(714c) Atomic- and Molecular-Scale Simulations of Energy Transport and Shock Dynamics in Crystalline TATB

Brief vignettes of recent theoretical studies of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) will be presented, namely, anisotropic thermal and energy transport properties of oriented crystals, predicted using all-atom molecular dynamics (MD), and the response of the crystal to shock wave loading, predicted using energy-conserving dissipative particle dynamics (DPDE, a coarse-grained MD-like simulation method). (1) Anisotropic thermal conductivity coefficients were determined for initially defect-free and defective TATB crystals at various temperatures and pressures. The room temperature, atmospheric pressure thermal conductivity for TATB is predicted to be generally greater and more anisotropic than the thermal conductivities of other molecular explosives; conduction within the hydrogen-bonded layers in the crystal is at least 68% greater than conduction between them. The phonon mean free path length is predicted to be less than 1 nm. Decreases in thermal conductivity induced by molecular vacancy defects are also anisotropic and exhibit a linear dependence on defect density. (2) Direction-dependent energy transfer from idealized 1-D hot spots was studied. Results from the hot-spot relaxation simulations were fit to an analytical solution for the 1-D continuum heat equation by treating the thermal diffusivity as a parameter. Temperature-dependent thermal diffusivity was also studied, albeit at the cost of having to resort to a numerical solution scheme for the heat equation. Validity of the continuum heat equation predictions for TATB is assessed for length scales below 20 nm.   (3) A DPDE coarse-grain model for TATB was developed in which individual TATB molecules are treated as rigid bodies. The intramolecular vibrational heat capacity, which would otherwise be a casualty of the coarse-graining, is incorporated using a simple “implicit degrees of freedom” model that accurately captures the quantum-mechanical temperature dependence of the heat capacity. The value for the adjustable parameter σ that couples the implicit intramolecular and explicit intermolecular degrees of freedom in the DPDE model was determined by fitting to the results of all-atom MD simulations for idealized energy relaxation scenarios. The resulting DPDE model was used to simulate the shock response of (001)-oriented crystal. Sensitivity of the results to σ, including profiles of temperature and stress and the nature of the inelastic deformation, will be summarized.