**2.1 Fabrication of CNTs**

The preparation methods of CNTs include chemical vapor deposition (CVD), arc discharge (AD), and laser ablation (LA) [26]. CVD is the most commonly used method to prepare CNTs in the laboratory. Generally, CNTs are grown under the action of the catalyst after carbon source cracking at a certain temperature. This method has a series of advantages, such as simple equipment, fast preparation speed, large output, and controllable quality. The catalysts are generally transition metals such as iron, cobalt, and nickel, and the carbon sources are generally carbon-containing organics such as methane, ethylene, acetylene, ethanol, and xylene. The morphology (diameter, wall layer, length, density, curvature, crystallinity, etc.) of CNTs can be tuned by controlling the type and concentration of catalyst, the ratio of carbon source and injection speed, the temperature, pressure, and time of CVD [27–30].

#### **Figure 1.**

*Schematic diagrams of fullerene single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) [19].*

**65**

180 F · g<sup>−</sup><sup>1</sup>

*Fiber Composites Made of Low-Dimensional Carbon Materials*

The AD method is also the main method to produce CNTs. Usually, in the lowpressure arc chamber of inert gas, hydrogen, or other gases, the graphite material is used as the electrode to generate a continuous arc between the electrodes, which makes the graphite react with the catalyst to generate CNTs. The AD method has a

Laser ablation is a method to prepare CNTs by bombarding the surface of graphite target doped with iron, cobalt, nickel, and other transition metals in an inert gas environment at 1200°C [33]. The advantage of this method is that the CNTs produced are of high purity and convenient for continuous production, but this method is not suitable for large-scale macro production due to its high energy consumption, complex equipment, and high preparation cost [34, 35]. In addition to the above three main preparation methods, CNTs can also be prepared by template method, flame method, solar energy method, and electrolytic alkali

Because of its special tubular structure and the strong binding force between

show good mechanical strength and fatigue resistance [10, 41].

the defect content, which leads to more phonon scattering.

of SWNTs can reach 240–1250 m<sup>2</sup>

a high specific surface area (about 430 m<sup>2</sup>

, a power density of 8 kW · kg<sup>−</sup><sup>1</sup>

capacitance, 20 kW · kg<sup>−</sup><sup>1</sup>

The carbon atoms in CNTs are arranged in a six-membered ring network structure, which is very conducive to phonon vibration. Therefore, CNTs have good thermal conductivity. Due to the anisotropy of the structure, the thermal conductivity of CNTs along the length direction is much higher than that in the vertical direction. Theoretically, the thermal conductivity of SWCNTs can reach 10,000 W/mK at room temperature. Due to the presence of impurities, the highest experimental values of SWCNTs and MWCNTs are 3500 and 3000 W/mK, respectively [42–44]. Theoretical calculation and experimental results show that with the increase of CNT diameter, the thermal conductivity of CNTs shows a downward trend (**Figure 2**) [45]. This is because the increase of diameter inevitably increases

CNTs are widely used in various electronic devices due to their high conductivity and chemical stability [46]. For SWCNTs, the specific surface area

the same time, high-temperature heat treatment can reduce the electrode impedance and increase the specific capacitance of SWNTs. The increase of capacitance is considered to be caused by the increase of specific surface area and a large number of 3–5 nm pore distribution [47, 48]. For MWCNTs, they usually have

CNTs of different shapes (such as direct growth, porous, array, and crimp) have

power density, and 6.5–7 Wh · kg<sup>−</sup><sup>1</sup>

· g<sup>−</sup><sup>1</sup>

, which can generate 180 F · g<sup>−</sup><sup>1</sup>

specific

.

energy density. At

), a specific capacitance of up to

, and an energy density of 0.56 Wh · kg<sup>−</sup><sup>1</sup>

· g<sup>−</sup><sup>1</sup>

 hybrid carbon atoms, CNTs have high strength, fracture toughness, and elastic modulus, which are superior to any one-dimensional fiber [37]. The tensile strength of CNTs can reach 50–800 GPa, nearly 100 times of that standard steel, about 200 times higher than that of other polymer fibers, and its structure can be kept intact under 1 million atmospheric pressure. CNTs will not break obviously under large bending, while graphite fiber will break when bending 1% (volume fraction). The maximum elastic modulus of CNTs is 1 TPa, which is equivalent to that of diamond and about five times to that of steel. Due to defects, the actual elastic modulus of MWCNTs is in the range of 20–50 GPa [38–40]. Fiber is usually used to strengthen composite materials. In addition to its own strength, a high aspect ratio (>20) is also a key factor to obtain high-strength composite materials. The aspect ratio of CNTs is generally >1000. Therefore, through CNT-reinforced composite materials, it can

high yield, and the CNT's crystal structure is relatively complete [31, 32].

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

metal halide method [36].

**2.2 Properties of CNTs**

SP2

*Fiber Composites Made of Low-Dimensional Carbon Materials DOI: http://dx.doi.org/10.5772/intechopen.92092*

The AD method is also the main method to produce CNTs. Usually, in the lowpressure arc chamber of inert gas, hydrogen, or other gases, the graphite material is used as the electrode to generate a continuous arc between the electrodes, which makes the graphite react with the catalyst to generate CNTs. The AD method has a high yield, and the CNT's crystal structure is relatively complete [31, 32].

Laser ablation is a method to prepare CNTs by bombarding the surface of graphite target doped with iron, cobalt, nickel, and other transition metals in an inert gas environment at 1200°C [33]. The advantage of this method is that the CNTs produced are of high purity and convenient for continuous production, but this method is not suitable for large-scale macro production due to its high energy consumption, complex equipment, and high preparation cost [34, 35]. In addition to the above three main preparation methods, CNTs can also be prepared by template method, flame method, solar energy method, and electrolytic alkali metal halide method [36].

