A comparison of the effect of graphitization on microstructures and properties of polyacrylonitrile and mesophase pitch-based carbon fibers

Polyacrylonitrile (PAN) and mesophase pitch (MPP)-based carbon fibers were heat treated in the temperature range of 1300–2700 °C. After high-temperature heat treatment (HHT), the microstructures and mechanical properties of PAN and MPP-based carbon fibers were investigated. For both series of carbon fibers, the Young’s modulus increased with heat treatment temperature increasing. The tensile strength of PAN-based carbon fibers decreased, while that of MPP-based carbon fibers increased. After HHT at 2700 °C, the tensile strength of MPP-based carbon fibers exceeded that of PAN-based carbon fibers. The results could be ascribed to the variously original structures and the different routines of structural evolution. The physical entanglements and covalent cross-links of carbon ribbons in PAN-based carbon fibers contributed to a higher shear stress between the graphene layers, however, tended to generate voids and cracks during HHT due to an extensive transformation from turbostratic to ordered structure along with nitrogen removing. For MPP-based carbon fibers, they displayed a radial texture with ordered and parallel packing of layers in the transverse section. Thus, it was easier for the graphene layers to stack and bond to the adjacent ones without strong rotations, leading to fewer voids and cracks.