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A new approach to identify chemical species from molecular dynamics (MD) simulations of reacting materials under extreme temperatures and pressures is presented. The approach is based on bond-distance and vibrational criteria, derived from the examination of atomic behavior during a density functional theory MD simulation of an overdriven shock of the explosive pentaerythritol tetranitrate. For comparison, the trajectory was analyzed using popular bonding criteria commonly used in analysis of reactive MD simulations, including distance, distance–time, and bond-order criteria. Cluster analyses using the new time-dependent bond definition approach presented here and a bond-order approach revealed that species and their corresponding lifetimes were strongly dependent on the chosen approach, indicating significant implications for the development of chemical mechanisms and chemical kinetics models using the results of reactive MD simulations. [ABSTRACT FROM AUTHOR]

For many years there has been significant interest in polymeric phases of nitrogen at low pressure for potential application as an energetic material. This was the result of years of theoretical work indicating potentially meta-stable polymeric nitrogen. Experimental evidence of both an amorphous phase and a cubic-gauche phase has added greatly to this interest [A. F. Goncharov, E. A. Gregoryanz, H. K. Mao, Z. Liu, and R. J. Hemley, Phys. Rev. Lett. 85, 1262 (2000); M. I. Eremets, R. J. Hemley, H. K. Mao, and E. Gregoryanz, Nature (London) 411, 170 (2001)]. While most of the theoretical work has been done on the many crystal phases of nitrogen, little work has been done on simulating amorphous polymeric nitrogen. The original goal of this work was to simulate amorphous polymeric nitrogen at low pressure; however, we unexpectedly found a new form of polymeric nitrogen. Starting from first principles dynamic shock simulation of cubic-gauche nitrogen [W. D. Mattson and R. Balu, Phys. Rev. B 83, 174105 (2011)] we demonstrate a new low pressure porous form that exhibits stability at low temperatures. We describe the detailed procedure of obtaining this structure as well as some of its physical characteristics. Finally, we explore composite structures of this new form of polymeric nitrogen and their possible relationship to an amorphous form. [ABSTRACT FROM AUTHOR]

To elucidate the mechanisms of energy release in a reacting nickel/aluminum bilayer, we simulate the exothermic alloying reactions using both microcanonical and isoenthalpic-isobaric molecular dynamics simulations and an embedded-atom method type potential. The mechanism of the mixing consists of a sequence of steps in which mixing and reaction first occurs at the interface; the resulting heat generated from the mixing then melts the Al layer; subsequent mixing leads to further heat generation after which the Ni layer melts. The mixing continues until the alloying reactions are completed. The results indicate that pressure has a significant influence on the rates of atomic mixing and alloying reactions. Local pressures and temperatures within the individual layers at the time of melting are calculated, and these results are compared with the pressure-dependent melting curves determined for pure Al and pure Ni using this interaction potential. [ABSTRACT FROM AUTHOR]

We present the results of a molecular dynamics simulation study designed to calculate the melting temperatures of pure nickel and pure aluminum at various system pressures using an embedded atom method type potential. The melting points are determined using a two-phase coexistence method, where the liquid and solid phases are modeled simultaneously at a fixed pressure and temperature, allowing us to bracket the value within a desired range of accuracy. The values obtained for the melting points of aluminum are consistently higher than expected based on experiment, while those for nickel are lower. Other thermal properties of aluminum and nickel were determined in order to fit the melting temperature data into a standard theoretical framework. Also, planar material defects, such as twin boundaries and stacking faults, were observed in crystals grown from the melt, occurring more often in aluminum systems than in nickel. Planar defect energies were calculated for both systems in order to explain these observations. [ABSTRACT FROM AUTHOR]

Quantum-mechanical calculations based on density functional theory are used to study the shock response of the polymeric cubic gauche phase of nitrogen (cg-N), proposed as an alternative energetic ingredient to those used in conventional explosive formulations. The shocked polymeric nitrogen undergoes multiple complex phase transformations and spontaneously forms defects. The occurrence of these dynamic phenomena absorbs the shock energy which subsequently slows the compression wave. Additionally, no reaction occurs immediately behind the shock front; rather reactions result from the unraveling of the material at the free edge of the filament opposite to shock propagation. As the material unravels, numerous polyatomic transients are formed, including five-membered rings and polymeric chains, which subsequently undergo secondary reactions to form the final diatomic products. The speed at which these reactions propagate through the material is much slower than the sound speed, and combined with the slowing compression wave, indicates that the material may not detonate under these conditions. [ABSTRACT FROM AUTHOR]

A variety of computational procedures used to predict properties of energetic materials is presented. These procedures, based on standard atomistic simulation methods, demonstrate the ability to predict key properties of these materials related to performance or hazard. Several applications of the various methods for nitrogen-rich materials are provided to illustrate capabilities. Also, an overview of theoretical efforts in computational design of novel all-nitrogen materials is given. [ABSTRACT FROM AUTHOR]