem that patterns of electrodes are revealed. Therefore, there is a necessity of studying how the chromaticity characteristic of CNTs changes or can be effectively adjusted according to the mixture with other materials (e.g., conductive polymers) for producing the hybrid-type electrodes.

In this section, hybrid-type transparent electrodes with characteristics desirable for flexible TCEs are introduced by coating CNTs with PEDOT:PSS via various methods such as spin coating, electrophoretic deposition (EPD), and electropolymerization. Their surface morphologies, sheet resistances, visible transmittances, and color properties have been characterized as functions of their preparation conditions. Also, to identify the flexible capabilities of the PEDOT:PSS-coated hybrid-type CNT-TCEs, the changes in their sheet resistances due to repetitive bending (up to 30,000 times) have been measured and compared with the results measured from the CNT-TCEs.

#### **4.1. PEDOT:PSS coating via spin coating**

16,000 times bending was almost more than 300% larger than the initial R□ value before the bending. The increase in the CNTs' R□ value due to bending may be attributed to the weak adhesion between the CNTs and the PET substrate. In contrast, the increase in the R□ due to bending was observed to have been substantially alleviated for the CNTs that were deposited on the corona-treated PET substrates. As shown in **Figure 8(b)**, the similar results were observed in the case of the inner bending test. For both bending tests, the increase in R□ after 16,000 bending repetitions was observed to have been minimal when the CNTs were deposited on the PET substrates that were [2 + 2]-times corona treated. Accordingly, the results shown in **Figure 8** confirm that the corona pretreatment improved the adhesion of

**Figure 8.** The changes in R□ of CNTs during bending: (a) for outer bending and (b) for inner bending. [(c)–(f)] The comparison of FE-SEM images for the CNTs before and after bending. The photos (c) and (d) indicate the CNTs deposited on the nontreated PET substrates. The photos (e) and (f) indicate the CNTs deposited on the [2+2]-times corona-treated

**Figure 8(c)** and **(e)** shows the FE-SEM surface morphologies of the CNTs before they were bent, and **Figure 8(d)** and **(f)** displays the surface morphologies of the CNTs after they underwent bending. As shown in **Figure 8(d)**, the repetitive bending caused the marked reduction in the CNTs' density for the CNTs deposited on the untreated PET substrate. In contrast, as shown in **Figure 8(f)**, the CNTs deposited on the [2 + 2]-times corona-treated PET substrate revealed insignificant changes in the surface morphology even after the bending of 16,000 times. As a result, the higher surface roughness, the greater contact angles, and the higher surface energies, which the corona-treated PET substrates revealed, indicated that the surfaces of the PET substrates became hydrophilic from hydrophobic and thereby led to the improve-

the CNTs to the PET substrate.

PET substrates.

218 Carbon Nanotubes - Recent Progress

ment of adhesion between the CNTs and the PET substrates.

For manufacturing hybrid-type CNT-TCEs, the CNTs were deposited on the PET substrates via spray coating by varying the spraying time from 10 to 80 s. Then, PEDOT:PSS layers were deposited on the CNTs via spin coating (for 1 min) by changing the spin speed within the range of 200–1500 rpm. The PEDOT:PSS-coated CNTs were dried in the oven for 5 min at 100°C and atmospheric pressure. The hybrid-type CNT-TCEs with various thickness combinations of CNTs and PEDOT:PSS layers were manufactured by controlling the spray time (for CNTs) and the spin speed (for PEDOT:PSS).

**Figure 9(a)** and **(c)** displays the electrical sheet resistances (R□) and visible range transmittances (Tvis) for CNTs (Cx ) according to the increased spray time from 20 (C<sup>1</sup> ) to 80 s (C7 ) and for PEDOT:PSS films (Py ) according to the decreased spin speed from 1500 (P<sup>1</sup> ) to 200 rpm (P7 ), respectively. For the CNTs, both R□ and Tvis monotonically decreased (e.g., R□ from 3035 to 379 Ω/□ and Tvis from 93.2 to 80.4%) as the spray time increased. Therefore, there is a tradeoff between R□ and Tvis. For the PEDOT:PSS films, the similar trends in R□ and Tvis appeared

**Figure 9.** The sheet resistance and transmittance measured from the (a) CNTs and (c) PEDOT:PSS according to deposition thickness. The FE-SEM images (b) and (d) indicate the cross-sectional morphology of the CNTs specimen of C<sup>7</sup> and PETOD:PSS specimen of P<sup>7</sup> .

(e.g., R□ from 330 to 90 Ω/□ and Tvis from 97.5 to 89.2%) as the spin speed decreased. Also, the FE-SEM cross-sectional morphology of the CNTs and PEDOT:PSS electrodes are displayed in the inset of **Figure 9(b)** and **(d)**. The thickness of CNTs increased from 40.5 to 112 nm by increasing the spraying time from 20 to 80 s and the thickness of PEDOT:PSS films increased from 28.4 to 111 nm by decreasing the spinning speed from 1500 to 200 rpm. Accordingly, the decrease of Tvis as shown in **Figure 9(a)** and **(c)** was attributed to the increase in the thicknesses of CNTs and PEDOT:PSS layers.

(redness) value is the positive definiteness (+) that is close to the color red as it increases and it is close to the color green as it increases with the negative definiteness (−). Also, the b\* (yellowness) value is close to yellow color as it increases with the positive definiteness and it appears

**Figure 10.** (a) The sheet resistance and average transmittance measured from the PEDOT:PSS-coated CNTs hybrid electrodes as a function of the spinning speed used to coat the PEDOT:PSS films via spin coating. The FE-SEM surface

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**Figure 11(b)**–**(d)** shows the measured results of chromaticity characteristics for three kinds of TCEs such as (b) CNTs, (c) PEDOT:PSS, and (d) PEDOT:PSS-coated CNTs. For the CNTs, the b\* gradually increased in the negative direction of sign from about 0.58 to 1.58 as the spraying time increased, whereas the a\* showed neighboring zero regardless of the spraying time. For the PEDOT:PSS films, both a\* and b\* monotonically increased in the negative direction of

**Figure 11.** (a) Color space (L\*, a\*, and b\*). [(b)–(d)] The measured chromatic parameters (a\* and b\*) for various electrodes:

), and (d) for PEDOT:PSS-coated CNTs (Cx

Py ).

to be blue color as it increases with the negative definiteness.

(b) for CNTs (Cx

), (c) for PEDOT:PSS (Py

and cross-sectional images: (b) and (c) for CNTs, (d) and (e) for PEDOT:PSS-coated CNTs.

For the hybrid electrodes (S<sup>z</sup> ), the R□ and Tvis were also measured and the results are shown in **Figure 10(a)**. Here, the hybrid electrodes were fabricated by depositing the CNTs via spray coating under the same condition as the specimen C<sup>3</sup> and coating them with PEDOT:PSS by decreasing the spinning speed from 1500 (S<sup>1</sup> ) to 200 rpm (S<sup>5</sup> ) via spin coating. For the hybridtype specimens (S<sup>z</sup> ), both R□ and Tvis decreased (e.g., R□ from 298 to 73.49 Ω/□ and Tvis from 87.52 to 78.84%) as the spinning speed decreased because the thickness of the PEDOT:PSS films increased from 28.4 to 114.9 nm with a decrease in the spinning speed from 1500 to 200 rpm. It was observed that the decrease in the Tvis due to the coating of the PEDOT:PSS films was trivial, while the R□ of CNTs was substantially reduced by the coating of PEDOT:PSS. For instance, the hybrid electrode of S<sup>1</sup> , which was fabricated by coating the CNTs (C<sup>3</sup> ) with the PEDOT:PSS film at the spinning speed of 1500 rpm, had an R□ of approximately 298 Ω/□ and a Tvis of 87.52%, whereas the CNTs (C<sup>3</sup> ) showed a much higher R□ (959.3 Ω/□) and a slightly higher Tvis (88.80%). This was because the PEDOT:PSS particles filled up the voids between the tubes in the CNTs as shown in **Figure 10(e)** and thereby formed a conduction bridge for electron transfer, inducing a decrease in the tube-tube junction resistance and the percolation threshold, eventually enhancing the conductivity of the hybrid electrodes [31].

In addition to low electric sheet resistance and high transmittance, TCEs should also satisfy the chromatic characteristics approaching no color. Generally, the chromaticity of a TCE can be determined by using the three-dimensional color space (namely, L\*, a\*, and b\*) where each coordinate axis expresses specific color components [32], as shown in **Figure 11(a)**. Here, L\* means brightness having the numerical value between 0 (lowest) and 100 (highest). The a\*

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**Figure 10.** (a) The sheet resistance and average transmittance measured from the PEDOT:PSS-coated CNTs hybrid electrodes as a function of the spinning speed used to coat the PEDOT:PSS films via spin coating. The FE-SEM surface and cross-sectional images: (b) and (c) for CNTs, (d) and (e) for PEDOT:PSS-coated CNTs.

(e.g., R□ from 330 to 90 Ω/□ and Tvis from 97.5 to 89.2%) as the spin speed decreased. Also, the FE-SEM cross-sectional morphology of the CNTs and PEDOT:PSS electrodes are displayed in the inset of **Figure 9(b)** and **(d)**. The thickness of CNTs increased from 40.5 to 112 nm by increasing the spraying time from 20 to 80 s and the thickness of PEDOT:PSS films increased from 28.4 to 111 nm by decreasing the spinning speed from 1500 to 200 rpm. Accordingly, the decrease of Tvis as shown in **Figure 9(a)** and **(c)** was attributed to the increase in the thicknesses

**Figure 9.** The sheet resistance and transmittance measured from the (a) CNTs and (c) PEDOT:PSS according to deposition thickness. The FE-SEM images (b) and (d) indicate the cross-sectional morphology of the CNTs specimen of C<sup>7</sup>

in **Figure 10(a)**. Here, the hybrid electrodes were fabricated by depositing the CNTs via spray

87.52 to 78.84%) as the spinning speed decreased because the thickness of the PEDOT:PSS films increased from 28.4 to 114.9 nm with a decrease in the spinning speed from 1500 to 200 rpm. It was observed that the decrease in the Tvis due to the coating of the PEDOT:PSS films was trivial, while the R□ of CNTs was substantially reduced by the coating of PEDOT:PSS. For

PEDOT:PSS film at the spinning speed of 1500 rpm, had an R□ of approximately 298 Ω/□ and

higher Tvis (88.80%). This was because the PEDOT:PSS particles filled up the voids between the tubes in the CNTs as shown in **Figure 10(e)** and thereby formed a conduction bridge for electron transfer, inducing a decrease in the tube-tube junction resistance and the percolation

In addition to low electric sheet resistance and high transmittance, TCEs should also satisfy the chromatic characteristics approaching no color. Generally, the chromaticity of a TCE can be determined by using the three-dimensional color space (namely, L\*, a\*, and b\*) where each coordinate axis expresses specific color components [32], as shown in **Figure 11(a)**. Here, L\* means brightness having the numerical value between 0 (lowest) and 100 (highest). The a\*

threshold, eventually enhancing the conductivity of the hybrid electrodes [31].

) to 200 rpm (S<sup>5</sup>

), both R□ and Tvis decreased (e.g., R□ from 298 to 73.49 Ω/□ and Tvis from

, which was fabricated by coating the CNTs (C<sup>3</sup>

) showed a much higher R□ (959.3 Ω/□) and a slightly

), the R□ and Tvis were also measured and the results are shown

and coating them with PEDOT:PSS by

) via spin coating. For the hybrid-

) with the

and

of CNTs and PEDOT:PSS layers.

instance, the hybrid electrode of S<sup>1</sup>

a Tvis of 87.52%, whereas the CNTs (C<sup>3</sup>

coating under the same condition as the specimen C<sup>3</sup>

decreasing the spinning speed from 1500 (S<sup>1</sup>

.

For the hybrid electrodes (S<sup>z</sup>

type specimens (S<sup>z</sup>

PETOD:PSS specimen of P<sup>7</sup>

220 Carbon Nanotubes - Recent Progress

(redness) value is the positive definiteness (+) that is close to the color red as it increases and it is close to the color green as it increases with the negative definiteness (−). Also, the b\* (yellowness) value is close to yellow color as it increases with the positive definiteness and it appears to be blue color as it increases with the negative definiteness.

**Figure 11(b)**–**(d)** shows the measured results of chromaticity characteristics for three kinds of TCEs such as (b) CNTs, (c) PEDOT:PSS, and (d) PEDOT:PSS-coated CNTs. For the CNTs, the b\* gradually increased in the negative direction of sign from about 0.58 to 1.58 as the spraying time increased, whereas the a\* showed neighboring zero regardless of the spraying time. For the PEDOT:PSS films, both a\* and b\* monotonically increased in the negative direction of

**Figure 11.** (a) Color space (L\*, a\*, and b\*). [(b)–(d)] The measured chromatic parameters (a\* and b\*) for various electrodes: (b) for CNTs (Cx ), (c) for PEDOT:PSS (Py ), and (d) for PEDOT:PSS-coated CNTs (Cx Py ).

sign as the spinning speed decreased (e.g., the a\* from −0.31 to −1.09 and the b\* from −0.24 to −2.05). Namely, increasing the thicknesses of CNTs and PEDOT:PSS layers led to the decrease in their sheet resistances, but it resulted in the deterioration of their chromaticity characteristics, especially for the yellowness. Here, it should be noted that the b\* values of the CNTs and PEDOT:PSS layers changed in the mutually opposite directions of sign according to the thicknesses of CNTs and PEDOT:PSS layers. **Figure 11(d)** displays the measured results of characteristic values of a\* and b\* of the PEDOT:PSS-coated CNTs manufactured by variously combining the deposition conditions of CNTs (i.e., the spraying times) and the coating conditions of PEDOT:PSS (i.e., the spinning speeds). For the C<sup>3</sup> P5 specimen, the a\* value of −0.28 was located between those of CNTs and PEDOT:PSS (i.e., −0.08 for C<sup>3</sup> and −0.64 for P<sup>5</sup> ) and the b\* value of 0.57 was also between those of CNTs and PEDOT:PSS (i.e., 0.96 for C<sup>3</sup> and −1.22 for P<sup>5</sup> ). This indicates that hybrid-type electrodes with CNTs and PEDOT:PSS combined) can possess the better color properties compared with those of single electrodes (e.g., CNTs or PEDOT:PSS) due to their complementary color relation.

via spin coating and the E<sup>z</sup>

films via EPD. For the hybrid electrodes E<sup>z</sup>

and **(e)** (i.e., the hybrid electrodes of E<sup>1</sup>

CNTs either using EPD (E<sup>z</sup>

the acid-treated CNTs [35].

the higher transmittance as compared to the sample S<sup>1</sup>

where the PEDOT:PSS films were hardly coated.

of the coated PEDOT:PSS layer (i.e., from S<sup>1</sup>

sured from the electrodes of CNTs (C<sup>3</sup>

the density of the CNTs in the S<sup>3</sup>

measured after bending of 30,000 times. For the C<sup>3</sup>

was significantly decreased from about 1.97 to 1.52 tubes/μm<sup>2</sup>

and E3

specimens fabricated by coating the CNTs (C<sup>3</sup>

Generally, the film deposition via EPD is generally achieved by the motion of the charged particles that are dispersed in a suitable solvent toward an electrode under an applied electric field [33]. Accordingly, the results of **Figure 12(a)** can be explained by comparing the surface morphologies of the hybrid electrodes, which are shown in **Figure 12(b)**–**(e)**. It was observed

the PEDOT:PSS layer was deposited not only on the region where the CNTs existed but also on the region where no CNTs existed. On the other hand, the surface images of **Figure 12(c)**

To compare the flexibility of the CNTs and the hybrid electrodes (i.e., PEDOT:PSS-coated

results are shown in **Figure 13(a)** as functions of the number of bending tests (up to 30,000). In the case of the CNTs, the R□ increased continuously as the number of bending times increased and appeared to almost have doubled after the completion of the bending test. On the contrary, the increase in the R□ was mitigated for the hybrid electrodes, indicating an improvement in flexibility. The flexibility-enhancement effect was higher by increasing the thickness

to S<sup>3</sup>

PEDOT:PSS film induced the physical gripping of the CNTs to enable them to adhere better to the underlying PET substrate [34]. This may also be due to the chemical bonds between the PEDOT:PSS film and the corona-treated PET substrate or between the PEDOT:PSS film and

**Figure 13(b)**–**(g)** shows the changes of FE-SEM surface morphologies due to bending, mea-

indicates their images measured before bending and **Figure 13(e)**, **(f)**, and **(g)** shows images

This may confirm that the hybrid electrodes have more potential in flexibility as compared to the CNTs. In addition, the hybrid electrode fabricated via spin coating showed less change in resistance than the hybrid electrode fabricated via EPD. The deposition of PEDOT:PSS occurs mainly on the surface of the CNTs in EPD, whereas in spin coating, the deposition of PEDOT:PSS occurs not only on the CNTs' surface but also on the PET substrate where no CNTs exist. In the case of the EPD-produced hybrid electrodes, accordingly, if the CNTs are

) and hybrids (S<sup>3</sup>

(or S<sup>3</sup>

or from E<sup>1</sup>

to E3

and E3

from the surface images of **Figure 12(b)** and **(d)** (i.e., the hybrid electrodes of S<sup>1</sup>

and E3

ered the area near the CNTs. Therefore, the observation that the sample E<sup>1</sup>

) or spin coating (S<sup>z</sup>

were observed to have been similar to those of the hybrid electrodes S<sup>z</sup>

ing). Here, it may be noted that the Tvis of the hybrid electrode E<sup>3</sup>

had almost the same R□ values such as approximately 114.4 Ω/□ for E<sup>3</sup>

slightly higher than that (84.36%) of the hybrid electrode S<sup>3</sup>

similar trend was also observed for the hybrid electrodes of S<sup>1</sup>

) with the PEDOT:PSS

(i.e., using spin coat-

and S<sup>3</sup>

. The

223

) that

) displayed

was measured as 85.52%,

, whereas both hybrid electrodes

http://dx.doi.org/10.5772/intechopen.72002

and 110.5 Ω/□ for S<sup>3</sup>

(or E3

). This may be because the

). **Figure 13(b)**, **(c)**, and **(d)**

after bending. On the contrary,

electrode (CNTs), the density of the CNTs

electrodes (hybrid) hardly changed after bending.

) was because there were regions

(i.e., using EPD), the changes in the R□ and Tvis

) showed that most of the PEDOT:PSS layer cov-

)), the bending tests were analyzed, and the

 and E1 .

Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes

#### **4.2. PEDOT:PSS coating via electrophoretic deposition (EPD)**

The coating of CNTs with PEDOT:PSS films via EPD was performed using a mixed solution of PEDOT:PSS (5 ml) and isopropyl alcohol (IPA, 100 ml). In the EPD process, the PEDOT:PSS films were deposited on the anode electrode where the spacing between the anode (i.e., CNTs) and the cathode (i.e., Cu) was kept at 10 mm and the DC bias voltage of 100 V was applied during the changing of the process time within the range of 20–40 s.

For the hybrid electrodes (E<sup>z</sup> ) which were fabricated by coating the CNTs (C<sup>3</sup> ) with the PEDOT:PSS films via the EPD method, the R□ and Tvis were measured. The results are shown in **Figure 12(a)**, according to the increase in the EPD time from 20 (E<sup>1</sup> ) to 40 s (E5 ). **Figure 12(a)** shows the relationships between the R□ and Tvis measured from the two different hybrid electrodes such as the S<sup>z</sup> specimens fabricated by coating the CNTs (C<sup>3</sup> ) with the PEDOT:PSS films

**Figure 12.** (a) The sheet resistances and average transmittances of the hybrid electrodes, where the PEDOT:PSS films were coated on the CNTs via spin coating (i.e., the specimens denoted as S<sup>z</sup> ) or EPD (i.e., the specimens denoted as E<sup>z</sup> ), along with the FE-SEM images of the S<sup>1</sup> and E1 specimens. [(b)–(e)] The surface morphologies of the specimens (b) S<sup>1</sup> , (c) E1 , (d) S<sup>3</sup> , and (e) E<sup>3</sup> .

via spin coating and the E<sup>z</sup> specimens fabricated by coating the CNTs (C<sup>3</sup> ) with the PEDOT:PSS films via EPD. For the hybrid electrodes E<sup>z</sup> (i.e., using EPD), the changes in the R□ and Tvis were observed to have been similar to those of the hybrid electrodes S<sup>z</sup> (i.e., using spin coating). Here, it may be noted that the Tvis of the hybrid electrode E<sup>3</sup> was measured as 85.52%, slightly higher than that (84.36%) of the hybrid electrode S<sup>3</sup> , whereas both hybrid electrodes had almost the same R□ values such as approximately 114.4 Ω/□ for E<sup>3</sup> and 110.5 Ω/□ for S<sup>3</sup> . The similar trend was also observed for the hybrid electrodes of S<sup>1</sup> and E1 .

Generally, the film deposition via EPD is generally achieved by the motion of the charged particles that are dispersed in a suitable solvent toward an electrode under an applied electric field [33]. Accordingly, the results of **Figure 12(a)** can be explained by comparing the surface morphologies of the hybrid electrodes, which are shown in **Figure 12(b)**–**(e)**. It was observed from the surface images of **Figure 12(b)** and **(d)** (i.e., the hybrid electrodes of S<sup>1</sup> and S<sup>3</sup> ) that the PEDOT:PSS layer was deposited not only on the region where the CNTs existed but also on the region where no CNTs existed. On the other hand, the surface images of **Figure 12(c)** and **(e)** (i.e., the hybrid electrodes of E<sup>1</sup> and E3 ) showed that most of the PEDOT:PSS layer covered the area near the CNTs. Therefore, the observation that the sample E<sup>1</sup> (or E3 ) displayed the higher transmittance as compared to the sample S<sup>1</sup> (or S<sup>3</sup> ) was because there were regions where the PEDOT:PSS films were hardly coated.

To compare the flexibility of the CNTs and the hybrid electrodes (i.e., PEDOT:PSS-coated CNTs either using EPD (E<sup>z</sup> ) or spin coating (S<sup>z</sup> )), the bending tests were analyzed, and the results are shown in **Figure 13(a)** as functions of the number of bending tests (up to 30,000). In the case of the CNTs, the R□ increased continuously as the number of bending times increased and appeared to almost have doubled after the completion of the bending test. On the contrary, the increase in the R□ was mitigated for the hybrid electrodes, indicating an improvement in flexibility. The flexibility-enhancement effect was higher by increasing the thickness of the coated PEDOT:PSS layer (i.e., from S<sup>1</sup> to S<sup>3</sup> or from E<sup>1</sup> to E3 ). This may be because the PEDOT:PSS film induced the physical gripping of the CNTs to enable them to adhere better to the underlying PET substrate [34]. This may also be due to the chemical bonds between the PEDOT:PSS film and the corona-treated PET substrate or between the PEDOT:PSS film and the acid-treated CNTs [35].

**Figure 13(b)**–**(g)** shows the changes of FE-SEM surface morphologies due to bending, measured from the electrodes of CNTs (C<sup>3</sup> ) and hybrids (S<sup>3</sup> and E3 ). **Figure 13(b)**, **(c)**, and **(d)** indicates their images measured before bending and **Figure 13(e)**, **(f)**, and **(g)** shows images measured after bending of 30,000 times. For the C<sup>3</sup> electrode (CNTs), the density of the CNTs was significantly decreased from about 1.97 to 1.52 tubes/μm<sup>2</sup> after bending. On the contrary, the density of the CNTs in the S<sup>3</sup> and E3 electrodes (hybrid) hardly changed after bending. This may confirm that the hybrid electrodes have more potential in flexibility as compared to the CNTs. In addition, the hybrid electrode fabricated via spin coating showed less change in resistance than the hybrid electrode fabricated via EPD. The deposition of PEDOT:PSS occurs mainly on the surface of the CNTs in EPD, whereas in spin coating, the deposition of PEDOT:PSS occurs not only on the CNTs' surface but also on the PET substrate where no CNTs exist. In the case of the EPD-produced hybrid electrodes, accordingly, if the CNTs are

**Figure 12.** (a) The sheet resistances and average transmittances of the hybrid electrodes, where the PEDOT:PSS films

sign as the spinning speed decreased (e.g., the a\* from −0.31 to −1.09 and the b\* from −0.24 to −2.05). Namely, increasing the thicknesses of CNTs and PEDOT:PSS layers led to the decrease in their sheet resistances, but it resulted in the deterioration of their chromaticity characteristics, especially for the yellowness. Here, it should be noted that the b\* values of the CNTs and PEDOT:PSS layers changed in the mutually opposite directions of sign according to the thicknesses of CNTs and PEDOT:PSS layers. **Figure 11(d)** displays the measured results of characteristic values of a\* and b\* of the PEDOT:PSS-coated CNTs manufactured by variously combining the deposition conditions of CNTs (i.e., the spraying times) and the coating condi-

P5

) which were fabricated by coating the CNTs (C<sup>3</sup>

specimen, the a\* value of −0.28

) and the

and −1.22

) with the

). **Figure 12(a)**

and −0.64 for P<sup>5</sup>

) to 40 s (E5

) with the PEDOT:PSS films

) or EPD (i.e., the specimens denoted as E<sup>z</sup>

specimens. [(b)–(e)] The surface morphologies of the specimens (b) S<sup>1</sup>

),

,

were coated on the CNTs via spin coating (i.e., the specimens denoted as S<sup>z</sup>

tions of PEDOT:PSS (i.e., the spinning speeds). For the C<sup>3</sup>

PEDOT:PSS) due to their complementary color relation.

**4.2. PEDOT:PSS coating via electrophoretic deposition (EPD)**

during the changing of the process time within the range of 20–40 s.

in **Figure 12(a)**, according to the increase in the EPD time from 20 (E<sup>1</sup>

was located between those of CNTs and PEDOT:PSS (i.e., −0.08 for C<sup>3</sup>

b\* value of 0.57 was also between those of CNTs and PEDOT:PSS (i.e., 0.96 for C<sup>3</sup>

). This indicates that hybrid-type electrodes with CNTs and PEDOT:PSS combined) can possess the better color properties compared with those of single electrodes (e.g., CNTs or

The coating of CNTs with PEDOT:PSS films via EPD was performed using a mixed solution of PEDOT:PSS (5 ml) and isopropyl alcohol (IPA, 100 ml). In the EPD process, the PEDOT:PSS films were deposited on the anode electrode where the spacing between the anode (i.e., CNTs) and the cathode (i.e., Cu) was kept at 10 mm and the DC bias voltage of 100 V was applied

PEDOT:PSS films via the EPD method, the R□ and Tvis were measured. The results are shown

shows the relationships between the R□ and Tvis measured from the two different hybrid elec-

specimens fabricated by coating the CNTs (C<sup>3</sup>

and E1

along with the FE-SEM images of the S<sup>1</sup>

For the hybrid electrodes (E<sup>z</sup>

trodes such as the S<sup>z</sup>

, and (e) E<sup>3</sup> .

(c) E1 , (d) S<sup>3</sup>

for P<sup>5</sup>

222 Carbon Nanotubes - Recent Progress

Cu films were deposited on glass substrates (Corning, Eagle 2000, 3 × 3 cm<sup>2</sup>

walled CNT powder (NanoAmor 1284YJ, 15 mg) was placed in a mixture of H<sup>2</sup>

HNO<sup>3</sup>

(Mg(NO<sup>3</sup>

)2 6H<sup>2</sup>

by varying the post-washing time from 0 to 30 s.

Cu meshes measured before and after post-washing (30 s).

perature via RF sputtering at 80 W power and 30 mTorr working pressure. Cu meshes were formed via lithography to produce a 200 μm line spacing and a 10 μm line width. Single-

meshes via the EPD process, conditions of which were previously reported in the authors' recent paper. The thickness of the CNTs was controlled by varying the EPD time from 5 to 15 s. Finally, for post-washing, the CNT-coated Cu meshes were dipped in deionized water

**Figure 14(a)** and **(b)** displays the FE-SEM photos of the fabricated Cu mesh and the CNTcoated Cu mesh along with the enlarged photo of the surface morphology of the CNT coating layer, respectively. It can be clearly seen that CNTs were used to selectively coat only the surfaces where the Cu mesh patterns had been formed. The thickness profiles of the CNT-coated Cu meshes measured before and after post-washing are shown in the inset of **Figure 14(c)**. The thickness of the Cu mesh was approximately 106 nm, and it was increased to approximately 913 nm after the Cu mesh was coated with CNTs via the 15 s EPD process. Meanwhile, the thickness of the CNT-coated Cu mesh was considerably decreased as the post-washing process progressed, and it was approximately 237 nm after 30 s post-washing, as shown in **Figure 14(c)**. This was because some of the CNTs were detached from the Cu mesh during the washing process. All of the Cu mesh specimens considered in this study are summarized in **Table 2**.

**Figure 14.** [(a) and (b)] The FE-SEM photos for a pristine Cu mesh with a 200 μm line spacing and a 10 μm line width and a CNT-coated Cu mesh including the surface morphology of the CNT coating layer. (c) The change in the thickness of the CNT-coated Cu mesh as a function of the post-washing time and the thickness profiles (inset) of the CNT-coated

 (10 ml) at a 2:1 volume ratio and was ultrasonicated for 30 min to eliminate the impurities such as the metal catalysts and amorphous carbons. The CNT solution was dispersed for 15 min using an ultrasonic generator after adding a dispersant of magnesium nitrate

O, 15 mg) and isopropyl alcohol (IPA, 100 ml). CNTs were deposited on the Cu

Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes

) at room tem-

(20 ml) and

225

SO<sup>4</sup>

http://dx.doi.org/10.5772/intechopen.72002

**Figure 13.** (a) The changes in the sheet resistances due to bending tests (up to 30,000 times), measured from the various electrodes such as CNTs (i.e., C<sup>3</sup> and C7 ) and PEDOT:PSS-coated CNTs (i.e., S<sup>1</sup> , S<sup>3</sup> , E<sup>1</sup> , and E<sup>3</sup> ). [(b)–(g)] The changes of FE-SEM surface morphologies due to bending, measured from the electrodes of CNTs (C<sup>3</sup> ) and hybrids (S<sup>3</sup> and E3 ). Here, (b), (c), and (d) indicate their images obtained before bending (left photos), and (e), (f), and (g) obtained after bending (right photos) of 30,000 times.

detached during the bending test, then the PEDOT:PSS layer on the CNTs is also detached and thus, the resistance of the electrodes will inevitably increase. In contrast, in the case of the hybrid electrodes fabricated via spin coating, the increase in resistance may be insignificant even if they experience the detachment of CNTs due to bending because the conductive PEDOT:PSS layers still remain.

#### **5. Effects of CNT-coating and washing on properties of copper meshes**

Metal meshes have been developed as the alternative transparent electrodes to replace the conventional ITO due to their low electric sheet resistance and high visible transmittance. However, metal meshes have a relatively high reflectance generated by metals, and several metals, such as gold (Au) and copper (Cu), are generally tinged with specific colors, leading to a visibility problem. The most commonly used materials for the metal-based TCEs are silver (Ag) and Cu. Compared with Ag, Cu is more abundant and less expensive, but it has higher resistivity. Also, Cu is generally tinged with a red-orange color [36], requiring it to be coated with black materials capable of absorbing light effectively. CNTs are known as excellent absorbers of visible light [37] and have advantages in terms of chemical stability, thermal and electric conductivity, mechanical strength, and flexibility. To improve the visibility of metal mesh electrodes, the authors recently introduced hybrid-type TCEs fabricated via the combination of metal meshes with CNTs [38].

Cu films were deposited on glass substrates (Corning, Eagle 2000, 3 × 3 cm<sup>2</sup> ) at room temperature via RF sputtering at 80 W power and 30 mTorr working pressure. Cu meshes were formed via lithography to produce a 200 μm line spacing and a 10 μm line width. Singlewalled CNT powder (NanoAmor 1284YJ, 15 mg) was placed in a mixture of H<sup>2</sup> SO<sup>4</sup> (20 ml) and HNO<sup>3</sup> (10 ml) at a 2:1 volume ratio and was ultrasonicated for 30 min to eliminate the impurities such as the metal catalysts and amorphous carbons. The CNT solution was dispersed for 15 min using an ultrasonic generator after adding a dispersant of magnesium nitrate (Mg(NO<sup>3</sup> )2 6H<sup>2</sup> O, 15 mg) and isopropyl alcohol (IPA, 100 ml). CNTs were deposited on the Cu meshes via the EPD process, conditions of which were previously reported in the authors' recent paper. The thickness of the CNTs was controlled by varying the EPD time from 5 to 15 s. Finally, for post-washing, the CNT-coated Cu meshes were dipped in deionized water by varying the post-washing time from 0 to 30 s.

**Figure 14(a)** and **(b)** displays the FE-SEM photos of the fabricated Cu mesh and the CNTcoated Cu mesh along with the enlarged photo of the surface morphology of the CNT coating layer, respectively. It can be clearly seen that CNTs were used to selectively coat only the surfaces where the Cu mesh patterns had been formed. The thickness profiles of the CNT-coated Cu meshes measured before and after post-washing are shown in the inset of **Figure 14(c)**. The thickness of the Cu mesh was approximately 106 nm, and it was increased to approximately 913 nm after the Cu mesh was coated with CNTs via the 15 s EPD process. Meanwhile, the thickness of the CNT-coated Cu mesh was considerably decreased as the post-washing process progressed, and it was approximately 237 nm after 30 s post-washing, as shown in **Figure 14(c)**. This was because some of the CNTs were detached from the Cu mesh during the washing process. All of the Cu mesh specimens considered in this study are summarized in **Table 2**.

detached during the bending test, then the PEDOT:PSS layer on the CNTs is also detached and thus, the resistance of the electrodes will inevitably increase. In contrast, in the case of the hybrid electrodes fabricated via spin coating, the increase in resistance may be insignificant even if they experience the detachment of CNTs due to bending because the conductive

**Figure 13.** (a) The changes in the sheet resistances due to bending tests (up to 30,000 times), measured from the various

) and PEDOT:PSS-coated CNTs (i.e., S<sup>1</sup>

(b), (c), and (d) indicate their images obtained before bending (left photos), and (e), (f), and (g) obtained after bending

, S<sup>3</sup> , E<sup>1</sup> , and E<sup>3</sup>

). [(b)–(g)] The changes of

and E3

). Here,

) and hybrids (S<sup>3</sup>

Metal meshes have been developed as the alternative transparent electrodes to replace the conventional ITO due to their low electric sheet resistance and high visible transmittance. However, metal meshes have a relatively high reflectance generated by metals, and several metals, such as gold (Au) and copper (Cu), are generally tinged with specific colors, leading to a visibility problem. The most commonly used materials for the metal-based TCEs are silver (Ag) and Cu. Compared with Ag, Cu is more abundant and less expensive, but it has higher resistivity. Also, Cu is generally tinged with a red-orange color [36], requiring it to be coated with black materials capable of absorbing light effectively. CNTs are known as excellent absorbers of visible light [37] and have advantages in terms of chemical stability, thermal and electric conductivity, mechanical strength, and flexibility. To improve the visibility of metal mesh electrodes, the authors recently introduced hybrid-type TCEs fabricated via the

**5. Effects of CNT-coating and washing on properties of copper** 

PEDOT:PSS layers still remain.

electrodes such as CNTs (i.e., C<sup>3</sup>

224 Carbon Nanotubes - Recent Progress

(right photos) of 30,000 times.

and C7

FE-SEM surface morphologies due to bending, measured from the electrodes of CNTs (C<sup>3</sup>

combination of metal meshes with CNTs [38].

**meshes**

**Figure 14.** [(a) and (b)] The FE-SEM photos for a pristine Cu mesh with a 200 μm line spacing and a 10 μm line width and a CNT-coated Cu mesh including the surface morphology of the CNT coating layer. (c) The change in the thickness of the CNT-coated Cu mesh as a function of the post-washing time and the thickness profiles (inset) of the CNT-coated Cu meshes measured before and after post-washing (30 s).


**Table 2.** The sample ID's considered in this study, along with their preparation conditions and the summary of measured quantities.

**Figure 15(a)**–**(c)** shows the results of the sheet resistance, transmittance, and reflectance measured from the pristine Cu mesh and the CNT-coated Cu meshes before and after post-washing, in terms of the CNT's thicknesses. The sheet resistance and transmittance of the Cu meshes were hardly affected by the CNT coating and the CNT's thickness, and was also marginally changed by post-washing. For all of the specimens, their sheet resistance and transmittance values were in the range of 20.2–21.5 Ω/□ and 89.8–90.7%, respectively. On the other hand, the reflectance of the Cu mesh was significantly reduced by the CNT coating. This indicated that the CNT coating layer played the role of suppressing the visible light reflected from the Cu mesh. In **Figure 15(c)**, it is noted that the reflectance of the CNT-coated Cu mesh was further reduced after it under-

**Figure 16.** (Right) (a) The reflectance spectra of primitive Cu-mesh (p-Cu), and CNT-coated Cu meshes before washing (CCu-3) and after washing (W-CCu-3). (b) The estimated chromatic parameters of redness (a\*) and yellowness (b\*) for

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http://dx.doi.org/10.5772/intechopen.72002

227

Generally, the color property of a metal mesh is closely related to its reflectance spectrum, from which the chromatic parameters, redness (a\*) and yellowness (b\*), can be determined. Both the a\* and b\* components being closer to zero indicate that the metal mesh appears nearly colorless. **Figure 16(a)** shows the reflectance spectra measured from the p-Cu, CCu-3, and W-CCu-3 specimens, and **Figure 16(b)** shows the redness and yellowness values estimated from all of the specimens. The pristine Cu mesh revealed a stair-like spectrum where the level of reflectance was demarcated at a wavelength of around 560 nm, and its redness

went post-washing (e.g., 7.89% for C-Cu-3 and 7.37% for W-CCu-3, respectively).

all the specimens fabricated in this study.

**Figure 15.** (Left) The results of: (a) sheet resistance, (b) transmittance, and (c) reflectance, measured from pristine Cu mesh (denoted by "p-Cu"), and CNT-coated Cu-meshes before washing and after 30 s post-washing (denoted by "CCux" and "W-CCu-x" ("x" = 1, 2, and 3), respectively), in terms of the CNT's thicknesses.

Applications of Carbon Nanotubes to Flexible Transparent Conductive Electrodes http://dx.doi.org/10.5772/intechopen.72002 227

**Figure 16.** (Right) (a) The reflectance spectra of primitive Cu-mesh (p-Cu), and CNT-coated Cu meshes before washing (CCu-3) and after washing (W-CCu-3). (b) The estimated chromatic parameters of redness (a\*) and yellowness (b\*) for all the specimens fabricated in this study.

**Figure 15(a)**–**(c)** shows the results of the sheet resistance, transmittance, and reflectance measured from the pristine Cu mesh and the CNT-coated Cu meshes before and after post-washing, in terms of the CNT's thicknesses. The sheet resistance and transmittance of the Cu meshes were hardly affected by the CNT coating and the CNT's thickness, and was also marginally changed by post-washing. For all of the specimens, their sheet resistance and transmittance values were in the range of 20.2–21.5 Ω/□ and 89.8–90.7%, respectively. On the other hand, the reflectance of the Cu mesh was significantly reduced by the CNT coating. This indicated that the CNT coating layer played the role of suppressing the visible light reflected from the Cu mesh. In **Figure 15(c)**, it is noted that the reflectance of the CNT-coated Cu mesh was further reduced after it underwent post-washing (e.g., 7.89% for C-Cu-3 and 7.37% for W-CCu-3, respectively).

Generally, the color property of a metal mesh is closely related to its reflectance spectrum, from which the chromatic parameters, redness (a\*) and yellowness (b\*), can be determined. Both the a\* and b\* components being closer to zero indicate that the metal mesh appears nearly colorless. **Figure 16(a)** shows the reflectance spectra measured from the p-Cu, CCu-3, and W-CCu-3 specimens, and **Figure 16(b)** shows the redness and yellowness values estimated from all of the specimens. The pristine Cu mesh revealed a stair-like spectrum where the level of reflectance was demarcated at a wavelength of around 560 nm, and its redness

**Figure 15.** (Left) The results of: (a) sheet resistance, (b) transmittance, and (c) reflectance, measured from pristine Cu mesh (denoted by "p-Cu"), and CNT-coated Cu-meshes before washing and after 30 s post-washing (denoted by "CCu-

x" and "W-CCu-x" ("x" = 1, 2, and 3), respectively), in terms of the CNT's thicknesses.

**Sample EPD time (min) Washing time (s) Thickness of** 

226 Carbon Nanotubes - Recent Progress

quantities.

**CNTs (nm)**

**Table 2.** The sample ID's considered in this study, along with their preparation conditions and the summary of measured

Cu(p) — — 106.3 90.71 11.83 1.91 1.92 CCu-1 5 — 323.6 90.24 8.91 1.54 1.88 CCu-2 10 — 546.2 89.80 8.44 1.15 1.61 CCu-3 15 — 807.0 89.79 7.89 0.88 1.13 W-CCu-1 5 30 95.6 90.18 8.51 1.44 1.51 W-CCu-2 10 30 110.9 89.98 7.79 0.96 0.58 W-CCu-3 15 30 130.7 89.83 7.37 0.16 −0.55

**Tvis (%) Rvis (%) a\* b\***

and yellowness values were approximately 1.91 and 1.92, respectively. This may be the cause of the red-orange tint of the pristine Cu mesh. In contrast, such disparity in reflectance nearly disappeared when the Cu meshes were coated with CNTs, and the flat-type spectra with much lower reflectance levels were observed for the CNT-coated Cu meshes. Both a\* and b\* values considerably decreased as the CNT's thickness was increased, and further decreased when the CNT-coated Cu meshes underwent post-washing. For instance, the a\* and b\* values of the CCu-3 mesh were approximately 0.88 and 1.13, respectively, and those of the W-CCu-3 mesh were approximately 0.16 and −0.55, respectively.

used as the dispersant for the CNT suspension. On the other hand, it was discovered that the Mg content was almost negligible in the W-CCu-3 specimen, indicating that most of the Mg atoms were removed by post-washing. **Figure 17(c)** compares the XPS Mg 2p spectra of the CCu-3 and W-CCu-3 specimens. The XPS peak corresponding to Mg-O/OH [39] was observed at around 49 eV, and the peak intensity of the W-CuC-3 specimen was much smaller than that of the CuC-3 specimen, indicating that the Mg component was removed by post-washing. Accordingly, it was suggested that the lower reflectance and the smaller chromatic parameters, which were observed when the CNT-coated Cu meshes underwent

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229

CNTs have excellent chemical stability, thermal and electrical conductivity (high intrinsic conductivity), mechanical strength, flexibility, solution processability, and potential for production at a low cost. Based on these advantages, the CNT-based TCEs are presented. In Section 3, it was illustrated that the adhesion of the CNTs was remarkably improved after the surface modification via corona pretreatment of the PET substrates. Then, in Sections 4 and 5, the hybrid-type TCEs, which can be commercialized in various applications, were fabricated and their characteristics were demonstrated. In particular, the studies about improving the electrical conductivity and transmittance of CNTs-TCEs may warrant increased interest. Finally, in Section 5, the metal-based TCEs coated with CNTs were considered as an effective structure to resolve the high reflectance generated by the intrinsic properties of metals. Until now, the TCEs fabricated using only CNTs had insufficient properties for applications to electronic devices. Various hybrid-types of CNT-based TCEs, however, could have potential in the application to next-generation flexible and stretchable

Department of Electronic Systems Engineering, Hanyang University, Ansan, Gyeonggi-do,

[1] Cann M, Large MJ, Henley SJ, Milne D, Sato T, Chan H, Jurewicz I, Dalton AB. High performance transparent multi-touch sensors based on silver nanowires. Materials Today

Communications. 2016;**7**:42-50. DOI: 10.1016/j.mtcomm.2016.03.005

post-washing, resulted from the removal of the residual Mg element.

**6. Conclusion**

**Author details**

Republic of Korea

**References**

electronics to overcome various issues.

\*Address all correspondence to: jinsp@hanyang.ac.kr

Bu-Jong Kim and Jin-Seok Park\*

In order to discover why the CNT-coated Cu meshes exhibited such lower reflectance and smaller a\* and b\* after they underwent post-washing, the changes in their atomic components and surface chemistry were investigated. **Figure 17(a)** and **(b)** shows the Auger electron spectroscopy (AES) measurement results, which indicate the relative atomic contents of C, O, Mg, and Cu for the CNT-coated Cu meshes before and after post-washing, respectively, as a function of the sputtering time (0–25 min) used for the AES analysis. For the CCu-3 specimen as shown in **Figure 17(a)**, the C atom content was always greater than the Cu atom content, while for the W-CCu-3 specimen as shown in **Figure 17(b)**, the Cu atom content exceeded the C atom content as the sputtering time was around 15 min, indicating that the layer was changed from a CNT region to a Cu mesh region. This was attributed to the decrease in the Cu mesh's thickness due to the detachment of the CNTs after post-washing. It was also observed that the CCu-3 specimen contained the Mg atoms with an atomic content higher than about 10%. This was because magnesium nitrate (Mg(NO<sup>3</sup> )2 6H<sup>2</sup> O) was

**Figure 17.** The relative atomic contents (measured by AES) of C, O, Mg, and Cu incorporated in the CNT-coated Cu mesh: (a) before washing (CCu-3) and (b) after washing (W-CCu-3). (c) The XPS Mg 2p spectra for CCu-3 and W-CCu-3 specimens.

used as the dispersant for the CNT suspension. On the other hand, it was discovered that the Mg content was almost negligible in the W-CCu-3 specimen, indicating that most of the Mg atoms were removed by post-washing. **Figure 17(c)** compares the XPS Mg 2p spectra of the CCu-3 and W-CCu-3 specimens. The XPS peak corresponding to Mg-O/OH [39] was observed at around 49 eV, and the peak intensity of the W-CuC-3 specimen was much smaller than that of the CuC-3 specimen, indicating that the Mg component was removed by post-washing. Accordingly, it was suggested that the lower reflectance and the smaller chromatic parameters, which were observed when the CNT-coated Cu meshes underwent post-washing, resulted from the removal of the residual Mg element.

#### **6. Conclusion**

and yellowness values were approximately 1.91 and 1.92, respectively. This may be the cause of the red-orange tint of the pristine Cu mesh. In contrast, such disparity in reflectance nearly disappeared when the Cu meshes were coated with CNTs, and the flat-type spectra with much lower reflectance levels were observed for the CNT-coated Cu meshes. Both a\* and b\* values considerably decreased as the CNT's thickness was increased, and further decreased when the CNT-coated Cu meshes underwent post-washing. For instance, the a\* and b\* values of the CCu-3 mesh were approximately 0.88 and 1.13, respectively, and those of the W-CCu-3

In order to discover why the CNT-coated Cu meshes exhibited such lower reflectance and smaller a\* and b\* after they underwent post-washing, the changes in their atomic components and surface chemistry were investigated. **Figure 17(a)** and **(b)** shows the Auger electron spectroscopy (AES) measurement results, which indicate the relative atomic contents of C, O, Mg, and Cu for the CNT-coated Cu meshes before and after post-washing, respectively, as a function of the sputtering time (0–25 min) used for the AES analysis. For the CCu-3 specimen as shown in **Figure 17(a)**, the C atom content was always greater than the Cu atom content, while for the W-CCu-3 specimen as shown in **Figure 17(b)**, the Cu atom content exceeded the C atom content as the sputtering time was around 15 min, indicating that the layer was changed from a CNT region to a Cu mesh region. This was attributed to the decrease in the Cu mesh's thickness due to the detachment of the CNTs after post-washing. It was also observed that the CCu-3 specimen contained the Mg atoms with an atomic

content higher than about 10%. This was because magnesium nitrate (Mg(NO<sup>3</sup>

**Figure 17.** The relative atomic contents (measured by AES) of C, O, Mg, and Cu incorporated in the CNT-coated Cu mesh: (a) before washing (CCu-3) and (b) after washing (W-CCu-3). (c) The XPS Mg 2p spectra for CCu-3 and W-CCu-3 specimens.

