Predicted Melt Curve and Liquid Shear Viscosity of RDX up to 30 GPa

Abstract

Recent grain‐scale simulations of HMX and TATB have shown that predictions for hot spot formation in high explosives are particularly sensitive to accurate determinations of the pressure‐dependent melt curve and the shear viscosity of the liquid phase. These physics terms are poorly constrained beyond ambient pressure for the explosive RDX. We adopt an all‐atom modeling approach using molecular dynamics (MD) simulations to predict the melt curve of RDX near to detonation conditions (30 GPa) and determine the shear viscosity of the liquid as a function of temperature and pressure above the melt curve. Phase‐coexistence simulations were used to determine the melt curve, which is predicted to vary by almost 1100 K as the pressure increases from 0 GPa to 30 GPa. Equilibrium MD simulations and the Green‐Kubo formalism were used to obtain the pressure‐temperature‐dependent shear viscosity. The shear viscosity of RDX is predicted to be of similar magnitude to the viscosity of TATB at low GPa‐range pressures, and to be roughly an order of magnitude lower than the viscosity of HMX. The temperature dependence of the shear viscosity is Arrhenius at a given pressure, and the exponential pre‐factor and activation term exhibit a strong, yet complicated, pressure dependence. An empirical pressure‐temperature dependent function for RDX shear viscosity is developed that simultaneously captures a wide range of MD predictions while taking an analytic form that extrapolates smoothly beyond the fitted regime. The relative strength of the pressure and temperature dependencies of these two physics terms is found to be of similar magnitude for RDX, HMX, and TATB, which motivates incorporating these results in future RDX grain‐scale modeling.

New Nonreactive Force Field for Accurate Molecular Dynamics Simulations of TATB at Extreme Conditions

“…We obtained corresponding estimates of T m at atmospheric pressure for the three nonreactive FFs using the phase-coexistence approach. These simulations followed the same general simulation protocol as was applied to obtain the pressure-dependent melting curves for TATB, HMX, and RDX . Our simulations considered melting from the (100) crystal face and used a simulation cell that initially contained a 50 × 5 × 5 crystal supercell (2500 molecules) in contact with a liquid region comprising 2500 molecules.…”

“…These simulations followed the same general simulation protocol as was applied to obtain the pressure-dependent melting curves for TATB, 52 HMX, 88 and RDX. 89 Our simulations considered melting from the (100) crystal face and used a simulation cell that initially contained a 50 × 5 × 5 crystal supercell (2500 molecules) in contact with a liquid region comprising 2500 molecules. The (100) facet was chosen based on predictions from the Bedrov FF made by Mathew et al, 51 who found that this facet exhibits the fastest melting kinetics and is the least susceptible to overheating among the three "primary" facets (100), (010), and (001).…”

United atom and coarse grained models for crosslinked polydimethylsiloxane with applications to the rheology of silicone fluids

“…14 Indeed, even the oft-followed Arrhenius temperature dependence of liquid shear viscosity cannot be derived from first principles nor the prefactor or activation energy term related to fundamental molecular processes. This shortcoming in theory leaves molecular dynamics (MD) simulations 15,16 and experiments 17–19 as the main methods by which one can obtain data to fit empirical viscosity models.…”