We have used UV-Raman spectroscopy to study the sp3 structure in polyacrylonitrile- and pitch-based carbon fibres with Young’s modulus values in the range of 90-830 GPa. The composite microstructure of these carbon fibres comprises an amorphous carbon phase that transfers the load to a reinforcing graphite crystalline phase. The intensity ratio of the T, D and G Raman bands showed that the low modulus polyacrylonitrile-based carbon fibres (Young’s modulus=200-300 GPa) contained 10-25% sp3 carbon, whereas the stiffer polyacrylonitrile-based carbon fibres (Young’s modulus >350 GPa) contained ∼5% sp3 material. This result suggests that the proportion of sp3 carbon decreases as the amorphous structure is converted into a crystalline structure during the carbonization treatment, leading to the high performance fibres. This observation is consistent with our current understanding of the relationship between structure and mechanical properties in carbon fibres.
- Carbon fibres, UV-Raman, sp3 carbon, Crystallinity