Atomistic Simulations of Detonation Instabilities in Condensed Phase Systems

We report the results of simulations of condensed phase detonation phenomena using a model diatomic system: 2AB -$>$ A$_{2}$ + B$_{2}$. The initial set of parameters for this system corresponded to the Model 0 set of C. White et al, which exhibits a steady, Chapman-Jouget (CJ) detonation structure with a reaction zone length of 30-100 {\AA}. This has a highly compressed CJ state (V/V$_{0}\sim $0.5) that does not consist of discrete molecular species. The potential form was modified so that a more molecular CJ state resulted, consistent with the models for conventional organic explosives. The new system has a less dense CJ state (V/V$_{0}\sim $0.8), and the reaction zone was substantially extended. The reaction rate fits Arrhenius-type kinetics with an activation energy of $\sim $2 eV, with a minor density dependence. In contrast, the original Model 0 system had a lower activation energy ($\sim $1 eV) with a stronger density dependence. The new system exhibits quite marked two dimensional instability structures with well-defined wavelengths similar to what has been observed for gas-phase detonations and for nitromethane. Depending on the exothermicity and the width of the periodic simulations, these instabilities can result in either detonation failure or quasi-steady propagation. The observed propagation velocities are several per cent higher than CJ values derived from thermodynamic analyses.