EFFECT OF CARBON CRYSTALLINITY ON THE NUCLEATION AND GROWTH OF DIAMOND UNDER HIGH-PRESSURE AND HIGH-TEMPERATURE

Using graphite with high crystallinity and carbon black with very poor crystallinity, we have investigated the effect of crystallinity on th e formation (nucleation) of diamond crystals and the growth of diamond seeds. In the case of graphite, both the synthesized diamond crystals and grown diamond seeds showed well-faceted planes. On the contrary, the morphologies of synthesized crystals and grown seeds from carbon b lack were very different from each other: well-faceted for the synthes ized crystals and irregular for the grown seeds. This result indicates that the driving force was much different between the two cases, even though the starting material was the same. When diamond seed was not present, carbon black appeared to transform first into crystalline gra phite and then into diamond. The difference in the driving force was e xplained in terms of the difference in solubility of various phases of carbon in metal liquid on a free energy-composition diagram. (C) 1998 Elsevier Science S.A.