Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session H5: First-Principles & Molecular Dynamics Calculations III |
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Chair: Ramon Ravelo, University of Texas, El Paso Room: Hyatt Regency Constellation F |
Tuesday, August 2, 2005 9:00AM - 9:30AM |
H5.00001: Atomistic and mesoscale modeling of the response of high energy materials to dynamical loading Invited Speaker: The mechano-chemical response of high energy density materials to dynamical loading involves a variety of complex processes that remain to be characterized at the molecular level. For example the translational kinetic energy of the shock-wave is transformed into intra-molecular excitations; the shock-wave leads to plastic deformation in the material, and defects (such as voids) and interfaces interact with the propagating wave causing energy and temperature localization (hot spots) that play a crucial role in the initiation and propagation of detonation. I will describe recent reactive and un-reactive MD simulations using accurate interatomic potentials designed to characterize the chemical decomposition and plastic response of various HE materials under dynamical loading. Non-equilibrium shock simulations enable the characterization of the induced plastic deformation and the initial chemical events while equilibrium simulations at various temperatures and densities enable us to follow the reactions to completion. While providing a very detailed description, all-atom MD has a serious limitation: being based on classical mechanics it leads to classical (rather than quantum) statistical mechanics. This results in a significant overestimation of the specific heat in molecular crystals for the temperatures relevant in HE materials. To address this shortcoming we have recently developed a new mesodynamical method (where a single particle describes groups of atoms) that enables a thermodynamically accurate description of energy transfer between mesoparticles (molecules in this case) and their internal degrees of freedom (DoFs). The thermal properties of the implicit DoFs are described via their specific heat. The mesodynamics results are in excellent agreement with all-atom MD simulations when a classical expression for the specific heat is used but also enables the accurate quantum mechanical-based treatment of the thermal role of the implicit DoFs. [Preview Abstract] |
Tuesday, August 2, 2005 9:30AM - 9:45AM |
H5.00002: Classical and Quantum Dynamics of Energy Transfer under Shock Conditions R.C. Mowrey, M.L. Elert, C.T. White Classical molecular dynamics (MD) simulations of shocks in molecular solids predict rapid excitation of bond motion indicating efficient translational to vibrational coupling. The validity of the MD description of collisional energy transfer near shock fronts has not been carefully tested. The importance of quantum effects under shock conditions is explored in classical MD and quantum mechanical (QM) simulations of a molecular lattice model consisting of three collinear diatomic molecules and a stationary wall. A fast-moving diatom collides with its neighbor initiating a collision cascade. The multiplicity of collisions precludes \textit{a priori} prediction of the detailed collision dynamics. The time-dependence of the six degrees-of-freedom wave function describing the system is determined using QM time-dependent wave packet methods. The intra- and inter-molecular interactions are described using nearest-neighbor potentials. Predicted vibrational and translational energy changes and bond breaking and formation from QM and MD calculations are compared. The dependence of the dynamics on the characteristics of the model (e.g., heavy vs. light atoms, homonuclear vs. heteronuclear diatoms, intra- and inter-molecular bond strengths) is investigated. [Preview Abstract] |
Tuesday, August 2, 2005 9:45AM - 10:00AM |
H5.00003: Nonequilibrium Atomistic Polymer Simulations Under Shear Steven Valone, Vivek Kapila The present effort is intended to provide insights into acceleration-driven instabilities in complex fluids. The vast majority of studies into Rayleigh-Taylor (steady acceleration), Richtmyer-Meshkov (impulsive acceleration or shock loading), and Kelvin-Helmholtz (shear) instabilities have been limited to simple fluids with relatively simple constitutive properties. Some fluids of interest originate from polymeric materials that will behave like complex fluids under sufficiently strong shock or shear loading. The resulting fluids possess very complicated shear-rate dependent viscous behavior. This shear-rate dependence is explored through nonequilibrium molecular dynamics simulations using shear boundary conditions [1] applied to an atomistic model of polyethylene. The shear-rate dependence is determined over a range of rates, not just for low shear rates. The viscosity data are fit to a generalized Lorentzian and rate-theory models. The low shear-rate results are compared to experimental data for the two models. \vskip \noindent [1] B.L. Holian, J. Chem. Phys., 117, 1173 (2002). [Preview Abstract] |
Tuesday, August 2, 2005 10:00AM - 10:15AM |
H5.00004: High Density Sliding at Ta/Al and Al/Al Interfaces J.E. Hammerberg, R. Ravelo, T.C. Germann We present 3D-NEMD results for the velocity dependence of the frictional force at smooth and roughened interfaces for Ta and Al single crystals. For Ta/Al we consider Ta (100)/Al (100) and Ta (110)/Al (111) interfaces sliding along [001] and $[1\mathop 1\limits^- 0]_{fcc} /[001]_{bcc} $ respectively. These are compared with Al (111)/Al (100) interfaces at the same loads, corresponding to a pressure of 15 GPa. Both interfacial pairs show similar behavior in the velocity dependence of the frictional force: a low velocity regime with an increasing frictional force, followed by a strain induced transformation regime at velocities above approximately 1/10 the transverse sound speed, followed by a fluidized interface at high velocities. For both interfacial pairs, the high velocity dependence of the frictional force exhibits power law behavior, v$^{-\beta }$, with $\beta $ = 3/4. We discuss the structural changes that influence dissipation in these regimes. [Preview Abstract] |
Tuesday, August 2, 2005 10:15AM - 10:30AM |
H5.00005: A Molecular Dynamics Study of Solid Gallium Using a Modified Embedded Atom Model Frank Cherne, Kai Kadau, Timothy Germann Gallium is a complex material that has been simulated using the literature modified embedded atom method (MEAM) potential [Baskes et al., PRB 66, 104107 (2002)]. This potential captures some of the unique characteristics of this ubiquitous metal. Here we report on the structural transformations based upon the crystallographic direction. In order to characterize the nature of the transition we have performed both non-equilibrium and equilibrium molecular dynamics (MD) simulations. These simulations provide insight into the nature of the solid-solid and solid-liquid phase transitions in addition to the kinetic behavior of this complex material. The Hugoniot for solid gallium will be presented. This research is being done in collaboration with an experimental ultrafast X-ray diffraction effort here at Los Alamos National Laboratory. Supported by the US DOE under contract W-7405-ENG-36. [Preview Abstract] |
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