Observing Hot Spot Formation in Individual Explosive Crystals under Shock Compression

The formation of hot spots in dynamically compressed, plastic-bonded explosives is known to be the primary mechanism by which these materials ignite and initiate, but hot spots are small, fleeting, and hard to observe. Using a microscope equipped with laser-launched, miniflyer plates, we have studied hot spots in small grains of cyclotetramethylene-tetranitramine (HMX) embedded in a polyurethane binder, shocked to about 20 GPa. A nanosecond video with 4 μm spatial resolution is used to observe hot spot formation and growth, while nanosecond optical pyrometry measured temperature. Using individual ∼200 μm nominally single crystals of HMX (HMX-SC), we observed hot spots forming preferentially on corners or edges. These hot spots are about 4000 K. When there are multiple hot spots, the flame propagated along crystal edges, and the crystal is mostly combusted after about 300 ns. Using polycrystalline grains (HMX-PC), 6000 K hot spots are created near internal defects or crystal junctions. However, the thermal mass of the material at 6000 K is quite small, so after those hot spots cool down, the HMX combustion is similar to the single crystals. Comparing a HMX-based polymer-bonded explosive (PBX) to the individual polymer-bonded HMX-SC and HMX-PC grains shows that the myriad hot spots in the PBX are hotter than HMX-SC and colder than HMX-PC, but they persist for a longer time in PBX than in the individual grains.