Synthesis and morphological control mechanism of boron nitride fibers in closed systems

Boron nitride (BN) fiber has broad application prospects in aerospace, metallurgy, nuclear industry, and other fields because of its high-temperature resistance, chemical corrosion resistance, excellent processability, and thermal conductivity. However, the difference in fiber morphology greatly affects the performance and limits its applications. Hence, it was essential to elucidate the morphological regulation mechanism of BN fibers. In this paper, different morphologies of BN fibers were obtained in a closed system by adjusting the pH, cooling temperature, additives, type and content of solvent, and ultrasonic power of the precursor solution. The results show that the diameter of BN fiber can be controlled by adjusting pH and cooling temperature (nucleation competition evolution mechanism). The introduction of additives, multi-solvents, and ultrasonic treatment can reduce the width of the fibers or increase the size of the direction of thickness to increase fiber length (interlayer adsorption barrier mechanism). It is both mechanisms that jointly affect the shape of the fibers. The long BN fibers with a diameter of about 25 µm and a length of 4 cm were successfully prepared in the closed system, which is 80 to 200 times longer than the traditional precursor method, which provided theoretical support and technical guidance for the regulation and preparation of boron nitride fiber morphology in a closed system.