高温高压触媒法金刚石生长的热力学分析

 利用热力学中经典的ΔG<0判定法，探讨了Fe基触媒合成金刚石晶体生长中的碳源问题，在计算中考虑了各物相的体积随温度和压力的变化。结果表明：在金刚石形成之前，就有大量Fe₃C形成，而在触媒法合成金刚石的温度和压力范围内，Fe₃C→C(金刚石)+3γ-Fe反应自由能和石墨→金刚石相变自由能均为负值，但前者比后者的绝对值更大，这说明前者更容易发生。因此，从热力学角度看，Fe₃C的形成降低了石墨转变为金刚石所要越过的势垒，使用Fe基触媒合成金刚石单晶的生长来源于Fe₃C的分解而不是石墨的直接转化。同时推导出在1200 K以上石墨-金刚石的平衡p-T关系：p_eq(GPa)=1.036+0.00236T (K)，与F.P.Bundy的平衡线非常接近，证明了本热力学计算方法的可行性。

Thermodynamic Analysis of Diamond Growth with Catalyst at HPHT

The diamond growth with Fe-based catalyst at high pressure and high temperature(HPHT) was analyzed with the determinant method of ΔG<0 in thermodynamics theory,and the changes of volume with temperature and pressure were involved in the calculation.The results show that the Fe3C phases have been formed before diamond nucleation;at the temperature and pressure range of the diamond synthesis with catalyst,the Gibbs free energies of Fe3C(→)C(diamond)+3γ-Fe and graphite(→)diamond are all negative,but the former's absolute value is much larger than the latter's,which means the former will take place more easily.Therefore,from the viewpoint of thermodynamics the formation of Fe3C reduces the potential energy of the transformation from graphite to diamond,and the diamond crystal growth with Fe based catalyst comes from the decomposition of Fe3C instead of the direct transformation from graphite.Moreover,the p-T (Pressure-Temperature) equilibrium of peq(GPa)=1.036+0.00236T(K) was gained,which was closer to that presented by F.P.Bundy.Thereby the thermodynamic calculation used in this paper is feasible.