Self-Propagating Reaction Mechanisms in TiB2 Integrated Al/CuO Nanothermites
Abstract
Unary nanoscale metallic fuels such as Al, Ti, Mg, Zr, and B with an oxidizer have been massively explored in reactive energetic formulations commonly used in explosives and propellants owing to their high combustion energy. While Al has routinely been preferred, its low melting point and the resultant sintering effects curtail the reaction surface area. Despite the high gravimetric and volumetric enthalpies of B, low combustion rates and poor diffusion across the liquid boron oxide barrier restrict its use as a fuel. Although Ti oxidation can be induced at low temperatures, its high sensitivity limits the integration in nanolaminates. To overcome some of these common limitations imposed by single fuel thermites, binary fuel systems are seen as a very desirable strategy to prepare insensitive safe to handle nanothermites exhibiting fast energy release and high propagation rates. Whereas TiB2 may not be the most obvious choice of fuel, the fuel-oxidizer surface synergistic effects brought by its integration in Al/CuO nanolaminates resulted in enhanced reactivity characterized by relatively faster burn rate and shorter ignition delay. Beyond the excellent heat conducting properties, TiB2 demonstrated a strong affinity towards oxygen boosting low temperature CuO decomposition concomitantly forming TiO and boron oxide, following which Al underwent oxidation via both liquid boron oxide and vapor phase oxygen. There is however a lack of fundamental understanding of the self-propagating mechanisms in such ternary thermites due to the complexity associated with the material heterogeneities and multiphase flow occurring during combustion. Herein, sputter-deposited ternary Al-TiB2/CuO nanolaminates are fabricated as model structures, and a high-speed imaging system is utilized to study the flame-front propagation at sub-millimeter scales. DC magnetron sputtering is specially chosen to obtain precisely controlled layer-by-layer structures. To assess the extent of variation in the combustion regime, TiB2:Al molar ratio is varied between 0 and 1. We examine the entire material life cycle of Al-TiB2 bi-fuel component comprising of microfabrication, placement of TiB2 in Al/CuO multilayer stack as well as the individual effects of Al and TiB2 on the self-propagating combustion mechanism. While the individual placement of TiB2 nanolayer in the stack only slightly affects the ignition and the combustion properties, the measure attests to be critical in precluding nano-structural losses. Finally, various combustion stages and flame front behaviors are discussed, followed by a scenario of the combustion process.