Preface

In recent years, new insulating materials are in great need due to the complexity of the pow‐ er system and the development of science and technology. There are many domestic and foreign scholars doing the related research now. Traditionally, inorganic substances such as mica and silicon dioxide were used as dielectric materials. However, with the rapid devel‐ opment of engineering dielectric, polymers are playing a more important role in dielectric materials. With the further research of polymer dielectrics, increasingly more principles need to be revised, and more problems that have not been well recognized are focused on.

As the polymer industry gradually turns into large-scale industry, polymers get more appli‐ cation, offering an alternative to the traditionally inorganic and ceramic material. This is due to their properties such as highly flexible, easier handling, good chemical stability, and cus‐ tomization for specific applications. Moreover, the engineering dielectrics, especially nano‐ dielectrics, have been put into use in the fields of electrical energy, electronic communications, and military aviation. Their properties have scientific significance and en‐ gineering value. In this book, several important thoughts and information are put forward.

The book gives the reader an overview on electrical properties in Dr. Abdullahi's chap‐ ter. More knowledge on other applications such as converter transformer, transistor, and energy storage is provided in the section of polymer properties and application. Besides, this book also presents some recent researches on typical polymer material such as sili‐ con rubber and LDPE. This may provide some clues of advanced polymer properties for both engineers and researches.

I have long been engaged in research work in the field of engineering dielectric. Polymer dielectrics started for me in/at/when and has become an important project of my lab in the last few years. It is a collection of our team's efforts. I wish to express a grateful acknowl‐ edgment to the authors of all these chapters. In addition, the cooperation of InTech is also highly recognized.

> **Boxue Du** Key Laboratory of Smart Grid of Ministry of Education School of Electrical Engineering and Automation Tianjin University Tianjin, China

**Polymer Properties and Application**

#### **Polymer Dielectric in Organic Field‐Effect Transistor Polymer Dielectric in Organic Field**‐**Effect Transistor**

Wei Shi, Yifan Zheng and Junsheng Yu Wei Shi, Yifan Zheng and Junsheng Yu

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

http://dx.doi.org/10.5772/65916

#### **Abstract**

In this chapter, we aim to present an overview of the polymer dielectrics in organic field‐ effect transistors and their applications. In the first section, we give a short introduction of polymer dielectrics in organic field‐effect transistors. We illustrate multilayer, hybrid, and cross‐linked polymer dielectrics adopted in organic field‐effect transistors. Then we introduce the available biomaterials engaged as polymer dielectrics in organic field‐ effect transistors. We mainly focus on the utilization of silk fibroin, DNA, and DNA base pair dielectrics. We end the chapter by presenting the applications of polymer dielectrics. We elaborate that the polymer dielectrics can function as the electrode buffer layer, as well as the organic field‐effect transistor‐based gas sensor, inverter, and memory.

**Keywords:** polymer dielectric, organic field‐effect transistor, biomaterial, gas sensor

## **1. Introduction**

Organic field‐effect transistor (OFET) is an indispensable component in the field of organic electronics, which has been developed to realize low‐cost, flexible large‐area products, and biodegradable electronics [1–3]. Compared with the conventional silicon dioxide–based device, OFETs with polymer dielectrics are ideally compatible with flexible substrates and solution process [4]. Apparently, the solution processable polymer dielectric is very attractive, for it is compatible with spin‐coating, casting, and printing at room temperature and under ambient conditions. Meanwhile, this capability has practical advantages when coupled with large‐scale production using the patterning technique [5]. Moreover, as the function dependent on the structure of polymer dielectric is readily available, to design and synthesize a structure with certain function becomes feasible [6], which results in complementary kinds of polymer dielectrics. Recently, with more and more polymer biomaterials engaged in the OFETs to serve

© 2017 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. © 2017 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.

as the dielectric, the resource of polymer dielectric has become environmentally friendly and very broad. Due to such kinds of polymer dielectrics, OFETs are available in versatile applications including sensor, inverter, and memory [7].

## **2. Introduction of polymer dielectric in organic field‐effect transistor**

#### **2.1. Polymer dielectric in organic field‐effect transistor**

In a common configuration, the OFET consists of the source and drain electrodes, semicon‐ ductor, dielectric, and gate. The functional part of an OFET device is the current channel, which exists in the first few monolayers of the semiconductor upon the dielectric. Therefore, the interface between the dielectric and the semiconductor plays a crucial role in the device performance, which makes the requirements for polymer dielectric material for OFET rather stringent [8, 9]. The crucial parameter of a dielectric material is the maximum possible electric displacement (*D*max) the dielectric can sustain:

$$D\_{\text{max}} = \varepsilon\_0 k E\_{\text{B}} \tag{1}$$

where *ɛ*0 is the vacuum permittivity, *k* is the dielectric constant, *EB* is the dielectric breakdown field, and the capacitance per area *Ci* , which is defined as:

$$C\_i = \varepsilon\_0 \frac{k}{d} \tag{2}$$

where *d* is the thickness of the dielectric. It is obvious that the capacitance magnitude is not only governed by the *k* value but also by the thickness of the dielectric.

The first detailed study of different polymer dielectrics in OFET was reported by Peng et al. in 1990 [10]. The OFET was fabricated on glass using five kinds of polymer dielectrics. They found that there was a strong correlation between the insulator's *k* value and the field‐effect mobility. Then, in 1997, the first high performance plastic transistor was realized by Bao et al. [11]. They employed polyimide as the dielectric and all the essential components were printed directly on the plastic. The OFET had a field‐effect mobility of 0.01–0.03 cm2 /Vs. In 2002, Klauk et al. made a step further achievement as the "all‐polymer" circuit which integrated a 250 nm thick melamine cross‐linked poly(vinyl pyrrolidone) (PVP) [12]. The cross‐linked PVP with the capacitance of ~11–12 nF/cm2 made the OFET yield a high carrier mobility of 3 cm2 /Vs. In the next year, Veres et al. reported that the interaction between the dielectric and the semiconductor plays a crucial role in the charge carrier transport. They found out that for a larger permittivity of the dielectric, the more charge carrier was localized at the surface of the dielectric [13]. Thus, there is a contradictory selection between the high dielectric material and the low permittivity material. In order to obtain such a balance, in 2004, Park et al. introduced double polymer layers as the dielectric in OFET [14]. This structure consisted of a thin PVP layer in contact with the semiconductor, which could induce good charge transport properties and a thick poly(vi‐ nyl acetate) (PVAc) layer as the bottom layer to realize good dielectric properties.

In recent years, the tendency of the research on polymer dielectrics has been toward printable, flexible, and biocompatible materials. In 2015, Huang et al. developed a versatile self‐healing polymer blend dielectric without salts and integrated it into the OFET [15]. This high capaci‐ tance of polymer blend dielectric could even induce the healing of the functional layer coated above it. In 2016, Jung et al. introduced a kind of cross‐linked poly(methyl methacrylate) (PMMA) in an n‐type OFET, which examined the application of the OFET for flexible circuit [16]. Schmidt et al. fabricated a low‐voltage fully printed flexible OFET using three layers of dielectric of CYTOP (low‐k), PVA (intermediate), and P(VDF‐TrFE‐CTFE) (high‐k) [17]. Moreover, more and more biomaterials, such as glucose, indigo, and nucleic acid–based materials, are used in the OFETs [18]. Among them, DNA and silk fibroin (SF) are two widely researched dielectric biomaterial in OFET. For example, Liang et al. had adopted DNA‐ hexadecyltrimethyl ammonium chloride (CTMA) as the dielectric layer in OFET to success‐ fully realize a nonvolatile memory [19]. Wang et al. employed the SF as the gate dielectric in OFET and obtained a high mobility of 23.2 cm2 /Vs and a low operating voltage of ‐3 V [20].

### **2.2. Multilayer polymer dielectric**

as the dielectric, the resource of polymer dielectric has become environmentally friendly and very broad. Due to such kinds of polymer dielectrics, OFETs are available in versatile

In a common configuration, the OFET consists of the source and drain electrodes, semicon‐ ductor, dielectric, and gate. The functional part of an OFET device is the current channel, which exists in the first few monolayers of the semiconductor upon the dielectric. Therefore, the interface between the dielectric and the semiconductor plays a crucial role in the device performance, which makes the requirements for polymer dielectric material for OFET rather stringent [8, 9]. The crucial parameter of a dielectric material is the maximum possible electric

**2. Introduction of polymer dielectric in organic field‐effect transistor**

*D kE* max 0 B = e

> i 0 *<sup>k</sup> <sup>C</sup> <sup>d</sup>* <sup>=</sup> e

only governed by the *k* value but also by the thickness of the dielectric.

on the plastic. The OFET had a field‐effect mobility of 0.01–0.03 cm2

where *ɛ*0 is the vacuum permittivity, *k* is the dielectric constant, *EB* is the dielectric breakdown

, which is defined as:

where *d* is the thickness of the dielectric. It is obvious that the capacitance magnitude is not

The first detailed study of different polymer dielectrics in OFET was reported by Peng et al. in 1990 [10]. The OFET was fabricated on glass using five kinds of polymer dielectrics. They found that there was a strong correlation between the insulator's *k* value and the field‐effect mobility. Then, in 1997, the first high performance plastic transistor was realized by Bao et al. [11]. They employed polyimide as the dielectric and all the essential components were printed directly

made a step further achievement as the "all‐polymer" circuit which integrated a 250 nm thick melamine cross‐linked poly(vinyl pyrrolidone) (PVP) [12]. The cross‐linked PVP with the

next year, Veres et al. reported that the interaction between the dielectric and the semiconductor plays a crucial role in the charge carrier transport. They found out that for a larger permittivity of the dielectric, the more charge carrier was localized at the surface of the dielectric [13]. Thus, there is a contradictory selection between the high dielectric material and the low permittivity material. In order to obtain such a balance, in 2004, Park et al. introduced double polymer layers as the dielectric in OFET [14]. This structure consisted of a thin PVP layer in contact with

made the OFET yield a high carrier mobility of 3 cm2

(1)

(2)

/Vs. In 2002, Klauk et al.

/Vs. In the

applications including sensor, inverter, and memory [7].

4 Properties and Applications of Polymer Dielectrics

**2.1. Polymer dielectric in organic field‐effect transistor**

displacement (*D*max) the dielectric can sustain:

field, and the capacitance per area *Ci*

capacitance of ~11–12 nF/cm2

In OFETs, the bulk and interface properties of gate dielectric are both crucial in determining device performance. Therefore, two prerequirements have to be met for the dielectric material: (1) a high gate capacitance, which reduces the required gate voltage for sufficient charge accumulation in the channel, can be obtained by decreasing the film thickness or increasing the dielectric constant; (2) a trap‐free interface between semiconductor and dielectric, which enhances the carrier mobility [17, 21]. A high *k* material has been considered as a good choice for the dielectric in OFET due to the excellent bulk insulating properties. However, the surface polarization of the high *k* material is much more higher than that of the low *k* material, which can significantly deteriorate the device performance. Therefore, to achieve high performance device, there should be a meticulous consideration between the high *k* and the low *k* materials. In this case, the utilization of the multilayer dielectric, combining a very thin low *k* layer at the interface to the semiconductor with a thick high *k* material for sufficient gate insulation is a reliable approach.

