Anisotropic Relaxation of Idealized Hot Spots in Crystalline 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB)
MP Kroonblawd and TD Sewell, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 17214-17223 (2016).
Relaxation of nanoscopic idealized hot spots in the layered molecular crystalline explosive 1,3,5-triamino-2,4,6-rinitrobenzene (TATB) was studied along two crystallo-graphically relevant directions using all- atom molecular dynamics (MD) simulations and continuum-level models based on the diffusive heat equation. Characteristics of relaxation from initial one-dimensional, nonequilibrium temperature distributions in the crystal were determined using MD simulations. Results from these MD simulations were fit to and compared with solutions for the one- dimensional diffusive heat equation by treating the thermal diffusivity as a parameter to assess the validity of using continuum models to describe heat transport in TATB on length scales below approximately 10 nm. It is found that energy transport is predominantly diffusive both within and between the crystal layers, even on the length scale of the unit cell. Continuum-level predictions are in excellent agreement with MD results for relaxation of a 2.5 nm wide hot spot along the direction normal to the crystal layers. The corresponding best-fit diffusivity is found to agree with independent predictions for the thermal conductivity along that direction to within 5%. Modest discrepancies in the continuum predictions are present for the direction nominally within the layers, and the apparent conductivity along that direction is found to be approximately 25% lower than expected. While some of these discrepancies are reconciled by treating the thermal conductivity as a function of temperature, the lower apparent conductivity indicates that some phonon modes might transfer energy ballistically within the layers on length scales below 10 nm.
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