Session B23: Materials in Extremes: Energetic Materials - II

Live

DFT calculation and data analysis on molecular and crystal level features of 60 reported EMs allowed us to define key descriptors that are characteristics of these compounds’ thermostability. We see these descriptors as reminiscent of “Lipinski’s rule of 5”, which revolutionized the design of new orally active pharmaceutical molecules. The proposed descriptors for thermostable EMs are of a type of molecular design, location, and type of the weakest bond in the energetic molecule, as well as specific ranges of oxygen balance, crystal packing coefficient, Hirshfeld surface hydrogen bonding, and crystal lattice energy. On this basis, we designed three new thermostable EMs HL3, HL7, and HL9, which were synthesized, characterized, and evaluated in small-scale detonation experiments. The best overall performing compound HL7 exhibited an onset decomposition temperature of 341 °C and has a density of 1.865 g cm^–3, and calculated VOD and Pd were 8517 m s^–1 and 30.6 GPa, respectively. Considering HL7’s safety parameters [IS = 22 J; FS = 352; and ESD = 1.05 J] and the results of detonation experiments, the proposed guidelines should further promote the rational design of novel thermostable EMs, suitable for deep well drilling, space exploration, and other defense and civil applications.   

Live

Anisotropic single crystal effects play a critical role in the mesoscale behavior of energetic materials in response to external stimuli like dynamic loading. Properties like stress, strain, and temperature are strongly influenced by these orientation effects. In particular, anisotropic thermal conductivity effects can lead to phenomena like hot spot generation during impact scenarios, and potentially lead to deflagration or detonation. Models can predict the behavior of energetic materials in response to external stimuli; however, the accuracy of these models depends on the forcefields and inputs used in the simulation. Experimental validation capable of measuring predicted phenomena is a necessity. Impulsively stimulated thermal scattering, a pump-probe technique, can simultaneously measure the crystal orientation dependent sound speed and thermal conductivity. Experimental measurements of the thermal conductivity for HMX crystals suggest that the thermal conductivity varies between 0.39-0.67 W/m-K. Thermal conductivities were measured for (100), (010), and (011) orientations. Models have predicted the thermal conductivity to have values between 0.39-0.5 W/m-K. Experimental results will be used to refine the forcefields present in the models   

Live

Shock loading of plasticly bonded high explosives leads to energy localization into hotspots, which are known to govern the initiation of detonation. Hotspots, and their criticality, have generally been characterized in terms of only their size and temperature. Yet, our recent results show that more energy is localized in potential energy (PE) than in kinetic energy (KE), and this PE rise is more persistent in time. This PE rise is a result of intra-molecular deformation that are unable to relax. However, modeling the effects of mechanical strain on the reaction rates of materials near detonation conditions is still a grand challenge. We use unsupervised clustering to group hotspot molecules by temperature and deformation. We find that for similar temperatures, as molecules reach higher levels of deformation, not only do they react faster, but the distribution of their lifetimes shifts from a Poisson distribution to an exponential decay. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Approved for unlimited release LLNL-ABS-815963.   

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Thin films of pentaerythritol tetranitrate (PETN) were shock compressed from 3-55 GPa using the laser driven shock apparatus at Los Alamos National Laboratory. Spectra were measured during the first 250 ps using visible transient absorption (VIS) from 400 to 700 nm and mid-infrared transient absorption (MIR) from 1150 to 3800 cm^–1. VIS and MIR results suggest irreversible chemistry was induced in PETN at pressures above 30 GPa. Under reactive conditions, the antisymmetric NO₂ stretch mode had a significantly increased absorption while the symmetric NO₂ stretch did not. No increased absorption occurred in the frequency regions where new CH-, NH-, and OH- bond absorptions would be expected. A new absorption appeared around 2200 cm^–1. In addition to the experiments, reactive molecular dynamics were carried out under equivalent shock conditions to correlate the evolution of the infrared spectrum to molecular processes. The simulations show results consistent to experiments up to 30 GPa but suggest that NO and NO₂ related features provided the strongest contributions to the shocked infrared changes. Our experiments suggest C≡O or N₂O bond formation, nitrite formation, and absence of significant hydroxyl or amine concentrations in the initial chemistry steps in shocked PETN.   