Yu et al. made an effective approach on multilayer dielectrics of PMMA and PVA in OFET [22]. Through analyzing the electrical characteristics of OFETs with various PVA/PMMA arrange‐ ments as shown in **Figure 1(a)**, it was found that one of the origins of the hysteresis was the trap in PVA bulk as well as at the interface of pentacene/PVA. Meanwhile, the results showed that the memory window was proportional to the amount of traps in PVA and the charge density at the interfaces of gate/PVA or PVA/pentacene. Then, the pentacene OFETs based on bilayer dielectrics of PMMA/SF was developed, as shown in **Figure 1(b)** [23]. The PMMA/SF bilayer dielectric exhibited a high field‐effect mobility of 0.21 cm2 /Vs and a high current on/off ratio of 1.5 × 104 . The performance enhancement was mainly attributed to the crystallization improvement of the pentacene and the smaller interface trap density at the SF/pentacene interface. Meanwhile, a low contact resistance also indicated that a good contact of electrode/ organic was formed.

**Figure 1.** Configuration and performance of OFETs with (a) PVA and PMMA multi‐layer dielectrics; (b) PMMA/SF bi‐ layer dielectrics [22, 23]. Copyright 2013, American Institute of Physics; Copyright 2014, IOP Publishing.

#### **2.3. Hybrid polymer dielectric**

An easily processable polymer typically has a low dielectric constant and good mechanical properties but requires large gate dielectric thicknesses due to high leakage current. One of possible solutions is to combine inorganic‐organic or organic‐organic materials as hybrid gate dielectrics. These complementary constituents ideally combine high permittivity of the inorganic inclusions with the high breakdown strength, mechanical flexibility, and easy processability of the organic counterparts [24]. Sun et al. explored a blending polymer of dielectric of polyphenyleneoxide (PPO) and polystyrene (PS) to enhance the performance of pentacene OFET [25], which could control both the inter‐grain‐enhancing process and the nucleation‐controlling process. The optimized morphology of pentacene thus led to the enhancement of mobility to 3.6 cm2 /Vs. Yu and coworkers also employed the PMMA/zinc oxide (ZnO) hybrid as the dielectric for OFET [26]. The resulted morphology of the pentacene grown on the hybrid dielectric was responsible for the enhanced sensing performance to ammonia (NH3) of the OFET.

Moreover, the polymer dielectric can not only serve as the dielectric layer in OFET but can also play a role in the semiconducting layer [27]. Yu et al. made it a step further to adopt the poly(3‐ hexylthiophene) (P3HT)/PS hybrid as the semiconductor [28]. The relationship between the molecular arrangement, aggregation, and charge transport in P3HT:PS blends with the boiling points and solubility in different solvents like chloroform (CF), o‐xylene (XY), chlorobenzene (CB), and 1,2‐dichlorobenzene (DCB) was systematically analyzed, as shown in **Table 1**. The result showed that DCB with the highest boiling temperature was beneficial to achieve distinct lateral aggregation of P3HT in the blend film. An optimized composition of 1.6 wt% P3HT in the PS matrix with three times increase of mobility and two times increase of current on/off ratio were obtained, compared to that of the pure P3HT (8 wt%). Li et al. adopted a blend of 6, 13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) and PS as the semiconducting layer [29]. They synthesized the PMMA and PS functionalized with both propargyl and azido groups by free radical copolymerization to serve as the dielectric to obtain a high performance OFET with a field‐effect mobility of 0.59 cm2 /Vs, and an on/off current ratio of 105 . Feng et al. fabricated a solution processed bottom‐gate bottom‐contact OFET, which could be able to sustain hybrid low‐/high‐voltage operation [30, 31]. In their devices, a channel engineering approach was used to obtain low‐voltage operation, by inducing phase separation with a blend of TIPS‐pentacene and PS to form an ultrathin high crystalline channel. Since the approach did not rely on enlarging the gate dielectric capacitance, the low‐voltage OFET with a relatively thick dielectric layer was shown to be able to sustain high‐voltage operation.


**Table 1.** Characteristics of pure P3HT and P3HT:PS blend OFETs based on different solvents [27]. Copyright 2015, Elsevier.

#### **2.4. Cross‐linked polymer dielectric**

interface. Meanwhile, a low contact resistance also indicated that a good contact of electrode/

**Figure 1.** Configuration and performance of OFETs with (a) PVA and PMMA multi‐layer dielectrics; (b) PMMA/SF bi‐

An easily processable polymer typically has a low dielectric constant and good mechanical properties but requires large gate dielectric thicknesses due to high leakage current. One of possible solutions is to combine inorganic‐organic or organic‐organic materials as hybrid gate dielectrics. These complementary constituents ideally combine high permittivity of the inorganic inclusions with the high breakdown strength, mechanical flexibility, and easy processability of the organic counterparts [24]. Sun et al. explored a blending polymer of dielectric of polyphenyleneoxide (PPO) and polystyrene (PS) to enhance the performance of pentacene OFET [25], which could control both the inter‐grain‐enhancing process and the nucleation‐controlling process. The optimized morphology of pentacene thus led to the

(ZnO) hybrid as the dielectric for OFET [26]. The resulted morphology of the pentacene grown on the hybrid dielectric was responsible for the enhanced sensing performance to ammonia

Moreover, the polymer dielectric can not only serve as the dielectric layer in OFET but can also play a role in the semiconducting layer [27]. Yu et al. made it a step further to adopt the poly(3‐ hexylthiophene) (P3HT)/PS hybrid as the semiconductor [28]. The relationship between the

/Vs. Yu and coworkers also employed the PMMA/zinc oxide

layer dielectrics [22, 23]. Copyright 2013, American Institute of Physics; Copyright 2014, IOP Publishing.

organic was formed.

6 Properties and Applications of Polymer Dielectrics

**2.3. Hybrid polymer dielectric**

enhancement of mobility to 3.6 cm2

(NH3) of the OFET.

Commonly, there are two problems that exist in polymer gate dielectrics. One is the stability and the other one is the electrical robustness [29]. Meanwhile, when multilayer polymer dielectrics are applied, the elimination of the dissolution or swelling problem of the upper layer toward the under layer is desired. Cross‐linking method can not only enhance solvent resistance and thermal stability, but also improve the electrical robustness of dielectric materials [32]. Moreover, the interference of each dielectric in the multilayer structure can be solved through the cross‐link process. Commonly, the cross‐linking in the dielectric layer can be achieved by photo or thermal reactions [33–35].

Li et al. synthesized the PMMA and PS functionalized with both propargyl and azido groups by free radical copolymerization, which could be effectively cross‐linked by thermal azide‐ alkyne cycloaddition reaction at a relatively low temperature of 100°C as shown in **Figure 2** [29, 36]. This bifunctional approach significantly improved the efficiency of cross‐linking reactions in the solid state and substantially enhanced the solvent resistance of the cross‐linked dielectric layers. The OFET exhibited a high device performance with a field‐effect mobility of 0.59 cm2 /Vs, and an on/off current ratio of 105 as mentioned above in the "hybrid polymer dielectric" part. At the same time, they synthesized two azide functionalized polymers by free radical copolymerization [37, 38]. Each new polymer was effectively cross‐linked with a small molecule cross‐linker by a thermally activated reaction at 100°C. This cross‐linking method is compatible with plastic substrates for flexible electronic applications.

**Figure 2.** Schematic of the OFET and the chemical structures of the relevant materials [29, 36]. Copyright 2015, RSC Publishing; Copyright 2014, RSC Publishing.

From the above discussion, we can see that polymer dielectric is widely researched in OFET due to its inherent advantages of solution processability and flexibility. With the abundant resources, we can choose polymer dielectric with certain properties to serve different functions in hybrid and multilayer structures. Moreover, through cross‐linking method, the atmosphere stability of polymer dielectric can be significantly enhanced, which brings a wider utilization for OFET and its electronic device.

## **3. Biomaterial‐engaged polymer dielectric**

#### **3.1. Biomaterial in organic field‐effect transistor**

Nowadays, the research field of organic electronics is becoming more and more interdiscipli‐ nary, since biomaterial possesses superior attributes of chemical abundance, biodegradability, and low cost. OFETs are frequently used in the detection of biomaterials, whereas biomaterials are engaged in OFETs to serve as one of the functional layers as shown in **Figure 3** [18, 39–41]. Substrates made of caramelized glucose, edible hard gelatin, and commercially available plastics based on potato and corn starch provide examples for metabolizable or biodegradable substrates [18]. DNA and DNA base pairs have been used as gate dielectric in OFETs and memory elements [42–44]. Beta‐carotene and indigo have been employed as the p‐type and n‐ type materials in OFETs, exhibiting the charge mobility of ∼10‐4 cm2 /Vs [45, 46].

**Figure 3.** Utilization of natural materials or materials inspired by nature in OFETs [18]. Copyright 2010, Wiley.

## **3.2. Silk fibroin dielectric**

Li et al. synthesized the PMMA and PS functionalized with both propargyl and azido groups by free radical copolymerization, which could be effectively cross‐linked by thermal azide‐ alkyne cycloaddition reaction at a relatively low temperature of 100°C as shown in **Figure 2** [29, 36]. This bifunctional approach significantly improved the efficiency of cross‐linking reactions in the solid state and substantially enhanced the solvent resistance of the cross‐linked dielectric layers. The OFET exhibited a high device performance with a field‐effect mobility

dielectric" part. At the same time, they synthesized two azide functionalized polymers by free radical copolymerization [37, 38]. Each new polymer was effectively cross‐linked with a small molecule cross‐linker by a thermally activated reaction at 100°C. This cross‐linking method is

**Figure 2.** Schematic of the OFET and the chemical structures of the relevant materials [29, 36]. Copyright 2015, RSC

From the above discussion, we can see that polymer dielectric is widely researched in OFET due to its inherent advantages of solution processability and flexibility. With the abundant resources, we can choose polymer dielectric with certain properties to serve different functions in hybrid and multilayer structures. Moreover, through cross‐linking method, the atmosphere stability of polymer dielectric can be significantly enhanced, which brings a wider utilization

Nowadays, the research field of organic electronics is becoming more and more interdiscipli‐ nary, since biomaterial possesses superior attributes of chemical abundance, biodegradability, and low cost. OFETs are frequently used in the detection of biomaterials, whereas biomaterials are engaged in OFETs to serve as one of the functional layers as shown in **Figure 3** [18, 39–41].

compatible with plastic substrates for flexible electronic applications.

