Thermodynamic analysis on the synthetic process of diamond under high temperature and high pressure: phase equilibria and their application

On the basis of the information about the stable and meta-stable phase equilibria among liquid (Liq.), Graphite (Gra.) and Diamond (Dia.) phases from Fe-C binary phase diagram at 5.7 GPa, the synthetic process of diamond under high temperature and high pressure was analyzed thermodynamically with Graphite taken as the carbon source and Iron as the solvent. Results show that by introducing Graphite phase into the two phase equilibrium region between Diamond and Liquid, the liquation reaction of Gra. → Dia. + Liq. can be resulted in. This kind of transformation proceeds in a way of the static stable reaction chain of three phases at an isothermal state or under a certain temperature gradient, from which the thermodynamic analysis and assessment can be made respectively on the one-time nucleation and growth process by way of the iso-thermal method and the homogeneous epitaxy from a single seed or a group of seeds by way of the temperature gradient method. The direction and the limitation of transformation can also be given. One of the noticeable results of the present analysis is that in the way of the static stable reaction chain of three phases, the so-called inclusion is actually the transformation product from the liquid phase after unloading pressure and decreasing temperature. The present analysis is applicable to not only the Fe-C and the Ni-C binary systems but also the Fe-Ni-C ternary system with the composition ratios of Fe∶ Ni between 5∶5-7∶3.