### **2.2 Properties of CNTs**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

are summarized.

material made of SP<sup>2</sup>

**2.1 Fabrication of CNTs**

**2. Carbon nanotubes (CNTs)**

CF, and GBFs, which are commonly used to assemble macro carbon composites. The preparation methods of GBFs and their composite fibers, as well as their applications in sensors, energy storage, energy conversion, and other aspects, such as supercapacitors, lithium-ion batteries (LIBs), actuators, and solar cells, are mainly introduced. Finally, the existing problems and future development of carbon-matrix composites

CNTs were first discovered under TEM in 1991. It is a one-dimensional tubular

meters to tens of nanometers, and its length can reach centimeter-level at most. According to the wall layer, it can be divided into single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) (**Figure 1**). It is the most commercialized nanofiber with the highest strength and the smallest diameter [19–21]. Moreover, CNTs have good toughness, which can withstand 40% of tensile strain without brittle behavior or fracture phenomenon, thus improving the toughness of matrix composite [22]. CNTs with super high aspect ratio and excellent mechanical and physical properties, such as high strength, high thermal conductivity, high conductivity, and low thermal expansion coefficient, are regarded as the ideal functional

modifier for preparing high-performance composite materials [23–25].

The preparation methods of CNTs include chemical vapor deposition (CVD), arc discharge (AD), and laser ablation (LA) [26]. CVD is the most commonly used method to prepare CNTs in the laboratory. Generally, CNTs are grown under the action of the catalyst after carbon source cracking at a certain temperature. This method has a series of advantages, such as simple equipment, fast preparation speed, large output, and controllable quality. The catalysts are generally transition metals such as iron, cobalt, and nickel, and the carbon sources are generally carbon-containing organics such as methane, ethylene, acetylene, ethanol, and xylene. The morphology (diameter, wall layer, length, density, curvature, crystallinity, etc.) of CNTs can be tuned by controlling the type and concentration of catalyst, the ratio of carbon source and injection speed, the temperature, pressure, and time of CVD [27–30].

*Schematic diagrams of fullerene single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube* 

hybrid carbon atoms. Its diameter ranges from several nano-

**64**

**Figure 1.**

*(MWCNT) [19].*

Because of its special tubular structure and the strong binding force between SP2 hybrid carbon atoms, CNTs have high strength, fracture toughness, and elastic modulus, which are superior to any one-dimensional fiber [37]. The tensile strength of CNTs can reach 50–800 GPa, nearly 100 times of that standard steel, about 200 times higher than that of other polymer fibers, and its structure can be kept intact under 1 million atmospheric pressure. CNTs will not break obviously under large bending, while graphite fiber will break when bending 1% (volume fraction). The maximum elastic modulus of CNTs is 1 TPa, which is equivalent to that of diamond and about five times to that of steel. Due to defects, the actual elastic modulus of MWCNTs is in the range of 20–50 GPa [38–40]. Fiber is usually used to strengthen composite materials. In addition to its own strength, a high aspect ratio (>20) is also a key factor to obtain high-strength composite materials. The aspect ratio of CNTs is generally >1000. Therefore, through CNT-reinforced composite materials, it can show good mechanical strength and fatigue resistance [10, 41].

The carbon atoms in CNTs are arranged in a six-membered ring network structure, which is very conducive to phonon vibration. Therefore, CNTs have good thermal conductivity. Due to the anisotropy of the structure, the thermal conductivity of CNTs along the length direction is much higher than that in the vertical direction. Theoretically, the thermal conductivity of SWCNTs can reach 10,000 W/mK at room temperature. Due to the presence of impurities, the highest experimental values of SWCNTs and MWCNTs are 3500 and 3000 W/mK, respectively [42–44]. Theoretical calculation and experimental results show that with the increase of CNT diameter, the thermal conductivity of CNTs shows a downward trend (**Figure 2**) [45]. This is because the increase of diameter inevitably increases the defect content, which leads to more phonon scattering.

CNTs are widely used in various electronic devices due to their high conductivity and chemical stability [46]. For SWCNTs, the specific surface area of SWNTs can reach 240–1250 m<sup>2</sup> · g<sup>−</sup><sup>1</sup> , which can generate 180 F · g<sup>−</sup><sup>1</sup> specific capacitance, 20 kW · kg<sup>−</sup><sup>1</sup> power density, and 6.5–7 Wh · kg<sup>−</sup><sup>1</sup> energy density. At the same time, high-temperature heat treatment can reduce the electrode impedance and increase the specific capacitance of SWNTs. The increase of capacitance is considered to be caused by the increase of specific surface area and a large number of 3–5 nm pore distribution [47, 48]. For MWCNTs, they usually have a high specific surface area (about 430 m<sup>2</sup> · g<sup>−</sup><sup>1</sup> ), a specific capacitance of up to 180 F · g<sup>−</sup><sup>1</sup> , a power density of 8 kW · kg<sup>−</sup><sup>1</sup> , and an energy density of 0.56 Wh · kg<sup>−</sup><sup>1</sup> . CNTs of different shapes (such as direct growth, porous, array, and crimp) have

#### **Figure 2.**

*The relationship between thermal conductivity and diameter of CNTs [45].*

been tested as electrodes. The array CNT is the most suitable electrode because of its small internal resistance, good reaction rate, regular gap structure, and stable conductive channel [49–51].