/Vs, and an on/off current ratio of 105 as mentioned above in the "hybrid polymer

of 0.59 cm2

8 Properties and Applications of Polymer Dielectrics

Publishing; Copyright 2014, RSC Publishing.

for OFET and its electronic device.

**3. Biomaterial‐engaged polymer dielectric**

**3.1. Biomaterial in organic field‐effect transistor**

Silk fibroin is one of the silk proteins emitted by the silkworm, which forms the structural center of silk with sericin around it. It is a natural biopolymer consisting of the repeated amino acid sequence of alternating glycine (gly) and alanine (ala) as shown in **Figure 4(a)** [47]. Until now, silk fibroin has been employed in various electronic fields, including the contact lenses [48], the platform for transistors [49], and photonic devices [50]. These applications are benefited from its unique characteristics of optical transparency, electrical insulation, and flexibility.

Yu et al. reported an enhanced performance pentacene OFETs consisting of PMMA/SF bilayer dielectric in 2014 [23]. The SF had good dielectric properties as shown in **Figure 4(b)**. The surface morphology of SF is very uniform with the root‐mean‐square surface roughness value of 1.3 nm as shown in **Figure 4(d)** (we can compare it with the smooth surface of PMMA shown in **Figure 4(c)**). The OFETs had a relatively high mobility of 0.21 cm2 /Vs with an enhanced on/ off ratio of 1.5 × 104 . This was mainly attributed to the crystallization improvement of the pentacene grown on SF (as shown in **Figure 4(e)** and **(f)**) and the smaller interface trap density at the SF/pentacene interface. Then, the utilization of SF‐engaged OFET to function as the nitrogen dioxide (NO2) sensor was further studied [51]. In this research, SF was deposited on the top of PMMA to act as the dielectric layer, resulting in an increase of NO2 sensing per‐ formance.

**Figure 4.** (a) Molecular structure of SF; (b) capacitance versus frequency (C‐F) property of SF, the inset shows the C‐F image in a frequency range of 0–5 KHz and structure of the device; AFM images of (c) PMMA; (d) SF; pentacene grown on (e) PMMA and (f) SF [23]. Copyright 2014, IOP Publishing.

### **3.3. DNA and DNA base pair dielectric**

DNA is a complicated macromolecule consisting of four base pairs of guanine, adenine, thymine, and cytosine. Singh et al. first reported the utilization of DNA in OFET as a dielectric layer in 2006 [52]. They used a DNA‐based biopolymer, derived from salmon milt and roe sac waste by‐products for the gate dielectric, in which the current was modulated over three orders of magnitude using gate voltages less than ‐10 V. In most studies, since the purified DNA dissolves only in water, it should be modified through a cationic surfactant (hexadecyltri‐ methyl ammonium chloride, CTMA) cation exchange reaction to enhance solubility, process‐ ing, and stability.

Stadler et al. reported the utilization of DNA‐CTMA as the gate dielectric in n‐type methano‐ fullerene as well as p‐type pentacene‐based OFETs working at low‐voltage levels and low gate leakage currents [43]. They further realized a nonvolatile memory element based on the large hysteresis in the transfer characteristics of these DNA‐based OFET. As DNA is soluble only in water, which is not compatible with most organic solvents, Yu et al. first introduced the spray coating method to fabricate DNA film. In 2016, spray‐coated DNA on top of the PMMA dielectric was used to fabricate a DNA‐functioned NO2 sensor based on OFET as shown in **Figure 5(a)** and **(b)** [53]. The high‐sensing performance is ascribed to the negatively‐charged phosphate groups in DNA molecules, which can interact with NO2 analytes as shown in **Figure 5(c)**.

**Figure 5.** (a) Schematic representation of spray‐coating technique; (b) device architecture of OFET sensor and the mo‐ lecular structures of DNA and PMMA; (c) representation of the sensing mechanism of OFET‐based NO2 chemical sen‐ sor [53]. Copyright 2016, Elsevier.

Nature provides an overwhelming diversity of materials for human being. Hence, looking for natural or nature‐inspired materials appears to be a promising route for the fabrication of fully biodegradable and biocompatible organic electronics. In the future work, to make the all biomaterial‐ based device, which is compatible with human body, is an attractive area of the biomaterial‐engaged polymer dielectrics.

## **4. Application‐related polymer dielectric**

pentacene grown on SF (as shown in **Figure 4(e)** and **(f)**) and the smaller interface trap density at the SF/pentacene interface. Then, the utilization of SF‐engaged OFET to function as the nitrogen dioxide (NO2) sensor was further studied [51]. In this research, SF was deposited on the top of PMMA to act as the dielectric layer, resulting in an increase of NO2 sensing per‐

**Figure 4.** (a) Molecular structure of SF; (b) capacitance versus frequency (C‐F) property of SF, the inset shows the C‐F image in a frequency range of 0–5 KHz and structure of the device; AFM images of (c) PMMA; (d) SF; pentacene

DNA is a complicated macromolecule consisting of four base pairs of guanine, adenine, thymine, and cytosine. Singh et al. first reported the utilization of DNA in OFET as a dielectric layer in 2006 [52]. They used a DNA‐based biopolymer, derived from salmon milt and roe sac waste by‐products for the gate dielectric, in which the current was modulated over three orders of magnitude using gate voltages less than ‐10 V. In most studies, since the purified DNA dissolves only in water, it should be modified through a cationic surfactant (hexadecyltri‐ methyl ammonium chloride, CTMA) cation exchange reaction to enhance solubility, process‐

Stadler et al. reported the utilization of DNA‐CTMA as the gate dielectric in n‐type methano‐ fullerene as well as p‐type pentacene‐based OFETs working at low‐voltage levels and low gate leakage currents [43]. They further realized a nonvolatile memory element based on the large hysteresis in the transfer characteristics of these DNA‐based OFET. As DNA is soluble only in water, which is not compatible with most organic solvents, Yu et al. first introduced the spray coating method to fabricate DNA film. In 2016, spray‐coated DNA on top of the PMMA dielectric was used to fabricate a DNA‐functioned NO2 sensor based on OFET as shown in **Figure 5(a)** and **(b)** [53]. The high‐sensing performance is ascribed to the negatively‐charged phosphate groups in DNA molecules, which can interact with NO2 analytes as shown in

grown on (e) PMMA and (f) SF [23]. Copyright 2014, IOP Publishing.

**3.3. DNA and DNA base pair dielectric**

ing, and stability.

**Figure 5(c)**.

formance.

10 Properties and Applications of Polymer Dielectrics

### **4.1. Electrode buffer layer in organic field‐effect transistor**

Dielectric polymers can not only serve as the dielectric layer in the OFETs, but can also be adopted as the modification layer at the interface between the semiconductor and the electro‐ des, due to their smooth and hydrophobic surface properties. Especially, the high performance bottom‐contact OFET is required for its potential application in the large‐scale industrialized production due to the remarkable advantage in the size‐controlled fine lithography process‐ ing [54]. One of the major limits in fabricating high performance OFET in bottom‐contact configuration is the large contact resistance at organic/electrode interface, which results in a pronounced current loss. This makes the employment of the electrode buffer layer necessary.

Yu et al. fabricated bottom‐contacted OFET by using PMMA as an electrode buffer layer between P3HT layer and gold electrodes in OFETs and obtained a five‐fold enhancement of whole mobility [55]. The relatively rough surface of gold (**Figure 6(a)**) is modified by PMMA (**Figure 6(b)**). The uniformity and hydrophobicity of PMMA surface were responsible for the remarkable reduction of contact resistance at P3HT/electrode interface while they enhanced the crystallinity of P3HT as shown in **Figure 6(c)**. Three polymer dielectrics of PMMA, PS, and polyvinylidene fluoride (PVDF) as the electrode buffer layers were further studied in top‐ contacted OFET to decrease the contact resistance (**Figure 6(d)**) [30]. All the OFETs incorpo‐ rating with the buffer layers obtained a significant enhancement of the device performance, whereas the device employing PMMA exhibited the highest charge mobility of 0.59 cm2 /Vs. This was due to the optimal surface energy and appropriate dielectric constant of PMMA, which are favorable for the growth of pentacene crystal.

**Figure 6.** AFM images of (a) gold and (b) PMMA; (c) XRD analysis of P3HT; (d) contact resistance of OFETs [55]. Copy‐ right 2013, American Institute of Physics.

#### **4.2. Organic field‐effect transistor‐based gas sensor**

OFET‐based sensors have been intensively researched by virtue of their incomparable advantage of abundant material resource, physical flexibility, and elaborate array compatibil‐ ity [56–59]. Moreover, OFET holds the unique capability of gate bias modulation, by which the signal can be easily amplified over orders of magnitude. This provides a promising future of OFETs for the ultra‐low detection [60–64]. The controllable multiparameters of charge mobility, threshold voltage, and current on/off ratio make OFETs more suitable for selective detection [65]. Until now, OFET has been used to detect the solution, gas as well as the biomaterials [66].

Different kinds of polymer dielectrics perform differently in the OFET sensors. Yu et al. investigated the effect of various kinds of polymer dielectrics on the sensing performance of the OFET sensors. NH3 gas sensors were fabricated based on pentacene OFETs using polymers including PVA, PVP, PMMA, or PS as the gate dielectric as shown in **Figure 7(a)**[67]. The OFETs with PS as the gate dielectric could achieve the detection limit as low as 1 ppm. Meanwhile, the recovery properties of OFETs with PS were also enhanced. The variation of the sensing performance of OFET sensors with different dielectrics was proved to be mainly induced by the different properties of dielectric/pentacene interfaces. Furthermore, the low‐trap density of PS dielectric surface and the absence of polar groups in PS dielectric were responsible for the high performance of NH3 sensors. Moreover, the OFET incorporating ZnO/PMMA hybrid dielectric was fabricated to detect NH3 as shown in **Figure 7(b)** [26]. The OFETs exhibited a 23% current change under 75 ppm NH3, as well as a remarkable shift of threshold voltage and field‐effect mobility. The sensing mechanism was ascribed to the decreased grain size of pentacene formed on the ZnO/PMMA hybrid dielectric, facilitating NH3 to diffuse into the conducting channel.

whereas the device employing PMMA exhibited the highest charge mobility of 0.59 cm2

which are favorable for the growth of pentacene crystal.

12 Properties and Applications of Polymer Dielectrics

right 2013, American Institute of Physics.

**4.2. Organic field‐effect transistor‐based gas sensor**

This was due to the optimal surface energy and appropriate dielectric constant of PMMA,

**Figure 6.** AFM images of (a) gold and (b) PMMA; (c) XRD analysis of P3HT; (d) contact resistance of OFETs [55]. Copy‐

OFET‐based sensors have been intensively researched by virtue of their incomparable advantage of abundant material resource, physical flexibility, and elaborate array compatibil‐ ity [56–59]. Moreover, OFET holds the unique capability of gate bias modulation, by which the signal can be easily amplified over orders of magnitude. This provides a promising future of OFETs for the ultra‐low detection [60–64]. The controllable multiparameters of charge mobility, threshold voltage, and current on/off ratio make OFETs more suitable for selective detection [65]. Until now, OFET has been used to detect the solution, gas as well as the biomaterials [66].

