Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R38: Materials in Extremes: Energetic MaterialsFocus Session
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Sponsoring Units: DCOMP GSCCM DMP Chair: Rebecca Kristine Lindsey, Lawrence Livermore National Laboratory Room: LACC 501A |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R38.00001: Formulating Reduced Order Models for the Reaction Chemistry of High Explosives Edward Kober The reaction chemistry of high explosives has been analyzed phenomenologically using models containing only a few reaction steps. This is the level of model desired for use in higher length scale simulations. For explicit models, one should nominally consider hundreds of reaction intermediates and thousands of possible reactions, but the reductions of these to several step models is difficult. An alternative approach is to perform and analyze condensed phase reactive molecular dynamics (MD) simulations on these systems. The typical method for quantifying these simulations relies on identifying explicit molecular species and tracking their concentrations. A different approach is demonstrated here which focuses on quantifying changes in the coordination chemistry / oxidation state of the atoms during the reaction processes. This classifies all atoms for every timestep of the simulation and produces a countable number of geometries that characterize the reaction progress. Initial reduced order models can then be formulated from these results using the non-negative matrix factorization method. Illustrations will be given for cook-off simulations using both the classical ReaxFF MD approach applied to HMX, and the DFT-TB LATTE code applied to nitromethane and PETN. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R38.00002: Coupling thermal response and shock physics in secondary explosives Bryan Henson, Laura Smilowitz We have recently produced a family of chemical decomposition models applicable to the thermal response of secondary explosives. In this talk we present applications of this model to the response of HMX and PETN. We use the pressurization rate upon ignition for these explosives in two different regimes of heating rate as the source term for ramp wave pressurization of material in the far field. By a simple application of the method of characteristics we calculate wave coalescence times and distances as a function of the applied thermal boundary condition. We compare results with observations from historical work on the Deflagration to Detonation Transtition (DDT) for HMX and the function of Explosive Bridgewire Detonators composed of both PETN and HMX that have been studied in this laboratory. |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R38.00003: Search for promising heterocyclic high energy density materials: A holistic approach for design of novel energetic materials Philip Pagoria, Roman Tsyshevskiy, Aleksandr Smirnov, Maija Kuklja Simple heterocyclic molecules with a five- or six-atom ring, functioning as a backbone preparatory compound, offer a large number of options for the synthesis of promising energetic materials. Despite many remarkable successes with synthesized compounds, the search for new energetics remains intuitive because of insufficient information on clear correlations between the structure of the material, its performance and sensitivity. We propose a comprehensive multilevel approach to design novel energetic materials which links an intensive analysis of data on existing materials, synthesis, experimental measurements and characterization, computational modeling, and validation. We demonstrate how this approach can be used for design of new heterocyclic oxadiazole-based energetic materials. We also show how multiscale computational modeling that combines first principles calculations, analytical theory, and empirical statistical analysis serves to investigate the structure-stability-performance relationship and design new energetic materials with tailored properties. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R38.00004: Photo-initiation of energetic interfaces: new perspectives in composite high explosives Maija Kuklja, Roman Tsyshevskiy, Sergey Rashkeev, Anton Zverev, Anatoly Mitrofanov, Nadezda Aluker Optical initiation to detonation of energetic materials (EMs) opens up new ways for safe handling, storage, and use of high explosives. The key in achieving tunable sensitivity of EM-metal oxide interface to external perturbation is the proper alignment of the regular and defect-generated electronic states of an oxide surface and EMs. We show how electronic and optical properties on EM-oxide interfaces are different from the properties of individual components and demonstrate how these changes can be effectively manipulated to enable control of both the energy absorption and release. We demonstrate how surface defects (e.g., oxygen vacancies, F-centers) and charge transfer present a rich set of additional opportunities to ensure highly controllable photo-initiation of chemistry at the interfaces. We discuss potential mechanisms of thermally and optically simulated decomposition reactions of energetic molecules adsorbed on oxide surfaces. We show that knowledge of chemical stability of ionized energetic molecules could be critical for manufacturing interfaces with tunable optical sensitivity. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R38.00005: Bimolecular Initial Reactions in γ-RDX Igor Schweigert Shock compression of energetic molecular crystals often triggers solid-solid phase transitions to denser polymorphs (such as the γ polymorph of RDX) prior to onset of reactivity and shock-to-detonation transition. The more compact molecular arrangement in the unit cell of γ-RDX should affect the initial reactions leading to molecular decomposition. In this presentation, I will describe density functional theory (DFT) based molecular dynamics simulations and reaction path optimizations of initial decomposition reactions in γ-RDX subjected to hydrostatic pressures up to 20 GPa. These simulations predict that the dominant decomposition mechanism is bimolecular H transfer and simultaneous NO2 loss rather than unimolecular N-N homolysis or HONO elimination |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R38.00006: Ultrafast Shock-Induced Reactions in PETN and RDX Thin Films Samuel Park, Michael Armstrong, Ian Kohl, Joseph Zaug, Robert Knepper, Alexander Tappan, Sorin Bastea, Jeffrey Kay Ultrafast shock interrogation (USI) and ultrafast absorption spectroscopy are used to examine shock compression of PETN and RDX thin films on the picosecond timescale. The USI measurements probe the thermodynamic state (stress and density) of shocked materials interferometrically. At low degrees of compression (V/V0 > 0.8), the USI data for both materials lie along the unreacted Hugoniots, and agree with previous experimental equation-of-state measurements. At higher degrees of compression (V/V0 < 0.8), the USI data lie well above the unreacted Hugoniots, with sharp rises near V/V0 ~ 0.8. These results appear to indicate rapid exothermic reaction of the materials. The temporal evolution of the interferometric signals indicates onset of reaction within ~50 ps after arrival of the shock. Ultrafast optical absorption experiments on shocked RDX support these conclusions. |
