Kinetics and thermodynamic behavior of carbon clusters under high pressure and high temperature

Abstract

Physical processes that govern the growth kinetics of carbon clusters at high pressure and high temperature are: (a) structural sp 3-to-sp 2 and sp 2-to-sp bonding changes and (b) cluster diffusion. Our study on item (a) deals with ab initio and semi-empirical quantum mechanical calculations to examine effects of cluster size on the relative stability of graphite and diamond clusters and the energy barrier between the two. We have also made molecular dynamics simulations using the Brenner bond order potential to show that the melting line of diamond based on the Brenner potential is reasonable and that the liquid structure changes from mostly sp-bonded carbon chains to mostly sp 3-bonding over a narrow pressure interval. We examined item (b) by using the time-dependent cluster size distribution function obtained from the relevant Smoluchowski equations. The resulting surface contribution to the Gibbs free energy of carbon clusters was implemented in a thermochemical equilibrium code and also to a new detonation model in a hydrodynamic code to examine the behavior of carbon-rich explosives. We find that carbon-rich explosives are sensitive to the metastability of graphitic carbon clusters and also to a delayed release of the surface energy of carbon clusters by the slow diffusive clustering processes.