Thermodynamic Analysis of the V-Shaped Area of High Pressure and High Temperature in Cubic Boron Nitride Synthesis with Li3N as a Catalyst

The possibilities of different phase transitions to cBN with Li3N as catalyst at high temperature and high pressure (1600-2200 K, 4.8-6.0 GPa) are analyzed, in the framework of the second law of thermodynamics. The Gibbs free energy (Delta G) of three reactions which may happen in the Li3N-BN system: hBN + Li3N -> Li3BN2, hBN -> cBN, and Li3BN2 -> cBN + Li3N, is calculated, with the influence of high temperature and high pressure on volume included. We show that Delta G of hBN + Li3N -> Li3BN2 and hBN -> cBN are between -35 similar to-10 KJ.mol(-1) and -25 similar to-19 KJ.mol(-1), respectively. However, Delta G of Li3BN2 -> cBN + Li3N can be positive or negative. The area formed by the positive data is a V-shaped area, which covers the most part of the cBN growing V-shaped area. It confirms that Li3BN2 is stable in the P-T area of cBN synthesis, and cBN is probably transformed directly from hBN. Analysis suggests that Li3BN2 promotes the transition from hBN to cBN.