Ammonia borane (AB) has attracted significant attention due to its high hydrogen content (19.6% by mass). To investigate the reaction mechanism associated with the combustion of AB, a reactive force field (ReaxFF) has been developed for use in molecular dynamics (MD) simulations. The ReaxFF parameters have been derived directly from quantum mechanical data (QM). NVT-MD simulations of single- and polymolecular AB thermolysis were conducted in order to validate the force field. The release of the first equivalent H(2) is a unimolecular reaction, and MD simulations show an activation energy of 26.36 kcal mol(-1), which is in good agreement with experimental results. The release of the second H(2) is also a unimolecular reaction; however, the release of a third H(2) requires the formation of a B-N polymer. Similar simulations were conducted with a boron and oxygen system, since the oxidation of boron will be an integral step in AB combustion, and show good agreement with the established mechanism for this system. At low temperatures, boron atoms agglomerate into a cluster, which is oxidized at higher temperatures, eventually forming condensed and gas phase boron-oxide-species. These MD results provide confidence that ReaxFF can properly model the oxidation of AB and provide mechanistic insight into the AB dehydrogenation and combustion reactions.

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Sonochemically generated reactive metal nanopowders containing Ti, Al, and B represent a new class of high-energy-density nanopowder fuels with superior energy content and air stability as compared to nano-aluminum. In this work, we optimize the energy density of a Ti–Al–B reactive metal nanopowder fuel by varying the Ti:Al:B ratios using a sonochemically mediated decomposition of a complex metal-hydride. After heating the recovered solids under vacuum to temperatures in the range between 150 to 300 °C, the powder’s air stability is significantly improved so that it can be handled in air. Variable-temperature vacuum heat treatment was used to produce fuels tuned to be stable with a gravimetric energy density exceeding that of pure bulk Al (>31 kJ/g). The density of the powder was found to be 2.62 g/cm3 by helium pycnometry, which translates to an impressive volumetric energy content of 89 kJ/cm3. In poly­(methyl methacrylate)-protected bomb calorimetry tests commercial nano-aluminum (SkySpring Nanomaterials, 20% oxide) only produced 25 kJ/g, while the sonochemically generated Ti–Al–B nanopowders released 24% more energy per unit mass and 19% more energy per unit volume in identical experiments.