Porosity: The key to initiating metallic composite particles under shock compression

The use of metallic composites as additives can potentially improve the energy density of explosives. Before employing them as additives it is imperative to design composites with physically separated metal fuel and oxidizer that can combust on short time scales, tens of nanoseconds, similar to high-performance molecular explosives. Towards that end, sensitizing composite powder particles to shock compression is crucial. In the current work, particle porosity is explored as means to induce hotspot formation within the particle through pore-collapse. Particles of two porous powders, Al-MoO3-KNO3 (equivalence ratio 3) and Al-CuO (equivalence ratio 4) prepared by arrested reactive milling with emulsion as a process control agent were tested. The composite particles were dispersed in polydimethylsiloxane (PDMS) polymer binder and shocked using a high-throughput tabletop laser-driven flyer apparatus. Shock ignition was verified using simultaneous high-speed thermal imaging and optical pyrometry. The spatial resolution offered by the high-speed camera enables us to assign emissions to specific particles in chosen time frames. The emission from a particle in the first 40 ns is interpreted as shock-driven hotspot formation. It was confirmed that particles with larger sizes and larger pores were more likely to be initiated by shock, while smaller, denser particles were less sensitive to shock but could be thermally initiated. It was found that sufficiently large pores (>20μm) resulted in certain hotspot formation, while smaller-sized pores, despite being present in larger numbers, did not lead to consistent hot spot formation.