Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session U2: First Principles and Molecular Dynamics Calculations V |
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Chair: D. Scott Stewart, University of Illinois at Urbana-Champaign Room: Fairmont Orchid Hotel Amphitheater |
Friday, June 29, 2007 8:00AM - 8:15AM |
U2.00001: Energetic materials under thermal shock: Molecular dynamics simulation with reactive force field Yi Liu, Sergey Zybin, Adri van Duin, William Goddard The physical and chemical response of energetic materials under thermal shock loading has been investigated for RDX, PETN and HMX by molecular dynamics method with ReaxFF reactive force field parameterized from first-principles calculations. We study the propagation of a thermal front and following reactive wave from the hot spot created by fast heating of a local region and keeping it at high constant temperature. The hot spot serves as heat source to heat up adjacent materials where no temperature constraint is imposed, and trigger the chemical decomposition of energetic molecules. The mechanism and evolution of chemical reactions induced by thermal shock is discussed along with the propagation of heat, mass, pressure, and reaction waves. [Preview Abstract] |
Friday, June 29, 2007 8:15AM - 8:30AM |
U2.00002: Energetic materials under mechanical shock and shear: Molecular dynamics simulation with reactive force field Sergey Zybin, Peng Xu, Adri van Duin, William Goddard The initial physical and chemical response of energetic materials under mechanical shock or shear loading has been investigated for RDX, PETN and HMX by molecular dynamics method with ReaxFF reactive force field parameterized from first-principles calculations. We study the propagation of a shock wave and shock-induced chemical reactions created by moving piston mimicked by a potential wall. We simulate both the continuous and impulsive piston loading to investigate its influence on the initiation and decomposition reactions in energetic materials as well as the orientational dependence using large-scale parallel ReaxFF-MD simulations. Besides, we perform a series of simulations of pure shear at high strain rate as well as static uniaxial compression of energetic crystals to study their transformation and decomposition under various loading conditions. The mechanism and evolution of chemical reactions induced by mechanical shock and pure shear is discussed along with the propagation of heat, mass, pressure, and reaction waves. [Preview Abstract] |
Friday, June 29, 2007 8:30AM - 8:45AM |
U2.00003: First-principles reactive molecular dynamics of initiation chemistry in energetic materials Aaron Landerville, Ivan Oleynik, Mortko Kozhushner, Carter White Understanding of initiation chemistry of shock-compressed energetic materials on the atomic scale is one of the outstanding problems for shock wave and energetic materials community. Using first-principles density functional theory, we have performed molecular dynamics simulations of the reactive molecular collisions of several energetic molecules such as PETN and RDX aimed at elucidating the first chemical events that trigger the chemistry behind the shock wave front. These results provide an insight into fundamental mechanisms responsible for the transformation of mechanical energy from the shock wave into molecular degrees of freedom that result in excitation of a reaction mode, bond breaking and subsequent events taking place under non-equilibrium conditions of the shock wave environment. [Preview Abstract] |
Friday, June 29, 2007 8:45AM - 9:00AM |
U2.00004: Theoretical studies of shock-induced plastic deformation and phase transformations in RDX Marc Cawkwell, Thomas Sewell The relationships between shock-induced plastic deformation and initiation sensitivity have been studied in RDX using MD simulations. Homogeneous defect nucleation in (100), (111) and (210) oriented single crystals has been studied under shock loading at particle velocities of 420, 630 and 840 m/s. Deformation by shear-bands was seen in shocks parallel to [100] at particle velocities $\ge $ 630 m/s. No evidence was found for homogeneous nucleation of defects such as dislocations or shear-bands in shocks normal to (111) and (210) at any particle velocity. These results are consistent with the steric hindrance model for initiation sensitivity and flyer plate studies of oriented RDX single crystals. The collapse of 20 nm diameter cylindrical voids was studied under the same conditions mentioned above. A variety of anisotropic material responses occur, along with significant increases in intramolecular temperatures, as the voids collapsed. Also, a previously unknown shock-induced phase transformation was observed during shocks normal to (210). This phase transformation occurs homogeneously for shocks parallel to [010] at particle velocities greater than 1 km/s. The Hugoniot for shock loading in this direction has been calculated for a defect free crystal, allowing for direct experimental assessment of this prediction. LA-UR-07-1016 [Preview Abstract] |
Friday, June 29, 2007 9:00AM - 9:15AM |
U2.00005: A semi-metallic layer in detonating nitromethane Evan Reed, Riad Manaa, Laurence Fried, Kurt Glaesemann, John Joannopoulos We present the first ever glimpse behind a detonation front in a chemically reactive quantum molecular dynamics simulation (up to 0.2 ns) of the explosive nitromethane (CH$_3$NO$_2$) represented by the density-functional-based tight-binding method (DFTB). This simulation is enabled by our recently developed multi-scale shock wave molecular dynamics technique (MSST) that opens the door to longer duration simulations by several orders of magnitude. The electronic DOS around the Fermi energy initially increases as metastable material states are produced but then later {\it decreases}, perhaps unexpectedly. These changes indicate that the shock front is characterized by an increase in optical thickness followed by a reduction in optical thickness hundreds of picoseconds behind the front, explaining recent experimental observations. We find that a significant population of intermediate metastable molecules are charged and charged species play an important role in the density of states evolution and a possible Mott metal-insulator transition. [Preview Abstract] |
Friday, June 29, 2007 9:15AM - 9:30AM |
U2.00006: Atomistic Simulations of Detonation Instabilities in Condensed Phase Systems Edward Kober, Andrew Heim, Timothy Germann, Niels Jensen We report the results of simulations of condensed phase detonation phenomena using a model diatomic system: 2AB -$>$ A$_{2}$ + B$_{2}$. The initial set of parameters for this system corresponded to the Model 0 set of C. White et al, which exhibits a steady, Chapman-Jouget (CJ) detonation structure with a reaction zone length of 30-100 {\AA}. This has a highly compressed CJ state (V/V$_{0}\sim $0.5) that does not consist of discrete molecular species. The potential form was modified so that a more molecular CJ state resulted, consistent with the models for conventional organic explosives. The new system has a less dense CJ state (V/V$_{0}\sim $0.8), and the reaction zone was substantially extended. The reaction rate fits Arrhenius-type kinetics with an activation energy of $\sim $2 eV, with a minor density dependence. In contrast, the original Model 0 system had a lower activation energy ($\sim $1 eV) with a stronger density dependence. The new system exhibits quite marked two dimensional instability structures with well-defined wavelengths similar to what has been observed for gas-phase detonations and for nitromethane. Depending on the exothermicity and the width of the periodic simulations, these instabilities can result in either detonation failure or quasi-steady propagation. The observed propagation velocities are several per cent higher than CJ values derived from thermodynamic analyses. [Preview Abstract] |
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