Crystallization of Diamond and Graphite

Abstract

The key features of the complete, stable nickel—carbon phase diagram have been established at pressures up to 54 kbar, and these features have been directly related to the nucleation and growth characteristics of diamonds in this system. At pressures in excess of 52.5 kbar, a liquid+graphite+diamond eutectic is formed at approximately 0.3 at. % nickel which results in an effective displacement of the diamond—graphite equilibrium line. This displacement is shown to be of considerable importance in the nucleation and growth of diamonds in this system. Under low and moderate driving forces, the rate-controlling mechanism in the growth on a speed diamond is the diffusion of carbon across the molten metal film surrounding the diamond. The observed diffusion coefficients at 54 kbar and 1660°<T<1800°K lie in the range: 2×10−5<D<4×10−5 cm2/sec.