Predicted melt curve and liquid shear viscosity of RDX and HMX up to detonation pressures

Recent grain-scale simulations have shown that hot spot formation mechanisms in high explosives are sensitive to the pressure-dependent melt curve and the shear viscosity of the liquid phase. Despite this importance, these physics terms are poorly constrained at GPa-range pressures due to a lack of experiments. Through molecular dynamics (MD) modeling, we provide the first direct predictions of the melting curve for RDX and HMX up to detonation-like pressures. Equilibrium MD simulations and the Green-Kubo formalism are applied to obtain the liquid shear viscosity as a function of temperature and pressure above the melt curve. Pressure sensitivity of the melt curve is found to be substantially greater than widely used estimates based on the Lindemann law. At the same time, the viscosity exhibits Arrhenius temperature dependence with a complicated pressure dependence. At a given pressure, the melting point of HMX is hundreds of Kelvin higher than RDX and the shear viscosity of HMX is similarly larger by an order of magnitude. Grain scale simulations showing the implications of these findings on hot spot formation processes are discussed.

 

Prepared by LLNL under Contract DE-AC52-07NA27344 and approved for unlimited release under document number LLNL-ABS-832269.