**3. Carbon fibers**

Carbon fiber is a kind of fiber material with high strength and high modulus. Its carbon content is more than 90%, and CF with carbon content more than 99% is also called graphite fiber, which is mainly composed of disordered graphite microcrystals stacked along the axial direction of the fiber [52]. CF is not only flexible and acid and alkali resistant but also stronger than steel, which makes it an important material for national defense, military industry, and civil use [53].

CF can be classified into polyacrylonitrile-based (PAN-based) CF, asphaltbased CF, viscose-based CF, and gas-phase growth CF according to the source of precursors [52]. As shown in **Figure 3**, according to the basic morphology, it can be divided into filament CF and short CF, wherein filament CF can be woven into twodimensional CF fabric and three-dimensional CF fabric. Based on the mechanical properties, it can be divided into general CF and high-performance CF which can also be divided into high-strength type (strength >2000 MPa) and high model type (modulus >300 GPa) CF. With the rapid development of aerospace, automobile manufacturing, and sports facilities, the performance of CF has been increased, and the outputs have been improved continuously. Currently, the largest amount of polyacrylonitrile-based CF is used in the real world [54].

#### **3.1 Fabrication of CFs**

The industrial production of CF mainly includes polyacrylonitrile-based CF, asphalt-based CF, and viscose-based CF. Among them, the preparation process of viscose-based CF must be graphitized by high-temperature stretching. Because of its complex equipment and technical difficulties, it has not been effectively developed. The production process of polyacrylonitrile-based CF mainly includes two processes:

**67**

**Figure 3.**

**3.2 Properties of CF**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

raw silk production and carbonization. The production process of raw silk mainly consists of polymerization of acrylonitrile monomer, solution defoaming, wire spraying, traction, water washing, oiling, drying, and reeling. Moreover, the carbonization process mainly includes pre-oxidation, low-temperature carbonization, high-temperature carbonization, surface treatment, sizing and drying, winding, and other processes. Note that the pre-oxidation refers to heating the precursor fiber in the air to about 270°C, holding for a period of time, so that the polyacrylonitrile linear polymer will be oxidized, pyrolyzed, cross-linked, and cyclized to form a heatresistant ladder polymer. In order to prevent melting and deformation of polyacrylonitrile fiber during high-temperature carbonization, the color of polyacrylonitrile fiber gradually changes from white to yellow, then brown, and finally black. The pre-oxidized fiber is carbonized in inert gas with high temperature, and then the cross-linking reaction arises further. With the removal of hydrogen, nitrogen, and

The raw material of asphalt-based CF is petroleum asphalt or coal asphalt. The preparation process mainly includes refining, spinning, pre-oxidation, carbonization, or graphitization of asphalt. Among them, mesophase asphalt is a kind of nematic liquid crystal (LC) material composed of disk-shaped or rod-shaped molecules formed by heavy aromatics during heat treatment. The asphalt-based CF prepared by mesophase asphalt is easy to graphitize and usually has a high modulus [52, 55–57].

Due to the carbonization and orientation at high temperatures, the carbon atoms of CF are arranged very closely, and the disordered graphite is closely connected. In addition, the diameter of CF is smaller, which can reduce the content of defects, so

oxygen atoms, CF with disordered graphite structure is formed.

*The pictures of (a) filament CF, (b) short CF, (c) CF cloth, and (d) 3D CF braid.*

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

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

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

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

Carbon fiber is a kind of fiber material with high strength and high modulus. Its carbon content is more than 90%, and CF with carbon content more than 99% is also called graphite fiber, which is mainly composed of disordered graphite microcrystals stacked along the axial direction of the fiber [52]. CF is not only flexible and acid and alkali resistant but also stronger than steel, which makes it an impor-

tant material for national defense, military industry, and civil use [53].

polyacrylonitrile-based CF is used in the real world [54].

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

CF can be classified into polyacrylonitrile-based (PAN-based) CF, asphaltbased CF, viscose-based CF, and gas-phase growth CF according to the source of precursors [52]. As shown in **Figure 3**, according to the basic morphology, it can be divided into filament CF and short CF, wherein filament CF can be woven into twodimensional CF fabric and three-dimensional CF fabric. Based on the mechanical properties, it can be divided into general CF and high-performance CF which can also be divided into high-strength type (strength >2000 MPa) and high model type (modulus >300 GPa) CF. With the rapid development of aerospace, automobile manufacturing, and sports facilities, the performance of CF has been increased, and the outputs have been improved continuously. Currently, the largest amount of

The industrial production of CF mainly includes polyacrylonitrile-based CF, asphalt-based CF, and viscose-based CF. Among them, the preparation process of viscose-based CF must be graphitized by high-temperature stretching. Because of its complex equipment and technical difficulties, it has not been effectively developed. The production process of polyacrylonitrile-based CF mainly includes two processes:

conductive channel [49–51].

**3. Carbon fibers**

**Figure 2.**

**3.1 Fabrication of CFs**

**66**

#### **Figure 3.** *The pictures of (a) filament CF, (b) short CF, (c) CF cloth, and (d) 3D CF braid.*

raw silk production and carbonization. The production process of raw silk mainly consists of polymerization of acrylonitrile monomer, solution defoaming, wire spraying, traction, water washing, oiling, drying, and reeling. Moreover, the carbonization process mainly includes pre-oxidation, low-temperature carbonization, high-temperature carbonization, surface treatment, sizing and drying, winding, and other processes. Note that the pre-oxidation refers to heating the precursor fiber in the air to about 270°C, holding for a period of time, so that the polyacrylonitrile linear polymer will be oxidized, pyrolyzed, cross-linked, and cyclized to form a heatresistant ladder polymer. In order to prevent melting and deformation of polyacrylonitrile fiber during high-temperature carbonization, the color of polyacrylonitrile fiber gradually changes from white to yellow, then brown, and finally black. The pre-oxidized fiber is carbonized in inert gas with high temperature, and then the cross-linking reaction arises further. With the removal of hydrogen, nitrogen, and oxygen atoms, CF with disordered graphite structure is formed.

The raw material of asphalt-based CF is petroleum asphalt or coal asphalt. The preparation process mainly includes refining, spinning, pre-oxidation, carbonization, or graphitization of asphalt. Among them, mesophase asphalt is a kind of nematic liquid crystal (LC) material composed of disk-shaped or rod-shaped molecules formed by heavy aromatics during heat treatment. The asphalt-based CF prepared by mesophase asphalt is easy to graphitize and usually has a high modulus [52, 55–57].
