Accelerating shock-driven reactions in metal nanocomposites*

Despite having nearly twice the energy density as HMX, aluminum-based thermites generally do not react fast enough to support high-pressure detonations because their chemical reactivity requires mixing separate fuel and oxidizer components.  In recent work, we shocked thermite particles in a porous powder bed and found that we could observe heat release and gas production on the desired time scale, which is roughly 30 ns, the duration of HMX reaction zone.  We hypothesized that the rapid reaction was enabled by pore collapse that resulted in fast shear mixing of fuel and oxidizer.  The thermites tested, were produced by arrested reactive milling (ARM), a technique which can generate myriad compositions, microstructures, and nanostructures.  With ARM we have the potential to engineer powder properties such as porosity, degree of mixing, shape, and size distributions for a range of fuel-oxidizer compositions.  For rapid assessment of these new materials, we have developed a high-throughput method to compare prototype metal composites to a benchmark reference, namely HMX crystals.  We embed the particles in a polymer binder and measure the time- and space-resolved optical emission after a flyer plate impact that produces shocks comparable to HMX detonation.  Recently, we shock compressed dispersed metal composite particles that have small (2 μm) internal pores and observed the particles exhibiting rapid energy release similar to HMX.  We showed that the particle-scale reactivity increases with the number of internal pores so the presence of porosity clearly accelerates the shock-induced chemistry.  Currently we are studying Al/CuO particles which can be produced with a range of sizes, shapes, and internal structures to determine which structural elements best accelerate the energy release.