Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session J4: TM First Principles Methods IV |
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Chair: Richard Needs, University of Cambridge Room: Vashon |
Tuesday, July 9, 2013 11:00AM - 11:30AM |
J4.00001: Equations of State for Mixtures: Results from DFT Simulations of Xenon/Ethane Mixtures Compared to High Accuracy Validation Experiments on Z Invited Speaker: Rudolph Magyar We report a computational and validation study of equation of state (EOS) properties of liquid / dense plasma mixtures of xenon and ethane to explore and to illustrate the physics of the molecular scale mixing of light elements with heavy elements. Accurate EOS models are crucial to achieve high-fidelity hydrodynamics simulations of many high-energy-density phenomena such as inertial confinement fusion and strong shock waves. While the EOS is often tabulated for separate species, the equation of state for arbitrary mixtures is generally not available, requiring properties of the mixture to be approximated by combining physical properties of the pure systems. The main goal of this study is to access how accurate this approximation is under shock conditions. Density functional theory molecular dynamics (DFT-MD) at elevated-temperature and pressure is used to assess the thermodynamics of the xenon-ethane mixture. The simulations are unbiased as to elemental species and therefore provide comparable accuracy when describing total energies, pressures, and other physical properties of mixtures as they do for pure systems. In addition, we have performed shock compression experiments using the Sandia Z-accelerator on pure xenon, ethane, and various mixture ratios thereof. The Hugoniot results are compared to the DFT-MD results and the predictions of different rules for combing EOS tables. The DFT-based simulation results compare well with the experimental points, and it is found that a mixing rule based on pressure equilibration performs reliably well for the mixtures considered.\\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, July 9, 2013 11:30AM - 11:45AM |
J4.00002: An Uncertainty Quantification System for Tabular Equations of State John H. Carpenter, Allen C. Robinson, Bert Debusschere, Ann E. Mattsson, Richard R. Drake, William J. Rider Providing analysts with information regarding the accuracy of computational models is key for enabling predictive design and engineering. Uncertainty in material models can make significant contributions to the overall uncertainty in calculations. As a first step toward tackling this large problem, we present an uncertainty quantification system for tabular equations of state (EOS). First a posterior distribution of EOS model parameters is inferred using Bayes rule and a set of experimental and computational data. EOS tables are generated for parameter states sampled from the posterior distribution. A new unstructured triangular table format allows for capturing multi-phase model behavior. A principal component analysis then reduces this set of tables to a mean table and most significant perturbations. This final set of tables is provided to hydrocodes for performing simulations using standard non-intrusive uncertainty propagation methods. A multi-phase aluminum model is used to demonstrate the system. \\[4pt] {*}Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, July 9, 2013 11:45AM - 12:00PM |
J4.00003: Predicting the crystalline and porous equations of state for secondary explosives Ryan Wixom, David Damm Accurate simulations of energetic material response necessitate accurate unreacted equations of state at pressures much higher than even the C-J state. Unfortunately, for reactive materials, experimental data at high pressures may be unattainable, and extrapolation from low-pressure data results in unacceptable uncertainty. In addition to being low-pressure, the available data is typically limited to the porous state. The fully-dense, or crystalline, equation of state is required for building mesoscale simulations of the dynamic response of energetic materials. We have used quantum molecular dynamics to predict the Hugoniots and 300K isotherms of crystalline PETN, HNS, CL-20 and TATB up to pressures not attainable in experiments. The porous Hugoniots for these materials were then analytically obtained and are validated by comparison with available data. Our calculations for TATB confirm the presence of a kink in the Hugoniot, and the softening of the shock response is explained in terms of a change in molecular conformation and the loss of aromaticity. [Preview Abstract] |
Tuesday, July 9, 2013 12:00PM - 12:15PM |
J4.00004: Density Functional Theory Characterization of Potential Poly-Nitrogen Precursors Ivan Oleynik, Aaron Landerville, Brad Steele The successful recovery of pure poly-nitrogen materials, such as cubic-gauche, from diamond anvil cells has proven both difficult and elusive. As it has been proposed that impurities within a polymeric nitrogen matrix may offer increased stability upon return of the material to ambient conditions, attention has turned to nitrogen-rich compounds, such as azides, as potential precursors to impure, but recoverable forms of poly-nitrogen compounds. To aid experimentalists in the search of novel poly-nitrogen compounds, thermo-physical properties and Raman spectra of the candidate precursor ammonium-azide, along with a theoretically predicted polymorph, are calculated using Density Functional Theory with and empirical van der Waals correction. Additionally, we present preliminary results for another proposed nitrogen-rich precursor cyanuric-triazide. [Preview Abstract] |
Tuesday, July 9, 2013 12:15PM - 12:30PM |
J4.00005: Chemistry of Al in Oxidizer Medium: From Shock Initiation to Post Detonation Kinetics Santanu Chaudhuri, Martin Losada, Shahryar Fotovati Reactive materials, propellants, and thermites are often constructed from Al/oxidizer composites. Al/oxidizer composites are also considered for self-sustaining reactions for deep space applications to reduce the need for carrying oxygen. In particular, Al/Teflon, Al/I$_{2}$O$_{5}$ and Al/RDX composites will be discussed as representative Al in oxidizer systems. Results of post-detonation kinetics using transition state theory and master equation based RRKM theory will be compared including discussion on some unresolved theoretical issues in collision theories and basis set effects in predicting the temperature/pressure-dependent kinetics. For Al/Teflon system, the RRKM theory calculated fall-off curves show a significant pressure dependence of rate constant in wide range of 0-1 MPa pressures at elevated temperatures. For Al/I$_{2}$O$_{5}$ systems, incorporation of spin-orbit coupling in DFT with various standard and augmented basis sets is important. A mechanism for generation of I$_{2}$ and O$_{2}$ during the reaction will be proposed. Finally, describing shock initiation reactions inside a condensed phase Al/RDX composites for a combustion reaction or detonation is currently a challenge for theoretical chemistry and chemical dynamics community. Especially, exact theoretical treatment for kinetics of reactants in confined hot-spots under high-pressure/temperature conditions is lacking. A new collision theoretical approach and reactive embedding possiblityies will be discussed as alternative to reactive force field based simulations of hot-spot growth. [Preview Abstract] |
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