高温高压下人造金刚石过程的热力学分析—相平衡信息及其应用

Thermodynamic Analysis on the Synthetic Process of Diamond under High Temperature and High Pressure:Phase Equilibria and Their Application

借助Fe-C二元系在5.7 GPa高压下等压相图提供的有关相平衡信息,以及石墨相Gra.和液相Liq.之间的介稳平衡信息,主要对以Fe作溶剂,以石墨为碳源,在高温高压下获得人造金刚石的主要工艺进行了热力学分析。分析结果显示：在Dia.＋Liq.两相平衡区里引入石墨相,则引发Gra.→Dia.＋Liq.析液反应过冷转变。这种转变是以三相定态反应链模式实现的。这种模式可以是等温的,亦可以呈一定温度梯度,分别对现行的一次性形核长大工艺和温度梯度法由籽晶/籽晶群同质外延工艺进行热力学分析和评述,并给出转变的指向和限度。分析结果还表明：在三相定态反应链模式下,人们通常所谓的包裹物都是液相卸压降温后的转变产物。本文的分析方法也适用于Fe-C体系、Ni-C体系和Fe∶Ni成分比约为5∶5~7∶3范围内的Fe-Ni-C体系。

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.