高温高压Li3N-hBN体系cBN转变的热力学分析

以经典热力学第二定律ΔG<0为依据,分析了静态高温高压触媒法合成立方氮化硼(cBN)过程中发生的可能反应.考虑温度和压强对反应物相体积的影响,计算了六方氮化硼(Li3N-hBN)体系中hBN+Li3N→Li3BN2,h BN→cBN及Li3BN2→Li3N+cBN反应在高温高压条件下的ΔG.结果证实,Li3BN2由Li3N与hBN在高温高压(T>1300 K,P>3.0 GPa)条件下反应得到,在cBN的合成(T=1600～1800 K,P=4.6～6.0 GPa)条件下,hBN和Li3BN2都有向cBN转化的倾向,但由hBN向cBN直接转变的反应自由能比Li3BN2分解生成cBN的反应自由能更负,反应的可能性更大.探讨了高温高压条件下立方氮化硼的转变机理。

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 ΔG<0 in second law of thermodynamics in the process of synthesizing cubic boron nitride(BN). The ΔG of reactions Li3N+hBNLi3BN2, hBNcBN and Li3BN2Li3N+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 Li3BN2Li3N+cBN and hBNcBN in subregion of the P-T area in thermodynamics. However, the ΔG of hBNcBN 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.