High-speed multi-spectral imaging of the hypergolic ignition of ammonia borane

Abstract

Recently, ammonia borane (a hydrogen-dense solid fuel) has received increased attention as a potential fuel for use in hybrid rocket systems due to its potential for higher performance. Ammonia borane (AB) is also highly hypergolic with white fuming nitric acid (WFNA) while lacking the health hazards associated with more conventional hypergolic fuels (e.g. hydrazine and its derivatives). To date, there remains a lack of information regarding the combustion behavior of ammonia borane. This work makes use of simultaneous visible and infrared multi-spectral imaging to study the combustion behavior of AB during a hypergolic ignition event (with reagent grade nitric acid droplets applied to the AB as the oxidizer). Using optical filters corresponding to previously reported spectral bands, emission from BO, BO2, HBO2, and the B-H stretch mode of AB were selectively imaged. A two-step ignition process consisting of an initial gas evolution step followed by a separate, seemingly premixed flame propagation step was observed. Observation of HBO2 emission during the pre-ignition/gas evolution phase suggests that rapid formation of HBO2 occurs even at the lower temperatures present prior to ignition. This initial formation of HBO2 (and the subsequent heat release associated with it) may be a major driver of AB/nitric acid decomposition. Emission intensity profiles obtained after the AB samples began burning with the ambient air suggests some degree of HBO2 consumption within the flame front, seemingly counter to the current understanding that HBO2 is a stable product species. This may indicate that boron combustion via boranes may proceed to more complete combustion than is typical for more conventional boron fuels. The ignition dynamics and sequence of boron product formation was well-captured by the multi-spectral imaging technique applied to this reaction system for the first time. This technique could be applied for further study of other boron systems.