Publications

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The chemical and physical processes involved in the shock-to-detonation transition of energetic solids are not fully understood due to difficulties in probing the fast dynamics involved in initiation. Here, we employ shock interferometry experiments with sub-20-ps time resolution to study highly textured (110) pentaerythritol tetranitrate (PETN) thin films during the early stages of shock compression using ultrafast laser-driven shock wave methods. We observe evidence of rapid exothermic chemical reactions in the PETN thin films for interface particle velocities above ∼1.05 km/s as indicated by shock velocities and pressures well above the unreacted Hugoniot. The time scale of our experiment suggests that exothermic reactions begin less than 50 ps behind the shock front for these high-density PETN thin films. Thermochemical calculations for partially reacted Hugoniots also support this interpretation. The experimentally observed time scale of reactivity could be used to narrow possible initiation mechanisms.

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Detonation corner turning describes the ability of a detonation wave to propagate into unreacted explosive that is not immediately in the path normal to the wave. The classic example of a corner turning test has a cylindrical geometry and involves a small diameter explosive propagating into a larger diameter explosive as described by Los Alamos' Mushroom test, where corner turning is inferred from optical breakout of the detonation wave. We present a complimentary method to study corner turning in millimeter-scale explosives through the use of vapor deposition to prepare the slab (quasi-2D) analog of the axisymmetric mushroom test. Because the samples are in a slab configuration, optical access to the explosive is excellent and direct imaging of the detonation wave and "dead zone" that results during corner turning is possible. Micromushroom test results are compared for two explosives that demonstrate different behaviors: pentaerythritol tetranitrate (PETN), which has corner turning properties that are nearly ideal; and hexanitroazobenzene (HNAB), which has corner turning properties that reveal a substantial dead zone.

Detonation corner turning describes the ability of a detonation wave to propagate into unreacted explosive that is not immediately in the path normal to the wave. The classic example of a corner turning test has a cylindrical geometry and involves a small diameter explosive propagating into a larger diameter explosive as described by Los Alamos' Mushroom test, where corner turning is inferred from optical breakout of the detonation wave. We present a complimentary method to study corner turning in millimeter-scale explosives through the use of vapor deposition to prepare the slab (quasi-2D) analog of the axisymmetric mushroom test. Because the samples are in a slab configuration, optical access to the explosive is excellent and direct imaging of the detonation wave and "dead zone" that results during corner turning is possible. Micromushroom test results are compared for two explosives that demonstrate different behaviors: pentaerythritol tetranitrate (PETN), which has corner turning properties that are nearly ideal; and hexanitroazobenzene (HNAB), which has corner turning properties that reveal a substantial dead zone.

The microstructure of pentaerythritol tetranitrate (PETN) films fabricated by physical vapor deposition can be altered substantially by changing the surface energy of the substrate on which they are deposited. High substrate surface energies lead to higher density, strongly textured films, while low substrate surface energies lead to lower density, more randomly oriented films. We take advantage of this behavior to create aluminum-confined PETN films with different microstructures depending on whether a vapor-deposited aluminum layer is exposed to atmosphere prior to PETN deposition. Detonation velocities are measured as a function of both PETN and aluminum thickness at near-failure conditions to elucidate the effects of microstructure on detonation behavior. The differences in microstructure produce distinct changes in detonation velocity but do not have a significant effect on failure geometry when confinement thicknesses are above the minimum effectively infinite condition.

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