Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session U5: First-Principles and MD VI: Chemical Reactivity |
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Chair: Marc Cawkwell and Joel Kress, Los Alamos National Laboratory Room: Grand I/J |
Thursday, June 18, 2015 2:15PM - 2:30PM |
U5.00001: Elucidation of high sensitivity of $\delta $-HMX: New insight from the first principle simulations Maija Kuklja, Onise Sharia, Roman Tsyshevsky Understanding of a tremendous difference in sensitivities of $\beta $ and $\delta $ phases of cyclotetramethylene-tetranitramine (HMX) has been long one of the stubborn challenges in the field of high energy density materials. Despite many experimental and theoretical efforts to explain the high sensitivity of the $\delta $ phase, convincing reasons behind the HMX behavior remained puzzling. We established that the presence of a polar surface in $\delta $-HMX has fundamental implications for stability and overall chemical behavior of the material. A comparative state-of-the-art quantum-chemical analysis of major decomposition mechanisms in polar $\delta $-HMX and nonpolar $\beta $-HMX discovered a dramatic difference in dominating dissociation reactions, activation barriers, and reaction rates. The polarization-induced charge transfer offered a logical explanation for different sensitivity of $\beta $-HMX and $\delta $-HMX polymorphs to detonation initiation. Our conclusions also removed long-standing contradictions and explained a large range of experimental data on thermal decomposition of HMX. [Preview Abstract] |
Thursday, June 18, 2015 2:30PM - 2:45PM |
U5.00002: \textit{Ab initio} calculations of nitramine dimers Sharon Koh-Fallet, Igor Schweigert Elevated temperatures and pressures are typically thought to have opposing effects on the reaction channels of nitramine decomposition. These high temperatures promote reactions with loose transition structures (positive activation entropies and volumes), such as N-N bond homolysis. Elevated pressures promote reactions with tight transition structures (negative activation entropies and volumes), such as intramolecular and intermolecular H transfer. However, no quantitative data exists regarding the range of temperatures and pressures at which these effects become pronounced. We are pursuing ab initio calculations of the corresponding unimolecular and bimolecular transition structures with the objective of estimating the relevant thermochemical parameters and quantifying the effects of elevated temperature and pressures on the corresponding rate constants. Here, we present density functional theory and complete active space calculations of gas-phase molecular dimers of nitramines as an intermediate step toward modeling transition structures directly in the condensed phase. [Preview Abstract] |
Thursday, June 18, 2015 2:45PM - 3:00PM |
U5.00003: Detonation initiation in solid explosive: MD simulation using AB interatomic potential Semen Murzov, Vasily Zhakhovsky Molecular dynamics simulation of impact detonation initiation in an AB model of condensed-phase explosive is performed with using a simplified reactive empirical bond order potential. Simulation of ultra-short piston-driven compression of AB explosive for the duration of a few picoseconds represents the indirect initiation of detonation by an ultra-short shock wave (SW) generated in a thin metal foil irradiated by a femtosecond laser pulse. Impact conditions required for transition of SW to detonation wave (DW) are studied. Variation of loading time and piston velocity, which controls piston pressure and input energy, is used to plot 2D regions of transition from SW to DW in pressure-time and energy-time planes. We demonstrate that the input energy required for initiation has a global minimum. Analysis of evolution of calculated pressure profiles gives a critical thickness of such AB explosive film in which transition from SW to DW can be completed. The effect of porosity and roughness of AB sample on impact detonation initiation is discussed. [Preview Abstract] |
Thursday, June 18, 2015 3:00PM - 3:15PM |
U5.00004: Mirrored continuum and molecular scale simulations of the ignition of gamma phase RDX D. Scott Stewart, Santanu Chaudhuri, Kaushik Joshi, Kiabek Lee We consider the ignition of a high-pressure gamma-phase of an explosive crystal of RDX which forms during overdriven shock initiation. Molecular dynamics (MD), with first-principles based or reactive force field based molecular potentials, provides a description of the chemistry as an extremely complex reaction network. The results of the molecular simulation is analyzed by sorting molecular product fragments into high and low molecular groups, to represent identifiable components that can be interpreted by a continuum model. A continuum model based on a Gibbs formulation, that has a single temperature and stress state for the mixture is used to represent the same RDX material and its chemistry. Each component in the continuum model has a corresponding Gibbs continuum potential, that are in turn inferred from molecular MD informed equation of state libraries such as CHEETAH, or are directly simulated by Monte Carlo MD simulations. Information about transport, kinetic rates and diffusion are derived from the MD simulation and the growth of a reactive hot spot in the RDX is studied with both simulations that mirror the other results to provide an essential, continuum/atomistic link. [Preview Abstract] |
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