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

Abstract

The radial microstructure development of polyacrylonitrile-based carbon fibers is traced during continuous carbonization from 400 to 1400 degrees 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 degrees C), (2) crosslinking by dehydrogenation (600-800 degrees C), (3) crosslinking by denitrogenation (1000-1200 degrees C), and (4) development of turbostratic structure (1200-1400 degrees 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. (c) 2022 Elsevier Ltd. All rights reserved.