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 (∆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 ∆G of hBN + Li₃N→Li₃BN₂ and hBN→cBN are between −35~−10 KJ·mol⁻¹ and −25~−19 KJ·mol⁻¹, respectively. However, ∆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. View Full-Text