Different kinds of polymer dielectrics perform differently in the OFET sensors. Yu et al. investigated the effect of various kinds of polymer dielectrics on the sensing performance of the OFET sensors. NH3 gas sensors were fabricated based on pentacene OFETs using polymers including PVA, PVP, PMMA, or PS as the gate dielectric as shown in **Figure 7(a)**[67]. The OFETs with PS as the gate dielectric could achieve the detection limit as low as 1 ppm. Meanwhile, the recovery properties of OFETs with PS were also enhanced. The variation of the sensing performance of OFET sensors with different dielectrics was proved to be mainly induced by the different properties of dielectric/pentacene interfaces. Furthermore, the low‐trap density of PS dielectric surface and the absence of polar groups in PS dielectric were responsible for

/Vs.

**Figure 7.** Real‐time response curves of (a) OFET with PS, PMMA, PVP, and PVA as dielectric; (b) OFET with ZnO/ PMMA and PMMA as dielectric exposed to NO2 [26, 67]. Copyright 2014, Elsevier; Copyright 2013, Elsevier.

**Figure 8.** Real‐time response curves of (a) OFET with and without DNA; (b) OFET with and without SF exposed to NO2 gas in the concentrations ranging from 10 to 50 ppm [51, 53]. Copyright 2015, Springer; Copyright 2016, Elsevier.

In terms of the biomaterials discussed above, they also exhibit good sensing performance in OFETs. A DNA‐based OFET sensor was realized for the detection of NO2 as shown in **Figure 8(a)**[53]. In this sensor, DNA was introduced between the gate dielectric and the organic semiconductor via spray‐coating to function as the detecting layer for NO2 analyte. There were remarkable shifts of 14.7% in saturation current and 14.4% in charge carrier mobility after exposure to 10 ppm NO2 analyte. With the concentration of NO2 increased to 50 ppm, the shifts of 22.8% in saturation current and 16.6% in charge carrier mobility had been obtained. The sensing performance was ascribed to the negatively‐charged phosphate groups in DNA molecules, which could interact with NO2 analytes and lead to a superior sensing performance of OFET incorporating with DNA (**Figure 5(c)**). Silk fibroin was also employed on top of PMMA dielectric in OFETs to detect NO2 as shown in **Figure 8(b)** [51]. When exposed to a high NO2 concentration of 50 ppm, the saturation current of the OFETs exhibited an increase of 16%. The superior sensing performance was due to the interaction between the hydroxyl and the amidogen of the SF biomaterial and NO2 molecules at the interface of dielectric/organic layer.

#### **4.3. Organic field‐effect transistor‐based inverter**

Inverters are the basic and indispensable parts in the case of electronic circuits. Especially, complementary inverters, which usually consist of both p‐type and n‐type transistors, are widely applied in silicon‐based circuits. Besides, due to the performance inconsistency of p‐ type and n‐type organic semiconductors, unipolar inverter, which can be functionalized with only one type of transistors, is extensively studied in the field of organic electronics [68, 69]. The OFETs with a significant controllable threshold voltage (*V*th) are indispensable to construct the high performance unipolar inverter. Ultraviolet (UV)/ozone (UVO) treatment can modify the physical and chemical characteristics of the polymer surface, leading to a shift of the *V*th without changing the properties of the bulk material [70, 71].

**Figure 9.** (a) Schematic configuration, (b) photograph, and (c) circuit diagram of unipolar inverter. (d) VTCs and (e) corresponding voltage gains of the inverters; (f) NMs of the inverter used OFET with 60 s UVO treated PS as load tran‐ sistor [72]. Copyright 2014, American Institute of Physics.

PS was also adopted as the gate dielectric in OFET to fabricate a unipolar inverter as shown in **Figure 9(a)**–**(c)** [72]. This inverter was based on a significant variation of threshold voltage (*V*th) of OFETs, which was realized by introducing UVO treatment to PS dielectric. A controllable *V*th shift of more than 10 V was obtained in the OFETs by adjusting the treating time, and the unipolar inverters exhibited an inverting voltage near 1/2 driving voltage and a noise margin of more than 70% of ideal value as shown in **Figure 9(d)**–**(f)**. The dramatic controllable *V*th of OFETs was attributed to the newly generated oxygen functional groups in the PS dielectric induced by UVO treatment. Guo et al. developed a novel organic/inorganic hybrid integration architecture to realize low‐voltage complementary inverters with low temperature (not exceeding 150°C) solution‐processed semiconductor and PVA dielectric layers [73]. The fabricated inverter had a voltage gain larger than 15 at an operation voltage of 3 V. The inverters were further fabricated on a polyethylene naphthalate plastic substrate with the bottom‐gate bottom‐contact configuration [28]. In the devices, UV cross‐linked PVA was used as the gate dielectric layer and the devices show a high DC voltage gain up to 67.3 at a supply voltage of 3 V. Also, the inverter with the low‐operation voltage based on a channel engineering approach was fabricated [74]. The relatively thick (even with a 400‐nm thick) and low dielectric constant polymer dielectric of PVA could be used in the inverter.

### **4.4. Organic field‐effect transistor‐based memory**

sensing performance was ascribed to the negatively‐charged phosphate groups in DNA molecules, which could interact with NO2 analytes and lead to a superior sensing performance of OFET incorporating with DNA (**Figure 5(c)**). Silk fibroin was also employed on top of PMMA dielectric in OFETs to detect NO2 as shown in **Figure 8(b)** [51]. When exposed to a high NO2 concentration of 50 ppm, the saturation current of the OFETs exhibited an increase of 16%. The superior sensing performance was due to the interaction between the hydroxyl and the amidogen of the SF biomaterial and NO2 molecules at the interface of dielectric/organic layer.

Inverters are the basic and indispensable parts in the case of electronic circuits. Especially, complementary inverters, which usually consist of both p‐type and n‐type transistors, are widely applied in silicon‐based circuits. Besides, due to the performance inconsistency of p‐ type and n‐type organic semiconductors, unipolar inverter, which can be functionalized with only one type of transistors, is extensively studied in the field of organic electronics [68, 69]. The OFETs with a significant controllable threshold voltage (*V*th) are indispensable to construct the high performance unipolar inverter. Ultraviolet (UV)/ozone (UVO) treatment can modify the physical and chemical characteristics of the polymer surface, leading to a shift of the *V*th

**Figure 9.** (a) Schematic configuration, (b) photograph, and (c) circuit diagram of unipolar inverter. (d) VTCs and (e) corresponding voltage gains of the inverters; (f) NMs of the inverter used OFET with 60 s UVO treated PS as load tran‐

PS was also adopted as the gate dielectric in OFET to fabricate a unipolar inverter as shown in **Figure 9(a)**–**(c)** [72]. This inverter was based on a significant variation of threshold voltage (*V*th) of OFETs, which was realized by introducing UVO treatment to PS dielectric. A controllable *V*th shift of more than 10 V was obtained in the OFETs by adjusting the treating time, and the unipolar inverters exhibited an inverting voltage near 1/2 driving voltage and a noise margin of more than 70% of ideal value as shown in **Figure 9(d)**–**(f)**. The dramatic controllable *V*th of OFETs was attributed to the newly generated oxygen functional groups in the PS dielectric induced by UVO treatment. Guo et al. developed a novel organic/inorganic hybrid integration architecture to realize low‐voltage complementary inverters with low temperature (not exceeding 150°C) solution‐processed semiconductor and PVA dielectric layers [73]. The fabricated inverter had a voltage gain larger than 15 at an operation voltage of 3 V. The inverters

**4.3. Organic field‐effect transistor‐based inverter**

14 Properties and Applications of Polymer Dielectrics

without changing the properties of the bulk material [70, 71].

sistor [72]. Copyright 2014, American Institute of Physics.

OFET‐based memory mostly relies on the hysteresis of the dielectric. As to a nonvolatile transistor, a voltage is added between the gate electrode and the semiconducting channel. Through the charge storage, the effective gate voltage within the device then differs from the applied voltage. The resulted polarization phenomenon creates an additional electronic state in the device. PVA is a commonly adopted polymer dielectric in OFET‐based memory. The hysteresis mechanism of OFETs with PVA dielectric can be concluded in two aspects: the charge transport in PVA bulk and the charge trapping/detrapping process in PVA bulk and/or at the interface of organic semiconductor/PVA.

The research on multilayer dielectric of PMMA and PVA in OFET was also carried out [22]. Through analyzing the electrical characteristics of OFETs with various PVA/PMMA arrange‐ ments, it was found that one of the origins of the hysteresis was the trap in PVA bulk as well as at the interface of pentacene/PVA. Meanwhile, the results showed that the memory window was proportional to the amount of traps in PVA and the charge density at the interfaces of gate/ PVA or PVA/pentacene. Therefore, the memory window could be controlled to around 0–10 V by tuning the thickness and combination of triple‐layer polymer dielectrics strategy. Meanwhile, as the dielectric interfaces also greatly influenced the number of densities and the mobility of charge carrier in adjacent semiconductors, the surface issue should also be taken into consideration. Feng et al. fabricated a novel type of OFET‐based write‐once read‐many memory (WORM) device [75]. The device used an ultraviolet cross‐linkable matrix dielectric polymer of poly (vinyl cinnamate) (PVC) mixed with ionic compounds 10‐methyl‐9‐phenyl‐ acridinium perchlorate (MPA+ ClO4 ‐ ) to form an ion‐dispersed gate dielectric layer. Under an applied gate voltage bias, the migration of cations and anions in opposite directions formed space charge polarization in the gate dielectric layer, resulting in a change of electrical characteristics. Through UV illumination method to cross‐link the matrix polymer, the stability of the formed space charge polarization could be enhanced. Hence, the OFET could function as a WORM with the applied voltage bias to define the polarization, and at the same time the UV illumination could stabilize the stored data.

There is a wide utilization of OFETs functionalized by polymer dielectric. The most common adopted application is the OFET‐based sensors. As the effective current channel lies at the interface between the semiconductor and the polymer dielectric, to modify the surface of the dielectric becomes an effective way to enhance the sensing performance. Moreover, polymer dielectric can also play a role in the electrode buffer layer as well as the inverter and memory, which makes the polymer dielectrics not simple function as the dielectric, but also as a versatile material.

## **5. Summary**

In this section, we make a concise introduction of polymer dielectric in OFETs, including the multilayer, hybrid, and cross‐linked polymer dielectric. In addition, we had a detailed overview of the available biomaterials of the silk fibroin and the DNA and DNA base pair as polymer dielectrics. As one of the most important layer in OFETs, polymer dielectric possesses a promising application as interface modification layer in OFETs, as well as the functional layer in OFET‐based sensor, inverter, and memory. Hence, the polymer dielectric holds versatile inherent properties to be explored and is paced toward the future in the field of organic electronics.

