Processing-controlled radial heterogeneous structure of carbon fibers and primary factors determining their mechanical properties

Commercial polyacrylonitrile (PAN)-based carbon fibers, T800S and T700S, were gradually etched from the surface to the inside by thermal oxidation at 600 °C up to 10 h for investigating microstructure and resulting mechanical properties depending on the position in the radial direction. While T800S showed a gradual decrease in mechanical properties with respect to the oxidation time, those of T700S showed an unusual behavior due to the existence of a transition zone between the skin and the core, where the structural development was shown to be as high as the surface. This transition zone is presumed to be due to the collisions (shear stress-induced alignment) between the skin and core as the difference in their degree of structural development is large. In addition, as a result of analyzing various microstructures over a wide range from macroscopic to atomic/bond scale, it was confirmed that microvoid diameter and crystalline orientation were dominant structural factors in determining the mechanical properties of carbon fibers. However, the graphitic crystal size and I_D/I_G ratio (the amounts of defects) showed little variation along the radial direction. Based on those overall analyses, more detailed structural sketches of two different types of CFs from the skin to cores are suggested. We believe that the integrated studies on radial heterogeneity and structure-property relationships of carbon fibers provide essential information for manufacturing high-performance carbon fibers.