Predicted Anisotropic Thermal Conductivity for Crystalline 1,3,5-Triamino-2,4,6-trinitobenzene (TATB): Temperature and Pressure Dependence and Sensitivity to Intramolecular Force Field Terms
MP Kroonblawd and TD Sewell, PROPELLANTS EXPLOSIVES PYROTECHNICS, 41, 502-513 (2016).
The anisotropic thermal conductivity of the layered molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), an insensitive secondary high explosive, is determined using classical molecular dynamics on the P = 0.0 GPa isobar for temperatures 200 K <= T <= 700K and on the T= 300 K isotherm for pressures 0.0 GPa < P < 2.5 GPa. Sensitivity of the predicted (300 K, 0.0 GPa) conductivity to intramolecular terms in the force field is investigated. Two conduction directions are considered, one nominally within and the other exactly perpendicular to the stacked planar single-molecule-thick layers comprising the TATB crystal. The thermal conductivity lambda(T,P) along both directions is found to decrease approximately as lambda proportional to 1/T with increasing temperature and increase approximately linearly lambda proportional to T with increasing pressure. The temperature dependence is found to be highly anisotropic with nearly twice as large a reduction in absolute conductivity within the molecular layers (Delta lambda = -0.67 W m(-1) K-1) compared to between them (Delta lambda = -0.35 Wm (-1) K-1). Anisotropy in the conductivity is predicted to decrease with increasing temperature; the P = 0.0 GPa conductivity is 68% greater within the layers than between them at 200 K, but only 49% greater at 700 K. The pressure dependence is also anisotropic, with a 51% and 76% increase in conductivity within and between the layers, respectively. Predicted values for the conductivity are found to differ by less than 12% for several instructive modifications to the intramolecular force field. Completely eliminating high-frequency N-H bond vibrations using the SHAKE algorithm leads to an isotropic reduction in the conductivity that scales as the corresponding reduction in the classical heat capacity, indicating that optical phonons are likely significant contributors to the total conductivity. Replacing harmonic bond potential energy functions with anharmonic Morse functions results in an isotropic. 6% reduction that is likely due to stronger phonon-phonon coupling and corresponding reduction in the phonon mean free path.
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