Preparation of Hexagonal Boron Nitride from Boric Acid and Characterization of the Materials

Fused boric acid reacted with urea to form a solid mass having an indefinite composition, B2O3.xNH3. It was further heated in an atmosphere of ammonia to 900°C, when the excess urea was removed, leaving a refractory white residue. The product was further heated in nitrogen atmosphere at temperatures ranging from 1100° to 1400°C for 1–4 hours. The powders obtained were boiled in water/HC1 and filtered. The washed samples were characterized by IR and X-ray studies and chemical analysis.At 900°C, two soluble phases were detected: one was isostructural with ammonium borate pentahydrate and the other with boric acid, wherein the OH groups were partially replaced by NH2. On further heating, reactions between NH2 and NH2OH led to the formation of B-NH-B and finally B-N bonds. With rising temperature, increasing amounts of the soluble phase were converted to boron oxynitride and a nitride of 90% purity was formed at 1400°C and of 95% purity at 1600°C in an inert atmosphere.