Evolution Mechanism of the Crystallite Geometry in PAN-Based Carbon Fibers during High-Temperature Heat Treatment

Ai Jun Gao; Zuo Guang Zhang; Liang Hua Xu

doi:10.4028/www.scientific.net/AMR.1090.26

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Evolution Mechanism of the Crystallite Geometry in PAN-Based Carbon Fibers during High-Temperature Heat Treatment

Abstract:

The mechanism for the change in the size of the geometry of graphite crystallites in polyacrylonitrile-based carbon fibers heat treated at 1500–2500 °C is proposed. The study indicated that the growth transition temperature of graphite crystallites in carbon fibers is 2000 °C. Above 2000 °C, the interlayer space (d₀₀₂) decreases slowly and an ordered graphitic structure forms. Below 2000 °C, the variation in the size of the crystallites along the directions of the axis (L_a_‖) and the radius (L_a_⊥) of the carbon fibers that were heat treated at various temperatures was almost the same and the crystallite form factor, L_a_‖/L_a_⊥, which is nearly 1. Above 2000 °C, L_a_‖/L_a_⊥ increases rapidly because the anisotropy of the arrangements of the crystallites in the carbon fibers and the geometry of the crystallites change from being square to becoming rectangular. The crystallite thickness (L_c) also increases with an increasing temperature and below 2000 °C the crystallite form factor (L_a_‖/L_c) increases rapidly. Above 2000 °C, the L_a_‖/L_c stabilized at 3.1. The different growth models in the different directions are the intrinsic root for the variations in the crystallite geometries that underwent high-temperature heat treatments.