Ryan R. Wixom

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.

First principles site occupation and migration of hydrogen, helium, and oxygen in β-phase erbium hydride

First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in β-phase ErH2. The ground state for hydrogen was indeed determined to be the tetrahedral position as commonly assumed, but if the surrounding tetrahedral sites are filled, any additional hydrogen will occupy the octahedral site. Only a small amount of thermally generated tetrahedral-vacancy octahedral-occupancy pairs are predicted at equilibrium since the formation energy is 1.21 eV. Other possible scenarios that result in octahedral hydrogen occupation include a H/Er ratio >2.0 and the presence of oxygen in the lattice. Our calculations indicate that oxygen impurities will reside in tetrahedral sites, even if that site is already occupied and hydrogen must be displaced into a neighboring octahedral site. Oxygen will migrate at moderate temperatures by jumping between tetrahedral and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahe...

Characterization of pore morphology in molecular crystal explosives by focused ion-beam nanotomography

The initiation and detonation properties of explosives are often empirically correlated to density, surface area, and particle size. Although these correlations are sometimes used successfully to predict the performance of bulk samples, the data are spatially averaged, which unfortunately muddles information critical to understanding fundamental processes. Density and surface area are essentially an indirect measure of porosity, which is arguably a more appropriate metric in many applications. We report the direct characterization of porosity in polycrystalline molecular crystal explosives by focused ion beam nanotomography, a technique that is typically reserved for robust materials such as ceramics and metals. The resulting three-dimensional microstructural data are incredibly rich, promising a substantial advance in our ability to unravel the processes governing initiation and detonation of molecular crystal explosives. In a larger context, this work demonstrates that focused ion beam nanotomography may be successfully extended to the investigation of nanoscale porosity in other molecular crystal or polymer materials.

MESOSCALE SIMULATIONS OF SHOCK INITIATION IN ENERGETIC MATERIALS CHARACTERIZED BY THREE‐DIMENSIONAL NANOTOMOGRAPHY

Three‐dimensional shock simulations of energetic materials have been conducted to improve our understanding of initiation at the mesoscale. Vapor‐deposited films of PETN and pressed powders of HNS were characterized with a novel three‐dimensional nanotomographic technique. Detailed microstructures were constructed experimentally from a stack of serial electron micrographs obtained by successive milling and imaging in a dual‐beam FIB/SEM. These microstructures were digitized and imported into a multidimensional, multimaterial Eulerian shock physics code. The simulations provided insight into the mechanisms of pore collapse in PETN and HNS samples with distinctly different three‐dimensional pore morphology and distribution. This modeling effort supports investigations of microscale explosive phenomenology and elucidates mechanisms governing initiation of secondary explosives.

Density-functional-theory calculations for silicon vacancy migration

The saddle-point configurations and associated formation energies of a migrating silicon vacancy in the +2, +1, 0, −1, and −2 charge states were computed using density-functional theory with a plane wave basis set, norm-conserving pseudopotentials, and the generalized-gradient approximation for exchange and correlation. Spurious electrostatic and strain contributions arising from use of periodic boundary conditions were removed by performing maximum likelihood fits on results from 215-, 511-, and 999-atom supercells, and thereby obtaining formation energies corresponding to isolated vacancies. Migration enthalpies were computed by subtracting similarly obtained formation energies for vacancies in local-energy minimum configurations. The results (0.27eV in the +2 charge state, 0.19eV in the +1 charge state, 0.36eV in the 0 charge state, 0.04eV in the −1 charge state, and 0.15eV in the −2 charge state) are in good overall agreement with experimental results obtained at low temperatures.

H enhancement of N vacancy migration in GaN.

We have used density functional theory to investigate diffusion of VN+ in the presence of H+. Optimal migration pathways were determined using the climbing image nudged elastic band and directed dimer methods. Our calculations indicate that the rate-limiting barrier for VN+ migration will be reduced by 0.58 eV by interplay with H+, which will enhance migration by more than an order of magnitude at typical GaN growth temperatures.

A Mie-Grüneisen EOS with non-constant specific heat

A complete and consistent equation of state based on Mie-Gruneisen assumptions was developed. This EOS assumes constant Gruneisen coefficient, but a variable specific heat that is modeled using Einsteins theory of heat capacities using two oscillators. The shock velocity particle velocity Hugoniot (Us-up) was derived from density functional theory molecular dynamics (DFT-MD). The EOS was formatted as a tabular EOS and checked for consistency using one-dimensional impact simulations in CTH.

Summary of Sandia research on metal tritides : FY 2007.

Sandia National Laboratories has cradle to grave responsibility for all neutron generators in the US nuclear weapons stockpile. As such, much research effort is exerted to develop a comprehensive understanding of all the major components of a neutron generator. One of the key components is the tritium containing target. The target is a thin metal tritide film. Sandias research into metal tritides began in the early 1960s with a collaboration with the Denver Research Institute (DRI) and continues to this day with a major in house research effort. This document is an attempt to briefly summarize what is known about the aging of erbium tritide and to review the major publications conducted at Sandia in FY 07. First, a review of our knowledge of helium in erbium tritide will be presented. Second, executive summaries of the six major SAND reports regarding neutron tube targets published in FY07 by Department 2735, the Applied Science and Technology Maturation Department, and research partners are presented.

N interstitial and its interaction with substitutional Mg in p-type GaN

Density-functional theory and the generalized gradient approximation were utilized to investigate the local-energy-minimum configurations and formation energies of N interstitials and their interaction with substitutional Mg in p-type GaN. Along with previously proposed configurations of the N interstitial, a new variant of the split interstitial is discussed. Split interstitials are more stable than the other configurations of the interstitial. The formation energies are such that N interstitials are not expected to form under equilibrium conditions, however, they may form during nonequilibrium processes and become mobile during annealing. The N interstitial is found to bind with substitutional Mg, with the binding energy of the complex being 0.75, 0.53, and 0.35 eV for the +2, +1, and neutral charge states of the complex.

Shock interactions with heterogeneous energetic materials

The complex physical phenomenon of shock wave interaction with material heterogeneities has significant importance and nevertheless remains little understood. In many materials, the observed macroscale response to shock loading is governed by characteristics of the microstructure. Yet, the majority of computational studies aimed at predicting phenomena affected by these processes, such as the initiation and propagation of detonation waves in explosives or shock propagation in geological materials, employ continuum material and reactive burn model treatment. In an effort to highlight the grain-scale processes that underlie the observable effects in an energetic system, a grain-scale model for hexanitrostilbene (HNS) has been developed. The measured microstructures were used to produce synthetic computational representations of the pore structure, and a density functional theory molecular dynamics derived equation of state (EOS) was used for the fully dense HNS matrix. The explicit inclusion of the microstr...