)2 6H<sup>2</sup>

O) was

mesh were approximately 0.16 and −0.55, respectively.

228 Carbon Nanotubes - Recent Progress

CNTs have excellent chemical stability, thermal and electrical conductivity (high intrinsic conductivity), mechanical strength, flexibility, solution processability, and potential for production at a low cost. Based on these advantages, the CNT-based TCEs are presented. In Section 3, it was illustrated that the adhesion of the CNTs was remarkably improved after the surface modification via corona pretreatment of the PET substrates. Then, in Sections 4 and 5, the hybrid-type TCEs, which can be commercialized in various applications, were fabricated and their characteristics were demonstrated. In particular, the studies about improving the electrical conductivity and transmittance of CNTs-TCEs may warrant increased interest. Finally, in Section 5, the metal-based TCEs coated with CNTs were considered as an effective structure to resolve the high reflectance generated by the intrinsic properties of metals. Until now, the TCEs fabricated using only CNTs had insufficient properties for applications to electronic devices. Various hybrid-types of CNT-based TCEs, however, could have potential in the application to next-generation flexible and stretchable electronics to overcome various issues.

#### **Author details**

Bu-Jong Kim and Jin-Seok Park\*

\*Address all correspondence to: jinsp@hanyang.ac.kr

Department of Electronic Systems Engineering, Hanyang University, Ansan, Gyeonggi-do, Republic of Korea

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**Chapter 12**

**Provisional chapter**

**Production of Water Dispersible Carbon Nanotubes**

Polymer wrapping methods have been used to disperse carbon nanotube (CNT) by using gelatin, an environment-friendly and easily decomposable biopolymer. The amino acid chain of gelatin becomes immobilized by the physical adsorption in the side wall of the CNTs through hydrophobic-hydrophobic interaction and results in the untangling of the CNT bundles. The dispersed solution remains stable for more than a month. Furthermore, this technique does not affect the physical properties of CNTs while enabling their dispersion in aqueous solutions. In addition, gelatin can be easily removed from the nanotubes after the dispersion of nanotubes by heating in water and filtration. Gelatin-dispersed CNTs are homogeneously mixed with the cellulose suspension and dried at room temperature to produce CNT/cellulose composite paper sheet. Adding multiwalled carbon nanotubes (MWNTs) in composite improves the mechanical, thermal, and electrical properties of cellulose. SEM investigation confirms the homogeneous distribution of MWNTs in the cellulose, which can be attributed to the improvement of its characteristics. Both sides of the CNT/cellulose sheet show uniform electrical conductivity, which is enhanced by increasing the MWNTs' content. IR image of the sheet clearly shows the temperature homogeneity of the surface. Thermal stability and the flame retardancy of the sheet are also found to be improved. The sheet has also strong absorbing of electromagnetic waves,

**Production of Water Dispersible Carbon Nanotubes** 

DOI: 10.5772/intechopen.70543

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Carbon nanotubes (CNTs) are realized to have a tremendous potential in many fields of applications because of their associated admirable chemical and physical properties [1, 2]. These molecular-scale

which make them important for microwave technology applications.

**Keywords:** carbon nanotube, polymer wrapping, water dispersibility, gelatin,

**and Nanotube/Cellulose Composite**

**and Nanotube/Cellulose Composite**

Kazi Hanium Maria and Tetsu Mieno

Kazi Hanium Maria and Tetsu Mieno

http://dx.doi.org/10.5772/intechopen.70543

CNT/cellulose composite

**1. Introduction**

**Abstract**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Provisional chapter**

#### **Production of Water Dispersible Carbon Nanotubes and Nanotube/Cellulose Composite and Nanotube/Cellulose Composite**

**Production of Water Dispersible Carbon Nanotubes** 

DOI: 10.5772/intechopen.70543

Kazi Hanium Maria and Tetsu Mieno Kazi Hanium Maria and Tetsu Mieno Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70543

#### **Abstract**

Polymer wrapping methods have been used to disperse carbon nanotube (CNT) by using gelatin, an environment-friendly and easily decomposable biopolymer. The amino acid chain of gelatin becomes immobilized by the physical adsorption in the side wall of the CNTs through hydrophobic-hydrophobic interaction and results in the untangling of the CNT bundles. The dispersed solution remains stable for more than a month. Furthermore, this technique does not affect the physical properties of CNTs while enabling their dispersion in aqueous solutions. In addition, gelatin can be easily removed from the nanotubes after the dispersion of nanotubes by heating in water and filtration. Gelatin-dispersed CNTs are homogeneously mixed with the cellulose suspension and dried at room temperature to produce CNT/cellulose composite paper sheet. Adding multiwalled carbon nanotubes (MWNTs) in composite improves the mechanical, thermal, and electrical properties of cellulose. SEM investigation confirms the homogeneous distribution of MWNTs in the cellulose, which can be attributed to the improvement of its characteristics. Both sides of the CNT/cellulose sheet show uniform electrical conductivity, which is enhanced by increasing the MWNTs' content. IR image of the sheet clearly shows the temperature homogeneity of the surface. Thermal stability and the flame retardancy of the sheet are also found to be improved. The sheet has also strong absorbing of electromagnetic waves, which make them important for microwave technology applications.

**Keywords:** carbon nanotube, polymer wrapping, water dispersibility, gelatin, CNT/cellulose composite

#### **1. Introduction**

Carbon nanotubes (CNTs) are realized to have a tremendous potential in many fields of applications because of their associated admirable chemical and physical properties [1, 2]. These molecular-scale

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

tubes of graphitic carbon are among the stiffest and strongest fibers known and have remarkable optical, electronic, and mechanical properties and many other unique characteristics [3]. For these reasons, there are already many studies using CNTs for electronic devices, including the use of CNTs as transistor channels, electrical wiring for lighting solutions industries (LSIs), and electron emitters [4–8]. Nevertheless, the as-produced material is extremely difficult to process. CNTs have a tendency to aggregate owing to the van der Waals attractive interaction between their sidewalls [9]. This tube-to-tube contact results in the hydrophobic nature of CNTs, which is responsible for their poor solubility in water and also incompatibility with a majority of solvents [10]. As a result, CNTs are precipitated in solvent. This lack of solubility and the difficulty of manipulating them in solvents limit the development of CNT-based devices or composites of interest for new applications. In order to obtain fine dispersion in the selected solutions, especially water, it is important to break the cohesion of aggregated CNTs. Development of efficient processes and chemical treatments that are able to control the quality of the CNT samples and to induce both their dispersion and partial or complete de-bundling remains highly challenging. Chemical modifications are the well-established method up to now, though this method modifies the desirable properties of CNTs [11]. On the other hand, polymer wrapping methods can preserve the intrinsic properties of CNTs. Usually, polymers and surfactants wrap the nanotube surface to enable the CNT dispersion in aqueous solutions [12, 13]. Also, this method produces impurity in the sample because wrapped polymers and surfactants become stuck in the sample after dispersion. Therefore, to overcome this problem, easily decomposable polymer should be considered for the dispersion of the CNTs. Here, in this chapter, various dispersion methods will be reviewed and an efficient procedure for the water dispersion will be reported by using gelatin, an environment-friendly material to wrap the surface of single walled carbon nanotubes (SWNTs) enabling their dispersion in water.

or bundles of only a few tubes. Dissolution of CNT in water has been facilitated by suitable

Production of Water Dispersible Carbon Nanotubes and Nanotube/Cellulose Composite

Generally, there are two mechanisms to disperse nanotube. In the mechanical mechanism, nanotube disentanglement is done by sonication and high-shear mixing. Sonication cut the longer nanotube into pieces to disperse CNT easily into water. As a result, the aspect ratio of nanotube also decreases in this way. Besides this, the stability of the dispersion is poor. Nevertheless, the chemical mechanism incorporates both covalent and non-covalent methods. In covalent functionalization, chemical modifications can be performed at the sidewall site,

sites via amidation or esterification reactions of carboxylic residues obtained on CNTs [15]. Usually, this sidewall modification or defect functionalization are the continuation of CNT

duce oxygenated functional groups such as carboxylic acid, ketone, alcohol, and ester groups. Therefore, they produce different types of chemical moieties onto the ends and defect sites of these tubes, which in turn significantly change the mechanical properties of CNTs by destroying the perfect structure of CNTs as well as disruption of π electron system in nanotubes, which is detrimental to transport properties of CNTs. Furthermore, strong oxidants or concentrated acids are environmentally unfriendly, which are often used for CNT functionalization.

A non-covalent mechanism associated with the adsorption of the chemical moieties onto the nanotube surface, either via π–π stacking interaction such as in DNA, uncharged surfactants, etc., or through coulomb attraction in the case of charged chemical moieties [16]. Compared to covalent functionalizations, non-covalent functionalizations are more capable of preserving

**Figure 1.** Different type of functionalization possibilities of CNTs: (a) covalent sidewall functionalization, (b) covalent defect sidewall functionalization, (c) non-covalent adsorption of surfactants, and (d) wrapping of polymers. Adapted

, ozone, and reactive plasma, which open the tubes and consequently pro-

hybridized carbon atoms [14] or at the defect

http://dx.doi.org/10.5772/intechopen.70543

237

or a mixture of them, or with strong oxidants

into sp3

, H2 SO4

enhancement of the surface of the nanotubes.

which causes the transformation of sp2

treatment with strong acid such as HNO3

**Figure 1** shows the different kinds of CNT functionalizations.

such as KMnO4

from Ávila-Orta et al. [16].

Next-generation computer devices, consumer electronics, wireless LAN devices, wireless antenna systems, cellular phone systems, high-performance sportswear, wearable displays, new classes of portable power, and embedded health monitoring devices are few portable device applications that have attracted a large interest in the scientific community because of their remarkable utilities [4–8]. The design of soft portable electronic equipment requires the development of a new class of materials with an array of novel functionalities, such as flexibility, stretch ability, and light-weight. Generally, this new class of material is termed as composites, which allow for many applications and designs previously impossible with traditional electronics technology. Nowadays, composites based on carbon nanotubes (CNTs) are very promising for the continuous growth of telecommunication market due to their many unique chemical and physical properties. Here, we introduced a convenient production method for the CNT composite, where cellulose is employing with multiwalled carbon nanotube (MWNTs) to enlarge the cellulose functionalities. This approach is illustrated by the blending of CNT with cellulose and characterization studies show that CNTs enable them to show outstanding electrical properties and thermal stabilities.

#### **2. Water dispersibility of CNTs**

Because of strong van der Waals attraction, the CNTs pack into bundles that aggregate into tangled networks, and thus do not disperse well in the water in their pristine state [9]. For optimal performance in most applications, the CNT should be separated into individual tubes or bundles of only a few tubes. Dissolution of CNT in water has been facilitated by suitable enhancement of the surface of the nanotubes.

tubes of graphitic carbon are among the stiffest and strongest fibers known and have remarkable optical, electronic, and mechanical properties and many other unique characteristics [3]. For these reasons, there are already many studies using CNTs for electronic devices, including the use of CNTs as transistor channels, electrical wiring for lighting solutions industries (LSIs), and electron emitters [4–8]. Nevertheless, the as-produced material is extremely difficult to process. CNTs have a tendency to aggregate owing to the van der Waals attractive interaction between their sidewalls [9]. This tube-to-tube contact results in the hydrophobic nature of CNTs, which is responsible for their poor solubility in water and also incompatibility with a majority of solvents [10]. As a result, CNTs are precipitated in solvent. This lack of solubility and the difficulty of manipulating them in solvents limit the development of CNT-based devices or composites of interest for new applications. In order to obtain fine dispersion in the selected solutions, especially water, it is important to break the cohesion of aggregated CNTs. Development of efficient processes and chemical treatments that are able to control the quality of the CNT samples and to induce both their dispersion and partial or complete de-bundling remains highly challenging. Chemical modifications are the well-established method up to now, though this method modifies the desirable properties of CNTs [11]. On the other hand, polymer wrapping methods can preserve the intrinsic properties of CNTs. Usually, polymers and surfactants wrap the nanotube surface to enable the CNT dispersion in aqueous solutions [12, 13]. Also, this method produces impurity in the sample because wrapped polymers and surfactants become stuck in the sample after dispersion. Therefore, to overcome this problem, easily decomposable polymer should be considered for the dispersion of the CNTs. Here, in this chapter, various dispersion methods will be reviewed and an efficient procedure for the water dispersion will be reported by using gelatin, an environment-friendly material to wrap the surface of single walled

Next-generation computer devices, consumer electronics, wireless LAN devices, wireless antenna systems, cellular phone systems, high-performance sportswear, wearable displays, new classes of portable power, and embedded health monitoring devices are few portable device applications that have attracted a large interest in the scientific community because of their remarkable utilities [4–8]. The design of soft portable electronic equipment requires the development of a new class of materials with an array of novel functionalities, such as flexibility, stretch ability, and light-weight. Generally, this new class of material is termed as composites, which allow for many applications and designs previously impossible with traditional electronics technology. Nowadays, composites based on carbon nanotubes (CNTs) are very promising for the continuous growth of telecommunication market due to their many unique chemical and physical properties. Here, we introduced a convenient production method for the CNT composite, where cellulose is employing with multiwalled carbon nanotube (MWNTs) to enlarge the cellulose functionalities. This approach is illustrated by the blending of CNT with cellulose and characterization studies show that CNTs enable them to

Because of strong van der Waals attraction, the CNTs pack into bundles that aggregate into tangled networks, and thus do not disperse well in the water in their pristine state [9]. For optimal performance in most applications, the CNT should be separated into individual tubes

carbon nanotubes (SWNTs) enabling their dispersion in water.

236 Carbon Nanotubes - Recent Progress

show outstanding electrical properties and thermal stabilities.

**2. Water dispersibility of CNTs**

Generally, there are two mechanisms to disperse nanotube. In the mechanical mechanism, nanotube disentanglement is done by sonication and high-shear mixing. Sonication cut the longer nanotube into pieces to disperse CNT easily into water. As a result, the aspect ratio of nanotube also decreases in this way. Besides this, the stability of the dispersion is poor. Nevertheless, the chemical mechanism incorporates both covalent and non-covalent methods. In covalent functionalization, chemical modifications can be performed at the sidewall site, which causes the transformation of sp2 into sp3 hybridized carbon atoms [14] or at the defect sites via amidation or esterification reactions of carboxylic residues obtained on CNTs [15]. Usually, this sidewall modification or defect functionalization are the continuation of CNT treatment with strong acid such as HNO3 , H2 SO4 or a mixture of them, or with strong oxidants such as KMnO4 , ozone, and reactive plasma, which open the tubes and consequently produce oxygenated functional groups such as carboxylic acid, ketone, alcohol, and ester groups. Therefore, they produce different types of chemical moieties onto the ends and defect sites of these tubes, which in turn significantly change the mechanical properties of CNTs by destroying the perfect structure of CNTs as well as disruption of π electron system in nanotubes, which is detrimental to transport properties of CNTs. Furthermore, strong oxidants or concentrated acids are environmentally unfriendly, which are often used for CNT functionalization. **Figure 1** shows the different kinds of CNT functionalizations.

A non-covalent mechanism associated with the adsorption of the chemical moieties onto the nanotube surface, either via π–π stacking interaction such as in DNA, uncharged surfactants, etc., or through coulomb attraction in the case of charged chemical moieties [16]. Compared to covalent functionalizations, non-covalent functionalizations are more capable of preserving

**Figure 1.** Different type of functionalization possibilities of CNTs: (a) covalent sidewall functionalization, (b) covalent defect sidewall functionalization, (c) non-covalent adsorption of surfactants, and (d) wrapping of polymers. Adapted from Ávila-Orta et al. [16].

intrinsic properties of CNTs when improving exfoliation and interfacial bonding. Though, the concern of this approach is the presence of physically bound modifier molecules on the surface, such anomalies can be removed easily by washing. Besides, this method increases the solubility of CNTs, making them less toxic and preserving the electrical properties of CNTs. Polymer wrapping method are widely used non-covalent approach that attributed to the physical association of polymers with CNTs. Polymers can wrap around CNTs, forming supramolecular complexes and most of the nanotubes are found as single tubes associated with at most a single layer of polymer. A smaller number of aggregates consisting of more than one nanotube are also remained at the dispersion. The advantage is that a stable suspension is observed for months. Stabilizing dispersions of carbon nanotubes (CNTs) is a major challenge, since it is a prerequisite to high performance of CNT in a large field of applications.

> There are two types of gelatin depending on acid or alkaline treatment. Type A gelatin (isoionic point of 6–9) is collected from acid-treated collagen, whereas alkali-treated precursor is responsible for manufacture type B (isoionic point of 5) gelatin. Usually type A gelatin

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The properties of gelatin are influenced by the intrinsic factors such as the source, age of the animal, and type of collagen. Gelatins produced from cattle and pigs' bones and skins are mostly used in commercial. Recently, gelatin from fish and poultry has received considerable attention and may serve as important commercial gelatin sources into the future [20]. This is because proteins derived from these sources may possess similar functional properties like mammalian. Moreover, fish gelatin is free from bovine spongiform encephalopathy (BSE)

Gelatin macromolecules can display a wide variety of conformations under specific conditions such as temperature, solvent, and pH. It can form a physical thermos-reversible gel at room temperature or below 40°C for relatively low concentrations. If the temperature is raised, the gel "melts," becoming liquid again [19]. This reversible conformational nature of gelatin makes it a suitable dispersing agent of carbon nanotube dispersion, which can be removed thermally. Gelatin has different amino acid composition and the molecular weight distribution based on their type. The gel strength, viscosity, pH, and melting point of gelatin depend on their structure and the amino acid chain [18–20]. Generally, gelatin with the higher number of amino acids chains have higher gel strength, which in turn increases the melting point. Over a wide range of pH values, gelatin behaves like polyampholyte, where the gelatin macromolecule has mixed anion and cation character, which also has uncharged hydrophilic and hydrophobic groups [20]. This in turn makes gelatin molecules soluble in water but may also cause

Pristine SWNT powder (Nanostructure & Amorphous Materials Inc., diameter = 1–2 nm, length = 5–30 μm, purity = 90%), MWNTs (Sigma-Aldrich, outer diameter = 10–30 nm, inner

and type B gelatin are successively extracted from pigskin and beef skin [19].

**Figure 2.** A typical Structural unit of gelatin. Adapted from Ref [18].

which is commonly called mad cow disease and more religiously acceptable.

them to be adsorbed at charged or hydrophobic surfaces.

**3.3. Preparation of gelatin dispersed CNTs**

**3.2. Properties of gelatin**

#### **3. Methodology of gelatin-wrapped CNTs**

Water dispersible CNTs without changing their physical properties are a significant challenge and a prerequisite for their applications [4–8]. For this reasons, polymer wrapping method can be the best option for CNT dispersion, because, in this method, polymer and surfactants are used to exfoliate the nanotube from the nanotube bundles by wrapping the individual nanotube surface while enabling their dispersion in aqueous solutions [12, 13]. As a result of polymer wrapping, polymers and surfactants stay in the sample synchronously to intact the physical properties of CNTs. The remaining polymers and surfactants create impurity in the samples, which in turn influence the CNT application. Therefore, easily decomposable polymer can be used to get rid of this issue as well as to produce safe nanotube materials for the nano-bioscience and technology. Here, we used the biocompatible, non-immunogenic gelatin to wrap the surface of CNTs enabling their dispersion in water [17]. Moreover, gelatin is an environment-friendly material and easy to remove from the CNTs by heating in water and filtering. This method is safe compared to the other methods available in the literature, as no hazardous reagents are used here.

#### **3.1. Molecular structure of gelatin**

Gelatin has a noticeable property of forming elastic gels at room temperature for relatively low concentrations (a few per cent of gelatin in water). It is a denatured collagen and contains different amino acid content. Collagen, which is the major protein present in animal bones, skin, and connective tissue, consists of rigid bar-like molecules that arranged in fibers are interconnected by covalent bonds. These molecules have three polypeptide chains such as glycine, proline, and 4-hydroxyproline, and they are arranged in a triple helix that is stabilized by hydrogen and hydrophobic bonds. Gelatin is a mixture of α-chains (one polymer/ single chain), β-chains (two α-chains covalently cross-linked), and γ-chains (three covalently cross-linked α-chains) [18]. Gelatin also has a mixture of single and double unfolded chains of hydrophilic character. The physical properties of gelatin depend on amino acid compositions, relative content of α-chains, β- or γ-components, and higher molecular weight aggregates. A typical structure of gelatin is -Ala-Gly-Pro-Arg-Gly-Glu-4Hyp-Gly-Pro- which is shown in **Figure 2**.

Production of Water Dispersible Carbon Nanotubes and Nanotube/Cellulose Composite http://dx.doi.org/10.5772/intechopen.70543 239

**Figure 2.** A typical Structural unit of gelatin. Adapted from Ref [18].

There are two types of gelatin depending on acid or alkaline treatment. Type A gelatin (isoionic point of 6–9) is collected from acid-treated collagen, whereas alkali-treated precursor is responsible for manufacture type B (isoionic point of 5) gelatin. Usually type A gelatin and type B gelatin are successively extracted from pigskin and beef skin [19].

#### **3.2. Properties of gelatin**

intrinsic properties of CNTs when improving exfoliation and interfacial bonding. Though, the concern of this approach is the presence of physically bound modifier molecules on the surface, such anomalies can be removed easily by washing. Besides, this method increases the solubility of CNTs, making them less toxic and preserving the electrical properties of CNTs. Polymer wrapping method are widely used non-covalent approach that attributed to the physical association of polymers with CNTs. Polymers can wrap around CNTs, forming supramolecular complexes and most of the nanotubes are found as single tubes associated with at most a single layer of polymer. A smaller number of aggregates consisting of more than one nanotube are also remained at the dispersion. The advantage is that a stable suspension is observed for months. Stabilizing dispersions of carbon nanotubes (CNTs) is a major challenge, since it is a prerequisite to high performance of CNT in a large field of applications.

Water dispersible CNTs without changing their physical properties are a significant challenge and a prerequisite for their applications [4–8]. For this reasons, polymer wrapping method can be the best option for CNT dispersion, because, in this method, polymer and surfactants are used to exfoliate the nanotube from the nanotube bundles by wrapping the individual nanotube surface while enabling their dispersion in aqueous solutions [12, 13]. As a result of polymer wrapping, polymers and surfactants stay in the sample synchronously to intact the physical properties of CNTs. The remaining polymers and surfactants create impurity in the samples, which in turn influence the CNT application. Therefore, easily decomposable polymer can be used to get rid of this issue as well as to produce safe nanotube materials for the nano-bioscience and technology. Here, we used the biocompatible, non-immunogenic gelatin to wrap the surface of CNTs enabling their dispersion in water [17]. Moreover, gelatin is an environment-friendly material and easy to remove from the CNTs by heating in water and filtering. This method is safe compared to the other methods available in the literature, as no

Gelatin has a noticeable property of forming elastic gels at room temperature for relatively low concentrations (a few per cent of gelatin in water). It is a denatured collagen and contains different amino acid content. Collagen, which is the major protein present in animal bones, skin, and connective tissue, consists of rigid bar-like molecules that arranged in fibers are interconnected by covalent bonds. These molecules have three polypeptide chains such as glycine, proline, and 4-hydroxyproline, and they are arranged in a triple helix that is stabilized by hydrogen and hydrophobic bonds. Gelatin is a mixture of α-chains (one polymer/ single chain), β-chains (two α-chains covalently cross-linked), and γ-chains (three covalently cross-linked α-chains) [18]. Gelatin also has a mixture of single and double unfolded chains of hydrophilic character. The physical properties of gelatin depend on amino acid compositions, relative content of α-chains, β- or γ-components, and higher molecular weight aggregates. A typical structure of gelatin is -Ala-Gly-Pro-Arg-Gly-Glu-4Hyp-Gly-Pro- which is

**3. Methodology of gelatin-wrapped CNTs**

238 Carbon Nanotubes - Recent Progress

hazardous reagents are used here.

**3.1. Molecular structure of gelatin**

shown in **Figure 2**.

The properties of gelatin are influenced by the intrinsic factors such as the source, age of the animal, and type of collagen. Gelatins produced from cattle and pigs' bones and skins are mostly used in commercial. Recently, gelatin from fish and poultry has received considerable attention and may serve as important commercial gelatin sources into the future [20]. This is because proteins derived from these sources may possess similar functional properties like mammalian. Moreover, fish gelatin is free from bovine spongiform encephalopathy (BSE) which is commonly called mad cow disease and more religiously acceptable.

Gelatin macromolecules can display a wide variety of conformations under specific conditions such as temperature, solvent, and pH. It can form a physical thermos-reversible gel at room temperature or below 40°C for relatively low concentrations. If the temperature is raised, the gel "melts," becoming liquid again [19]. This reversible conformational nature of gelatin makes it a suitable dispersing agent of carbon nanotube dispersion, which can be removed thermally.

Gelatin has different amino acid composition and the molecular weight distribution based on their type. The gel strength, viscosity, pH, and melting point of gelatin depend on their structure and the amino acid chain [18–20]. Generally, gelatin with the higher number of amino acids chains have higher gel strength, which in turn increases the melting point. Over a wide range of pH values, gelatin behaves like polyampholyte, where the gelatin macromolecule has mixed anion and cation character, which also has uncharged hydrophilic and hydrophobic groups [20]. This in turn makes gelatin molecules soluble in water but may also cause them to be adsorbed at charged or hydrophobic surfaces.

#### **3.3. Preparation of gelatin dispersed CNTs**

Pristine SWNT powder (Nanostructure & Amorphous Materials Inc., diameter = 1–2 nm, length = 5–30 μm, purity = 90%), MWNTs (Sigma-Aldrich, outer diameter = 10–30 nm, inner diameter = 3–10 nm, length = 1–10 μm, purity >90%), and geletin (Wako 1st Grade, appearance: yellowish-brown and crystalline powder) are used as received to prepare the mixture and the mixture is observed to check the dispersion ability. 5 mg raw carbon soot and 20 mg gelatin were mixed with 20 ml of pure water (Wako Pure Chemical Industries Ltd.). Both MWNT and SWNT soot was used with gelatin to prepare the mixture. An ultrasonic homogenizer equipped with a microtip sonotrode (VC 130, Vibra Cell, Sonics & Materials Inc., *f* = 20 kHz, 6 mmφ probe) was intermittently operated to stir the mixture. The input power of homogenizer was always kept at 20 W and the mixing was for 60 min, where the duty ratio was 60%. The collected gelatin-SWNT and gelatin-MWNT dispersion last above a month at room temperature.

#### **4. Evaluation of dispersibility of the CNTs with gelatin**

The CNTs have a tendency to aggregate owing to the attractive interaction between their sidewalls. CNTs can be hydrated by gelatin, which can wrap around their sidewalls [17], as schematically shown in **Figure 3**.

**4.2. TEM observation of gelatin-CNT dispersion**

dispersion after 1 month.

To observe the dispersion state of the wrapped CNTs in water, one drop of the solution is placed on a copper grid coated with a collodion film for the image observation from the transmission electron microscope (TEM, JEOL JEM1400, accelerating voltage 100 kV). **Figure 5** is a typical TEM image where it is shown that the CNTs are well wrapped with gelatin. It is seen that the gelatin uniformly wrapped around the entire surface of tubes making the SWNTs opaque and surfaces rough as compared with unwrapped SWNTs. The wrapping mechanism of polymer is believed to be driven largely by thermodynamics to eliminate hydrophobic interface between the tubes and the aqueous medium. In case of growth technique of NT, metal particle is used in the electrode as a catalyst to grow SWNTs and then remain attached to the SWNTs as impurity. Usually, amorphous carbon and metal nanoparticles are produced

**Figure 5.** Typical TEM images of (a) pure MWNTs and (b) pure SWNTs. Left side shows the pristine nanotubes and right

side shows the dispersed nanotube bundles in aqueous gelatin solution.

**Figure 4.** Photographs of (a) SWNT dispersion in water immediately after sonication, (b) SWNT precipitate in water after a few hours, (c) SWNT dispersion in gelatin-water solution immediately after sonication, and (d) stable SWNT-gelatin

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#### **4.1. Visual inspection of gelatin-CNT dispersibility**

The resulting hydrophobic nature is responsible for their poor solubility in water, and CNTs are precipitated in water as shown in **Figure 4(a)** and **(b)**. Most polymer wrapping methods depend on the physical adsorption of the polymer on the surface of the CNTs to increase the solubility of the nanotubes.

Gelatin is a natural water-soluble biopolymer that is composed of 18 types of amino acid chains [21, 22]. The amino acid chains of the gelatin wrap around the sidewalls of the CNTs through a hydrophobic-hydrophobic interaction [17, 21, 23]. Gelatin has zwitterionic structure and the amino acid chain of the gelatin has hydrophobic nature. CNTs' surfaces are also hydrophobic. This hydrophobic-hydrophobic interaction may include a hydrophilic group to the CNTs' surface, which in turn disperses CNT into water, as shown in **Figure 4(c)** and **(d)**. The CNT dispersion is found to last over a month.

**Figure 3.** Schematic showing wrapping of SWNTs with gelatin. Adapted from Maria and Mieno [17].

**Figure 4.** Photographs of (a) SWNT dispersion in water immediately after sonication, (b) SWNT precipitate in water after a few hours, (c) SWNT dispersion in gelatin-water solution immediately after sonication, and (d) stable SWNT-gelatin dispersion after 1 month.

#### **4.2. TEM observation of gelatin-CNT dispersion**

diameter = 3–10 nm, length = 1–10 μm, purity >90%), and geletin (Wako 1st Grade, appearance: yellowish-brown and crystalline powder) are used as received to prepare the mixture and the mixture is observed to check the dispersion ability. 5 mg raw carbon soot and 20 mg gelatin were mixed with 20 ml of pure water (Wako Pure Chemical Industries Ltd.). Both MWNT and SWNT soot was used with gelatin to prepare the mixture. An ultrasonic homogenizer equipped with a microtip sonotrode (VC 130, Vibra Cell, Sonics & Materials Inc., *f* = 20 kHz, 6 mmφ probe) was intermittently operated to stir the mixture. The input power of homogenizer was always kept at 20 W and the mixing was for 60 min, where the duty ratio was 60%. The collected gelatin-SWNT and gelatin-MWNT dispersion last above a month at

The CNTs have a tendency to aggregate owing to the attractive interaction between their sidewalls. CNTs can be hydrated by gelatin, which can wrap around their sidewalls [17], as

The resulting hydrophobic nature is responsible for their poor solubility in water, and CNTs are precipitated in water as shown in **Figure 4(a)** and **(b)**. Most polymer wrapping methods depend on the physical adsorption of the polymer on the surface of the CNTs to increase the

Gelatin is a natural water-soluble biopolymer that is composed of 18 types of amino acid chains [21, 22]. The amino acid chains of the gelatin wrap around the sidewalls of the CNTs through a hydrophobic-hydrophobic interaction [17, 21, 23]. Gelatin has zwitterionic structure and the amino acid chain of the gelatin has hydrophobic nature. CNTs' surfaces are also hydrophobic. This hydrophobic-hydrophobic interaction may include a hydrophilic group to the CNTs' surface, which in turn disperses CNT into water, as shown in **Figure 4(c)** and **(d)**. The CNT

**Figure 3.** Schematic showing wrapping of SWNTs with gelatin. Adapted from Maria and Mieno [17].

**4. Evaluation of dispersibility of the CNTs with gelatin**

room temperature.

240 Carbon Nanotubes - Recent Progress

schematically shown in **Figure 3**.

solubility of the nanotubes.

dispersion is found to last over a month.

**4.1. Visual inspection of gelatin-CNT dispersibility**

To observe the dispersion state of the wrapped CNTs in water, one drop of the solution is placed on a copper grid coated with a collodion film for the image observation from the transmission electron microscope (TEM, JEOL JEM1400, accelerating voltage 100 kV). **Figure 5** is a typical TEM image where it is shown that the CNTs are well wrapped with gelatin. It is seen that the gelatin uniformly wrapped around the entire surface of tubes making the SWNTs opaque and surfaces rough as compared with unwrapped SWNTs. The wrapping mechanism of polymer is believed to be driven largely by thermodynamics to eliminate hydrophobic interface between the tubes and the aqueous medium. In case of growth technique of NT, metal particle is used in the electrode as a catalyst to grow SWNTs and then remain attached to the SWNTs as impurity. Usually, amorphous carbon and metal nanoparticles are produced

**Figure 5.** Typical TEM images of (a) pure MWNTs and (b) pure SWNTs. Left side shows the pristine nanotubes and right side shows the dispersed nanotube bundles in aqueous gelatin solution.

with the SWNTs as byproducts. These byproducts can be dispersed by gelatin also and can be removed from SWNTs dispersion by filtration. Another worth mentioning advantage of using gelatin as a dispersing agent is that dried gelatin-wrapped CNTs are easily re-dissolved in water with sonication.

**5. CNT/polymer composites**

be possible in the future.

**5.1. Objective of CNT/cellulose composite**

Generally, composites are composed of two or more materials, which exhibit the unique properties of combined materials. These composites, such as polymer composites, have enhanced mechanical properties because they combine the properties of high modulus carbon fibers and polymer [24]. The additive has great effect in polymer composites because the modulus, strength, and high-fracture toughness of composites are built upon on the nature of additive. By employing the nanoscale CNT into a polymer system results in very short distance between the fillers; thus, the properties of composites can be largely modified even at an extremely low content of filler. Thus, CNT/polymer composites, consisting of CNT as additives and polymer matrices, are considered to be an important group of relatively inexpensive materials for many engineering applications. In order to successfully introduce CNTs in large-scale industrial applications, they need to be homogeneously embedded into light-weight polymer to make nanocomposites. The combination of mechanical and electrical properties of individual nanotubes makes them the ideal reinforcing agents in a number of applications. CNT-polymer composites are promising as new generation materials, and the importance for nanocomposites will increase as miniaturization becomes more important in areas, such as computing, sensors, biomedical, and many other applications [25]. Therefore, CNT/polymer nanocomposites are one of the utmost investigated composite because of the easy fabrication of polymer matrix without damaging CNTs. Another advantage is the low cost of polymer for which mass production of nanocomposites would

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Recently, the rapid development of portable device hardware has aroused huge demand for lighter and more efficient heat-transfer materials [25]. As the overweighted heat-transfer materials have delayed the development of portable device hardware, it is required to develop light, flexible, inexpensive, and even roll up or wearable devices for multifunctional portable electronics [6, 26, 27]. In this case, composite materials consisting of carbon nanotubes and cellulose can be the ideal solution [28, 29], because nanocomposites have the potential to significantly surpass the properties of conventional bulk materials. The difficulties in developing such nanocomposites are dispersion of nanotubes, CNT-polymer interactions, manufacturing cost, and performance of the composite. Cellulose have attracted great attention with expertise in diverse areas [30, 31] as a renewable source-based biodegradable polymer and becomes a major medium for displaying and transmitting information owing to its potential compostibility, mechanical stability under atmospheric conditions, and ability to absorb ink [32]. Carbon nanotubes (CNTs) are regarded as one of the most versatile additives of composites because it can improve mechanical and electrical characteristics of cellulose [33]. Especially, homogeneous distribution of CNTs in the cellulose matrix has predominantly contributed to improve its characteristics. CNT forms a conducting network using its electron donating and accepting ability in the composite [6, 34–36]. These CNT/cellulose composite materials have some outstanding properties such as being highly strong, electrically

#### **4.3. Investigation by UV–visible spectrometer**

A UV–visible spectrometer (JASCO Co., V-630) was used to examine the dispersion stability of the dispersed solution. For this experiment, 1 mg of raw soot and 5 mg of gelatin were mixed with 10 ml of pure water to make the sample. The sample was placed in a quartz cuvette (1.0 × 1.0 × 4.5 cm<sup>3</sup> ) and time variation of absorbance (Abs) at a wavelength of 233 nm was measured for 4 h. Here, the wavelength was chosen corresponding to the maximum absorbance region of the UV–visible spectra. **Figure 6** shows the time variation of absorption spectrum for gelatin-SWNT dispersing liquid at a wavelength of 233 nm. It is noticed that the absorbance started to decrease for more than 1 h and then shows a stable nature with the increase of time, which indicates the stability of gelatin-SWNT dispersion.

From the above analysis, it is found that the gelatin-CNT dispersion is very stable and CNTs do not precipitate in water. Therefore, it can claim that the gelatin molecules wrap around CNTs and CNTs are separated and dispersed in water. This easy way to make dispersed solution of CNT has great impact on the application field. CNT composite material can be made by using this gelatin-CNT dispersion as gelatin can be easily removed by heating and filtration.

**Figure 6.** Absorbance vs. time at λ = 233 nm to measure the stability of the gelatin-SWNT dispersion. Inset shows the UV absorption spectrum of the solution.

#### **5. CNT/polymer composites**

with the SWNTs as byproducts. These byproducts can be dispersed by gelatin also and can be removed from SWNTs dispersion by filtration. Another worth mentioning advantage of using gelatin as a dispersing agent is that dried gelatin-wrapped CNTs are easily re-dissolved

A UV–visible spectrometer (JASCO Co., V-630) was used to examine the dispersion stability of the dispersed solution. For this experiment, 1 mg of raw soot and 5 mg of gelatin were mixed with 10 ml of pure water to make the sample. The sample was placed in a quartz

was measured for 4 h. Here, the wavelength was chosen corresponding to the maximum absorbance region of the UV–visible spectra. **Figure 6** shows the time variation of absorption spectrum for gelatin-SWNT dispersing liquid at a wavelength of 233 nm. It is noticed that the absorbance started to decrease for more than 1 h and then shows a stable nature with the

From the above analysis, it is found that the gelatin-CNT dispersion is very stable and CNTs do not precipitate in water. Therefore, it can claim that the gelatin molecules wrap around CNTs and CNTs are separated and dispersed in water. This easy way to make dispersed solution of CNT has great impact on the application field. CNT composite material can be made by using this gelatin-CNT dispersion as gelatin can be easily removed by heating and filtration.

0 0.5 1 1.5 2 2.5 3 3.5 4

**Figure 6.** Absorbance vs. time at λ = 233 nm to measure the stability of the gelatin-SWNT dispersion. Inset shows the UV

233

0 0.5 1 1.5 2 2.5 3 3.5 4

Abs

*t* (h)

200 400 600 800 1000 1200

l (nm)

increase of time, which indicates the stability of gelatin-SWNT dispersion.

) and time variation of absorbance (Abs) at a wavelength of 233 nm

in water with sonication.

242 Carbon Nanotubes - Recent Progress

cuvette (1.0 × 1.0 × 4.5 cm<sup>3</sup>

**4.3. Investigation by UV–visible spectrometer**

3.2

absorption spectrum of the solution.

3.4

3.6

3.8

4

4.2

Abs (at 233 nm)

4.4

4.6

4.8

Generally, composites are composed of two or more materials, which exhibit the unique properties of combined materials. These composites, such as polymer composites, have enhanced mechanical properties because they combine the properties of high modulus carbon fibers and polymer [24]. The additive has great effect in polymer composites because the modulus, strength, and high-fracture toughness of composites are built upon on the nature of additive. By employing the nanoscale CNT into a polymer system results in very short distance between the fillers; thus, the properties of composites can be largely modified even at an extremely low content of filler. Thus, CNT/polymer composites, consisting of CNT as additives and polymer matrices, are considered to be an important group of relatively inexpensive materials for many engineering applications. In order to successfully introduce CNTs in large-scale industrial applications, they need to be homogeneously embedded into light-weight polymer to make nanocomposites. The combination of mechanical and electrical properties of individual nanotubes makes them the ideal reinforcing agents in a number of applications. CNT-polymer composites are promising as new generation materials, and the importance for nanocomposites will increase as miniaturization becomes more important in areas, such as computing, sensors, biomedical, and many other applications [25]. Therefore, CNT/polymer nanocomposites are one of the utmost investigated composite because of the easy fabrication of polymer matrix without damaging CNTs. Another advantage is the low cost of polymer for which mass production of nanocomposites would be possible in the future.

#### **5.1. Objective of CNT/cellulose composite**

Recently, the rapid development of portable device hardware has aroused huge demand for lighter and more efficient heat-transfer materials [25]. As the overweighted heat-transfer materials have delayed the development of portable device hardware, it is required to develop light, flexible, inexpensive, and even roll up or wearable devices for multifunctional portable electronics [6, 26, 27]. In this case, composite materials consisting of carbon nanotubes and cellulose can be the ideal solution [28, 29], because nanocomposites have the potential to significantly surpass the properties of conventional bulk materials. The difficulties in developing such nanocomposites are dispersion of nanotubes, CNT-polymer interactions, manufacturing cost, and performance of the composite. Cellulose have attracted great attention with expertise in diverse areas [30, 31] as a renewable source-based biodegradable polymer and becomes a major medium for displaying and transmitting information owing to its potential compostibility, mechanical stability under atmospheric conditions, and ability to absorb ink [32]. Carbon nanotubes (CNTs) are regarded as one of the most versatile additives of composites because it can improve mechanical and electrical characteristics of cellulose [33]. Especially, homogeneous distribution of CNTs in the cellulose matrix has predominantly contributed to improve its characteristics. CNT forms a conducting network using its electron donating and accepting ability in the composite [6, 34–36]. These CNT/cellulose composite materials have some outstanding properties such as being highly strong, electrically and thermally conductive, and the usage of these materials have been widely investigated [35–38]. CNT/pulp has been prepared by a paper making process, which is a fast, simple, robust, low-cost, and readily scalable process. The properties of the resulting CNT/cellulose paper sheet are also identified and reported here.

#### **5.2. Fabrication of CNT/cellulose composites**

The most important part of preparation of CNT/cellulose composites is to improve the interaction between the pulp fibers, gelatin and multiwalled carbon nanotubes (MWNTs) to optimize the paper making process for CNT/cellulose composites. MWNTs and geletin are purchased from Sigma-Aldrich and Wako Pure Chemical, respectively, which are used as received. Cellulose fibers used here are obtained directly from manufacturing facility, where cellulose is processed with a beater to get uniform pulp in the sheet.

At first, the cellulose suspension is made by soaking the fibers into pure water and blended for 20 min. 10 mg of MWNTs (outer diameter = 10–30 nm, inner diameter = 3–10 nm, length = 1–10 μm, purity >90%) and 30 mg of gelatin are sonicated with 10 ml of pure water by using a supersonic homogenizer (Sonic vibra cell, VC 130, Sonic & Materials Inc., *f* = 20 kHz, 6 mmϕ probe) at an input power of 20 W for 60 min. Then, 300 mg of cellulose suspension is added to the CNT dispersion and blended for 20 min. The final solution is poured into a tray dish and dried at room temperature to get a paper sheet. A possible paper making process is schematically shown in **Figure 7**. The obtained paper sheets have a thickness of about 0.2–0.4 mm. **Figure 8** shows the image of the cellulose pulp, SEM image of the pulp, and CNT/cellulose coposit sheet.

**5.3. Presence of CNTs on the composite sheet**

**5.4. Properties of CNT/cellulose composites**

*5.4.1. Electrical properties of the CNT/cellulose composite sheet*

as shown in **Figure 10**.

The image of the uniform distribution of MWNTs in the CNT/cellulose composite is taken by a SEM (JEOL JSM 6510LV, operating voltage of 10 kV) to observe the morphology of the sheet as well as the presence of CNT into the composites. It is observed from **Figure 9** that CNTs cover surface of almost all fibers of the sheet. Some CNTs seem to form bridge-like structure with paper fibers. The SEM images clearly reveal that the interconnected networks established by the individual CNTs so that numerous electrical paths can be formed. In contrast with the SEM observation, Raman measurement (JASCO NR-1800, using a laser for excitation, *λ* = 532 nm) also confirmed the presence of CNTs in the paper sheets. It is observed that Raman peaks of both G-band and D-band and radial breathing modes generated from CNTs,

**Figure 8.** (a) Image of the cellulose pulp that used for making the sheet, (b) SEM image of the pure cellulose pulp at low

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magnification (×5000), (c) photographs of a pure cellulose sheet (left), and (d) a CNT/cellulose sheet (right).

As CNT forms the conducting paths into the cellulose sheet, composites shows considerable resistance. Resistance is measured of 10 randomly chosen measuring points on both sides (A side and B side) of

**Figure 7.** A schematic of the paper making process for the fabrication of CNT/cellulose composite sheets. Adopted from Maria and Mieno [37].

**Figure 8.** (a) Image of the cellulose pulp that used for making the sheet, (b) SEM image of the pure cellulose pulp at low magnification (×5000), (c) photographs of a pure cellulose sheet (left), and (d) a CNT/cellulose sheet (right).

#### **5.3. Presence of CNTs on the composite sheet**

and thermally conductive, and the usage of these materials have been widely investigated [35–38]. CNT/pulp has been prepared by a paper making process, which is a fast, simple, robust, low-cost, and readily scalable process. The properties of the resulting CNT/cellulose

The most important part of preparation of CNT/cellulose composites is to improve the interaction between the pulp fibers, gelatin and multiwalled carbon nanotubes (MWNTs) to optimize the paper making process for CNT/cellulose composites. MWNTs and geletin are purchased from Sigma-Aldrich and Wako Pure Chemical, respectively, which are used as received. Cellulose fibers used here are obtained directly from manufacturing facility, where

At first, the cellulose suspension is made by soaking the fibers into pure water and blended for 20 min. 10 mg of MWNTs (outer diameter = 10–30 nm, inner diameter = 3–10 nm, length = 1–10 μm, purity >90%) and 30 mg of gelatin are sonicated with 10 ml of pure water by using a supersonic homogenizer (Sonic vibra cell, VC 130, Sonic & Materials Inc., *f* = 20 kHz, 6 mmϕ probe) at an input power of 20 W for 60 min. Then, 300 mg of cellulose suspension is added to the CNT dispersion and blended for 20 min. The final solution is poured into a tray dish and dried at room temperature to get a paper sheet. A possible paper making process is schematically shown in **Figure 7**. The obtained paper sheets have a thickness of about 0.2–0.4 mm. **Figure 8** shows the image of the cellulose pulp, SEM image of the pulp,

**Figure 7.** A schematic of the paper making process for the fabrication of CNT/cellulose composite sheets. Adopted from

paper sheet are also identified and reported here.

cellulose is processed with a beater to get uniform pulp in the sheet.

**5.2. Fabrication of CNT/cellulose composites**

244 Carbon Nanotubes - Recent Progress

and CNT/cellulose coposit sheet.

Maria and Mieno [37].

