LA-UR-01-3541
Thomas D. Sewell, Dmitry Bedrov, Ralph Menikoff, and Grant D. Smith
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. Additionally, the isothermal compression curve was computed for beta-HMX for 0-10.6 GPa; the bulk modulus K and its pressure derivative K' 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.
To appear in Proceedings of the 2001 APS Topical Conference on the Shock Compression of Condensed Matter, June 2001, Atlanta, GA.
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LA-UR-01-3489
Dmitry Bedrov, Grant D. Smith, and Thomas D. Sewell
Molecular dynamics simulations using a recently developed quantum chemistry-based atomistic force field were performed in order to obtain unit cell parameters, coefficients of thermal expansion, and heats of sublimation for the three pure crystal polymorphs of HMX. The predictions for beta-, alpha-, and delta-HMX showed good agreement with the available experimental data.
To appear in Proceedings of the 2001 APS Topical Conference on the Shock Compression of Condensed Matter, June 2001, Atlanta, GA.
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LA-UR-01-0091
K.O. Rasmussen, T.D. Sewell, T. Lookman, and A. Saxena
We compare for multiblock copolymers the results of mean field calculations with those from Monte Carlo simulations based on the bond fluctuation method and experimental results from scattering data. The application of Leibler's theory for copolymers and the results of Monte Carlo simulations indicate that the microphase separation transition occurs at larger chi-N as the number of blocks is increaesd 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 is in qualitative agreement with experimental results for model multiblock poly(styrene-isoprene) systems and recent results for a segmented poly(ester-urethane).
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LA-UR-00-2377
D. Bedrov, C. Ayyagari, G.D. Smith, T.D. Sewell, R. Menikoff, and J.M. Zaug
Molecular dynamics simulations using a recently developed quantum chemistry-based atomistic force field (J. Phys. Chem. B, 1999, 103, 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 beta-, alpha-, and delta-HMX showed good agreement with the available experimental data. For the case of beta-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.
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LA-UR-00-3804-revised
Ralph Menikoff and Thomas D. Sewell
Plastic-bonded explosives are heterogeneous materials. Initiation of a PBX is dominated by hot spots which are subgrain in size. Consequently, simulations of hot spots require resolving individual explosive grains. Computations on the grain scale are called meso-scale simulations. At the grain level and explosive is crystalline and, by its very nature, anisotropic. This has an effect on the dissipative mechanisms leading to the formation and evolution of hot spots. Here we focus on the explosive HMX. Properties of HMX needed for meso-scale simulations are discussed and the available data are reviewed.
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LA-UR-00-3608-revised
Ralph Menikoff and Thomas D. Sewell
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 the data used for the fit. This point is illustrated by re-analyzing two data sets for beta-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.
To appear in High Pressure Research
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LA-UR-98-3820
Carl M. Bennett and Thomas D. Sewell
Isothermal-isobaric Monte Carlo calculations are used in conjunction with an expression that relates the elastic stiffness tensor to the mean-square fluctuations of the strain tensor to obtain ``first principles'' predictions of the Young's moduli, shear moduli, and Poisson's ratios for room-temperature crystalline RDX. The results are based on numerical data obtained during previously reported calculations of the hydrostatic compression of RDX over the pressure domain 0 GPa < p < 4 GPa [J. Appl. Phys. 83, 4142 (1998)]. Although there are no experimental data available for comparison, the predicted values of the engineering coefficients are in accord with general expectations for brittle molecular crystals. The calculations reported here are preliminary: more extensive Monte Carlo realizations are needed to yield well-converged predictions; these are underway for RDX and beta-HMX.
To appear Proceedings of the Eleventh Symposium (International) on
Detonation
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