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 N2 gas at 1500 οC. 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 N2 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 N2, results in more BN formation in CaCO3-added and CaOadded B2O3-activated C mixtures subjected to N2. BN particles were also found to grow in carbon-free CaO-B2O3 melts not only in the presence of N2 when additional BN forms but also in the absence of N2 (under Ar) when no additional BN forms.

Keywords: Grain growth, Powders-chemical preparation, Nitrides, Alkaline earth oxides, Boron nitride