Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes. To close this gap, a novel multiphase shock tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 20%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 are reported. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30-40% by the presence of the particle field.

An experimental apparatus has been developed to determine thermal accommodation coefficients for a variety of gas-surface combinations. Results are obtained primarily through measurement of the pressure dependence of the conductive heat flux between parallel plates separated by a gas-filled gap. Measured heat-flux data are used in a formula based on Direct Simulation Monte Carlo (DSMC) simulations to determine the coefficients. The assembly also features a complementary capability for measuring the variation in gas density between the plates using electron-beam fluorescence. Surface materials examined include 304 stainless steel, gold, aluminum, platinum, silicon, silicon nitride, and polysilicon. Effects of gas composition, surface roughness, and surface contamination have been investigated with this system; the behavior of gas mixtures has also been explored. Without special cleaning procedures, thermal accommodation coefficients for most materials and surface finishes were determined to be near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Surface cleaning by in situ argon-plasma treatment reduced coefficient values by up to 0.10 for helium and by ∼0.05 for nitrogen and argon. Results for both single-species and gas-mixture experiments compare favorably to DSMC simulations.

High resolution photoionization efficiency curves have been obtained for CH3COCH3+ and CD3COCD3+ using supersonic molecular beam sampling. As a result of adiabatic cooling during the nozzle expansion, sufficient concentrations of (CH3COCH3)2, (CD3COCD3)2, (CH3COCH3)3, and (CH3COCH3)4 were formed to permit the study of their photoion yield curves as well. Appearance potential curves have been determined for CH3CO+, CD3CO+, and (CH3COCH3) ⋅CH3CO+ fragments. The measured ionization potentials of acetone and acetone-d6 monomers are 9.694±0.006 and 9.695±0.006 eV, respectively. Transitions to higher vibrational levels in CH3COCH3+ are seen at 320, 695, and 930−1370 cm−1 above threshold. The effect of perdeutero substitution is to reduce these frequencies to 260 and 660–1100 cm−1. Appearance potentials of CH3CO+ and CD3CO+ fragments are observed at 10.52±0.02 and 10.56±0.02 eV, respectively. The measured ionization energies for (CH3COCH3)n, n=1–4, are found to decrease linearly as a function of 1/n. Observed ionization thresholds for (CH3COCH3)2, (CH3COCH3)3, and (CH3COCH3)4 are 9.26±0.03, 9.10±0.03, and 9.02±0.03 eV, respectively. Within experimental resolution, the ionization potentials of (CH3COCH3)2 and (CD3COCD3)2 are identical. The appearance potential of the process (CH3COCH3)2 → (CH3COCH3) ⋅CH3CO++CH3+e− is found to be 10.08±0.05 eV. By consideration of appropriate thermodynamic cycles, a lower bound for the acetone dimer ion binding energy is calculated to be 0.538 eV (12.4 kcal/mole) and the desolvation energy of (CH3COCH3) ⋅CH3CO+ is estimated to be 0.544 eV (12.5 kcal/mole).

Small metal bridgewires are commonly used to ignite energetic powders such as pyrotechnics, propellants, and primary or secondary explosives. In this paper we describe a new means for igniting explosive materials using a semiconductor bridge (SCB). When driven with a short (20 μs), low-energy pulse (less than 3.5 mJ), the SCB produces a hot plasma that ignites explosives. The SCB, a heavily n-doped silicon film, typically 100 μm long by 380 μm wide by 2 μm thick, is 30 times smaller in volume than a conventional bridgewire. SCB devices produce a usable explosive output in a few tens of microseconds and operate at one-tenth the input energy of metal bridgewires. In spite of the low energies for ignition, SCB devices are explosively safe. We describe SCB processing and experiments evaluating SCB operation. Also discussed are the SCB vaporization process, plasma formation, optical spectra from the discharge, heat transfer mechanisms from the SCB to the explosive powders, and SCB device applications.

The mesoscopic scale response of low-density pressings of granular sugar (sucrose) to shock loading has been examined in gas-gun impact experiments using both VISAR and a line-imaging, optically recording velocity interferometer system in combination with large-volume-element, high-resolution, three-dimensional numerical simulations of these tests. Time-resolved and spatially resolved measurements of material motion in waves transmitted by these pressings have been made as a function of impact velocity, sample thickness, and sample particle size distribution. Observed wave profiles exhibit a precursor regime arising from elastic stress wave propagation and a dispersive compaction wave with superimposed localized particle velocity fluctuations of varying amplitude. Material motion associated with dynamic stress bridging leads compaction wave arrival by ∼2μs at the lowest impact velocity (0.25kms−1) employed in this study and <200ns at the higher values (0.7–0.8kms−1). Over the same range, the compaction wave becomes markedly less dispersive with wave ramp durations declining from ∼500to∼50ns. For impact velocities near 0.5kms−1 and samples varying in thickness from 2.27to8.03mm, a roughly steady wave behavior is obtained at the thinner end of the range; however, evidence of subtle wave evolution is apparent over this thickness range. Pressings of sieved sugar with different particle size distributions exhibit distinguishable differences in stress bridging and compaction wave behavior. These pressings somewhat limit stochastic behavior and provide favorable conditions for the development of quasiperiodic fluctuations in particle velocity, particularly in impacts generating incomplete compaction. The experimental results are consistent with the exceedingly complex wave field behavior evident in the numerical simulations and provide useful benchmark wave profiles (at the sample boundary) for validation of material models used in these calculations.

A previously-developed experimental facility has been used to determine gas-surface thermal accommodation coefficients from the pressure dependence of the heat flux between parallel plates of similar material but different surface finish. Heat flux between the plates is inferred from measurements of temperature drop between the plate surface and an adjacent temperaturecontrolled water bath. Thermal accommodation measurements were determined from the pressure dependence of the heat flux for a fixed plate separation. Measurements of argon and nitrogen in contact with standard machined (lathed) or polished 304 stainless steel plates are indistinguishable within experimental uncertainty. Thus, the accommodation coefficient of 304 stainless steel with nitrogen and argon is estimated to be 0.80 and 0.87 , respectively, independent of the surface roughness within the range likely to be encountered in engineering practice. Measurements of the accommodation of helium showed a slight variation with 304 stainless steel surface roughness: 0.36 for a standard machine finish and 0.40 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.0.02 ± 0.02 ± 0.02 ± 0.02 ± 4

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Pulsed-laser-excited Raman scattering methods have been used to examine the dynamic molecular-level response of an explosive molecule (triaminotrinitrobenzene, TATB) to sustained shock loading at a fused silica window interface. The anomalous behavior of Raman modes associated with nitro groups in the molecule (the 881-cm"1 N02 deformation mode, the 1146-cm"1 symmetric C-N02 stretching mode, and the 1170-cm"1 totally symmetric N02 stretching mode) is compared to results obtained under static high pressure. The shock compression data indicate that elevated temperatures act to restrain pressure-enhanced coupling of N02 and NH2 groups in the molecule. Differences in the spectra obtained under static and dynamic high-pressure conditions are discussed in relation to the known insensitivity of TATB to shock initiation.