Indexed on: 22 May '20Published on: 21 May '20Published in: Journal of Physical Chemistry A
The formation of hot spots in dynamically compressed, plastic-bonded explosives are 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, mini-flyer plates, we have studied hot spots in small grains of HMX (cyclotetramethylene-tetranitramine) embedded in a polyurethane binder, shocked to about 20 GPa. 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 4000K. 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), 6000K hot spots are created near internal defects or crystal junctions. However, the thermal mass of the material at 6000K is quite small, so after those hot spots cool down the HMX combustion is similar to the single crystals. Comparing an HMX-based polymer-bonded explosive (PBX) to the individual polymer-bonded HMX-SC and HMX-PC grains shows 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.