Doped δ-bismuth oxides to investigate oxygen ion transport as a metric for condensed phase thermite ignition.

Nanothermites offer high energy density and high burn rates, but are mechanistically only now being understood. One question of interest is how initiation occurs and how the ignition temperature is related to microscopic controlling parameters. In this study, we explored the potential role of oxygen ion transport in BiO as a controlling mechanism for condensed phase ignition reaction. Seven different doped δ-BiO were synthesized by aerosol spray pyrolysis. The ignition temperatures of Al/doped BiO, C/doped BiO and Ta/doped BiO were measured by temperature-jump/time-of-flight mass spectrometer coupled with a high-speed camera respectively. These results were then correlated to the corresponding oxygen ion conductivity (directly proportional to ion diffusivity) for these doped BiO measured by impedance spectroscopy. We find that ignition of thermite with doped BiO as oxidizer occurs at a critical oxygen ion conductivity (∼0.06 S cm) of doped BiO in the condensed-phase so long as the aluminum is in a molten state. These results suggest that oxygen ion transport limits the condensed state BiO oxidized thermite ignition. We also find that the larger oxygen vacancy concentration and the smaller metal-oxide bond energy in doped BiO, the lower the ignition temperature. The latter suggests that we can consider the possibility of manipulating microscopic properties within a crystal, to tune the resultant energetic properties.