The Reactivity of Energetic Materials Under High Pressure and Temperature

Chemical transformations that occur at the reactive shock front of energetic materials determine many aspects of material properties and performance. One major shortcoming of current explosive models is the lack of chemical kinetics data of the reacting explosive at the high pressures and, temperatures experienced under detonation conditions. In the absence of experimental data, long time-scale atomistic molecular dynamics simulations with reactive chemistry provide insight into the decomposition mechanisms of explosives and allow us to obtain effective reaction rates. These rates can then be incorporated into a thermochemical continuum code for accurate and predictive description of grain- and continuum-scale dynamics of reacting explosives. During the course of the past decade, we have examined the chemistry of several reacting explosive materials, such as the high-performing HMX and the very insensitive TATB explosives, both of which are organic molecular solids at ambient conditions. We have used quantum-based, self-consistent charge density functional tight-binding method to calculate the interatomic forces in molecular dynamics simulations either for thermal decomposition studies (constant volume temperature) or dynamical shock studies using the multiscale shock simulation technique (MSST). These studies allow us to investigate the chemical reactivity of explosives and to examine electronic properties at extreme conditions of temperature and pressure for a relatively long timescale on the order of several hundreds of picoseconds. In this chapter, we discuss challenges in simulating the reactions of shocked energetic materials and review specific examples of our recent simulations on HMX, PETN, and shocked TATB. Each of these studies revealed interesting aspects associated with known macroscopic properties of these materials. We also discuss simulations on nonenergetic materials such as shocked carbon and methane.