Boron nitride fiber, a polycrystalline fiber primarily composed of boron nitride. Typically has a diameter of 4–6 micrometers, a density of 1.8–1.9 g/cm³, a strength of approximately 2 GPa, an elastic modulus of 340–350 GPa, and a melting point of 300°C. It exhibits excellent thermal insulation and high-temperature resistance. It is resistant to nuclear radiation, chemical corrosion, and infrared radiation. Mainly used as a reinforcement agent for ceramic matrix composites, as well as for micro-antenna window components in spacecraft and aircraft.