Evolution of microstructure and property during industrial continuous heat treatment of polyacrylonitrile fibers up to 1900 °C

Abstract

The evolution of the microstructures of polyacrylonitrile (PAN) fibers during successive industrial heat treatments was systematically traced. It was found that the cyclization reaction began in the amorphous zone around the PAN crystals and continued until the pre-low-temperature carbonization phase. From 400 to 1900 °C, the carbonization process was separated into four stages, where the two-dimensional aromatic layers formed by the cyclization cross-links progressively evolve into three-dimensionally ordered carbon microcrystals. The microcrystals tended to in-plane growth during the high-temperature carbonization stage. The aspect ratio of the microcrystals of carbon fiber exhibited the same variation trend as its tensile strength, both reaching their maximum value at 1700 °C. Small-angle X-ray scattering (SAXS) revealed the hereditary evolution from the microfiber structure in the precursor fiber to the banded graphite-like layered structure within the carbon fibers after heat treatment. As the stabilization and carbonization processes progressed, the microvoids within the fibers showed a decrease in cross-sectional dimensions and transitioned to a slit-like configuration accompanied by a decrease in the curvature of the graphitic-like carbon layer. The fluctuating changes in microvoid length and orientation are influenced by the structural relaxation in the middle stage of carbonization, as well as the regular rearrangement of graphite-like structures in the late stage of carbonization. The results of relevant analysis can provide further insights for the development of ultra-high-strength medium-modulus carbon fibers.