PREFERRED ORIENTATION OF HIGH-PERFORMANCE CARBON-FIBERS

The preferred orientation of polyacrylnitrile (PAN)-based carbon fibers, mesophase pitch-derived carbon fibers, and pitch precursor fibers was studied by using x-ray diffraction technique. The half width at half maximum (HWHM) intensity of the phi scan x-ray diffraction profiles of these fibers was a minimum at around 2-theta = 26-degrees. The result implies that a crystallite with a larger coherence length of crystallite size L(c)(002) is better aligned along the fiber axis than that with a smaller L(c)(002) in these fibers. Further, theta-2-theta scan profiles depending on phi showed that a better aligned crystallite possesses a larger L(c)(002) than a misaligned one. L(c)(002) of a significantly misoriented crystallite remained constant at about 2 nm even after heat-treatment, though L(c)(002) of a well-aligned crystallite was easily changed by heat-treatment for both PAN and pitch-based fibers. The pitch precursor fiber exhibited a clear peak at about 2-theta = 7-degrees in the theta-2-theta profile and unusual phi scan profiles for 2-theta around 7-degrees, which were explained by assuming columnar structures formed by molecule stacking along the c-axis with periodic arrangements of the columns perpendicular to the c-axis. The periodic column stacking structure observed in the pitch precursor fiber was also detected in pitch-based carbon fibers heat-treated at lower temperatures.