Chapter 4 - High Performance Carbon Fibers

Abstract

Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers—polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market, while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. Both fiber types are subjected to spinning, oxidative stabilization, and high temperature carbonization steps. Also, in both cases, although the ultimate properties are not developed until carbonization, the property limitations and the final structure are mainly because of the chemical nature of the precursor and the morphology of the as-spun fiber. There are many similarities in the processing techniques used for all continuous carbon fibers—the structure and properties of the final products are highly variable, depending on the chemical nature of the precursor. Since PAN-based fibers are turbostratic in nature, they are limited in developing ultra-high stiffnesses or thermal conductivities, but the absence of large graphitic crystallites is well-suited for developing extremely high strength. Mesophase pitch fibers, by contrast, are graphitizable and can develop extremely high stiffnesses and thermal conductivities. Unfortunately, the large crystallites necessary to develop these properties carry a cost in tensile strength. Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber.