Theoretical determination of anisotropic thermal conductivity for initially defect-free and defective TATB single crystals

Abstract

The anisotropic thermal conductivity was determined for initially defect-free and defective crystals of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a material that exhibits a graphitic-like packing structure with stacked single-molecule-thick layers, using the reverse non-equilibrium molecular dynamics method and an established TATB molecular dynamics force field. Thermal conduction in TATB is predicted to be substantially higher and more anisotropic than in other related organic molecular explosives, with conduction along directions nominally in the plane of the molecular layers at least 68% greater than conduction along the direction exactly perpendicular to the layers. Finite-size effects along the conduction directions were assessed. The conductivity along directions nominally in the plane of the molecular layers was found to be insensitive to the supercell length along the conduction direction-a result commensurate with the estimated phonon mean free path, ∼6 Å. A small decrease in the conductivity normal to the layers was found for longer supercells and is likely due to increased phonon scattering as a result of dynamic structural transitions in the crystal. The thermal conductivity of TATB crystals containing vacancy defects was also determined and the variation of conductivity with crystal density was found to be both linear and anisotropic, with the introduction of vacancy defects leading to a greater percentage reduction in conduction for the direction perpendicular to the molecular layers.

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

“…Parameters for the U bond and U bend potentials in the HB-FF were adjusted to approximately reproduce experimental spectral frequenciesf or TATB crystal and normal modes for an isolated molecule [ 6].R eference normal modes were obtainedu sing ad ensity functional theory (DFT)e lectronic structure calculation. RNEMD simulations and the HB-FFw ere used to predict the room-temperature atmospheric-pressure (300 K, 0.0 GPa) anisotropic thermal conductivity of initially defect-free TATB crystals [6,7] and TATB crystals with molecularv acancy defects [7]. TATB crystals tacking-fault energiesw ere also determined [25] using the HB-FF;t he exceptionally low barrier to slid-ing of the planar molecularl ayers was associated [7] with the formation of subtle dynamic structural transitions at (300 K, 0.0 GPa).…”

“…In this report we use the reversen on-equilibriumm olecular dynamics method( rNEMD) [1,40,41] and Fourier's heat law to determine the thermal conductivity of TATB crystal as functions of temperature and pressure and assesst he sensitivity of predictions for l to intramolecular forcef ield terms. The rNEMDm ethodi se stablished [1,[4][5][6][7][40][41][42][43][44], has well-understoodl imitations whena pplied to crystals [42,44],a nd is particularly easy to implement in comparison to othera pproaches [2][3][4]45],f or instance those using Green-Kubo fluctuation theory.W ith the rNEMD method one imposes an exactly known energy flux J betweens patially separated hot and cold regions in the material and measures the resulting average temperature gradient < r x T > that develops between them.F ourier's heat law is used to obtain the conductivity l as the proportionality constant between J and < r x T > .O ne particularl imitation of rNEMD applied to crystals is the introduction of artificial crystal boundaries due to the hot and cold regions, which leads to an additional phonon scattering source [42].I nt he case of TATB crystalw eh ave already determined [6,7] that rNEMD predictions for l are largely insensitive to the crystal supercell dimensions and therefore assume l bound is negligible compared to other scattering mechanisms. The reader is referred to Refs.…”

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1IntroductionCharacterizationo fe nergy transport processes in molecular materials usinga ll-atom molecular dynamics( MD) simulations provides anecessaryand physical basis for the prediction and understanding of experimentally undetermined properties [1][2][3][4][5][6][7],m any of whicha re needed for the parameterization of the kinds of continuum-based mesoscale engineeringm odels widely used to simulate energetic materials [8][9][10][11][12][13][14][15]. Accurate predictionsf or anisotropic bulk material properties such as the thermal conductivity and rate coefficientsf or energyt ransfer processes are necessary if parameterized models are to yield reliable predictions for dynamic processes such as shock loading [9][10][11][12][14][15][16][17][18],h ot spot formation and relaxation [9-12, 14, 15, 19],a nd initiation of chemistry [11,14,19].R ecentM D-based predictions for the thermal conductivity of the insensitivem olecular crystalline explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) revealed significanta nisotropy for energy transport at T = 300 Ka nd P = 0.0 GPa [6,7]. ( Here and for all subsequent instances, pressures reported as 0.0 GPa formally correspond to 1atm, which is 0.0 GPa within the precision of our calculations.)

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Tandem Molecular Dynamics and Continuum Studies of Shock‐Induced Pore Collapse in TATB

“…The crystal is triclinic with two molecules per unit cell. The structure of the crystal is strongly anisotropic and this is reflected in pronounced anisotropy for many of the thermal [39][40][41][42][43][44][45][46][47][48][49][50], mechanical [51][52][53][54][55][56][57][58][59][60], and optical properties [61].…”

Dislocation Core Structure at Finite Temperature Inferred by Molecular Dynamics Simulations for 1,3,5-Triamino-2,4,6-trinitrobenzene Single Crystal

“…24 The nonpolarizable, 20% scaled-charges force field for TATB initially developed by Bedrov et al 2 was used with planar, symmetric rigid molecules. They predicted for the first time TATB elastic constants and crystal structure evolution in T and P. This force field was modified by Kroonblawd and Sewell 3,4 and used to investigate the thermal behavior of TATB and more recently to obtain the first generalized stacking fault surface and to explore the mechanisms of deformation under nanoindentation. 5 Instead of the Ewald summation for the computation of longrange electrostatic interactions, the Reaction Field 25−27 approximation with a 13 Å cutoff and 100.0 for the dielectric constant ε was used.…”