Thermite Initiation Processes and Thresholds
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Thermite Initiation Processes and Thresholds Curtis E. Johnson,1 Kelvin T. Higa,1 Thao T. Tran,2 and W. Rick Albro2 1 NAVAIR, Research Division, 1900 N Knox Rd, Stop 6303, China Lake, CA 93555, U.S.A. 2 NAVAIR, Energetics Research Division, 1900 N Knox Rd, China Lake, CA 93555, U.S.A. ABSTRACT The objectives of this work are to characterize thermite initiation processes and thresholds, and to develop thermite reactive trains, where a sensitive nanothermite ignites an insensitive micron thermite, which produces little gas. Nanothermites, including Al/AgIO3, Al/Bi2O3, Al/MoO3, Al/Fe3O4, and Ti/AgIO3, were characterized for their ignition behavior by spark and resistive heating. Energies for spark and thermal initiation were as low as 9 and 140 µJ, respectively. Thermal initiation results were consistent with local temperature as the main controlling factor. The propagation rate of the Al/Fe3O4 nanothermite was about 100X slower than that of the other nanothermites. This low reactivity is attributed to the high volatilization temperature and high melting point of the oxidizer. Mixing of 90% Al/Fe3O4 nanothermite with 10% of a more sensitive, high-gas-producing nanothermite gave materials with the same sensitivity as the sensitive nanothermite. Thus, the mixture provides a safer sensitive nanothermite. Thermites with micron-scale ingredients were pressed into pellets and ignited with small amounts of nanothermite. Gas production of micron thermite compositions was reduced by adding the intermetallic composite, Ti/2B, or excess iron. In both cases, a single hot mass was produced, while the pure micron Al/Fe2O3 produced a dispersion of particles. INTRODUCTION Nanothermites, defined as thermite mixtures where at least one component has a dimension in the range of about 1-100 nm, exhibit greatly enhanced reactivity compared to their micronscale counterparts[1-4]. Their high reaction velocities, approaching or exceeding 1000 m/s in some cases, have been attributed to small mass transport distances and a convective reaction mechanism, where hot gaseous species rapidly heat surrounding unreacted material [2,5]. In this work, our focus is on quantifying initiation thresholds for nanothermites, and on developing thermite reactive trains, where a sensitive nanothermite ignites an insensitive micron-scale thermite, which produces little gas. EXPERIMENT Commercial reagents: Al powders (80 nm, Novacentrix, metallic Al content ~74%; ~2 μm and ~5 μm, Valimet H-2 and H-5), MoO3 (45 nm, Climax), Bi2O3 (50 nm, Nanophase Tech.; 320 nm, Aldrich), Fe3O4 (56 nm,
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