Session Q1: MD-4: Molecular Dynamics IV - Energetic Materials

Compressive shear reactive dynamics to evaluate the anisotropic sensitivity of single-crystal energetic materials

Complex coupling between mechanical, thermal, and chemical effects are at the heart of many important but not understood phenomena, including the shock sensitivity of materials to detonation. We propose a general protocol (Compressive Shear Reactive Dynamics, CS-RD) for predicting the mechanic, thermal, and chemical processes and show that this protocol predicts correctly the relative sensitivities observed experimentally for single crystal PETN [C(CH2ONO2)4]. We find that sensitive directions lead to close molecular contacts (steric hindrance) resulting in severe deformation that leads to large stress overshoots and increases in temperature that results in bond-breaking processes whereas insensitive directions exhibit little distortion or stress overshoot, delayed temperature increases and less dissociation. This insight that a planar shock fails because of shear in a plane oblique from the shock direction and that the essential features controlling the failure mechanisms must be sought in this shear phenomena should be useful in elucidating the mechanisms for more complex multigranular multicomponent systems including defects and it may be useful for other complex collision phenomena.   

Initiation of PETN decomposition under shock compression: Reactive molecular dynamics simulation

The initial physical and chemical response of energetic materials under mechanical shock has been investigated for PETN by molecular dynamics method with ReaxFF reactive force field parameterized from first-principles calculations. We study the propagation of a shock wave and shock-induced chemical reactions created by moving piston mimicked by a potential wall. We simulate both the continuous and impulsive piston loading to investigate its influence on the initiation and decomposition reactions in energetic materials as well as the orientational dependence using large-scale parallel ReaxFF-MD simulations. The mechanism and evolution of chemical reactions induced by mechanical shock and pure shear is discussed along with the propagation of heat, mass, pressure, and reaction waves.   

Novel Catalytic Behavior of Water in High Explosive Decomposition

Water under conditions of extremely high pressure and temperature is known to exhibit fascinating physical behaviors. Its remarkable structural and phase complexity strongly suggests that its chemical properties, which are largely unstudied, may be unusual as well. Detonations of high explosives containing oxygen and hydrogen provide unique systems for exploring the chemistry of ``extreme water,'' because detonations form water at conditions similar to those of giant planetary interiors. Contrary to the current view of water as a stable final product, we show here that water plays an unexpected role in catalyzing complex explosive reactions. From first-principles atomistic simulations of the high explosive pentaerythritol tetranitrate (PETN), we discovered that $\rm H_2O$ (source), H (reducer) and OH (oxidizer) act as a dynamic team that transports oxygen between reactive centers. Our finding suggests that $\rm H_2O$ may catalyze reactions in other explosives and in planetary interiors. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Ab initio molecular dynamics simulations on sensitivity of nitrogen-rich poly-tetrazolo energetic compounds

Polyazido nitrogen-rich heterocyclic energetic compounds have gained attention recently due to their high heats of formation and less hazardous effects on the environment. These compounds are generally featured with C-N binary nitrogen-rich heteocycles. It was also found that they may serve as precursors for the preparation of carbon nanotubes and nitrogen-rich carbon nitrides. However, these high energetic compounds can be extremely sensitive to shock, friction, impact, and electrostatic discharge. In the present paper, an azide-tetrazole chain-ring tautomerism was proposed to form new compounds structured with tetrazole rings. The sensitivity and the heats of formation for these poly-tetrazolo compounds have been investigated using \textit{ab initio} quantum chemistry and \textit{ab inito} biasing potential molecular dynamics simulations. The results have shown that these poly-tetrazolo compounds are potentially new, environmentally friendly and high-performance energetic materials that are characterized with low sensitivities and good thermal stabilities.   

