New discovery on the relationship between microstructure and tensile strength of PAN-based carbon fibers

Based on the “double Debye” theory, the microvoid and amorphous structure parameters of T and MJ series carbon fibers could be obtained by SAXS analysis. Results indicated that tensile strength of T and MJ series carbon fibers increased with the decrease of microvoid size, and the relationship between them basically followed the Griffith microcrack theory. Importantly, it was found that the fitting result from the Griffith formula which was supplemented by introducing amorphous structure parameters was much better than the reports before, and it indicated that tensile strength of MJ series increased with the increase of amorphous size, but T series showed an opposite trend. It was because that amorphous region in MJ series provided the path for the propagation of crack and consumed more energy, which increased the tensile strength. For T series, nitrogen atoms in amorphous region might cause the distortion and deformation of graphite lamellar, thus introducing more defects. And the effect on tensile strength caused by defects from nitrogen atoms might be much greater than the contribution of crack propagation in amorphous region. Therefore, the decrease of amorphous size might represent the reduction of defects, thus increasing the tensile strength.