Synthesis and optical ignition of aluminum and silicon-based energetic materials [electronic resource]
Keywords: Mechanical Engineering.

Energetic materials, aluminum (Al) and silicon (Si), due to their large volumetric energy densities, earth abundance, and low cost, have broad applications in propulsion, thermal batteries, waste disposal and power generation for microsystems. The energetic materials are commonly prepared by mixing fuel and oxidizer powders, however, the energy release rates are slow and difficult to ignite. Furthermore, the large portion of the reactants remains unburned due to the formation of the oxide layer during the reaction. Optimized energetic materials would have the reactive components mixed on a scale as fine as possible to reduce the mass transport distance and facilitate the ignition. This leads to the idea of reducing the sizes of energetic materials down to nanoscale to increase the surface area and contact area between the fuel and oxidizer. In this study, we investigated two new areas: 1) the effects of the nanostructured morphology on the exothermic reaction of Al and CuO, 2) demonstrate and understand the flash ignition of Al nanoparticles (NPs), and extending the flash ignition to Al microparticles (MPs) and porous Si. First, it remains a challenge to create energetic materials, a mixture of Al and metal oxides, with nanoscale uniformity. Here, we report synthesis and ignition studies on thermites (mixtures of Al and metal oxides) with unique nanostructures, i.e., CuO/Al core/shell nanowires (NWs) and Al/CuO core/shell micro and nano particles. The CuO NW cores were synthesized by the thermal annealing of copper films and served as templates for the deposition of Al shells by subsequent sputtering. Similarly, for core/shell particles, the Al particles were coated with a very thin CuO shell using a solution phase method. The advantage of such core/shell structures are that CuO and Al are uniformly mixed at the nanoscale with no aggregation. The onset temperatures of the exothermic reaction of the core/shell NWs were similar to those of nanoparticle NP-based thermites in terms of magnitude, and insensitivity to equivalence ratios. Moreover, the core/shell NW thermites, compared to NP-based thermites, exhibit greatly improved mixing uniformity and reduced activation energy for the thermite reaction. For Al/CuO core/shell particles, in comparison to mixtures of Al particles and CuO NPs, have better chemical homogeneity and physical contact between Al and CuO, so that the core/shell particles exhibit much larger burning rates. The core/shell structure is a general and effective structure to tailor the combustion performance of energetic materials. Second, nonintrusive optical flash ignition is attractive for many applications due to its simplicity, and flexibility in controlling the area exposed to the flash. However, the oxidation mechanism of Al NPs at large heating rates remains inconclusive due to the lack of direct experimental evidence. We studied the oxidation mechanism of Al NPs under large heating rate (on the order of 106 K/s or higher) by a simple flash ignition method, which uses a xenon flash to ignite Al NPs. The flash ignition occurs when the Al NPs have suitable diameters and sufficient packing density to increase the temperature above their ignition temperatures. We then extended the flash ignition to Al MPs. Flash ignition of Al MPs is challenging due to their higher minimum flash ignition energy, which originates from weaker light absorption and higher ignition temperature compared to Al NPs. By the addition of WO3 NPs to Al MPs, the minimum flash ignition energy of Al MPs was reduced and we studied the roles of WO3 NPs upon flash ignition. Finally, we demonstrate that freestanding porous Si films can also be optically ignited in ambient air by a xenon flash. Our complementary experimental and numerical studies reveal that the minimum flash ignition energy increases with increasing the thickness due to heat loss through the porous Si layer. The minimum flash ignition energy is lower for higher porosity Si film since higher porosity reduces the heat capacity and thermal conductivity, facilitating the temperature rise. We believe that these results will be of great importance to reliably ignite energetic materials and to prevent unwanted combustion for practical energetic applications.