HIGH-PRESSURE SYNTHESES OF CUBIC BN USING WATER, UREA, AND BORIC-ACID CATALYSTS

Cubic BN is successfully synthesized under high pressure by using three nonmetal catalysts, i.e. water, urea, and boric acid, and the mechanism is explained. The synthesis is accomplished with the aid of ammonium borate flux which is produced by the reaction between starting hexagonal BN and catalysts at pressures above 4.3-4.8 GPa and temperatures above 800-1000°C. The resulting cubic BN is a white powder, with grains having a tetrahedral crystal morphology and a 0.1-0.3 μm size. At pressures below 4.2 GPa cubic BN is found to reconvert to hexagonal BN in the presence of ammonium borate, showing that hexagonal BN is stable in this region. The lower pressure limit in the synthesis of cubic BN is surmised to be based on the cubic-hexagonal BN equilibrium line in the ammonium borate coexisting systems. © 1979 American Institute of Physics.