The effects and roles of CaCO3 and CaO in the carbothermic formation of h-BN

The effects of CaCO3 and CaO on the carbothermic formation of h-BN were investigated by subjecting plain, CaCO3-added and CaO-added B2O3-activated C mixtures to N-2 gas at 1500 degrees C. Both additions were found to result in the formation of more BN with larger particle sizes in a given time. It was shown that BN forms in the absence of carbon in CaO-B2O3 melts but not in a plain B2O3 melt subjected to N-2 by a mechanism involving nitrogen dissolution in and formation of BN from the melt. This mechanism, in addition to the reaction of B2O3 with C and N-2, results in more BN formation in CaCO3-added and CaO-added B2O3-activated C mixtures subjected to N-2. BN particles were also found to grow in carbon-free CaO-B2O3 melts not only in the presence of N-2 when additional BN forms but also in the absence of N-2 (under Ar) when no additional BN forms.