Non-equilibrium molecular dynamics simulation study of heat transport in hexahydro-1,3,5-trinitro-s-triazine (RDX)

[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...

Section: Discussionmentioning

confidence: 58%

Section: Hb-ff Vs Hb-ff + + Fixnhmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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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

“…The PGM is the prevalent approach for estimating bulk thermal conductivity in crystalline solids. However, it has been shown to breakdown for some perovskites and selenide compounds with highly anharmonic Hamiltonians . This is because the PGM is based on an assumption of weakly interacting phonons thus resulting in phonon mean free paths (MFP), denoted by

Λ

, that are larger than the lattice constant.…”

Section: Methodsmentioning

confidence: 99%

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

Kumar

VanGessel

Chung

2019

Self Cite

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.

Predicted Melt Curve and Liquid Shear Viscosity of RDX up to 30 GPa

Kroonblawd

Springer

2022

Recent grain‐scale simulations of HMX and TATB have shown that predictions for hot spot formation in high explosives are particularly sensitive to accurate determinations of the pressure‐dependent melt curve and the shear viscosity of the liquid phase. These physics terms are poorly constrained beyond ambient pressure for the explosive RDX. We adopt an all‐atom modeling approach using molecular dynamics (MD) simulations to predict the melt curve of RDX near to detonation conditions (30 GPa) and determine the shear viscosity of the liquid as a function of temperature and pressure above the melt curve. Phase‐coexistence simulations were used to determine the melt curve, which is predicted to vary by almost 1100 K as the pressure increases from 0 GPa to 30 GPa. Equilibrium MD simulations and the Green‐Kubo formalism were used to obtain the pressure‐temperature‐dependent shear viscosity. The shear viscosity of RDX is predicted to be of similar magnitude to the viscosity of TATB at low GPa‐range pressures, and to be roughly an order of magnitude lower than the viscosity of HMX. The temperature dependence of the shear viscosity is Arrhenius at a given pressure, and the exponential pre‐factor and activation term exhibit a strong, yet complicated, pressure dependence. An empirical pressure‐temperature dependent function for RDX shear viscosity is developed that simultaneously captures a wide range of MD predictions while taking an analytic form that extrapolates smoothly beyond the fitted regime. The relative strength of the pressure and temperature dependencies of these two physics terms is found to be of similar magnitude for RDX, HMX, and TATB, which motivates incorporating these results in future RDX grain‐scale modeling.