Microstructure of mesophase pitch-based carbon fiber and its control

Abstract

The macro as well as microscopic structure of four mesophase pitch-based carbon fibers of typical textures is observed using optical, scanning electron, transmission electron, and scanning tunneling microscopes. The four fibers had transverse textures of radial, radial-skin/random-core, random and quasi-onion. High resolution SEM clarified the domains to have typical shapes of linear, bent, multi-fold bent and loop. The shapes and alignment of the domains depended upon the spinning conditions and mesophase pitch properties, which defined the texture in a rather complex manner, varying in the transverse locations of the fibers of any overall textures. Each domain appeared to possess micro-domains which were arranged to define the size and shape of the domain. The shape of the micro-domains was basically unchanged by carbonization and graphitization, while the carbon plane cluster, or graphitic unit grew during carbonization and graphitization within the micro-domain. Such crystal growth brings about shrinkage of the micro-domain to change the size of the domains, strongly influencing overall structure and the properties of the carbon and graphitized fibers. The unique pleat shaped structural units, which are aligned in a zig-zag manner in the longitudinal section of the carbon and graphitized fibers, were found by high resolution SEM and STM. Such structures were formed in the spinning step with the fiber of liquid crystal polymers and mesophase pitch-based carbon fiber. Neither PAN, isotropic pitch-based fibers nor needle coke exhibited such a structure. The size and alignment in the pleat-like structure were also dependent upon the nature of mesophase pitch and spinning conditions, although the formation mechanism is not clarified yet. Such overall structure is briefly compared to the properties of the carbon fiber to find guidelines for the preparation of carbon fibers with better performances.