Fast reactions of shocked energetic microporous metallic composites

Metal/oxidizer composites can potentially release more energy than organic explosives such as HMX (1,3,5,7-Tetranitro-1,3,5,7-tetrazocane), but are generally unsuitable for high-performance explosives due to slow energy release. These composites have physically separated fuel and oxidizer components, and their energetic combustion reactions are usually rate-limited by diffusive mass transfer. Here we describe studies of metal composite microparticles produced by arrested reactive milling (ARM) in an emulsion process fluid, which produces particles with internal pores intended to facilitate rapid shear mixing of fuel and oxidizer under shock compression. The particles were sectioned with a microtome to determine the extent of internal porosity versus particle size. We describe a tabletop high-throughput method for evaluating novel ARM composites, where a small number of composite microparticles are embedded in a transparent polymer and shocked by 4 km/s laser-launched flyer plates. We compared the thermal emission from the composites to an HMX standard to determine whether these particles had potential to release energy fast enough to support a detonation. The thermal emission was analyzed by an optical pyrometer yielding nanosecond time-dependent graybody temperatures and radiances. Thermal images were also obtained with a nanosecond camera in order to determine which particles were ignited by the shock. We found that the larger more porous particles ignited more readily, showing that internal porosity can be engineered to increase chemical reactivity under shock in metal/oxidizer composites. Both the composite and the HMX were ignited by shock on a <20 ns time scale, although the composite temperature of 5000 K was significantly greater than the 3500 K HMX temperature.