(692b) Boron-Aluminum Nanoenergetics: Substantial Improvement of Energy Release in Blended Nanomaterials

Boron (B) has attracted considerable attention as a potential high energetic material in propulsion, solid fuels, and explosives due to its exceptionally high gravimetric (58 kJ/g) and volumetric (140 kJ/mL) energy densities. However, the native boron oxide layer on its surface reduces the energy release and slows down the oxidation through a diffusion-controlled process. Reactive metals such as aluminium (Al), when mixed with B have shown to improve the energy release, though by relatively small amounts. We hypothesize that the main effect of blending Al as an additive is to chemically reduce the native boron oxide during combustion leading to two beneficial outcomes: more B is available for oxidation at the expense of less energetic Al, and since the reduction of boron oxide by Al is exothermic, there is the additional energetic benefit of the redox reaction. For such benefit to materialize, one must employ nanoparticles, which carry substantially more oxide than microparticles, and the amount of Al added must match the stoichiometric requirement for the boron oxide present.

Here we present a simple but highly efficient method to extract higher amounts of chemical energy from B using Al than from B alone. We study the effect of Al (~70 nm) addition on the energy release and oxidation efficiency of B (~500 nm). The investigation of Al/B blends with different compositions using the thermochemical analysis is performed to discover the blend with a maximum energy release. Stoichiometric calculations with the help of HRTEM and XPS analyses also indicate the similar optimum composition of Al in B in order to get maximum energy release. This composition provides maximum energy release and a higher degree of oxidation due to the synergistic effect of B oxidation and redox reaction between Al and B₂O₃. HAADF-STEM-EDS analysis on oxidation products of Al/B proves the formation of ternary oxide in the system, which provides porous channels for oxidation of B, thereby increasing its contact with the oxidizer. Similar analysis of the blends demonstrates both qualitatively and quantitatively that Al and B particles stay closer in hydrocarbons, and therefore, they can also benefit propellants as fuel additives.