RESUMO
The skin-core evolvement of the carbon fibers was studied as a function of heat-treatment temperature though the analysis of Raman spectroscopy of the carbon fibers surface and core. It was found that the change of the Raman spectra of the carbon fibers core was similar to that on the surface with the increase in heat-treatment temperature. At 1600 degrees C, the Rs and Rc values were almost equal, indicating that the degrees of the graphitization of the carbon fibers surface and core were almost uniform. The Rs and Rc values decreased dramatically with the increase in heat-treatment temperature, and Rs decreased more. At 2800 degrees C, the Rs value came to 0.429, lowered 77.2%, while the Rc value then came to 1.101, lowered 38.7% only. It implied that the graphitization degree of the carbon fibers was enhanced with increasing the heat treatment temperature, and that of carbon fibers surface was enhanced more. The graphite characters of the carbon of the carbon fibers surface were different from that of the carbon fibers core. The former is close to soft carbon, which is easy to graphitize, while the latter is close to hard carbon, which is difficult to graphitize, and it may be resin carbon Skin-core structure gene Rsc (= Rs/Rc) which denoted the skin-core degree of the carbon fibers was first brought forward and adopted. The Rsc value is between 0 and 1. When the Rsc value is equal to 1, the carbon fibers are homogenous. When the Rsc value is close to zero, there are serious skin-core structures in the carbon fibers. The Rsc value reduced linearly with the increase in heat-treatment temperature, indicating that the homogeneous degrees of the carbon fibers decreased and the skin-core degrees of the carbon fibers increased. The crystallite size of the carbon fibers surface and core increased gradually with the increase in heat-treatment temperature, but the surface's increased more quickly, indicating that the carbon of the carbon fibers surface was easier to graphitize than the carbon fibers core. Serious skin-core structure was one of the reasons that caused the reducing of the carbon fibers' tensile strength.
RESUMO
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.