Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D11: Focus Session: Simulations of Matter at Extreme Conditions I |
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Sponsoring Units: DCOMP DMP GSCCM Chair: Carter White, Naval Research Lab Room: LACC 153C |
Monday, March 21, 2005 2:30PM - 3:06PM |
D11.00001: Atomistic and mesoscale modeling of mechanical and chemical processes in energetic materials Invited Speaker: New-generation reactive interatomic potentials with molecular dynamics enable the full-physics, full-chemistry description complex thermal, mechanical, and chemical process in a wide variety of materials. I will describe the use of the first principles-based reactive force field ReaxFF to describe shock and thermal induced decomposition of various molecular energetic materials, including PETN, RDX, TATB and nitromethane. Non-equilibrium shock simulations enable the characterization of the initial chemical events under dynamical loading while equilibrium simulations at various temperatures and densities enable us to study phenomena at longer time-scales and follow the reactions to completion. These simulations provide information not only about chemical rates but also a molecular-level understanding of the complex multi-molecular chemistry involved. Furthermore, we find that decomposition time-scales of various materials have a strong correlation with their intrinsic sensitivity to ignition. While providing a very detailed description, all-atom MD simulations are computationally too intensive to simulate many important processes in molecularly complex materials such as shockwaves. Thus, 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. We describe the thermal role of the internal degrees of freedom of each mesoparticle using local, finite thermostats that are coupled to the local energy of the mesoparticles. The parameters entering the mesoscopic description can be obtained from all-atom simulations. We exemplify the new method via shock simulations of the crystalline polymer poly(vinylidene fluoride); the mesodynamics results are in excellent agreement with all-atom MD simulations. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D11.00002: Thermal decomposition of condensed nitromethane from molecular dynamics using the ReaxFF Reactive force field Si-ping Han, Adri van Duin, William A. Goddard, III, Alejandro Strachan We study the thermal decomposition and subsequent reaction of the energetic material nitromethane (CH$_3$NO$_2$) using molecular dynamics (MD) with the ReaxFF first principles-based reactive force field. We characterize the chemistry of liquid and solid nitromethane at high temperatures (2000-3000 K) and density 1.97 g/cm$^3$ for times up to 200 picosec. At T=3000 K the first reaction in the decomposition of nitromethane is an inter-molecular proton transfer leading to CH$_3$NOOH and CH$_2$NO$_2$. For lower temperatures (T=2500 and 2000 K) the first reaction during decomposition is often an isomerization reaction involving the scission of the C-N bond the formation of a C-O bond to form methyl nitrate (CH$_3$ONO) . Also at very early times we observe intra-molecular proton transfer events. The main product of these reactions is H$_2$O, which starts forming following the initiation steps. The appearance of H$_2$O marks the beginning of the exothermic chemistry. Recent quantum mechanics-based MD simulations on the chemical reactions for a crystalline sample heated to T=3000 K for 10 ps are in excellent agreement our ReaxFF MD, providing a direct validation of ReaxFF. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D11.00003: Thermal Decomposition of Plastic Bonded Explosives by Molecular Dynamic Simulations with the ReaxFF Force Field Luzheng Zhang, Adri van Duin, Siddharth Dasgupta, William Goddard III Plastic bonded explosives (PBX) are a type of composite energetic materials in which a high explosive is dispersed in a polymer matrix. The main purpose of making such high explosive polymer bound is to reduce their sensitivity to shock, friction, impact, etc. Thermal decomposition is an essential process to characterize an energetic material, because it is one of main causes of initiation of the explosives. In this work, we used MD simulations with the reactive force field (ReaxFF) to study the thermal decomposition of RDX crystal boned with polyurethane chains (Estane) and with nitrocellulose chains. The simulation results showed that RDX's thermal decomposition processing varies when a polymer binder was bonded to the crystal. With addition of polymer binders, RDX's sensitivity is reduced. In all cases studied, the products such as N$_{2}$, H$_{2}$O, CO, CO$_{2}$, OH, etc. can be identified. However, the contributions to these individual species are different: nitrocellulose has much more contributions to N$_{2} $, CO$_{2}$, and CO; but Estane has a little contributions to H$_{2}$O and almost no contributions to N$_{2}$, CO and CO$_{2}$. In addition, we found that the decomposition of RDX with Estane along the Y-direction is slower than that along the X-direction. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D11.00004: First principles studies of PETN molecular crystal under uniaxial compression Jijun Zhao, J.M. Winey, Y.M. Gupta, Warren Perger First principles calculations are important tools to understand changes of atomic structures and physical properties of energetic molecular crystals under compression. Using the plane-wave pseudopotential technique (CASTEP program), we have performed first principles calculations to determine the structural and vibrational properties of pentaerythritol tetranitrate (PETN) crystals under uniaxial compression to about 5 GPa along the [001], [100], and [110] orientations. Our results under uniaxial elastic strains are discussed and compared with those from hydrostatic compression. For a given compression, the geometry of various chemical groups in the PETN molecule show different changes in terms of their bond length and bond angle. The deformation of PETN crystal under compression is also found to be sensitive to the loading conditions. The shift of vibrational frequencies of PETN crystal due to uniaxial compression along different orientations is discussed. