Microstructures and properties of polymer-derived hexagonal boron nitride fibers with initial gradient oxygen contents

Oxygen plays a crucial role in the synthesis of boron nitride (BN) and the preparation of BN ceramics. The effects of oxygen content on the microstructure and properties of BN fibers treated at 1600 °C have not been studied yet. In this work, polymer-derived BN fibers are cured at 300 °C under oxygen and pyrolyzed at 600 °C, 700 °C and 800 °C under ammonia as the pre-heated BN fibers. Then, these pre-heated BN fibers are treated at 1600 °C under nitrogen as the final BN fibers. The compositions and microstructures of the pre-heated and final BN fibers are investigated. As the pre-heated temperature increases, the oxygen contents of the pre-heated BN fibers decrease to 24 wt%, 17 wt% and 5 wt%, respectively. The structures of the pre-heated BN fibers transform from amorphous BN to turbostratic BN. The oxygen in the pre-heated BN fiber volatilizes as boron oxide during treatment at 1600 °C, promoting the formation and growth of h-BN in the final BN fiber, with the maximum grain size reaching 74 × 20 nm (La × Lc). As the grain size of the final BN fiber increases, the Young's modulus, density and oxidation resistance level of the BN fiber improve, and the dielectric properties decrease. The final BN fiber that underwent pre-heating at 700 °C has the best comprehensive properties, with a tensile strength of 1.11 GPa and a Young's modulus of 265 GPa. This study researches the relationship between composition, structure and performance, and provides a scientific basis for the preparation process of BN fibers.