Pre-stressing aluminum nanoparticles as a strategy to enhance reactivity of nanothermite composites

Aluminum (Al) fuel particles are used in a variety of energetic formulations yet harvesting their full chemical potential energy and increasing their energy release rate upon ignition have been a challenge and are key motivators to advancing energy generation technologies. One approach to improving combustion performance is to alter the mechanical properties of the Al particle by inducing an elevated stress state through prestressing. This study examines the combustion performance of prestressed nanoscale aluminum (nAl) particles that were annealed to temperatures ranging from 200 to 400 °C and quenched at slow (exponential) and faster (linear) cooling rates. Powder X-ray diffraction measurements show that prestressing nAl particles at 300 °C increases the strain by an order of magnitude. Constant volume combustion cell tests on nAl combined with copper oxide nanopowder (nAl + CuO) revealed higher peak pressures and pressurization rates for prestressed nAl + CuO composites compared to their untreated counterpart. High speed emission spectroscopy was employed to deduce condensed phase temperatures from the reaction confined within the combustion cell. Burn time measurements, obtained by integrating the emission spectra, were observed to correlate inversely with generated pressure. High heating rate (∼5 × 10⁵ K/s) in-situ TEM results augment the combustion cell results. The results imply that prestressing mechanically alters the nanoparticles which subsequently accelerate the release of aluminum core through outward diffusion. This results in the rapid loss of nanostructure which was observed at the nanoscale through in-situ electron microscopy. The released aluminum thus reacts rapidly with the oxidizer in the condensed phase resulting in a faster and more violent reaction. Improved performance of prestressed nAl coupled with the simplicity of processing provides a low cost and scalable approach to improving metal fuel particle combustion.