Abstract
The phase relation of the system Ca-B-N was investigated at a pressure of 2.5 GPa by both differential thermal analysis (DTA) and a quenching method. When BN reacted with Ca at temperatures higher than 1150° C, Ca3B2N4 was formed together with small amounts of Ca3N2 and CaB6. The system of Ca3B2N4-BN has a eutectic relationship at 1316° C and 2.5 GPa. The synthesis of cBN was established by using Ca3B2N4 under the thermodynamic stable conditions of cBN. It could be seen that cBN crystals grew repeatedly in Ca3B2N4 melt, which advanced into the hBN layer. The cBN crystals obtained in this way were of outstanding purity and of good quality. This indicated that hBN dissolved and precipitated as cBN in Ca3B2N4 melt without any appreciable change of molar boron/nitrogen ratio. In the system of Ca-B-N, the low-temperature limit of cBN formation was closely related to the eutectic relationship between BN and Ca3B2N4.
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