## **Acknowledgements**

We acknowledge the National Natural Science Foundation of China (NSFC) (Grant no. 61177032 and No. 61675041), the Fundamental Research Funds for the Central Universities (Grant no. ZYGX2010Z004), the Foundation of Innovation Groups of NSFC (Grant no. 61421002), the China Scholarship Council (File no. 201506070085 and no. 201506070069), and Science & Technology Department of Sichuan Province (Grant no. 2016HH0027).

## **Author details**

Wei Shi, Yifan Zheng and Junsheng Yu\*

\*Address all correspondence to: jsyu@uestc.edu.cn

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu, People's Republic of China

## **References**


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**5. Summary**

16 Properties and Applications of Polymer Dielectrics

electronics.

**Acknowledgements**

**Author details**

**References**

Wei Shi, Yifan Zheng and Junsheng Yu\*

*nications* 2016, *7*, 10908.

*Society Reviews* 2015, *44* (8), 2087–2107.

\*Address all correspondence to: jsyu@uestc.edu.cn

(UESTC), Chengdu, People's Republic of China

In this section, we make a concise introduction of polymer dielectric in OFETs, including the multilayer, hybrid, and cross‐linked polymer dielectric. In addition, we had a detailed overview of the available biomaterials of the silk fibroin and the DNA and DNA base pair as polymer dielectrics. As one of the most important layer in OFETs, polymer dielectric possesses a promising application as interface modification layer in OFETs, as well as the functional layer in OFET‐based sensor, inverter, and memory. Hence, the polymer dielectric holds versatile inherent properties to be explored and is paced toward the future in the field of organic

We acknowledge the National Natural Science Foundation of China (NSFC) (Grant no. 61177032 and No. 61675041), the Fundamental Research Funds for the Central Universities (Grant no. ZYGX2010Z004), the Foundation of Innovation Groups of NSFC (Grant no. 61421002), the China Scholarship Council (File no. 201506070085 and no. 201506070069), and

Science & Technology Department of Sichuan Province (Grant no. 2016HH0027).

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China

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#### **High-***k* **Polymer Nanocomposites for Energy Storage Applications High-***k* **Polymer Nanocomposites for Energy Storage Applications**

Asad Mahmood, Abdul Naeem and Tahira Mahmood Asad Mahmood, Abdul Naeem and Tahira Mahmood

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

http://dx.doi.org/10.5772/65944

#### **Abstract**

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High dielectric (high*-k*) polymer nanocomposites that can electrostatically store energy are widely used in electronics and electric power systems due to their high breakdown strengths (*Eb*), durability, and ability to configure in various shapes. However, these nanocomposites suffer from a limited working temperature regime, thus limiting their extreme applications, such as hybrid and electric vehicles, aerospace power electronics, and deep ground fuel exploration. Furthermore, the *Eb* and the electric displacement (*D*) of polymer nanocomposites must be simultaneously enhanced for high-density capacitor applications, which prove to be difficult to modify concurrently. This chapter thoroughly reviews (investigates) the recent developments in the high-*k* polymer nanocomposites synthesis, characterization, and energy storage applications. Consequently, the aim of this chapter is to provide an overview of the novel developmental strategies in order to develop high-dielectric nanocomposites perovskite ceramics that can be incorporated in high-energy-density (HED) applications.

**Keywords:** polymer nanocomposites, capacitors, perovskite ceramics, dielectric constant, energy density applications

## **1. Introduction**

Highdielectric constant (*high-k*) materials and polymer nanocomposites are under extensive investigation because of their potential applications in organic field-effect transistors (OFETs), invertors, electro-optics, energy, humidity and temperature sensor applications. The magnitude of *k* for silicon dioxide (SiO2) is given as 3.9. Materials that exhibit *k* > 3.9 are classified as

© 2017 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. © 2017 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.

high-*k*, and materials whose *k*< 3.9 are classified as low-*k* materials. Polymer dielectric nanocomposites are generally composed of dielectric polymers as the matrix material, and inorganic/organic fillers as the reinforcement, utilizing the properties of both. Polymers have been found to demonstrate high breakdown strengths along with high energy density (HED), while the fillers, especially dielectric ceramics, have a high dielectric constant (*k*). The combination of both provides superior dielectric properties depending on the type and nature of the polymer matrices as well as the fillers. The total energy storage density (U) of a capacitor is given as Eq. (1).

$$\mathbf{U} = \begin{bmatrix} \mathbf{E} \mathbf{d} \mathbf{D} \end{bmatrix} \tag{1}$$

where **E** and D are the applied electric field and electric displacement, respectively. The magnitude of U for the linear dielectrics is defined as Eq. (2).

$$\mathbf{U} = \mathbf{l} / \,\mathbf{2DE} + \mathbf{l} / \,\mathbf{2k\varepsilon}\_0 \mathbf{E}^2 \tag{2}$$

Where, *k* is the dielectric constant and ɛ0 = 8.854 × 10−12 F/m is the permittivity of free space. Therefore, the magnitude of U in Eq. (2) depends on the values of both *k* and **E**. However, limitations exist in ceramic and electrolytic capacitors, such as the applied **E** of a material is restricted to the breakdown strength of the same material. Due to these deficiencies, polymer capacitors have proven to be better alternatives for high energy density applications because of their high *Eb*, easy and economical processing, and flexibility in designing [1–3].

For high energy density applications, various ceramic fillers have been reported in the recent literature, such as barium titanate (BaTiO3, BT), barium strontium titanate (Ba1−*x*Sr*x*TiO3), barium zirconate titanate (BaZr*x*Ti1−*x*O3), lead zirconate titanate (PbZr*x*Ti1−*x*O3), metal phenylphosphonates ATi(O3PC6H5)3, where A = Mg, Ca, Sr, Ba, Pb, MnCoFe2O4, and MnCuFe2O4 [4–6]. The inorganic-organic nanocomposites have been processed by various techniques. Generally, the presynthesized filler nanoparticles are dispersed in the polymer matrix. Another method known as the *in situ* sol-gel involves the hydrothermal treatment of the filler precursor/polymer system in hydroxide solution. The filler particles are synthesized at the precursor sites in the polymeric system. Despite several modifications in these synthetic procedures, the final materials still exhibit conglomeration, which is responsible for the inconsistent behavior of the system. Furthermore, due to the incompatibility of the polymer-precursor and possible gel formation overtime, the system properties are compromised [7]. Zhu et al. [8] investigated the core-shell structured polymer@BT nanoparticles by using a polymer shells with different elemental properties in order to study the effect of core-shell in detail for surface modifications. A surface-initiated reversible-addition-fragmentation chain transfer (RAFT) polymerization method was used, where the thicknesses of the polymer shells were maintained to the same values. Results of this study indicated that for high energy density applications, high dielectric constant and low electrical conductivity are required. The importance of conductivity of the shell material in the final device performance was also reported in this investigation [8]. Other filler materials that are used include graphene oxide, carbon nanotubes (CNTs), and metal nanoparticles.

This chapter provides a comprehensive understanding of the recent developments in the high*k* nanocomposites without detailing the complex mathematical models and formulas that have been excessively quoted in various books and reviews. We provide an understanding of the nanocomposites processing, strategies to optimize their dielectric properties, and various filler materials that have been recently used. This section also strives to provide a future perspective in the high-*k* nanocomposites and conclude with some summarized remarks about the topic.

## **2. Electrical properties**

high-*k*, and materials whose *k*< 3.9 are classified as low-*k* materials. Polymer dielectric nanocomposites are generally composed of dielectric polymers as the matrix material, and inorganic/organic fillers as the reinforcement, utilizing the properties of both. Polymers have been found to demonstrate high breakdown strengths along with high energy density (HED), while the fillers, especially dielectric ceramics, have a high dielectric constant (*k*). The combination of both provides superior dielectric properties depending on the type and nature of the polymer matrices as well as the fillers. The total energy storage density (U) of a capacitor

where **E** and D are the applied electric field and electric displacement, respectively. The

Therefore, the magnitude of U in Eq. (2) depends on the values of both *k* and **E**. However, limitations exist in ceramic and electrolytic capacitors, such as the applied **E** of a material is restricted to the breakdown strength of the same material. Due to these deficiencies, polymer capacitors have proven to be better alternatives for high energy density applications because

For high energy density applications, various ceramic fillers have been reported in the recent literature, such as barium titanate (BaTiO3, BT), barium strontium titanate (Ba1−*x*Sr*x*TiO3), barium zirconate titanate (BaZr*x*Ti1−*x*O3), lead zirconate titanate (PbZr*x*Ti1−*x*O3), metal phenylphosphonates ATi(O3PC6H5)3, where A = Mg, Ca, Sr, Ba, Pb, MnCoFe2O4, and MnCuFe2O4 [4–6]. The inorganic-organic nanocomposites have been processed by various techniques. Generally, the presynthesized filler nanoparticles are dispersed in the polymer matrix. Another method known as the *in situ* sol-gel involves the hydrothermal treatment of the filler precursor/polymer system in hydroxide solution. The filler particles are synthesized at the precursor sites in the polymeric system. Despite several modifications in these synthetic procedures, the final materials still exhibit conglomeration, which is responsible for the inconsistent behavior of the system. Furthermore, due to the incompatibility of the polymer-precursor and possible gel formation overtime, the system properties are compromised [7]. Zhu et al. [8] investigated the core-shell structured polymer@BT nanoparticles by using a polymer shells with different elemental properties in order to study the effect of core-shell in detail for surface modifications. A surface-initiated reversible-addition-fragmentation chain transfer (RAFT) polymerization method was used, where the thicknesses of the polymer shells were maintained to the same values. Results of this study indicated that for high energy density applications, high dielectric constant and low electrical conductivity are required. The importance of conductivity of the shell material in the final device per-

<sup>2</sup> U 1/ 2D 1/ 2 = + **E E** *k*

ɛ

of their high *Eb*, easy and economical processing, and flexibility in designing [1–3].

0

e

magnitude of U for the linear dielectrics is defined as Eq. (2).

Where, *k* is the dielectric constant and

U dD <sup>=</sup> ò**<sup>E</sup>** (1)

(2)

0 = 8.854 × 10−12 F/m is the permittivity of free space.

is given as Eq. (1).

