Catalyzed synthesis of rhombohedral boron nitride in sodium chloride molten salt

Abstract

Although a variety of methodologies/techniques have been used to prepare h-BN powders with different sizes and purities, only a few methods are reported to synthesize r-BN. In this work, highly crystalline r-BN with a purity of 94wt% was successfully synthesized in sodium chloride molten salt using Na2B4O7 and Mg powders as starting materials at 1000°C in nitrogen atmosphere. The sodium (Na) produced by the reaction of Mg and Na2B4O7 has a positive effect on the formation r-BN in the molten salt. The effect of Na as a crystallization promoter to produce crystallized r-BN was demonstrated by heating a mixture of t-BN and Na at 800–1200°C. The formation, dissolution and evaporation of Na in the melt was discussed. The influence of synthesis temperature on the phase composition and morphology of the final products in the melt was also investigated. The possible formation mechanism of r-BN is proposed.