Thomas D. Sewell

A molecular dynamics simulation study of elastic properties of HMX

Atomistic simulations were used to calculate the isothermal elastic properties for β-, α-, and δ-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The room-temperature isotherm for each polymorph was computed in the pressure interval 0⩽p⩽10.6 GPa and was used to extract the initial isothermal bulk modulus Ko and its pressure derivative using equations of state employed previously in experimental studies of the β-HMX isotherm. The complete elastic tensor for each polymorph was calculated at room temperature and atmospheric pressure. For the case of β-HMX, the calculated elastic tensor is compared to one based on a fit to sound speed data yielding reasonably good agreement. The bulk modulus of β-HMX obtained from equation-of-state fits to the room-temperature isotherm agrees well with that determined from the complete elastic tensor and from volume fluctuations at atmospheric pressure. However, the value of Ko obtained from the isotherm is sensitive to choice of equation of state fitting form and to t...

Constituent properties of HMX needed for mesoscale simulations

Plastic-bonded explosives are heterogeneous materials. Improved burn models for weak initiation relevant to accident scenarios require a better understanding of the physics associated with the formation and growth of hot spots. Since the relevant length scale is subgrain in extent, mesoscale simulations are needed to study hot spots. Mesoscale simulations require as input constitutive properties of an explosive grain. In addition, it is essential to account for physical dissipative mechanisms since hot spots represent local peaks in the fluctuations of the temperature field. Here, constitutive properties of the explosive HMX needed for mesoscale simulations are discussed and experimental data reviewed. Because some decomposition may occur during a measurement, it is difficult to account for systematic error in the data. To get a sense of the uncertainties in material parameters, it is necessary to examine all the available data. In addition, we discuss results from molecular dynamics simulations of some properties for which experimental data are lacking.

Molecular dynamics simulations of HMX crystal polymorphs using a flexible molecule force field

Molecular dynamics simulations using a recently developed quantum chemistry-based atomistic force field [J. Phys. Chem. B, 103 (1999) 3570 ] were performed in order to obtain unit cell parameters, coefficients of thermal expansion, and heats of sublimation for the three pure crystal polymorphs of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The predictions for β-, α-, and δ-HMX showed good agreement with the available experimental data. For the case of β-HMX, anisotropic sound speeds were calculated from the molecular dynamics simulation-predicted elastic coefficients and compared with recent Impulsive Stimulated Light Scattering (ISLS) sound speed measurements. The level of agreement is encouraging.

Fitting forms for isothermal data

Abstract Isothermal data in the (V, P)-plane are generally not sufficiently precise to determine the bulk modulus and its pressure derivative using finite differences. Instead the data are fit to an analytic expression and the derivatives of the analytic expression are used. The derivatives obtained in this fashion may be sensitive to the fitting form and the domain of data used for the fit. This point is illustrated by re-analyzing two data sets for β-HMX. With the third order Birch-Murnaghan equation and a Hugoniot based fitting form we show that the uncertainty in the modulus due to the fitting forms is greater than the statistical uncertainty of the fits associated with the experimental error bars. Moreover, there is a systematic difference between the two data sets. Both fitting forms give statistically good fits for both experiments, although the modulus at ambient pressure ranges from 10.6 to 17.5 GPa. The large variation in the initial value of the modulus is due in part to the lack of data in the low pressure regime (below 1 GPa) and to the property of a molecular crystal, in contrast to a metal or atomic crystals, to stiffen substantially under a small amount of compression. The values of the modulus and its derivative are an important issue for an explosive like HMX because they affect predictions of the Hugoniot locus in the regime of the Chapman-Jouget detonation pressure.

Elastic Properties of HMX

Atomistic molecular dynamics simulations have been used to calculate isothermal elastic properties for {beta}-, {alpha}-, and {delta}-HMX. The complete elastic tensor for each polymorph was determined at room temperature and pressure via analysis of microscopic strain fluctuations using formalism due to Rahman and Parrinello [J. Chem. Phys. 76,2662 (1982)]. Additionally, the isothermal compression curve was computed for {beta}-HMX for 0 {le} p {le} 10.6 GPa; the bulk modulus K and its pressure derivative K{prime} were obtained from two fitting forms employed previously in experimental studies of the {beta}-HMX equation of state. Overall, the results indicate good agreement between the bulk modulus predicted from the measured and calculated compression curves. The bulk modulus determined directly from the elastic tensor of {beta}-HMX is in significant disagreement with the compression curve-based results. The explanation for this discrepancy is an area of current research.

