Effect of fiber microstructure studied by Raman spectroscopy upon the mechanical properties of carbon fibers

The microstructure of polyacrylonitrile (PAN)-based high-performance carbon fibers, including high-strength carbon fibers (HSCFs), high-modulus carbon fibers (HMCFs), and ultrahigh-modulus carbon fibers (UHMCFs), was systematically characterized by the Raman spectroscopy. Two characteristic bands, D-line and G-line, showed up in the Raman spectra of HSCFs, HMCFs, and UHMCFs. However, the wavenumber of the G-line peak of HSCFs shifted to higher wavenumber (about 1,595 cm(-1)) and that of the D-line peak of HMCFs and UHMCFs shifted to lower wavenumber (about 1,350 cm(-1)). The relationship between the microstructure and mechanical properties of carbon fibers was also studied in detail. It was of significant relevance between surface disordered structure and the mechanical properties of HSCFs, and decreases in the full width at half maximum values of the disorder-induced D-line and A-line could result in higher tensile strength and tensile modulus of HSCFs. As for HMCFs and UHMCFs, the disorder-induced D '-line more easily affected the tensile strength. A higher tensile modulus of HMCF and UHMCF was obtained as a result of decreases in the disordered structure and increases in the graphite structure. An increase of the intensity ratio I-D/I-G together with I-A/I-G (HSCFs) or I-D '/I-G (HMCFs and UHMCFs) could result in increases in the tensile strength and tensile modulus of carbon fibers.