**3.2 Properties of CF**

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

it has very high mechanical strength and modulus. The tensile strength and modulus of CF can reach 7 and 700 GPa, which are much higher than those of glass fiber and Kevlar fiber. CF can withstand high temperature above 3000°C without contact with air. Therefore, CF has outstanding heat-resistant performance. The higher is the temperature, the greater is the fiber strength. After graphitization, the density of mesophase asphalt-based CF increases, and the carbon content exceeds 99%. Most of the carbon atoms in the fiber form a large area of graphite sheet structure along the fiber axis by SP<sup>2</sup> hybridization, which is very conducive to the phonon vibration. Therefore, the thermal conductivity of the graphite fiber can reach up to 1000 W/mK [45, 58, 59]. It is worth noting that the electrical properties of CF are not ideal, because of the inherent polycrystalline structure and a large number of grain boundaries inevitably formed during the pyrolysis of organic precursors [21].
