Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J26: Focus Session: Materials in Extremes: Energetic Materials |
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Sponsoring Units: GSCCM DCOMP DMP Chair: Evan Reed, Stanford University Room: 502 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J26.00001: Shock induced chemistry in liquids on picosecond timescales Invited Speaker: Shawn McGrane While great progress has been made in theory and simulation of shock induced chemical reactivity, there are few experiments sensitive to the time and length scales necessary to validate these theories. In this talk, we will report the results of experiments on liquids exposed to ultrafast laser driven shocks observed by interferometry and spectroscopy with picosecond time resolution. These time and length scales correspond to those accessible to reactive molecular dynamics simulations, and are often required to observe chemical kinetics using optical methods prior to sample opacity caused by product formation. We will report interferometric and transient absorption data for times up to 300 ps on nitromethane, carbon disulfide, phenylacetylene, acrylonitrile, and several other liquids shocked to initial states between 5 and 22 GPa. Indications of volume increasing and decreasing chemical reactions are observed interferometrically. Chemical products are observed via transient absorption signatures. Efforts to identify these products with vibrational spectroscopies will be reported. We will also compare the results observed in these small scale experiments with literature results from experiments acquired on time and length scales larger by orders of magnitude. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J26.00002: The Raman spectrum of ammonium nitrate at high pressures from first principles calculations Ivan Oleynik, Brad Steele, Aaron Landerville The pressure induced phase transitions in sodium azide, which include a potential polymeric nitrogen phase transition, are investigated using evolutionary crystal structure prediction methods coupled with density functional theory calculations. Two new phases are predicted to be stable above 53 GPa that have an inequivalent ratio of sodium to nitrogen atoms as compared to sodium azide. The Raman spectrum is calculated from 0-100 GPa using these newly predicted structures, as well as the newly discovered I4/mcm phase of sodium azide. The predicted Raman spectrum is shown to give good agreement to experimental data above 30 GPa and below 15 GPa. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J26.00003: Toward a reaction rate model of condensed-phase RDX decomposition under high temperatures Igor Schweigert Shock ignition of energetic molecular solids is driven by microstructural heterogeneities, at which even moderate stresses can result in sufficiently high temperatures to initiate material decomposition and the release of the chemical energy. Mesoscale modeling of these ``hot spots'' requires a chemical reaction rate model that describes the energy release with a sub-microsecond resolution and under a wide range of temperatures. No such model is available even for well-studied energetic materials such as RDX. In this presentation, I will describe an ongoing effort to develop a reaction rate model of condensed-phase RDX decomposition under high temperatures using first-principles molecular dynamics, transition-state theory, and reaction network analysis. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J26.00004: Optical characterization of shock-induced chemistry in the explosive nitromethane using DFT and time-dependent DFT Lenson Pellouchoud, Evan Reed With continual improvements in ultrafast optical spectroscopy and new multi-scale methods for simulating chemistry for hundreds of picoseconds, the opportunity is beginning to exist to connect experiments with simulations on the same timescale. We compute the optical properties of the liquid phase energetic material nitromethane (CH$_{\mathrm{3}}$NO$_{\mathrm{2}})$ for the first 100 picoseconds behind the front of a simulated shock at 6.5km/s, close to the experimentally observed detonation shock speed. We utilize molecular dynamics trajectories computed using the multi-scale shock technique (MSST) for time-resolved optical spectrum calculations based on both linear response time-dependent DFT (TDDFT) and the Kubo-Greenwood (KG) formula within Kohn-Sham DFT. We find that TDDFT predicts optical conductivities 25-35{\%} lower than KG-based values and provides better agreement with the experimentally measured index of refraction of unreacted nitromethane. We investigate the influence of electronic temperature on the KG spectra and find no significant effect at optical wavelengths. With all methods, the spectra evolve non-monotonically in time as shock-induced chemistry takes place. We attribute the time-resolved absorption at optical wavelengths to time-dependent populations of molecular decomposition products, including NO, CNO, CNOH, H$_{\mathrm{2}}$O, and larger molecules. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J26.00005: Micron-scale Reactive Atomistic Simulation of Void Collapse and Hotspot Growth in PETN Aidan Thompson, Tzu-Ray Shan Material defects and heterogeneities such as dislocations, grain boundaries, and micro-porosity play key roles in the shock-induced initiation of detonation in energetic materials. Non-equilibrium molecular dynamics simulations (NEMD) with the ReaxFF force field (ReaxFF) in LAMMPS were performed to explore the effect of nanoscale voids on hotspot growth and initiation in pentaerythritol tetranitrate (PETN) crystals under weak shock conditions. Previously, we have performed reactive NEMD simulations of weak shocks in a $(20nm)^3$ PETN crystal containing a spherical void. We observed hotspot formation and an exothermic reaction zone. To observe growth of the hotspot, we have now greatly extended the time and lengthscale of the simulation. We created a cylindrical pore in a $0.3\times0.2\times0.001\mu m^3$ crystal. Once the shockwave reached the free surface we continued the simulation using the shock-front absorbing boundary condition. Results show steadily increasing axial and lateral spatial extent of the hotspot and a complex coupling of exothermic chemistry to hotspot growth. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J26.00006: Unreacted Equations of State of Shocked Single Crystal PETN and Beta-HMX Joseph Zaug, Michael Armstrong, Jonathan Crowhurst, Louis Ferranti, Sorin Bastea, Lawrence Fried We report results from ultrafast shockwave experiments conducted on single crystal high explosives. Ultrafast shock studies can enable high throughput characterizations of unreacted equations of state to higher pressures than previously reported and also quantify the magnitude of anisotropic mechanical response to shock waves. Our ultrafast results yield --as of this writing- [110] PETN data up to a pressure of 26 GPa, which is 1.6x higher than published mid-scale gun results. Published HMX shock data are strikingly sparse; seven points up to approximately 10 GPa are reported from shocked solvent-pressed beta-HMX and Robert Craig reported three single crystal points (undisclosed crystal orientation) between 34 and 42 GPa. Two nonhydrostatic cold-compression diamond-anvil cell studies, u-Raman $+$ u-XRD, and u-Raman $+$ deflagration rates, report a transition in HMX, possibly shear induced, beginning at 26-27 GPa. A previously posed question is whether Craig's data are affected by this transition.$\backslash $pard An analysis of our results for [010] beta-HMX indicate it is less compressible than portrayed by the commonly accepted Hugoniot, which is based on a parameterized third-order Birch-Murnaghan model EoS using the ten before mentioned shock wave measurements and the more recent cold-compression u-XRD study by Yoo et al. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J26.00007: Structural diversity in the ammonium azide molecular crystal at high pressures Aaron Landerville, Brad Steele, Ivan Oleynik Ammonium azide (NH$_{4}$N$_{3})$ seems to undergo phase transitions under compression as indicated by the experimentally measured Raman spectrum. However, X-ray diffraction studies of the NH$_{4}$N$_{3}$ crystal beyond the known first phase transition at $\sim$ 3 GPa have yet to be performed. Additionally, first-principles density functional perturbation theory calculations of the known phase of NH$_{4}$N$_{3}$ have been unsuccessful at reproducing Raman spectral evolution with pressure seen in experiment, while no evidence has been found that NH$_{4}$N$_{3}$ transitions to hydronitrogen solid at the predicted pressure of 36 GPa. This may indicate that the true lowest enthalpy configuration has yet to be discovered. Here, evolutionary structure prediction method coupled with density functional calculations are employed to calculate the lowest enthalpy phases of ammonium azide as a function of pressure. Novel structures are predicted, and ground state enthalpies and the Raman spectra are calculated as a function of pressure and compared with the experimental Raman spectra. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J26.00008: Effect of subcritical damage on sensitivity of a plastic bonded explosive George Sunny, Thomas Krawietz, John Cox, Jennifer Jordan, Chad Rumchik As energetic materials are subjected to increasingly extreme environments, a more thorough understanding of the relationships between mechanical insult and changes in explosive sensitivity is desired. To that end, a Shock Wave Apparatus, originally developed at TDW (Schrobenhausen, Germany), has been employed to induce subcritical shocks of up to 0.7 GPa in a plastic bonded explosive sample while preserving the sample for further study. Changes in density due to the subcritical shocks are measured, and the sensitivity of the damaged explosive is determined through a TDW/AFRL Modified Gap Test configuration that allows the run-to-detonation (RTD) to be determined for a given shock loading. Changes in sensitivity are determined by comparing the RTD for each damaged sample with corresponding RTD for pristine (i.e. undamaged) samples. Confined Split-Hopkinson Pressure Bar experiments are also conducted in order to understand the effects of damage at lower strain-rates and pressures. Finally, the effects on sensitivity due to multiple shocks are also investigated in this study. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J26.00009: A comparison of reactive burn models to gap/rate-stick experiments Christina Scovel, Ralph Menikoff, Elizabeth Francois, Dana Dattelbaum, Jay Kucko We present a numerical study of shock initiation for the case when a detonation wave in a donor PBX 9502 passes through an inert polymer material (epoxy) and then into an acceptor of PBX 9502. The pressure gradient behind the detonation wave causes the lead shock in the epoxy to decay and strongly influences whether the acceptor detonates. We compare four reactive burn models and discuss difference in their predictions. