Accelerating shock-driven reactions in metal nanocomposites

• Main Authors: Valluri, Siva Kumar, Dreizin, Edward L., Dlott, Dana D.
• Format: Conference Proceeding
• Language: English
• Subjects: Acetonitrile; Deflagration; Emission; Emulsion polymerization; Energetic materials; Explosive compacting; Fuels; Hexanes; High speed cameras; HMX; Laminates; Metal powders; Nanocomposites; Oxidizing agents; Particulate composites; Porosity; Reaction kinetics; Redox reactions
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/12.0032372

Metal powders are sought as energetic additives to conventional explosives. However, due to their sluggish reaction kinetics with external gaseous oxidizers, pure metals are replaced with metallic composites with intimately mixed condensed phase oxidizers prepared by arrested reactive milling (ARM). Such composites can be initiated by non-thermal, mechanical means; through shear mixing of fuel and oxidizer under a shock compressive load. Since the ARM preparatory technique allows for tuning multiple powder attributes such as fuel/oxidizer of interest, their degree of mixing, amount of oxidizer, particle porosity, among others, a parametric study is crucial in identifying suitable traits for fast reactions in metal composites. Using our high-throughput benchtop experimentation we deliver shocks to isolated powder particles suspended in transparent polymer by impacting them with laser driven flyer plates. The emission from the shock compressed particles is followed using an optical emission pyrometer as well as a high-speed camera to follow evolution of reactivity in individual particles. In this manner a variety of samples can be tested quickly to explore the parametric space. In recent work, to probe the role of porosity, porous Al-MoO3-KNO3 prepared by milling in a hexane acetonitrile emulsion was shock compressed. In particles where sufficiently large pores over 2µm were identified, shock-initiation was achieved consistently. In this work, the same composite powder’s reactive behavior was characterized to judge the improvement in pore collapse-driven ignition and subsequent combustion kinetics. When compared to Octogen’s (HMX) deflagration under the same experimental conditions, the composite was found to have equally fast hotspot formation and onset of redox reaction within nanoseconds of HMX. Using intensities of emission plumes observed within particles as proxy for temperatures, we find that even larger porous particles with poor surface area to volume ratio showed fast rise in temperature. This is suggested to be due to the more effective mixing of fuel-oxidizer laminates in large particles that have a higher number of pores and larger sized pores.