Small-angle X-ray scattering in carbon fibers

Abstract

Small-angle X-ray scattering (SAXS) has been used as an analytical tool to study microporosity in polyacrylonitrile-(PAN-) and pitch-based carbon fibers. Microporosity in carbon fibers consists primarily of needle-shaped voids whose long axes are oriented preferentially along the fiber axis. This preferred orientation is seen to be independent of the nature of the fiber precursor and is the major deciding factor in controlling the modulus of carbon fibers. As the modulus increases, the general trend for both PAN-and pitchbased fibers is to move from a system consisting of many small pores to one with few larger pores. At very small angles, the steep rise in scattered intensity is due to reflection effects for PAN-based fibers and multiple scattering effects from voids for pitch-based fibers.in which ribbons of crystallites were aligned parallel to the fiber axis. The irregular contours of these crystailites create voids that are long and narrow or slit shaped and have a preferred orientation along the fiber axis. The SAXS pattern of such a structure shows a fanlike scattering along the equator. Fig. 2 is a schematic representation of the intensity distribution of SAXS in reciprocal space for a carbon fiber of high HTT and moderate preferred orientation. If one assumes that the fan-like intensity distribution is produced by a random distribution of needle-like pores with a preferred orientation of the principal axes of the pores lying parallel to the fiber axis, then the intensity distribution in reciprocal space consists of a distribution of discs centered on the origin of reciprocal space with disc normals lying

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