Quantum Chemistry–Based Force Field for Simulations of Energetic Dinitro Compounds

ABSTRACT A quantum chemistry–based force field for molecular dynamics simulations of energetic dinitro compounds has been developed, based on intermolecular binding energies, molecular geometries, molecular electrostatic potentials, and conformational energies obtained from quantum chemistry calculations on model compounds. Nonbonded parameters were determined by fitting experimental densities and heats of vaporizations of model compounds. Torsional parameters were parameterized to reproduce accurately the relative conformational energy minima and barriers in 2,2-dinitropropane, di-methoxy di-methyl ether, 2,2-dinitro-3-methoxypropane, and bis(2,2-dinitropropyl)formal. Molecular dynamics simulations using the developed force field accurately reproduce thermodynamic and transport properties of 1,1-dinitroethane, 2,2-dinitropropane, and a eutectic mixture of bis(2,2-dinitropropyl)formal and bis(2,2-dinitropropyl)acetal.

Complete Equation of State for β‐HMX and Implications for Initiation

A thermodynamically consistent equation of state for β‐HMX, the stable ambient polymorph of HMX, is developed that fits isothermal compression data and the temperature dependence of the specific heat computed from molecular dynamics. The equation of state is used to assess hot‐spot conditions that would result from hydrodynamic pore collapse in a shock‐to‐detonation transition. The hot‐spot temperature is determined as a function of shock strength by solving two Riemann problems in sequence: first for the velocity and density of the jet formed when the shock overtakes the pore, and second for the stagnation state when the jet impacts the far side of the pore. For a shock pressure below 5 GPa, the stagnation temperature from the jet is below the melt temperature at ambient pressure and hence insufficient for rapid reaction. Consequently, for weak shocks a dissipation mechanism in addition to shock heating is needed to generate hot spots. When the stagnation temperature is sufficiently high for rapid reacti...

Monte Carlo Simulations of Crystalline TATB

We are performing constant-NPT Monte Carlo calculations of the physical properties of crystalline TATB. Our approach is to employ an atomistic model in which the individual molecules are treated as semi-rigid entities. Each molecule is allowed to undergo rigid translations and rotations, and in some cases limited intramolecular flexibility is conferred on the molecules via exocyclic torsions. Additionally, the size and shape of the simulation box is allowed to vary. Our immediate interest is in computing the density, lattice energy, lattice constants, and other structural parameters as a function of temperature. Preliminary results indicate that simulations involving only two molecules suffice for calculations of the energy and density, but that more molecules are required to compute the lattice constants. Intramolecular flexibility is important, particularly at higher temperatures.

Molecular Dynamics Simulations of HMX Crystal Polymorphs Using a Flexible Molecule Force Field

Molecular dynamics simulations using a recently developed quantum chemistry‐based atomistic force field [J. Phys. Chem. B 103 (1999) 3570] were performed in order to obtain unit cell parameters, coefficients of thermal expansion, and heats of sublimation for the three pure crystal polymorphs of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX). The predictions for β‐, α‐, and δ‐HMX showed good agreement with the available experimental data.

Mean Field and Monte Carlo Modeling of Multiblock Copolymers

The authors discuss and apply extensions needed to treat multiblock copolymers within the mean field theoretical framework for microphase separation in diblock copolymer metals, originally due to Leibler. The mean field calculations are complemented by lattice Monte Carlo realizations using the bond fluctuation model. They find that the microphase separation transition occurs at larger {sub {chi}}N as the number of blocks in increased beyond two (i.e., beyond diblock), and that the characteristic length scale of the emerging morphology decreases as the number of blocks increases. The latter prediction is in qualitative agreement with published experimental results due to Sontak and co-workers for model multiblock poly(styrene-isoprene) systems and recent results due to Hjelm and co-workers for a segmented poly(ester-urethane) relevant to Los Alamos interests. Additionally, the mean field predictions and bond fluctuation realizations yield consistent results.