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D11.00005: Atomistic-scale simulations of the initial chemical events in triacetonetriperoxide (TATP) detonation Adri van Duin, Yehuda Zeiri, William Goddard To study the initial chemical events related to the detonation of triacetonetriperoxide (TATP) we have performed a series of molecular dynamics (MD) simulations using the ReaxFF reactive force field [1,2], extended to reproduce the quantumchemical (QM)-derived relative energies of the reactants, products, intermediates and transition states related to the TATP unimolecular decomposition. We find excellent agreement between the reaction products predicted from QM and those observed from ReaxFF unimolecular cookoff simulations. Furthermore, the primary reaction products observed in the unimolecular cookoff simulations match closely with those observed from a TATP-condensed phase cookoff simulation, indicating that unimolecular decomposition dominates TATP-condensed phase initiation. [1] A.C.T. van Duin, S. Dasgupta, F. Lorant and W.A. Goddard (2001), \textit{J. Phys. Chem. A} \textbf{105}, 9396-9409.$.$ [2] A. Strachan, A.C.T. van Duin, D. Chakraborty, S. Dasgupta and W.A. Goddard III (2003) \textit{Phys. Rev. Letters} \textbf{91}, 09301. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D11.00006: Directional-dependence in shock-induced melting of fcc metals Ramon Ravelo, B.L. Holian, T.C. Germann, P.S. Lomdahl Shock-induced melting in single crystals was investigated as a function of shock direction utilizing a new equilibrium molecular dynamics method for following the dynamical evolution of condensed matter subjected to shock waves\footnote{R. Ravelo, B.L. Holian, T.C. Germann and P.S. Lomdahl, Phys Rev B, 70, 014103 (2004).}. The solid-liquid Hugoniots of Al, Cu and Lennard-Jones single crystals were enerated as a function of shock direction .The interatomic potentials describing Cu and Al are described by the embedded atom method (EAM). It is foound that in all these systems, the shear stresses at the shock-front dominate the melting process. As a function of orientation, melting occurs at lower pressures (temperatures) for (110) shocks and at higher pressures (temperatures) for (100) shocks. The magnitude of the shear stress at the melting pressure correlates with the orientations: (100):(111):(110) (smaller to largest value as a function of orientation). [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D11.00007: A multi-scale method enabling long-duration molecular dynamics simulations of steady shock waves Evan Reed, Laurence Fried, William Henshaw A multi-scale simulation method is formulated and applied to the study of steadily propagating shock waves in materials. The method combines molecular dynamics and the Euler equations for compressible flow to provide up to 8 demonstrated orders of magnitude of computational savings over non-equilibrium molecular dynamics simulations of steady shock waves. The molecular dynamics system is constrained to obey the Rayleigh stress condition and the Hugoniot energy condition to sample the sequence of thermodynamic states of a steady shock. Utilizing a coarse- grained approach with analytical equations of state, we explicitly show that spatial profiles of shock waves yielded by the method are identical to those of fully hydrodynamic simulations of steady shock waves in chemically reactive systems. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D11.00008: Deformation and excitation of molecules during shock compression of defects Sergey V. Zybin, Mark L. Elert, Carter T. White Shock initiation of chemical reactions in solids often starts at hot spots that are created during interaction of a shock wave with pre-existing defects and interfaces in the material. The formation of hot spots can involve the vaporization of material into a pore (or crack) followed by recompression of the ejected gas during the collapse of the pore with fast temperature increase (local overheating). However, in presence of large temperature gradients and non-equilibrium energy transfer at the front, the analysis of molecule deformation and excitation should not be limited to the Arrhenius kinetics only, but can also involve non-thermal mechanisms. Here, we report results of molecular dynamics simulations of shock passage through the planar gap (crack) in hydrocarbon molecular solids and analyze the temperature rise, energy distributions, and bond deformations. Our analysis shows that the region of most intense molecular deformation is located in vicinity of the collision front of the rarefaction and reflected (from the opposite side of the crack) waves, where the velocity distributions deviate widely from the equilibrium Maxwellian. