Evolution of the skin-core structure of PAN-based carbon fibers with high temperature treatment

PAN-based carbon fibers were subjected to high temperature heat treatment (HTT) at 1500-2500℃ and the evolution of the skin-core structure of the carbon fibers was investigated by elemental analysis, XRD, TEM and Raman spectroscopy. Results showed that nitrogen content and d002 decreased, and carbon content and La increased with HTT temperature. La increased sharply with temperature above 1900℃. The Raman spectra taken at four points in radial directions indicated that the intensity of the G band at the skin was stronger than at the core with no change in peak position at 1700℃. However, the G peak position decreased from 1588.2 to 1582.2cm-1, the width of the G band at half maximum intensity decreased from 46.3 to 28.9 and that of D band from 46.9 to 36.7cm-1 from core to skin after HTT at 2500℃. The differences of microstructure in the core and skin at different temperatures are related closely to the differences in nitrogen removal efficiency and temperature. At temperatures below 1900℃, nitrogen near the skin is more easily removed due to a shorter diffusion path and a higher temperature on the skin, leading to a slower increase of La with temperature on the skin than in the core. At temperatures above 1900℃, nitrogen is removed completely and graphitization on the skin is more rapid than in the core, leading to a large skin-core difference with the skin portion having a high modulus and the core portion a high strength.