Hypervelocity reactive dynamics in detonating PETN, RDX, and HMX energetic materials

Despite intensive experimental and theoretical efforts, the first chemical events that trigger the chemistry behind the shock wave front in detonating materials are still largely unknown. We investigate the chemical initiation of detonation in shock compressed PETN, RDX, and HMX which results from intermolecular collisions behind the shock wave using first-principles reactive molecular dynamics. The reaction dynamics of bimolecular collisions were studied as a function of collision velocities and crystallographic orientations. For each orientation, threshold collision velocities of reaction, and products of decomposition were determined. The timescale of reaction was determined and used to understand whether these initial chemical events are largely driven by reaction dynamics, or temperature. Bond dissociation energies were calculated for each molecule and used to rationalize the outcome of the chemical events in the course of the reaction dynamics. A correlation between the order in which products are formed and the relative strengths of the bonds within the bimolecular complexes was investigated. Finally, the relationship between orientation dependent sensitivities and steric factors is discussed.   

First-principles investigation of reactive molecular dynamics in detonating TATB

TATB is a highly insensitive energetic material (EM) with a unique planar structure similar to that of graphene. Though hydrostatic compression studies have been performed, little is known about the reasons for the high degree of insensitivity that makes TATB an ideal EM for many important military applications. To elucidate the factors responsible for its high insensitivity, we performed first-principles molecular dynamics simulations of TATB bimolecular collisions at different velocities for various crystallographic orientations. We determine threshold collision velocities for each orientation and discuss steric properties of the bulk crystal as they relate to the inferred anisotropic sensitivities. We also investigate the role of bond dissociation energies in the insensitivity of TATB, and discuss the possible contributions of in-plane hydrogen bonding and van-der-Waals forces.   

Shock and detonation processes studied at the nanoscale

We report recent advances on shock and detonation processes studied at the nanoscale using several simulation techniques. Concerning shock physics, example of the investigation of spallation phenomena in copper and tantalum using large scale Molecular Dynamics will be given. Concerning High Energetic materials, example of the use of Monte Carlo techniques to build the equation of state (EOS) of nitromethane is shown. Thermodynamic properties of detonation products are computed using Reactive Monte Carlo simulations, including an explicit treatment of carbon clusters based on reactive potential results. Finally, we develop equilibrium methods for the simulation of isentropic processes (either during the compression or the release of a material), that we applied to study the release of a monoatomic liquid. These results are compared to the direct non-equilibrium MD simulations of the same process, showing that the isentropic approximation is not strictly valid.   

Eckart-Sayvetz Alignment: A Useful Tool for the Analysis of Molecular-Scale Simulation Data for Studies of Material Under ExtremeThermodynamic Conditions

Molecules subjected to shock wave loading will, in general, be highly distorted and arbitrarily displaced compared to their (0 K, 0 GPa) structures. This complicates the analysis for such systems, particularly when the goal is to express dynamic molecular-scale properties in a frame indexed to the static equilibrium geometry. The Eckart-Sayvetz condition (ESC) provides a rigorous approach by which the optimal alignment between frames for such analyses can be achieved. We illustrate the use of the ESC in a MD study of shocked nitromethane crystal, for initial temperatures of 50 and 200 K and impact velocities ranging from 0.5 to 3.0 km/s. The results provide clear evidence of melting for impacts of at least 2 km/s and provide insights into the fundamental events associated with its onset. Energy redistribution in the shocked crystal was studied by monitoring the time dependence of the molecular normal mode kinetic energies. The energy-transfer results are sensitive to shock strength and initial temperature, but are generally consistent with the notion of vibrational up-pumping.   

Shock Compression Calculation of RDX and PETN Molecular Crystals Using the Hugoniostat Method

Parameters of uniaxial shock compression were calculated for RDX molecular crystal in crystallographic direction [100] and for PETN in directions [100], [110], [001] by Hugoniostat method using the non-reactive interatomic force field [G. D. Smith and co-workers, J. Phys. Chem. B 112 734 (2008)]. P-V, D-Up, T-P and T-Up dependences have been obtained for each crystallographic direction on the Hugoniot curve. Pressure and degree of compression were obtained for which the transformation of the molecular crystal lattice takes place at MD simulation of uniaxial shock compression. The structural transformation of the RDX crystal lattice was identified at 25 GPa pressure and 0.66 compression ratio for simulation of uniaxial shock compression in [100] direction. The structural transformation of the PETN crystal lattice was identified at 8-12 GPa and compression ratio of 0.80-0.73 for MD simulation of uniaxial shock compression in [110] direction.