Preparation of Polyacrylonitrile-Based Carbon Fibers With Radial Homogeneous Structure and Improved Mechanical Properties Through Boron Modification

The boric acid modification offers an effective approach to improve the radial homogeneity and mechanical performance of polyacrylonitrile (PAN)-based carbon fibers by affecting stabilization behavior. However, the mechanism of how boron affects the stabilization process remains unclear. Herein, the impact of boron on the cyclization and oxidation reactions is investigated through experimental data and computational calculations. The results reveal that boron oxide exhibits a stronger electron-withdrawing effect on cyano nitrogen compared to hydroxyl oxygen, effectively inhibiting both ionic and free radical cyclization reactions, as well as subsequent oxidation reactions in the skin regions. This inhibitory effect enhances the cross-sectional oxygen uptake and stabilization. The boric acid also improves the preferred orientation in precursor fibers by reducing the polarity of cyano groups and promotes the inheritance of structural characteristics from precursor fibers through “cross-linking” effect. Consequently, the structural characteristics and mechanical properties of modified carbon fibers are significantly improved. These findings could provide fundamental insights into the mechanisms of thermal reaction regulation and microstructure evolution in the preparation of high-performance PAN-based carbon fibers.