High temperature evolution of the microstructure in the radial direction of PAN-based carbon fibers and its relationship to mechanical properties

The radial microstructural in carbon fibers is important for their mechanical properties. Microstructures at different locations in the radial direction produced by heat treatments from 1350 to 2400 °C and their relationship to the mechanical properties were investigated by Raman spectroscopy, elemental analysis, X-ray diffraction, high-resolution transmission electron microscopy and mechanical testing. Results indicated that the differences of the shift and full widths at half maxima (FWHM) of the Raman G and D bands at different points in the radial direction from the skin to the core, showed minima while the tensile strength of the fibers was a maximum at 1700 °C heat treatment temperature. The maximum tensile strength of carbon fibers at 1700°C was ascribed to the smallest skin-core variation at this temperature. The FWHM of the G and D bands from the same points in the radial direction decreased with heat treatment temperature, indicating an increased uniformity of the vibration modes of both G and D bands The sizes and the orientation degree of the crystallites increased with heat treatment temperature, leading to an increase of tensile modulus. Non-carbon elements were preferentially released, and crystallites were preferentially grown and orientated in the skin region compared to those in the core due to the preferential heat absorption in the skin, leading to the lower values of FWHM of G and D bands in the skin than in the core. The Raman shift of the G band had a maximum at 1700 °C while that of D band decreased with heat treatment temperature at the same radial points of the fibers. The fact that the release rate of non-carbon elements increased with heat treatment temperature below 1700 °C could be responsible for the increased G band shift below 1700 °C. With a continuous decrease in the non-carbon element content, this effect was lessened and the improvement of the perfection of the graphite structure became dominant above 1700 °C.