Graphitization of diamond at zero pressure and at a high pressure

Abstract Natural diamonds have been heated in the temperature range of 1850 to 2000 °C at zero pressure and the rates at which diamond transforms to graphite measured. For 111 and 110 surfaces activation energies of 253+18 and 174+12 kcal mol-1 (1159 + 75 and 728 + 50 kJ/mol) respectively have been obtained. Diamonds have also been heated in the temperature range of 1950 to 2200 °C under a pressure of 48 + 3 kbar (4. 8 + 0. 3 GPa) and an activation volume of about 10 cm3 mol-1 obtained for both 111 and 110 surfaces. It is proposed that the rate controlling process in the graphitization of diamond is the detachment of a single atom from the diamond surface. This is contrary to previous proposals in which the detachment of groups of atoms have been considered to be the rate-controlling process. In the present work, it is suggested that the rate-controlling step for graphitization is the detachment of a triply bonded atom from a 111 surface and of a doubly bonded atom from a 110 surface.