Structure property relations in a range of mesophase pitch-based carbon fibres

The purpose of this work was to study the structure and compressive behaviour of mesophase pitch (MP) -based carbon fibres, in an attempt to understand the relations between fibre structure and compressive strength. For this purpose, a range of Du Pont MP-based carbon fibres were used, including high modulus fibres (E35, E55, E75, E105, E120 and E130) and high thermal conductivity fibres (E35C, C700).Compressive strengths of the fibres were measured using a tensile recoil testing method, and a failure mechanism was proposed on the basis of recoil-failure face observations with the scanning electron microscope. It was considered that Du Pont fibres fail due to a combination of tensile and compressive forces arising from bending. Both macrostructure and microstructure of the fibres were investigated using X-ray diffraction, scanning and transmission electron microscopy. The macrostructure is of folded pseudo-radial layers arranged in domains which become more perfect and singular with increasing Young’s modulus. Moreover, high thermal conductivity fibres have both circular and missing-sector cross sections. The microstructure ranges from turbostratic to three-dimensional graphitic order with different degrees of imperfection as observed directly in dark-field and lattice-fringe micrographs. For the first time, image analysis techniques were applied to TEM micrographs (mainly dark-field and lattice-fringe images) in an attempt to determine quantitatively the degree of lattice imperfection as well as other structural parameters. Following the analysis of images, new parameters to measure lattice imperfection have been introduced, particularly the crystallite imperfection factor (1C) and lattice tortuosity (S). The influence of both macrostructure and microstructure on the fibre compressive strength has been discussed thoroughly. It was found that increases in the lattice imperfections (both in-plane imperfection and stacking imperfection) increase the fibre compressive strength significantly. In addition, the layer stacking size and crystallite orientation should be limited to a relatively lower level in order to achieve a good compressive performance of carbon fibres. With regards to the effect of macrostructure, generally fibres with folded layers and domain structures tend to have a higher compressive strength than fibres with flat layers and uniform structure.