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J26.00010: Investigations of PBX 9502 relight phenomena using a modified gap test Elizabeth Francois, Christina Scovel, Dana Dattelbaum We present a series of experimental results on PBX 9502 relight gap tests where epoxy gaps of varying thickness and material were placed within between equal lengths of PBX 9502. Piezo pins were used to record velocity before and after the gap. Relight location was measured and subsequent velocity calculated. These results were used to validate and improve models, and support gas-gun shock initiation experiments. The design for these tests utilized a modified gap test where the donor and the acceptor explosives are the same, and separated by an epoxy gap of varying thickness. The epoxy used was comprised of Epon-828 and Jeffamine T-403. The explosive studied was PBX 9502. The goal of the experiment was to initially reach steady state detonation behavior, and then retard it with the gap, and measure the velocity and re-initiation behavior. The results were then compared to existing models. Other gap materials were studied as well, and the approach and results of all materials will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J26.00011: Multi-physics Meso-scale Finite Element Simulation of HMX-based Solid Propellant Subjected to Thermal Insults Gaurav Srivastava, Karel Matous A large strain chemo-thermo-mechanical numerical framework has been developed to model the coupled chemical, thermal and mechanical behavior of solid propellant at the meso-scale. The mechanical behavior is modeled using a hyperelastic material model with viscous damage and J2 plasticity. The model admits a general nonlinear coefficient of thermal expansion to capture the thermo-mechanical behavior. The chemical model considers a system of chemical reactions with the rate kinetics being governed by a modified Arrhenius law. The thermal model considers thermodynamically consistent energy contributions from the inelastic mechanical deformations and the chemical reactions. The finite element method has been employed to discretize the continuum equations. Some simulation results will be presented to demonstrate the use of the developed framework in modeling the behavior of HMX-based solid propellant under thermal loads. The developed framework captures the large volumetric strains that are a characteristic of the $\beta$-$\delta$ phase transition of the HMX crystals and is able to predict locations of potential cracks in the binder. Such a simulation tool may prove to be useful in determining optimal conditions for the safe storage of such materials. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J26.00012: Near Absolute Equation of State Measurements of CH using Velocimetry and Radiography Dayne Fratanduono, Peter Celliers, Amy Lazicki, Jim Hawreliak, Gilbert Collins The OMEGA EP laser was used to conduct absolute near equation of state measurement along the principal Hugoniot of CH to 6 Mbar. A 6 ns long, 3700 J laser pulse in direct drive was used to launch a cylindrical shock in a multi-layered aluminum/CH target which was imaged using a Fe backlighter. The technique presented here incorporated VISAR shock velocity measurements with shock compression measured using side-on radiography to determine the Hugoniot. Experimental uncertainties of less than 10{\%} in density were obtained in these experiments. The measured Hugoniot values of this study are consistent with previous measurements that were impedance matched to quartz (Barrios et al. PoP 2010). These experiments were conducted, as proof of principle, for future absolute EOS measurements on the NIF. Future experimental work will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J26.00013: Modeling Shock Desensitization of Composition B Explosive Charles Mader The NOBEL multimaterial adaptive grid Eulerian hydrodynamic code was used to model a shaped charge jet formation, its interaction with a steel plate, and shock formation of a bow shock in front of the jet that shocks and desensitizes a cylinder of Composition B (60/40 RDX/TNT at 1.715 g/cc) explosive so that when the jet arrives it fails to initiate detonation in the desensitized explosive. The jet passes through the Composition B explosive cylinder, an air gap, and then initiates propagating detonation in a second Composition B explosive cylinder that has not been desensitized by a preshock. The experimental arrangement was studied using X-ray radiography at the Material Research Laboratory in Melbourne, Australia. [Preview Abstract] |
Tuesday, March 4, 2014 5:30PM - 5:42PM |
J26.00014: X-ray induced mobility of molecular oxygen at extreme conditions Michael Pravica, Dimitry Popov, Stanislav Sinogeikin, Daniel Sneed, Quinn Smith, Griffin Guardala We report an in situ Raman study of KClO4 irradiated with x-rays in a diamond anvil cell. Decomposition via KClO4 $+$ hv $\to $ KCl $+$ 2O2 was monitored via the O2 vibron at 2 GPa, 6 GPa and 9 GPa. For all pressures, the vibron grew in intensity and then diminished after successive irradiation suggesting that O2 was diffusing away from the irradiated region. Surprisingly, the diffusion rate accelerated with pressure increase, indicating that the nonhydrostatic pressure gradient was likely driving molecular diffusion of oxygen. At 9 GPa, the vibron bifurcated suggesting that O2 exists as two forms: interstitial and bulk solid. This method can be employed to study molecular diffusion under extreme conditions. [Preview Abstract] |
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