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D11.00009: Shock-induced collapse of nano-bubbles embedded in a solid matrix E. Bringa, P. Erhart Molecular-dynamics (MD) simulations of shocks interacting with He bubbles in embedded atom method Cu will be presented. At relatively low shock pressures the stress field of the bubble helps nucleating significant dislocation activity around it, with relatively small deformation of the bubble. At higher pressures the bubble shows compression along the shock direction. This compression reaches a value slightly above 50{\%} before the solid matrix melts and the He dissolves in the liquid. Simulations using elliptical bubbles show some compression enhancement respect to the spherical bubble case, but no jetting was ever observed for the He pressures studied. The work at LLNL was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48 [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D11.00010: Shock-induced Plasticity and Brittle Cracks in Aluminum Nitride Paulo Branicio, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta Two hundred and nine million atom molecular-dynamics simulation of hypervelocity projectile impact in aluminum nitride reveals strong interplay between shock-induced structural phase transformation, plastic deformation and brittle cracks. The shock wave splits into an elastic precursor and a wurtzite-to-rocksalt structural transformation wave. When the elastic wave reflected from the boundary of the sample interacts with the transformation wave front, nanocavities are generated along the penetration path of the projectile and dislocations in adjacent regions. The nanocavities coalesce to form mode I brittle cracks while dislocations generate kink bands that give rise to mode II cracks. These simulations provide a microscopic view of defects associated with simultaneous tensile and shear cracking at the structural phase transformation boundary due to shock impact in high-strength ceramics. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D11.00011: A Reactive Bond Order Potential for NxHy Species: Molecular Dynamics Simulations of Hydrazine Decomposition Yanhong Hu, Edward Byrd, Betsy Rice, Donald Brenner A reactive empirical bond order potential for N$_{x}$H$_{y}$ species has been developed that describes bonding properties over a wide range of molecular and solid-state structures. The function includes non-bonded interactions, and allows bond breaking and forming with appropriate changes in atomic hybridization in a computationally efficient manner, features that facilitate large-scale atomic simulations of condensed-phase reactive chemistry. Initial simulations of the chemical decomposition of hydrazine under extreme conditions will be presented and compared with available experimental results and \textit{ab initio} calculations. Because many of the properties of hydrazine are reasonably well known from experiment, the results of our simulations will help validate the potential, as well as lend important new insights into the reactive dynamics of these systems. This work is supported by a DOD MURI grant managed by the Army Research Office. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D11.00012: Massive parallel simulation of phenomena in condensed matter at high energy density Vladimir Fortov, Igor Lomonosov, Vadim Kim This talk deals with computational hydrodynamics, advanced material properties and phenomena at high energy density. New results of massive parallel 3D simulation done with method of individual particles in cells have been obtained. The gas dynamic code includes advanced physical models of matter such as multi-phase equations of state, elastic-plastic, spallation, optic properties and ion-beams stopping. Investigated are the influence on hypervelocity impact processes effects of equation of state, elastic-plastic and spallation. We also report results of numerical modeling of the action of intense heavy ion beams on metallic targets in comparison with new experimental data. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D11.00013: Atomistic simulations of orientation effects during shock compression and decomposition of energetic materials Sergey V. Zybin, Luzheng Zhang, Adri van Duin, Siddharth Dasgupta, William A. Goddard III Several experiments have indicated that the shock sensitivity of single crystal energetic materials can depend on the crystallographic direction. For example, sensitivity of PETN strongly correlates with orientational anisotropy of elastic precursor strength as well as steric hindrance to shear in some slip directions. In particular, deformation and excitation of energetic molecules can be affected by different slip systems and mechanisms of elastic-plastic transition for different directions. To study the orientational effects in material transformation and initiation of chemical events related to the detonation we have performed a series of reactive molecular dynamics (MD) simulations using the ReaxFF reactive force field, capable to reproduce the quantum chemical (QM)-derived relative energies of the reactants, products, intermediates and transition states related to the RDX and HMX unimolecular decomposition. Our analysis shows that the sensitivity, pathways, and products of shock-induced decomposition in these single energetic crystals are dependent on the shock orientation. [Preview Abstract] |
Monday, March 21, 2005 5:30PM - 5:42PM |
D11.00014: Nonlinear anisotropic response for shocked single energetic crystals Sergey V. Zybin, Tahir Cagin, William A. Goddard III The response of energetic materials to the shock loading determines the initiation of chemical reactions and transition to detonation. The sensitivity of single energetic crystals can depend on shock orientation and correlate with their elastic properties, the strength of elastic precursor, and the mechanisms of shock-induced plasticity. In a continuum framework, the elastic-plastic response of materials is usually described by the constitutive relations coupling linear elastic response to a viscoplastic flow in a single crystal. However, for stronger shocks or more compliant crystals the resulting response at the shock front may be complicated by: 1) large compressions leading to a nonlinear elastic response, 2) highly anisotropic strains requiring the use of non-hydrostatic constitutive relations (equation of states), 3) very high strain rates which may affect the mechanism of plasticity. Here, we present the results of ab initio and reactive force fields (ReaxFF) calculations of nonlinear mechanical response of energetic materials under large anisotropic (e.g. uniaxial) strains typical for the conditions at the shock front. [Preview Abstract] |
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