Theoretical determination of anisotropic thermal conductivity for crystalline 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

All‐atom molecular dynamics (MD) and Eulerian continuum simulations, performed using the LAMMPS and SCIMITAR3D codes, respectively, were used to study thermo‐mechanical aspects of the shock‐induced collapse of an initially cylindrical 50 nm diameter pore in single crystals of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Three impact speeds, 0.5 km s−1, 1.0 km s−1 and 2.0 km s−1, were used to generate the shocks. These impact conditions are expected to yield collapse mechanisms ranging from predominantly visco‐plastic to hydrodynamic. For the MD studies, three crystal orientations (relative to shock‐propagation direction) were examined that span the limiting cases with respect to the crystalline anisotropy in TATB. An isotropic constitutive model was used for the continuum simulations, thus crystal anisotropy is absent. The evolution of spatiotemporally resolved quantities during collapse is reported including local pressure, temperature, pore size and shape, and material flow. Multiple models for the melting curve and specific heat were studied. Within the isotropic elastic/perfectly plastic continuum framework and for the range of impact conditions studied, the specific heat and melting curve sub‐models are shown to have modest effects on the continuum hotspot predictions in the present inert calculation. Treating the MD predictions as ‘ground truth’, albeit with a classical rather than quantum‐like heat capacity, it is clear that – at a minimum – an extension of the constitutive model to account for crystal plasticity and anisotropic strength will be required for a high‐fidelity continuum description.

“…The all‐atom, nonreactive, fully flexible force field originally due to Bedrov et al . and later refined by Kroonblawd and Sewell and Mathew et al . was used for the MD simulations.…”

Section: Methodsmentioning

confidence: 99%

Section: Force-field Modelmentioning

confidence: 99%

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

Zhao

Lee

Sewell

et al. 2020

“…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.…”

Section: Introductionmentioning

confidence: 99%

“…

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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mentioning

confidence: 99%

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

Kroonblawd

Sewell

2015

Self Cite

[a] 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.) Many other thermomechanical properties of TATB crystala lso exhibit significant anisotropy [20][21][22][23][24][25][26]. Anisotropy in TATB crystal physical properties is thought to be ac onsequence of the crystal packings tructure, which is triclinic and exhibits al ayered structure, wherein nearly planar TATB molecules form planar single-molecule-thick Abstract:T he 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 classicalm olecular dynamics on the P = 0.0 GPa isobar for temperatures 200 K T 700 Ka nd on the T = 300 Ki sotherm for pressures0 .0 GPa P 2.5 GPa. Sensitivity of the predicted( 300 K, 0.0 GPa) conductivity to intramolecular terms in the forcef ield is investigated.T wo conduction directions are considered, one nominally within and the other exactlyp erpendicular to the stackedp lanar single-molecule-thick layers comprising the TATB crystal. The thermal conductivity l(T,P)a long both directionsi s foundt od ecrease approximately as l / 1/T with increasing temperature and increase approximately linearly l / T with increasingp ressure. The temperature dependence is found to be highly anisotropic with nearly twice as large ar eduction in absolutec onductivity within the molecularl ayers (Dl = À0.67 Wm À1 K À1)c ompared to betweent hem (Dl = À0.35 Wm À1 K À1). Anisotropy in the conductivity is predicted to decrease with increasing temperature;t he P = 0.0 GPa conductivity is 68 %g reater within the layers than betweent hem at 200 K, but only 49 %g reater at 700 K. The pressure dependence is also anisotropic, with a5 1% and 76 %i ncrease in conductivity within and between the layers, respectively.P redictedv alues for the conductivity are found t...

Bond Strain and Rotation Behaviors of Anharmonic Thermal Carriers in ‐RDX

Kumar

VanGessel

Chung

2019

In this paper, we examine how bond strain and rotation carry heat in the molecular crystal α‐RDX. We calculate anharmonic lifetimes and then rank order the phonons and their distortions to the lattice by their importance as carriers of thermal energy. The motions of the atoms that constitute the strain and rotation are determined using the phonon mode energy and mode shapes under the harmonic approximation. To draw the distinction between propagating and diffusive carriers, we additionally compare the thermal conductivity estimates from three microscale models: phonon gas model, Cahill‐Watson‐Pohl formula and Allen‐Feldman harmonic theory. We found that the modes that contribute substantially to the thermal conductivity are also the dominant contributors to the strain and rotation of all bonds in α ‐RDX, among which N−N and N−O bonds were found to exhibit the largest strains and rotations. We also found that on average, the N−N bonds exhibit the largest deformation, ∼35% more strain and ∼6% more rotation than N−O bonds. Our calculations confirm that large straining of the N−N and N−O bonds results from relatively larger displacement of the nitrogen atom in the nitro group −NO2.