The image of the uniform distribution of MWNTs in the CNT/cellulose composite is taken by a SEM (JEOL JSM 6510LV, operating voltage of 10 kV) to observe the morphology of the sheet as well as the presence of CNT into the composites. It is observed from **Figure 9** that CNTs cover surface of almost all fibers of the sheet. Some CNTs seem to form bridge-like structure with paper fibers. The SEM images clearly reveal that the interconnected networks established by the individual CNTs so that numerous electrical paths can be formed. In contrast with the SEM observation, Raman measurement (JASCO NR-1800, using a laser for excitation, *λ* = 532 nm) also confirmed the presence of CNTs in the paper sheets. It is observed that Raman peaks of both G-band and D-band and radial breathing modes generated from CNTs, as shown in **Figure 10**.

#### **5.4. Properties of CNT/cellulose composites**

#### *5.4.1. Electrical properties of the CNT/cellulose composite sheet*

As CNT forms the conducting paths into the cellulose sheet, composites shows considerable resistance. Resistance is measured of 10 randomly chosen measuring points on both sides (A side and B side) of

**Figure 9.** SEM images of the CNT-paper sheet at different magnifications taken at an operating voltage of 10 kV.

a CNT/cellulose paper sheet by the needle-electrodes, and the distance between needle-electrodes are kept constant at 20 mm. The distributions of the resistances are shown as histograms in **Figure 11**.

It is found that the electrical conductivity of the CNT/cellulose paper sheets is highly uniform. In case of 5 mg MWNTs added sheet, the A side has resistance, *R* = 2.61 MΩ (standard deviations, *s* = ±0.08, electrical conductivity, *σ* = 1.91 mS/m) and the B side has resistance, *R* = 2.60 MΩ (standard deviations, *s* = ±0.05, electrical conductivity, *σ* = 1.92 mS/m). When 10 mg of MWNTs are added, the A side has resistance, *R* = 1.14 MΩ (standard deviations, *s* = ±0.04, electrical

conductivity, *σ* = 4.38 mS/m) and the B side has resistance, *R* = 1.103 MΩ (standard deviations, *s* = ±0.05, electrical conductivity, *σ* = 4.53 mS/m). Therefore, it is noticed that the conductivity can be improved by increasing the amount of CNTs added to the cellulose [37]. *I*-*V*in character-

**Figure 11.** Histograms of measured resistance on the both sides of the CNT/cellulose paper sheet. Resistances are

0

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*R* (MΩ)

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TG/DTA (Rigaku Thermoplus Analyzer, TG8120) experiments for the cellulose sheet and the CNT/cellulose sheet are carried out at a heating rate of 10°C/min from room temperature to 800°C and the curves are shown in **Figure 13**. DTA curves show that the thermal degradation of the both sheets occurs in two different steps in air atmosphere. In the first step of degradation, the cellulose sheet and the CNT/cellulose sheet begin to lose weight at 332 and 355°C, respectively, and the final combustion in second step occurs at 409 and 507°C, respectively. The final combustion of the CNT/cellulose sheet delayed by ca. 100°C is clearly observed from

istic curve shows the ohmic conduction on the paper sheet as shown in **Figure 12**.

*5.4.2. Thermal properties of the CNT/cellulose composite sheet*

0.8 1 1.2 1.4 1.6

*R* (MΩ)

5 mg MWNTs- A side

2 2.5 3 3.5

decreased with the increase of MWNTs content.

*R* (MΩ)

10 mg MWNTs- A side

0

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Number of points

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4

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2

Number of points

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**Figure 10.** Raman spectra of the CNT/cellulose paper sheet. RBM modes are shown on the left side and the G and D bands are shown on the right side.

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**Figure 11.** Histograms of measured resistance on the both sides of the CNT/cellulose paper sheet. Resistances are decreased with the increase of MWNTs content.

conductivity, *σ* = 4.38 mS/m) and the B side has resistance, *R* = 1.103 MΩ (standard deviations, *s* = ±0.05, electrical conductivity, *σ* = 4.53 mS/m). Therefore, it is noticed that the conductivity can be improved by increasing the amount of CNTs added to the cellulose [37]. *I*-*V*in characteristic curve shows the ohmic conduction on the paper sheet as shown in **Figure 12**.

#### *5.4.2. Thermal properties of the CNT/cellulose composite sheet*

a CNT/cellulose paper sheet by the needle-electrodes, and the distance between needle-electrodes are kept constant at 20 mm. The distributions of the resistances are shown as histograms in **Figure 11**.

**Figure 9.** SEM images of the CNT-paper sheet at different magnifications taken at an operating voltage of 10 kV.

It is found that the electrical conductivity of the CNT/cellulose paper sheets is highly uniform. In case of 5 mg MWNTs added sheet, the A side has resistance, *R* = 2.61 MΩ (standard deviations, *s* = ±0.08, electrical conductivity, *σ* = 1.91 mS/m) and the B side has resistance, *R* = 2.60 MΩ (standard deviations, *s* = ±0.05, electrical conductivity, *σ* = 1.92 mS/m). When 10 mg of MWNTs are added, the A side has resistance, *R* = 1.14 MΩ (standard deviations, *s* = ±0.04, electrical

460

1350 1400 1450 1500 1550 1600 1650

*k* (cm-1)

<sup>G</sup> *<sup>I</sup>*

465

470

475

*I* (arb. units)

**Figure 10.** Raman spectra of the CNT/cellulose paper sheet. RBM modes are shown on the left side and the G and D

480

D

D/*I* G = 0.67

485

490

1.11 nm

1.28 nm

*I* (arb. units)

100 150 200 250 *k* (cm-1)

1.36 nm

1.52 nm

1.77 nm

1.91 nm

246 Carbon Nanotubes - Recent Progress

2.15 nm

2.41 nm

bands are shown on the right side.

TG/DTA (Rigaku Thermoplus Analyzer, TG8120) experiments for the cellulose sheet and the CNT/cellulose sheet are carried out at a heating rate of 10°C/min from room temperature to 800°C and the curves are shown in **Figure 13**. DTA curves show that the thermal degradation of the both sheets occurs in two different steps in air atmosphere. In the first step of degradation, the cellulose sheet and the CNT/cellulose sheet begin to lose weight at 332 and 355°C, respectively, and the final combustion in second step occurs at 409 and 507°C, respectively. The final combustion of the CNT/cellulose sheet delayed by ca. 100°C is clearly observed from

of the sheet is improved by adding MWNTs [35, 37, 39]. There is also an additional peak at 477°C on the CNT/cellulose curve before the final combination peak, which corresponds to

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The flame retardancy test is carried out for both the cellulose and CNT/cellulose sheets. The combustion process is shown in **Figure 14**. Both the sheets are hanged on a metal supporter and ignited simultaneously by a gas flame. It is observed that the cellulose sheet burns quickly to ashes completely within 20 s, but the CNT/cellulose takes more than half minutes to burn and turns to charcoal, which indicates that added MWNTs improved the flame retardancy of

**Figure 15(b)** and **(c)** shows the temperature vs. time curves of the CNT/cellulose sheet by a thermocouple and by an IR camera during heating and cooling process. The schematic of the measurement are shown in **Figure 15(a)**. The experiment is carried out by measuring the variation in the temperature under the applied voltage, *V*in = 80–220 V. Three layers of CNT/ cellulose sheet has been used for reducing resistance low enough by which the passage of an electric current through a CNT/cellulose sheet releases heat due to Joule heating process [40]. Heating and cooling temperatures of the sheet are measured at the center of the sheet. It is observed that the heat starts to increase with time by applying a DC voltage and radiate from the sheet. The surface of the CNT/cellulose sheet reached above a maximum temperature of 120°C within 5 min depending on the applied voltage and become saturate at that maximum

**Figure 14.** Photographs of flame retardancy test for the pure cellulose sheet (left-white sample) and CNT/cellulose sheet

(right-black sample). At *t* = 0 s, a flow is added on the bottom of the samples.

the combustion of the gelatin.

the sheet [37].

**Figure 12.** *I*-*V*in curve for *L* = 50 mm showing the linear conduction (inset: Schematic of the *I*-*V*in characteristic measurement setup).

the DTA curves, which indicate that cellulose sheets losses weight faster than CNT/cellulose sheet. From the TG curves, it is observed that the combustion rate of CNT/cellulose sheet is 22% less than that of the pure cellulose sheet. This result signifies that the thermal stability

**Figure 13.** TG/DTA curves for the pure cellulose sheet (dotted line) and the CNT/cellulose sheet solid line).

of the sheet is improved by adding MWNTs [35, 37, 39]. There is also an additional peak at 477°C on the CNT/cellulose curve before the final combination peak, which corresponds to the combustion of the gelatin.

The flame retardancy test is carried out for both the cellulose and CNT/cellulose sheets. The combustion process is shown in **Figure 14**. Both the sheets are hanged on a metal supporter and ignited simultaneously by a gas flame. It is observed that the cellulose sheet burns quickly to ashes completely within 20 s, but the CNT/cellulose takes more than half minutes to burn and turns to charcoal, which indicates that added MWNTs improved the flame retardancy of the sheet [37].

**Figure 15(b)** and **(c)** shows the temperature vs. time curves of the CNT/cellulose sheet by a thermocouple and by an IR camera during heating and cooling process. The schematic of the measurement are shown in **Figure 15(a)**. The experiment is carried out by measuring the variation in the temperature under the applied voltage, *V*in = 80–220 V. Three layers of CNT/ cellulose sheet has been used for reducing resistance low enough by which the passage of an electric current through a CNT/cellulose sheet releases heat due to Joule heating process [40]. Heating and cooling temperatures of the sheet are measured at the center of the sheet. It is observed that the heat starts to increase with time by applying a DC voltage and radiate from the sheet. The surface of the CNT/cellulose sheet reached above a maximum temperature of 120°C within 5 min depending on the applied voltage and become saturate at that maximum

the DTA curves, which indicate that cellulose sheets losses weight faster than CNT/cellulose sheet. From the TG curves, it is observed that the combustion rate of CNT/cellulose sheet is 22% less than that of the pure cellulose sheet. This result signifies that the thermal stability

0 100 200 300 400 500 600 700 800

*T* ( 0 C)

**Figure 13.** TG/DTA curves for the pure cellulose sheet (dotted line) and the CNT/cellulose sheet solid line).

0

160

320

Heat Flow (arb.unit)

480

640

Pure sheet CNT sheet

DTA

**Figure 12.** *I*-*V*in curve for *L* = 50 mm showing the linear conduction (inset: Schematic of the *I*-*V*in characteristic measurement

*V* in (V)

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TG

120

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setup).

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248 Carbon Nanotubes - Recent Progress

**Figure 14.** Photographs of flame retardancy test for the pure cellulose sheet (left-white sample) and CNT/cellulose sheet (right-black sample). At *t* = 0 s, a flow is added on the bottom of the samples.

temperature. Much higher temperature can be obtained by increasing the layers of the CNT/

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During the test, all the samples are heated to maximum equilibrium temperature and then cooled naturally, and this heat releasing ability of the sheets is also measured. It is observed that temperature decreases exponentially to the room temperature within few minutes. Thermocouple-based measurements show a little lower value of temperature than the infrared-based measurements. This difference arises from the slow response of the thermocouple due to the slow heat propagation [41]. The IR image of the CNT/cellulose sheet as shown in **Figure 15(d)** indicates the uniformity of the heat dissipation. It is clearly shows that high temperature area (glow area) is located at the center of the sheet. To study the flexibility of the CNT/cellulose sheet, the temperature and current are also measured by bending the sheet more than 90° for 20 times and the total time elapsed for the whole process is 80 min. **Figure 15(e)** shows the flexible nature of the paper sheet, because repeated bending of paper sheet does not make any significant change of the current conduction through the paper sheet, and temperature also remains almost constant. These results mentioned above indicate that CNT/cellulose sheet can be used as an electrothermal heating

A gun-diode microwave transmitter and a recorder (Pasco Scientific WA-9314B, *P*<sup>μ</sup> = 10 mW) of 10.5 GHz with horn antenna are used to measure the microwave absorption properties of

and pure cellulose sheet are made for comparison. The pure cellulose sheet and the CNT/ cellulose sheet are placed on a plastic substrate during the experiment. A schematic representation of the measurement of microwave absorption ability is shown in **Figure 16(a)**.

In this absorption ability experiment, the incident microwave is divided into two parts: the transmitted microwave and the absorbed one. To determine the reflection ability of the sheet, transmitter and receiver is set in such a way that the incident and reflected wave makes equal angles to the surface normal, as shown in **Figure 16(b)**. The experimental result of the absorption and transmission ability vs. the content of MWNTs of the CNT/cellulose sheet is shown in **Figure 17(a)**. The sheet with low content of MWNTs shows weak absorbing ability and the

The absorption test is also carried out with the increase of thickness of the sheet as shown in **Figure 17(b)**. Few sheets of same thickness and same content of MWNTs (30 mg) are used for this experiment. During the experiment, thickness is increased by putting the sheet one after one on the plastic sheet. Absorption ability is found to increase with the increase of thickness as well as MWNTs content. This increase of absorption may occur due to the increase of con-

The CNT/cellulose sheets exhibit very small reflection and the reflections slightly improve with the increase of MWNTs content as shown in **Figure 18**. It is conjectured that interfacial electric polarization improves with the increase of MWNTs content due to the interaction of

of the both CNT/cellulose sheet

cellulose sheet and the sheet could be used as a flat-type electric heater.

element [37].

*5.4.3. Absorbing properties of the composite sheet*

the sheet. For this experiment, a size of 100 × 100 × 0.3 mm<sup>3</sup>

absorption increases with the increase of MWNTs content [37].

ductivity with the increase of MWNTs content [37].

**Figure 15.** (a) Schematic of the heating test measurement of the CNT/cellulose sheet by a thermocouple and an IR camera. Temperature vs time curves during heating and cooling of the CNT/cellulose sheet measured (b) by a thermocouple and (c) by an IR camera. (d)Thermal image at *V*in = 220 V showing the uniform heating of the sheet where the temperature scales shown at the bottom part. (e) Variation of temperature and current after repeated bending, where the input voltage *V*in = 100 V.

temperature. Much higher temperature can be obtained by increasing the layers of the CNT/ cellulose sheet and the sheet could be used as a flat-type electric heater.

During the test, all the samples are heated to maximum equilibrium temperature and then cooled naturally, and this heat releasing ability of the sheets is also measured. It is observed that temperature decreases exponentially to the room temperature within few minutes. Thermocouple-based measurements show a little lower value of temperature than the infrared-based measurements. This difference arises from the slow response of the thermocouple due to the slow heat propagation [41]. The IR image of the CNT/cellulose sheet as shown in **Figure 15(d)** indicates the uniformity of the heat dissipation. It is clearly shows that high temperature area (glow area) is located at the center of the sheet. To study the flexibility of the CNT/cellulose sheet, the temperature and current are also measured by bending the sheet more than 90° for 20 times and the total time elapsed for the whole process is 80 min. **Figure 15(e)** shows the flexible nature of the paper sheet, because repeated bending of paper sheet does not make any significant change of the current conduction through the paper sheet, and temperature also remains almost constant. These results mentioned above indicate that CNT/cellulose sheet can be used as an electrothermal heating element [37].

#### *5.4.3. Absorbing properties of the composite sheet*

*T*(

0C)

80 V 100 V

(b)

250 Carbon Nanotubes - Recent Progress

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Turn on Turn off

(a)

*t* (min)

180 V 200 V 220 V 240 V

IR image

(d) (e)

*T*(

0C)

80 V 100 V

25

**Figure 15.** (a) Schematic of the heating test measurement of the CNT/cellulose sheet by a thermocouple and an IR camera. Temperature vs time curves during heating and cooling of the CNT/cellulose sheet measured (b) by a thermocouple and (c) by an IR camera. (d)Thermal image at *V*in = 220 V showing the uniform heating of the sheet where the temperature scales shown at the bottom part. (e) Variation of temperature and current after repeated

30

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0C)

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40

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Turn on Turn off

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120 V 150 V

bending, where the input voltage *V*in = 100 V.

A gun-diode microwave transmitter and a recorder (Pasco Scientific WA-9314B, *P*<sup>μ</sup> = 10 mW) of 10.5 GHz with horn antenna are used to measure the microwave absorption properties of the sheet. For this experiment, a size of 100 × 100 × 0.3 mm<sup>3</sup> of the both CNT/cellulose sheet and pure cellulose sheet are made for comparison. The pure cellulose sheet and the CNT/ cellulose sheet are placed on a plastic substrate during the experiment. A schematic representation of the measurement of microwave absorption ability is shown in **Figure 16(a)**.

In this absorption ability experiment, the incident microwave is divided into two parts: the transmitted microwave and the absorbed one. To determine the reflection ability of the sheet, transmitter and receiver is set in such a way that the incident and reflected wave makes equal angles to the surface normal, as shown in **Figure 16(b)**. The experimental result of the absorption and transmission ability vs. the content of MWNTs of the CNT/cellulose sheet is shown in **Figure 17(a)**. The sheet with low content of MWNTs shows weak absorbing ability and the absorption increases with the increase of MWNTs content [37].

The absorption test is also carried out with the increase of thickness of the sheet as shown in **Figure 17(b)**. Few sheets of same thickness and same content of MWNTs (30 mg) are used for this experiment. During the experiment, thickness is increased by putting the sheet one after one on the plastic sheet. Absorption ability is found to increase with the increase of thickness as well as MWNTs content. This increase of absorption may occur due to the increase of conductivity with the increase of MWNTs content [37].

The CNT/cellulose sheets exhibit very small reflection and the reflections slightly improve with the increase of MWNTs content as shown in **Figure 18**. It is conjectured that interfacial electric polarization improves with the increase of MWNTs content due to the interaction of

addition of carbon nanotubes in the cellulose. The pure cellulose sheet and CNT/cellulose sheet

**Figure 17.** Variation of transmission property of the CNT/cellulose sheet (a) with the increase of MWNT content and (b)

44

3

**Figure 18.** Variation of reflection property of the CNT/cellulose sheet (a) with the increase of MWNT content and (b) with

4

5

6

Reflection (%)

7

8

(a) (b)

48

52

56

Transmission (%)

60

64

68

(a) (b)

Production of Water Dispersible Carbon Nanotubes and Nanotube/Cellulose Composite

samples are immersed in color and saline. Both the cellulose and CNT/cellulose sheets start to absorb aqueous solution immediately after the immersion and keep the sheets in the solution until they get wet completely. The schematic of the absorption speed measurement is shown in **Figure 19(a)**. The absorption speed, weight changes of the CNT/cellulose sheets are measured from this experiment and compared with the pure cellulose sheet. Absorbed amount of saline by the sheets are calculated by measuring the weight changes of the sheets after the experiment.

for this experiment. In this experiment, the

0 0.4 0.8 1.2 1.6 2 Thickness (mm)

0 0.4 0.8 1.2 1.6 2

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Thickness (mm)

are prepared with a size of 100 × 20 × 0.3 mm<sup>3</sup>

5 10 15 20 25 30 35

the thickness, where MWNT content increases with the thickness.

Amount of MWNTs (mg)

5 10 15 20 25 30 35

with the thickness, where MWNT content increases with the thickness.

Amount of MWNTs (mg)

55

0 0.5 1 1.5 2 2.5 3 3.5

Reflection (%)

60

65

70

Transmission (%)

75

80

85

**Figure 16.** Schematic representation of the (a) microwave absorption measurement and (b) reflection measurement of CNT/cellulose sheet.

microwave radiation with the large number of charge multipoles between the MWNTs and cellulose, which causes reflection rather absorption as shown in **Figure 18(b)**. The results can be comparable with the previous examinations with the other composites of MWNT [42]. This means, there are several considerable factors, like the MWNTs content, thickness of sheet, the interfacial electric polarization, etc. to design a microwave absorbing material [37, 43]. It is also found that a zigzag setup of four sheets of same dimensions, each contains 30 mg MWNTs can absorb about 50% of the incident microwave. Therefore, this composite sheet may have great potential for microwave absorbing applications taking into account all the above results.

#### *5.4.4. Aqueous absorption speed of CNT/cellulose composites*

Generally, cellulose sheets have high absorption speed, which in turn degrades their functional, structural, and mechanical properties. CNT/cellulose composite shows the significant improvement on the aqueous absorption properties without degradation resulted from the

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**Figure 17.** Variation of transmission property of the CNT/cellulose sheet (a) with the increase of MWNT content and (b) with the thickness, where MWNT content increases with the thickness.

addition of carbon nanotubes in the cellulose. The pure cellulose sheet and CNT/cellulose sheet are prepared with a size of 100 × 20 × 0.3 mm<sup>3</sup> for this experiment. In this experiment, the samples are immersed in color and saline. Both the cellulose and CNT/cellulose sheets start to absorb aqueous solution immediately after the immersion and keep the sheets in the solution until they get wet completely. The schematic of the absorption speed measurement is shown in **Figure 19(a)**. The absorption speed, weight changes of the CNT/cellulose sheets are measured from this experiment and compared with the pure cellulose sheet. Absorbed amount of saline by the sheets are calculated by measuring the weight changes of the sheets after the experiment.

microwave radiation with the large number of charge multipoles between the MWNTs and cellulose, which causes reflection rather absorption as shown in **Figure 18(b)**. The results can be comparable with the previous examinations with the other composites of MWNT [42]. This means, there are several considerable factors, like the MWNTs content, thickness of sheet, the interfacial electric polarization, etc. to design a microwave absorbing material [37, 43]. It is also found that a zigzag setup of four sheets of same dimensions, each contains 30 mg MWNTs can absorb about 50% of the incident microwave. Therefore, this composite sheet may have great potential for microwave absorbing applications taking into account all

**Figure 16.** Schematic representation of the (a) microwave absorption measurement and (b) reflection measurement of

Generally, cellulose sheets have high absorption speed, which in turn degrades their functional, structural, and mechanical properties. CNT/cellulose composite shows the significant improvement on the aqueous absorption properties without degradation resulted from the

the above results.

CNT/cellulose sheet.

252 Carbon Nanotubes - Recent Progress

*5.4.4. Aqueous absorption speed of CNT/cellulose composites*

**Figure 18.** Variation of reflection property of the CNT/cellulose sheet (a) with the increase of MWNT content and (b) with the thickness, where MWNT content increases with the thickness.

The high voltage is also applied by changing the polarity and it does not have any noticeable effect in absorption speed. It is also evident from **Figure 20** that absorption speed and absorbed amount increases with the increase of MWNT content and by applying voltage. This may indicate that the absorption speed and absorbed aqueous amount of the CNT/cellulose sheet is controlled by the MWNT loading [37]. Addition of MWNTs in the sheet or and high voltage may responsible to increase the diffusive energy of the aqueous molecule to penetrate through the sheet. Absorption speed is also governed by other factors, for example cellulose pulp content, dispersion of MWNT, and homogeneous mixing of MWNT with the cellulose in the sheet. This increasing behavior of absorption speed of the composite sheet indicates the mechanical reli-

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255

This work provides a simple and effective method for dispersing CNTs in water by using gelatin in order to broaden the application of CNTs. Gelatin wraps the surface of the nanotube to form a stable dispersion, which remains stable for more than a month. This wrapping method does not affect the physical properties of the nanotube. In addition, it can be easily removed from the nanotubes after the dispersion of nanotubes by heating in water and filtration.

The applications of CNTs as composites offer new opportunities to produce cost-effective electronics. CNT-based sheet has been prepared by using a paper making process. MWNTs improve the mechanical, thermal, and electrical properties of cellulose. SEM investigation confirms the homogeneous distribution of MWNTs in the cellulose, which can be attributed to the improvement of its characteristics. Both sides of the CNT/cellulose sheet show uniform electrical conductivity, which is enhanced by increasing the MWNTs content. IR image of the sheet clearly shows the temperature homogeneity of the surface. Thermal stability and the flame retardancy of the sheet are also found to be improved. The sheet has also strong absorbing of electromagnetic waves, which makes them important for microwave technology applications. These electrically conductive and electromagnetic-wave-absorption properties can be useful in radar wave absorbing, electro thermal heating elements, and 2D electric circuit applications, electromagnetic

This study was supported by the Promotion of Nano-Biotechnology Research to Support Aging, Welfare Society from Ministry of Education, Culture, Sports, Science and Technology, Japan. TEM, SEM, and TG/DTA measurements were done at Research Institute of Green Science & Technology, Shizuoka University. Raman measurement was done at Research

ability of the composite.

**6. Summary**

shields, etc.

**Acknowledgements**

Institute of Electronics, Shizuoka University.

**Figure 19.** (a) Schematic of the absorption speed measurement of the CNT/cellulose sheet. (b) Absorption speed of the cellulose sheet and CNT/cellulose sheet in different solutions and conditions.

**Figure 19(b)** shows that the absorption speed of the cellulose sheet is very high in both color and saline. On the other hand, the CNT/cellulose sheet has low absorption speed. A high voltage, ~300 V, is applied through Ni electrode to the CNT/cellulose sheet to investigate the changes in speed, and it is found that the absorption speed increases significantly in both color and saline.

**Figure 20.** (a) Absorption speed of the cellulose sheet and CNT/cellulose sheet with the increase of MWNT content, (b) amount of absorbed saline with the increase of MWNT content.

The high voltage is also applied by changing the polarity and it does not have any noticeable effect in absorption speed. It is also evident from **Figure 20** that absorption speed and absorbed amount increases with the increase of MWNT content and by applying voltage. This may indicate that the absorption speed and absorbed aqueous amount of the CNT/cellulose sheet is controlled by the MWNT loading [37]. Addition of MWNTs in the sheet or and high voltage may responsible to increase the diffusive energy of the aqueous molecule to penetrate through the sheet. Absorption speed is also governed by other factors, for example cellulose pulp content, dispersion of MWNT, and homogeneous mixing of MWNT with the cellulose in the sheet. This increasing behavior of absorption speed of the composite sheet indicates the mechanical reliability of the composite.

#### **6. Summary**

**Figure 19(b)** shows that the absorption speed of the cellulose sheet is very high in both color and saline. On the other hand, the CNT/cellulose sheet has low absorption speed. A high voltage, ~300 V, is applied through Ni electrode to the CNT/cellulose sheet to investigate the changes in speed, and it is found that the absorption speed increases significantly in both color and saline.

**Figure 19.** (a) Schematic of the absorption speed measurement of the CNT/cellulose sheet. (b) Absorption speed of the

0

A

**Figure 20.** (a) Absorption speed of the cellulose sheet and CNT/cellulose sheet with the increase of MWNT content,

Cellulose

sheet

CNT

(a) (b)

sheet

10 mg

15 mg

20 mg

voltage apply

Amount of MWNTs increases

30 mg

1

2

3

) g( eni al as debr os ba f o t nuo m

Speed x 10-3(cm sec-1)

Cellulose

sheet

254 Carbon Nanotubes - Recent Progress

CNT

sheet

10 mg

15 mg

(b) amount of absorbed saline with the increase of MWNT content.

20 mg

Amount of MWNTs increases

voltage apply

cellulose sheet and CNT/cellulose sheet in different solutions and conditions.

30 mg

This work provides a simple and effective method for dispersing CNTs in water by using gelatin in order to broaden the application of CNTs. Gelatin wraps the surface of the nanotube to form a stable dispersion, which remains stable for more than a month. This wrapping method does not affect the physical properties of the nanotube. In addition, it can be easily removed from the nanotubes after the dispersion of nanotubes by heating in water and filtration.

The applications of CNTs as composites offer new opportunities to produce cost-effective electronics. CNT-based sheet has been prepared by using a paper making process. MWNTs improve the mechanical, thermal, and electrical properties of cellulose. SEM investigation confirms the homogeneous distribution of MWNTs in the cellulose, which can be attributed to the improvement of its characteristics. Both sides of the CNT/cellulose sheet show uniform electrical conductivity, which is enhanced by increasing the MWNTs content. IR image of the sheet clearly shows the temperature homogeneity of the surface. Thermal stability and the flame retardancy of the sheet are also found to be improved. The sheet has also strong absorbing of electromagnetic waves, which makes them important for microwave technology applications. These electrically conductive and electromagnetic-wave-absorption properties can be useful in radar wave absorbing, electro thermal heating elements, and 2D electric circuit applications, electromagnetic shields, etc.

#### **Acknowledgements**

This study was supported by the Promotion of Nano-Biotechnology Research to Support Aging, Welfare Society from Ministry of Education, Culture, Sports, Science and Technology, Japan. TEM, SEM, and TG/DTA measurements were done at Research Institute of Green Science & Technology, Shizuoka University. Raman measurement was done at Research Institute of Electronics, Shizuoka University.

#### **Author details**

Kazi Hanium Maria1,2\* and Tetsu Mieno2,3

\*Address all correspondence to: kazimaria@du.ac.bd


[10] Krueger A. Carbon nanotubes. In: Carbon Materials and Nanotechnology. Weinheim:

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**Section 5**

**State-of-the-Art Energy**

**State-of-the-Art Energy**

**Chapter 13**

**Provisional chapter**

**The Porous Carbon Nanotube-Cellulose Papers as**

**The Porous Carbon Nanotube-Cellulose Papers as** 

**and Supercapacitor Applications**

**and Supercapacitor Applications**

Xiaogang Sun, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen

Xiaogang Sun, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen

Additional information is available at the end of the chapter

black to satisfy the multifunctional requirements for LIBs.

**Keywords:** carbon nanotube, cellulose, lithium ion battery, collector, paper,

**1. Applications of carbon nanotubes (CNTs) in lithium ion battery** 

Lithium ion batteries (LIBs) and supercapacitors (SCs) are considered to be the most promising energy storage devices because of their relatively high energy density, good cycle

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71916

**Abstract**

supercapacitor

**and supercapacitor**

**Current Collector and Electrode for Lithium Ion Battery**

**Current Collector and Electrode for Lithium Ion Battery** 

Lithium ion batteries (LIB) and supercapacitors (electric double-layer capacitors (EDLCs) and lithium ion capacitors (LIC)) are the most energy storage service for mobile application. Lithium ion batteries are currently the most popular type of battery for powering portable electronic devices and are growing in popularity for defense, automotive and aerospace applications. The investigation of supercapacitors (SCs) has also achieved significant progresses. Although they have shown remarkable commercial successes, the electrodes and their constituent materials are still the subject of intensive research. Our research focused on a new type of carbon nanotube-cellulose composite materials as current collector of LIBs and as electrodes of SCs for improving and enhancing the energy/ power density and cyclic performance of them. Carbon nanotubes (CNTs) have been widely used as conductive agent for both anodes and cathodes to replace super carbon

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.71916

#### **The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium Ion Battery and Supercapacitor Applications The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium Ion Battery and Supercapacitor Applications**

DOI: 10.5772/intechopen.71916

Xiaogang Sun, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen Xiaogang Sun, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71916

#### **Abstract**

Lithium ion batteries (LIB) and supercapacitors (electric double-layer capacitors (EDLCs) and lithium ion capacitors (LIC)) are the most energy storage service for mobile application. Lithium ion batteries are currently the most popular type of battery for powering portable electronic devices and are growing in popularity for defense, automotive and aerospace applications. The investigation of supercapacitors (SCs) has also achieved significant progresses. Although they have shown remarkable commercial successes, the electrodes and their constituent materials are still the subject of intensive research. Our research focused on a new type of carbon nanotube-cellulose composite materials as current collector of LIBs and as electrodes of SCs for improving and enhancing the energy/ power density and cyclic performance of them. Carbon nanotubes (CNTs) have been widely used as conductive agent for both anodes and cathodes to replace super carbon black to satisfy the multifunctional requirements for LIBs.

**Keywords:** carbon nanotube, cellulose, lithium ion battery, collector, paper, supercapacitor

#### **1. Applications of carbon nanotubes (CNTs) in lithium ion battery and supercapacitor**

Lithium ion batteries (LIBs) and supercapacitors (SCs) are considered to be the most promising energy storage devices because of their relatively high energy density, good cycle

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

performance, power performance, longer service life, and green environmental characteristics. LIBs have a broad application prospects in portable electronic equipment, electric vehicles, and hybrid vehicles [1, 2].

The electrodes of LIB commonly use copper foil or aluminum foil as a current collector, and the active material is coated on the current collector. Carbon nanotubes (CNTs) have unique features of one-dimension, large surface area and high conductivity, which contribute to electrochemical performance of LIBs and SCs [3–5]. Thus, many investigations in terms of electrode materials are focused on CNTs. Compared with aluminum foil or copper foil, metal-free current collector is a research hotspot, CNT film or cellulose papers used as negative electrode, or anode current collector was applied in flexible lithium ion battery in many researches [6, 7]. CNT-cellulose papers (CNTCPs) are very good host for activated materials, which have a porous structure, easy formability, flexibility, and excellent physical and chemical properties. The porous CNTCP has excellent flexibility and conductivity. It has a broad application prospect for LIB applications including lithium-sulfur battery [1–15], Nano-SnO<sup>2</sup> anode [16–43], silicone anode [44–56], Li/CFx battery [57–63], and supercapacitors [64–80]. In this paper, we report the porous CNTCP applications as current collector and electrode for lithium ion battery and supercapacitor, respectively. Moreover, the CNTs used in this study are whisker multiwalled carbon nanotubes.

The fabrication process of the papers is simple and scalable, similar to those technology widely used for paper making in industry. The porous CNTCP with interconnected channel has high surface area and high electrical conductivity. It was considered as potential current collector and electrode for LIBs and SCs.

#### **2. Fabrication and characterization of CNTs and CNTCPs**

The synthesis of CNTs in the present investigation was synthesized in a vertical tubular reactor. An organic-metallic compound (ferrocene) was used as catalyst precursor, toluene as carbon feedstock, sulfur as growth promotion agent, and hydrogen as carrier gas.

The CNTs were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-6701F), transmission electron microscopy (TEM, JEOL JEM-2010FEF), X-ray diffraction (XRD, DI SYSTEM), Raman spectrometry (SENTERRA), and thermogravimetry (TGA, PYRIS DIAMOND). The microstructure of the electrodes was investigated by scanning electron microscopy (FE-SEM, JSM-6701F). Fourier transformation infrared spectra (FTIR, Nicolet 5700) were recorded. Electrical conductivity was measured by the conventional four-probe

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium…

http://dx.doi.org/10.5772/intechopen.71916

265

**3. The CNTCP used as current collector and electrode for LIBs and** 

CNTs were synthesized by chemical vapor deposition and treated at a high temperature of 3000°C for graphitization. The CNTs powder was dispersed in deionized water with a dispersant

DC method (St2258C four-point probe meter).

**3.1. CNTCP used to replace aluminum foils for Li-S cells**

**Figure 1.** (a) SEM of CNTs, (b) HRTEM of CNT, (c) SEM of CNTCP, and (d) FCNT.

**SCs application**

*3.1.1. Experiment*

*3.1.1.1. Preparation of composite host*

The microfeed pump was used to feed the toluene solution and organic metal compound into reactor at a rate of 30 ml/min. The mass flow instrument was used to add hydrogen to the reactor at a rate of 40 ml/min. The generation temperature of CNTs is about 1200°C. The graphitization treatment of CNTs was operated in graphite resistance furnace.

CNTCPs were prepared by the following process: The CNT powders were dispersed by sonication and high speed cutting in distilled water for 2 h, respectively. The surfactant of polyvinylpyrrolidone (PVP) was added to improve the dispersion performance. Cellulose fibers were prepared through smashing napkins by high-speed shearing for 3 h. Both were fully dispersed and mixed by high-speed shear and sand milling in deionized water to form a suspension with a ratio of 1:1. The mixed slurry of CNTs and cellulose fibers was infiltrated by vacuum filtration, and the as-produced CNT paper (AMP) was obtained by peeling it off from the filter paper after drying at 60°C for 12 h. A randomly interwoven fiber mats were peeled from filter cloth and carbonized at 1460°C in vacuum condition. **Figure 1(a, b)** shows the SEM and HRTEM images of CNTs, and **Figure 1(c, d)** shows the SEM images of CNTCP and FCNT.

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium… http://dx.doi.org/10.5772/intechopen.71916 265

**Figure 1.** (a) SEM of CNTs, (b) HRTEM of CNT, (c) SEM of CNTCP, and (d) FCNT.

The CNTs were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-6701F), transmission electron microscopy (TEM, JEOL JEM-2010FEF), X-ray diffraction (XRD, DI SYSTEM), Raman spectrometry (SENTERRA), and thermogravimetry (TGA, PYRIS DIAMOND). The microstructure of the electrodes was investigated by scanning electron microscopy (FE-SEM, JSM-6701F). Fourier transformation infrared spectra (FTIR, Nicolet 5700) were recorded. Electrical conductivity was measured by the conventional four-probe DC method (St2258C four-point probe meter).

#### **3. The CNTCP used as current collector and electrode for LIBs and SCs application**

#### **3.1. CNTCP used to replace aluminum foils for Li-S cells**

#### *3.1.1. Experiment*

performance, power performance, longer service life, and green environmental characteristics. LIBs have a broad application prospects in portable electronic equipment, electric

The electrodes of LIB commonly use copper foil or aluminum foil as a current collector, and the active material is coated on the current collector. Carbon nanotubes (CNTs) have unique features of one-dimension, large surface area and high conductivity, which contribute to electrochemical performance of LIBs and SCs [3–5]. Thus, many investigations in terms of electrode materials are focused on CNTs. Compared with aluminum foil or copper foil, metal-free current collector is a research hotspot, CNT film or cellulose papers used as negative electrode, or anode current collector was applied in flexible lithium ion battery in many researches [6, 7]. CNT-cellulose papers (CNTCPs) are very good host for activated materials, which have a porous structure, easy formability, flexibility, and excellent physical and chemical properties. The porous CNTCP has excellent flexibility and conductivity. It has a broad application prospect for LIB applica-

Li/CFx battery [57–63], and supercapacitors [64–80]. In this paper, we report the porous CNTCP applications as current collector and electrode for lithium ion battery and supercapacitor, respectively. Moreover, the CNTs used in this study are whisker multiwalled carbon nanotubes.

The fabrication process of the papers is simple and scalable, similar to those technology widely used for paper making in industry. The porous CNTCP with interconnected channel has high surface area and high electrical conductivity. It was considered as potential current

The synthesis of CNTs in the present investigation was synthesized in a vertical tubular reactor. An organic-metallic compound (ferrocene) was used as catalyst precursor, toluene as car-

The microfeed pump was used to feed the toluene solution and organic metal compound into reactor at a rate of 30 ml/min. The mass flow instrument was used to add hydrogen to the reactor at a rate of 40 ml/min. The generation temperature of CNTs is about 1200°C. The

CNTCPs were prepared by the following process: The CNT powders were dispersed by sonication and high speed cutting in distilled water for 2 h, respectively. The surfactant of polyvinylpyrrolidone (PVP) was added to improve the dispersion performance. Cellulose fibers were prepared through smashing napkins by high-speed shearing for 3 h. Both were fully dispersed and mixed by high-speed shear and sand milling in deionized water to form a suspension with a ratio of 1:1. The mixed slurry of CNTs and cellulose fibers was infiltrated by vacuum filtration, and the as-produced CNT paper (AMP) was obtained by peeling it off from the filter paper after drying at 60°C for 12 h. A randomly interwoven fiber mats were peeled from filter cloth and carbonized at 1460°C in vacuum condition. **Figure 1(a, b)** shows the SEM and HRTEM images of CNTs, and **Figure 1(c, d)** shows the SEM images of CNTCP

**2. Fabrication and characterization of CNTs and CNTCPs**

bon feedstock, sulfur as growth promotion agent, and hydrogen as carrier gas.

graphitization treatment of CNTs was operated in graphite resistance furnace.

anode [16–43], silicone anode [44–56],

vehicles, and hybrid vehicles [1, 2].

264 Carbon Nanotubes - Recent Progress

tions including lithium-sulfur battery [1–15], Nano-SnO<sup>2</sup>

collector and electrode for LIBs and SCs.

and FCNT.

#### *3.1.1.1. Preparation of composite host*

CNTs were synthesized by chemical vapor deposition and treated at a high temperature of 3000°C for graphitization. The CNTs powder was dispersed in deionized water with a dispersant of sodium dodecylsulfate (SDS) (CH<sup>3</sup> (CH<sup>2</sup> ) <sup>11</sup>OSO<sup>3</sup> Na). The paper cellulose is made of soaked soft wood pulp in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by high-shear emulsifier to form suspension for 3 h. The suspension liquid of cellulose and CNTs pulp was infiltrated by vacuum filtration. A randomly interwoven fiber mat was obtained. The paper was rolled and tailored as current collector for activated materials.

#### *3.1.1.2. Preparation of sulfur electrodes*

Sulfur and conductive additive of carbon black (CB) were mixed by balling for 2 h at 230 r/ min. Then, the slurry of sulfur and CB was prepared by ultrasonication and high-shear process with N-methyl-2-pyrrolidone (NMP) as solution and PVDF as binder. The ratio of S:CB:PVDF = 36:54:10. The blended slurry was coated on to aluminum foils (S-Al electrode) and porous composite host (S-CNTs/CP electrode). The electrodes were dried under vacuum at 60°C for 24 h.

**Figure 2.** SEM (a) and TEM (b) images of CNTs.

**Figure 3.** SEM images of CNTs/CF host (a) surface, (b) cross-section, and (c) pore size distribution.

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#### *3.1.1.3. Assembling of cell and electrochemical measurements*

The tailored of S-Al electrode and S-CNTs/CP electrode were respectively used as working electrodes. Li metal foils were used as the counter electrodes, and Celgard 2300 was used as the separator. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUN LABSTAR, Germany). The electrolyte was prepared by dissolving lithium bistrifluoromethane sulfonylimide (LITFSI) and lithium nitrate (LiNO<sup>3</sup> ) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 ratio, by volume). The electrochemical characterization of the cells was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed within a potential window of 1.8–2.6 V by an electrochemical workstation (CHI 660B). The specific capacity was calculated according to the mass of the sulfur in the host. The charge and discharge current density was set at 0.05 C to 2C.

#### *3.1.2. Results and discussion*

SEM image in **Figure 2(a)** shows that the CNTs have a distinct one-dimension line structure and were not entangled. They can be more easily dispersed than common carbon nanotubes (CCNTs). TEM image of the CNTs is shown in **Figure 2(b)**. It can be observed that the CNTs have a straight and clear texture with high crystallinity.

**Figure 3(a)** shows SEM image of top surface of as-produced CNTs/CP host with a randomin-plane web-like network structure. The CNTs/CP host demonstrated homogenous incorporation of CNTs in the cellulose fibers network. **Figure 3(b)** shows the cross-section image of CNTs/CP host. The figures clearly displayed porous network structure which provided sufficient void space for loading a large amount of sulfur and accommodating the volume expansion of sulfur during cycles. The coarse surface and interconnected channel permit good penetration of the electrolyte and sulfur. The pore size distribution of as-produced CNTs/CP host is shown in **Figure 3(c)**. From the picture, we can see that the host has big porosity. The special surface area of the host reached 25.6 m<sup>2</sup> /g.

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium… http://dx.doi.org/10.5772/intechopen.71916 267

**Figure 2.** SEM (a) and TEM (b) images of CNTs.

of sodium dodecylsulfate (SDS) (CH<sup>3</sup>

266 Carbon Nanotubes - Recent Progress

*3.1.1.2. Preparation of sulfur electrodes*

at 60°C for 24 h.

(CH<sup>2</sup> ) <sup>11</sup>OSO<sup>3</sup>

paper was rolled and tailored as current collector for activated materials.

*3.1.1.3. Assembling of cell and electrochemical measurements*

have a straight and clear texture with high crystallinity.

special surface area of the host reached 25.6 m<sup>2</sup>

imide (LITFSI) and lithium nitrate (LiNO<sup>3</sup>

*3.1.2. Results and discussion*

wood pulp in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by high-shear emulsifier to form suspension for 3 h. The suspension liquid of cellulose and CNTs pulp was infiltrated by vacuum filtration. A randomly interwoven fiber mat was obtained. The

Sulfur and conductive additive of carbon black (CB) were mixed by balling for 2 h at 230 r/ min. Then, the slurry of sulfur and CB was prepared by ultrasonication and high-shear process with N-methyl-2-pyrrolidone (NMP) as solution and PVDF as binder. The ratio of S:CB:PVDF = 36:54:10. The blended slurry was coated on to aluminum foils (S-Al electrode) and porous composite host (S-CNTs/CP electrode). The electrodes were dried under vacuum

The tailored of S-Al electrode and S-CNTs/CP electrode were respectively used as working electrodes. Li metal foils were used as the counter electrodes, and Celgard 2300 was used as the separator. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUN LABSTAR, Germany). The electrolyte was prepared by dissolving lithium bistrifluoromethane sulfonyl-

(DOL) (1:1 ratio, by volume). The electrochemical characterization of the cells was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed within a potential window of 1.8–2.6 V by an electrochemical workstation (CHI 660B). The specific capacity was calculated according to the mass of the

SEM image in **Figure 2(a)** shows that the CNTs have a distinct one-dimension line structure and were not entangled. They can be more easily dispersed than common carbon nanotubes (CCNTs). TEM image of the CNTs is shown in **Figure 2(b)**. It can be observed that the CNTs

**Figure 3(a)** shows SEM image of top surface of as-produced CNTs/CP host with a randomin-plane web-like network structure. The CNTs/CP host demonstrated homogenous incorporation of CNTs in the cellulose fibers network. **Figure 3(b)** shows the cross-section image of CNTs/CP host. The figures clearly displayed porous network structure which provided sufficient void space for loading a large amount of sulfur and accommodating the volume expansion of sulfur during cycles. The coarse surface and interconnected channel permit good penetration of the electrolyte and sulfur. The pore size distribution of as-produced CNTs/CP host is shown in **Figure 3(c)**. From the picture, we can see that the host has big porosity. The

/g.

sulfur in the host. The charge and discharge current density was set at 0.05 C to 2C.

Na). The paper cellulose is made of soaked soft

) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane

**Figure 3.** SEM images of CNTs/CF host (a) surface, (b) cross-section, and (c) pore size distribution.

It was known that a sufficient amount of electrolyte is especially required for a high sulfurloaded cathode during the electrochemical transformation between sulfur and polysulfides [81, 82]. Therefore, the superior electrolyte absorbability of the S-CNTs/CP contributes to stabilized electrochemical reactions in high sulfur-loaded electrodes. S-CNTs/CP can also suppress the diffusion of polysulfides, which resulted in superior cycling stability.

