[Raman spectra of PAN-based carbon fibers during graphitization]

Laser Raman spectroscopy was employed to characterize the structure of PAN-based carbon fibers during graphitization (2 000-3 000 degrees C), and the spectra of the surface and the cross section of the fibers were compared. The results show that the Raman spectra of the fibers after graphitization can be separated as three bands (D, G and D'). The degree of disorder of the fibers can be measured by Raman spectra parameter, such as the full-widths at half maximum (FWHM) of D and G bands, Raman shift of G band, and the integrated intensity ratio in the form of R(I(D) I(G)). Further investigation demonstrated that the FWHM of D and G bands, Raman shift of G band and the value of R decrease with increasing heat treatment temperature (HTT). The D band can be seen and the value of R is 0.19 even after being heat treated at 3 000 V, indicting that the fibers still have disordered carbons. In addition, the value of R is linearly related to the reciprocal of the basal plane length of the crystallites (L(a)). The spectra of the surface and the cross-section of the fibers after graphitization show obvious difference. So the degree of graphitization and preferred orientation of carbon fibers can be quantitatively characterized by laser Raman spectroscopy.