Energy release mechanisms of solid-state hydride containing hybrid propellant systems*

Solid-state hydrides possess higher enthalpy of combustion than commonly employed fuels in energetic materials such as Al, Ti, Si, and Mg, on both gravimetric and molar basis. Ammonia borane (NH₃BH₃/AB) possesses the highest gravimetric hydrogen content and has the potential to release high amount of energy from the oxidation of its energetic elemental constituents (B and H). We have characterized AB-nanoflakes, synthesized by anti-solvent crystallization method, using various techniques such as in-situ time-of-flight mass spectrometry, FTIR, TGA/DSC, and NMR, accompanied by DFT calculations, to study the reaction mechanism of AB with different solid-state oxidizers (KClO₄, CuO, Bi₂O₃, AP). We found that AB/AP reaction pathway differs from the reaction mechanism of AB with all other oxidizers. This alternative reaction route causes the AB/AP system to exhibit remarkably higher energy release rates over that of AB/KClO4 (∼27x) and the standard Al/AP propellant (∼7x). These findings motivated us to use aerosol route to synthesize a hybrid propellant composite consisting of Al/AP/AB, with optimized gravimetric and volumetric energy release profile, having a ~400 K lower ignition onset than Al/AP system.