**Figure 4(a)** shows the cross-section images of S-CNTs/CP electrode. It can be observed that sulfur embedded into micropores of host has a uniform distribution. This made an intimate contact between sulfur and host. The interfacial contact area enhanced and interface resistance decreased substantially, which are highly desirable for high rate charge-discharge cycles. The energy dispersive spectra (EDS) of cross-section of S-CNTs/CP electrode (**Figure 5**) further demonstrate that sulfur embedded into CNTs/CP host which was attributed to good cycle stability. **Figure 4(b)** shows the cross-section images of S-Al electrode. It can also be observed

> that there will be visible pores between the interface of sulfur and Al foil, which resulted in increasing interface resistance. **Figure 4** shows the morphologies of two electrodes of S-Al electrode (c) and S-CNTs/CP electrode (d) after 60 cycles. It was observed that S-CNTs/ CP electrode maintains a smooth and unbroken surface, while the S-Al electrode shows an uneven surface. Due to weak binding force and low affinity between sulfur and Al foil, sulfur was exfoliated from Al foil during cycles. **Figure 6** shows the first three cyclic voltammetry (CV) curve of the S-CNTs/CP electrode at a scan rate of 0.05 mV /S in the voltage range of 1.8–2.6 V. In the first electrode scan, two characteristic reduction peaks at 2.29 and 1.99 V can be observed. The reduction peak at about 2.3 V belongs to the reduction of element sul-

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S2 , (Li<sup>2</sup>

Sn, 4﹤n﹤8). The peak at 2.0 V indicated the reduction of lithium

S) [81–84]. The two oxidation peaks were

fur to long-chain LiPSs (Li<sup>2</sup>

polysulfide to short-chain lithium sulfide (Li<sup>2</sup>

**Figure 6.** The first three cycles of CV curves of S-CNTs/CP electrode.

**Figure 5.** The EDS images of cross section of S-CNTs/CP electrode.

**Figure 4.** SEM of CNTs/CP and Al foil loaded with sulfur (a) S-Al cross section, (b) S-CNTs/CP cross section, (c) S-Al top after cycles, (d) S-CNTs/CP top after cycles.

**Figure 5.** The EDS images of cross section of S-CNTs/CP electrode.

It was known that a sufficient amount of electrolyte is especially required for a high sulfurloaded cathode during the electrochemical transformation between sulfur and polysulfides [81, 82]. Therefore, the superior electrolyte absorbability of the S-CNTs/CP contributes to stabilized electrochemical reactions in high sulfur-loaded electrodes. S-CNTs/CP can also sup-

**Figure 4(a)** shows the cross-section images of S-CNTs/CP electrode. It can be observed that sulfur embedded into micropores of host has a uniform distribution. This made an intimate contact between sulfur and host. The interfacial contact area enhanced and interface resistance decreased substantially, which are highly desirable for high rate charge-discharge cycles. The energy dispersive spectra (EDS) of cross-section of S-CNTs/CP electrode (**Figure 5**) further demonstrate that sulfur embedded into CNTs/CP host which was attributed to good cycle stability. **Figure 4(b)** shows the cross-section images of S-Al electrode. It can also be observed

**Figure 4.** SEM of CNTs/CP and Al foil loaded with sulfur (a) S-Al cross section, (b) S-CNTs/CP cross section, (c) S-Al top

after cycles, (d) S-CNTs/CP top after cycles.

press the diffusion of polysulfides, which resulted in superior cycling stability.

268 Carbon Nanotubes - Recent Progress

that there will be visible pores between the interface of sulfur and Al foil, which resulted in increasing interface resistance. **Figure 4** shows the morphologies of two electrodes of S-Al electrode (c) and S-CNTs/CP electrode (d) after 60 cycles. It was observed that S-CNTs/ CP electrode maintains a smooth and unbroken surface, while the S-Al electrode shows an uneven surface. Due to weak binding force and low affinity between sulfur and Al foil, sulfur was exfoliated from Al foil during cycles. **Figure 6** shows the first three cyclic voltammetry (CV) curve of the S-CNTs/CP electrode at a scan rate of 0.05 mV /S in the voltage range of 1.8–2.6 V. In the first electrode scan, two characteristic reduction peaks at 2.29 and 1.99 V can be observed. The reduction peak at about 2.3 V belongs to the reduction of element sulfur to long-chain LiPSs (Li<sup>2</sup> Sn, 4﹤n﹤8). The peak at 2.0 V indicated the reduction of lithium polysulfide to short-chain lithium sulfide (Li<sup>2</sup> S2 , (Li<sup>2</sup> S) [81–84]. The two oxidation peaks were

**Figure 6.** The first three cycles of CV curves of S-CNTs/CP electrode.

represented at 2.3 and 2.4 V. Li<sup>2</sup> Sn (n > 2) was formed at 2.3 V, and elemental sulfur was generated at 2.4 V. The electrodes displayed the almost identical CV curves from beginning of the second cycle. The overlap of subsequent oxide and reduction peaks suggested that the electrodes have high electrochemical stability and good reversibility. It also indicated that the interconnected porous 3D structure is a quite good support of sulfur to prevent shuttle effect and maintain high utilization of sulfur.

**Figure 7(b)** shows the discharge-charge curves of S-CNTs/CP electrode. The galvanostatic discharge-charge profiles displayed a similar curve as S-Al electrode. These plateaus are also consistent well with the current peaks in the CV curves (**Figure 6**). But the discharge profiles of S-CNTs/CP electrode displayed highly overlapped voltage plateaus at 2.1 V. It was considered that the short-chain sulfides taken place in almost same potential upon cycling, which revealed little kinetic barriers for conformation of short-chain sulfides [85–87]. The first discharge specific capacity of S-CNTs/CP electrode reached around 1282 mAh/g, which represented a 76.5% utilization of sulfur (1675 mAh/g). The capacity still maintained at 991.7mAh/g after 10 cycles, which is 77.3% of first discharge capacity. The Coulombic efficiency maintained over 98% from second cycle beginning (**Figure 7c**). After 20 cycles, the Coulombic efficiency of S-Al electrode dropped sharply to 90%, while S-CNTs/cellulose electrode still held 98%. The S-CNTs/cellulose electrode achieved better electrochemical performance than S-Al electrode. The cycle and rate performance of the both electrodes were shown in **Figure 7(d)**. It was shown that the discharge capacity of S-Al electrode declined rapidly. The capacity only maintained around 180 mA/g at the rate of 2C after 50 cycles, while S-CNTs/CP electrode exhibited a stabile discharge capacity of 400mAh/g at 2C. When the charge-discharge rate returned to 0.5C, the discharge capacity recovered to 600mAh/g with a Coulombic efficiency of 97.5%, which suggested excellent redox stability of the S-CNTs/ CP electrode. The excellent electrochemical performance of S-CNTs/CP electrode could be ascribed to its good conduction, abundant active interface, and porous interconnected structure. The unique structure can not only increase the absorption of electrolyte and loading of sulfur but also provide stable transfer channels for ions and electrons. The diffusion of LiPSs

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**Figure 8** illustrates the electrochemical impedance analyses of the S-CNTs/cellulose and S-Al electrode before cycles and after cycles, respectively. The experimental data demonstrated that the electrochemical impedance spectra of the two electrodes consist of a single-depressed

**Figure 8.** The electrochemical impedance spectra before (a) and after (b) cycles.

also was confined effectively.

**Figure 7(a)** shows the discharge-charge curves of S-Al electrode at first, second, and 10th at the current rate of 0.05 C, respectively. It can be seen that there are two plateaus for each discharge curve at 2.3 and 2.1 V, respectively. The plateaus of 2.3 V represented the formation of long-chain polysulfides (Li<sup>2</sup> Sn, 4﹤n﹤8) with shorter flat range. The plateaus of 2.1 V represented conformation of short-chain sulfides of Li<sup>2</sup> S2 and Li<sup>2</sup> S [13] with longer flat range. The first discharge specific capacity reached around 1085 mAh/g, which represented a 64.7% utilization of sulfur (1675 mAh/g). But the capacity rapidly faded to 884.5 mAh/g at second cycle. The capacity only maintained at 714 mAh/g after 10th cycle, which is only 65.8% of first discharge capacity. The utilization of sulfur was also declined to 42.6%.

**Figure 7.** Charge/discharge of S-Al electrode (a) and S-CNTs/CP electrode (b), coulombic efficiency (c), cycle and rate performance of (d).

**Figure 7(b)** shows the discharge-charge curves of S-CNTs/CP electrode. The galvanostatic discharge-charge profiles displayed a similar curve as S-Al electrode. These plateaus are also consistent well with the current peaks in the CV curves (**Figure 6**). But the discharge profiles of S-CNTs/CP electrode displayed highly overlapped voltage plateaus at 2.1 V. It was considered that the short-chain sulfides taken place in almost same potential upon cycling, which revealed little kinetic barriers for conformation of short-chain sulfides [85–87]. The first discharge specific capacity of S-CNTs/CP electrode reached around 1282 mAh/g, which represented a 76.5% utilization of sulfur (1675 mAh/g). The capacity still maintained at 991.7mAh/g after 10 cycles, which is 77.3% of first discharge capacity. The Coulombic efficiency maintained over 98% from second cycle beginning (**Figure 7c**). After 20 cycles, the Coulombic efficiency of S-Al electrode dropped sharply to 90%, while S-CNTs/cellulose electrode still held 98%. The S-CNTs/cellulose electrode achieved better electrochemical performance than S-Al electrode. The cycle and rate performance of the both electrodes were shown in **Figure 7(d)**. It was shown that the discharge capacity of S-Al electrode declined rapidly. The capacity only maintained around 180 mA/g at the rate of 2C after 50 cycles, while S-CNTs/CP electrode exhibited a stabile discharge capacity of 400mAh/g at 2C. When the charge-discharge rate returned to 0.5C, the discharge capacity recovered to 600mAh/g with a Coulombic efficiency of 97.5%, which suggested excellent redox stability of the S-CNTs/ CP electrode. The excellent electrochemical performance of S-CNTs/CP electrode could be ascribed to its good conduction, abundant active interface, and porous interconnected structure. The unique structure can not only increase the absorption of electrolyte and loading of sulfur but also provide stable transfer channels for ions and electrons. The diffusion of LiPSs also was confined effectively.

represented at 2.3 and 2.4 V. Li<sup>2</sup>

270 Carbon Nanotubes - Recent Progress

and maintain high utilization of sulfur.

tion of long-chain polysulfides (Li<sup>2</sup>

performance of (d).

represented conformation of short-chain sulfides of Li<sup>2</sup>

discharge capacity. The utilization of sulfur was also declined to 42.6%.

Sn (n > 2) was formed at 2.3 V, and elemental sulfur was gen-

Sn, 4﹤n﹤8) with shorter flat range. The plateaus of 2.1 V

S [13] with longer flat range.

and Li<sup>2</sup>

S2

erated at 2.4 V. The electrodes displayed the almost identical CV curves from beginning of the second cycle. The overlap of subsequent oxide and reduction peaks suggested that the electrodes have high electrochemical stability and good reversibility. It also indicated that the interconnected porous 3D structure is a quite good support of sulfur to prevent shuttle effect

**Figure 7(a)** shows the discharge-charge curves of S-Al electrode at first, second, and 10th at the current rate of 0.05 C, respectively. It can be seen that there are two plateaus for each discharge curve at 2.3 and 2.1 V, respectively. The plateaus of 2.3 V represented the forma-

The first discharge specific capacity reached around 1085 mAh/g, which represented a 64.7% utilization of sulfur (1675 mAh/g). But the capacity rapidly faded to 884.5 mAh/g at second cycle. The capacity only maintained at 714 mAh/g after 10th cycle, which is only 65.8% of first

**Figure 7.** Charge/discharge of S-Al electrode (a) and S-CNTs/CP electrode (b), coulombic efficiency (c), cycle and rate

**Figure 8** illustrates the electrochemical impedance analyses of the S-CNTs/cellulose and S-Al electrode before cycles and after cycles, respectively. The experimental data demonstrated that the electrochemical impedance spectra of the two electrodes consist of a single-depressed

**Figure 8.** The electrochemical impedance spectra before (a) and after (b) cycles.

semicircle in the high-to-medium frequency region and an inclined line at low frequencies. Before cycle, the charge-transfer resistance of S-CNTs/CP electrode reached 110 Ω, which is lower than 240 Ω of S-Al electrode. After 60 cycles, the charge-transfer resistance of S-CNTs/CP electrode declined to 60 Ω, while the charge-transfer resistance of S-Al electrode only reduced to 210 Ω (**Figure 8b**). This can be attributed to the porous structure of the electrode and 3D conductive network of the electrode, which provides good electrolyte wetting and infiltrating and fast charge transport channel. The polysulfides in the S-CNTs/CP electrode suffer from less dissolution in the electrolyte due to the confinement which prevent and relieve shuttle effect. EDS (**Figure 5**) also identify that sulfur was well distributed in S-CNTs/CP electrode. The strong interaction between sulfur encapsulated in the micropores of the electrode and CNTs is beneficial to improve the electron transfer. The electrolyte and sulfur incorporated in the interconnected channel of porous electrode enhanced the mobility of lithium ions for the electrochemical reaction. The novelty host can also accommodate the volume change of sulfur and alleviate stress generated by volume expansion during cycling.

*3.2.1.2. Assembling of cell*

carbonate (DMC) (1:1 in weight).

*3.2.2. Results and discussion*

/CNT electrode:

Discharge and charge for CNTs.

Discharge and charge for SnO<sup>2</sup>

Charge:Li<sup>x</sup>

Charge:Li<sup>x</sup>

SnO<sup>2</sup>

*3.2.1.3. Electrochemical measurements*

culated according to the total mass of the CNT and SnO<sup>2</sup>

The morphology and microstructures of SnO<sup>2</sup>

Hierarchical open channel can store a large of SnO<sup>2</sup>

reduce interface resistance. **Figure 9(b)** shows the SnO<sup>2</sup>

CNTs have line structure and good aligned carbon atoms.

Discharge:6C + xLi<sup>+</sup> + xe− → Li<sup>x</sup>

.

Discharge:SnO<sup>2</sup> + 4Li+ + 4e− → 2Li2

Sn + xLi<sup>+</sup> + xe− → Li<sup>x</sup>

The tailored CNT papers were used as working electrodes, and Li metal plate was used as the counter electrode. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUNLABSTAR, Germany) by stacking a porous polypropylene separator. The liquid electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl

The electrochemical characterization of the cells was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed by an electrochemical workstation (CHI 660B). The specific capacity was cal-

**Figure 9(a)** shows cross-sectional area of CNT paper. It was exhibited that the CNT papers have a random-in-plane web-like network structure with interconnected channel. The

similar ball structure. **Figure 9(c,d)** exhibits the morphology of CNTs. It can be observed the

The electrochemical process can be summarized as following in charge-discharge for the

scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

paper has high special surface and can increase interface contact area of SnO<sup>2</sup>

.

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium…

/CNT/Cellulose electrodes were examined by

to increase energy density. The porous

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273

has an average diameter of 60 nm and

C<sup>6</sup> (1)

O + Sn (3)

Sn (0≤x ≤4.4) (4)

C<sup>6</sup> → 6C + xLi<sup>+</sup> + xe− (2)

Sn → Sn + xLi<sup>+</sup> + xe− (0≤x ≤4.4) (5)

and support to

#### *3.1.3. Conclusions*

The S-CNTs/CP electrode with high porosity and interconnected channel was fabricated as host of sulfur for Li-S batteries. This unique hierarchical porous structure with 3D electric conductive network formed by CNTs can provide good path for the penetration of electrolyte and sulfur. The conduction of ions and electrons also were significantly improved. The unique architecture can trap elemental sulfur and LiPSs to suppress shuttle effect. The dissolution and diffusion of LiPSs were effectively checked during cycles. S-CNTs/CP electrode also demonstrated an excellent electrolyte uptake characteristic with four times higher than S-Al electrode. The high electrolyte absorbability of S-CNTs/CP electrode improved the utilization of sulfur. The porous structure can accommodate the volumetric expansion of sulfur during lithiation and alleviate stress generated by volume expansion [88]. This work provided a facile and environment-friendly technology for fabricating high-performance Li-S batteries and exhibited a promising future for practical applications.

#### **3.2. CNTCP used to replace aluminum foils as collector and scaffold for nano-SnO<sup>2</sup> anode**

#### *3.2.1. Experiment*

#### *3.2.1.1. Preparation of electrode pieces anodes*

The nanosized tin oxide (SnO<sup>2</sup> ) and carbon nanotubes (CNTs) were milling in a ball mill (250 r/min) for about 1 h. Solvent 1-methyl-2-pyrrolidone (NMP) and binder (PVDF) was wetted ball mill for 2 h to obtain a negative electrode slurry. Solid content is about 50 vol%. The ratio of SnO2: CNT: PVDF = 1.8 g: 0.6 g: 0.3 g was uniformly coated to a 3 μm copper foil and approximately 100 μm of CNT conductive paper, respectively. Then, drying at 60°C in vacuum atmosphere cut into φ14mm pole pieces. The tin oxide lithium battery pole piece was prepared.

#### *3.2.1.2. Assembling of cell*

semicircle in the high-to-medium frequency region and an inclined line at low frequencies. Before cycle, the charge-transfer resistance of S-CNTs/CP electrode reached 110 Ω, which is lower than 240 Ω of S-Al electrode. After 60 cycles, the charge-transfer resistance of S-CNTs/CP electrode declined to 60 Ω, while the charge-transfer resistance of S-Al electrode only reduced to 210 Ω (**Figure 8b**). This can be attributed to the porous structure of the electrode and 3D conductive network of the electrode, which provides good electrolyte wetting and infiltrating and fast charge transport channel. The polysulfides in the S-CNTs/CP electrode suffer from less dissolution in the electrolyte due to the confinement which prevent and relieve shuttle effect. EDS (**Figure 5**) also identify that sulfur was well distributed in S-CNTs/CP electrode. The strong interaction between sulfur encapsulated in the micropores of the electrode and CNTs is beneficial to improve the electron transfer. The electrolyte and sulfur incorporated in the interconnected channel of porous electrode enhanced the mobility of lithium ions for the electrochemical reaction. The novelty host can also accommodate the volume change of sulfur

The S-CNTs/CP electrode with high porosity and interconnected channel was fabricated as host of sulfur for Li-S batteries. This unique hierarchical porous structure with 3D electric conductive network formed by CNTs can provide good path for the penetration of electrolyte and sulfur. The conduction of ions and electrons also were significantly improved. The unique architecture can trap elemental sulfur and LiPSs to suppress shuttle effect. The dissolution and diffusion of LiPSs were effectively checked during cycles. S-CNTs/CP electrode also demonstrated an excellent electrolyte uptake characteristic with four times higher than S-Al electrode. The high electrolyte absorbability of S-CNTs/CP electrode improved the utilization of sulfur. The porous structure can accommodate the volumetric expansion of sulfur during lithiation and alleviate stress generated by volume expansion [88]. This work provided a facile and environment-friendly technology for fabricating high-performance Li-S batteries and

**3.2. CNTCP used to replace aluminum foils as collector and scaffold for nano-SnO<sup>2</sup>**

(250 r/min) for about 1 h. Solvent 1-methyl-2-pyrrolidone (NMP) and binder (PVDF) was wetted ball mill for 2 h to obtain a negative electrode slurry. Solid content is about 50 vol%. The ratio of SnO2: CNT: PVDF = 1.8 g: 0.6 g: 0.3 g was uniformly coated to a 3 μm copper foil and approximately 100 μm of CNT conductive paper, respectively. Then, drying at 60°C in vacuum atmosphere cut into φ14mm pole pieces. The tin oxide lithium battery pole piece

) and carbon nanotubes (CNTs) were milling in a ball mill

and alleviate stress generated by volume expansion during cycling.

exhibited a promising future for practical applications.

*3.2.1.1. Preparation of electrode pieces anodes*

The nanosized tin oxide (SnO<sup>2</sup>

*3.1.3. Conclusions*

272 Carbon Nanotubes - Recent Progress

**anode**

*3.2.1. Experiment*

was prepared.

The tailored CNT papers were used as working electrodes, and Li metal plate was used as the counter electrode. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUNLABSTAR, Germany) by stacking a porous polypropylene separator. The liquid electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight).

#### *3.2.1.3. Electrochemical measurements*

The electrochemical characterization of the cells was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed by an electrochemical workstation (CHI 660B). The specific capacity was calculated according to the total mass of the CNT and SnO<sup>2</sup> .

#### *3.2.2. Results and discussion*

The morphology and microstructures of SnO<sup>2</sup> /CNT/Cellulose electrodes were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

**Figure 9(a)** shows cross-sectional area of CNT paper. It was exhibited that the CNT papers have a random-in-plane web-like network structure with interconnected channel. The Hierarchical open channel can store a large of SnO<sup>2</sup> to increase energy density. The porous paper has high special surface and can increase interface contact area of SnO<sup>2</sup> and support to reduce interface resistance. **Figure 9(b)** shows the SnO<sup>2</sup> has an average diameter of 60 nm and similar ball structure. **Figure 9(c,d)** exhibits the morphology of CNTs. It can be observed the CNTs have line structure and good aligned carbon atoms.

The electrochemical process can be summarized as following in charge-discharge for the SnO<sup>2</sup> /CNT electrode:

Discharge and charge for CNTs.

$$\text{Discharge:} \mathfrak{6C + xLi^{+} + xe^{-} \to Li\_{x}C\_{6}} \tag{1}$$

$$\text{Charge:} \text{Li}\_{\text{x}}\text{C}\_{6} \rightarrow \text{ } \mathsf{6C} + \mathsf{x}\mathrm{Li}^{+} + \mathsf{x}\mathrm{e}^{-}\tag{2}$$

Discharge and charge for SnO<sup>2</sup> .

$$\text{Discharge:}\,\text{SnO}\_2 + 4\,\text{Li}^+ + 4\text{e}^- \rightarrow 2\,\text{Li}\_2\text{O} + \text{Sn} \tag{3}$$

$$\text{Sn} + \text{xLi}^{+} + \text{xe}^{-} \rightarrow \text{Li}\_{\text{x}} \text{Sn} \ (0 \text{\&x} \ \text{\&4.4}) \tag{4}$$

$$\text{Charge:}\,\text{Li}\_{\text{x}}\text{Sn} \rightarrow \text{ Sn} + \text{x}\,\text{Li}^{\*} + \text{xe}^{-} \begin{pmatrix} 0 \leq \text{x} \ \textsf{4}.4 \end{pmatrix} \tag{5}$$

**Figure 10(b)** shows the initial three galvanostatic discharge and charge profiles of the SnO<sup>2</sup>

**Figure 10.** (a) Cyclic voltammetry curves at a scan rate of 0.1 mV/s over the potential window of 0–2.4 V. (b) the initial

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium…

the formation of Sn and Li<sup>2</sup>

with more interface areas.

shows the rate capabilities of SnO<sup>2</sup>

The SnO<sup>2</sup>

are higher than the theoretical capacity of SnO<sup>2</sup>

three galvanostatic discharge and charge profiles of the SnO<sup>2</sup>

The electrochemical performances of pure SnO<sup>2</sup>

was hold at 100 mA/g. **Figure 11** shows the capacity of pure SnO<sup>2</sup>

beneficial to enhance the kinetics of electrochemical reactions between SnO<sup>2</sup>

a first discharge capacity of 1050mAh/g of copper foil collector. SnO<sup>2</sup>

evaluated (**Figure 11**). The pure SnO<sup>2</sup>

CNT electrode at current density of 100 mA/g. The first discharge and charge capacity reached around 2240 and 1250 mAh/g, respectively, with first coulombic efficiency of 56%. A voltage plateau at around 0.90 V can be noticed in the initial discharge process, which is attributed to

CNTs that have a specific capacity of 200–3000mAh/g [94], and the oxygen containing functional groups reacted with lithium ions to exhibit certain capacity and the irreversible formation of the SEI layer [95]. A discharge capacity of 1140 and 1000 mAh/g was achieved in the 2nd and 3rd cycle. With the increase of the cycle numbers, the capacity fades very slowly.

and slurry (1.8 g SnO<sup>2</sup>

/CNT paper electrode presents the excellent electrochemical performance. **Figure 12(a)**

100 mA/g to 500 mA/g. It exhibits a first discharge capacity of 1218 100mAh/g compared with

showed remarkably enhanced rate capability, and much better capacity retention was attained, particularly at high current densities during charging. The capacity retention holds 680 mAh/g

coated on copper foil and CNT paper collector, respectively. The charge-discharge current

capacity fading at the initial stage of cycling. After 20 cycles, the capacity almost faded away completely. The cathode-doped CNTs coated on copper foil collector maintained a capacity of 200mAh/g after 20 cycles. The cathode with CNT paper as collector exhibited excellent performance. The capacity was tend to stabilization after 6 cycles and still holds a capacity of about 600mAh/g after 20 cycles. The result suggested that the porous CNT paper collector is

O [28]. Moreover, the initial discharge capacities of 2240 mAh/g

/CNT electrode at current density of 100 mA/g.

(1494 mAh/g) [93], which can be attributed to

http://dx.doi.org/10.5772/intechopen.71916

and CNT porous composite electrode were

/CNT paper electrode at various current densities from

: 0.6 g CNT: 0.3 g PVDF) were

cathode exhibited abrupt

and lithium ions

/CNT/cellulose electrode

/

275

**Figure 9.** SEM images of porous collector (a), SnO<sup>2</sup> (b), CNTs (c), and HRTEM image of CNTs (d).

Cyclic voltammetry curves were recorded for SnO<sup>2</sup> /CNT electrodes as shown in **Figure 10(a)**. The first cycle curve in the anodic sweep is substantially different from the subsequent curves. During the first oxidation scan, three characteristic oxidation peaks were observed at 0.3, 0.5, and 1.3 V. The strong peak at 0.3 V can be ascribed to lithium insertion into CNTs (Eq.(1)), which have been treated at 3000°C and obtained at very high crystallinity degree [89]. The strong peak at 0.5 V was corresponding to alloying process for forming Lix Sn alloy (Eq. (4)) [90]. The broad peak at 1.3 V can be considered to be the partly reversible reaction representing the conversion of Sn to SnO<sup>2</sup> (Eq. (3)) [91]. In first cathodic sweep process, sharp characteristic peaks were observed at 0.65 V. The sharp peak at 0.65 V can be ascribed to the formation of the solid electrolyte interphase (SEI) film on surface of CNTs and SnO<sup>2</sup> [92]. The irreversible transformation of SnO<sup>2</sup> to Sn was also contributed to the prominent reduction peak (Eq. (3)) [92]. The sharp reduction peak only appeared in first discharge cycle, which was originated from the initial irreversible capacity [92]. During subsequent 2nd and 3rd cycles, the CV curves were overlapped fully. This implied that the reactions in Eqs. (3) and (4) are highly reversible.

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium… http://dx.doi.org/10.5772/intechopen.71916 275

**Figure 10.** (a) Cyclic voltammetry curves at a scan rate of 0.1 mV/s over the potential window of 0–2.4 V. (b) the initial three galvanostatic discharge and charge profiles of the SnO<sup>2</sup> /CNT electrode at current density of 100 mA/g.

**Figure 10(b)** shows the initial three galvanostatic discharge and charge profiles of the SnO<sup>2</sup> / CNT electrode at current density of 100 mA/g. The first discharge and charge capacity reached around 2240 and 1250 mAh/g, respectively, with first coulombic efficiency of 56%. A voltage plateau at around 0.90 V can be noticed in the initial discharge process, which is attributed to the formation of Sn and Li<sup>2</sup> O [28]. Moreover, the initial discharge capacities of 2240 mAh/g are higher than the theoretical capacity of SnO<sup>2</sup> (1494 mAh/g) [93], which can be attributed to CNTs that have a specific capacity of 200–3000mAh/g [94], and the oxygen containing functional groups reacted with lithium ions to exhibit certain capacity and the irreversible formation of the SEI layer [95]. A discharge capacity of 1140 and 1000 mAh/g was achieved in the 2nd and 3rd cycle. With the increase of the cycle numbers, the capacity fades very slowly.

The electrochemical performances of pure SnO<sup>2</sup> and CNT porous composite electrode were evaluated (**Figure 11**). The pure SnO<sup>2</sup> and slurry (1.8 g SnO<sup>2</sup> : 0.6 g CNT: 0.3 g PVDF) were coated on copper foil and CNT paper collector, respectively. The charge-discharge current was hold at 100 mA/g. **Figure 11** shows the capacity of pure SnO<sup>2</sup> cathode exhibited abrupt capacity fading at the initial stage of cycling. After 20 cycles, the capacity almost faded away completely. The cathode-doped CNTs coated on copper foil collector maintained a capacity of 200mAh/g after 20 cycles. The cathode with CNT paper as collector exhibited excellent performance. The capacity was tend to stabilization after 6 cycles and still holds a capacity of about 600mAh/g after 20 cycles. The result suggested that the porous CNT paper collector is beneficial to enhance the kinetics of electrochemical reactions between SnO<sup>2</sup> and lithium ions with more interface areas.

Cyclic voltammetry curves were recorded for SnO<sup>2</sup>

senting the conversion of Sn to SnO<sup>2</sup>

**Figure 9.** SEM images of porous collector (a), SnO<sup>2</sup>

274 Carbon Nanotubes - Recent Progress

irreversible transformation of SnO<sup>2</sup>

highly reversible.

The first cycle curve in the anodic sweep is substantially different from the subsequent curves. During the first oxidation scan, three characteristic oxidation peaks were observed at 0.3, 0.5, and 1.3 V. The strong peak at 0.3 V can be ascribed to lithium insertion into CNTs (Eq.(1)), which have been treated at 3000°C and obtained at very high crystallinity degree [89]. The

(b), CNTs (c), and HRTEM image of CNTs (d).

[90]. The broad peak at 1.3 V can be considered to be the partly reversible reaction repre-

characteristic peaks were observed at 0.65 V. The sharp peak at 0.65 V can be ascribed to the

peak (Eq. (3)) [92]. The sharp reduction peak only appeared in first discharge cycle, which was originated from the initial irreversible capacity [92]. During subsequent 2nd and 3rd cycles, the CV curves were overlapped fully. This implied that the reactions in Eqs. (3) and (4) are

formation of the solid electrolyte interphase (SEI) film on surface of CNTs and SnO<sup>2</sup>

strong peak at 0.5 V was corresponding to alloying process for forming Lix

/CNT electrodes as shown in **Figure 10(a)**.

(Eq. (3)) [91]. In first cathodic sweep process, sharp

to Sn was also contributed to the prominent reduction

Sn alloy (Eq. (4))

[92]. The

The SnO<sup>2</sup> /CNT paper electrode presents the excellent electrochemical performance. **Figure 12(a)** shows the rate capabilities of SnO<sup>2</sup> /CNT paper electrode at various current densities from 100 mA/g to 500 mA/g. It exhibits a first discharge capacity of 1218 100mAh/g compared with a first discharge capacity of 1050mAh/g of copper foil collector. SnO<sup>2</sup> /CNT/cellulose electrode showed remarkably enhanced rate capability, and much better capacity retention was attained, particularly at high current densities during charging. The capacity retention holds 680 mAh/g

6 cycles. After then, the capacity fading was quite gentle. It should be noted that the retained capacity of the electrode was approximately 500 mAh/g after 100 cycles, which is much higher than that of the commercial graphite electrode. Furthermore, the CNT electrode maintained a

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium…

trode before cycles and after cycles, respectively. The experimental data show that the electrochemical impedance spectra of the electrodes consist of a single depressed semicircle in the high-to-medium frequency region and an inclined line at low frequencies. Before cycles, the charge-transfer resistance of the electrode reached 260 Ω. But after 100 cycles, the chargetransfer resistance of the electrode drastically declined to 60 Ω. This can be attributed to the porous structure of the electrode and 3D conductive network formed in the electrode by CNTs, which provide good electrolyte wetting and infiltrating and fast charge transport chan-

Sn can be dispersed and infiltrated into the space of the collector. This resulted in the increase

the micropores of the electrode enhanced effectively the electron transfer. This can account for

trical resistance decreased substantially, which are beneficial to high-rate charge-discharge

aggregation. **Figure 14(c, d)** shows the image of cross section of both electrodes. **Figure 14(c)**

was embedded into micropores of CNT/cellulose electrodes. This made an intimate

and CNTs and increases interfacial contact areas. The interface elec-

/CNT/cellulose can bear big

http://dx.doi.org/10.5772/intechopen.71916

/CNT/cellulose elec-

277

, and reduced element

/CNT/cellulose electrodes. It can be observed

/CNT/cellulose electrodes after cycles.

nanoparticles do not show obvious

) encapsulated in

stable coulombic efficiency of over 98%. This suggested the SnO<sup>2</sup>

**Figure 13** illustrates the electrochemical impedance analyses of the SnO<sup>2</sup>

nel. The porous collector provides a large of space for storage of SnO<sup>2</sup>

the decline of charge-transfer resistance after charge/discharge cycles.

of contact interface areas for activated materials and collector. The Sn (SnO<sup>2</sup>

current shock and maintain good cycle performance.

**Figure 14(a)** shows top surface images of SnO<sup>2</sup>

cycles. **Figure 14(b)** shows the morphologies of SnO<sup>2</sup>

coated on CNT collector.

A smooth layer on the surface can be observed. The SnO<sup>2</sup>

that SnO<sup>2</sup>

contact between SnO<sup>2</sup>

**Figure 13.** EIS of SnO<sup>2</sup>

**Figure 11.** Cycling performance of SnO<sup>2</sup> coated on different collector.

**Figure 12.** Cycling and rate performance of SnO<sup>2</sup> coated on CNT porous collector.

at 100 mA/g, 580 mAh/g at 200 mA/g, 420 mAh/g at 300 mA/g, 310 mAh/g at 400 mA/g, and 220 mAh/g at 500 mA/g. When the current density returned to 100 and 50 mA/g, respectively, the discharge capacity was recovered to 610 and 620 mAh/g in spite of subjecting to cycling at high current densities. The outstanding rate retention identified that the composite electrode is robust and highly stable even so high rate charge-discharge was tested.

The cycling performance and coulombic effect of SnO<sup>2</sup> /CNT electrode were evaluated at 200 mA/g. As presented in **Figure 12(b)**, the capacity of electrode reduces rapidly during first 6 cycles. After then, the capacity fading was quite gentle. It should be noted that the retained capacity of the electrode was approximately 500 mAh/g after 100 cycles, which is much higher than that of the commercial graphite electrode. Furthermore, the CNT electrode maintained a stable coulombic efficiency of over 98%. This suggested the SnO<sup>2</sup> /CNT/cellulose can bear big current shock and maintain good cycle performance.

**Figure 13** illustrates the electrochemical impedance analyses of the SnO<sup>2</sup> /CNT/cellulose electrode before cycles and after cycles, respectively. The experimental data show that the electrochemical impedance spectra of the electrodes consist of a single depressed semicircle in the high-to-medium frequency region and an inclined line at low frequencies. Before cycles, the charge-transfer resistance of the electrode reached 260 Ω. But after 100 cycles, the chargetransfer resistance of the electrode drastically declined to 60 Ω. This can be attributed to the porous structure of the electrode and 3D conductive network formed in the electrode by CNTs, which provide good electrolyte wetting and infiltrating and fast charge transport channel. The porous collector provides a large of space for storage of SnO<sup>2</sup> , and reduced element Sn can be dispersed and infiltrated into the space of the collector. This resulted in the increase of contact interface areas for activated materials and collector. The Sn (SnO<sup>2</sup> ) encapsulated in the micropores of the electrode enhanced effectively the electron transfer. This can account for the decline of charge-transfer resistance after charge/discharge cycles.

**Figure 14(a)** shows top surface images of SnO<sup>2</sup> /CNT/cellulose electrodes. It can be observed that SnO<sup>2</sup> was embedded into micropores of CNT/cellulose electrodes. This made an intimate contact between SnO<sup>2</sup> and CNTs and increases interfacial contact areas. The interface electrical resistance decreased substantially, which are beneficial to high-rate charge-discharge cycles. **Figure 14(b)** shows the morphologies of SnO<sup>2</sup> /CNT/cellulose electrodes after cycles. A smooth layer on the surface can be observed. The SnO<sup>2</sup> nanoparticles do not show obvious aggregation. **Figure 14(c, d)** shows the image of cross section of both electrodes. **Figure 14(c)**

**Figure 13.** EIS of SnO<sup>2</sup> coated on CNT collector.

at 100 mA/g, 580 mAh/g at 200 mA/g, 420 mAh/g at 300 mA/g, 310 mAh/g at 400 mA/g, and 220 mAh/g at 500 mA/g. When the current density returned to 100 and 50 mA/g, respectively, the discharge capacity was recovered to 610 and 620 mAh/g in spite of subjecting to cycling at high current densities. The outstanding rate retention identified that the composite electrode

coated on different collector.

coated on CNT porous collector.

200 mA/g. As presented in **Figure 12(b)**, the capacity of electrode reduces rapidly during first

/CNT electrode were evaluated at

is robust and highly stable even so high rate charge-discharge was tested.

The cycling performance and coulombic effect of SnO<sup>2</sup>

**Figure 12.** Cycling and rate performance of SnO<sup>2</sup>

**Figure 11.** Cycling performance of SnO<sup>2</sup>

276 Carbon Nanotubes - Recent Progress

contact interface areas of collector and tin dioxide and lowed interface electrical resistance. This is in full accord with EIS spectra (**Figure 13a**), which indicated that the charge transfer resistance became smaller after cycles. The diffusion also improved reversible capacity and

electrode for lithium ion battery electrode. The 3D structure with interconnected channel can

much better capacity retention. The capacity retention holds 680 mAh/g at 100 mA/g and 220 mAh/g at 500 mA/g. When the current density returned to 100 mA/g, the discharge capacity

shock and maintain good cycle performance. The CNT electrode also shows a stable coulom-

Commercial nanosilicon powders (purity: 99.0%, average diameter: 100 nm) and cellulose fibers were purchased in market. CNTs were synthesized by floating reactant method in a vertical tubular reactor. Benzene was used as carbon feedstock, organic-metallic compound (ferrocene) as a catalyst precursor, thiophene as growth promotion agent, and hydrogen as

**3.3. CNTCP used to replace aluminum foils as collector and scaffold for nanosilicon** 

for high energy density battery. The hierarchical pores and channels can

The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium…

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279

/CNT/ cellulose electrode showed remarkably enhanced rate capability and

during charge/discharge and maintain a good cycle

/CNT/cellulose can bear big current

as positive

In summary, we have demonstrated a porous CNT/cellulose host to support SnO<sup>2</sup>

/CNT porous electrodes (a) before and (b) after cycles.

maintained a good cycle stability from beginning of 40th cycles.

was recovered to 610 mAh/g. This suggested that the SnO<sup>2</sup>

*3.2.3. Conclusion*

**Figure 15.** EDS of SnO<sup>2</sup>

load ultrahigh SnO<sup>2</sup>

bic efficiency of over 98%.

stability. SnO<sup>2</sup>

**anode**

*3.3.1. Experiment*

*3.3.1.1. Raw materials*

accommodate the big expansion of SnO<sup>2</sup>

**Figure 14.** SEM images of SnO<sup>2</sup> /CNT/cellulose electrodes (a) top surface before cycles, (b) top surface after cycles, (c) cross section after cycles, and (d) SnO<sup>2</sup> /copper foil electrode cross section after cycles.

demonstrates that SnO<sup>2</sup> and CNT/Cellulose collector have close combination, and no cracks can be observed on interface of both after cycles. **Figure 14(c)** further revealed that SnO<sup>2</sup> was embedded into CNT/cellulose collector and made an intimate contact which was attributed to good cycle stability. However, SnO<sup>2</sup> /copper foil electrode shows an obvious crack and pores on interface after cycles. It can also be observed that SnO<sup>2</sup> and Cu foil collector have not good affinity, and there can be visible interface pores which resulted in the decreasing contact interface. This sharply increasing of interface resistance will result in rapid fading of capacity.

**Figure 15(a)** shows that SnO<sup>2</sup> was only observed on the top of CNT porous collector. After charge-discharge cycles, SnO<sup>2</sup> intercalated into the interior of CNT collector and fully dispersed in the collector (**Figure 15(b)**). This can be attributed to the special interconnected channel of the CNT collector which provided and enhanced diffusion of nano-tin reduced from SnO<sup>2</sup> in discharging process. The homogeneous diffusion of tin increased effectively

**Figure 15.** EDS of SnO<sup>2</sup> /CNT porous electrodes (a) before and (b) after cycles.

contact interface areas of collector and tin dioxide and lowed interface electrical resistance. This is in full accord with EIS spectra (**Figure 13a**), which indicated that the charge transfer resistance became smaller after cycles. The diffusion also improved reversible capacity and maintained a good cycle stability from beginning of 40th cycles.

#### *3.2.3. Conclusion*

In summary, we have demonstrated a porous CNT/cellulose host to support SnO<sup>2</sup> as positive electrode for lithium ion battery electrode. The 3D structure with interconnected channel can load ultrahigh SnO<sup>2</sup> for high energy density battery. The hierarchical pores and channels can accommodate the big expansion of SnO<sup>2</sup> during charge/discharge and maintain a good cycle stability. SnO<sup>2</sup> /CNT/ cellulose electrode showed remarkably enhanced rate capability and much better capacity retention. The capacity retention holds 680 mAh/g at 100 mA/g and 220 mAh/g at 500 mA/g. When the current density returned to 100 mA/g, the discharge capacity was recovered to 610 mAh/g. This suggested that the SnO<sup>2</sup> /CNT/cellulose can bear big current shock and maintain good cycle performance. The CNT electrode also shows a stable coulombic efficiency of over 98%.

#### **3.3. CNTCP used to replace aluminum foils as collector and scaffold for nanosilicon anode**

#### *3.3.1. Experiment*

**Figure 14.** SEM images of SnO<sup>2</sup>

278 Carbon Nanotubes - Recent Progress

demonstrates that SnO<sup>2</sup>

cross section after cycles, and (d) SnO<sup>2</sup>

**Figure 15(a)** shows that SnO<sup>2</sup>

charge-discharge cycles, SnO<sup>2</sup>

from SnO<sup>2</sup>

good cycle stability. However, SnO<sup>2</sup>

on interface after cycles. It can also be observed that SnO<sup>2</sup>

/CNT/cellulose electrodes (a) top surface before cycles, (b) top surface after cycles, (c)

and CNT/Cellulose collector have close combination, and no cracks

/copper foil electrode shows an obvious crack and pores

was only observed on the top of CNT porous collector. After

intercalated into the interior of CNT collector and fully dis-

and Cu foil collector have not

was

/copper foil electrode cross section after cycles.

can be observed on interface of both after cycles. **Figure 14(c)** further revealed that SnO<sup>2</sup>

embedded into CNT/cellulose collector and made an intimate contact which was attributed to

good affinity, and there can be visible interface pores which resulted in the decreasing contact interface. This sharply increasing of interface resistance will result in rapid fading of capacity.

persed in the collector (**Figure 15(b)**). This can be attributed to the special interconnected channel of the CNT collector which provided and enhanced diffusion of nano-tin reduced

in discharging process. The homogeneous diffusion of tin increased effectively

#### *3.3.1.1. Raw materials*

Commercial nanosilicon powders (purity: 99.0%, average diameter: 100 nm) and cellulose fibers were purchased in market. CNTs were synthesized by floating reactant method in a vertical tubular reactor. Benzene was used as carbon feedstock, organic-metallic compound (ferrocene) as a catalyst precursor, thiophene as growth promotion agent, and hydrogen as carrier gas. The reaction temperature is around 1200°C. The following thermal treatment of CNTs was carried out at 3000°C for 20 h using a graphite-resistance furnace operating in a vacuum atmosphere.

#### *3.3.1.2. Preparation of ternary electrodes*

The CNTs powder was dispersed in deionized water with a surfactant of polyvinylpyrrolidone (PVP). The paper cellulose is made of napkins in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by acute stirring. The mixed slurry of nanosilicon, CNTs, and cellulose fibers was infiltrated by vacuum. A porous mats of CNTs and cellulose fibers embedded with nanosilicon were obtained for anode applications.

#### *3.3.1.3. Characterizations and testing*

The CNTs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrometry, and thermogravimetry (TGA). Nanosilicon and the microstructure of the electrodes were observed by SEM.

The CR2025 coin-type cells were assembled in an Ar-filled glove box with a metallic lithium foil as the counter electrode. The electrochemical performance was measured by a cell tester with 1 M LiPF<sup>6</sup> in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as electrolyte. The galvanostatic charge-discharge was tested within a cut-off voltage window from 0 to 3.0 V at a current density of 80 mA/g.

some irreversible capacity loss. All those resulted in continuous capacity loss in cycles [101–103]. **Figure 18a** also exhibits an obvious charge voltage plateau at 0.5 V in first 10 cycles, which was connected to the characteristic of dealloying of Li/Si alloy. **Figure 18b** shows the discharge and charge profile of CNTCP electrode. The first discharge capacity and Coulomb efficiency of CNTCP electrode reached 2107mAh/g and 85%, respectively, which are 106 and 6% higher than those of CF electrode. CNTCP electrode becomes stable after 3 cycles, and Coulomb efficiency is closed to 100%. After 30 cycles, the specific capacity still maintained 1000mAh/g compared with only 180mAh/g of CP electrode. The cycling performances of both electrodes are showed in **Figure 18**. CNTCP electrode exhibited a 1000, 900, 500, and 200 mAh/g at a current density of 80, 200, 1000, and 2000 mA/g, respectively. When the current density reduced to 200 mA/g, the specific capacity restored 850 mAh/g after 160 cycles, which illustrated the highly reversible electrochemical behavior of CNTCP electrode. As a contrast, the capacity of CF electrode rapidly faded during cycles. The capacity declined to around 200 mAh/g at a current density of 80 mA/g after 30 cycles. The capacity

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281

**Figure 16.** SEM images of nanosilicon (a) and composite Si/CNT anode (b).

**Figure 17.** SEM images of CNT paper (a) top and (b) single cellulose fiber coated by MWCNTs.

#### *3.3.2. Results and discussion*

**Figure 16a, b** shows the morphology and structure of nanosilicon and composite anode of Si/ CNT. The SEM image exhibits that the nanosilicon powders behave a polyhedral structure and an average diameter distribution of about 100 nm.