Thursday, March 8, 2018 9:12AM - 9:48AM |
R38.00007: Molecular Dynamics Simulations of Shock Induced Chemistry in Organic Materials Invited Speaker: Romain Perriot The chemistry of energetic materials (EM) is characterized by cascades of exothermic reactions on the ps to ns timescales, which generate numerous intermediates between the explosive and final products. Experiments have struggled to resolve the intermediates (species and reaction rates), which dictate the state of the material at longer time scales. It is imperative to obtain a quantitative understanding of the reaction pathways as a function of temperature and pressure during the early stages of detonation, so that we can move beyond the use of heavily calibrated, though non-transferable, rate laws in mesoscale simulations. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R38.00008: Thermal Explosion of HMX Using a Subscale Frank-Kamenetskii Model with Detailed Chemical Kinetics Nikolai Petsev, Xia Ma, Bryan Henson, Brad Clements We present a subscale model for conductive burning in an explosive material featuring a complex kinetic network. Our approach is based on the Frank-Kamenetskii (FK) thermal explosion model, with numerical solutions obtained using the method of weighted residuals (MWR), as originally proposed by Bennet et al. [J. Mech. Phys. Solids, 46, 2303, 1998]. The subscale model incorporates localized heating of the solid energetic material due to grain-scale phenomena like frictional heating of small cracks, resulting in ignition. This framework, coupled to a physically-based kinetic model for the reaction, is applied to the problem of conductive burning of HMX. We present preliminary results from our studies, including propagation velocities for the reaction front under varying conditions, and discuss future extensions and applications of this model. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R38.00009: Status of Lab-scale Asynchronous Radiography of Spontaneous Thermal Explosions and Detonations Laura Smilowitz, Bryan Henson The Lab-scale Asynchronous Radiography System (LARS) facility at Los Alamos has been evolving in capability over the past decade and has been applied to problems of materials in extremes ranging from slow thermal phase changes to the initiation of detonation. By trading off spatial resolution, time resolution, field of view, and contrast sensitivity, the system is able to capture continuous movies of x-ray transmission over broad ranges of time resolution and event duration. At one extreme, it has been used with 50 micron spatial and 10 ms time resolution to follow solid-solid and solid-liquid-gas transitions in explosives heated from room temperature through to thermal explosion. At the other end of the spectrum, it has been applied with few hundred micron spatial resolution and hundreds of nanosecond time resolution to observe the initiation of detonation in a commercial EBW detonator. A summary of the capability and current applications and future directions will be discussed. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R38.00010: Imaging of explosions using a pulsed light source
by K.L. McNesby, E.S. Collins, R.A. Benjamin
Army Research Laboratory, Aberdeen Proving Ground, MD Kevin McNesby A high-repetition rate laser is used as an illumination source for shadowgraph and for schlieren imaging of explosions. The laser is a diode pumped solid state device (Nd:YAG) operating at 200 kHz, outputting 0.33 milliJoules of energy per pulse (10 nanosecond) of 532 nanometer radiation. The laser pulse rate is synchronized to a high speed video camera, such that the light from the laser coincides with the onset of the exposure period of the camera. The camera is filtered at the laser wavelength, and the illumination and viewing lines-of-sight are made coincidental to minimize parallax. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R38.00011: Ultrafast Mid-Infrared Spectroscopy on Shocked Thin Film Explosive Crystals Michael Powell, Steven Son, Pamela Bowlan, Shawn McGrane The chemistry of energetic materials under shocked conditions is complex and poorly understood. Various theories exist that predict reaction mechanisms; however, direct experimental insight into the chemical reactions prevalent during shocks has had little attention. Mid-Infrared radiation measurement were used to investigate the chemistry of ultrafast laser shock loading of thin films of explosive crystals. Spectrally the leading edge of the shock drive beam was clipped to provide a more instantaneous shock to the sample. Generation of the mid-infrared pulse was accomplished via a four wave difference frequency generation (FWDFG) where the compressed pulse (ωp) was temporally and spatially overlapped in air with the second harmonic of the compressed pulse (ω2p) to generate a wide spectra of infrared laser energy (ωp + ωp – ω2p = ωMIR) over ~600 cm-1 to ~1400 cm-1. Mid-infrared spectra are collected by an infrared spectrometer and analyzed for chemical bond identification under different shock conditions. This experiment aims to identify the chemistry of shocked explosive crystals to compare to molecular dynamic (MD) simulations. These results should be useful in guiding molecular dynamics simulations that probe the same time and length scales as the experiments. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R38.00012: Detonation on a tabletop in Nitromethane Mithun Bhowmick, Dana Dlott We report generating a shock in nitromethane that is sustained by the explosive energy of reaction, using a convenient tabletop apparatus. This is effectively a microscopic detonation. Our tabletop shock apparatus uses a laser-driven flyer plate assembly and an optical pyrometer, where we can launch flyer plates to an array of mass-produced tiny cells containing liquids. Using a photon Doppler velocimeter apparatus, we can measure temporal profiles and energy of the incoming and outgoing shocks to study shock wave energy dissipation/amplification below and above emission thresholds, respectively. Graybody fits to the measured emission extract temperatures ~3000K from shock-initiated nitromethane. The ability to conveniently shock liquid cells on a tabletop with high throughput provides opportunity to study detonation in many energetic materials. |
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