Shock initiation of explosives under the microscope

This dissertation explores the shock compression of solid state explosives by thin, laser-driven flyer plates. These flyer plates consist of aluminum foils, typically 25 – 75 μm thick, which are launched using a Nd:YAG laser with a 20 ns pulse width in a technique developed within the Dlott group. A full description of the novel instrumentation used to monitor the temperature history of shock compressed explosives and its integration into the existing flyer plate apparatus is detailed. The system consists of 32 individual photomultiplier tubes to which light is fed through a prism spectrograph with fiber optic coupling. These photomultiplier tubes collect the spectrally-dispersed light and record intensity with a high range of linearity to a digitizing oscilloscope. The emission collected over 32-channels is then used to calculate the temperature and spatially-averaged emissivity with nanosecond resolution in what we dub a 32-channel pyrometer. This technique allows for the first high-fidelity direct measurement of the temperature in heterogeneous reactive sites which are created during shock compression, known as hot spots, on the nanosecond time scale. The 32-channel pyrometer described herein was used to explore the temperature histories of solid explosives ranging from low-density powder compacts to polymer-bound explosives which has no void space on the microscale. The temperature histories have given us valuable insight into the dynamics and reaction kinetics of explosive materials under shock compression by thin flyers. Short duration shocks into thin samples are invaluable for theoretical constructs for which no experimental data was previously available as well as their similarity to practical systems such as those used in detonator systems such as the exploding bridge wire and exploding foil initiator. In our first foray into fast pyrometry we used low density octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) samples which were hand packed into wells containing ~20 μg of explosive material. This low density sample produced hot spots which were ~7000 K, a temperature which was incredibly high by all expectations. We determined that the high temperatures always preceded a burst in reactive volume which indicated that the spikes in temperature caused increased reaction in the reactive sites surroundings. Vastly improved samples, created by adding a significant weight fraction of an elastomeric polymer, poly(dimethyl siloxane) PDMS, reduced the extreme hot spot temperatures which were seen upon impact. In these so-called PBXs (polymer-bound explosives) we substituted HMX for PETN, an explosive known to have extremely fast chemistry. We used these samples to determine that the hot spot temperatures observed in the low density HMX were due to the in-situ gas generation from decomposition of explosive into micron-scale voids and the subsequent compression of these gas-filled pores. This discovery allowed us to examine the underlying hot spot temperature evolutions which occurred due to the solid state chemistry of our explosive compounds. We have determined that the chemistry which generates hot spots occurs on timescales which are faster than our pyrometer can measure due to the instantaneous appearance of hot spots at their maximum temperature and that slower chemistry can be observed by exchanging PETN for various other explosive compounds