Li_3N-hBN体系合成cBN单晶“V”形区的相变热力学

Phase Transition Thermodynamics of Cubic Boron Nitride in the V-shaped Area Synthesized in Li_3N-hBN System

利用静态高温高压触媒法合成优质cBN单晶,合成温度和压强范围形成一"V"形区域。从Gibbs自由能(△G)角度分析了采用Li3N为触媒合成cBN单晶时,不同物相在"V"形区及其扩大区域(1600~2200 K、4.8~6.0 GPa)内向cBN相变的可能性。分别计算了高温高压下hBN+Li3N→Li3BN2、hBN→cBN和Li3BN2→cBN+Li3N三个反应的△G。结果表明:在"V"形区及其扩大区域内,前二个反应的△G均为负值,分别为-35^-10 kJ/mol和-25^-19kJ/mol;而第三个反应的△G有正有负,其正值范围形成了一个温度、压强的"V"形区域。该"V"形区基本覆盖了以前文献中提到的"V"形区。这说明在合成优质cBN单晶的温度压强范围内,Li3BN2稳定存在,不能分解出cBN。Li3N触媒体系内的cBN可能源自hBN的直接相变,而非Li3BN2的分解。但分析表明Li3BN2促进了hBN→cBN的相变。

A V-shaped area was formed no matter what kind of catalyst was adopted in the synthesizing of cubic boron nitride（cBN）, with static high pressure catalytic synthesis method. The possibilities of different phases transformed to cBN in the V-shaped area and its expanded region （1600-2200 K, 4. 8-6.0 GPa） in Li3N-hBN system were analyzed from the point of Gibbs free energy（AG）. The AG of reactions hBN ＋ Li3N→Li3BN2, hBN→cBN and Li3BN2→cBN ＋Li3N was calculated, respectively. The results show that, in the V-shaped area and its expanded region, the A G of the first two reactions were negative, and they were between -35- -10 kJ/mol and -25- -19 kJ/mol, respectively. However, the AG of the third one were positive or negative. The area formed by the positive data was a V-shaped area, which covered the most part of V area from previous literatures. Li3 BN2 was stable in the P-T area of cBN synthesis, and cBN was probably transformed directly from hBN. It was Li3BN9 which promoted the transformation of hBN-eBN.