A COEXISTENCE OF BORON-NITRIDE AND BORIC OXIDE

Crystallization of turbostratic boron nitride, prepared from a mixture with varying ratios of boric acid and urea, was studied by heating from 220 to 1100-degrees-C. Formation of boron nitride layers was observed at temperatures as low as 220-degrees-C. Two ways to crystallization were revealed by further heating depending on the urea ratio in the starting mixture. Above 70%, the system remained monophasic and consisted of only one turbostratic phase. Below this ratio the system split into three phases at temperatures above 700-degrees-C. These were assigned as turbostratic boron nitride, boron oxynitride, and crystalline boric oxide. Boron oxynitride has been interpreted as an interfacial zone between turbostratic boron nitride crystallites and residual boric oxide resulting from an imperfect nitriding of starting oxidic precursors. Existence of this transition zone promotes the crystallization when heated at elevated temperatures. (C) 1994 Academic Press, Inc.