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 E5: First-Principles & Molecular Dynamics Calculations II |
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Chair: Thomas Sewell, Los Alamos National Laboratory Room: Hyatt Regency Constellation F |
Monday, August 1, 2005 3:30PM - 3:45PM |
E5.00001: Atomistic-scale simulations of energetic materials with ReaxFF reactive force fields W.A. Goddard III, S.V. Zybin, A.C. van Duin, L. Zhang, S. Dasgupta, S-P. Han, A. Strachan Understanding the response of energetic materials to thermal or shock loading at the atomistic level demands a highly accurate description of the reaction dynamics of million atom systems to capture the complex chemical and mechanical behavior involved: nonequilibrium energy/mass transfer, molecule excitation and decomposition under high strain/heat rates, formation of defects, plastic flow, and phase transitions. To enable such simulations, we developed the ReaxFF reactive force fields based on quantum mechanics (QM) calculations of reactants, products, high-energy intermediates and transition states, but using functional forms suitable for large-scale molecular dynamics simulations of chemical reactions under extreme conditions. We will present an overview of recent progress in ReaxFF developments, including the extension of ReaxFF to new nitramine- based (nitromethane, HMX, PETN, TATB) and peroxide-based (TATP) explosives. To demonstrate the versatility and transferability of ReaxFF, we will present applications to solid composite propellants such as Al/Al2O3-metal nanoparticles embedded into solid explosive matrices (RDX, PETN). [Preview Abstract] |
Monday, August 1, 2005 3:45PM - 4:00PM |
E5.00002: Comparative study of energetic materials by classical interatomic potential ReaxFF and first-principles density functional theory Ivan Oleynik, Sergey Zybin, L. Zhang, William Goddard Prediction of properties of energetic materials using atomic-scale simulations techniques is one of the challenging areas of energetic materials (EM) research. Molecular dynamics (MD) simulation of EM using classical reactive interatomic potentials is a powerful modeling technique that is capable of addressing sub-nanometer and sub-picosecond length and time scales of shock compression and detonation phenomena. However, the results of computer simulations can only be as reliable as the ability of the interatomic potentials to describe properly a variety of chemical effects including bond-breaking and bond-making. Recently, the reactive force field ReaxFF has been developed based on fitting of an ab-initio database of HCNO chemistry and is currently being actively used for MD simulations of EM. We performed a comparative study of static and thermodynamic properties of PETN, RDX and HMX using both density functional theory (DFT) and ReaxFF including static properties of different crystalline phases and equation of states (EOS). The transferability issues are discussed in the region of physical parameters relevant for MD simulation of initial decomposition and detonation in EM. [Preview Abstract] |
Monday, August 1, 2005 4:00PM - 4:15PM |
E5.00003: Shock induced decomposition and sensitivity of energetic materials by ReaxFF molecular dynamics S.V. Zybin, L. Zhang, A.C. van Duin, S. Dasgupta, W.A. Goddard III 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 influence of shock/impact orientation on initiation and decomposition of energetic materials 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 as well as crystalline phases of energetic materials. [Preview Abstract] |
Monday, August 1, 2005 4:15PM - 4:30PM |
E5.00004: Thermal decomposition of energetic materials by ReaxFF reactive molecular dynamics L. Zhang, S.V. Zybin, A.C. van Duin, S. Dasgupta, W.A. Goddard III Understanding the complex physicochemical processes that govern the initiation and decomposition kinetics of energetic materials can pave the way for modifying the explosive or propellant formulation to improve their performance and reduce the sensitivity. In this work, we used molecular dynamics (MD) simulations with the reactive force field (ReaxFF) to study the thermal decomposition of pure crystals (RDX, HMX) as well as crystals bonded with polyurethane chains (Estane). The preliminary simulation results show that pure RDX and HMX crystals exhibit similar decomposition kinetics with main products (e.g., N2, H2O, CO2, and CO) and intermediates (NO2, NO, HONO, OH) in a good agreement with experiment. We also studied the effect of temperature on decomposition rate which increases at higher temperatures. With addition of polymer binders, we found that the reactivity of these energetic materials is reduced, and the polymer chains packing along different planes may also influence their thermal decomposition. In addition, we studied the thermal decomposition of TATP and hydrazine which are examples of ReaxFF development for non- nitramine based energetic materials. [Preview Abstract] |
