Radial microstructure development of polyacrylonitrile (PAN)-based carbon fibers

The radial microstructure development of polyacrylonitrile-based carbon fibers is traced during continuous carbonization from 400 to 1400 °C. Based on the changes in the microstructure and mechanical properties of the fibers, it is confirmed that they undergo four distinctive stages depending on the temperature: (1) further cyclization in a remained unstabilized structure (400–600 °C), (2) crosslinking by dehydrogenation (600–800 °C), (3) crosslinking by denitrogenation (1000–1200 °C), and (4) development of turbostratic structure (1200–1400 °C). A structural analysis of the surface, skin, and core regions reveals that the innermost structure (core) originates from a high degree of structural relaxation. In contrast, densification is dominated in the outermost structure (surface). Such a radial heterogeneity is attributed to gas evolution along the radial direction of the fibers during the high-temperature carbonization and fiber shrinkage. The tensile strength exhibits a strong relationship with the crystal sizes as compared to the degree of disorder and amorphous structures, indicating that the crystal growth by densification has a greater influence on the tensile properties than the structure relaxation.