ANALYTICAL STUDY OF THE THERMODYNAMIC FACTORS CONTROLLING DIAMOND SYNTHESIS AND GROWTH AT HIGH PRESSURES AND TEMPERATURES.

A method for predicting a priori the conditions for indirect diamond synthesis, i.e., synthesis via a metal catalyst or solvent, was developed. The method is based upon thermodynamic calculation of the high pressure phase equilibria for a given metal-carbon system, in which the liquid phase is approximated as a regular solution, i.e., a linear dependence of the volume of liquid metal-carbon alloys with composition has been assumed. Calculations were carried out for the nickel-carbon and manganesecarbon phase equilibria, representing non-carbide forming and carbide forming systems at 1 atmosphere pressure, respectively. For comparison to the calculations, the eutectic temperature of the nickel-carbon system was measured experimentally at 40 kb pressure by means of in situ electrical resistances vs. temperature measurements and post-mortem metallographic observations the calculated and experimental values are in agreement. Comparison of the calculated nickel-carbon phase equilibria with the experimental diamond synthesis data of other workers indicates that the lower temperaturepressure limits for indirect diamond synthesis are controlled by the eutectic temperature at pressure and the diamond-graphite equilibrium line. Author