Thermodynamic analysis of the V-shaped area of high pressure and high temperature in cubic boron nitride synthesis with Li3N as a catalyst

Abstract

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 (ΔG) 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 ΔG of hBN + Li3N→Li3BN2 and hBN→cBN are between -35~-10 KJ·mol-1 and -25~-19 KJ·mol-1, respectively. However, ΔG 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. © 2014 by the authors.