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 φ scan x-ray diffraction profiles of these fibers was a minimum at around 2θ = 26°. 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, θ-2θ scan profiles depending on φ 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 2θ = 7° in the θ-2θ profile and unusual ° scan profiles for 2θ around 7°, 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.