Small angle X-ray scattering from voids within fibers during the stabilization and carbonization stages

Five kinds of commercial carbon fibers heat-treated at various temperatures were adopted to investigate the scattering from voids within the fibers by small angle X-ray scattering (SAXS), since the voids produced during stabilization and carbonized processes provide unfavorable influence on mechanical properties of the carbon fibers. The theoretical calculation was carried out for SAXS by assuming ellipsoidal voids with different shapes. In this model system, the long axis of all ellipsoidal voids on the two-dimensional plane was assumed to be oriented predominantly with respect to the machine direction but the center of gravity of each void are assumed to be arranged in the transverse direction assuring one-dimensional lattice. To analyze inter-particle interference effect from ellipsoidal voids, the function H-1(y) proposed by Hoseman was introduced to represent the probability of finding the nearest-neighbor void at a displacement vector. In comparison with observed and calculated SAXS patterns, the inter-particle interference effect was observed for the fibers carbonized at 1200-1700 degreesC but the effect disappeared with further heat-treatment at 3000 degreesC. Namely, the scattering from the fiber heat-treated at 3000 degreesC, which is so-called graphite fiber, showed no inter-particle interference effect. Namely, void distribution of high modulus carbon fiber with graphite crystallites was different from that of high strength carbon fiber with turbostratic structures. (C) 2002 Elsevier Science Ltd. All rights reserved.