24 Properties and Applications of Polymer Dielectrics

Currently, pure polymers such as biaxially oriented polypropylene (BOPP) are used in energy storage applications, because of their high breakdown strength and low cost. However, pure polymers suffer from low magnitudes of dielectric constant, which results in low performance for energy density applications [2]. To overcome low dielectric constant limitations, high dielectric constant electroceramic nanoparticles are used as filler materials. Generally, due to the small band gap in semiconductors, thermal excitation is responsible for the generation of charge carriers, while the band gap of dielectric materials is relatively large, so electrical contacts or an external sources are responsible for the injection of charge carriers. Transition from one band to another in dielectrics requires high energy [9]. Electrons present in the valence shell of an atom are responsible for the dielectric phenomena, which interacts with externally applied fields, such as an electric or magnetic field. In polar dielectrics, the +ve and –ve charges are responsible for the production of electric dipole, whereas nonpolar dielectric materials lack this inherent dipole at the zero electric field, so an external potential is applied to shift the electron cloud, resulting in dipole phenomena [10]. Despite the use of high-*k* ceramics as filler materials in the development of nanocomposites, another approach is to introduce electrically conductive nanofillers. Such a device configuration has shown superior dielectric response; however, low breakdown strength and high dissipation factor both are disadvantages to be considered [11]. Ceramics became the backbone of the electronic industry after the discovery of ferroelectricity in these materials in 1946. Ferroelectricity was first observed in BT in the mid-1940s and since then, BT has become one of the most essential ferroelectric materials [12]. Before the discovery of BT, steatite, mica, MgTiO3, CaTiO3, and TiO2 were widely used in capacitors. These materials exhibit ɛ*<sup>r</sup>* ≤ 100, which limits their use in the industry of dielectric capacitors [13–15]. During World War II, the need of high dielectric constant materials increased in capacitor applications [16, 17]. Ferroelectric materials exhibit nonlinear dielectric polarization against an external *E* due to the presence of permanent dipoles [18]. The polarization in these materials increases with the increasing strength of *E*, which corresponds to the alignment of dipoles in an applied electric field. The state (*Ps*) in which the dipoles of the ferroelectrics align with the applied field is called saturation. The polarization decreases with the reduction of *E* as well as with the reversal of the applied field's direction, however, when the applied electric field is removed (*E* = 0), ferroelectric materials still exhibit some polarization. When the value of the field reaches to a certain point, the materials exhibit zero polarization, which is called coercive field (*Ec*). Dielectric properties of ferroelectric materials change significantly with temperature near *Tc*. Generally, magnitude of *k* increases with an increasing temperature below *Tc*, reaching a maximum value and then decreases with a further increase in temperature, for example, barium titanate *k* ≈ 2000 at room temperature reaches 7000 at *Tc*(~120°C) [19, 20]. Due to the high dielectric constant of ceramic materials, they are deemed as an efficient filler material for energy storage applications.

An ideal dielectric material resists the flow of charge completely; only allowing displacement of charges, resulting in polarization. In a typical capacitor arraignment, when an alternating *E* is applied to an ideal dielectric, the current will carry the voltage by a phase angle of π/2 (i.e., 90o ), where no power will be absorbed by the dielectric, and the capacitor will exhibit dielectric loss (tan *δ* = 0). On the other hand, the practical materials exhibit tan *δ*, and it is because the current-voltage phase angle is not exactly 90o ; therefore, the current is slightly lagging behind. The angle and magnitude of the lag are defined as *δ* and tan *δ*, respectively. Dielectric losses are associated with various mechanisms occurring in materials, such as electronic polarization, ion vibration, deformation, and ion migration. Generally, dielectric loss in ceramics is due to ion migration. Temperature and frequency are important factors that influence tan *δ* [21]. Compared to dielectric ceramics, the low magnitudes of *k* and tan *δ* in polymer dielectrics are attractive for transistor applications and they can be used in the form of thin layers (nanometer layers) as they exhibit low leakage current and high breakdown strength, which contributes to the miniaturization of electrical devices. **Table 1** summarizes the breakdown strength of some common polymers. Pure polymers are not suitable for high energy density applications because of their low dielectric constant.


**Table 1.** Dielectric strength of commonly used polymers [22].

High dielectric constants in ceramics originated from ionic polarization, which is associated with the asymmetric migration of the central metal cation in the crystal lattice. In contrast, polymers exhibit various types of polarization, i.e., ionic, electronic, and orientation, that are responsible for their dielectric constants [23, 24]. Typically, depending on the nature of the material and applied field, at least one kind of polarization mechanism is present in dielectrics. Electronic polarization is induced in a dielectric material when subjected to an external electric field, where the electron cloud is displaced relative to the nucleus. This type of polarization may be stimulated in all dielectrics when they are placed under an external electric field. In ionic polarization, the cations and anions in an atom are displaced in opposite directions, resulting in a net dipole moment. The magnitude of the dipole moment generated for each ion pair is equivalent to the product of relative displacement and respective ionic charges. This type of polarization occurs only in ionic materials. Orientation polarization occurs in materials that possess permanent dipole moments, and it is induced when these permanent dipoles align in the direction of an applied electric field. This type of polarization decreases with an increase of temperature [25].

Another polarization characteristic of the multicomponent dielectric systems (such as semicrystalline polymers, inorganic-organic nanocomposites, and polymer blends) is Maxwell-Wagner-Sillars (MWS) interfacial polarization (IP), which is associated with the reorganization of the interface charges, i.e., electrons and hole accumulated at the interfaces in a heterogeneous system. According to the MWS effect, when an electric field is applied to a heterogeneous dielectric system, the charges accumulate on the surfaces of the different dielectrics exhibiting different retention time. Such a configuration is desirable in achieving a high dielectric constant in the system. However, a drawback to this inhomogeneous distribution of charge in a multicomponent system is that the whole system may fail, therefore, it is important to optimize all the parameters that will define the final properties of the system for better performance [3].

## **3. Filler materials/polymers**

materials exhibit zero polarization, which is called coercive field (*Ec*). Dielectric properties of ferroelectric materials change significantly with temperature near *Tc*. Generally, magnitude of *k* increases with an increasing temperature below *Tc*, reaching a maximum value and then decreases with a further increase in temperature, for example, barium titanate *k* ≈ 2000 at room temperature reaches 7000 at *Tc*(~120°C) [19, 20]. Due to the high dielectric constant of ceramic materials, they are deemed as an efficient filler material for energy storage applications.

An ideal dielectric material resists the flow of charge completely; only allowing displacement of charges, resulting in polarization. In a typical capacitor arraignment, when an alternating *E* is applied to an ideal dielectric, the current will carry the voltage by a phase angle of π/2

dielectric loss (tan *δ* = 0). On the other hand, the practical materials exhibit tan *δ*, and it is

lagging behind. The angle and magnitude of the lag are defined as *δ* and tan *δ*, respectively. Dielectric losses are associated with various mechanisms occurring in materials, such as electronic polarization, ion vibration, deformation, and ion migration. Generally, dielectric loss in ceramics is due to ion migration. Temperature and frequency are important factors that influence tan *δ* [21]. Compared to dielectric ceramics, the low magnitudes of *k* and tan *δ* in polymer dielectrics are attractive for transistor applications and they can be used in the form of thin layers (nanometer layers) as they exhibit low leakage current and high breakdown strength, which contributes to the miniaturization of electrical devices. **Table 1** summarizes the breakdown strength of some common polymers. Pure polymers are not suitable for high

**Polymer Dielectric strength (V/μm) Polymer Dielectric strength (V/μm)**

High dielectric constants in ceramics originated from ionic polarization, which is associated with the asymmetric migration of the central metal cation in the crystal lattice. In contrast, polymers exhibit various types of polarization, i.e., ionic, electronic, and orientation, that are responsible for their dielectric constants [23, 24]. Typically, depending on the nature of the material and applied field, at least one kind of polarization mechanism is present in dielectrics. Electronic polarization is induced in a dielectric material when subjected to an external electric field, where the electron cloud is displaced relative to the nucleus. This type of polarization may be stimulated in all dielectrics when they are placed under an external electric field. In ionic polarization, the cations and anions in an atom are displaced in opposite directions,

because the current-voltage phase angle is not exactly 90o

energy density applications because of their low dielectric constant.

Polyethylene (LD) 200 Polypropylene (biaxially oriented) 200 Polyethylene (HD) 200 Polystyrene 200 Polyethylene (XL) 220 Poly(vinylidene fluoride) 10.2 Polycarbonate 252 Polyester 300 Polyimide 280 Epoxy resin 25–45

**Table 1.** Dielectric strength of commonly used polymers [22].

), where no power will be absorbed by the dielectric, and the capacitor will exhibit

; therefore, the current is slightly

(i.e., 90o

26 Properties and Applications of Polymer Dielectrics

The dielectric properties of polymer nanocomposites are affected by various factors such as the type, size, concentration, and shape of the filler materials and polymer matrix. For high energy density applications, the semiconductive fillers such as titanium dioxide (TiO2), zinc oxide (ZnO), molybdenum sulfide (MoS2), and silicon carbide (SiC) have been studied. These fillers have shown considerable promise as candidates for high voltage applications in polymer dielectrics [26]. Among these, TiO2 presents excellent physiochemical properties and it can be crystallized in the form of rutile, anatase, and brookite structures. Tomara et al. [27] reported the processing of anatase TiO2-epoxy resin nanocomposites. Various concentrations from 3 to 12% parts per hundred resins per weight were investigated. The broadband dielectric spectroscopy measurements showed five relaxation modes designated as *γ*, β, intermediate dipolar effect (IDE), *α*, and interfacial polarization (IP) with varying temperatures and constant frequencies. The *γ* and β modes were associated with the reorientation of smaller segments and rearrangement of the polarized groups. The IDE, *α* and IP were attributed to the TiO2 inclusions, glass to rubber transitions, and electric heterogeneity of the nanocomposites, respectively. The TiO2-polymer composites were reported to have a superior dielectric response compared to pure polymers [27]. Xie et al. [28] prepared core-shell structured hyperbranched aromatic polyimide grafted BaTiO3 (BT-HBP) hybrid nanofillers. Poly(vinylideneflouride)-trifluoroethylene-chlorofluoethylene(PVDF-TrFE-CFE) was used as the polymer matrix. The dielectric study showed that the 40 vol% of BT-HBP had a high dielectric constant (1485.5 at 1000 Hz) compared to untreated BaTiO3, which was recorded as 206.3 [28]. To further investigate and optimize the dielectric and energy storage properties of polymer nanocomposites, dielectric ceramics with perovskite structures are used as filler materials. **Table 2** presents various ceramic fillers and polymers with respective dielectric constants.


\*PLZT, lead lanthanum zirconium titanate.

**Table 2.** Dielectric constant of commonly used ceramics and polymers for energy storage applications [22].

The problem associated with perovskite ceramics as fillers in polymer matrix is the low breakdown strength of the final system.Due to higher dielectric constant of the ceramic materials than polymers, a significant electric field is concentrated in the polymer matrix compared to the ceramic nanoparticles. This behavior is responsible for the decrease of breakdown strength in the ceramic-polymer nanocomposite systems. To address this issue, dielectric fillers with large aspect ratios, such as (1D) BaTiO3, hexagonal boron nitride nanosheets, and (2D) clay nanosheets are used. These strategies are the tradeoff between the permittivity values and dielectric breakdown. These systems have shown high magnitudes of breakdown strength that may be due to the presence of order traps, an increase in the number of scattering traps to injected charges, and the path tortuosity in the electrical treeing process during breakdown [29]. Some recently used ceramic-polymer composites with their corresponding electrical properties are summarized in **Table 3**.


