Experiments Probing Fundamental Mechanisms of Energetic Material Initiation and Ignition
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Experiments Probing Fundamental Mechanisms of Energetic Material Initiation and Ignition
Christopher M. Berg, Kathryn E. Brown, Rusty W. Conner, Yuanxi Fu, Hiroki Fujiwara1, Alexei Lagutchev, William L. Shaw, Xianxu Zheng and Dana D. Dlott School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 S. Goodwin Avenue, Urbana, IL 61801
ABSTRACT Two fundamental processes associated with shock compression of energetic materials (EM) are initiation and ignition. Initiation occurs just behind a shock front and ignition occurs anywhere from a few nanoseconds to hundreds of nanoseconds later. Experiments are described that probe the fundamental mechanisms of these processes on relevant length and time scales: picosecond vibrational spectroscopy of nanometer thick layers of energetic materials (EM) with laser-driven shock waves, and nanosecond emission spectroscopy of micrometer thick layers of EM using laser-driven flyer plates. INTRODUCTION Two fundamental processes associated with shock compression of energetic materials (EM) are initiation and ignition [1-3]. Initiation occurs just behind a shock front, where molecules endothermically disintegrate into smaller fragments. The multiplication of molecular species causes the local pressure to spike. The time and length scales associated with initiation may be estimated from the few measurements of this von Neumann spike: ~200 ps and < 1 μm [4,5]. Atomistic simulations of initiation behind a shock front show extensive fragmentation of molecules such as RDX occurring within a few picoseconds [6]. Quantum mechanical simulations of nitromethane suggest a different fragmentation mechanism: the formation of a transient semimetallic layer where the molecules dissociate into atoms [7]. Ignition occurs farther behind a shock front where the volume expansion causes the dissociated molecular fragments to combine to form highly excited states of much lower-energy, stable species such as NO, CO, H2O, CN, CO2 and so on. A great deal of chemical energy is released by converting higher-energy molecules such as HMX into these stable molecules, on the order of 10 kJ cm-3, and after the nascent excited electronic and vibrational state energies are converted into translational energy, this energy chemically sustains the shock front. The time and length scales associated with ignition can be estimated from reaction zone parameters, 1-100 ns and 5-500 μm. In these proceedings, we describe two experimental efforts underway in our laboratories to investigate initiation and ignition on the relevant time and length scales. The length scale is particularly significant for two reasons. First, shock wave excitations travel about 105 slower 1
Current address, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
than optical probes, so unless the sample is very thin, the time resolution of the experiment is limited by the shock wave transit time across the sample. For typical shock velocities of, say, 5 km/s (5 nm/ps or 5 μm/ns) picosecond initiat
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