Effect of amorphous carbon on the tensile behavior of polyacrylonitrile (PAN)-based carbon fibers

The effects of the microstructure evolution of amorphous carbon on the tensile behavior of polyacrylonitrile (PAN)-based carbon fibers were investigated. The microstructure as a function of heat treatment temperature was characterized by means of XRD, HRTEM and Raman spectra. It is found that the amorphous carbon content decreases with increasing heat treatment temperature and that the densities of the carbon fibers increase is due to the removal of the impurity elements and the shrinking of the graphite planes. The amorphous carbon parallel to the graphite planes transforms into graphite planes and stacks on the graphite crystallites, leading to the increase in the graphite crystallite thickness. And the graphite crystallite length is increased through the amorphous-to-crystallite transition which occurs at the edges of graphite planes and the coalescence between two adjacent graphite crystallites. It is found that the tensile behavior of PAN-based carbon fibers mainly depends on the microstructure evolution of amorphous carbon. The reactions between sp² carbon clusters and graphite planes improve the cross-linking among graphite crystallites, which has a positive effect on the tensile strength of the carbon fibers. However, a large number of structural defects and residual stresses, introduced by the rearrangement of graphite planes, are the main reasons for the degradation of the tensile strength. The tensile strains of the carbon fibers decrease and the tensile modulus increase with the decrease in the amorphous carbon content, which are mainly due to the amorphous-to-crystallite transition in the skin region.