Tzu-Ray Shan

Atomistic Simulation of Orientation Dependence in Shock-Induced Initiation of Pentaerythritol Tetranitrate

The dependence of the reaction initiation mechanism of pentaerythritol tetranitrate (PETN) on shock orientation and shock strength is investigated with molecular dynamics simulations using a reactive force field and the multiscale shock technique. In the simulations, a single crystal of PETN is shocked along the [110], [001], and [100] orientations with shock velocities in the range 3-10 km/s. Reactions occur with shock velocities of 6 km/s or stronger, and reactions initiate through the dissociation of nitro and nitrate groups from the PETN molecules. The most sensitive orientation is [110], while [100] is the most insensitive. For the [001] orientation, PETN decomposition via nitro group dissociation is the dominant reaction initiation mechanism, while for the [110] and [100] orientations the decomposition is via mixed nitro and nitrate group dissociation. For shock along the [001] orientation, we find that CO-NO(2) bonds initially acquire more kinetic energy, facilitating nitro dissociation. For the other two orientations, C-ONO(2) bonds acquire more kinetic energy, facilitating nitrate group dissociation.

Classical interatomic potential for orthorhombic uranium.

A classical interatomic potential for uranium metal is derived within the framework of the charge optimized many body (COMB) formalism. The potential is fitted with a database obtained from experiment and density functional theory (DFT) calculations. The potential correctly predicts orthorhombic α-U to be the ground state. Good agreement with experimental values is obtained for the lattice parameters, nearest neighbor distances, and elastic constants. Molecular dynamics simulations also correctly show the anisotropy in the coefficient of thermal expansion and the temperature dependence of the nearest neighbor distances.

A charge optimized many-body (comb) potential for titanium and titania

This work proposes an empirical, variable charge potential for Ti and TiO(2) systems based on the charge-optimized many-body (COMB) potential framework. The parameters of the potential function are fit to the structural and mechanical properties of the Ti hcp phase, the TiO(2) rutile phase, and the energetics of polymorphs of both Ti and TiO(2). The relative stabilities of TiO(2) rutile surfaces are predicted and compared to the results of density functional theory (DFT) and empirical potential calculations. The transferability of the developed potential is demonstrated by determining the adsorption energy of Cu clusters of various sizes on the rutile TiO(2)(1 1 0) surface using molecular dynamics simulations. The results indicate that the adsorption energy is dependent on the number of Cu-Cu bonds and Cu-O bonds formed at the Cu/TiO(2) interface. The adsorption energies of Cu clusters on the reduced and oxidized TiO(2)(1 1 0) surfaces are also investigated, and the COMB potential predicts enhanced bonding between Cu clusters and the oxidized surface, which is consistent with both experimental observations and the results of DFT calculations for other transition metals (Au and Ag) on this oxidized surface.

Development of a ReaxFF Reactive Force Field for Ammonium Nitrate and Application to Shock Compression and Thermal Decomposition

We have developed a new ReaxFF reactive force field parametrization for ammonium nitrate. Starting with an existing nitramine/TATB ReaxFF parametrization, we optimized it to reproduce electronic structure calculations for dissociation barriers, heats of formation, and crystal structure properties of ammonium nitrate phases. We have used it to predict the isothermal pressure-volume curve and the unreacted principal Hugoniot states. The predicted isothermal pressure-volume curve for phase IV solid ammonium nitrate agreed with electronic structure calculations and experimental data within 10% error for the considered range of compression. The predicted unreacted principal Hugoniot states were approximately 17% stiffer than experimental measurements. We then simulated thermal decomposition during heating to 2500 K. Thermal decomposition pathways agreed with experimental findings.

Shock-induced hotspot formation and chemical reaction initiation in PETN containing a spherical void

We present results of reactive molecular dynamics simulations of hotspot formation and chemical reaction initiation in shock-induced compression of pentaerythritol tetranitrate (PETN) with the ReaxFF reactive force field. A supported shockwave is driven through a PETN crystal containing a 20 nm spherical void at a sub-threshold impact velocity of 2 km/s. Formation of a hotspot due to shock-induced void collapse is observed. During void collapse, NO2 is the dominant species ejected from the upstream void surface. Once the ejecta collide with the downstream void surface and the hotspot develops, formation of final products such as N2 and H2O is observed. The simulation provides a detailed picture of how void collapse and hotspot formation leads to initiation at sub-threshold impact velocities.

Reactive atomistic simulations of shock-induced initiation processes in mixtures of ammonium nitrate and fuel oil

Ammonium nitrate mixed with fuel oil (ANFO) is a commonly used blasting agent. In this paper we investigated the shock properties of pure ammonium nitrate (AN) and two different mixtures of ammonium nitrate and n-dodecane by characterizing their Hugoniot states. We simulated shock compression of pure AN and ANFO mixtures using the Multi-scale Shock Technique, and observed differences in chemical reaction. We also performed a large-scale explicit sub-threshold shock of AN crystal with a 10 nm void filled with 4.4 wt% of n-dodecane. We observed the formation of hotspots and enhanced reactivity at the interface region between AN and n-dodecane molecules.

Nanostructure-enhanced Chemical Reactivity and Detonation in Energetic Materials: End of Year Summary.

End of year summary including report on project milestones, project productivity, and next steps.