(532by) Investigation of the Thermochemical Reaction Pathways of Solid-State Ammonia Borane with Chemical Oxidizers

Ammonia Borane (NH₃BH₃/AB), a widely explored molecule for low temperature hydrogen storage applications, is also an attractive fuel for energetic materials, provided that its constituent boron and hydrogen can be fully oxidized. Investigation on the high temperature thermochemical reaction kinetics of AB with solid state oxidizers used in energetic materials have not been performed earlier. Using in-situ time-of-flight mass spectrometry supported by ATR-FTIR, DSC, NMR, and DFT calculations, we show that the reactions of AB with oxidizers like KClO₄, CuO, and Bi₂O₃ are mediated by [NH₃BH₂NH₃]⁺[BH₄]^− (DADB) formation, resulting in its kinetic entrapment into low-energy BNH_x clusters that are resistant to further oxidation, thus limiting complete energy extraction from AB. On the contrary, we find that with NH₄ClO₄ as an oxidizer, the presence of NH₄⁺ ions causes AB to follow an alternative reaction route forming [NH₃BH₂NH₃]⁺[ClO₄]^- instead of DADB, thus inhibiting the formation of BNH_x species, hence enabling its complete oxidation. This alternative reaction route enables the extraction of the complete energy content of AB, causing the AB/AP system to release energy ~27 times faster than the AB/KClO₄ and ~7 times faster than standard propellant Al/AP system.