Oxidative Stabilization Mechanism of Synthesized Pitch-Based Carbon Fiber by Combining In Situ Analysis and Molecular Simulation

Understanding the mechanisms of oxidative stabilization is a prerequisite for enhancing the efficiency of oxidation techniques and optimizing the production of carbon fibers. In this study, the characteristics of pitch fibers after oxidative stabilization as well as the formation and cross-linking reaction mechanism of the oxygen-containing functional groups by combining in situ analysis and molecular simulations were systematically elucidated. The results revealed that the oxidative stabilization process is highly dependent on the functional group variation and oxidizing conditions. A higher heating temperature, a slower heating rate, and a longer holding time are beneficial for the introduction and diffusion of oxygen from the surface to the interior of the fiber. Increasing the temperature from 180 to 300 °C can activate additional reactions, leading to exponential formation of oxygen-containing groups. A lower heating rate accelerated the formation of Ar–O–CO–Ar and R–O–CO–R. The proportion of aromatic C═C is related to the cross-linking and aromatization processes during oxidative stabilization. A higher heating rate was unfavorable for the formation of C═O, whereas more polycyclic aromatic hydrocarbons were cross-linked at longer holding times. Oxygen is distributed homogeneously from the surface to the interior, which is a critical factor influencing the exceptional performance of carbon fibers. Additionally, the reaction of methylene with oxygen requires a higher temperature, compared to the reactions of oxygen and hydroxyl radicals with aliphatic functional groups.