Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session P5: First-Principles and MD V: Melting & Energy Transport |
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Chair: Eduardo Bringa, Universidad Nacional de Cuyo, Igor Schweigert, Naval Research Laboratory Room: Grand I/J |
Wednesday, June 17, 2015 11:15AM - 11:30AM |
P5.00001: Molecular Dynamics Simulations of Shock Wave Propagation across the Nitromethane Crystal-Melt Interface Shan Jiang, Thomas D. Sewell, Donald L. Thompson We are interested in understanding the fundamental processes that occur during propagation of shock waves across the crystal-melt interface in molecular substances. We have carried out molecular dynamics simulations of shock passage from the nitromethane (100)-oriented crystal into the melt and \textit{vice versa} using the fully flexible, non-reactive Sorescu, Rice, and Thompson force field. A stable interface was established for a temperature near the melting point by using a combination of isobaric-isothermal (NPT) and isochoric-isothermal (NVT) simulations. The equilibrium bulk and interfacial regions were characterized using spatial-temporal distributions of molecular number density, kinetic and potential energy, and C-N bond orientations. Those same properties were calculated as functions of time during shock propagation. As expected, the local temperatures (intermolecular, intramolecular, and total) and stress states differed significantly between the liquid and crystal regions and depending on the direction of shock propagation. Substantial differences in the spatial distribution of shock-induced defect structures in the crystalline region were observed depending on the direction of shock propagation. [Preview Abstract] |
Wednesday, June 17, 2015 11:30AM - 11:45AM |
P5.00002: Theoretical studies of anisotropic energy transport in TATB crystals Matthew Kroonblawd, Thomas Sewell Anisotropic thermal transport properties were determined theoretically for single crystals of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) using molecular dynamics. TATB exhibits a graphitic-like layered packing structure with a two-dimensional hydrogen-bonding network within, but not between, the molecule-thick layers that comprise the crystal. Anisotropic thermal conductivity coefficients were determined for initially defect-free and defective TATB crystals at various temperatures and pressures, and direction-dependent relaxation of idealized hot spots was studied. The room temperature, atmospheric pressure thermal conductivity for TATB is predicted to be generally greater and more anisotropic than the thermal conductivities of other molecular explosives; conduction within the layers is at least 68{\%} greater than conduction between them. The phonon mean free path length is predicted to be less than 1 nm. Decreases in thermal conductivity induced by molecular vacancy defects are also anisotropic and exhibit a linear dependence on defect density. Results from the hot-spot relaxation simulations were compared with and fit to an analytical solution for the one-dimensional continuum heat equation by treating the thermal diffusivity as a parameter. Validity of the continuum heat equation predictions for TATB is assessed for length scales below 20 nm. [Preview Abstract] |
Wednesday, June 17, 2015 11:45AM - 12:00PM |
P5.00003: Melting curves of metals by ab initio calculations Dmitry Minakov, Pavel Levashov In this work we used several ab initio approaches to reproduce melting curves and discussed their abilities, advantages and drawbacks. We used quasiharmonic appoximation and Lindemann criterion to build melting curves in wide region of pressures. This approach allows to calculate the total free energy of electrons and phonons, so it is possible to obtain all thermodynamic properties in the crystalline state. We also used quantum molecular dynamics simulations to investigate melting at various pressures. We explored the size-effect of the heat until it melts (HUM) method in detail. Special attention was paid to resolve the boundaries of the melting region on density. All calculations were performed for aluminum, copper and gold. Results were in good agreement with available experimental data. Also we studied the influence of electronic temperature on melting curves. It turned out that the melting temperature increased with the rise of electron temperature at normal density and had non-monotonic behavior at higher densities. [Preview Abstract] |
Wednesday, June 17, 2015 12:00PM - 12:15PM |
P5.00004: Graphite melting: atomistic kinetics bridges theory and experiment Nikita Orekhov, Vladimir Stegailov Unique thermophysical properties of graphite result in its important role in science and engineering. However, the experimental data on graphite melting temperature (T$_{\mathrm{m}})$ still remain controversial despite the long history of investigation. The experimental results of several works cover the wide span from 3800 to 5000 K that is an essentially larger uncertainty than the errors of individual experiments. In this work we deploy the molecular dynamics (MD) method and study the kinetics of graphite melting, concerning the aspects of defect formation, single graphene layer melting and the rates of spontaneous liquid nuclei formation. Our MD calculations show an unexpectedly weak kinetics of the melting front propagation in graphite that is several orders slower than that in metals. We demonstrate that at sufficiently high heating rates (higher than 10$^{5} - $ 10$^{6}$ K/s) the temperatures 500-1000 K above the graphite melting temperature can be reached before the crystal decay. It allows us to explain long-standing problem of the discrepancy in the experimental data making a hypothesis that there is a strong dependence between experimentally measured graphite melting temperatures and corresponding rates of heating. [Preview Abstract] |
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