X-ray scattering studies of graphite fibers

Abstract

The structural features of three different graphite fibers were studied via small- and wide-angle x-ray techniques. The experimental evidence is consistent with a sheath/core fiber morphology. Graphitization, degree of orientation, crystallite size, and microporosity were analyzed. Samples included low (AS4) and high (HMS) modulus poly(acrylonitrile) (PAN) and melt-spun pitch-based (VSB-16) fibers. By wide-angle x-ray diffraction (WAXD) VSB-16 was found to have the highest degree of graphitization, the highest degree of orientation, and the largest crystallite regions, and AS4 the poorest graphitized structure. The void system in these graphite fibers was investigated by small-angle x-ray scattering (SAXS). SAXS from glycerin-soaked fibers indicates the scattering at very small angles (2θ<10 mrad) is dominated by total reflection of x rays at the fiber surface. The pores in HMS and VSB-16 fibers are inaccessible to glycerin and the pores in AS4 fiber are partially accessible. The pores in PAN-based HMS and AS4 fibers are of needlelike shape and those in VSB-16 are ellipsoidal. The porosity is 12.6%, 8.4%, and 4.5% in HMS, AS4, and VSB-16 fibers, respectively. Deviations from Porod’s law were observed at large angles and attributed to scattering from fractal aggregates of carbon atoms in the graphite crystallites and/or fractal boundaries of pores. The fractal dimension of the aggregates is 2.3±0.1, 2.8±0.2, and 3.0±0.2 for AS4, HMS, and VSB-16 fibers, respectively. Speculations about the fractal nature of aggregation may stimulate some new insight to the graphitization process, paracrystallinity, and the strength of graphite fibers.