Although aluminum (Al) nanoparticles have been widely explored as fuels in energetic applications, researchers are still exploring approaches for tuning their energy release profile via microstructural alteration. In this study, we show that a nanocomposite (∼70 nm) of a metal ammine complex, such as tetraamine copper nitrate (Cu(NH3)4(NO3)2/TACN), coated Al nanoparticles containing only 10 wt. % TACN, demonstrates a ∼200 K lower reaction initiation temperature coupled with an order of magnitude enhancement in the reaction rate. Through time/temperature-resolved mass spectrometry and ignition onset measurements at high heating rates, we show that the ignition occurs due to a condensed phase reaction between Al and copper oxide (CuO) crystallized on TACN decomposition. TEM and XRD analyses on the nanoparticles at an intermediate stage show that the rapid heat release from TACN decomposition in-situ enhances the strain on the Al core with induction of nonuniformities in the thickness of its AlOx shell. The thinner region of the nonuniform shell enables rapid mass transfer of Al ions to the crystallized CuO, enabling their condensed phase ignition. Hence, the thermochemical shock from TACN coating induces stresses at the Al/AlOx interface, which effectively switches the usual gas phase O2 diffusion-limited ignition process of Al nanoparticles to become condensed phase Al ion transfer controlled, thereby enhancing their reactivity.
Keywords: energetic materials; interfaces; lattice strain; metal; metal oxides; nanoparticles; nanothermite; redox reactions.