Quantifying the enhanced combustion characteristics of electrospray assembled aluminum mesoparticles

Aluminum particles have been extensively used to enhance the combustion characteristics of propellant, pyrotechnic and explosive formulations. Unfortunately the relatively high ignition temperatures of aluminum result in severe sintering prior to combustion, leading to early loss of nanostructure and thus a smaller power law exponent for size dependent burning than expected. One such scheme we explore, to defeat sintering, is to create low temperature gas-generation, which helps in breaking up the soft agglomerates before/during combustion. In this article, we characterize the combustion characteristics of electrospray assembled micron scale particles composed of commercial nano-aluminum (ALEX), bound in an energetic polymer matrix composed of nitrocellulose. The nitrocellulose not only acts as a binder for the nanoparticles but also as a dispersant owing to its dissociation at low temperatures (ca. 450 K). Combustion characteristics were measured by direct injection of the electrospray assembled particles into the post flame region of a CH4/O2 diffusion flame. We find that the composite meso-particles show an order of magnitude reduction in average burn times when compared to that of the commercial nanoaluminum (ALEX), and are as fast as the shortest nanoparticle burn time. Scanning electron microscopy of quenched post-combustion particles clearly shows smaller sized products in the combustion of electrospray generated composite particles when compared to ALEX powder. This latter point should also lead to a more complete reaction and certainly demonstrates that the concept of using a two-stage reacting system: one at low temperatures to generate gas to separate particles followed by the nominal oxidation reaction is at the least a strategy that is worthy of further exploration.