**Figure 16b** shows that nanosilicon is uniformly embedded in the network of CNTs. **Figure 17a** shows the morphology and structure of CNTCP with rough and porous surface. CNTs is uniformly dispersed and attached on paper fiber (**Figure 17b**). A good three-dimensional conductive network was constructed with cellulose fiber as the framework. **Figure 18** shows the galvanostatic charge-discharge curves of the Si/CNT composite anode at a current density of 80 mA/g with current collector of copper foil (CF) and CNTCP, respectively. It can be seen that CF electrode exhibited an initial discharge and charge specific capacity of 1020 mAh/g and 817 mAh/g, respectively (**Figure 18a**). The capacity rapidly faded with cycling. The capacity dropped to less than 200 mAh/g after 30 cycles. It was very evident that CF electrode exhibited very high irreversible capacity loss in cycles. The irreversible capacity is mainly from the growth of solid-electrolyte interface (SEI) films on surface of nanosilicon. The lithiation of silicon, which resulted in accumulation of Li + in silicon matrix to form Lix Si alloy, also contributes to the big irreversible capacity. Further, owing to big volume change over 400% and press [96–98] during lithium ion insertion and extraction, the nanosilicon crystals were cracked and pulverized by press. Those produce new interface for forming SEI film. The pulverization of nanosilicon will also result in incomplete dealloying [99, 100]. The phase change of crystalline nanosilicon into amorphous state also caused The Porous Carbon Nanotube-Cellulose Papers as Current Collector and Electrode for Lithium… http://dx.doi.org/10.5772/intechopen.71916 281

**Figure 16.** SEM images of nanosilicon (a) and composite Si/CNT anode (b).

carrier gas. The reaction temperature is around 1200°C. The following thermal treatment of CNTs was carried out at 3000°C for 20 h using a graphite-resistance furnace operating in a

The CNTs powder was dispersed in deionized water with a surfactant of polyvinylpyrrolidone (PVP). The paper cellulose is made of napkins in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by acute stirring. The mixed slurry of nanosilicon, CNTs, and cellulose fibers was infiltrated by vacuum. A porous mats of CNTs and cellulose

The CNTs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrometry, and thermogravimetry

The CR2025 coin-type cells were assembled in an Ar-filled glove box with a metallic lithium foil as the counter electrode. The electrochemical performance was measured by a cell tester

trolyte. The galvanostatic charge-discharge was tested within a cut-off voltage window from

**Figure 16a, b** shows the morphology and structure of nanosilicon and composite anode of Si/ CNT. The SEM image exhibits that the nanosilicon powders behave a polyhedral structure

**Figure 16b** shows that nanosilicon is uniformly embedded in the network of CNTs. **Figure 17a** shows the morphology and structure of CNTCP with rough and porous surface. CNTs is uniformly dispersed and attached on paper fiber (**Figure 17b**). A good three-dimensional conductive network was constructed with cellulose fiber as the framework. **Figure 18** shows the galvanostatic charge-discharge curves of the Si/CNT composite anode at a current density of 80 mA/g with current collector of copper foil (CF) and CNTCP, respectively. It can be seen that CF electrode exhibited an initial discharge and charge specific capacity of 1020 mAh/g and 817 mAh/g, respectively (**Figure 18a**). The capacity rapidly faded with cycling. The capacity dropped to less than 200 mAh/g after 30 cycles. It was very evident that CF electrode exhibited very high irreversible capacity loss in cycles. The irreversible capacity is mainly from the growth of solid-electrolyte interface (SEI) films on surface of nanosilicon. The lithiation of silicon, which resulted in accumu-

Further, owing to big volume change over 400% and press [96–98] during lithium ion insertion and extraction, the nanosilicon crystals were cracked and pulverized by press. Those produce new interface for forming SEI film. The pulverization of nanosilicon will also result in incomplete dealloying [99, 100]. The phase change of crystalline nanosilicon into amorphous state also caused

in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as elec-

Si alloy, also contributes to the big irreversible capacity.

(TGA). Nanosilicon and the microstructure of the electrodes were observed by SEM.

fibers embedded with nanosilicon were obtained for anode applications.

vacuum atmosphere.

280 Carbon Nanotubes - Recent Progress

with 1 M LiPF<sup>6</sup>

*3.3.2. Results and discussion*

*3.3.1.2. Preparation of ternary electrodes*

*3.3.1.3. Characterizations and testing*

0 to 3.0 V at a current density of 80 mA/g.

lation of Li + in silicon matrix to form Lix

and an average diameter distribution of about 100 nm.

some irreversible capacity loss. All those resulted in continuous capacity loss in cycles [101–103]. **Figure 18a** also exhibits an obvious charge voltage plateau at 0.5 V in first 10 cycles, which was connected to the characteristic of dealloying of Li/Si alloy. **Figure 18b** shows the discharge and charge profile of CNTCP electrode. The first discharge capacity and Coulomb efficiency of CNTCP electrode reached 2107mAh/g and 85%, respectively, which are 106 and 6% higher than those of CF electrode. CNTCP electrode becomes stable after 3 cycles, and Coulomb efficiency is closed to 100%. After 30 cycles, the specific capacity still maintained 1000mAh/g compared with only 180mAh/g of CP electrode. The cycling performances of both electrodes are showed in **Figure 18**. CNTCP electrode exhibited a 1000, 900, 500, and 200 mAh/g at a current density of 80, 200, 1000, and 2000 mA/g, respectively. When the current density reduced to 200 mA/g, the specific capacity restored 850 mAh/g after 160 cycles, which illustrated the highly reversible electrochemical behavior of CNTCP electrode. As a contrast, the capacity of CF electrode rapidly faded during cycles. The capacity declined to around 200 mAh/g at a current density of 80 mA/g after 30 cycles. The capacity

**Figure 17.** SEM images of CNT paper (a) top and (b) single cellulose fiber coated by MWCNTs.

**Figure 18.** Discharge and charge curves of (a) copper foil cell and (b) CNT paper cell.

lowered to around 100 mAh/g after 69 cycles at a current density of 200 mA/g (**Figure 19**). The rapid decay of capacity of CF electrode was ascribed to the cracking and pulverization of silicone, which resulted in more SEI forming on interface and inefficacy of silicone, owing to big volume change and high press cannot be buffered on copper foil current collector. **Figure 20a** and **b** shows the cross-section images of CF electrode and CNTCP electrode. It can be observed in **Figure 20a** that Cu foil collector has no good affinity and nanosilicon easily produces pores on interface and big interface resistance. The sharply increasing of interface resistance will result in rapid fading of capacity. The CNTCP collector has high-porous structure and interconnected channel. Carbon and cellulose fiber have intrinsic affinity with activated materials. Nano-Si was embedded into surface

pores of CNTCP, which contributes to the low interface resistance and fast transfer of electrons and ions. The rate performance and utilization ratio of the active material were further improved. To better understand the volume change and the stability of SEI layer, the impedance analysis was performed. All the EIS curves (**Figure 21**) show a depressed semicircle in the high frequency region and an oblique line in the low frequency domain. The CF electrode and CNTCP electrode exhibited a charge -transfer resistance 820 Ω (**Figure 21a**) and 580 Ω (**Figure 21c**) respectively. CNTCP electrode decreased by 29% than CF electrode because nano-Si was embedded into CNTCP host and effectively reduced the resistance of the coated activated material. After 30 cycles, the CNTCP electrode demonstrated significantly superiority than CF electrode. Owing to excellent accommodation of volume expansion and electrolyte absorptivity of CNTCP electrode, the charge-transfer resistance of CNTCP electrode decreased to around 30 Ω (**Figure 21c**). On the contrary, the charge-transfer resistance of CF electrode rapidly increased to around 2000 Ω (**Figure 21d**). This may be ascribed to the exfoliation of activated materials from copper collector owing to big volume expansion of silicone in cycles. Here, CNTCP not only was used as current collector but also provides the battery with a certain capacity. **Figure 22b** revealed that CNTCP contributes 200mAh/g reversible capacity, while CF only provides a capacity of 45mAh/g [100, 104–117]. The CNTCP also consumes an

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**Figure 20.** SEM image of interface of copper foil electrode (a), CNT paper electrode (b).

In the summary, the porous CNTCP was used as host of nano-Si for high-performance Li-ion battery. The CNTCP exhibited high conduction and excellent absorptivity of electrolyte. Nano-Si penetrated into the network of CNTCP to achieve a low interface resistance for high rate performance. The porous CNTCP with interconnected channel can absorb an amount of electrolyte and was wetted fully by electrolyte. All those contribute to the electrochemical performance of CNTCP electrode. The CNTCP electrode obtained a reversible capacity, as high as 1000mAh/g after 50 charge-discharge cycles. The good cycle performance is ascribed to

irreversible capacity of 170 mAh/g (**Figure 22a**).

*3.3.3. Conclusions*

**Figure 19.** Cycle performance in different rate of copper foil cell and CNT paper cell.

**Figure 20.** SEM image of interface of copper foil electrode (a), CNT paper electrode (b).

pores of CNTCP, which contributes to the low interface resistance and fast transfer of electrons and ions. The rate performance and utilization ratio of the active material were further improved.

To better understand the volume change and the stability of SEI layer, the impedance analysis was performed. All the EIS curves (**Figure 21**) show a depressed semicircle in the high frequency region and an oblique line in the low frequency domain. The CF electrode and CNTCP electrode exhibited a charge -transfer resistance 820 Ω (**Figure 21a**) and 580 Ω (**Figure 21c**) respectively. CNTCP electrode decreased by 29% than CF electrode because nano-Si was embedded into CNTCP host and effectively reduced the resistance of the coated activated material. After 30 cycles, the CNTCP electrode demonstrated significantly superiority than CF electrode. Owing to excellent accommodation of volume expansion and electrolyte absorptivity of CNTCP electrode, the charge-transfer resistance of CNTCP electrode decreased to around 30 Ω (**Figure 21c**). On the contrary, the charge-transfer resistance of CF electrode rapidly increased to around 2000 Ω (**Figure 21d**). This may be ascribed to the exfoliation of activated materials from copper collector owing to big volume expansion of silicone in cycles.

Here, CNTCP not only was used as current collector but also provides the battery with a certain capacity. **Figure 22b** revealed that CNTCP contributes 200mAh/g reversible capacity, while CF only provides a capacity of 45mAh/g [100, 104–117]. The CNTCP also consumes an irreversible capacity of 170 mAh/g (**Figure 22a**).

#### *3.3.3. Conclusions*

lowered to around 100 mAh/g after 69 cycles at a current density of 200 mA/g (**Figure 19**). The rapid decay of capacity of CF electrode was ascribed to the cracking and pulverization of silicone, which resulted in more SEI forming on interface and inefficacy of silicone, owing to big volume change and high press cannot be buffered on copper foil current collector. **Figure 20a** and **b** shows the cross-section images of CF electrode and CNTCP electrode. It can be observed in **Figure 20a** that Cu foil collector has no good affinity and nanosilicon easily produces pores on interface and big interface resistance. The sharply increasing of interface resistance will result in rapid fading of capacity. The CNTCP collector has high-porous structure and interconnected channel. Carbon and cellulose fiber have intrinsic affinity with activated materials. Nano-Si was embedded into surface

**Figure 18.** Discharge and charge curves of (a) copper foil cell and (b) CNT paper cell.

282 Carbon Nanotubes - Recent Progress

**Figure 19.** Cycle performance in different rate of copper foil cell and CNT paper cell.

In the summary, the porous CNTCP was used as host of nano-Si for high-performance Li-ion battery. The CNTCP exhibited high conduction and excellent absorptivity of electrolyte. Nano-Si penetrated into the network of CNTCP to achieve a low interface resistance for high rate performance. The porous CNTCP with interconnected channel can absorb an amount of electrolyte and was wetted fully by electrolyte. All those contribute to the electrochemical performance of CNTCP electrode. The CNTCP electrode obtained a reversible capacity, as high as 1000mAh/g after 50 charge-discharge cycles. The good cycle performance is ascribed to the accommodation and buffering effect to huge volume expansion of silicone during cycles. Therefore, the author believes that CNTCP has a good application prospect as current collector for silicon-based lithium ion batteries.

**3.4. Primary battery based on fluorinated CNTs as cathode with CNTCP collector**

ductivity and outer layers formed the structure of fluorinated carbon.

*3.4.1.3. Primary Li/CFx assemble and electrochemical characterization*

performed on a CHI660B electrochemical workstation.

Three samples of fluorinated CNTs (FCNTs) were synthesized by direct fluorination of multiwalled carbon nanotubes [118]. The fluorination temperature was set at around 400°C. F/C ratio was adjusted to 0.28, 0.59, and 0.81. The FCNTs maintained a core-shell-hole structure after fluorination. The core still maintained pristine MWCNT structure with high electric con-

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Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were performed to observe the core-shell-hole structure of FCNTs and analyzed their composition (EDX). F/C ratio was obtained by chemical titration method and EDX spectrometry. The change of atomic and electronic structures of FCNTs was investigated using X-ray photoelectron spectroscopy (XPS). The thermal stability of the FCNTs was investigated by thermogravimetric analysis (TGA). The weight loss of the material under argon atmosphere was recorded, while it was heated at a rate of 10°C/min from ambient temperature to 900°C. X-ray diffraction (XRD) and Raman were performed to identify the structure of FCNTs

The primary Li/CFx (x = 0.81) batteries were assembled with FCNTs as cathode against Li foil as anode. The electrochemical performance of the batteries was measured by cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) measurements were

SEM and HRTEM images of the FCNTs (CF0.81) are presented in **Figure 23a** and **b**, respectively. After fluorination, the FCNTs exhibited a classic core-shell-hole structure. **Figure 24** shows the microstructure of FCNTs (CF0.81). It can be clearly observed that the outer layers of CNTs were inserted by fluorine to form fluorinated carbides and inner layers hold pristine highly crystallized structure of CNTs. The exterior shell thickness of carbon fluorides (CF0.81) was around 12 nm and internal layers of CNTs retained pristine graphitized structure with a thickness around 10 nm. The novel structure suggested that FCNTs still hold good interior electron conduction channel, even though exterior CNTs transformed into electrical insulator of carbon fluorides (CF0.81). The central hole of CNT still remained with a diameter of 6 nm. The core-shell-hole structure means that the FCNTs maintained better electric conductivity than those of FG which is electric insulator. The central holes were contributed to the absorption of electrolyte and the diffusion of

lithium ions. The novel structure can improve the conduction of both electrons but ions.

*3.4.1. Experimental*

*3.4.1.2. Physical characterization*

and primary CNTs.

*3.4.2. Results and discussion*

*3.4.1.1. Fluorination of multiwalled carbon nanotubes*

**Figure 21.** Impedance spectra of CF and CNT electrodes (a) fresh CF electrode (b) CF electrode after 30 cycles (c) fresh CNT electrodes (d) CNT electrodes after 30 cycles.

**Figure 22.** (a) First 30 discharge and charge curves and (b) cycle performance of CNTCP.

#### **3.4. Primary battery based on fluorinated CNTs as cathode with CNTCP collector**

#### *3.4.1. Experimental*

the accommodation and buffering effect to huge volume expansion of silicone during cycles. Therefore, the author believes that CNTCP has a good application prospect as current collec-

**Figure 21.** Impedance spectra of CF and CNT electrodes (a) fresh CF electrode (b) CF electrode after 30 cycles (c) fresh

**Figure 22.** (a) First 30 discharge and charge curves and (b) cycle performance of CNTCP.

tor for silicon-based lithium ion batteries.

284 Carbon Nanotubes - Recent Progress

CNT electrodes (d) CNT electrodes after 30 cycles.

#### *3.4.1.1. Fluorination of multiwalled carbon nanotubes*

Three samples of fluorinated CNTs (FCNTs) were synthesized by direct fluorination of multiwalled carbon nanotubes [118]. The fluorination temperature was set at around 400°C. F/C ratio was adjusted to 0.28, 0.59, and 0.81. The FCNTs maintained a core-shell-hole structure after fluorination. The core still maintained pristine MWCNT structure with high electric conductivity and outer layers formed the structure of fluorinated carbon.

#### *3.4.1.2. Physical characterization*

Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were performed to observe the core-shell-hole structure of FCNTs and analyzed their composition (EDX). F/C ratio was obtained by chemical titration method and EDX spectrometry. The change of atomic and electronic structures of FCNTs was investigated using X-ray photoelectron spectroscopy (XPS). The thermal stability of the FCNTs was investigated by thermogravimetric analysis (TGA). The weight loss of the material under argon atmosphere was recorded, while it was heated at a rate of 10°C/min from ambient temperature to 900°C. X-ray diffraction (XRD) and Raman were performed to identify the structure of FCNTs and primary CNTs.

#### *3.4.1.3. Primary Li/CFx assemble and electrochemical characterization*

The primary Li/CFx (x = 0.81) batteries were assembled with FCNTs as cathode against Li foil as anode. The electrochemical performance of the batteries was measured by cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660B electrochemical workstation.

#### *3.4.2. Results and discussion*

SEM and HRTEM images of the FCNTs (CF0.81) are presented in **Figure 23a** and **b**, respectively. After fluorination, the FCNTs exhibited a classic core-shell-hole structure. **Figure 24** shows the microstructure of FCNTs (CF0.81). It can be clearly observed that the outer layers of CNTs were inserted by fluorine to form fluorinated carbides and inner layers hold pristine highly crystallized structure of CNTs. The exterior shell thickness of carbon fluorides (CF0.81) was around 12 nm and internal layers of CNTs retained pristine graphitized structure with a thickness around 10 nm. The novel structure suggested that FCNTs still hold good interior electron conduction channel, even though exterior CNTs transformed into electrical insulator of carbon fluorides (CF0.81). The central hole of CNT still remained with a diameter of 6 nm. The core-shell-hole structure means that the FCNTs maintained better electric conductivity than those of FG which is electric insulator. The central holes were contributed to the absorption of electrolyte and the diffusion of lithium ions. The novel structure can improve the conduction of both electrons but ions.

weight can be ascribed to the forming of gaseous carbon-nitrogen compounds ((CN)x

ment with XRD (**Figure 25a**).

temperature from 620–900°C [1]. The last remaining weight of four samples hold 89.4, 31.8, 29.5, and 15.8%, respectively, at 900°C. XRD patterns are shown for CNTs and FCNTs (CF0.28, CF0.56, CF0.81) in **Figure 25a** and **b**, respectively. The stronger graphite peak (002) is observed at around 26.5° for CNTs (**Figure 25a**). But the graphite peak of FCNTs gradually weaken and finally disappeared with the enhancing fluorination of CNTs (**Figure 25a**). The outer layer structure of CNTs was destroyed with the formation of FCNTs. Fluorination only affects the outer layers of the CNTs which was resulted in destruction of graphite structure. The FCNT inner layers maintained graphite structure (**Figure 23b**). **Figure 25b** shows the Raman spectra of CNTs and FCNTs respectively. The characteristic D, G, and 2D bands are seen in two samples. Each spectrum consists of a distinctive pair of broadband at 1580 cm−1 (G band) and 1360 cm−1 (D band). The Raman peaks correspond to sp2 and sp3 carbon stretching modes, respectively. The intensity ratio of the G-band to the D-band (IG/ID) can be used to evaluate the graphitized degree of the samples [121, 122]. The IG/ID ratio of the CNTs reaches 4.3, which means a very high crystallinity. The ratio of IG/ID was seen to decrease to 0.66 for the FMWCNTs, as fluorine atoms are incorporated into the sp2 carbon lattice of outer layers to form amorphous sp3 structure in fluorination progresses. These results are in good agree-

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The X-ray photoelectron spectroscopy (XPS) of the CNTs were obtained with a MultiLab 3000 spectrometer. The spectra were analyzed in order to investigate the nature of FCNTs. XPS of pure CNTs and FCNT samples are plotted in **Figure 26a**. The distinct carbon peaks of spectra were around 285.0 eV. The fluorine peaks in FCNTs were also found around 687.7 eV. With the fluorination enhanced, the F1 s peak was increased, while the intensity of the C1s peak was decreased. The detailed features for C1s peak are shown in **Figure 26b**. The peak at a binding energy (284.6 eV) in C1s spectra was assigned to sp2 hybridized carbon in CNTs. To format C-F bonds as a result of fluorination, the peak at a binding energy (289.4 eV) was assigned to sp3 hybridized carbon [118]. The peak (sp2) becomes broader and finally faded away, while the peak (sp3) was strengthened as the fluorination progresses [123]. Based on EDAX data, the four samples have been identified as CF0.28, CF0.56, CF0.81, and pure CNTs.

**Figure 25.** (a) XRD of CF0.28, CF0.59, CF0.81 and CNT and (b) Raman of CF0.81 and CNT.

, etc.) at

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**Figure 23.** SEM and HRTEM of FCNTs (CF0.28).

Carbon fluorides (CF*x*) are a type of nonstoichiometric compounds, whose thermal stability depends on the content of fluorine, the nature of carbon precursor, and the method of synthesis [8, 9]. The TGA of pristine CNTs and three FCNTs (CF0.28,CF0.56, and CF0.81) are shown in **Figure 24**. It was founded that there is little loss for MWCNTs before 700°C. The weight loss of CNTs slowly began at 700°C and only has a weight loss of 10.6% at 900°C. It suggested a high heat stability of CNTs which have been treated at a temperature of 3000 and obtained very high crystallinity over 90%. The three FCNTs maintained stable before 460°C. But they began to decompose slowly when the temperature continues to increase. The decomposing of FCNTs accelerated at 500°C and terminated basically at 620°C. The primary volatile products of the thermal decomposition of the FCNTs contained CF<sup>4</sup> and C<sup>2</sup> F4 , which are formed through a fluorocarbon radical process [119, 120]. It can be concluded that the CF0.28, CF0.56, and CF0.81 contained around 40, 55, and 70 wt% fluorinated carbons. The following loss of

**Figure 24.** TGA curves of CNTs and FCNTs (CF0.28, CF0.56, CF0.81) in nitrogen atmosphere from room temperature to 900°C.

weight can be ascribed to the forming of gaseous carbon-nitrogen compounds ((CN)x , etc.) at temperature from 620–900°C [1]. The last remaining weight of four samples hold 89.4, 31.8, 29.5, and 15.8%, respectively, at 900°C. XRD patterns are shown for CNTs and FCNTs (CF0.28, CF0.56, CF0.81) in **Figure 25a** and **b**, respectively. The stronger graphite peak (002) is observed at around 26.5° for CNTs (**Figure 25a**). But the graphite peak of FCNTs gradually weaken and finally disappeared with the enhancing fluorination of CNTs (**Figure 25a**). The outer layer structure of CNTs was destroyed with the formation of FCNTs. Fluorination only affects the outer layers of the CNTs which was resulted in destruction of graphite structure. The FCNT inner layers maintained graphite structure (**Figure 23b**). **Figure 25b** shows the Raman spectra of CNTs and FCNTs respectively. The characteristic D, G, and 2D bands are seen in two samples. Each spectrum consists of a distinctive pair of broadband at 1580 cm−1 (G band) and 1360 cm−1 (D band). The Raman peaks correspond to sp2 and sp3 carbon stretching modes, respectively. The intensity ratio of the G-band to the D-band (IG/ID) can be used to evaluate the graphitized degree of the samples [121, 122]. The IG/ID ratio of the CNTs reaches 4.3, which means a very high crystallinity. The ratio of IG/ID was seen to decrease to 0.66 for the FMWCNTs, as fluorine atoms are incorporated into the sp2 carbon lattice of outer layers to form amorphous sp3 structure in fluorination progresses. These results are in good agreement with XRD (**Figure 25a**).

Carbon fluorides (CF*x*) are a type of nonstoichiometric compounds, whose thermal stability depends on the content of fluorine, the nature of carbon precursor, and the method of synthesis [8, 9]. The TGA of pristine CNTs and three FCNTs (CF0.28,CF0.56, and CF0.81) are shown in **Figure 24**. It was founded that there is little loss for MWCNTs before 700°C. The weight loss of CNTs slowly began at 700°C and only has a weight loss of 10.6% at 900°C. It suggested a high heat stability of CNTs which have been treated at a temperature of 3000 and obtained very high crystallinity over 90%. The three FCNTs maintained stable before 460°C. But they began to decompose slowly when the temperature continues to increase. The decomposing of FCNTs accelerated at 500°C and terminated basically at 620°C. The primary volatile prod-

through a fluorocarbon radical process [119, 120]. It can be concluded that the CF0.28, CF0.56, and CF0.81 contained around 40, 55, and 70 wt% fluorinated carbons. The following loss of

**Figure 24.** TGA curves of CNTs and FCNTs (CF0.28, CF0.56, CF0.81) in nitrogen atmosphere from room temperature to 900°C.

 and C<sup>2</sup> F4

, which are formed

ucts of the thermal decomposition of the FCNTs contained CF<sup>4</sup>

**Figure 23.** SEM and HRTEM of FCNTs (CF0.28).

286 Carbon Nanotubes - Recent Progress

The X-ray photoelectron spectroscopy (XPS) of the CNTs were obtained with a MultiLab 3000 spectrometer. The spectra were analyzed in order to investigate the nature of FCNTs. XPS of pure CNTs and FCNT samples are plotted in **Figure 26a**. The distinct carbon peaks of spectra were around 285.0 eV. The fluorine peaks in FCNTs were also found around 687.7 eV. With the fluorination enhanced, the F1 s peak was increased, while the intensity of the C1s peak was decreased. The detailed features for C1s peak are shown in **Figure 26b**. The peak at a binding energy (284.6 eV) in C1s spectra was assigned to sp2 hybridized carbon in CNTs. To format C-F bonds as a result of fluorination, the peak at a binding energy (289.4 eV) was assigned to sp3 hybridized carbon [118]. The peak (sp2) becomes broader and finally faded away, while the peak (sp3) was strengthened as the fluorination progresses [123]. Based on EDAX data, the four samples have been identified as CF0.28, CF0.56, CF0.81, and pure CNTs.

**Figure 25.** (a) XRD of CF0.28, CF0.59, CF0.81 and CNT and (b) Raman of CF0.81 and CNT.

**Figure 26.** (a) XPS showing both F1 s and C1s peaks for pristine and FCNTs (b) detailed features for C1s peak.

The discharge profile of FG (Li/CF<sup>1</sup> , theoretic capacity: 864mAh/g) cells is shown in **Figure 27a**. The discharge capacity reached 840 mAh/g (0.05 C), 744 mAh/g (0.1 C), 663 mAh/g (0.5 C), and 520 mAh/g (1 C). The graphite monofluoride exhibited the characteristic voltage plateau around 2.5 V at 0.05 C rate. The discharge plateau slowly dropped to about 2.0 V at discharge rate of 1 C. **Figure 27b** shows the discharge curves of FCNTs (Li/CF0.81). The discharge starts at a higher voltage of about 3.2 V and then drops to about 2.7 V at discharge rate of 0.05 C. The discharge plateau maintained at 2.7 V with a specific capacity of 782mAh/g, which reached 98.7% of theoretic capacity of FCNT (792mAh/g for CF0.81). The discharge plateau dropped to 2.45 V, when the discharge rate increased to 1 C from 0.05 C with a capacity of 620 mAh/g. The discharge efficiency lowered to 78.28% of theoretic capacity for carbon fluorides (CF0.81) (**Figure 27**).

Nyquist plots are composed of a semi-circle at high frequency and a linear line at low frequency. The intersection with the x-axis was corresponded to the charge-transfer resistance

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noteworthy that the charge-transfer resistance of the electrodes increased with the increasing content of fluorine. The higher ratio of fluorine/carbon provided more activated points for the reduction of carbon fluorides to lithium fluorides and resulted in higher discharge capacity

FCNTs exhibits a classic core-shell-hole structure. The outer layers of CNTs were Fluorinated and inner layers still maintained pristine graphitized structure integrity. The FCNTs hold good interior electron conduction channel, even though exterior CNTs transformed into electrical insulator. The core-shell-hole structure means that the FCNT maintained better electric conductivity than those of FG. FCNT exhibited better electrochemical performance than those of FG. The discharge plateau of FCNT maintained at 2.7 V (0.05C) with a specific capacity of 782 mAh/g, which reached 98.7% of theoretic capacity of FCNT (792mAh/g for CF0.81). The discharge plateau still maintained 2.45 V at 1 C with a capacity of 620 mAh/g, and the discharge efficiency reached 78.28% compared with FG of 2 V, 520 mAh/g, and 60%, respectively. FCNTs were considered as a promise cathode materials for high performance of lithium fluo-

electrode, respectively. It was

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.

of the electrode with 100 and 180 Ω for CF0.81 electrode and CF<sup>1</sup>

**Figure 28.** Electrochemical impedance spectroscopy (EIS) of CF0.28, CF0.56, CF0.81 and CF<sup>1</sup>

[124–128].

ride battery.

*3.4.3. Conclusions*

Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 100 kHz to 0.01 Hz at an excitation amplitude of 5 mV (**Figure 28**). The

**Figure 27.** Discharge profile of FG and three FCNT electrodes at different rates.

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**Figure 28.** Electrochemical impedance spectroscopy (EIS) of CF0.28, CF0.56, CF0.81 and CF<sup>1</sup> .

Nyquist plots are composed of a semi-circle at high frequency and a linear line at low frequency. The intersection with the x-axis was corresponded to the charge-transfer resistance of the electrode with 100 and 180 Ω for CF0.81 electrode and CF<sup>1</sup> electrode, respectively. It was noteworthy that the charge-transfer resistance of the electrodes increased with the increasing content of fluorine. The higher ratio of fluorine/carbon provided more activated points for the reduction of carbon fluorides to lithium fluorides and resulted in higher discharge capacity [124–128].

#### *3.4.3. Conclusions*

The discharge profile of FG (Li/CF<sup>1</sup>

288 Carbon Nanotubes - Recent Progress

(**Figure 27**).

, theoretic capacity: 864mAh/g) cells is shown in **Figure 27a**.

The discharge capacity reached 840 mAh/g (0.05 C), 744 mAh/g (0.1 C), 663 mAh/g (0.5 C), and 520 mAh/g (1 C). The graphite monofluoride exhibited the characteristic voltage plateau around 2.5 V at 0.05 C rate. The discharge plateau slowly dropped to about 2.0 V at discharge rate of 1 C. **Figure 27b** shows the discharge curves of FCNTs (Li/CF0.81). The discharge starts at a higher voltage of about 3.2 V and then drops to about 2.7 V at discharge rate of 0.05 C. The discharge plateau maintained at 2.7 V with a specific capacity of 782mAh/g, which reached 98.7% of theoretic capacity of FCNT (792mAh/g for CF0.81). The discharge plateau dropped to 2.45 V, when the discharge rate increased to 1 C from 0.05 C with a capacity of 620 mAh/g. The discharge efficiency lowered to 78.28% of theoretic capacity for carbon fluorides (CF0.81)

**Figure 26.** (a) XPS showing both F1 s and C1s peaks for pristine and FCNTs (b) detailed features for C1s peak.

Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 100 kHz to 0.01 Hz at an excitation amplitude of 5 mV (**Figure 28**). The

**Figure 27.** Discharge profile of FG and three FCNT electrodes at different rates.

FCNTs exhibits a classic core-shell-hole structure. The outer layers of CNTs were Fluorinated and inner layers still maintained pristine graphitized structure integrity. The FCNTs hold good interior electron conduction channel, even though exterior CNTs transformed into electrical insulator. The core-shell-hole structure means that the FCNT maintained better electric conductivity than those of FG. FCNT exhibited better electrochemical performance than those of FG. The discharge plateau of FCNT maintained at 2.7 V (0.05C) with a specific capacity of 782 mAh/g, which reached 98.7% of theoretic capacity of FCNT (792mAh/g for CF0.81). The discharge plateau still maintained 2.45 V at 1 C with a capacity of 620 mAh/g, and the discharge efficiency reached 78.28% compared with FG of 2 V, 520 mAh/g, and 60%, respectively. FCNTs were considered as a promise cathode materials for high performance of lithium fluoride battery.

#### **3.5. CNTCP used as electrodes for supercapacitors**

#### *3.5.1. Experiment*

#### *3.5.1.1. Synthesis of multiwalled carbon nanotubes (CNTs)*

CNTs were synthesized by floating reactant method in a vertical tubular reactor (**Figure 29**). Benzene was used as carbon feedstock, organic-metallic compound (ferrocene) as a catalyst precursor, thiophene as growth promotion agent, and hydrogen as carrier gas. The reaction temperature is around 1200°C. The following thermal treatment of CNTs was carried out at 3000°C for 10 h using a graphite resistance furnace operating in a vacuum atmosphere.

electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl

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The SEM (**Figure 30a**) images show that the CNTs have a distinct one-dimension line structure and were not entangled. They can be more easily dispersed than common carbon nanotubes (CCNTs). TEM image of the CNTs is shown in **Figure 30(b)**. It can be observed that the MWCNTs have a straight and clear texture with high crystallinity. The Raman spectra is demonstrated in **Figure 31a**. The intensity ratio of G and D band presents the degree of graphitization [121]. The IG/ID ratio of graphitized-CNTs was 1.81, which means a higher degree of graphitization than the raw CNTs with a 0.67 ratio of IG/ID. The XRD pattern (**Figure 31b**)

The CNTCP papers are shown in **Figure 32(a, b)** and presented an obvious structure of fibers network with a 50 wt% CNT addition. **Figure 32(c, d)** shows the SEM images of carbonized CNTCP papers (CNT/CCP). It can be seen that carbonized paper fibers were coated with multiwalled carbon nanotubes. CNTs were uniformly dispersed on the surface of paper fibers. There are five strong bands in the Raman spectra (**Figure 33**) of CNT/CP; the bands of 1100 and 2900 cm−1 represented the cellulose characteristic band and the bands of 1350, 1580, and 2700 cm−1 corresponded to the D band, G band, and 2D band of CNTs, respectively. The G band presents the ordered carbon, and D band presents the disordered carbon. The ratio of IG/ ID was 1.18, which means CNTs have a high degree of graphitization. The CNT/CCP only has three bands of D, G, and 2D, which can be related to the reason of carbonization. The ratio of

IG/ID was decreased to 1.02 due to higher amorphous carbon content [121, 129, 130].

Both of the CNT/CP and CNT/CCP electrodes exhibited a good layered porous structure (**Figure 34**). Most of the pore size is distributed in 2–4 nm, and CNT/CP has a much smaller pore size than CNT/CCP. Nanopores can improve the specific surface area of two papers but also conducive to the absorption of electrolytes to form electric double layer. More importantly, nanopores have played a role in helping the diffusion of electrolyte. Resulting in charged ions can pass through the 3D porous electrode, which can greatly reduce the charge

carbonate (DMC) (1:1 in weight).

shows the degree of graphitization as 97.25%.

*3.5.2. Results and discussion*

transfer resistance.

**Figure 30.** SEM and TEM images of CNTs.

#### *3.5.1.2. Preparation of electrodes*

The CNTs powder was dispersed in deionized water with a surfactant of polyvinylpyrrolidone (PVP). The paper cellulose is made of napkins in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by acute stirring. A vacuum filtration was used to infiltrate the mixed slurry of CNTs and cellulose fiber. Then, the formed CNTCP was carbonized at 1460°C in vacuum condition.

#### *3.5.1.3. Material characterizations*

The electrochemical characterization of the supercapacitors was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) was tested by electrochemical workstation (PARSTAT4000). Cyclic voltammetry (CV) was performed by an electrochemical workstation (CHI 660B).

#### *3.5.1.4. Assembling of supercapacitors and testing*

The tailored CNT-cellulose papers were used as the electrodes for symmetric electrochemical supercapacitors. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUNLABSTAR, Germany) by stacking a porous polypropylene separator. The liquid

**Figure 29.** Schematic diagram of the setup for synthesis of CNTs.

electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight).

#### *3.5.2. Results and discussion*

**3.5. CNTCP used as electrodes for supercapacitors**

*3.5.1.1. Synthesis of multiwalled carbon nanotubes (CNTs)*

CNTs were synthesized by floating reactant method in a vertical tubular reactor (**Figure 29**). Benzene was used as carbon feedstock, organic-metallic compound (ferrocene) as a catalyst precursor, thiophene as growth promotion agent, and hydrogen as carrier gas. The reaction temperature is around 1200°C. The following thermal treatment of CNTs was carried out at 3000°C for 10 h using a graphite resistance furnace operating in a vacuum atmosphere.

The CNTs powder was dispersed in deionized water with a surfactant of polyvinylpyrrolidone (PVP). The paper cellulose is made of napkins in deionized water. The CNTs dispersion liquid and the cellulose pulp were mixed by acute stirring. A vacuum filtration was used to infiltrate the mixed slurry of CNTs and cellulose fiber. Then, the formed CNTCP was carbon-

The electrochemical characterization of the supercapacitors was measured by a cell tester (CT-3008 W-5V5mA-S4). Electrochemical impedance spectroscopy (EIS) was tested by electrochemical workstation (PARSTAT4000). Cyclic voltammetry (CV) was performed by an

The tailored CNT-cellulose papers were used as the electrodes for symmetric electrochemical supercapacitors. CR2025 coin-type cells were assembled in an Ar-filled glove box (MBRAUNLABSTAR, Germany) by stacking a porous polypropylene separator. The liquid

*3.5.1. Experiment*

290 Carbon Nanotubes - Recent Progress

*3.5.1.2. Preparation of electrodes*

ized at 1460°C in vacuum condition.

electrochemical workstation (CHI 660B).

*3.5.1.4. Assembling of supercapacitors and testing*

**Figure 29.** Schematic diagram of the setup for synthesis of CNTs.

*3.5.1.3. Material characterizations*

The SEM (**Figure 30a**) images show that the CNTs have a distinct one-dimension line structure and were not entangled. They can be more easily dispersed than common carbon nanotubes (CCNTs). TEM image of the CNTs is shown in **Figure 30(b)**. It can be observed that the MWCNTs have a straight and clear texture with high crystallinity. The Raman spectra is demonstrated in **Figure 31a**. The intensity ratio of G and D band presents the degree of graphitization [121]. The IG/ID ratio of graphitized-CNTs was 1.81, which means a higher degree of graphitization than the raw CNTs with a 0.67 ratio of IG/ID. The XRD pattern (**Figure 31b**) shows the degree of graphitization as 97.25%.

The CNTCP papers are shown in **Figure 32(a, b)** and presented an obvious structure of fibers network with a 50 wt% CNT addition. **Figure 32(c, d)** shows the SEM images of carbonized CNTCP papers (CNT/CCP). It can be seen that carbonized paper fibers were coated with multiwalled carbon nanotubes. CNTs were uniformly dispersed on the surface of paper fibers.

There are five strong bands in the Raman spectra (**Figure 33**) of CNT/CP; the bands of 1100 and 2900 cm−1 represented the cellulose characteristic band and the bands of 1350, 1580, and 2700 cm−1 corresponded to the D band, G band, and 2D band of CNTs, respectively. The G band presents the ordered carbon, and D band presents the disordered carbon. The ratio of IG/ ID was 1.18, which means CNTs have a high degree of graphitization. The CNT/CCP only has three bands of D, G, and 2D, which can be related to the reason of carbonization. The ratio of IG/ID was decreased to 1.02 due to higher amorphous carbon content [121, 129, 130].

Both of the CNT/CP and CNT/CCP electrodes exhibited a good layered porous structure (**Figure 34**). Most of the pore size is distributed in 2–4 nm, and CNT/CP has a much smaller pore size than CNT/CCP. Nanopores can improve the specific surface area of two papers but also conducive to the absorption of electrolytes to form electric double layer. More importantly, nanopores have played a role in helping the diffusion of electrolyte. Resulting in charged ions can pass through the 3D porous electrode, which can greatly reduce the charge transfer resistance.

**Figure 30.** SEM and TEM images of CNTs.

**Figure 31.** XRD pattern and Raman spectrometry of CNTs.

The electrochemical performance of the electrodes has been investigated by cyclic voltammetry (CV) (**Figure 35a,b**) with the voltage range of 0–2.5 V at four different scan rates of 5, 30, 50, and 100 mV/s. The CV curve of electric double layer capacitor is rectangular [131, 132].

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**Figure 33.** Raman spectra of CNT/CP and CNT/CCP electrodes.

**Figure 34.** Pore diameter distribution for the CNT/CP and the CNT/CCP.

**Figure 32.** SEM images of (a,b) CNT/CP and (c,d) CNT/CCP electrodes.

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**Figure 33.** Raman spectra of CNT/CP and CNT/CCP electrodes.

**Figure 31.** XRD pattern and Raman spectrometry of CNTs.

292 Carbon Nanotubes - Recent Progress

**Figure 32.** SEM images of (a,b) CNT/CP and (c,d) CNT/CCP electrodes.

**Figure 34.** Pore diameter distribution for the CNT/CP and the CNT/CCP.

The electrochemical performance of the electrodes has been investigated by cyclic voltammetry (CV) (**Figure 35a,b**) with the voltage range of 0–2.5 V at four different scan rates of 5, 30, 50, and 100 mV/s. The CV curve of electric double layer capacitor is rectangular [131, 132]. However, the CV curve of CNT/CP (**Figure 35a**) electrodes presented a reduction peak without oxidation peak. The reduction reaction was related to the iron-based catalyst metals and reaction of lithium ions in electrolyte. Moreover, the irreversible redox reaction of iron and irreversible capacity of CNTs contributed to the absence of oxidation peak [121, 133–135]. The CV curve of CNT/CCP (**Figure 35b**) electrodes presented a perfect redox peak, which was related to faraday reaction. The no-carbon elements of CNT/CCP electrodes were evaporated in the process of carbonization. This is beneficial to the intercalate and de-intercalate of lithium ions in amorphous carbon particles [64, 131–136].

Specific capacitance and scan rate curves of CNT/CP and CNT/CCP are shown in **Figure 36**. The specific capacitance (Cscv) of the symmetric supercapacitor was calculated according to the following equation: Cscv = 2S/[(U2-U1) mν]. Cscv is the specific capacitance of the capacitor in F/g, S is the total voltammetric sweeps area during CV, (U2–U1) is the range of voltage, m is the active material mass per electrode, and ν is the scan rate.

With the increase of scan rate, the capacity of supercapacitor was decreased. The capacitance of MWCNT/CF electrode was 24 and 4.55 F/g at a scan rate 5 and 100 mV/s, respectively. By contrast, the capacitance of CNT/CCF electrode corresponds to 43 and 26 F/g at a scan rate 5 and 100 mV/s, respectively. The results show that carbonized electrode (CNT/CCF) exhibited a better capacitance performance than uncarbonized electrodes (CNT/CF).

**Figure 37** shows the electrochemical impedance spectroscope (EIS) of the supercapacitors, and the EIS was measured at the same open circuit potential of 2.5 V. The interception of the real axis represents the solution resistance. The solution resistance of the supercapacitor is 16 Ω. The semicircle in the high frequency region presented the charge transfer resistance of electrode. The charge transfer resistance of carbonized electrode (CNT/CCF) is only 3 Ω, which suggested an excellent transfer resistance performance. The impedance curves showed a straight line slopes at low frequency region, which also suggested a good capacitive performance of carbonized electrodes. Absence of semicircle at low frequency means low contact resistance between collector and electrode plates [9]. The electrical conductivity of CNT/CCF electrode was tested by four-point probe resistance meter. The electrical conductivity was obviously raised from 714.3 to 325.1 S/m after carbonization.

The specific capacitance (Cscv) of the two capacitors was calculated based on the charge/ discharge profiles according to the following equation: Cscd = [2I]/[m(dV/dt)], where Cscd is the specific capacitance of the capacitors in F/g, I is the current in A, m is the mass of the active material in the capacitors in gram, and dV/dt is the slope of the discharge curve after

**Figure 37.** Electrochemical impedance spectroscope (EIS) of supercapacitor with carbonized electrode.

**Figure 36.** The specific capacitance of the supercapacitors vs. scan rate for CNT/Cp electrode and CNT/CCp electrode.

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The charge/discharge curves of the CNT/CF and CNT/CCF electrodes were compared using different current density of 50, 100, 200, and 400 mA/g (**Figure 38**). The Cscd decreased as the current density increased. For CNT/CF electrode (**Figure 38a**), the capacity decreased from 17

the IR drop.

**Figure 35.** Cyclic voltammetry curves of (a) CNT/CP and (b) CNT/CCP electrodes.

However, the CV curve of CNT/CP (**Figure 35a**) electrodes presented a reduction peak without oxidation peak. The reduction reaction was related to the iron-based catalyst metals and reaction of lithium ions in electrolyte. Moreover, the irreversible redox reaction of iron and irreversible capacity of CNTs contributed to the absence of oxidation peak [121, 133–135]. The CV curve of CNT/CCP (**Figure 35b**) electrodes presented a perfect redox peak, which was related to faraday reaction. The no-carbon elements of CNT/CCP electrodes were evaporated in the process of carbonization. This is beneficial to the intercalate and de-intercalate of lithium ions

Specific capacitance and scan rate curves of CNT/CP and CNT/CCP are shown in **Figure 36**. The specific capacitance (Cscv) of the symmetric supercapacitor was calculated according to the following equation: Cscv = 2S/[(U2-U1) mν]. Cscv is the specific capacitance of the capacitor in F/g, S is the total voltammetric sweeps area during CV, (U2–U1) is the range of voltage,

With the increase of scan rate, the capacity of supercapacitor was decreased. The capacitance of MWCNT/CF electrode was 24 and 4.55 F/g at a scan rate 5 and 100 mV/s, respectively. By contrast, the capacitance of CNT/CCF electrode corresponds to 43 and 26 F/g at a scan rate 5 and 100 mV/s, respectively. The results show that carbonized electrode (CNT/CCF) exhibited

**Figure 37** shows the electrochemical impedance spectroscope (EIS) of the supercapacitors, and the EIS was measured at the same open circuit potential of 2.5 V. The interception of the real axis represents the solution resistance. The solution resistance of the supercapacitor is 16 Ω. The semicircle in the high frequency region presented the charge transfer resistance of electrode. The charge transfer resistance of carbonized electrode (CNT/CCF) is only 3 Ω, which suggested an excellent transfer resistance performance. The impedance curves showed a straight line slopes at low frequency region, which also suggested a good capacitive performance of carbonized electrodes. Absence of semicircle at low frequency means low contact resistance between collector and electrode plates [9]. The electrical conductivity of CNT/CCF electrode was tested by four-point probe resistance meter. The electrical conductivity was obviously raised from 714.3 to 325.1 S/m after carbonization.

in amorphous carbon particles [64, 131–136].

294 Carbon Nanotubes - Recent Progress

m is the active material mass per electrode, and ν is the scan rate.

**Figure 35.** Cyclic voltammetry curves of (a) CNT/CP and (b) CNT/CCP electrodes.

a better capacitance performance than uncarbonized electrodes (CNT/CF).

**Figure 36.** The specific capacitance of the supercapacitors vs. scan rate for CNT/Cp electrode and CNT/CCp electrode.

**Figure 37.** Electrochemical impedance spectroscope (EIS) of supercapacitor with carbonized electrode.

The specific capacitance (Cscv) of the two capacitors was calculated based on the charge/ discharge profiles according to the following equation: Cscd = [2I]/[m(dV/dt)], where Cscd is the specific capacitance of the capacitors in F/g, I is the current in A, m is the mass of the active material in the capacitors in gram, and dV/dt is the slope of the discharge curve after the IR drop.

The charge/discharge curves of the CNT/CF and CNT/CCF electrodes were compared using different current density of 50, 100, 200, and 400 mA/g (**Figure 38**). The Cscd decreased as the current density increased. For CNT/CF electrode (**Figure 38a**), the capacity decreased from 17 F/g (50 mA/g), 12.8 F/g (100 mA/g), and 13 F/g (200 mA) to 12 F/g (400 mA/g). For CNT/CCF electrode (**Figure 38b**), the capacity decreased from 39 F/g (50 mA/g), 38 F/g (100 mA/g), 38 F/g (200 mA/g), 32.9 F/g (400 mA/g) to 29.2F/g (800 mA/g). The testing results indicated that the performance of supercapacitor was greatly improved after carbonization of electrodes. The curves of charge/discharge are not straight lines due to Faraday reaction.

*3.5.3. Conclusions*

**capacitors**

**Author details**

**References**

after 500 cycles at a current density of 400 mA/g.

applications of CNTCP in LIBs and SCs.

