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The aggregate behavior of hot spots in shocked plastic-bonded explosives (PBX) was studied by nanosecond optical pyrometry. The averaged thermal emission spectra from at least 25 tiny (50 μg) explosive charges of a pentaerythritol tetranitrate PBX, at several impact velocities from 1.5 to 4.5 km/s, was used to determine average temperatures and emissivities. Individual spectra were analyzed to determine the distribution of hot spot temperatures in individual charges with unique microstructures. Understanding shocks in tiny charges with different microstructures is needed to understand shocks in large PBX charges which sample many microstructures as they propagate. The initial hot spot density was several percent, and the average initial hot spot temperature of 4000 K was, surprisingly, independent of impact velocity. With underdriven shocks, the initial hot spot temperatures clustered around 4000 K, but with overdriven shocks, there were both hotter and colder hot spots. The initial hot spot density increased quadratically with impact velocity. The generation of hot spots was described by a model with a threshold energy to trigger hot spot formation and a distribution of energetic barriers to hot spot formation. [ABSTRACT FROM AUTHOR]

A new emission apparatus with high time resolution and high dynamic range was used to study shock-induced ignition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine in the form of ultrafine powder (4 ± 3 μm particle size), over a range of impact velocities (0.8-4.3 km s-1) and impact durations (2.5-16 ns). A graybody model was used to extract graybody emissivities and time-dependent temperatures from a few ns to 100 μs. The emission transients consisted of three parts: a 6700K nanosecond burst during the shocks, a 4000-4500K temperature spike near 0.3 μs followed by a ~3300K tail extending out to ~100 μs. These temperatures varied remarkably little with impact velocity and duration, while the emission intensities and emissivities changed by over an order of magnitude. The emissivity changes were interpreted with a hot spot model, where hot spot temperatures reached a maximum of 6700K and the hot spot volume fractions increased from 5% to 100% as impact velocity increased from 1 to 3 km s-1. Changing shock durations in the 2.5-16 ns range had noticeable effects on the microsecond emission. The 0.3 μs temperature spike was much smaller or absent with 2.5 ns shocks, but prominent with longer durations. An explanation for these effects was put forth that invoked the formation of carbon-rich clusters during the shock. In this view, cluster formation was minimal with 2.5 ns shocks, but longer-duration shocks produced increasingly larger clusters, and the 0.3 μs temperature spikes represented cluster ignition. [ABSTRACT FROM AUTHOR]

Initial hot spot temperatures and temperature evolutions for 4 polymer‐bound explosives under shock compression by laser‐driven flyer plates at speeds from 1.5–4.5 km s−1 are presented. A new averaging routine allows for improved signal to noise in shock compressed impactor experiments and yields temperature dynamics which are more accurate than has been previously available. The PBX formulations studied here consist of either pentaerythritol tetranitrate (PETN), 1,3,5‐trinitro‐1,3,5‐triazinane (RDX), 2,4,6‐trinitrotoluene (TNT), or 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) in a 80/20 wt.% mixture with a silicone elastomer binder. The temperature dynamics demonstrate a unique shock strength dependence for each base explosive. The initial hot spot temperature and its evolution in time are shown to be indicative of chemistry occurring within the reaction zone of the four explosives. The number density of hot spots is qualitatively inferred from the spatially‐averaged emissivity and appears to increase exponentially with shock strength. An increased emissivity for formulations consisting of TNT and TATB is consistent with carbon‐rich explosives and in increased hot spot volume. Qualitative conclusions about sensitivity were drawn from the initial hot spot temperature and rate at which the number of hot spots appear to grow. [ABSTRACT FROM AUTHOR]

For the first time, mesoscale simulations of shocked explosives are validated with hot spot temperature data. Following recent experiments, we simulate a 25 μm aluminum flyer impacting a powder bed of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) with an initial speed of 1.4 to 3.6 km s−1. The powder bed has a density of 0.56 g cm3, or 29 % crystal density, and an average grain size of 4 μ m. Simulations are performed in the multi‐physics code, ALE3D. We employ a comprehensive HMX material model that accounts for the chemistry, unreacted and product equation‐of‐states (EOSs), elastic‐plastic response, as well as melting and melt viscosity. Simulations show compaction, reaction, and gas jetting during the initial stages of shock in the powder bed. Shock reflection at the glass window interface increases hot spot temperatures in HMX product gas in excess of 7000 K at higher impact speeds which is readily observable using pyrometry techniques. We extract a temperature histogram from simulations at discrete times, using it to calculate an effective emission spectrum and effective temperature, Teff, for comparison to data. Simple T‐ and T4‐based methods inadequately weight the higher temperature regions and result in a low Teff that is in poor agreement with data. However, calculating Teff based on Planck's Law with a gray body assumption demonstrates good agreement with hot spot temperature data. A modified Arrhenius model is fit to the normalized reaction rates predicted by mesoscale simulations and inferred temperatures range from 1544 K to 1779 K; these temperatures are approximately 4x lower than the peak hot spot temperatures but 2‐3x higher than the bulk shock temperatures. The agreement with experimental results validates our mesoscale simulations as useful tools for elucidating the role of hot spot mechanisms in the shock initiation of heterogeneous explosives. [ABSTRACT FROM AUTHOR]

The explosive triacetone triperoxide(TATP) is produced by theacid-catalyzed reaction of hydrogen peroxide and acetone, and methodsfor its identification are important in forensic and military scenarios.Previous work has documented concomitant crystalline polymorphismof the crude reaction product. Here, we have investigated the crystallinephases resulting from TATP syntheses from aqueous hydrogen peroxideas well as from sodium percarbonate and urea peroxide as oxidant sources,under different concentrations and types of acid. Powder X-ray diffraction(PXRD) patterns of the as-prepared powders indicate that, in contrastto previously published conditions, rapid precipitations afford apure crystalline phase of TATP. High concentrations of Brønstedacid lead to the selective formation of a single kinetic crystallinephase, while a Lewis acid (SnCl4) gives rise to a purespecimen of the thermodynamic phase. Additionally, this study highlightsthe utility of PXRD as a rapid and high-confidence identificationtool for TATP which has only now become feasible due to the fairlyrecent disclosure of crystal structures of the various TATP crystallinepolymorphs. [ABSTRACT FROM AUTHOR]