Thermal Explosions of Polymer-Bonded Explosives with High Time and Space Resolution

It has always been difficult to observe thermally induced explosions, because the onset is unpredictable. By use of ultrasound to induce intense, localized frictional heating at the surface of crystals embedded in a flexible polymer, we have created a new method for the initiation of microexplosions under conditions where temporal and spatially resolved observations can be made. Specifically, we report the use of ultrasound to flash-heat polymer-embedded <500 μm RDX (CH₂NNO₂)₃ and HMX (CH₂NNO₂)₄ crystals at rates >10 000 K/s. By using this extremely rapid heating on small samples, we were able to confine the explosion to narrow regions in time and space. The explosion was measured using dual thermal imagers providing temporal and spatial resolutions of 1 μs and 15 μm. Surprisingly, the explosions always occurred in <i>two stages</i>, an initial 0.1 ms stage and a subsequent 100 ms stage. The first stage of RDX explosion (2500 K lasting 140 μs) was less violent than that for HMX (4400 K lasting 70 μs), which is consistent with the general observation that HMX is regarded as a higher-performance explosive. The origin of the two-stage explosion originates from how the explosive chemistry is modulated by the mechanical behavior of the flexible polymer at the interface with the explosive crystal. The crystal explosion created a blast that produced a cavity in the surrounding polymer filled with reactive gases; subsequent ignition of the gases in that cavity caused the second-stage explosion