\*Address all correspondence to: xiaogangsun@163.com

Alloys and Compounds. 2015;**619**:298-302

MWCNT/cellulose fiber electrodes provide 3D porous network structure and large interfacial area between electrode and electrolyte. Capacitance performance could be improved by the electrode and electrolyte's intimate contact. After carbonization treatment, the CNT/CCP electrodes have a higher performance of capacitance. The improvement is contributed to a better absorption of electrolyte, lower resistance of carbonized electrodes, and increase of effective interface area for forming of double layers. When the current density is 50 mA/g, the maximum specific capacitance of a unit capacitor reached 39 F/g by galvanization charge/ discharge measurement, while the minimum specific capacitance of unit capacitor reached 24 F/g at a scan rate 5 mV/s of cyclic voltammetry measurement. The capacitance holds 74.6%

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297

**4. Outlook for CNTs and CNTCP in primary/second batteries and** 

In summary, our researches were reported of CNTs and CNTCP for primary/second batteries and supercapacitors applications. It has a great potential application value for the porous carbon nanotube-cellulose papers as current collectors and electrodes in lithium ion battery and supercapacitors. However, there are still some problems to be solved. The pore size and porosity and carbonization process of CNTCPs needed to be innovated to improve the strength and electrical conductivity. New high flexibility and strength of nanofibers need to be developed to adapt to electrolytes, as the cellulose papers are easily destroyed in liquid. The further investigations need to be done to overcome technological barriers for industrial

Xiaogang Sun\*, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen

[1] Zhang Y, Zhao Y, Konarov A, et al. Effect of mesoporous carbon microtube prepared by carbonizing the poplar catkin on sulfur cathode performance in Li/S batteries. Journal of

[2] Liang J, Sun ZH, Li F, et al. Carbon materials for Li–S batteries: Functional evolution and

School of Mechatronics Engineering, Nanchang University, Nanchang, China

performance improvement. Energy Storage Materials. 2015;**2**:76-106

Continuous charge-discharge was carried out for the CNT/CCP supercapacitor at constant current of 400 mA/g for 500 cycles (**Figure 39**). **Figure 39** shows the change in specific capacitance in 500 cycles. The CNT/CCP supercapacitor showed a progressive decrease in cycles. The specific capacitance still holds 74.6% after 500 cycles, which may be contributed to good wetting of electrolyte to electrodes and big interface areas between electrodes and electrolyte.

**Figure 38.** Galvanostatic charge/discharge curves of (a) CNT/CF and (b) CNT/CCP electrodes.

**Figure 39.** Capacitance retention of the carbonized CNT/CCP electrodes.

#### *3.5.3. Conclusions*

F/g (50 mA/g), 12.8 F/g (100 mA/g), and 13 F/g (200 mA) to 12 F/g (400 mA/g). For CNT/CCF electrode (**Figure 38b**), the capacity decreased from 39 F/g (50 mA/g), 38 F/g (100 mA/g), 38 F/g (200 mA/g), 32.9 F/g (400 mA/g) to 29.2F/g (800 mA/g). The testing results indicated that the performance of supercapacitor was greatly improved after carbonization of electrodes.

Continuous charge-discharge was carried out for the CNT/CCP supercapacitor at constant current of 400 mA/g for 500 cycles (**Figure 39**). **Figure 39** shows the change in specific capacitance in 500 cycles. The CNT/CCP supercapacitor showed a progressive decrease in cycles. The specific capacitance still holds 74.6% after 500 cycles, which may be contributed to good wetting of electrolyte to electrodes and big interface areas between electrodes and

The curves of charge/discharge are not straight lines due to Faraday reaction.

**Figure 38.** Galvanostatic charge/discharge curves of (a) CNT/CF and (b) CNT/CCP electrodes.

**Figure 39.** Capacitance retention of the carbonized CNT/CCP electrodes.

electrolyte.

296 Carbon Nanotubes - Recent Progress

MWCNT/cellulose fiber electrodes provide 3D porous network structure and large interfacial area between electrode and electrolyte. Capacitance performance could be improved by the electrode and electrolyte's intimate contact. After carbonization treatment, the CNT/CCP electrodes have a higher performance of capacitance. The improvement is contributed to a better absorption of electrolyte, lower resistance of carbonized electrodes, and increase of effective interface area for forming of double layers. When the current density is 50 mA/g, the maximum specific capacitance of a unit capacitor reached 39 F/g by galvanization charge/ discharge measurement, while the minimum specific capacitance of unit capacitor reached 24 F/g at a scan rate 5 mV/s of cyclic voltammetry measurement. The capacitance holds 74.6% after 500 cycles at a current density of 400 mA/g.

### **4. Outlook for CNTs and CNTCP in primary/second batteries and capacitors**

In summary, our researches were reported of CNTs and CNTCP for primary/second batteries and supercapacitors applications. It has a great potential application value for the porous carbon nanotube-cellulose papers as current collectors and electrodes in lithium ion battery and supercapacitors. However, there are still some problems to be solved. The pore size and porosity and carbonization process of CNTCPs needed to be innovated to improve the strength and electrical conductivity. New high flexibility and strength of nanofibers need to be developed to adapt to electrolytes, as the cellulose papers are easily destroyed in liquid. The further investigations need to be done to overcome technological barriers for industrial applications of CNTCP in LIBs and SCs.

#### **Author details**

Xiaogang Sun\*, Manyuan Cai, Long Chen, Zhiwen Qiu, Jie Wang, Xu Li and Wei Chen

\*Address all correspondence to: xiaogangsun@163.com

School of Mechatronics Engineering, Nanchang University, Nanchang, China

#### **References**


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**Chapter 14**

**Provisional chapter**

**Potential Application of Photo-thermal Volumetric**

**Potential Application of Photo-thermal Volumetric** 

**Engines**

**Engines**

Luciano Strafella

**Abstract**

for the HCCI engines.

Luciano Strafella

Antonio Paolo Carlucci, Bruce Chehroudi, Antonio Ficarella, Domenico Laforgia and

Antonio Paolo Carlucci, Bruce Chehroudi, Antonio Ficarella, Domenico Laforgia and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70887

**Ignition of Carbon Nanotubes in Internal Combustion**

In internal combustion engines, an ignition source is required to initiate the combustion process. This is commonly obtained either through an electric spark generation or by physical act of compression-ignition. In order to improve performance and lower pollutants levels, researchers have proposed alternatives to conventional ignition or combustion processes, such as homogeneously-charge compression-ignition (HCCI) combustion, whose critical operational requirement is precise control of the autoignition timing within the engine operating cycle. In this work, an innovative volumetricallydistributed ignition approach is proposed to control the onset of the autoignition process, by taking advantage of the optical ignition properties of carbon nanotubes when exposed to a low-consumption light source. It is shown that this ignition method enhanced the combustion of methane, hydrogen, LPG, and gasoline (injected to chamber in liquid phase). The results for this new ignition method show that pressure gradient and combustion efficiency are increased, while combustion duration and ignition delay time are decreased. A direct observation of the combustion process indicates that these benefits are due to the spatially-distributed ignition followed by a faster initial consumption of the air/fuel mixture. The use of this ignition system is therefore proposed as a promising technology for the combustion management in internal combustion engines, specifically

**Ignition of Carbon Nanotubes in Internal Combustion** 

DOI: 10.5772/intechopen.70887

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Keywords:** carbon nanotubes, combustion control, internal combustion engines, metal nanoparticles, photo-thermal ignition, HCCI, autoignition, photo-ignition


**Provisional chapter**

#### **Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal Combustion Engines Ignition of Carbon Nanotubes in Internal Combustion Engines**

**Potential Application of Photo-thermal Volumetric** 

DOI: 10.5772/intechopen.70887

Antonio Paolo Carlucci, Bruce Chehroudi, Antonio Ficarella, Domenico Laforgia and Luciano Strafella Antonio Ficarella, Domenico Laforgia and Luciano Strafella Additional information is available at the end of the chapter

Antonio Paolo Carlucci, Bruce Chehroudi,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70887

#### **Abstract**

[125] Nagasubramanian G, Sanchez B. A new chemical approach to improving discharge capacity of Li/(CF x) n, cells. Journal of Power Sources. 2007;**165**(2):630-634

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In internal combustion engines, an ignition source is required to initiate the combustion process. This is commonly obtained either through an electric spark generation or by physical act of compression-ignition. In order to improve performance and lower pollutants levels, researchers have proposed alternatives to conventional ignition or combustion processes, such as homogeneously-charge compression-ignition (HCCI) combustion, whose critical operational requirement is precise control of the autoignition timing within the engine operating cycle. In this work, an innovative volumetricallydistributed ignition approach is proposed to control the onset of the autoignition process, by taking advantage of the optical ignition properties of carbon nanotubes when exposed to a low-consumption light source. It is shown that this ignition method enhanced the combustion of methane, hydrogen, LPG, and gasoline (injected to chamber in liquid phase). The results for this new ignition method show that pressure gradient and combustion efficiency are increased, while combustion duration and ignition delay time are decreased. A direct observation of the combustion process indicates that these benefits are due to the spatially-distributed ignition followed by a faster initial consumption of the air/fuel mixture. The use of this ignition system is therefore proposed as a promising technology for the combustion management in internal combustion engines, specifically for the HCCI engines.

**Keywords:** carbon nanotubes, combustion control, internal combustion engines, metal nanoparticles, photo-thermal ignition, HCCI, autoignition, photo-ignition

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

#### **1. Introduction**

The system for triggering and controlling the ignition process is the heart of the robust and effective combustion process in internal combustion engines (ICEs).

mixture into the combustion chamber. The greatest benefits were reductions of NOx

emissions [2].

(e.g., by adding n-butane).

into the shape of a scroll.

generally not suitable for the HCCI engines.

However, there were little or no reductions in fuel consumption and increases in CO and HC

Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal…

Finally, in natural gas engines, the HCCI combustion mode can be reached by increasing the intake temperature, boost pressure, and compression ratio, or varying the fuel composition

In order to implement the above techniques, substantial modifications have been necessary on the air, exhaust, and fuel paths, as well as on the engine structure and layout. Most of all, the critical requirement for the proper operation of the HCCI engines was found to be a precise control of the autoignition process, namely, the control of the time at which autoignition of the gaseous air/fuel mixture takes place inside the combustion chamber [2, 3]. Using a variety of complex control systems, based on parameters influencing the beginning of the autoignition process, it is possible to efficiently operate a HCCI engine. However, these controlling systems are still extremely complex, expensive, and onerous [2, 4]. This stems from the fact that the onset of autoignition for premixed air/fuel mixtures is very sensitive to engine operating and design parameters. Unfortunately, the ignition processes initiated by conventional sources of external energy are sensitive to surrounding environmental conditions and

Therefore, an innovative light-activated approach has been proposed by Chehroudi [3] in order to obtain a volumetrically distributed ignition. This approach, which takes advantage of the optical ignition properties of carbon nanotubes (CNTs), is based on the observation that carbon nanotubes, bounded with other nanoenergetic materials (nEMs), that is, metallic nanopowders, ignite collectively and burn when exposed to low-consumption short-duration light sources. Therefore, they could act as autoignition nuclei when mixed with a homogenous fuel/oxidizer mixture and exposed to a pulsed light source, such as an ordinary camera

A CNT is a hollow nanostructure like a hollow cage, essentially consisting of a graphitic plane rolled into a thin tube, both ends of which can be closed by a fullerene-type dome structure. The existence of CNTs was originally discovered by Iijima [5]. Since their discovery, CNTs have been a subject of intense study: this material, in fact, exhibits various interesting mechanical, thermal, optical, and electrical properties. Many of these properties have been proposed for countless applications, for example, for the hydrogen storage, as additives in structural materials and for the production of ad-hoc biosensors for the diagnosis of diseases. There are two forms of carbon nanotubes, namely single-walled carbon nanotube (SWCNT) consisting of a single layer of graphene rolled into a tube whose diameter depends on the chirality of the nanotube, and multi-walled carbon nanotube (MWCNT) that can appear in a coaxial assembly of SWCNT similar to a coaxial cable, or as a single sheet of graphite rolled

The mechanism by which CNTs bonded with nEMs' photo-ignite is difficult to analyze and has not been fully explained. However, there are some theories based on experimental observations. The first observation of this phenomenon was documented by Ajayan et al. [6].

flash. This phenomenon is usually referred to as "photo-thermal ignition" (PTI).

and soot.

309

http://dx.doi.org/10.5772/intechopen.70887

The electric spark used in gasoline engines generates a relatively slow flame front, which then propagates burning the air/fuel mixture. This ignition system is, by its nature, a single point ignition system and is characterized by a relatively high energy input. The benefits of having multiple ignition nuclei and ability to vary their positions on the fuel conversion efficiency and reduction of pollutants levels have been demonstrated in the literature at different operating conditions. However, the practical applications have been limited to a few points, that is, a maximum of two spark plugs per cylinder in automotive engines.

In diesel engines, on the other hand, the ignition is achieved through the compression process, realized by the piston movement during the compression stroke. The compression ignition process, being sensitive to operating conditions such as ambient temperature and pressure, affects the following combustion development, characterized by two different burning phases. The first phase is a fast-burning combustion of the injected liquid fuel that was previously atomized, vaporized, and mixed with air during the ignition delay period, called premixed phase, which is followed by a second phase, characterized by a much slower mixture burning rate called the mixing-controlled or diffusive combustion.

In the literature, the potentials of a third combustion mode, called homogeneous-charge compression-ignition (HCCI), in simultaneously reducing fuel consumption and pollutant emissions are extensively documented. HCCI is an alternative combustion mode for ICE, in which a premixed homogeneous mixture of fuel and oxidizer (auto-)ignites, ideally all at the same time. In this manner, the combustion process instantly involves the entire mixture trapped inside the cylinder, leading to a remarkable reduction in fuel consumption [1]. The ignition (or autoignition) process in the HCCI engines exhibits multiple ignition points distributed throughout the combustion chamber. This, when combined with the combustion of a lean premixed air/fuel mixture, lowers burned-gas temperatures, which prevents the formation of nitrogen oxides (NOx ). Furthermore, combustion of premixed lean mixtures forms virtually no soot. However, the HCCI combustion mode, simple from a conceptual point of view, is hard to be implemented. In gasoline engines, waste thermal energy contained in exhaust gases and coolant, hot residuals, and internal exhaust gas recirculation (EGR) have been demonstrated to be effective ways to heat the intake air and hence initiate mixture autoignition. In this way, and under HCCI combustion mode, it is possible to achieve a reduction of the fuel consumption up to 50%, a reduction of two orders of magnitude of NOx , comparable levels of carbon monoxide (CO), and an acceptable increase in hydrocarbons (HC)—that can be eliminated by means of an already mature technology such as the three-way catalyst—compared to conventional gasoline spark ignition engines [2].

In diesel engines, on the other hand, the critical issue is represented by the preparation of a homogeneous mixture. In this view, port injection as well as the number and timing of injections and spray and combustion chamber shape can help obtaining a more homogeneous air/fuel mixture into the combustion chamber. The greatest benefits were reductions of NOx and soot. However, there were little or no reductions in fuel consumption and increases in CO and HC emissions [2].

**1. Introduction**

308 Carbon Nanotubes - Recent Progress

mation of nitrogen oxides (NOx

NOx

The system for triggering and controlling the ignition process is the heart of the robust and

The electric spark used in gasoline engines generates a relatively slow flame front, which then propagates burning the air/fuel mixture. This ignition system is, by its nature, a single point ignition system and is characterized by a relatively high energy input. The benefits of having multiple ignition nuclei and ability to vary their positions on the fuel conversion efficiency and reduction of pollutants levels have been demonstrated in the literature at different operating conditions. However, the practical applications have been limited to a few points, that

In diesel engines, on the other hand, the ignition is achieved through the compression process, realized by the piston movement during the compression stroke. The compression ignition process, being sensitive to operating conditions such as ambient temperature and pressure, affects the following combustion development, characterized by two different burning phases. The first phase is a fast-burning combustion of the injected liquid fuel that was previously atomized, vaporized, and mixed with air during the ignition delay period, called premixed phase, which is followed by a second phase, characterized by a much slower mix-

In the literature, the potentials of a third combustion mode, called homogeneous-charge compression-ignition (HCCI), in simultaneously reducing fuel consumption and pollutant emissions are extensively documented. HCCI is an alternative combustion mode for ICE, in which a premixed homogeneous mixture of fuel and oxidizer (auto-)ignites, ideally all at the same time. In this manner, the combustion process instantly involves the entire mixture trapped inside the cylinder, leading to a remarkable reduction in fuel consumption [1]. The ignition (or autoignition) process in the HCCI engines exhibits multiple ignition points distributed throughout the combustion chamber. This, when combined with the combustion of a lean premixed air/fuel mixture, lowers burned-gas temperatures, which prevents the for-

virtually no soot. However, the HCCI combustion mode, simple from a conceptual point of view, is hard to be implemented. In gasoline engines, waste thermal energy contained in exhaust gases and coolant, hot residuals, and internal exhaust gas recirculation (EGR) have been demonstrated to be effective ways to heat the intake air and hence initiate mixture autoignition. In this way, and under HCCI combustion mode, it is possible to achieve a reduction of the fuel consumption up to 50%, a reduction of two orders of magnitude of

, comparable levels of carbon monoxide (CO), and an acceptable increase in hydrocarbons (HC)—that can be eliminated by means of an already mature technology such as the

In diesel engines, on the other hand, the critical issue is represented by the preparation of a homogeneous mixture. In this view, port injection as well as the number and timing of injections and spray and combustion chamber shape can help obtaining a more homogeneous air/fuel

three-way catalyst—compared to conventional gasoline spark ignition engines [2].

). Furthermore, combustion of premixed lean mixtures forms

effective combustion process in internal combustion engines (ICEs).

is, a maximum of two spark plugs per cylinder in automotive engines.

ture burning rate called the mixing-controlled or diffusive combustion.

Finally, in natural gas engines, the HCCI combustion mode can be reached by increasing the intake temperature, boost pressure, and compression ratio, or varying the fuel composition (e.g., by adding n-butane).

In order to implement the above techniques, substantial modifications have been necessary on the air, exhaust, and fuel paths, as well as on the engine structure and layout. Most of all, the critical requirement for the proper operation of the HCCI engines was found to be a precise control of the autoignition process, namely, the control of the time at which autoignition of the gaseous air/fuel mixture takes place inside the combustion chamber [2, 3]. Using a variety of complex control systems, based on parameters influencing the beginning of the autoignition process, it is possible to efficiently operate a HCCI engine. However, these controlling systems are still extremely complex, expensive, and onerous [2, 4]. This stems from the fact that the onset of autoignition for premixed air/fuel mixtures is very sensitive to engine operating and design parameters. Unfortunately, the ignition processes initiated by conventional sources of external energy are sensitive to surrounding environmental conditions and generally not suitable for the HCCI engines.

Therefore, an innovative light-activated approach has been proposed by Chehroudi [3] in order to obtain a volumetrically distributed ignition. This approach, which takes advantage of the optical ignition properties of carbon nanotubes (CNTs), is based on the observation that carbon nanotubes, bounded with other nanoenergetic materials (nEMs), that is, metallic nanopowders, ignite collectively and burn when exposed to low-consumption short-duration light sources. Therefore, they could act as autoignition nuclei when mixed with a homogenous fuel/oxidizer mixture and exposed to a pulsed light source, such as an ordinary camera flash. This phenomenon is usually referred to as "photo-thermal ignition" (PTI).

A CNT is a hollow nanostructure like a hollow cage, essentially consisting of a graphitic plane rolled into a thin tube, both ends of which can be closed by a fullerene-type dome structure. The existence of CNTs was originally discovered by Iijima [5]. Since their discovery, CNTs have been a subject of intense study: this material, in fact, exhibits various interesting mechanical, thermal, optical, and electrical properties. Many of these properties have been proposed for countless applications, for example, for the hydrogen storage, as additives in structural materials and for the production of ad-hoc biosensors for the diagnosis of diseases. There are two forms of carbon nanotubes, namely single-walled carbon nanotube (SWCNT) consisting of a single layer of graphene rolled into a tube whose diameter depends on the chirality of the nanotube, and multi-walled carbon nanotube (MWCNT) that can appear in a coaxial assembly of SWCNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll.

The mechanism by which CNTs bonded with nEMs' photo-ignite is difficult to analyze and has not been fully explained. However, there are some theories based on experimental observations. The first observation of this phenomenon was documented by Ajayan et al. [6]. They suggested that the optically black SWCNT fibers absorbed visible and infrared light and transmitted that energy as heat to Fe (iron) nanoparticle sites (leftovers from the SWCNT growth phase), which subsequently gained enough activation energy to oxidize and support a combustion reaction with the surrounding air. Tseng et al. [7] proposed that the photo-acoustic and ignition effects are attributable to rapid increase in temperature over 457°C (ignition point of Ferrocene, Fc), resulting from both absorption of the light flash by CNTs and the presence of catalyst particles in fluffy SWCNTs, generating an acoustic wave and oxidation of the CNTs. Bockrath et al. [8] showed that the phenomenon is not isolated to SWCNTs but, in fact, other carbonaceous compounds synthesized on metal catalysts can ignite upon exposure to a flash lamp. Braidy et al. [9] not only confirmed the flash ignition effect on SWCNTs but also reported the presence of iron oxide particles in the combustion by-products. In [10], the authors concluded that the metal nanoparticle impurities in the SWCNT samples are responsible for the photo-ignition phenomenon.

when exposed to a light source, was compared with the one obtained with a conventional spark plug ignition system (SI), limiting the analysis only to air/methane mixtures. For all air/fuel ratios (AFRs) tested, it was demonstrated that the combustion triggered by PTI of the MWCNTs with Fc (i.e., Ferrocene) was characterized by a higher combustion pressure gradient and a higher peak pressure than the one started by the SI. Moreover, a volumetrically distributed combustion process was observed instead of the classic flame front propagation seen in conventional gasoline engines. Therefore, in this chapter, the application of "CNT bounded with nEMs powders," collectively referred here as nanoignition agents (nIAs), for the ignition

Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal…

gasoline, inside a constant volume chamber is described. A constant volume chamber was used to simulate the combustion chamber of an ICE, highlighting the benefits, in terms of reduction of combustion duration and its completeness, compared with the combustion trig-

A system was designed and realized on purpose to demonstrate volumetric ignition of gaseous fuels (methane, hydrogen, and LPG) and gasoline through PTI of MWCNTs with 75% by weight of Fc (it was demonstrated in [23] that this composition had the lowest power density required for ignition). MWCNTs and Fc properties are reported, respectively, in **Table 1**. A schematic diagram of the complete experimental layout for both air/gaseous fuel mixtures

**MWCNTs Fc**

/g –

Form, color Powder, black Powder, yellow-orange

Purity ≥98% 98%

O.D. × I.D. × L. 10 nm ± 1 nm × 4.5 nm ± 0.5 nm × 3–6 μm –

Bulk density 0.068 g/cm3 –

Density ~2.1 g/mL at 25°C (lit.) 1.49 g/cm3

Melting point 3652–3697°C (lit.) 172–175°C (lit.) Specific heat (~300 K) 242 kJ/kg 344 kJ/kg Autoignition point – >150°C Boiling point – 249°C (lit.)

Vapor pressure – 0.03 mmHg (40 °C)

Absorption – λmax = 358 nm

), liquified petroleum gas (LPG), hydrogen (H<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.70887

), and

311

of different fuels such as methane (CH<sup>4</sup>

and air/gasoline mixtures is shown in **Figure 1**.

Surface area 280–350 m2

**Table 1.** Properties of MWCNTs and ferrocene.

gered by a conventional SI system.

**2. Experimental setup**

Despite the intensive analysis of the PTI phenomenon, few applications have been proposed so far. Among them, the idea of igniting various fuel/oxidizer mixtures through PTI has rather been investigated. For example, Berkowitz et al. [11] introduced and mixed SWCNTs (containing 70% iron by weight) in an air/ethylene mixture inside a combustion chamber and exposed them to the camera flash, triggering in this way the combustion process. Another application was proposed by Manaa et al. [12] who demonstrated that the flash ignition and initiation of explosive-nanotubes mixture—SWCNTs with at least 29% by weight of iron impurity—in solid fuels are possible. However, in order to attain SWCNTs-nEMs mixture photo-ignition in liquid fuels, the CNTs-nEMs must be separated from the liquid until the moment of ignition. Badakshan et al. [13] have encapsulated a small sample of SWCNTs and solid additives—aluminum nanoparticles and ammonium perchlorate as solid oxidizers—inside a gelatin capsule through which they have been flashed. It was observed that the photo-ignition of nanopowder speeded up the combustion process of hexane + acetone as fuels (50% each). Furthermore, the authors have also tested PTI of simulated solid rocket fuel as an alternative to classic electric ignition. These tests essentially showed a great potential for obtaining the volumetrically distributed ignition of liquid and solid fuels.

A further application was first pioneered by Chehroudi [14], who studied the PTI of liquid fuels with CNTs, was limited to atmospheric conditions. Chehroudi [15–20] believes that this phenomenon could be used to ignite multiple points in an air/fuel mixture simultaneously, for example in ICE, and hence realizing the so-called volumetrically-distributed ignition for HCCI combustion.

The use of PTI system would lead to the following advantages compared to the other ignition systems:


A first investigation in this direction has been described in [21, 22], where the combustion process initiated by CNT and nEMs, collectively referred to as nanoignition agents (nIAs), when exposed to a light source, was compared with the one obtained with a conventional spark plug ignition system (SI), limiting the analysis only to air/methane mixtures. For all air/fuel ratios (AFRs) tested, it was demonstrated that the combustion triggered by PTI of the MWCNTs with Fc (i.e., Ferrocene) was characterized by a higher combustion pressure gradient and a higher peak pressure than the one started by the SI. Moreover, a volumetrically distributed combustion process was observed instead of the classic flame front propagation seen in conventional gasoline engines. Therefore, in this chapter, the application of "CNT bounded with nEMs powders," collectively referred here as nanoignition agents (nIAs), for the ignition of different fuels such as methane (CH<sup>4</sup> ), liquified petroleum gas (LPG), hydrogen (H<sup>2</sup> ), and gasoline, inside a constant volume chamber is described. A constant volume chamber was used to simulate the combustion chamber of an ICE, highlighting the benefits, in terms of reduction of combustion duration and its completeness, compared with the combustion triggered by a conventional SI system.

#### **2. Experimental setup**

They suggested that the optically black SWCNT fibers absorbed visible and infrared light and transmitted that energy as heat to Fe (iron) nanoparticle sites (leftovers from the SWCNT growth phase), which subsequently gained enough activation energy to oxidize and support a combustion reaction with the surrounding air. Tseng et al. [7] proposed that the photo-acoustic and ignition effects are attributable to rapid increase in temperature over 457°C (ignition point of Ferrocene, Fc), resulting from both absorption of the light flash by CNTs and the presence of catalyst particles in fluffy SWCNTs, generating an acoustic wave and oxidation of the CNTs. Bockrath et al. [8] showed that the phenomenon is not isolated to SWCNTs but, in fact, other carbonaceous compounds synthesized on metal catalysts can ignite upon exposure to a flash lamp. Braidy et al. [9] not only confirmed the flash ignition effect on SWCNTs but also reported the presence of iron oxide particles in the combustion by-products. In [10], the authors concluded that the metal nanoparticle impurities in the

Despite the intensive analysis of the PTI phenomenon, few applications have been proposed so far. Among them, the idea of igniting various fuel/oxidizer mixtures through PTI has rather been investigated. For example, Berkowitz et al. [11] introduced and mixed SWCNTs (containing 70% iron by weight) in an air/ethylene mixture inside a combustion chamber and exposed them to the camera flash, triggering in this way the combustion process. Another application was proposed by Manaa et al. [12] who demonstrated that the flash ignition and initiation of explosive-nanotubes mixture—SWCNTs with at least 29% by weight of iron impurity—in solid fuels are possible. However, in order to attain SWCNTs-nEMs mixture photo-ignition in liquid fuels, the CNTs-nEMs must be separated from the liquid until the moment of ignition. Badakshan et al. [13] have encapsulated a small sample of SWCNTs and solid additives—aluminum nanoparticles and ammonium perchlorate as solid oxidizers—inside a gelatin capsule through which they have been flashed. It was observed that the photo-ignition of nanopowder speeded up the combustion process of hexane + acetone as fuels (50% each). Furthermore, the authors have also tested PTI of simulated solid rocket fuel as an alternative to classic electric ignition. These tests essentially showed a great potential for obtaining the volumetrically

A further application was first pioneered by Chehroudi [14], who studied the PTI of liquid fuels with CNTs, was limited to atmospheric conditions. Chehroudi [15–20] believes that this phenomenon could be used to ignite multiple points in an air/fuel mixture simultaneously, for example in ICE, and hence realizing the so-called volumetrically-distributed ignition for

The use of PTI system would lead to the following advantages compared to the other ignition

• the volume where ignition takes place can be adjusted to achieve both localized or volu-

A first investigation in this direction has been described in [21, 22], where the combustion process initiated by CNT and nEMs, collectively referred to as nanoignition agents (nIAs),

SWCNT samples are responsible for the photo-ignition phenomenon.

distributed ignition of liquid and solid fuels.

metrically distributed ignition.

• the ignition could be achieved remotely and timely; and

HCCI combustion.

310 Carbon Nanotubes - Recent Progress

systems:

A system was designed and realized on purpose to demonstrate volumetric ignition of gaseous fuels (methane, hydrogen, and LPG) and gasoline through PTI of MWCNTs with 75% by weight of Fc (it was demonstrated in [23] that this composition had the lowest power density required for ignition). MWCNTs and Fc properties are reported, respectively, in **Table 1**. A schematic diagram of the complete experimental layout for both air/gaseous fuel mixtures and air/gasoline mixtures is shown in **Figure 1**.


**Table 1.** Properties of MWCNTs and ferrocene.

fuels used in each case is determined in order to ensure a final pressure of 6 bar in the mixture chamber while at the same time achieve a desired air/gaseous-fuel mass ratio. From the mixture chamber, the air/gaseous-fuel mixture is introduced into the combustion chamber. A solenoid valve is used to determine the amount of air/gaseous fuel mixture introduced into the combustion chamber in order to reach a desired pressure of 3 bar at the onset of the experiment. Passing through the solenoid valve, the mixture flow also picks up and carries with it a desired amount of MWCNTs/nEMs (nIAs) into the combustion chamber previously

Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal…

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313

For the experiments carried out with air and gasoline, a liquid fuel injection system composed of a fuel tank pressurized with an inert gas (nitrogen) to a value equal to 4.5 bar is used for all tests. The fuel injection system is composed of a traditional automotive gasoline injector and a module to control the injector opening. In order to run the experiments, air and gasoline are separately introduced into the combustion chamber. The amount of the gasoline introduced into the combustion chamber is determined in order to ensure a desired value of the air/gasoline mass ratio and this is realized through on the pulse width applied to the electronics associated with the injector control system. Once the gasoline is injected into the combustion chamber, the air, at a pressure of 6 bar in the mixture chamber, is introduced into the combustion chamber by means of a solenoid valve. In this case too, the amount of air introduced into the combustion chamber is controlled by the opening time of the solenoid in order to reach the desired pressure of 3 bar for each test. Tests with the gasoline fuel were conducted by varying an additional parameter, that is, the "residence time" of the liquid gasoline injected into the combustion chamber before it is mixed with the air coming from the mixture chamber carrying the nIAs. Because the photoignition could possibly be negatively impacted due to wetting of the nIAs by the injected liquid gasoline spray, the effects of this residence time should offer a preliminary data as to the impacts of such a wetting process. The residence time is the time between the end of liquid fuel injection and the air solenoid valve opening time. Two different residence times have been tested: 0 and

500 ms, representing the lower and upper bounds for this parameter used in this study.

For each test, as described previously, once the desired pressure (3 bar) was reached in the combustion chamber, the solenoid valve was automatically closed and, after a constant delay, a TTL signal was generated for activating either the camera flash or the spark plug. Furthermore, the energy released by the flash unit was about 5 J for air-methane fuel tests (this value was assumed to be constant and taken as a reference for all air-gaseous fuel tests), 8 J for air-gasoline tests, while the energy released by the spark plug was about 20 J for all tests. Before each test, the combustion chamber was thoroughly purged with fresh air. In all the tests, nIAs sample, each consisting of 20 mg of MWCNTs with 75% of Fc by weight were used. The minimum amount of nanoparticles was obtained as in [22], which corresponds to a nIAs concentration of 159 ppmv in the combustion chamber. Out of 158 ppmv, 139 represented by the MWCNTs and 19 ppmv by the Fc, corresponds to an overall energy equal to 6.3 J, of which 1.1 J come from MWCNTs and 5.2 J from Fc (which collectively are less than 1% of the overall

For experiments with both ignition systems, tests were conducted by varying the relative air/

introduced in the CNTs holder, see **Figure 1**.

thermal energy released by the fuels).

fuel ratio, *λ*, defined as:

**Figure 1.** Schematic diagram of the experimental apparatus used during air/gaseous fuels and air/gasoline tests. Reprinted from [24], with permission from Elsevier.

The experimental setup was designed for performing a single combustion event at a time; the procedure for charging the air/fuel mixture as well as the ignition agents, therefore, must be repeated for each test, as detailed in the following. The constant volume combustion chamber is made of low-carbon steel and has a cylindrical shape with inner diameter of 53 mm and length of 270 mm. The chamber was equipped with a piezoresistive pressure sensor (KELLER type PA-21Y 0–200 bar). Pressure signal has been sampled at 2.5 kHz using a NI cDAQ 9178 acquisition board with a NI 9205 AI module. A longitudinal quartz rectangular optical window (172 mm length × 37 mm height × 20 mm thickness) was mounted along the side of the combustion chamber to allow visual access. By using a CCD Memrecam GX-1F High Speed framing camera, positioned in front of the quartz optical access, images of the ignition and burning processes have been acquired with a frame rate of 2.5 kHz.

An automotive spark plug (NGK model 4983 DCPR7E-N-10) and a Xe camera flash (Linear Xenon Flash Tubes model FT-L4040 for air/gaseous fuel tests, and a Linear Xenon Flash Tubes model FT-L6085 for air/gasoline tests) have been placed at appropriate locations inside the combustion chamber. In this manner, the combustion process via SI can be compared with that obtained through PTI of MWCNTs/nEMs mixed with air/fuel mixture. Both ignition methods have been activated by a relay, remotely controlled by means of a DIO 5 Volt TTL High Speed NI 9401 module. Once the desired pressure of 3 bar is reached in the combustion chamber, the solenoid valve automatically closes and, after a constant delay, a TTL signal is generated for activating either the camera flash or the spark plug. The maximum energy that can be released by Xe flash lamps used for photo-ignition is respectively 50 J for tests with gaseous fuels and 120 J for tests with gasoline since, supposedly, tests with gasoline would require more energy for nIAS ignition.

In order to run the experiments for air/gaseous fuel mixtures, air and gaseous fuel are separately introduced into the mixture chamber through different ducts. The amount of gaseous fuels used in each case is determined in order to ensure a final pressure of 6 bar in the mixture chamber while at the same time achieve a desired air/gaseous-fuel mass ratio. From the mixture chamber, the air/gaseous-fuel mixture is introduced into the combustion chamber. A solenoid valve is used to determine the amount of air/gaseous fuel mixture introduced into the combustion chamber in order to reach a desired pressure of 3 bar at the onset of the experiment. Passing through the solenoid valve, the mixture flow also picks up and carries with it a desired amount of MWCNTs/nEMs (nIAs) into the combustion chamber previously introduced in the CNTs holder, see **Figure 1**.

For the experiments carried out with air and gasoline, a liquid fuel injection system composed of a fuel tank pressurized with an inert gas (nitrogen) to a value equal to 4.5 bar is used for all tests. The fuel injection system is composed of a traditional automotive gasoline injector and a module to control the injector opening. In order to run the experiments, air and gasoline are separately introduced into the combustion chamber. The amount of the gasoline introduced into the combustion chamber is determined in order to ensure a desired value of the air/gasoline mass ratio and this is realized through on the pulse width applied to the electronics associated with the injector control system. Once the gasoline is injected into the combustion chamber, the air, at a pressure of 6 bar in the mixture chamber, is introduced into the combustion chamber by means of a solenoid valve. In this case too, the amount of air introduced into the combustion chamber is controlled by the opening time of the solenoid in order to reach the desired pressure of 3 bar for each test. Tests with the gasoline fuel were conducted by varying an additional parameter, that is, the "residence time" of the liquid gasoline injected into the combustion chamber before it is mixed with the air coming from the mixture chamber carrying the nIAs. Because the photoignition could possibly be negatively impacted due to wetting of the nIAs by the injected liquid gasoline spray, the effects of this residence time should offer a preliminary data as to the impacts of such a wetting process. The residence time is the time between the end of liquid fuel injection and the air solenoid valve opening time. Two different residence times have been tested: 0 and 500 ms, representing the lower and upper bounds for this parameter used in this study.

The experimental setup was designed for performing a single combustion event at a time; the procedure for charging the air/fuel mixture as well as the ignition agents, therefore, must be repeated for each test, as detailed in the following. The constant volume combustion chamber is made of low-carbon steel and has a cylindrical shape with inner diameter of 53 mm and length of 270 mm. The chamber was equipped with a piezoresistive pressure sensor (KELLER type PA-21Y 0–200 bar). Pressure signal has been sampled at 2.5 kHz using a NI cDAQ 9178 acquisition board with a NI 9205 AI module. A longitudinal quartz rectangular optical window (172 mm length × 37 mm height × 20 mm thickness) was mounted along the side of the combustion chamber to allow visual access. By using a CCD Memrecam GX-1F High Speed framing camera, positioned in front of the quartz optical access, images of the ignition and

**Figure 1.** Schematic diagram of the experimental apparatus used during air/gaseous fuels and air/gasoline tests.

An automotive spark plug (NGK model 4983 DCPR7E-N-10) and a Xe camera flash (Linear Xenon Flash Tubes model FT-L4040 for air/gaseous fuel tests, and a Linear Xenon Flash Tubes model FT-L6085 for air/gasoline tests) have been placed at appropriate locations inside the combustion chamber. In this manner, the combustion process via SI can be compared with that obtained through PTI of MWCNTs/nEMs mixed with air/fuel mixture. Both ignition methods have been activated by a relay, remotely controlled by means of a DIO 5 Volt TTL High Speed NI 9401 module. Once the desired pressure of 3 bar is reached in the combustion chamber, the solenoid valve automatically closes and, after a constant delay, a TTL signal is generated for activating either the camera flash or the spark plug. The maximum energy that can be released by Xe flash lamps used for photo-ignition is respectively 50 J for tests with gaseous fuels and 120 J for tests with gasoline since, supposedly, tests with gasoline would

In order to run the experiments for air/gaseous fuel mixtures, air and gaseous fuel are separately introduced into the mixture chamber through different ducts. The amount of gaseous

burning processes have been acquired with a frame rate of 2.5 kHz.

require more energy for nIAS ignition.

Reprinted from [24], with permission from Elsevier.

312 Carbon Nanotubes - Recent Progress

For each test, as described previously, once the desired pressure (3 bar) was reached in the combustion chamber, the solenoid valve was automatically closed and, after a constant delay, a TTL signal was generated for activating either the camera flash or the spark plug. Furthermore, the energy released by the flash unit was about 5 J for air-methane fuel tests (this value was assumed to be constant and taken as a reference for all air-gaseous fuel tests), 8 J for air-gasoline tests, while the energy released by the spark plug was about 20 J for all tests.

Before each test, the combustion chamber was thoroughly purged with fresh air. In all the tests, nIAs sample, each consisting of 20 mg of MWCNTs with 75% of Fc by weight were used. The minimum amount of nanoparticles was obtained as in [22], which corresponds to a nIAs concentration of 159 ppmv in the combustion chamber. Out of 158 ppmv, 139 represented by the MWCNTs and 19 ppmv by the Fc, corresponds to an overall energy equal to 6.3 J, of which 1.1 J come from MWCNTs and 5.2 J from Fc (which collectively are less than 1% of the overall thermal energy released by the fuels).

For experiments with both ignition systems, tests were conducted by varying the relative air/ fuel ratio, *λ*, defined as:

$$
\lambda = \frac{\text{(AIF)}\_{\text{act}}}{\text{(AIF)}\_{\text{st}}} \tag{1}
$$

where (*A/F*) *act* is the ratio between air (*A*) and fuel (*F*) mass actually fed into the combustion chamber, and (*A/F*)*st* is the stoichiometric amount of air for the type of the fuel per unit amount of fuel used. Based on this definition, *λ* = 1 represents an actual stoichiometric mixture, while *λ* > 1 indicates a mixture leaner than the stoichiometric as *λ* exceeds unity. In this work, (*A/F*) st for methane, hydrogen, LPG, and gasoline were assumed equal to 17.4, 34, 15.5, and 14.7, respectively. The air/fuel ratio, *λ*, was varied within an interval ranging from 1 to 3.5 and was estimated considering both the mixture composition in the mixture chamber and the filling process of the combustion chamber described earlier.

The ignition and combustion characteristics were analyzed through measurement of the combustion chamber pressure. In fact, for a constant volume chamber and under the hypotheses of homogeneous system and negligible wall heat transfer, the first law of thermodynamics allows the estimation of the heat release rate (*HRR*) by the fuel combustion as:

$$HRR(t) \, = \frac{dQ}{dt} = \frac{1}{\gamma - 1} V \frac{dp}{dt} \tag{2}$$

where *Q* is the heat released by the fuel during combustion (on first approximation, it is proportional to the fuel burned), *γ* is the mixture-specific heat ratio (assumed constant and equal to 1.38), *V* is the combustion chamber volume, and *p* is the pressure measured in the combustion chamber. Using the *HRR*, it is also possible to estimate the cumulative thermal energy released within a time interval between the start of combustion (*t ignition*) and any time *t* later as follows:

$$cumHRR(t) = \int\_{t\_{\text{peak}}}^{t} HRRdt\tag{3}$$

decreases (not shown in **Figure 2**) due to cooling of the exhaust gases through heat exchange with the chamber walls. Therefore, the portion of the *cumHRR(t)* curve useful for the combus-

**Figure 2.** *cumHRR(t)* traces for PTI and SI ignition approaches using air/methane mixture at *λ* = 1.6. Reprinted from [24],

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315

• the *ignition delay*, defined as the difference between the time when the amount of heat released had reached a value equal to 10% of total heat released and the time when the trigger

• the *combustion duration*, defined as the difference between the time instants when the heat released had reached 90% and 10% of the total heat released. Note that the "total" combus-

In **Figure 3**, the *cumHRR(t)* curves related to PTI (a, a′, and a″) and SI (b, b′, and b″) combustion processes are shown. **Figure 3** also separates results for methane, LPG, and hydrogen for different values of *λ*. **Table 2** shows percentages of CNT and Fc for different fuels and differ-

From the *cumHRR(t)* curve, it was possible to estimate the following parameters:

tion period is the sum of ignition delay and combustion duration.

tion analysis is only the rising one as shown.

**3. Performance of PTI combustion**

**3.1. Air/gaseous fuel mixtures**

ent values of *λ* in the tests.

has been activated;

with permission from Elsevier.

The maximum value of the *cumHRR(t)* is obtained by integrating the *HRR* between *tignition* and the time *t pmax*, which corresponds to the maximum pressure reached in the combustion chamber. This value is an indicator of the total heat released. Furthermore, knowing *cumHRR(t)*, it is also possible to estimate the combustion efficiency, here defined as:

ber. This value is an indicator of the total heat released. Furthermore, knowing  $cumHRR(t)$ , it is also possible to estimate the combustion efficiency, here defined as: 
$$\eta\_b = \frac{\int\_{t\_{\text{min}}}^{t\_{\text{-}}} HRRdt}{(m\_{\text{min}} \, ^\circ H\_{\text{(sh)}}) + (m\_{\text{nlts}} \, ^\circ H\_{\text{(nlts)}})} \tag{4}$$

in which the numerator is equal to the total heat released during combustion, while the denominator is equal to the thermal energy supplied to the system in the form of liquid or gaseous fuels. The contribution of the nanoparticles combustion energy in the denominator is equal to 6.3 J, which is less than 0.3% for each condition tested, and has been neglected.

**Figure 2** shows *cumHRR* traces of the fuel burning process when initiated through the ignition of nIAs via a flash and a spark plug in an air/methane mixture at a *λ* value of 1.6. It is possible to see that, after the trigger signal (flash or spark activation), both curves exhibit a rising phase due to the heat released by the fuel during the combustion and reach a peak value when all the fuel is fully oxidized. Afterward, the pressure, and *cumHRR* as a consequence, slowly Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal… http://dx.doi.org/10.5772/intechopen.70887 315

**Figure 2.** *cumHRR(t)* traces for PTI and SI ignition approaches using air/methane mixture at *λ* = 1.6. Reprinted from [24], with permission from Elsevier.

decreases (not shown in **Figure 2**) due to cooling of the exhaust gases through heat exchange with the chamber walls. Therefore, the portion of the *cumHRR(t)* curve useful for the combustion analysis is only the rising one as shown.

From the *cumHRR(t)* curve, it was possible to estimate the following parameters:


#### **3. Performance of PTI combustion**

#### **3.1. Air/gaseous fuel mixtures**

*<sup>λ</sup>* <sup>=</sup> (*A*/*F*) \_\_\_\_\_\_*act*

filling process of the combustion chamber described earlier.

*HRR*(*t*) <sup>=</sup> *\_\_\_ dQ*

within a time interval between the start of combustion (*t*

is also possible to estimate the combustion efficiency, here defined as:

*cumHRR*(*t*) = ∫

*η<sup>b</sup>* =

where (*A/F*)

314 Carbon Nanotubes - Recent Progress

work, (*A/F*)

the time *t*

neglected.

(*A*/*F*) *st*

tion chamber, and (*A/F*)*st* is the stoichiometric amount of air for the type of the fuel per unit amount of fuel used. Based on this definition, *λ* = 1 represents an actual stoichiometric mixture, while *λ* > 1 indicates a mixture leaner than the stoichiometric as *λ* exceeds unity. In this

and 14.7, respectively. The air/fuel ratio, *λ*, was varied within an interval ranging from 1 to 3.5 and was estimated considering both the mixture composition in the mixture chamber and the

The ignition and combustion characteristics were analyzed through measurement of the combustion chamber pressure. In fact, for a constant volume chamber and under the hypotheses of homogeneous system and negligible wall heat transfer, the first law of thermodynamics

where *Q* is the heat released by the fuel during combustion (on first approximation, it is proportional to the fuel burned), *γ* is the mixture-specific heat ratio (assumed constant and equal to 1.38), *V* is the combustion chamber volume, and *p* is the pressure measured in the combustion chamber. Using the *HRR*, it is also possible to estimate the cumulative thermal energy released

*dt* <sup>=</sup> \_\_\_1

*tignition t*

*pmax*, which corresponds to the maximum pressure reached in the combustion cham-

The maximum value of the *cumHRR(t)* is obtained by integrating the *HRR* between *tignition* and

ber. This value is an indicator of the total heat released. Furthermore, knowing *cumHRR(t)*, it

<sup>∗</sup> *Hi*,*fuel*) + (*mnIAs*

in which the numerator is equal to the total heat released during combustion, while the denominator is equal to the thermal energy supplied to the system in the form of liquid or gaseous fuels. The contribution of the nanoparticles combustion energy in the denominator is equal to 6.3 J, which is less than 0.3% for each condition tested, and has been

**Figure 2** shows *cumHRR* traces of the fuel burning process when initiated through the ignition of nIAs via a flash and a spark plug in an air/methane mixture at a *λ* value of 1.6. It is possible to see that, after the trigger signal (flash or spark activation), both curves exhibit a rising phase due to the heat released by the fuel during the combustion and reach a peak value when all the fuel is fully oxidized. Afterward, the pressure, and *cumHRR* as a consequence, slowly

∫*t ignition tpmax HRRdt* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (*mfuel*

*<sup>γ</sup>* <sup>−</sup> <sup>1</sup> *<sup>V</sup> dp \_\_\_*

*dt* (2)

*ignition*) and any time *t* later as follows:

*HRRdt* (3)

<sup>∗</sup> *Hi*,*nIAs*) (4)

allows the estimation of the heat release rate (*HRR*) by the fuel combustion as:

*act* is the ratio between air (*A*) and fuel (*F*) mass actually fed into the combus-

st for methane, hydrogen, LPG, and gasoline were assumed equal to 17.4, 34, 15.5,

(1)

In **Figure 3**, the *cumHRR(t)* curves related to PTI (a, a′, and a″) and SI (b, b′, and b″) combustion processes are shown. **Figure 3** also separates results for methane, LPG, and hydrogen for different values of *λ*. **Table 2** shows percentages of CNT and Fc for different fuels and different values of *λ* in the tests.

