Chapter 10 Thermodynamic and mechanical properties of HMX from atomistic simulations

Abstract

The goal of studies to understand the physical response of plastic-bonded high explosives (PBXs) is to identify and characterize the essential dissipative processes that must be captured to provide reliable, physically based subgrid physics models for use in predictive, macroscale engineering codes. The high explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is the energetic material in a number of high performance military explosive and propellant formulations. HMX exhibits three crystal polymorphs at ambient pressure denoted β-, α-, and ∂-HMX and listed in terms of stability with increasing temperature. At temperature T∼550 K the crystalline structure becomes unstable and HMX begins to melt. The liquid phase of HMX is unstable and, therefore, no direct measurements of hermophysical properties of liquid HMX have been conducted. Nevertheless, melt properties as well as mechanical and thermodynamic properties of crystalline HMX are crucial for a complete modeling of HMX decomposition under external stimuli. Judicious application of molecular simulation tools for the calculation of thermophysical and mechanical properties is a viable strategy for obtaining some of the information required as input to mesoscale equations of state. Given a validated potential-energy surface, simulations can serve as a complement to experimental data by extending intervals in pressure and temperature for which information is available. This chapter also details force field—general philosophy, quantum chemistry, and force field parameterization and validation. There are descriptions of simulations of liquid HMX—its viscosity and self-diffusion coefficient, and thermal conductivity—crystalline HMX—the structural properties, enthalpy of sublimation, and hydrostatic compression. There are notes on the properties of melt curve, the specific heat and Gruneisen coefficient, and solid–solid phase transitions. β-∂ transition involves conformational rearrangement of the molecular structure and is accompanied by damage that arises of in-commensurate structures.