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 fr om 220 to 1100-degrees-C. Formation of boron nitride layers was observ ed at temperatures as low as 220-degrees-C. Two ways to crystallizatio n were revealed by further heating depending on the urea ratio in the starting mixture. Above 70%, the system remained monophasic and consis ted of only one turbostratic phase. Below this ratio the system split into three phases at temperatures above 700-degrees-C. These were assi gned as turbostratic boron nitride, boron oxynitride, and crystalline boric oxide. Boron oxynitride has been interpreted as an interfacial z one between turbostratic boron nitride crystallites and residual boric oxide resulting from an imperfect nitriding of starting oxidic precur sors. Existence of this transition zone promotes the crystallization w hen heated at elevated temperatures. (C) 1994 Academic Press, Inc.