Key microstructure for high-performance pitch-based carbon fibers

Pitch-based carbon fibers were fabricated from petroleum mesophase pitch under systematically varied spinning conditions and graphitization temperatures to investigate manufacturing conditions that maximize key mechanical properties, namely Young’s modulus, compressive strength, and adhesion. The microstructural characteristics were analyzed using mainly X-ray diffraction and laser Raman spectroscopy. The results revealed that although pitch molecules exhibit enhanced alignment within the spinning nozzle, this orientation becomes partially disrupted in the downstream draw zone. The Young’s modulus showed a strong correlation not only with the alignment of graphitic layers along the fiber axis but also with the coherent stacking thickness (Lc) of graphite lamellae. This behavior is attributed to a geometric confinement (“shape”) effect, whereby the fiber diameter is orders of magnitude smaller than its length. The compressive strength was found to depend on the balance between average microstructural parameters, specifically Lc and the in-plane crystallite size (La). Increasing Lc relative to La was associated with enhanced compressive strength. In contrast, the surface crystallite size (surface La) was governed primarily by the final graphitization temperature. Consequently, the graphitization temperature also directly influenced the saturated adhesive strength to the resin matrix. Based on these results, the author proposes that promoting Lc growth under relatively low-temperature graphitization conditions can improve the Lc/La balance. This approach is expected to yield carbon fibers exhibiting both high modulus and high compressive strength. Furthermore, the small surface La facilitates strong fiber–resin adhesion.