\*nf, nanofibers; np, nanoparticles; nc, nanocubes.

To further investigate and optimize the dielectric and energy storage properties of polymer nanocomposites, dielectric ceramics with perovskite structures are used as filler materials. **Table 2** presents various ceramic fillers and polymers with respective dielectric constants.

**Fillers** *k* **Polymers** *k* BaTiO3 1700 Nonfluorinated aromatic polyimides 3.2–3.6 PLZT (7/60/40) 2590 Fluorinated polyimide 2.6–2.8 PbNb2O6 225 Poly(phenyl quinoxaline) 2.8 SrTiO3 2000 Poly(arylene ether oxazole) 2.6–2.8 CaCu3Ti4O12 ~60,000 Poly(arylene ether) 2.9 La1.8Sr0.2NiO4 ~100,000 Polyquinoline 2.8 TiO2 80 Silsesquioxane 2.8–3.0 ZrO2 25 Poly(norborene) 2.4 SiO2 3.9 Perfluorocyclobutane polyether 2.4 Al2O3 9 Fluorinated poly(arylene ether) 2.7 Ta2O5 22 Polynaphthalene 2.2 HfO2 25 Poly(tetrafluoroethylene) 1.9 HfSiO4 11 Polystyrene 2.6 Y2O3 15 Poly(vinylidene fluoride-co-hexafluoropropylene) ~12 La2O3 30 Poly(ether ketone ketone) ~3.5

**Table 2.** Dielectric constant of commonly used ceramics and polymers for energy storage applications [22].

sponding electrical properties are summarized in **Table 3**.

The problem associated with perovskite ceramics as fillers in polymer matrix is the low breakdown strength of the final system.Due to higher dielectric constant of the ceramic materials than polymers, a significant electric field is concentrated in the polymer matrix compared to the ceramic nanoparticles. This behavior is responsible for the decrease of breakdown strength in the ceramic-polymer nanocomposite systems. To address this issue, dielectric fillers with large aspect ratios, such as (1D) BaTiO3, hexagonal boron nitride nanosheets, and (2D) clay nanosheets are used. These strategies are the tradeoff between the permittivity values and dielectric breakdown. These systems have shown high magnitudes of breakdown strength that may be due to the presence of order traps, an increase in the number of scattering traps to injected charges, and the path tortuosity in the electrical treeing process during breakdown [29]. Some recently used ceramic-polymer composites with their corre-

\*PLZT, lead lanthanum zirconium titanate.

28 Properties and Applications of Polymer Dielectrics

**Table 3.** Energy density and breakdown strength of the ceramic-polymer nanocomposites recently reported in the literature.

Another complication associated with the nanocomposites is the high dielectric loss. To reduce the dielectric loss, a dielectric layer coated with conductive nanoparticles is artificially synthesized. Such efforts have been reported to be successful, however, a decrease in the dielectric constant was also observed. Moreover, the processing of such systems may cause conductive networks or tunneling effect between the conductive nanoparticles coated on the surface of the dielectric layer, which results in high dielectric loss and high current leakage. One method to overcoming this problem is to distribute the fillers in the polymer matrix homogeneously in order to avoid contact between the filler particles [38]. Another problem that may cause high current leakage, high dielectric loss, low dielectric constant, and low breakdown strength are the compatibility issues between the inorganic and organic nanocomposites. To resolve this problem, "all polymer" approach has been introduced, where a dielectric polymer such as PVDF and its derivatives are used in the form of polymer blends. These attempts have shown superior dielectric properties; however, these polymer blends are known to cause high dielectric loss. To further modify these systems, a "percolative" dielectric composite approach has been developed. In this method, a conductive organic domain is dispersed in a dielectric polymer matrix. The space charge accumulates at the interface of the conductive organic and dielectric domains. A high dielectric response is achieved because the conductive domain acts as the "super dipole", resulting in high polarization. The magnitude of the dielectric constant reaches a maximum, with an increasing volume fraction of the organic domain, up to just below the percolation threshold. All polymer systems have the advantage over inorganic-organic polymer composites, such as easy processing, economical, and light weight; however, such systems also suffer from high leakage current and low breakdown strength [39].

The extensive studies that have been reported for various filler materials in the polymer matrix only highlight the advantages of the final systems. It is important to critically analyze the underlining physics and chemistry of the filler materials, polymer matrix, and the dielectric response of the multicomponent systems. An essential aspect of the reported results is the reproducibility for practical device performance.

## **4. Interfacial chemistry**

The dielectric constant of the polymer nanocomposites can be increased by loading a high concentration of the dielectric constant fillers. However, high constant, i.e., BT (60%) leads to agglomeration of the filler particles in the polymer matrix, which results in pore formation and subsequently low dielectric constant and high dielectric loss. In order to homogeneously load high concentration of the dielectric fillers, the interfacial chemistry between the filler particles and the polymer matrix must be optimized. Furthermore, the intergranular interaction should also be investigated for homogeneous distribution and conglomerates formed in the polymer matrix. Generally, the interfacial polarization is considered as the primary polarization mechanism in the polymer nanocomposites. The energy density capacity of the polymer nanocomposites can be increased by modifying the interfacial chemistry between the filler materials. By applying this technique, high magnitudes of dielectric constant and breakdown strength can be achieved. Therefore, for better performance of the device, surface interaction between the filler materials itself and the filler-polymer matrix is important [29]. To do so, it is vital to understand the affinity between the polymer matrix and filler particles.

One strategy for addressing this issue is modifying the surface of the filler materials. The surface modifier consists of two major components: the first one is the functional group that attaches the modifier to the filler particle's surface such as, −PO4 3−, −OH, −SO3H− , −SO3 2−, −COOH, −NR3 z+, and –Coo− ; and the second component is the macromolecular chain, solvable and dispersible in different media such as polyether, polyester, polyolefin, and polyacrylate. For example, phosphoric acid and silane coupling agents are used for surface modification of the BT nanoparticles. Moreover, the molecular structure of the surface modifier must be given considerable attention, making sure it is similar to the polymer matrix; otherwise it will result in the formation of pores, voids, and cracks in the polymer nanocomposites. The problem of high leakage current and dielectric loss associated with the silane coupling agent is due to the unabsorbed residues species in the final system [40, 41]. Niu et al. [42] studied the relationship between the surface modifier structure and BT-PVDF nanocomposites for energy storage applications. The schematic surface modification, composites formation, and energy density graph are illustrated in **Figure 1**.

They reported the use of four types of modifiers which belong to the carboxylic acids, i.e., 2, 3, 4, 5-tetrafluorobenzoic acid, 4-(trifluoromethyl) phthalic acid, tetrafluorophthalic acid, and phthalic acid designated as F4C, F3C2, F4C2, and C2, respectively. The breakdown strength was observed to improve in all four cases. The energy density of the BT/PVDF nanocomposites with F4C and C2 was increased by 35.7 and 37.7%, respectively [42]. Zhou et al. [43] studied the surface hydroxylated BT (h-BT)/PVDF nanocomposites, where crude BT (c-BT) nanoparticles were transferred to h-BT nanoparticles using the H2O2 aqueous solution. Results showed that h-BT presented high thermal stability, low frequency dependent dielectric response, low dielectric loss, and high magnitude of dielectric constant compared to c-BT-based nanocomposites [43]. Mukherjee et al. [44] reported a surface modification of the BT nanoparticles by using the plasma enhanced chemical vapor deposition (CVD) method, where the reactive amine groups were implemented as surface modification agents. The final modified nanoparticles were treated with epoxide monomers to form nanocomposites. A comparison between the modified BT/epoxide and the unmodified BT/epoxide nanocomposites showed that the former exhibited superior thermal and electrical properties to the latter [44].

weight; however, such systems also suffer from high leakage current and low breakdown

The extensive studies that have been reported for various filler materials in the polymer matrix only highlight the advantages of the final systems. It is important to critically analyze the underlining physics and chemistry of the filler materials, polymer matrix, and the dielectric response of the multicomponent systems. An essential aspect of the reported results is the

The dielectric constant of the polymer nanocomposites can be increased by loading a high concentration of the dielectric constant fillers. However, high constant, i.e., BT (60%) leads to agglomeration of the filler particles in the polymer matrix, which results in pore formation and subsequently low dielectric constant and high dielectric loss. In order to homogeneously load high concentration of the dielectric fillers, the interfacial chemistry between the filler particles and the polymer matrix must be optimized. Furthermore, the intergranular interaction should also be investigated for homogeneous distribution and conglomerates formed in the polymer matrix. Generally, the interfacial polarization is considered as the primary polarization mechanism in the polymer nanocomposites. The energy density capacity of the polymer nanocomposites can be increased by modifying the interfacial chemistry between the filler materials. By applying this technique, high magnitudes of dielectric constant and breakdown strength can be achieved. Therefore, for better performance of the device, surface interaction between the filler materials itself and the filler-polymer matrix is important [29]. To do so, it is vital to understand the affinity between the polymer matrix and filler

One strategy for addressing this issue is modifying the surface of the filler materials. The surface modifier consists of two major components: the first one is the functional group that

and dispersible in different media such as polyether, polyester, polyolefin, and polyacrylate. For example, phosphoric acid and silane coupling agents are used for surface modification of the BT nanoparticles. Moreover, the molecular structure of the surface modifier must be given considerable attention, making sure it is similar to the polymer matrix; otherwise it will result in the formation of pores, voids, and cracks in the polymer nanocomposites. The problem of high leakage current and dielectric loss associated with the silane coupling agent is due to the unabsorbed residues species in the final system [40, 41]. Niu et al. [42] studied the relationship between the surface modifier structure and BT-PVDF nanocomposites for energy storage applications. The schematic surface modification, composites formation, and energy density

They reported the use of four types of modifiers which belong to the carboxylic acids, i.e., 2, 3, 4, 5-tetrafluorobenzoic acid, 4-(trifluoromethyl) phthalic acid, tetrafluorophthalic acid, and

; and the second component is the macromolecular chain, solvable

3−, −OH, −SO3H−

, −SO3 2−,

attaches the modifier to the filler particle's surface such as, −PO4

strength [39].

particles.

−COOH, −NR3

z+, and –Coo−

graph are illustrated in **Figure 1**.

reproducibility for practical device performance.

**4. Interfacial chemistry**

30 Properties and Applications of Polymer Dielectrics

**Figure 1.** Scheme process of nanoparticle modification and incorporation into the polymer matrix and discharged energy density of the nanocomposites filled with BT and m-BT nanoparticles [42].

