Effect of ambient pretreatment of graphite and solvent-catalyst on diamond formation

Diamond was formed from purified natural graphite under high pressure and temperature conditions (7 G Pa, 1700° C) using a solvent-catalyst in the unary (Fe) or binary (Fe-Ti) system. The effect of an ambient pretreatment of the starting mixed powder (graphite and solvent-catalyst) was investigated in relation to the formation and grain growth of diamond. An initial desorption of adsorbed water vapour or harmful gases from the starting powder in vacuum (2 × 10^–5 torr) at higher temperatures (>400° C) was required in order to increase the conversion ratio from graphite to diamond. The subsequent ambient pretreatment at 1000° C in different atmospheres was found to affect the grain growth process of diamond. The depression of grain growth was confirmed in both cases of pretreatments in vacuum (2 × 10^–5 torr) and in an argon atmosphere (1 × 10^–3 or 760 torr). The diamond grains were discrete in the vacuum pretreatment, while a particle joining between the diamond grains was promoted in the argon pretreatment. The pretreatment in an N₂ atmosphere (1 × 10^–3 or 760 torr) tended to accelerate the grain growth of diamond.