Monday, August 1, 2005 4:30PM - 4:45PM |
E5.00005: Thermal Decomposition of TATB at Extreme Conditions Riad Manaa, Laurence Fried Detailed description of chemical reaction mechanisms of solid energetic materials at high-pressure and temperatures is essential for understanding events that occur at the reactive front of these materials and the subsequent building of predictive models of materials properties. We report the results of two ab initio based molecular dynamic simulation of the chemistry of TATB, at density of 2.9 g/cm$^{3}$ and temperature of 1500K, and at density of 2.87 g/cm$^{3}$ and temperature of 2500 K. The molecular forces are determined using the self-consistent-charge, density-functional - based tight-binding method. Following the dynamics for a time scale of up to forty picoseconds allows the construction of approximate rate laws for typical products such as H$_{2}$O, N$_{2}$, CO, and CO$_{2}$. The reaction rates of these products will be compared to those obtained previously for HMX at similar conditions. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-Eng-48. [Preview Abstract] |
Monday, August 1, 2005 4:45PM - 5:00PM |
E5.00006: Shock-Wave Propagation Study in Fe2O3+Al Energetic Nanocomposites Using Classical Molecular Dynamics Simulations Vikas Tomar, Min Zhou Classical molecular dynamics (MD) simulation is an important technique for analyzing custom-designed nanostructured materials and nano-sized systems such as nanowires and nanobelts. This research focuses on the effect of shock wave propagation in Fe2O3+Al nanointerfaces. A generic potential form is used to describe the behavior of the Fe+Al+Fe2O3+Al2O3 system over a range of mechanical loading rate and temperature. The potential is able to describe bulk single crystal behavior of Fe, Al, Fe2O3, Al2O3 as well as interfacial transitions among them. Shock-wave propagation analyses over a range of plate impact velocities are used to reveal the effect of the correlation between nanoscale phase morphology and applied loading on the desired mechanical attributes. Effect of orientation of bicrystals interface on shock wave defect formations in various crystalline orientations of fcc-Al is also investigated. [Preview Abstract] |
Monday, August 1, 2005 5:00PM - 5:15PM |
E5.00007: Equation of State of 1,1-diamino-2,2-dinitroethylene from First Principles Frank Zerilli, Maija Kuklja In recent work, the 0 K isotherms of 1,1-diamino-2,2-dinitroethylene (FOX-7), and $\beta $ HMX were calculated with the ab-initio periodic structure code CRYSTAL. It was found that Hartree-Fock calculations gave the best agreement with experiment and a calculation with complete optimization of the internal molecular coordinates gives excellent agreement with experimental data. For the best results, especially for anisotropic materials, it was necessary to optimize both atomic coordinates and lattice parameters under a fixed volume constraint. The good results may be the result of cancellation of basis set superposition error with dispersion force errors. Here we report calculations of the full equation of state for the explosive FOX-7 that give results which compare well with experimental data reported by Peiris, et al. The above zero temperature contribution to the free energy was calculated from the phonon frequency spectrum obtained with density functional theory (DFT) in the local density approximation (LDA) with the code ABINIT. [Preview Abstract] |
Monday, August 1, 2005 5:15PM - 5:30PM |
E5.00008: The atomic and electronic structure of defects in 1,1-diamino-2,2-dinitroethylene Maija Kuklja, Sergey Rashkeev, Frank Zerilli The atomic and electronic structure of defects in the molecular crystal 1,1-diamino-2,2-dinitroethylene (FOX-7) is studied by means of both first-principles Hartree-Fock and density-functional theory methods. The defect-related local electronic states in the band gap and their contribution to the optical and transport properties of FOX-7 are modeled. The decomposition energy of the material in the solid phase in the presence of defects is accurately calculated using the nudged elastic band approach and compared with results obtained from other methods. The possible correlation between the lowering of the decomposition barrier and the electronic properties of the material has been investigated. [Preview Abstract] |
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