**Figure 3.** Plots of *cumHRR(t)* for PTI (a, a′, and a″) and SI (b, b′, and b″) ignition methods are shown. Plots in (c, c′, and c″) show percent reductions of ignition delay and total combustion duration for the PTI approach with respect (or reference) to the SI method for different values of *λ.* Results for methane, LPG, and hydrogen are shown. PTI and SI stand for photo-thermal ignition and spark ignition. Reprinted from [24], with permission from Elsevier.

Comparing the results for PTI column with those for SI column in **Figure 3**, it can be observed that the combustion initiated by the photo-thermal ignition of nIAs releases a total heat higher than the case in which the mixture is ignited by a spark plug. This effect is thought to be due to two reasons. On one hand, the wall heat transfer is expected to play a role in SI due to longer combustion duration. On the other hand, from the analysis of combustion process images (results are presented in the following section), the combustion triggered by the PTI involves the entire mixture in the combustion chamber when compared to the SI case. It is therefore expected that for the PTI case, the mixture at the peripheral areas of the combustion chamber (i.e., near-wall regions) is more easily reached and burned. While for the SI combustion, this would be unlikely because the propagating flame front cools as it approaches the walls of the chamber. Moreover, comparing results in PTI and SI columns in **Figure 3**, it can also be argued that, with PTI, both ignition delay and combustion duration are shortened. This conclusion is confirmed by results reported in **Figure 3(c)**, (**c′**), and (**c″**) for methane, LPG, and hydrogen, respectively, which show percent relative reductions in ignition delay and total combustion duration using PTI as compared to the SI approach. The same trend has also been reported in [11] where the authors have tested the photo-induced

**Methane**

> λ [−]

Air [g] Fuel [g] % CNT

% Fc **Table 2.**

0.84

0.83

0.83

0.83

0.83 Air, gaseous fuel, CNT and Fc quantities used for each test. Percentages are on mass basis.

0.82

0.8

0.8

0.8

0.8

0.8

1.00

0.99

0.97

0.95

0.93

0.90

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0.28

0.28

0.28

0.28

0.28

0.28

0.27

0.27

0.27

0.27

0.27

0.34

0.33

0.32

0.31

0.31

0.30

0.06

0.05

0.05

0.04

0.04

0.03

0.09

0.08

0.06

0.05

0.03

0.03

0.03

0.02

0.02

0.02

0.01

1.73

1.75

1.76

1.77

1.77

1.78

1.79

1.80

1.806

1.82

1.83

1.44

1.49

1.52

1.57

1.59

1.64

1.6

1.9

2.1

2.4

2.7

3.0

1.2

1.5

1.9

2.5

3.7

1.5

1.8

1.9

2.4

2.7

3.4

**LPG**

**Hydrogen**


Comparing the results for PTI column with those for SI column in **Figure 3**, it can be observed that the combustion initiated by the photo-thermal ignition of nIAs releases a total heat higher than the case in which the mixture is ignited by a spark plug. This effect is thought to be due to two reasons. On one hand, the wall heat transfer is expected to play a role in SI due to longer combustion duration. On the other hand, from the analysis of combustion process images (results are presented in the following section), the combustion triggered by the PTI involves the entire mixture in the combustion chamber when compared to the SI case. It is therefore expected that for the PTI case, the mixture at the peripheral areas of the combustion chamber (i.e., near-wall regions) is more easily reached and burned. While for the SI combustion, this would be unlikely because the propagating flame front cools as it approaches the walls of the chamber. Moreover, comparing results in PTI and SI columns in **Figure 3**, it can also be argued that, with PTI, both ignition delay and combustion duration are shortened. This conclusion is confirmed by results reported in **Figure 3(c)**, (**c′**), and (**c″**) for methane, LPG, and hydrogen, respectively, which show percent relative reductions in ignition delay and total combustion duration using PTI as compared to the SI approach. The same trend has also been reported in [11] where the authors have tested the photo-induced

photo-thermal ignition and spark ignition. Reprinted from [24], with permission from Elsevier.

316 Carbon Nanotubes - Recent Progress

**Figure 3.** Plots of *cumHRR(t)* for PTI (a, a′, and a″) and SI (b, b′, and b″) ignition methods are shown. Plots in (c, c′, and c″) show percent reductions of ignition delay and total combustion duration for the PTI approach with respect (or reference) to the SI method for different values of *λ.* Results for methane, LPG, and hydrogen are shown. PTI and SI stand for **Table 2.** Air, gaseous fuel, CNT and Fc quantities used for each test. Percentages are on mass basis. ignition of quiescent air/fuel mixtures containing suspended photo-sensitive nanomaterials. SWCNTs with 70% Fe impurity by weight were suspended in air/ethylene mixtures in a static combustion chamber and exposed to a camera flash to cause ignition of the mixture. For comparison purposes, traditional automotive spark ignition experiments were also carried out for air-ethylene mixtures.

The combustion peak pressures reached inside the combustion chamber, using both the PTI and SI methods, are reported in **Figure 4(a)**, (**b**), and (**c**), for methane, LPG, and hydrogen, respectively. Also, the total combustion duration and ignition delay times are shown in **Figure 4(a′)**

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**Figure 4.** Comparison of the peak pressures a, b, and c for CH4

, LPG, and H2

and ignition delay (a″) with PTI and total combustion duration (b′) and ignition delay (b″) with SI for different values of

*λ* are shown for the gaseous fuels used in this study. Reprinted from [24], with permission from Elsevier.

, respectively. Total combustion duration (a′)

Still referring to **Figure 3**, and for all gaseous fuel tests, increasing *λ*, the maximum *cumHRR(t)* decreases and is delayed. In fact, the combustion processes are limited in their heat capacity by the amount of air mixed with the gaseous fuel. Therefore, the maximum gaseous fuel amount is determined based on the maximum amount of air that the system is capable of handling. It is possible to note that the maximum *cumHRR(t)* value is higher at the nearstoichiometric ratio and decreases at higher *λ* values. This is because the amount of air to complete the combustion process is more than that necessary for the stoichiometric AFR (*λ* = 1). For all conditions tested, the combustion process with hydrogen is much faster, more intense, and shows a shorter ignition delay and combustion duration compared to other two gaseous fuels. This is mainly due to its laminar flame burning speed, being higher than that of methane and LPG (see data reported in **Table 3**), and more intense because the lower heating value (LHV) of hydrogen is higher than that of methane and LPG. Finally, the ignition delay is shorter because the minimum ignition energy required to ignite hydrogen is ≈0.02 mJ, while those for the methane and LPG are 0.3 and 0.26 mJ, respectively.

From **Figure 3(c)**, (**c′**), and (**c″**), it can be seen that in all cases, the ignition delay period is reduced when combustion was initiated using the PTI approach. Interestingly, it can also be observed that, for the methane and LPG, the advantage (in terms of % reduction in ignition delay period and total combustion duration) of PTI declines as the mixture AFR increases toward the leanest mixture. There is an exception in the case of methane and for the leanest mixture tested where this percent reduction actually increases substantially (from *λ* of 2.7–3.0). One sees a completely different trend for hydrogen, where the advantage of the photoignition continuously increases as the mixture is leaned out. The comparison of the percent reductions in ignition delay period to those for the total combustion duration indicates that the two values are generally very close to each other. This suggests that the benefits of PTI are primarily at the ignition delay phase of the combustion. However, one can see a slightly larger difference between the two values for LPG and only for the lowest values of the *λ*.


**Table 3.** Physical properties of tested gaseous fuels.

The combustion peak pressures reached inside the combustion chamber, using both the PTI and SI methods, are reported in **Figure 4(a)**, (**b**), and (**c**), for methane, LPG, and hydrogen, respectively. Also, the total combustion duration and ignition delay times are shown in **Figure 4(a′)**

ignition of quiescent air/fuel mixtures containing suspended photo-sensitive nanomaterials. SWCNTs with 70% Fe impurity by weight were suspended in air/ethylene mixtures in a static combustion chamber and exposed to a camera flash to cause ignition of the mixture. For comparison purposes, traditional automotive spark ignition experiments were also car-

Still referring to **Figure 3**, and for all gaseous fuel tests, increasing *λ*, the maximum *cumHRR(t)* decreases and is delayed. In fact, the combustion processes are limited in their heat capacity by the amount of air mixed with the gaseous fuel. Therefore, the maximum gaseous fuel amount is determined based on the maximum amount of air that the system is capable of handling. It is possible to note that the maximum *cumHRR(t)* value is higher at the nearstoichiometric ratio and decreases at higher *λ* values. This is because the amount of air to complete the combustion process is more than that necessary for the stoichiometric AFR (*λ* = 1). For all conditions tested, the combustion process with hydrogen is much faster, more intense, and shows a shorter ignition delay and combustion duration compared to other two gaseous fuels. This is mainly due to its laminar flame burning speed, being higher than that of methane and LPG (see data reported in **Table 3**), and more intense because the lower heating value (LHV) of hydrogen is higher than that of methane and LPG. Finally, the ignition delay is shorter because the minimum ignition energy required to ignite hydrogen is ≈0.02 mJ, while

From **Figure 3(c)**, (**c′**), and (**c″**), it can be seen that in all cases, the ignition delay period is reduced when combustion was initiated using the PTI approach. Interestingly, it can also be observed that, for the methane and LPG, the advantage (in terms of % reduction in ignition delay period and total combustion duration) of PTI declines as the mixture AFR increases toward the leanest mixture. There is an exception in the case of methane and for the leanest mixture tested where this percent reduction actually increases substantially (from *λ* of 2.7–3.0). One sees a completely different trend for hydrogen, where the advantage of the photoignition continuously increases as the mixture is leaned out. The comparison of the percent reductions in ignition delay period to those for the total combustion duration indicates that the two values are generally very close to each other. This suggests that the benefits of PTI are primarily at the ignition delay phase of the combustion. However, one can see a slightly larger difference between the two values for LPG and only for the lowest values

**Hydrogen Methane LPG**

those for the methane and LPG are 0.3 and 0.26 mJ, respectively.

LHV [mJ/kg] 119.9 50.0 46.2 Flammability limit 2.9–76% 5.3–15% 1.7–9.5% Flash point [°C] 585 540 480 Minimum ignition energy [mJ] 0.02 0.30 0.26 Laminar flame speed [m/s] 2.7–3.3 0.3–0.4 0.3–0.4

ried out for air-ethylene mixtures.

318 Carbon Nanotubes - Recent Progress

of the *λ*.

**Table 3.** Physical properties of tested gaseous fuels.

**Figure 4.** Comparison of the peak pressures a, b, and c for CH4 , LPG, and H2 , respectively. Total combustion duration (a′) and ignition delay (a″) with PTI and total combustion duration (b′) and ignition delay (b″) with SI for different values of *λ* are shown for the gaseous fuels used in this study. Reprinted from [24], with permission from Elsevier.

and (**a″**) for the PTI and **Figure 4(b′)** and (**b″**), for the SI approaches. In this way, differences between the three gaseous fuels are highlighted.

In summary, rapid rise of pressure and, therefore, a shorter pressure rise time, as well as a higher total heat released are seen in **Figure 3**. Also, mostly higher peak pressures were observed for the PTI approach as seen in **Figure 4**. This is due to the fact that the ignition of the mixture by flash exposure of the MWCNTs/nEMs leads to numerous ignition nuclei, burning "simultaneously," and hence speeding up the heat release during the combustion process. However, with the spark plug approach, as it is well known, the combustion process is triggered in only one point (i.e., within the spark plug gap) and then proceeds with the propagation of a flame front. The fuel mixture burning process is therefore too far from burning simultaneously as it is almost the case for the PTI.

This spatially-distributed ignition of the air/fuel mixture has been confirmed by observing the combustion process directly. **Figure 5** shows consecutive frames of the combustion process evolution in time. Two series of pictures related to the combustion process with PTI (left column) and spark plug (right column) are reported. Data refer to *λ* = 2.10 for air/methane mixture, and the time interval between two consecutive frames is equal to 5 ms. It is clearly visible that, with PTI process, the combustion is faster and the combustion chamber is entirely illuminated starting from the fifth frame indicating spatial burning. On the contrary, in the SI case, the propagation of a flame front is recognized in all the pictures. Moreover, the light radiated by the flame is weaker.

combustion efficiency obtained with the combustion process triggered by the PTI process is

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, (b) LPG, and

321

**Figure 6.** Comparison of combustion efficiency with PTI and SI at different values of *λ* and for (a) CH4

For the first time, the feasibility of igniting air/gasoline mixtures by means of PTI has been demonstrated in a constant volume combustion chamber. The combustion process has been compared with the one obtained by a traditional SI system. For these tests too, the amounts of air and gasoline as well as the CNT and Fc as percentages of the fuel mass in the mixture have

**Figure 7(a)** and (**b**) show *cumHRR(t)* traces for tests with liquid gasoline fuel injection by using PTI and SI approaches, respectively. Results for the percent reduction of combustion duration and ignition delay time using PTI as compared to SI approach are also shown in **Figure 7(c)** and (**c′**). Data shown in **Figure 7(a)**, (**b**) and (**c**) are related to tests when the residence time of gasoline fuel inside the combustion chamber (before mixing with the incoming air through the solenoid valve) was equal to 0 ms. On the other hand, results reported in **Figure 7(a′)**, (**b′**) and (**c′**) refer to tests with the residence time 500 ms. In the first set of tests, the liquid fuel, after injection into the combustion chamber, was immediately mixed

**λ [−] Air [g] Gasoline [g] % CNT % Fc** 0.4 1.97 0.34 0.22 0.65 0.5 1.97 0.26 0.22 0.67 0.8 1.97 0.16 0.23 0.70 1.2 1.97 0.11 0.24 0.72 2.2 1.97 0.06 0.25 0.74 2.7 1.97 0.05 0.25 0.74

**Table 4.** Air, gasoline, CNT and Fc quantities used of each test. Percentages are on mass basis.

slightly higher or comparable to that observed using the spark plug.

**3.2. Air/gasoline mixtures**

(c) H2 .

been calculated, and are reported in **Table 4**.

Finally, in **Figure 6**, the combustion efficiency *η<sup>b</sup>* with PTI and SI is reported for different values of *λ* for CH4 , LPG, and H2 . It is possible to note that, for each gaseous fuel tested, the

**Figure 5.** Pictures of the combustion process of an air/methane mixture with *λ* = 2.10; comparison between PTI and SI. Reprinted from [21], with permission from Elsevier.

**Figure 6.** Comparison of combustion efficiency with PTI and SI at different values of *λ* and for (a) CH4 , (b) LPG, and (c) H2 .

combustion efficiency obtained with the combustion process triggered by the PTI process is slightly higher or comparable to that observed using the spark plug.

#### **3.2. Air/gasoline mixtures**

and (**a″**) for the PTI and **Figure 4(b′)** and (**b″**), for the SI approaches. In this way, differences

In summary, rapid rise of pressure and, therefore, a shorter pressure rise time, as well as a higher total heat released are seen in **Figure 3**. Also, mostly higher peak pressures were observed for the PTI approach as seen in **Figure 4**. This is due to the fact that the ignition of the mixture by flash exposure of the MWCNTs/nEMs leads to numerous ignition nuclei, burning "simultaneously," and hence speeding up the heat release during the combustion process. However, with the spark plug approach, as it is well known, the combustion process is triggered in only one point (i.e., within the spark plug gap) and then proceeds with the propagation of a flame front. The fuel mixture burning process is therefore too far from burn-

This spatially-distributed ignition of the air/fuel mixture has been confirmed by observing the combustion process directly. **Figure 5** shows consecutive frames of the combustion process evolution in time. Two series of pictures related to the combustion process with PTI (left column) and spark plug (right column) are reported. Data refer to *λ* = 2.10 for air/methane mixture, and the time interval between two consecutive frames is equal to 5 ms. It is clearly visible that, with PTI process, the combustion is faster and the combustion chamber is entirely illuminated starting from the fifth frame indicating spatial burning. On the contrary, in the SI case, the propagation of a flame front is recognized in all the pictures. Moreover, the light radiated by the flame is weaker.

**Figure 5.** Pictures of the combustion process of an air/methane mixture with *λ* = 2.10; comparison between PTI and

with PTI and SI is reported for different

. It is possible to note that, for each gaseous fuel tested, the

between the three gaseous fuels are highlighted.

320 Carbon Nanotubes - Recent Progress

ing simultaneously as it is almost the case for the PTI.

Finally, in **Figure 6**, the combustion efficiency *η<sup>b</sup>*

SI. Reprinted from [21], with permission from Elsevier.

, LPG, and H2

values of *λ* for CH4

For the first time, the feasibility of igniting air/gasoline mixtures by means of PTI has been demonstrated in a constant volume combustion chamber. The combustion process has been compared with the one obtained by a traditional SI system. For these tests too, the amounts of air and gasoline as well as the CNT and Fc as percentages of the fuel mass in the mixture have been calculated, and are reported in **Table 4**.

**Figure 7(a)** and (**b**) show *cumHRR(t)* traces for tests with liquid gasoline fuel injection by using PTI and SI approaches, respectively. Results for the percent reduction of combustion duration and ignition delay time using PTI as compared to SI approach are also shown in **Figure 7(c)** and (**c′**). Data shown in **Figure 7(a)**, (**b**) and (**c**) are related to tests when the residence time of gasoline fuel inside the combustion chamber (before mixing with the incoming air through the solenoid valve) was equal to 0 ms. On the other hand, results reported in **Figure 7(a′)**, (**b′**) and (**c′**) refer to tests with the residence time 500 ms. In the first set of tests, the liquid fuel, after injection into the combustion chamber, was immediately mixed


**Table 4.** Air, gasoline, CNT and Fc quantities used of each test. Percentages are on mass basis.

From **Figure 8**, it is possible to note that the combustion efficiency obtained with the combustion process triggered by photo-thermal ignition is slightly higher or comparable to that

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**Figure 9** shows (a) peak pressure, (b) ignition delay, (c) combustion duration, and (d) combustion efficiency, for different values of *λ* and for the two residence times tested. Generally, it is seen that the combustion process generated for the longer residence time exhibits a higher peak of combustion pressure. The ignition delay for the tests with 500 ms residence time seems to be equal or less than the ones with 0 ms case. However, both ignition delay and combustion duration with 500 ms residence time are shorter compared to those at 0 ms. Furthermore, it was possible to note a slight improvement of the combustion efficiency and the total heat released (THR) for the tests carried out igniting the mixtures where the fuel has had a higher residence time before mixing with the air. This behavior could be due to more time for evaporation of the gasoline fuel droplets in the combustion chamber after liquid injection, leading to a more homogeneous and distributed air/fuel mixture inside the combustion chamber. This could facilitate the ignition process and cause a faster consumption of the air/gasoline mixture with a consequent increase of the combustion peak pressure

Finally, in each test with PTI, shorter pressure rise times and higher peaks of *cumHRR* and combustion pressures were observed because the ignition of the mixture was by a large number of ignition nuclei which occurred "simultaneously," thereby speeding up the combustion process. With the spark plug ignition system, combustion is initiated at single point and hence proceeds with flame propagation mechanism. The burning process is therefore far from being "simultaneous." This behavior has been confirmed by observing the combustion process through high-speed imaging. **Figure 10** shows high-speed images of a single combustion event in each case, initiated by the two ignition systems. Two series of pictures related to the combustion process with PTI (left column) and SI (right column) are reported. It is possible to note that even from the first frame an instantaneous and spatially

**Figure 8.** Combustion efficiency with PTI and SI for different values of *λ* and for tests carried out with a residence time

observed using the spark plug, for both residence times used in this study.

and efficiency.

equal to 0 ms (a) and 500 ms (b).

**Figure 7.** *cumHRR(t)* for combustion of liquid gasoline injection into the chamber initiated with PTI (a, a′) and SI (b, b′) for different values of λ and at two residence times of 0 and 500 ms. Percent reduction of the ignition delay and total combustion duration (c, c′) with PTI as compared to the SI approach for different values of *λ* and at two residence times of 0 ms and 500 ms are shown. Reprinted from [24], with permission from Elsevier.

with the air coming from the mixture chamber. In the second set of tests, the mixing process inside the combustion chamber took place after a fuel residence time equal to 500 ms before the air from the mixture chamber is brought into the combustion chamber. Note that because the gasoline is injected into the combustion chamber in liquid phase, the case with zero residence time implies ignition in the presence of fuel which is predominantly in liquid phase. The case with 500 ms attempts to allow time for more vaporization/mixing to approach a more homogenous mixture. However, the degree of mixing and homogeneity cannot be fully verified at this time.

Also in tests with the liquid fuel, it was observed that the combustion process triggered by the photo-thermal ignition system exhibited the same trend seen when gaseous fuels were used. That is, the combustion process triggered by the PTI generated higher or comparable *cumHRR* to that observed with the spark plug ignition system. Increasing *λ*, i.e., burning leaner mixtures, the peak *cumHRR* decreases and is delayed. Furthermore, it is seen in **Figure 7(c)** and (**c′**) that with PTI, both ignition delay and total combustion duration are shorter than those calculated when SI is used.

From **Figure 7**, it can also be noted that the combustion process for a very lean condition, that is, a *λ* value equal to 2.7, has occurred only for the photo-thermal ignition process, while a misfire took place when combustion was initiated by the spark plug system. This misfire could be either due to an excess of air in the air/fuel mixture or the nature of the SI process. Because the spark plug can only provide ignition at a single point, it is intrinsically incapable of initiating and sustaining the flame front under such a highly lean condition.

From **Figure 8**, it is possible to note that the combustion efficiency obtained with the combustion process triggered by photo-thermal ignition is slightly higher or comparable to that observed using the spark plug, for both residence times used in this study.

**Figure 9** shows (a) peak pressure, (b) ignition delay, (c) combustion duration, and (d) combustion efficiency, for different values of *λ* and for the two residence times tested. Generally, it is seen that the combustion process generated for the longer residence time exhibits a higher peak of combustion pressure. The ignition delay for the tests with 500 ms residence time seems to be equal or less than the ones with 0 ms case. However, both ignition delay and combustion duration with 500 ms residence time are shorter compared to those at 0 ms. Furthermore, it was possible to note a slight improvement of the combustion efficiency and the total heat released (THR) for the tests carried out igniting the mixtures where the fuel has had a higher residence time before mixing with the air. This behavior could be due to more time for evaporation of the gasoline fuel droplets in the combustion chamber after liquid injection, leading to a more homogeneous and distributed air/fuel mixture inside the combustion chamber. This could facilitate the ignition process and cause a faster consumption of the air/gasoline mixture with a consequent increase of the combustion peak pressure and efficiency.

Finally, in each test with PTI, shorter pressure rise times and higher peaks of *cumHRR* and combustion pressures were observed because the ignition of the mixture was by a large number of ignition nuclei which occurred "simultaneously," thereby speeding up the combustion process. With the spark plug ignition system, combustion is initiated at single point and hence proceeds with flame propagation mechanism. The burning process is therefore far from being "simultaneous." This behavior has been confirmed by observing the combustion process through high-speed imaging. **Figure 10** shows high-speed images of a single combustion event in each case, initiated by the two ignition systems. Two series of pictures related to the combustion process with PTI (left column) and SI (right column) are reported. It is possible to note that even from the first frame an instantaneous and spatially

with the air coming from the mixture chamber. In the second set of tests, the mixing process inside the combustion chamber took place after a fuel residence time equal to 500 ms before the air from the mixture chamber is brought into the combustion chamber. Note that because the gasoline is injected into the combustion chamber in liquid phase, the case with zero residence time implies ignition in the presence of fuel which is predominantly in liquid phase. The case with 500 ms attempts to allow time for more vaporization/mixing to approach a more homogenous mixture. However, the degree of mixing and homogeneity cannot be fully

**Figure 7.** *cumHRR(t)* for combustion of liquid gasoline injection into the chamber initiated with PTI (a, a′) and SI (b, b′) for different values of λ and at two residence times of 0 and 500 ms. Percent reduction of the ignition delay and total combustion duration (c, c′) with PTI as compared to the SI approach for different values of *λ* and at two residence times

of 0 ms and 500 ms are shown. Reprinted from [24], with permission from Elsevier.

Also in tests with the liquid fuel, it was observed that the combustion process triggered by the photo-thermal ignition system exhibited the same trend seen when gaseous fuels were used. That is, the combustion process triggered by the PTI generated higher or comparable *cumHRR* to that observed with the spark plug ignition system. Increasing *λ*, i.e., burning leaner mixtures, the peak *cumHRR* decreases and is delayed. Furthermore, it is seen in **Figure 7(c)** and (**c′**) that with PTI, both ignition delay and total combustion duration are shorter than those calcu-

From **Figure 7**, it can also be noted that the combustion process for a very lean condition, that is, a *λ* value equal to 2.7, has occurred only for the photo-thermal ignition process, while a misfire took place when combustion was initiated by the spark plug system. This misfire could be either due to an excess of air in the air/fuel mixture or the nature of the SI process. Because the spark plug can only provide ignition at a single point, it is intrinsically incapable

of initiating and sustaining the flame front under such a highly lean condition.

verified at this time.

322 Carbon Nanotubes - Recent Progress

lated when SI is used.

**Figure 8.** Combustion efficiency with PTI and SI for different values of *λ* and for tests carried out with a residence time equal to 0 ms (a) and 500 ms (b).

**Figure 9.** Comparison of (a) peak pressure, (b) ignition delay, (c) total combustion duration, (d) combustion efficiency, and total heat released with PTI for different values of *λ* and for tests carried out with a residence time equal to 0 and 500 ms are shown.

**4. Nanostructured materials for automotive propulsion**

could be used to direct the flash energy into the combustion chamber.

particulate abatement.

gasoline mixture.

Preliminary results from the PTI approach shows a potential for volumetrically distributed ignition of the fuels. However, the next step is to demonstrate the same results in a real engine. Indeed, the fact that a very small amount of energy (such as that emitting from an ordinary camera flash) is able to bring about volumetric ignition in a lean air/fuel mixture could lead to substantial improvements of engine performance. There are several ways the nanostructured materials could be introduced into the engine, for example, either by powder injection into the intake port or through mixing it with the fuel. A system of fiberoptic cables and flash lamp

**Figure 10.** Pictures of the combustion process at *λ* = 0.52; comparison between MWCNTs/nEMs PTI and SI for air/

Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal…

http://dx.doi.org/10.5772/intechopen.70887

325

But the main issue is possible environmental impacts of nanometric carbonaceous structures in the combustion process due to possible creation of condensation nuclei for further particulate formation in the engine and from some nanostructured materials escaping the combustion process, which can end up in the atmosphere. The potential impact of combustion products on health and environment cannot be easily predicted at the present stage of the research. In fact, it is expected that the in actual application, a more precise control of ignition timing and a shorter combustion would allow more flexibility to redesign the engine in terms of geometric as well as control parameters. This could have direct implications on the raw pollutant levels. Special concern exists because of emission of particles or particulate matters. On the other hand, many direct injection engines currently present on the market are already equipped with filtering systems, like diesel particulate filter, for the

distributed MWCNTs/nEMs particles ignition is occurring during the flash exposure, which leads to ignition of the entire charge in the combustion chamber. Indeed, the combustion chamber is fully illuminated starting from the third frame. The light radiated by the burning process is distributed throughout the entire chamber until the combustion process is completed. This could be due to the fact that the nanoignition agents, being spatially distributed inside the chamber, are able to ignite the mixture away from the actual location of the flash lamp ignition source. On the other hand, the combustion process triggered by the spark plug exhibits an ignition phase that is longer and filled by the presence of flame starting from the 5th to 6th frame. It initially involves only a small fraction of the charge within the chamber (i.e., near the spark plug electrodes) with the high-intensity areas located close to the trigger point (i.e., spark plug). The spark ignition process forms a flame kernel which subsequently propagates and consumes the fuel at the burning speed of the flame. Note that the air/gasoline ratio for **Figure 10** is at a rich mixture condition because it generates a much better contrast. However, images under lean condition also exhibited very similar burning behavior explained here.

**Figure 10.** Pictures of the combustion process at *λ* = 0.52; comparison between MWCNTs/nEMs PTI and SI for air/ gasoline mixture.

#### **4. Nanostructured materials for automotive propulsion**

distributed MWCNTs/nEMs particles ignition is occurring during the flash exposure, which leads to ignition of the entire charge in the combustion chamber. Indeed, the combustion chamber is fully illuminated starting from the third frame. The light radiated by the burning process is distributed throughout the entire chamber until the combustion process is completed. This could be due to the fact that the nanoignition agents, being spatially distributed inside the chamber, are able to ignite the mixture away from the actual location of the flash lamp ignition source. On the other hand, the combustion process triggered by the spark plug exhibits an ignition phase that is longer and filled by the presence of flame starting from the 5th to 6th frame. It initially involves only a small fraction of the charge within the chamber (i.e., near the spark plug electrodes) with the high-intensity areas located close to the trigger point (i.e., spark plug). The spark ignition process forms a flame kernel which subsequently propagates and consumes the fuel at the burning speed of the flame. Note that the air/gasoline ratio for **Figure 10** is at a rich mixture condition because it generates a much better contrast. However, images under lean condition also exhibited very similar

**Figure 9.** Comparison of (a) peak pressure, (b) ignition delay, (c) total combustion duration, (d) combustion efficiency, and total heat released with PTI for different values of *λ* and for tests carried out with a residence time equal to 0 and

burning behavior explained here.

500 ms are shown.

324 Carbon Nanotubes - Recent Progress

Preliminary results from the PTI approach shows a potential for volumetrically distributed ignition of the fuels. However, the next step is to demonstrate the same results in a real engine. Indeed, the fact that a very small amount of energy (such as that emitting from an ordinary camera flash) is able to bring about volumetric ignition in a lean air/fuel mixture could lead to substantial improvements of engine performance. There are several ways the nanostructured materials could be introduced into the engine, for example, either by powder injection into the intake port or through mixing it with the fuel. A system of fiberoptic cables and flash lamp could be used to direct the flash energy into the combustion chamber.

But the main issue is possible environmental impacts of nanometric carbonaceous structures in the combustion process due to possible creation of condensation nuclei for further particulate formation in the engine and from some nanostructured materials escaping the combustion process, which can end up in the atmosphere. The potential impact of combustion products on health and environment cannot be easily predicted at the present stage of the research. In fact, it is expected that the in actual application, a more precise control of ignition timing and a shorter combustion would allow more flexibility to redesign the engine in terms of geometric as well as control parameters. This could have direct implications on the raw pollutant levels. Special concern exists because of emission of particles or particulate matters. On the other hand, many direct injection engines currently present on the market are already equipped with filtering systems, like diesel particulate filter, for the particulate abatement.

#### **5. Conclusions**

The presented data here demonstrate the feasibility of a new ignition concept for initiating the combustion process of several air/fuel mixtures. The new ignition concept is called photothermal ignition (PTI) and consists of flashing a nanoenergetic material with a light flash. The results from the PTI approach were compared with those obtained using a traditional spark plug system, showing great potential for future applications in combustion processes, especially for implementing the HCCI combustion mode in internal combustion engines.

These results are considered to be of scientific and practical importance because the combustion process, initiated in mixtures with extremely lean air/fuel ratios of interest in lean-burn HCCI engines, would allow substantial reductions of fuel consumption, nitrogen oxides, and

, Antonio Ficarella1

Potential Application of Photo-thermal Volumetric Ignition of Carbon Nanotubes in Internal…

, Domenico Laforgia1

http://dx.doi.org/10.5772/intechopen.70887

and

327

\*, Bruce Chehroudi2

1 Department of Engineering for Innovation, University of Salento, Lecce, Italy

Publishing in Mechanical Engineering and CRC Press; 2007. p. 557

[5] Iijima S. Helical microtubules of graphitic carbon. Nature. 1991;**354**:56-58

2 Department of Mechanical Engineering, Arkansas Tech University, Russellville, Arkansas,

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[3] Chehroudi B. Activation and control of autoignition in HCCI engines using volumetrically-distributed ignition of as-produced single-walled carbon nanotubes. In: SAE

[4] Johansson B. Homogeneous charge compression ignition: the future of IC engines?.

[6] Ajayan PM, Terrones M, de la Guardia A. Nanotubes in a flash-ignition—Ignition and

[7] Tseng S, Tai N, Hsu W, Chen L, Wang J, Chiu C. Ignition of carbon nanotubes using a

[8] Bockrath B, Johnson JK, Sholl DS, Howard B, Matragna C, Shi W. Ignition nanotubes

[9] Braidy N, Botton GA, Adronov A. Oxidation of Fe nanoparticles embedded in single-walled carbon nanotubes by exposure to a bright flash of white light. Nano Letters. 2002;**8**:1277-1280

[10] Smits J, Wincheski B, Namkung M, Crooks R, Louie R. Response of Fe powder, purified and as-produced HiPco single-walled carbon nanotubes to flash exposure. Materials Science and Engineering: A (Structural Materials: Properties, Microstructure and

\*Address all correspondence to: paolo.carlucci@unisalento.it

Technical Paper. 2012. DOI: 10.4271/2012-01-1691

reconstruction. Science 2002;**296**:705

photoflash. Carbon. 2007;**45**(5):958-964

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International Journal of Vehicle Design. 2007;**44**:1-19

soot emissions.

**Author details**

Luciano Strafella1

USA

**References**

Antonio Paolo Carlucci1

In this chapter, the promising results in enhancing the combustion of methane, hydrogen, LPG, and gasoline applying this novel approach to initiate combustion are shown. In particular, the abovementioned fuels have been mixed with air in a constant volume vessel and ignited with nanopowder or a conventional spark ignition system.

The new light-activated distributed ignition demonstrated superior performance, which includes a shorter combustion duration, a shorter ignition delay period, an increased pressure peak, and improved combustion efficiency. A direct observation of the combustion process has established that benefits shown here are due to the fact that photo-thermal ignition system establishes a spatially distributed ignition, which consequently leads to a faster consumption of the air/fuel mixture in the test vessel. Higher pressure peaks and shorter rapid rising period are achieved by the fact that the new ignition system leads to numerous ignition nuclei that burn near-simultaneously, hence contributing to a volumetrically distributed combustion process in the combustion chamber. This is drastically different than the flame front propagation observed with the spark ignition.

Moreover, it was demonstrated for the first time that the proposed ignition system is able to ignite air/gasoline mixtures when liquid gasoline fuel is injected into the chamber, without isolating/encapsulating the nanoenergetic material.

Furthermore, a combustion process triggered by the photo-thermal ignition was possible even at a relative air/fuel ratio of 2.7. Ignition at this relative air/fuel ratio was impossible with the conventional spark-plug ignition system used here.

High-speed camera images acquired during the combustion process indicate that photo-thermal ignition resulted in volumetrically distributed quasi-homogeneous ignition followed by a better and faster consumption of the air/fuel mixture with no discernible flame front. This behavior is in contrast to what observed with the spark ignition, namely a single ignition point followed by a flame propagation across the combustion chamber.

The use of the photo-thermal ignition system is therefore a promising technology for the combustion management in internal combustion engines because it is characterized by the following advantages compared to the other ignition systems:


These results are considered to be of scientific and practical importance because the combustion process, initiated in mixtures with extremely lean air/fuel ratios of interest in lean-burn HCCI engines, would allow substantial reductions of fuel consumption, nitrogen oxides, and soot emissions.

#### **Author details**

**5. Conclusions**

326 Carbon Nanotubes - Recent Progress

The presented data here demonstrate the feasibility of a new ignition concept for initiating the combustion process of several air/fuel mixtures. The new ignition concept is called photothermal ignition (PTI) and consists of flashing a nanoenergetic material with a light flash. The results from the PTI approach were compared with those obtained using a traditional spark plug system, showing great potential for future applications in combustion processes, espe-

In this chapter, the promising results in enhancing the combustion of methane, hydrogen, LPG, and gasoline applying this novel approach to initiate combustion are shown. In particular, the abovementioned fuels have been mixed with air in a constant volume vessel and

The new light-activated distributed ignition demonstrated superior performance, which includes a shorter combustion duration, a shorter ignition delay period, an increased pressure peak, and improved combustion efficiency. A direct observation of the combustion process has established that benefits shown here are due to the fact that photo-thermal ignition system establishes a spatially distributed ignition, which consequently leads to a faster consumption of the air/fuel mixture in the test vessel. Higher pressure peaks and shorter rapid rising period are achieved by the fact that the new ignition system leads to numerous ignition nuclei that burn near-simultaneously, hence contributing to a volumetrically distributed combustion process in the combustion chamber. This is drastically different than the flame front

Moreover, it was demonstrated for the first time that the proposed ignition system is able to ignite air/gasoline mixtures when liquid gasoline fuel is injected into the chamber, without

Furthermore, a combustion process triggered by the photo-thermal ignition was possible even at a relative air/fuel ratio of 2.7. Ignition at this relative air/fuel ratio was impossible with

High-speed camera images acquired during the combustion process indicate that photo-thermal ignition resulted in volumetrically distributed quasi-homogeneous ignition followed by a better and faster consumption of the air/fuel mixture with no discernible flame front. This behavior is in contrast to what observed with the spark ignition, namely a single ignition

The use of the photo-thermal ignition system is therefore a promising technology for the combustion management in internal combustion engines because it is characterized by the

• the ignition could be achieved remotely and distributed spatially at a large number of loca-

• the volume where ignition takes place could be adjusted to achieve both localized and

cially for implementing the HCCI combustion mode in internal combustion engines.

ignited with nanopowder or a conventional spark ignition system.

propagation observed with the spark ignition.

isolating/encapsulating the nanoenergetic material.

the conventional spark-plug ignition system used here.

point followed by a flame propagation across the combustion chamber.

following advantages compared to the other ignition systems:

volumetrically distributed ignition.

tions; and

Antonio Paolo Carlucci1 \*, Bruce Chehroudi2 , Antonio Ficarella1 , Domenico Laforgia1 and Luciano Strafella1

\*Address all correspondence to: paolo.carlucci@unisalento.it

1 Department of Engineering for Innovation, University of Salento, Lecce, Italy

2 Department of Mechanical Engineering, Arkansas Tech University, Russellville, Arkansas, USA

#### **References**


[11] Berkowitz A, Oehlschlaeger M. Photo-induced ignition of quiescent ethylene/air mixtures containing suspended carbon nanotubes. Proceedings of the Combustion Institute. 2011;**33**(2):3359-3366

[23] Visconti P, Primiceri P, Longo D, Strafella L, Carlucci AP, Lomascolo M, et al. Photoignition process of multiwall carbon nanotubes and ferrocene by continuous wave Xe lamp illumination. Beilstein Journal of Nanotechnology. 2017;**8**:134-144. DOI: 10.3762/

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[24] Ficarella A, Carlucci AP, Chehroudi B, Laforgia D, Strafella L. Multi-Walled Carbon Nanotubes (MWCNTs) bonded with Ferrocene particles as ignition agents for air-fuel

mixtures. Fuel. 2017;**208**:734-745. DOI: https://doi.org/10.1016/j.fuel.2017.07.052

bjnano.8.14


[23] Visconti P, Primiceri P, Longo D, Strafella L, Carlucci AP, Lomascolo M, et al. Photoignition process of multiwall carbon nanotubes and ferrocene by continuous wave Xe lamp illumination. Beilstein Journal of Nanotechnology. 2017;**8**:134-144. DOI: 10.3762/ bjnano.8.14

[11] Berkowitz A, Oehlschlaeger M. Photo-induced ignition of quiescent ethylene/air mixtures containing suspended carbon nanotubes. Proceedings of the Combustion Institute.

[12] Manaa R, Mitchell A, Garza R. Flash ignition and initiation of explosives nanotubes mix-

[13] Badakhshan A, Danczyk SA, Wirth D, Pilon L. Ignition of liquid fuel spray and simulated solid rocket fuel by photo-ignition of carbon nanotubes utilizing a camera flash. In: JANNAF-Liquid Propulsion Conference 2011; December 5-9; Huntsville, Alabama. 2011

[14] Chehroudi B. Nanotechnology and applied combustion: Use of nanostructured materials for light-activated distributed ignition of fuels with propulsion applications. Recent

[15] Chehroudi B, Vaghjiani GL, Ketsdever A. Method for distributed ignition of fuels by

[16] Chehroudi B, Vaghjiani GL, Ketsdever A. Apparatus for distributed ignition of fuels by

[17] Chehroudi B, Badashan A, Danczyk SA, Morgan C. Ignition characteristics of Single-Walled Carbon Nanotubes (SWCNTs) utilizing a camera flash for distributed ignition of liquid sprays. In: Joint Army-Navy-NASA-Air Force (JANNAF) Propulsion Meeting (JPM) and 6th Modeling and Simulation/4th Liquid Propulsion/3ed Spacecraft Propulsion

[18] Chehroudi B, Danczyk SA. A novel distributed ignition method using single-walled carbon nanotubes (SWCNTs) and a low-power flash light. In: Global Powertrain Congress, World Powertrain Conference & Exposition; September 19-21; Novi, Michigan. 2006 [19] Chehroudi B, Danczyk SA, Ketsdever A, Vaghjiani GL. A low power, novel ignition of fuels using Single-Walled Carbon Nanotubes (SWCNTs) and a camera flash. In: 53ed JANNAF Interagency Propulsion Committee Meeting, 2nd Liquid Propulsion, 1st

Spacecraft Propulsion Subcommittee; December 5-8; Monterey, California. 2005

[20] Chehroudi B, Danczyk SA. An innovative ignition method using SWCNTs and a camera flash. In: Nano Science and Technology Institute (NSTI), Nanotechnology Conference

[21] Carlucci AP, Stafella L. Air-methane mixture ignition with Multi-Walled Carbon NanoTubes (MWCNTs) and comparison with spark ignition. Energy Procedia. 2015;

[22] Carlucci AP, Ciccarella G, Strafella L. Multi-Walled Carbon Nanotubes (MWCNTs) as ignition agents for air/methane mixtures. IEEE Transactions on Nanotechnology.

Joint Sub-committee Meeting; December 8-12; Orlando, Florida. 2008

and Trade Show; May 8-12; Anaheim, California. 2005. p. 226-229

ture. Journal of the American Chemical Society. 2005;**127**:13786-13787

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light sources. US Patent 7517215 B1. 2009

light sources. US Patent 7665985 B1. 2010

2011;**33**(2):3359-3366

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2016;**15**(5):699-704

[24] Ficarella A, Carlucci AP, Chehroudi B, Laforgia D, Strafella L. Multi-Walled Carbon Nanotubes (MWCNTs) bonded with Ferrocene particles as ignition agents for air-fuel mixtures. Fuel. 2017;**208**:734-745. DOI: https://doi.org/10.1016/j.fuel.2017.07.052

**Chapter 15**

Provisional chapter

**Reversible and Reproducible Hydrogen Storage in**

Reversible and Reproducible Hydrogen Storage in

Duraisamy Silambarasan, Velappa Jayaraman Surya,

Duraisamy Silambarasan, Velappa Jayaraman Surya,

Veerapandy Vasu and Kombiah Iyakutti

Veerapandy Vasu and Kombiah Iyakutti

http://dx.doi.org/10.5772/intechopen.75763

Abstract

target.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Borane**

Borane

**Single-Walled Carbon Nanotubes Functionalized with**

DOI: 10.5772/intechopen.75763

In this chapter, commercial single-walled carbon nanotubes (SWCNTs) were purified by standard methods and functionalized with borane (BH3). The morphology, presence of elements and vibrations of different functional groups are probed by transmission electron microscopy (TEM), energy dispersive (ED) spectroscopy, and Fourier transform infrared spectroscopy (FTIR), respectively. A Sievert-like hydrogenation setup has been designed and is employed for hydrogenating the functionalized SWCNTs for different time durations. The amount of hydrogen stored in the functionalized SWCNTs has been quantified using elemental analysis, carbon, hydrogen, nitrogen, sulfur (CHNS) combined with thermal (TG/TDS) measurements. A maximum of 4.77 wt.% of hydrogen has been stored at 50C and the samples become dehydrogenated in the temperature range 90– 125C. From the experiments, it has been found that the amount of hydrogen stored in functionalized SWCNTs increases with increasing hydrogenation duration. Moreover, the entire hydrogenation and dehydrogenation process was examined by Raman, thermal, and elemental analyses together. During the experiments, hydrogenation and dehydrogenation processes were stabilized and were found to be repeatable. Overall, the achieved hydrogen storage capacity of SWCNTS functionalized with BH3 is close to the US DOE

Keywords: single-walled carbon nanotubes, functionalization, borane, density functional

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

theory, hydrogen storage capacity, dehydrogenation

Single-Walled Carbon Nanotubes Functionalized with

#### **Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane** Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane

DOI: 10.5772/intechopen.75763

Duraisamy Silambarasan, Velappa Jayaraman Surya, Veerapandy Vasu and Kombiah Iyakutti Duraisamy Silambarasan, Velappa Jayaraman Surya, Veerapandy Vasu and Kombiah Iyakutti

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75763

#### Abstract

In this chapter, commercial single-walled carbon nanotubes (SWCNTs) were purified by standard methods and functionalized with borane (BH3). The morphology, presence of elements and vibrations of different functional groups are probed by transmission electron microscopy (TEM), energy dispersive (ED) spectroscopy, and Fourier transform infrared spectroscopy (FTIR), respectively. A Sievert-like hydrogenation setup has been designed and is employed for hydrogenating the functionalized SWCNTs for different time durations. The amount of hydrogen stored in the functionalized SWCNTs has been quantified using elemental analysis, carbon, hydrogen, nitrogen, sulfur (CHNS) combined with thermal (TG/TDS) measurements. A maximum of 4.77 wt.% of hydrogen has been stored at 50C and the samples become dehydrogenated in the temperature range 90– 125C. From the experiments, it has been found that the amount of hydrogen stored in functionalized SWCNTs increases with increasing hydrogenation duration. Moreover, the entire hydrogenation and dehydrogenation process was examined by Raman, thermal, and elemental analyses together. During the experiments, hydrogenation and dehydrogenation processes were stabilized and were found to be repeatable. Overall, the achieved hydrogen storage capacity of SWCNTS functionalized with BH3 is close to the US DOE target.