Live

Energetic materials are subjected to harsh environmental conditions including impact, vibration and varied temperatures. Under such loading conditions, local stress or strain concentrations may lead to the formation of hot spots and unintended reaction. To visualize the dynamic damage and reaction processes in polymer bonded energetic crystals under impact loading, a high-speed X-ray phase contrast imaging (PCI) setup was synchronized with projectile impact from a light gas gun. Impact compressive loading was applied on PBX specimens containing a single or multiple energetic crystals. The impact-induced, time-resolved damage and reaction processes were captured using the high-speed X-ray PCI at a frame rate of 50,000 fps. Numerical simulation models were built based on the experimental results and used to investigate the impact damage and local temperatures in PBX over a wider range of parameter variations.   

Live

As the primary mechanism for the ignition and initiation of polymer-bonded explosives during dynamic compression, the generation and propagation of hot spots in these materials are small and hardly observed with conventional experimental techniques. Here, we developed a hyperspectral imaging system that was combined with a shock compression microscope to observe the hot spots and obtain the spatial distribution of the temperature and emissivity of individual explosive microcrystals embedded in polymer matrix. We launched dynamic compressions with an aluminum flyer plate (500 µm diameter, 37 µm thick) that was accelerated to 3.6 km/s by a flat-top pulsed laser. The spatial distribution of the broadband thermal-emission signals from the explosive microcrystal denoted HMX (cyclotetramethylene-tetranitramine) of 200-300 µm in size, was captured by a multi-channel ultra-fast camera. By applying different color filters for different channels and capturing auto-emission images at different times, we generated videos of the spatial distributions of color temperature and emissivity on explosive microcrystals.   

Live

In this work, impact-induced failure of hydroxyl terminated polybutadiene (HTPB) – Cyclo tetra methylene tetra nitramine (HMX) energetic material samples is studied using the cohesive finite element method (CFEM). The CFEM model incorporates a viscoplastic constitutive model using experimentally measured parameters, interface level separation properties, and temperature increase due to mechanical impact. Nanoscale dynamic impact experiments were used to obtain parameters for a strain-rate dependent viscoplastic constitutive model for HTPB, HMX, and the HTPB-HMX interfaces. Mechanical Raman spectroscopy (MRS) was used to obtain cohesive zone model parameters to simulate interface separation. Microstructures having circular HMX particles were found to show higher local temperature rise as compared to those with diamond or irregular shaped HMX particles with sharp edges. Regions within the analyzed microstructures near high-volume fraction of HMX particles were found to have relatively high temperature spikes.   

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Experimental Area A at LANSCE is a large unused experimental area with overhead crane coverage, installed power, access to the LANSCE proton beam, and more than 30,000 square feet of floor space. In former times it was used for pion experiments. We propose to restore proton delivery to Area A and to build a new three axis proton radiography area designed to support high, peak-intensity, low-power, low-emittance beam delivery for studying small dynamic experiments aimed at studying dynamic materials physics at the meso-scale. With the addition of a responsive RF kicker currently under evaluation, such a facility could support multiple beamlines, including a multi-axis proton radiography installation. A refurbished Area A would also support other high-current experimental beamlines for materials studies and isotope production. We will present the technical options and some examples of physics opportunities where the temporal and spatial resolution offered by such a facility will help elucidate shock physics at the grain level.   

Live

A finite strain thermomechanical model developed by Luscher et al. is advanced and applied to the high rate deformation of β-cyclotetramethylene tetranitramine (β-HMX) single crystals. The model accounts for the plastic deformation due to the dislocation slip and deformation twinning. The viable set of slip systems for β-HMX was evaluated from quasistatic simulations of the Knoop indentation experiments by Gallagher et al. We calibrate the dislocation slip part of the model to the plate impact experiments by Dick et al. For the twinning part of the model we use the phase-field framework proposed by Clayton and Knap. We added anisotropy to the twin interface energy to accurately capture the elongated shape of twins observed in the experiments. We will report predictions of the twin growth for several orientations of β-HMX crystals under shock compression.
Luscher et al., Journal of the Mechanics and Physics of Solids 98 (2017) 63–86
Gallagher et al., Chemistry Central Journal (2015) 9:22
Dick et al., J. Appl. Phys. Vol. 96, No. 1 (2004)
Clayton and Knap, Physica D 240 (2011) 841–858   

