Hexagonal boron nitride was synthesized from boron oxide using super critical ammonia and solid CaO below the melting point of B2O3 by the following reaction: B2O3 (s)+2NH3 (super-critical fluid)+3CaO (s) = 2BN (s)+3Ca(OH)2 (s). The experiment was carried out at 623-753 K and the initial ammonia pressure was 3.0-40.0 MPa. The mean diameter of obtained BN particles was 0.2-0.3 μm. The yield of BN increased with increasing temperature, initial ammonia pressure and the addition of CaO. The thermochemical calculation showed that the CaO addition suppressed the decomposition loss of ammonia during the reaction.
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