Surface modifications of the filler material are important as it decreases the probability of contact between the filler materials, and it improves the compatibility between the fillers and the polymer matrix. Through this procedure, the properties of the polymer nanocomposites can be enhanced for energy storage applications.

## **5. Processing strategies**

The dielectric and energy storage properties of the polymer nanocomposites can also be tuned through the preparation methods. The processing techniques not only include the composite formation, but also the synthesis of the filler materials in various sizes and shapes. This section only summarizes the methods for the formation of filler-polymer nanocomposites. The most adaptable synthesis approach for the processing of the polymer composites in the recent literature is liquid-phase assisted dispersion. In this method, the filler materials and the polymer are mixed in a desired solvent, followed by stirring for a long duration, i.e., 24 h for homogeneous mixing. The mixture is then transferred into a glass substrate, where the thickness of the film is maintained. The film is further dried at various desired temperatures and times in a vacuum/air atmosphere to remove the solvent, then annealed at desired temperatures, i.e., BT@TiO2-nanofibers-P (VDF-HFP) at 200°C/7 min and subsequently quenched in an ice bath [45]. Another approach is the solution casting method, where a polymer is dissolved in a suitable solvent and a predetermined weight-percent of the filler material is added to the solution. The solution is thoroughly mixed using an ultrasonic bath or a magnetic stirrer, and then spin coated on a glass substrate. The process is repeated until the desired thickness is achieved, then the film is annealed at some low temperature (~80°C) for crystallization, and the solvent is removed by thermal dehydration in an oven.

Xu et al. [46] processed a three-phase system using polyimide (PI), BT, and multiwall carbon nanotubes (MWCNTs) by the electrospinning technique. MWCNTs were homogeneously dispersed in the PI matrix and subsequently collected by using the BT particles. The solution was subjected to electrospinning using 20 kV/20 cm from the spinneret to the collector, keeping the flow rate of 1.2 mL/h at room temperature. The resultant nanofibers were treated with heat, at 70°C/6h (vacuum), 250°C/1 h in N2. The composite nanofibers were further treated at 5 MPa pressure at 300°C in a vacuum for 5 min using hot-pressing techniques. A high dielectric constant of 1061.98 (at 100 Hz) and an energy density of 4.773 J/cm3 were recorded for the composition with 40 vol% BTNPs/10 vol% MWCNTs [46]. Zhang et al. [47] reported a novel strategy represented in **Figure 2** for the processing of the BT and PVDF nanocomposites. Nanofibers were synthesized using electrospinning, and the surfaces were modified using 1H,1H,2H,2H-perfluorooctyl trimethoxysilane. Finally, the modified BT and PVDF nanocomposites were processed using solution blending, where the dielectric properties of the final composites were improved. In addition, this technique has an advantage over the grafting method for processing the nanocomposites due to its low cost and simple procedure [47].

**Figure 2.** Schematic illustration of preparation of the nanocomposites of PVDF and fluorosilane-modified BT nanofibers [47].

## **6. Future trends**

Surface modifications of the filler material are important as it decreases the probability of contact between the filler materials, and it improves the compatibility between the fillers and the polymer matrix. Through this procedure, the properties of the polymer nanocomposites

The dielectric and energy storage properties of the polymer nanocomposites can also be tuned through the preparation methods. The processing techniques not only include the composite formation, but also the synthesis of the filler materials in various sizes and shapes. This section only summarizes the methods for the formation of filler-polymer nanocomposites. The most adaptable synthesis approach for the processing of the polymer composites in the recent literature is liquid-phase assisted dispersion. In this method, the filler materials and the polymer are mixed in a desired solvent, followed by stirring for a long duration, i.e., 24 h for homogeneous mixing. The mixture is then transferred into a glass substrate, where the thickness of the film is maintained. The film is further dried at various desired temperatures and times in a vacuum/air atmosphere to remove the solvent, then annealed at desired temperatures, i.e., BT@TiO2-nanofibers-P (VDF-HFP) at 200°C/7 min and subsequently quenched in an ice bath [45]. Another approach is the solution casting method, where a polymer is dissolved in a suitable solvent and a predetermined weight-percent of the filler material is added to the solution. The solution is thoroughly mixed using an ultrasonic bath or a magnetic stirrer, and then spin coated on a glass substrate. The process is repeated until the desired thickness is achieved, then the film is annealed at some low temperature (~80°C) for crystallization, and the solvent is removed by thermal dehydration

Xu et al. [46] processed a three-phase system using polyimide (PI), BT, and multiwall carbon nanotubes (MWCNTs) by the electrospinning technique. MWCNTs were homogeneously dispersed in the PI matrix and subsequently collected by using the BT particles. The solution was subjected to electrospinning using 20 kV/20 cm from the spinneret to the collector, keeping the flow rate of 1.2 mL/h at room temperature. The resultant nanofibers were treated with heat, at 70°C/6h (vacuum), 250°C/1 h in N2. The composite nanofibers were further treated at 5 MPa pressure at 300°C in a vacuum for 5 min using hot-pressing techniques. A high dielectric constant of 1061.98 (at 100 Hz) and an energy density of 4.773 J/cm3

recorded for the composition with 40 vol% BTNPs/10 vol% MWCNTs [46]. Zhang et al. [47] reported a novel strategy represented in **Figure 2** for the processing of the BT and PVDF nanocomposites. Nanofibers were synthesized using electrospinning, and the surfaces were modified using 1H,1H,2H,2H-perfluorooctyl trimethoxysilane. Finally, the modified BT and PVDF nanocomposites were processed using solution blending, where the dielectric properties of the final composites were improved. In addition, this technique has an advantage over the grafting method for processing the nanocomposites due to its low cost and simple

were

can be enhanced for energy storage applications.

**5. Processing strategies**

32 Properties and Applications of Polymer Dielectrics

in an oven.

procedure [47].

To further enhance the dielectric response and physiochemical stability of the high-*k* dielectric nanocomposites is the fabrication of high aspect ratio filler materials. Tang et al. [48] studied the effect of different aspect ratios of the BT nanowires on the dielectric properties of the BT/PVDF nanocomposites. The aspect ratios of the nanowires were tuned by controlling the temperature during hydrothermal synthesis, where they were varied from 9.3 to 45.8 corresponding to the temperature ranges from 150 to 240°C, respectively. A direct relationship was observed with the aspect ratio of nanocomposites and dielectric constant. A high dielectric constant of 44.3 was reported for the BT-30 vol% nanocomposites with an aspect ratio of 45.8, which was 30.7% higher than BT samples with an aspect ratio of 9.3 while 352% higher than polymer matrix samples [48]. Despite the high dielectric constant fillers for high dielectric nanocomposite applications, such as BT and PbZrTiO3-based derivatives, these filler materials still suffer from some disadvantages, which include low dielectric response even with high loading concentration, low breakdown strength, and deteriorated flexibility. Furthermore, lead-based compounds are toxic in nature and are expected to be banned in the near future. An alternative to ceramic fillers is the graphenebased nanocomposites for high-energy and ultracapacitor applications. Graphene is a one atom thick sheet of carbon, in which carbon atoms exhibit sp2 hybridization which correspond to excellent electron mobility, mechanothermal stability, and high flexibility. Graphene oxide (GO) can be easily dispersed in the aqueous solution and the electrical properties can be optimized by using various thermal and chemical approaches such as partial reduction. Thus, the GO/polymer nanocomposites can prove an efficient system for high dielectric applications [49, 50]. Recently, attention has been given to the carbon nanotubes (CNTs), a crystalline form of carbon because of their unique physical and mechanical properties. CNTs are considered to be potential candidates for the formation of nanocomposites, which can improve their electrical, thermal, and mechanical properties [51]. Although extensive studies have reported the dielectric properties of the nanocomposites, very few investigate the dielectric properties vs. temperature [52]. These properties must be thoroughly investigated under mechanical and dielectric thermal stability for practical device applications. Recent advances in the computational material science, physics, and chemistry have developed sophisticated algorithms for predicting the physical and chemical properties of the system. These methods and molecular dynamic techniques must be utilized to design a more reliable and comprehensive system for energy storage applications.

## **7. Conclusions**

For high-energy-density applications, materials should exhibit the high-energy storage capacity. For this purpose, materials should demonstrate high dielectric permittivity, low leakage current, low dielectric loss, and high breakdown strength. All these parameters are crucial for implementing laboratorial research in industrial processes. Furthermore, an economical device fabrication strategy should be adopted for practical and commercial applications; however, a single material does not ideally express all these properties as one system; therefore, a tradeoff is used for device performance between the material properties for different applications. From previous discussion, it has been established that the energy storage capability of the dielectric nanocomposites is strongly influenced by the choice of filler materials, shape and size, morphology, processing methods, use of surfactants or surface modifying agents, and polymer matrix. For better storage applications, all these parameters must be optimized and new strategies should be developed for better processing and understanding of the device fabrications.

## **Author details**

Asad Mahmood\* , Abdul Naeem and Tahira Mahmood

\*Address all correspondence to: naeem@upesh.edu.pk

National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan

## **References**

partial reduction. Thus, the GO/polymer nanocomposites can prove an efficient system for high dielectric applications [49, 50]. Recently, attention has been given to the carbon nanotubes (CNTs), a crystalline form of carbon because of their unique physical and mechanical properties. CNTs are considered to be potential candidates for the formation of nanocomposites, which can improve their electrical, thermal, and mechanical properties [51]. Although extensive studies have reported the dielectric properties of the nanocomposites, very few investigate the dielectric properties vs. temperature [52]. These properties must be thoroughly investigated under mechanical and dielectric thermal stability for practical device applications. Recent advances in the computational material science, physics, and chemistry have developed sophisticated algorithms for predicting the physical and chemical properties of the system. These methods and molecular dynamic techniques must be utilized to design a more reliable and comprehensive system for energy storage applica-

For high-energy-density applications, materials should exhibit the high-energy storage capacity. For this purpose, materials should demonstrate high dielectric permittivity, low leakage current, low dielectric loss, and high breakdown strength. All these parameters are crucial for implementing laboratorial research in industrial processes. Furthermore, an economical device fabrication strategy should be adopted for practical and commercial applications; however, a single material does not ideally express all these properties as one system; therefore, a tradeoff is used for device performance between the material properties for different applications. From previous discussion, it has been established that the energy storage capability of the dielectric nanocomposites is strongly influenced by the choice of filler materials, shape and size, morphology, processing methods, use of surfactants or surface modifying agents, and polymer matrix. For better storage applications, all these parameters must be optimized and new strategies should be developed for better processing and under-

tions.

**7. Conclusions**

34 Properties and Applications of Polymer Dielectrics

standing of the device fabrications.

, Abdul Naeem and Tahira Mahmood

National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar,

\*Address all correspondence to: naeem@upesh.edu.pk

**Author details**

Asad Mahmood\*

Pakistan


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