Synchrotron study on the evolution of the radial structural distribution of carbon fiber monofilaments during heat treatment process

Evolution of the radial distribution of a micro-crystalline structure within polyacrylonitrile monofilaments at various temperatures during carbonization was investigated via the method of synchrotron wide-angle X-ray diffraction (S-WAXD) and micro-scanning Raman spectroscopy. The result indicated that in the core region of the carbon fiber monofilaments, the L(c) is lower while the interplanar spacing (d(002)) is higher compared with those in the skin region at 1300 °C. The difference between the skin and core regions in L(c) and d(002) constantly decreases as the heat treatment temperature increases. The results of micro-scanning Raman indicated that the micro-crystallites grow faster along the L(a) direction, and the graphite degree is higher in the skin region than that in the core region. This is attributed to the fact that the removal of nitrogen element is the domain factor on the growth of a pseudo-graphite crystallite at relatively lower heat treatment temperatures. As the temperature rises, L(c) is developed mainly in the core region with the closed packing of a graphite layer, while the crystallites in the skin region grow in both L(a) and L(c) directions, which are mainly affected by the disordered and boundary graphite structure.