Thermodynamic Analysis of cBN Transformation in the Li3N-hBN System at High Pressure and High Temperature

The possibility of several reactions at high pressure and high temperature was analyzed by the determinant method of Delta G<0 in second law of thermodynamics in the process of synthesizing cubic boron nitride(BN). The Delta G of reactions Li3N+hBN -> Li3BN2, hBN -> cBN and Li3BN2 -> Li3N+cBN in the Li3N-hBN system, was calculated in considering of the change of volume with temperature and pressure, under the synthetic condition of cubic BN. It was confirmed that Li3BN2 was synthesized with Li3N and hexagonal BN in certain condition of high pressure and high temperature(T>1300 K and P>3.0 GPa). When temperature and pressure raise to the area of cubic BN(T=1600-1800 K and P=4.6-6.0 GPa), it was possible for two reactions Li3BN2 -> Li3N+cBN and hBN -> cBN in subregion of the P-T area in thermodynamics. However, the Delta G of hBN -> cBN was more negative than that of the former, and it was easier for the latter reaction. A preliminary discussion about the transition mechanism of cubic BN was made with the result of calculation.