Live

In both manufacturing and dynamic loading, the interplay between deviatoric stress, plastic strain, and heat generation at the mesoscale dictate the responses of plastic bonded explosives (PBX). In situ mesoscale insights are needed to quantify structure-property relationships, inform theory, and enable simulations. We have attempted such an effort and will present an overview of our progress so far.
Laser-driven shock, gas gun, and split-Hopkinson pressure bar experiments have been performed to span multiple orders of strain rate, using synchrotron and X-ray free electron laser radiation to measure time-resolved X-ray diffraction (XRD) and phase contrast imaging (PCI) in situ for single crystal and plastic bonded explosives. This range of strain rates enables investigation of coupling between crystal mechanics, thermal softening, and microsturcture that governs explosive response.
Multiphase single crystal plasticity models have been developed. They consist of non-linear thermo-elasticity, Orowan expressions for slip rate using the Austin-McDowell model for dislocation velocity, and multiphase equations of state (EOS) imposing phase transitions through Gibbs free-energy. Constitutive equations were parameterized with density functional theory and atomistic calculations for EOS and elastic constants along with experimental measurements of anisotropic deformation mechanisms and rates. These models are capable of predicting anisotropy, grain size, and pressure dependent effects remarkably well.
Combining the new capabilities, mesoscale thermomechanics can be investigated from the average lattice response up to PBX microstructures. For the first time, XRD quantify average lattice response and allows for direct comparison of experiments and simulations through measured and computed diagnostics. Using the experimentally validated models, simulation can be compared to PCI of heterogeneous micorstructure effects such as void collapse and grain boundaries.   

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Commonly accepted method for testing thermal stability of energetic materials is thermal analysis. The most popular method of thermal analysis are differential scanning calorimetry (DSC) and differential thermal analysis (DTA). Thermomicroscopy is an another valuable method for detection of physical and chemical changes during heating of the sample. A combination of the methods described above is the analysis of the change in the light reflectance from the sample as a function of its temperature. This method was applied for investigation of thermal stability of common explosives like 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), pentaerythritol tetranitrate (PETN) and relatively new explosive with low sensitivity 3-nitro-1,2,4-triazol-5-one (NTO). On the curves of reflectance changes as a function of temperature registered for RDX and NTO, new undescribed previously processes were observed. An attempt was made to explain these new phenomena by comparing the new data with well-known DSC curves for investigated compounds. Diphase model of the solid substance decomposition was proposed to describe the observed phenomena.   

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This presentation will demonstrate a novel Eulerian framework for modeling crystal plasticity in shock-loaded energetic materials. The specific focus will be to address challenges in Eulerian crystal plasticity and demonstrate a robust computational framework for large anisotropic plastic deformations in single-crystal β-HMX. Plastic-strain localization and associated heating around microstructural defects (voids, cracks etc.) in shock-loaded crystalline energetic materials are well-known to initiate exothermic reactions. Accurate initiation modeling demands frameworks which can handle extreme deformations as well as anisotropic plastic localizations in materials. While Eulerian formulations are naturally suited for large deformations, plasticity is typically modeled via isotropic J-2 models, which do not reflect the roles of preferential slip directions in plastic deformations. This presentation will demonstrate an Eulerian anisotropic plasticity framework with the example of void-collapse in HMX. Computations will be performed for different shock strengths and crystal orientations to demonstrate the roles of preferential directions during void-collapse.   

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We have recently shown that the input shock, subsequent initiation and detonation propagation in an Exploding Bridgewire (EBW) detonator exhibits complex internal structure and temporal behavior. Using flash radiography we have observed the prompt emanation of a relatively weak shock wave (3000 m/s) from the region of the bridgewire at the time of vaporization. Abel inversion of the images reveals a highly symmetric, hemishperical structure in the density that propagates through the initial pressing. Visible imaging of the cylindrical surface of the detonator reveals a luminous wavelike structure that appears radially symmetric but lacking the exact symmetry of the density feature. In this talk these structures will be directly compared spatially and temporally for a number of EBW detonators of different sizes and materials. The relationship between shock and detonation like features will be discussed in the context of the mechanism of function of EBW detonators and recent progress in modeling the chemistry of energy release in these applications.