Keywords: single-walled carbon nanotubes, functionalization, borane, density functional theory, hydrogen storage capacity, dehydrogenation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 1. Introduction

Hydrogen is evolving as a green fuel for transportation purposes [1, 2]. In recent times, the storage and transport of hydrogen remains a subject of scientific importance. Storage of hydrogen in the form of gas and liquid needs high pressurized environment and cryogenic temperatures which has general problems associated with leakage, safety, and storage capacity. Therefore, storage in solid-state materials provides an appropriate choice. However, the interaction between hydrogen and the host material is either strong (covalent or ionic) or weak (van der Waals forces). For instance, metal hydrides, metal organic frameworks, clathrates, and other nanostructures cannot provide synergic advantages of high gravimetric storage capacity, reliability, and suitable kinetics for applications [3–6]. Carbon-based nanostructures have attracted the scientific community as one of the promising materials for hydrogen storage [7– 10]. Among them, carbon nanotubes (CNTs) are widely reported to be potential materials for hydrogen storage [11–17]. CNTs possess unique properties such as hollowness, cylindrical shape, interstitial sites, nanometer scale diameter, and porosity that make them as one of the viable candidates for hydrogen storage [17]. An attempt of hydrogen storage in SWCNT bundles made by Dillon et al. [18] has led to broad investigation on CNTs for hydrogen storage. Further experimentations on hydrogen storage in CNTs indicated that bare CNTs are not suitable material for the storage of hydrogen [19–23]. This is because of the weak interaction (van der Waals interaction) between hydrogen molecules and CNTs; hence, more hydrogen can be stored only at lower temperatures through physisorption. Functionalization of CNTs by the addition of atoms or molecules enhances the interaction between hydrogen and CNTs which results in higher storage capacity [24–29]. Moreover, the functionalization of SWCNTs with transition-metal atoms itself occupies more weight percentage on SWCNTs [30]; and also they form strong metal hydrides while hydrogenation which inturn obviously reduce the space for hydrogen storage [31, 32].

2. Theoretical background

carbon atoms per unit cell.

2.1. Computational details

Figure 1. Front and top views of CNT (10, 0).

To understand the hydrogen storage behavior of CNT and the mechanism of hydrogen binding, we have investigated the hydrogen storage capacity of CNT (10, 0) functionalized with BH3 using DFT calculations implemented in VASP (Vienna Ab-initio Simulation Package). Figure 1 shows the front and top views of CNT (10, 0). CNT (10, 0) is an achiral type of zigzag nanotube, whose mirror image has an identical structure to the original one. The shape of the cross section is trans-type. The radius of the tube is 3.96 Å and it has 20 hexagons and 40

Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane

were used. The local density approximation (LDA) was adopted for the exchange-correlation functional. Two unit cells of (10, 0) were taken in a super cell of geometry 28 � 28 � 7.92 Å. The super cell was taken to be large enough since the periodic repetitions do not lead to any effective interactions between neighboring tubes. The super cell of the tube has 20 half and full hexagons. The energy cutoff for the plane wave basis set was 287 eV. The Brillouin zone was sampled by 1 � 1 � 8 mesh points with Monkhorst-pack k-point scheme [33]. Twenty BH3 molecules were attached on 20 full hexagons around the CNT; this corresponds to the full coverage case. The structures were optimized using conjugate gradient algorithm. Convergence was achieved when the energy was less than 1 � <sup>10</sup>�<sup>5</sup> eV/atom and 0.5 � <sup>10</sup>�<sup>5</sup> eV/Å. The hydrogen molecules were attached to the SWCNT–BH3 complex with a bond length of 0.74 Å

; B: 2s<sup>2</sup> p<sup>1</sup>

http://dx.doi.org/10.5772/intechopen.75763

; H: 1s<sup>1</sup>

333

Ultra-soft pseudopotentials treating the electronic states as valence C: 2s<sup>2</sup> 2p<sup>2</sup>

parallel to the tube axis. The binding energy of the BH3 molecule was calculated as

E<sup>B</sup> ð Þ¼ BH3 ½ � E<sup>T</sup> ðSWCNT � 20BH3Þ � E<sup>T</sup> ð Þ� SWCNT 20E<sup>T</sup> ð Þ BH3 =m (1)

In this chapter, a hydrogen storage medium (HSM) based on SWCNTs, capable of storing and delivering hydrogen in the temperature range suitable for fuel cell applications has been developed. Here, the SWCNTs are modified by means of functionalizing them with BH3. Our earlier computational studies [27] based on density functional theory (DFT) exhibited that functionalization of SWCNTs with BH3 increases the binding energy of hydrogen molecules, thereby improving the storage capacity too. The theoretical results inspired us to choose BH3 for functionalization of SWCNTs, experimentally. Moreover, it was pre-determined to carry out the hydrogenation experiments just above room temperature. This was due to the fact that hydrogen storage at very lower temperature and pressure conditions is not feasible for mobile applications. Hydrogenation of the functionalized SWCNTs has been performed for different time durations. Further, the hydrogenated samples were annealed to check the desorption of hydrogen. The amount of hydrogen uptake and desorption temperature range have been calculated. The binding energy of hydrogen and the nature of hydrogen binding were estimated based on the characterization results. Both hydrogenation and dehydrogenation experiments were repeated to examine the reproducibility.

#### 2. Theoretical background

1. Introduction

332 Carbon Nanotubes - Recent Progress

reduce the space for hydrogen storage [31, 32].

iments were repeated to examine the reproducibility.

Hydrogen is evolving as a green fuel for transportation purposes [1, 2]. In recent times, the storage and transport of hydrogen remains a subject of scientific importance. Storage of hydrogen in the form of gas and liquid needs high pressurized environment and cryogenic temperatures which has general problems associated with leakage, safety, and storage capacity. Therefore, storage in solid-state materials provides an appropriate choice. However, the interaction between hydrogen and the host material is either strong (covalent or ionic) or weak (van der Waals forces). For instance, metal hydrides, metal organic frameworks, clathrates, and other nanostructures cannot provide synergic advantages of high gravimetric storage capacity, reliability, and suitable kinetics for applications [3–6]. Carbon-based nanostructures have attracted the scientific community as one of the promising materials for hydrogen storage [7– 10]. Among them, carbon nanotubes (CNTs) are widely reported to be potential materials for hydrogen storage [11–17]. CNTs possess unique properties such as hollowness, cylindrical shape, interstitial sites, nanometer scale diameter, and porosity that make them as one of the viable candidates for hydrogen storage [17]. An attempt of hydrogen storage in SWCNT bundles made by Dillon et al. [18] has led to broad investigation on CNTs for hydrogen storage. Further experimentations on hydrogen storage in CNTs indicated that bare CNTs are not suitable material for the storage of hydrogen [19–23]. This is because of the weak interaction (van der Waals interaction) between hydrogen molecules and CNTs; hence, more hydrogen can be stored only at lower temperatures through physisorption. Functionalization of CNTs by the addition of atoms or molecules enhances the interaction between hydrogen and CNTs which results in higher storage capacity [24–29]. Moreover, the functionalization of SWCNTs with transition-metal atoms itself occupies more weight percentage on SWCNTs [30]; and also they form strong metal hydrides while hydrogenation which inturn obviously

In this chapter, a hydrogen storage medium (HSM) based on SWCNTs, capable of storing and delivering hydrogen in the temperature range suitable for fuel cell applications has been developed. Here, the SWCNTs are modified by means of functionalizing them with BH3. Our earlier computational studies [27] based on density functional theory (DFT) exhibited that functionalization of SWCNTs with BH3 increases the binding energy of hydrogen molecules, thereby improving the storage capacity too. The theoretical results inspired us to choose BH3 for functionalization of SWCNTs, experimentally. Moreover, it was pre-determined to carry out the hydrogenation experiments just above room temperature. This was due to the fact that hydrogen storage at very lower temperature and pressure conditions is not feasible for mobile applications. Hydrogenation of the functionalized SWCNTs has been performed for different time durations. Further, the hydrogenated samples were annealed to check the desorption of hydrogen. The amount of hydrogen uptake and desorption temperature range have been calculated. The binding energy of hydrogen and the nature of hydrogen binding were estimated based on the characterization results. Both hydrogenation and dehydrogenation experTo understand the hydrogen storage behavior of CNT and the mechanism of hydrogen binding, we have investigated the hydrogen storage capacity of CNT (10, 0) functionalized with BH3 using DFT calculations implemented in VASP (Vienna Ab-initio Simulation Package). Figure 1 shows the front and top views of CNT (10, 0). CNT (10, 0) is an achiral type of zigzag nanotube, whose mirror image has an identical structure to the original one. The shape of the cross section is trans-type. The radius of the tube is 3.96 Å and it has 20 hexagons and 40 carbon atoms per unit cell.

#### 2.1. Computational details

Ultra-soft pseudopotentials treating the electronic states as valence C: 2s<sup>2</sup> 2p<sup>2</sup> ; B: 2s<sup>2</sup> p<sup>1</sup> ; H: 1s<sup>1</sup> were used. The local density approximation (LDA) was adopted for the exchange-correlation functional. Two unit cells of (10, 0) were taken in a super cell of geometry 28 � 28 � 7.92 Å. The super cell was taken to be large enough since the periodic repetitions do not lead to any effective interactions between neighboring tubes. The super cell of the tube has 20 half and full hexagons. The energy cutoff for the plane wave basis set was 287 eV. The Brillouin zone was sampled by 1 � 1 � 8 mesh points with Monkhorst-pack k-point scheme [33]. Twenty BH3 molecules were attached on 20 full hexagons around the CNT; this corresponds to the full coverage case. The structures were optimized using conjugate gradient algorithm. Convergence was achieved when the energy was less than 1 � <sup>10</sup>�<sup>5</sup> eV/atom and 0.5 � <sup>10</sup>�<sup>5</sup> eV/Å. The hydrogen molecules were attached to the SWCNT–BH3 complex with a bond length of 0.74 Å parallel to the tube axis. The binding energy of the BH3 molecule was calculated as

$$E\_{\rm B} \text{ (BH}\_3\text{)}=[E\_{\rm T} \text{ (SWCNT}-20 \text{BH}\_3\text{)}-E\_{\rm T} \text{ (SWCNT)}-20E\_{\rm T} \text{ (BH}\_3\text{)}]/m \tag{1}$$

Figure 1. Front and top views of CNT (10, 0).

In a similar way, the binding energy per H2 molecule was calculated as

$$E\_{\rm B} \left( \rm H\_{2} \right) = \left[ E\_{\rm T} \left( \rm SWCNT - 20 \rm BH\_{3} + m \rm H\_{2} \right) - E\_{\rm T} \left( \rm SWCNT - 20 \rm BH\_{3} \right) - m \times E\_{\rm T} \left( \rm H\_{2} \right) \right] / m \tag{2}$$

where E<sup>T</sup> denotes the total energy of the respective systems. The integer m represents the number of H2 molecule adsorbed on the surface.

#### 2.2. Functionalization

Being light weight and hydrogen rich, BH3 is able to adsorb H2 molecules as adsorbates on the surface of CNT. Two unit cells of CNT (10, 0) contain 20 complete hexagons. It is reported that the center of a hexagon is the most stable state for adsorbates [34]. We have attached a single BH3 on the center of each hexagon at a distance of 2 Å from the surface of the tube. The system C20BH3 refers to CNT (10, 0) + 20BH3 complex. After relaxation, the equilibrium position of BH3 occurs at 2.6 Å from the surface of the tube with an increased bond length of 1.2 Å and an average bond angle of 119�. Figure 2a and b shows the structure of C20BH3 before and after relaxation, respectively. The binding energy per BH3 is 3.98 eV. In the figures, gray, parrot green and white color balls represent the carbon, boron, and hydrogen atoms, respectively.

2.3.2. C20(BH3 + 2H2)

2.3.3. C20(BH3 + 3H2)

2.3.4. C20(BH3 + 4H2)

Another H2 molecule is attached to each BH3. The C20(BH3 + 2H2) structure is relaxed and the H2 has been displaced from the distance of 2 to 4 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.78 Å. The relaxed structure of the system C20 (BH3 + 2H2) is shown in Figure 3b. The binding energy of a H2 molecule is 0.38 eV, the hydrogen storage capacity is 6.12 wt%, and the desorption temperature is calculated as 486 K.

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The binding energy of the system C20(BH3 + 2H2) lies in the range of ideal binding energy 0.2–0.4 eV. Hence, the system can adsorb more number of hydrogen. The third H2 adsorbed on BH3 is moved away from the BH3 after relaxation. It is adsorbed at 3.42 Å and the bond length of H2 molecule has been increased from 0.74 to 0.83 Å. The relaxed structure of the system C20 (BH3 + 3H2) is shown in Figure 4a. The binding energy of a H2 molecule is 0.24 eV, the storage

Another H2 molecule is added to each BH3. This structure is relaxed and the H2 has been adsorbed at a distance of 3.39 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.83 Å. The relaxed structure of the system C20(BH3 + 4H2) is shown in Figure 4b. In this case, the H2 molecules are weakly bound, and the binding energy is found to

All H2 are molecularly physisorbed through BH3 and oriented randomly on CNT. The binding energy of H2, storage capacity, and desorption temperature of all the systems are given in Table 1. The electrostatic interaction is responsible for the binding of H2 on the (CNT + BH3)

capacity of the system is 8.9 wt%, and the desorption temperature is 307 K.

Figure 3. (a) Relaxed structure of C20(BH3 + H2) (b) relaxed structure of C20(BH3 + 2H2).

be 0.16 eV, which lies below the ideal binding energy of H2.

#### 2.3. Hydrogenation

#### 2.3.1. C20(BH3 + H2)

A single H2 molecule is attached to each BH3 at 2 Å distance from the surface of the tube in the relaxed structure of C20BH3. This complex C20(BH3 + H2) is relaxed and the H2 close to BH3 has been displaced. Now the distance between BH3 and H2 reduces to 1.47 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.9 Å. The relaxed structure of the system C20(BH3 + H2) is shown in Figure 3a. The binding energy of a single H2 molecule is 0.86 eV, the storage capacity is 3.16 wt%, and the desorption temperature is 1100 K.

Figure 2. Structure of C20BH3 before relaxation and after relaxation.

Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane http://dx.doi.org/10.5772/intechopen.75763 335

Figure 3. (a) Relaxed structure of C20(BH3 + H2) (b) relaxed structure of C20(BH3 + 2H2).

#### 2.3.2. C20(BH3 + 2H2)

In a similar way, the binding energy per H2 molecule was calculated as

number of H2 molecule adsorbed on the surface.

2.2. Functionalization

334 Carbon Nanotubes - Recent Progress

2.3. Hydrogenation

2.3.1. C20(BH3 + H2)

E<sup>B</sup> ð Þ¼ H2 ½ � E<sup>T</sup> ðSWCNT � 20BH3 þ mH2Þ � E<sup>T</sup> ðSWCNT � 20BH3Þ � m � E<sup>T</sup> ð Þ H2 =m (2)

where E<sup>T</sup> denotes the total energy of the respective systems. The integer m represents the

Being light weight and hydrogen rich, BH3 is able to adsorb H2 molecules as adsorbates on the surface of CNT. Two unit cells of CNT (10, 0) contain 20 complete hexagons. It is reported that the center of a hexagon is the most stable state for adsorbates [34]. We have attached a single BH3 on the center of each hexagon at a distance of 2 Å from the surface of the tube. The system C20BH3 refers to CNT (10, 0) + 20BH3 complex. After relaxation, the equilibrium position of BH3 occurs at 2.6 Å from the surface of the tube with an increased bond length of 1.2 Å and an average bond angle of 119�. Figure 2a and b shows the structure of C20BH3 before and after relaxation, respectively. The binding energy per BH3 is 3.98 eV. In the figures, gray, parrot green and white color balls represent the carbon, boron, and hydrogen atoms, respectively.

A single H2 molecule is attached to each BH3 at 2 Å distance from the surface of the tube in the relaxed structure of C20BH3. This complex C20(BH3 + H2) is relaxed and the H2 close to BH3 has been displaced. Now the distance between BH3 and H2 reduces to 1.47 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.9 Å. The relaxed structure of the system C20(BH3 + H2) is shown in Figure 3a. The binding energy of a single H2 molecule is

0.86 eV, the storage capacity is 3.16 wt%, and the desorption temperature is 1100 K.

Figure 2. Structure of C20BH3 before relaxation and after relaxation.

Another H2 molecule is attached to each BH3. The C20(BH3 + 2H2) structure is relaxed and the H2 has been displaced from the distance of 2 to 4 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.78 Å. The relaxed structure of the system C20 (BH3 + 2H2) is shown in Figure 3b. The binding energy of a H2 molecule is 0.38 eV, the hydrogen storage capacity is 6.12 wt%, and the desorption temperature is calculated as 486 K.

#### 2.3.3. C20(BH3 + 3H2)

The binding energy of the system C20(BH3 + 2H2) lies in the range of ideal binding energy 0.2–0.4 eV. Hence, the system can adsorb more number of hydrogen. The third H2 adsorbed on BH3 is moved away from the BH3 after relaxation. It is adsorbed at 3.42 Å and the bond length of H2 molecule has been increased from 0.74 to 0.83 Å. The relaxed structure of the system C20 (BH3 + 3H2) is shown in Figure 4a. The binding energy of a H2 molecule is 0.24 eV, the storage capacity of the system is 8.9 wt%, and the desorption temperature is 307 K.

#### 2.3.4. C20(BH3 + 4H2)

Another H2 molecule is added to each BH3. This structure is relaxed and the H2 has been adsorbed at a distance of 3.39 Å. After relaxation, the bond length of H2 molecule has been increased from 0.74 to 0.83 Å. The relaxed structure of the system C20(BH3 + 4H2) is shown in Figure 4b. In this case, the H2 molecules are weakly bound, and the binding energy is found to be 0.16 eV, which lies below the ideal binding energy of H2.

All H2 are molecularly physisorbed through BH3 and oriented randomly on CNT. The binding energy of H2, storage capacity, and desorption temperature of all the systems are given in Table 1. The electrostatic interaction is responsible for the binding of H2 on the (CNT + BH3)

process) in few proportions. Hence, it is essential to purify the CNTs before conducting experiments. The purification steps involved heating at 300C for 1 hour, acid treatment, and

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A chamber made up of stainless steel has been designed for our hydrogen storage experiments and is named as hydrogen storage setup. The length of the chamber is about 45 cm with a diameter about 10 cm. The thickness of the stainless sheet is about 0.5 cm. The schematic diagram of the hydrogen storage setup is shown in Figure 5. In the figure, the size of the gas inlet valve is large compared to the gas outlet valve. This has been made intentionally to make some amount of gas exist in the chamber to react with samples during the hydrogenation experiment. Moreover, the sample is placed in the path of gas outlet. This is because of the reason that; the gas should meet the sample before leaving the chamber. The sample holder is also made up of stainless steel sheet. Commercial hydrogen gas (purity >90%) cylinders has been used for the experiments.

A well dispersed solution of SWCNTs was deposited on alumina substrates and a subsequent heating up to 200C was done. LiBH4 (the precursor for BH3) was mixed with diethyl ether ((C2H5)2O) in a ratio of 25 mg/ml, and it was drop casted over the surface of SWCNTs. Then the substrates were heated to 275C (decomposition temperature of LiBH4) for 1 hour, which yields borane. The released BH3 reacts with SWCNTs and forms a complex, SWCNT + BH3.

The functionalized samples were loaded in the hydrogenation setup and hydrogenated as detailed in this section. Initially, we would like to know whether the functionalized SWCNTs exhibit hydrogen storage capability at a temperature around 100 or 50C. For that, the functionalized samples were maintained at a particular temperature and the hydrogen gas was allowed to flow for some time interval at a known flow rate and then the samples were left in the chamber to attain room temperature. After hydrogenation, the hydrogen content present in the samples was again estimated using CHNS-elemental analysis. The initial hydrogenation results we got were encouraging. The results are presented in Table 2. After the confirmation of the hydrogen storage ability of functionalized SWCNTs, we improved our hydrogenation setup by introducing a few controlled valves to reduce the leakage of hydrogen gas. The

repeated washing.

3.3. Designing of hydrogen storage setup

3.4. SWCNT deposition and functionalization

Figure 5. Schematic diagram of hydrogen storage setup.

This complex acts as an HSM.

3.5. Hydrogenation

Figure 4. (a) Relaxed structure of C20(BH3 + 3H2) (b) relaxed structure of C20(BH3 + 4H2).


Table 1. Storage capacities for system C20(BH3 + nH2) (n = 1–4).

complex. Since the binding energy of BH3 is higher than the H2 binding energy range, only H2 molecules will be released during desorption and not the BH3 molecules.

#### 3. Experimental

#### 3.1. Materials

SWCNTs were purchased from Sigma Aldrich with a purity of >98% and purified using acid treatment. LiBH4 was purchased from Sigma Aldrich with an assay of >95%. The other reagents and chemicals used for experiments were purchased from Merck with 99% purity. Alumina substrates of dimension 19 19 0.65 mm were used. The substrates were cleaned with chromic acid, acetone, and distilled water by means of sonication for 30 minutes (alumina substrates were taken as they are stable at higher temperatures).

#### 3.2. Purification of SWCNTs

Generally, commercial CNTs contain a major component of CNTs along with amorphous carbon structures and metal catalyst impurities (that might be used during the synthesis process) in few proportions. Hence, it is essential to purify the CNTs before conducting experiments. The purification steps involved heating at 300C for 1 hour, acid treatment, and repeated washing.

#### 3.3. Designing of hydrogen storage setup

A chamber made up of stainless steel has been designed for our hydrogen storage experiments and is named as hydrogen storage setup. The length of the chamber is about 45 cm with a diameter about 10 cm. The thickness of the stainless sheet is about 0.5 cm. The schematic diagram of the hydrogen storage setup is shown in Figure 5. In the figure, the size of the gas inlet valve is large compared to the gas outlet valve. This has been made intentionally to make some amount of gas exist in the chamber to react with samples during the hydrogenation experiment. Moreover, the sample is placed in the path of gas outlet. This is because of the reason that; the gas should meet the sample before leaving the chamber. The sample holder is also made up of stainless steel sheet. Commercial hydrogen gas (purity >90%) cylinders has been used for the experiments.

#### 3.4. SWCNT deposition and functionalization

A well dispersed solution of SWCNTs was deposited on alumina substrates and a subsequent heating up to 200C was done. LiBH4 (the precursor for BH3) was mixed with diethyl ether ((C2H5)2O) in a ratio of 25 mg/ml, and it was drop casted over the surface of SWCNTs. Then the substrates were heated to 275C (decomposition temperature of LiBH4) for 1 hour, which yields borane. The released BH3 reacts with SWCNTs and forms a complex, SWCNT + BH3. This complex acts as an HSM.

#### 3.5. Hydrogenation

complex. Since the binding energy of BH3 is higher than the H2 binding energy range, only H2

System Binding energy/H2 (eV) Desorption temperature (K) Storage capacity (wt%)

3.16 6.12 8.90 11.5

SWCNTs were purchased from Sigma Aldrich with a purity of >98% and purified using acid treatment. LiBH4 was purchased from Sigma Aldrich with an assay of >95%. The other reagents and chemicals used for experiments were purchased from Merck with 99% purity. Alumina substrates of dimension 19 19 0.65 mm were used. The substrates were cleaned with chromic acid, acetone, and distilled water by means of sonication for 30 minutes (alumina

Generally, commercial CNTs contain a major component of CNTs along with amorphous carbon structures and metal catalyst impurities (that might be used during the synthesis

molecules will be released during desorption and not the BH3 molecules.

Figure 4. (a) Relaxed structure of C20(BH3 + 3H2) (b) relaxed structure of C20(BH3 + 4H2).

0.86 0.38 0.24 0.16

Table 1. Storage capacities for system C20(BH3 + nH2) (n = 1–4).

substrates were taken as they are stable at higher temperatures).

3. Experimental

3.2. Purification of SWCNTs

3.1. Materials

CNT20(BH3 + H2) CNT20(BH3 + 2H2) CNT20(BH3 + 3H2) CNT20(BH3 + 4H2)

336 Carbon Nanotubes - Recent Progress

The functionalized samples were loaded in the hydrogenation setup and hydrogenated as detailed in this section. Initially, we would like to know whether the functionalized SWCNTs exhibit hydrogen storage capability at a temperature around 100 or 50C. For that, the functionalized samples were maintained at a particular temperature and the hydrogen gas was allowed to flow for some time interval at a known flow rate and then the samples were left in the chamber to attain room temperature. After hydrogenation, the hydrogen content present in the samples was again estimated using CHNS-elemental analysis. The initial hydrogenation results we got were encouraging. The results are presented in Table 2. After the confirmation of the hydrogen storage ability of functionalized SWCNTs, we improved our hydrogenation setup by introducing a few controlled valves to reduce the leakage of hydrogen gas. The

Figure 5. Schematic diagram of hydrogen storage setup.


4. Results and discussion

1260 and 2350 cm�<sup>1</sup>

with the theoretical results [25].

Figure 6. (a) TEM image and (b) ED spectrum of SWCNTs.

4.1. Morphology, elemental and functional group analyses

Figure 6(a) shows the TEM image of SWCNTs. The image reveals the good distribution and separation of SWCNTs with an average diameter of 2–4 nm. The ED spectrum of SWCNTs shown in Figure 6(b) displays the existence of carbon and oxygen in the sample along with copper resulting from the supporting grid. The functional groups present in the functionalized SWCNTs are determined from the bands appearing in the IR spectrum. Figure 7 shows the IR spectrum of the functionalized SWCNTs. Absorption peaks at 1608, 1027, and 729 cm�<sup>1</sup> correspond to C=C stretching, C–C stretching, and skeleton vibrations of carbon atoms in CNTs, respectively. The electrostatic interaction between BH3 and CNT caused the deformation [35] and asymmetric stretching [36] of B–H bonds; the resultant respective peaks are observed at

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asymmetric deformation vibrations of the dissociative adsorption of BH3 on CNT. The results of the IR study authenticate that the SWCNTs functionalized with BH3 is stable and is agreed

The composition of elements present in the functionalized sample is found from the X-ray photoelectron spectroscopy (XPS) spectrum. The full survey spectrum of the functionalized sample is shown in Figure 8. The de-convoluted XPS spectra of the individual components, such as C, Li, B, and O are shown in Figure 9(a)–(d), respectively. The C 1s component at 286.3 eV is assigned to C–O or C–OH bond [37] and a small peak at 290.9 eV is due to Li2CO3. The peak at 56.7 eV is attributed to Li. The lower binding energy component (188.9 eV) is assigned to boron atoms bonded only to the other boron atoms [38, 39], whereas the higher energy component (193.8 eV) represents the oxidized boron [40, 41]. The main O 1s component is assigned to boron oxide [39]. The weight percentages are 44.7, 15.4, 7.1, and 28.7 for the elements C, B, Li, and O, respectively. From the XPS studies, it is observed that the incorporation of

. On the other hand, the appearance of a peak at 1430 cm�<sup>1</sup> is due to C–H

Table 2. Initial hydrogenation results.


Table 3. Optimized hydrogenation results.

functionalized samples were loaded in the hydrogenation setup and hydrogenated for different time durations by maintaining the substrate temperature at 50C and the hydrogen flow rate at ~0.5 liter/minute. The storage capacities were calculated as the difference of hydrogen content in the samples before and after the hydrogenation experiment and the results are presented in Table 3.

#### 3.6. Dehydrogenation

Thermal annealing was used to stimulate the dehydrogenation from the hydrogenated samples. The hydrogenated samples were annealed at 200C for 1 hour. The temperature was controlled by a digital proportional-integral-derivative (PID) controller. After annealing, the samples were left in the furnace to reach room temperature and then characterized.

#### 3.7. Characterizations

The morphology of SWCNTs was analyzed by transmission electron microscope (TEM) imaging using a JEOL JEM 2100 model unit with an accelerating voltage of 200 kV. The energy dispersive (ED) spectrum of SWCNTs was recorded using the JEOL model 6390 unit with an accelerating voltage of 5 kV. Infrared (IR) spectrum was recorded over the range 4000– 450 cm<sup>1</sup> using a Shimadzu model (FTIR-8400S, CE) spectrometer at room temperature with a resolution of 1 cm<sup>1</sup> . X-ray photoelectron spectroscopy (XPS) measurements were taken using a Scienta ESCA3000 model spectrometer. Raman measurements were carried out using a Labram HR800 model spectrometer. The samples were excited with a 17 mW (~633 nm) He–Ne laser. Before taking measurements, the spectrometer was calibrated using a silica wafer. An Elementar Vario EL III model analyzer was used for CHNS-elemental analysis. A Perkin Elmer-Diamond model unit was used to study the thermal response of the samples.

#### 4. Results and discussion

functionalized samples were loaded in the hydrogenation setup and hydrogenated for different time durations by maintaining the substrate temperature at 50C and the hydrogen flow rate at ~0.5 liter/minute. The storage capacities were calculated as the difference of hydrogen content in the samples before and after the hydrogenation experiment and the results are

Substrate temperature (C) Flow rate (liter/min) Flow duration (min) Storage capacity (wt.%)

Sample index Flow rate (liter/min) Flow duration (min) Storage capacity (wt.%)

40 45 4.35 4.77

100 1 4 0.68 100 1 20 0.87 50 3 20 1.50

CBH1 0.5 30 3.27 CBH2 0.5 35 3.78

Thermal annealing was used to stimulate the dehydrogenation from the hydrogenated samples. The hydrogenated samples were annealed at 200C for 1 hour. The temperature was controlled by a digital proportional-integral-derivative (PID) controller. After annealing, the

The morphology of SWCNTs was analyzed by transmission electron microscope (TEM) imaging using a JEOL JEM 2100 model unit with an accelerating voltage of 200 kV. The energy dispersive (ED) spectrum of SWCNTs was recorded using the JEOL model 6390 unit with an accelerating voltage of 5 kV. Infrared (IR) spectrum was recorded over the range 4000– 450 cm<sup>1</sup> using a Shimadzu model (FTIR-8400S, CE) spectrometer at room temperature with

using a Scienta ESCA3000 model spectrometer. Raman measurements were carried out using a Labram HR800 model spectrometer. The samples were excited with a 17 mW (~633 nm) He–Ne laser. Before taking measurements, the spectrometer was calibrated using a silica wafer. An Elementar Vario EL III model analyzer was used for CHNS-elemental analysis. A Perkin

Elmer-Diamond model unit was used to study the thermal response of the samples.

. X-ray photoelectron spectroscopy (XPS) measurements were taken

samples were left in the furnace to reach room temperature and then characterized.

presented in Table 3.

CBH3 CBH4

Table 2. Initial hydrogenation results.

338 Carbon Nanotubes - Recent Progress

0.5 0.5

CBH1–4 represents four different hydrogenated samples.

Table 3. Optimized hydrogenation results.

3.6. Dehydrogenation

3.7. Characterizations

a resolution of 1 cm<sup>1</sup>

#### 4.1. Morphology, elemental and functional group analyses

Figure 6(a) shows the TEM image of SWCNTs. The image reveals the good distribution and separation of SWCNTs with an average diameter of 2–4 nm. The ED spectrum of SWCNTs shown in Figure 6(b) displays the existence of carbon and oxygen in the sample along with copper resulting from the supporting grid. The functional groups present in the functionalized SWCNTs are determined from the bands appearing in the IR spectrum. Figure 7 shows the IR spectrum of the functionalized SWCNTs. Absorption peaks at 1608, 1027, and 729 cm�<sup>1</sup> correspond to C=C stretching, C–C stretching, and skeleton vibrations of carbon atoms in CNTs, respectively. The electrostatic interaction between BH3 and CNT caused the deformation [35] and asymmetric stretching [36] of B–H bonds; the resultant respective peaks are observed at 1260 and 2350 cm�<sup>1</sup> . On the other hand, the appearance of a peak at 1430 cm�<sup>1</sup> is due to C–H asymmetric deformation vibrations of the dissociative adsorption of BH3 on CNT. The results of the IR study authenticate that the SWCNTs functionalized with BH3 is stable and is agreed with the theoretical results [25].

The composition of elements present in the functionalized sample is found from the X-ray photoelectron spectroscopy (XPS) spectrum. The full survey spectrum of the functionalized sample is shown in Figure 8. The de-convoluted XPS spectra of the individual components, such as C, Li, B, and O are shown in Figure 9(a)–(d), respectively. The C 1s component at 286.3 eV is assigned to C–O or C–OH bond [37] and a small peak at 290.9 eV is due to Li2CO3. The peak at 56.7 eV is attributed to Li. The lower binding energy component (188.9 eV) is assigned to boron atoms bonded only to the other boron atoms [38, 39], whereas the higher energy component (193.8 eV) represents the oxidized boron [40, 41]. The main O 1s component is assigned to boron oxide [39]. The weight percentages are 44.7, 15.4, 7.1, and 28.7 for the elements C, B, Li, and O, respectively. From the XPS studies, it is observed that the incorporation of

Figure 6. (a) TEM image and (b) ED spectrum of SWCNTs.

Figure 7. IR spectrum of functionalized SWCNTs.

values reported by Ioannatos et al. [1], Im et al. [39], and Li et al. [42]. Figure 10 shows the Raman spectra of SWCNTs (C), SWCNTs functionalized with BH3 (CB), and hydrogenated functionalized SWCNTs (CBH). In the entire spectrum, the D and G bands appeared around

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Figure 9. De-convoluted XPS spectra of (a) carbon, (b) Lithium, (c) boron and (d) oxygen.

calculating the intensity ratio of the D to G band. As the defect density increased for the samples from SWCNTs, functionalized SWCNTs to hydrogenated SWCNTs, the corresponding D/G ratio has also increased from 0.08, 0.21, to 0.59. The Raman and CHNelemental analyses are together used to probe dehydrogenation (the dehydrogenated sample and is designated as DCB). The hydrogenation and dehydrogenation experiments are repeated a number of times and the corresponding D/G ratio is also measured. The deterioration level of the functionalized SWCNTs after repeated hydrogenation and dehydrogenation is only about 2.3%. A similar change of 3% was reported by Zhang et al. [43]. The hydrogen storage capacity

Figure 11 shows the TG spectra of all hydrogenated samples CBH1, CBH2, CBH3, CBH4 and Figure 12 shows the region associated with hydrogen desorption. The spectrum for the sample CBH1 shows a weight loss of about 3.27 wt% in the temperature range 105–140C and it corresponds to the desorption of stored hydrogen from the sample. The desorption profile shown in Figure 12 exactly measures the starting and end temperatures of the desorption. The primary weight loss is attributed to hydrogen desorption since the desorption of borane starts

above 275C. The findings obtained from TG analysis are in line with Raman results.

, respectively. The defect concentration in CNT can be measured by

1350 and 1570 cm<sup>1</sup>

varies 5% about the mean value.

4.3. Desorption analyses

Figure 8. XPS survey spectrum of functionalized SWCNTs.

oxygen present in the functionalized sample is ~30 wt%. This incorporation may be due to the deposition of samples carried out in open atmosphere.

#### 4.2. Hydrogenation analyses

The quantity of hydrogen stored in the functionalized samples is measured from the CHNSelemental analysis. The hydrogenation results are shown in Tables 2 and 3. The storage capacity at this temperature (50C) of the designed storage medium is much higher than the

Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane http://dx.doi.org/10.5772/intechopen.75763 341

Figure 9. De-convoluted XPS spectra of (a) carbon, (b) Lithium, (c) boron and (d) oxygen.

values reported by Ioannatos et al. [1], Im et al. [39], and Li et al. [42]. Figure 10 shows the Raman spectra of SWCNTs (C), SWCNTs functionalized with BH3 (CB), and hydrogenated functionalized SWCNTs (CBH). In the entire spectrum, the D and G bands appeared around 1350 and 1570 cm<sup>1</sup> , respectively. The defect concentration in CNT can be measured by calculating the intensity ratio of the D to G band. As the defect density increased for the samples from SWCNTs, functionalized SWCNTs to hydrogenated SWCNTs, the corresponding D/G ratio has also increased from 0.08, 0.21, to 0.59. The Raman and CHNelemental analyses are together used to probe dehydrogenation (the dehydrogenated sample and is designated as DCB). The hydrogenation and dehydrogenation experiments are repeated a number of times and the corresponding D/G ratio is also measured. The deterioration level of the functionalized SWCNTs after repeated hydrogenation and dehydrogenation is only about 2.3%. A similar change of 3% was reported by Zhang et al. [43]. The hydrogen storage capacity varies 5% about the mean value.

#### 4.3. Desorption analyses

oxygen present in the functionalized sample is ~30 wt%. This incorporation may be due to the

The quantity of hydrogen stored in the functionalized samples is measured from the CHNSelemental analysis. The hydrogenation results are shown in Tables 2 and 3. The storage capacity at this temperature (50C) of the designed storage medium is much higher than the

deposition of samples carried out in open atmosphere.

Figure 8. XPS survey spectrum of functionalized SWCNTs.

4.2. Hydrogenation analyses

Figure 7. IR spectrum of functionalized SWCNTs.

340 Carbon Nanotubes - Recent Progress

Figure 11 shows the TG spectra of all hydrogenated samples CBH1, CBH2, CBH3, CBH4 and Figure 12 shows the region associated with hydrogen desorption. The spectrum for the sample CBH1 shows a weight loss of about 3.27 wt% in the temperature range 105–140C and it corresponds to the desorption of stored hydrogen from the sample. The desorption profile shown in Figure 12 exactly measures the starting and end temperatures of the desorption. The primary weight loss is attributed to hydrogen desorption since the desorption of borane starts above 275C. The findings obtained from TG analysis are in line with Raman results.

Figure 10. Raman spectra of all the samples.

Figure 11. TG spectra of all the hydrogenated samples.

The activation energy of desorption, Ed can be calculated from the maximum desorption peak using the following Equation [1],

$$\ln\left(T\_m^{\ \ 2}/\beta\right) = E\_d/R\*T\_m\tag{3}$$

CBH1 is 20.66 kJ/mol. Van't Hoff Equation [2] is applied to estimate the binding energy (EB) of

CBH1 3.277 121 20.66 0.310 CBH2 3.785 113 20.11 0.302 CBH3 4.345 111 19.97 0.300 CBH4 4.770 107 19.69 0.287

Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane

where T is the desorption temperature, KB is Boltzmann's constant and ΔS is change in entropy. The estimated binding energy value is 0.31 eV per H2 for the temperature 121�C. The various factors found from the hydrogenation and dehydrogenation studies are presented in Table 4. From the desorption profile, it is apparent that the desorption peak temperature

The obtained binding energy values fall in the weak chemisorption region, and is in the suggested region (0.2–0.4 eV) for a practical hydrogen storage medium [7]. The functionalized SWCNTs and hydrogen interact through a combination of inductive, covalent, and electrostatic charge transfer mechanisms [7, 25] amidst CNTs and hydrogen, borane anchored as a bridge. The composition presence of hydrogen measured from CHNS analysis is equivalent to

reduces with the increase in hydrogen storage capacity.

Figure 12. Desorption spectra of all the hydrogenated samples.

Sample index H2 (wt.%) Tm (

Table 4. Hydrogen desorption characteristic parameters.

EB ¼ T∗ KB ð Þ ΔS=R (4)

�C) Ed (kJ/mol) EB (eV)

http://dx.doi.org/10.5772/intechopen.75763

343

hydrogen,

where Tm is the temperature at maximum peak (121�C), β is the heating rate (10�C/min), and R is the universal gas constant. The calculated activation energy of desorption for the sample

Reversible and Reproducible Hydrogen Storage in Single-Walled Carbon Nanotubes Functionalized with Borane http://dx.doi.org/10.5772/intechopen.75763 343

Figure 12. Desorption spectra of all the hydrogenated samples.


Table 4. Hydrogen desorption characteristic parameters.

The activation energy of desorption, Ed can be calculated from the maximum desorption peak

where Tm is the temperature at maximum peak (121�C), β is the heating rate (10�C/min), and R is the universal gas constant. The calculated activation energy of desorption for the sample

<sup>=</sup><sup>β</sup> <sup>¼</sup> Ed=R∗Tm (3)

ln Tm<sup>2</sup>

using the following Equation [1],

Figure 11. TG spectra of all the hydrogenated samples.

Figure 10. Raman spectra of all the samples.

342 Carbon Nanotubes - Recent Progress

CBH1 is 20.66 kJ/mol. Van't Hoff Equation [2] is applied to estimate the binding energy (EB) of hydrogen,

$$E\_B = T \ast K\_B \left(\Delta \mathcal{S} / \mathbb{R}\right) \tag{4}$$

where T is the desorption temperature, KB is Boltzmann's constant and ΔS is change in entropy. The estimated binding energy value is 0.31 eV per H2 for the temperature 121�C. The various factors found from the hydrogenation and dehydrogenation studies are presented in Table 4. From the desorption profile, it is apparent that the desorption peak temperature reduces with the increase in hydrogen storage capacity.

The obtained binding energy values fall in the weak chemisorption region, and is in the suggested region (0.2–0.4 eV) for a practical hydrogen storage medium [7]. The functionalized SWCNTs and hydrogen interact through a combination of inductive, covalent, and electrostatic charge transfer mechanisms [7, 25] amidst CNTs and hydrogen, borane anchored as a bridge. The composition presence of hydrogen measured from CHNS analysis is equivalent to the weight lost in the thermogravimetric study, i.e., the quantity of hydrogen desorbed is equal to the quantity of hydrogen adsorbed. Thus, our system exhibits 100% desorption in the temperature range 90–140�C, which may be more suitable for vehicular-based fuel cells than the desorption temperature reported by Arellano et al. [21] and Lee et al. [44]. This observation reveals that the adsorption sites on CNTs surface are relatively uniform and that there are no sites which form very weak or very strong adsorption bonds [1]. On the whole, the present investigation reveals the ability of reversible hydrogen uptake by the functionalized SWCNTs.

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#### 5. Summary

In this chapter, the hydrogenation and dehydrogenation studies of SWCNTs functionalized with BH3 are presented. The SWCNTs are successfully functionalized with BH3 using LiBH4 as the precursor. The deposition process involves a simple drop casting method. The presence of BH3 in the functionalized sample is confirmed using IR study. From XPS study, apart from C, the presence of Li, B, and O are also observed in the functionalized sample. Then, the functionalized samples are hydrogenated for different time durations. A maximum storage capacity of 4.77 wt% is achieved at 50�C, which is close to the U.S. DOE target of 5.5 wt% for an HSM to be used for on-board applications. Based on thermal annealing, a systematic investigation of desorption of hydrogen is carried out. The evidences for desorption is provided by Raman, CHNS-elemental, and TG measurements. The results show that thermal annealing treatment induces desorption of hydrogen from the hydrogenated functionalized SWCNTs. The stored hydrogen is released over the temperature range 90–140�C. The deterioration level of the sample is also checked using Raman analysis. Overall, this investigation shows that the SWCNTs functionalized with BH3 may be a suitable reversible hydrogen storage system that is capable of storing and releasing hydrogen under optimum conditions over repeated cycles suitable for hydrogen-based fuel cells used in vehicular applications.

#### Author details

Duraisamy Silambarasan1,2\*, Velappa Jayaraman Surya3,4, Veerapandy Vasu1 and Kombiah Iyakutti<sup>3</sup>

\*Address all correspondence to: simbuphysics@yahoo.com

1 School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu, India

2 PG and Research Department of Physics, The MDT Hindu College, Tirunelveli, Tamil Nadu, India

3 Department of Physics and Nanotechnology, SRM University, Kattankulathur, Tamil Nadu, India

4 SRM Research Institute, SRM University, Kattankulathur, Tamil Nadu, India

### References

the weight lost in the thermogravimetric study, i.e., the quantity of hydrogen desorbed is equal to the quantity of hydrogen adsorbed. Thus, our system exhibits 100% desorption in the temperature range 90–140�C, which may be more suitable for vehicular-based fuel cells than the desorption temperature reported by Arellano et al. [21] and Lee et al. [44]. This observation reveals that the adsorption sites on CNTs surface are relatively uniform and that there are no sites which form very weak or very strong adsorption bonds [1]. On the whole, the present investigation reveals the ability of reversible hydrogen uptake by the functionalized SWCNTs.

In this chapter, the hydrogenation and dehydrogenation studies of SWCNTs functionalized with BH3 are presented. The SWCNTs are successfully functionalized with BH3 using LiBH4 as the precursor. The deposition process involves a simple drop casting method. The presence of BH3 in the functionalized sample is confirmed using IR study. From XPS study, apart from C, the presence of Li, B, and O are also observed in the functionalized sample. Then, the functionalized samples are hydrogenated for different time durations. A maximum storage capacity of 4.77 wt% is achieved at 50�C, which is close to the U.S. DOE target of 5.5 wt% for an HSM to be used for on-board applications. Based on thermal annealing, a systematic investigation of desorption of hydrogen is carried out. The evidences for desorption is provided by Raman, CHNS-elemental, and TG measurements. The results show that thermal annealing treatment induces desorption of hydrogen from the hydrogenated functionalized SWCNTs. The stored hydrogen is released over the temperature range 90–140�C. The deterioration level of the sample is also checked using Raman analysis. Overall, this investigation shows that the SWCNTs functionalized with BH3 may be a suitable reversible hydrogen storage system that is capable of storing and releasing hydrogen under optimum conditions over repeated cycles

suitable for hydrogen-based fuel cells used in vehicular applications.

\*Address all correspondence to: simbuphysics@yahoo.com

Duraisamy Silambarasan1,2\*, Velappa Jayaraman Surya3,4, Veerapandy Vasu1 and

1 School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu, India

4 SRM Research Institute, SRM University, Kattankulathur, Tamil Nadu, India

2 PG and Research Department of Physics, The MDT Hindu College, Tirunelveli, Tamil Nadu,

3 Department of Physics and Nanotechnology, SRM University, Kattankulathur, Tamil Nadu,

5. Summary

344 Carbon Nanotubes - Recent Progress

Author details

Kombiah Iyakutti<sup>3</sup>

India

India


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nl0498785

348 Carbon Nanotubes - Recent Progress

j.ssc.2008.03.018

## *Edited by Mohammed Muzibur Rahman and Abdullah Mohamed Asiri*

The book *Carbon Nanotubes - Recent Progress* contains a number of recent researches on synthesis, growth, characterization, development, and potential applications on carbon materials especially CNTs in nanoscale. It is a promising novel research from top to bottom that has received a lot of interest in the last few decades. It covers the advanced topics on the physical, chemical, and potential applications of CNTs. Here, the interesting reports on cutting-edge science and technology related to synthesis, morphology, control, hybridization, and prospective applications of CNTs are concluded. This potentially unique work offers various approaches on the R&D implementation of carbon-related materials and their hybrids. Basically, we know that nanotechnology offers the regulating of substances at the nanoscale and nanodimension substances in few nanometers, in which exclusive phenomenon facilitates us to control the novel practical applications with carbon materials or CNTs. Here, in this book, an overview of current CNTs' fundamental and substantial applications and enactment of the research worldwide is presented. The techniques of CNT preparation, types of carbon material utilization, and possible applications related with CNTs or carbonhybrid material researches were investigated. It is an important booklet for research organizations, governmental research centers, academic libraries, and R&D affianced in recent research and development of carbon nanotubes.

Published in London, UK © 2018 IntechOpen © Stas Ovsky / unsplash

Carbon Nanotubes - Recent Progress

Carbon Nanotubes

Recent Progress

*Edited by Mohammed Muzibur Rahman* 

*and Abdullah Mohamed Asiri*