Direct Identification of Two Regimes of Metal Particle Combustion using Condense-luminescence

Experiments were designed to investigate two regimes of metal particle combustion: fast and slow burning regimes. Stress-altering aluminum particles had been shown to produce a distinctly faster burning rate compared to untreated aluminum particles. The root cause for the differences in burning rate had been unclear. In this study, stress-altered and untreated aluminum particles were reacted as dispersed powder in a closed bomb calorimeter designed to monitor the transient temperature changes resulting from energy release upon combustion. The product residue was analyzed for size and species concentration. Results showed metastable γ-alumina that is associated with nano-oxide formation was in substantially higher concentration for stress-altered particle reactions that produced greater energy transfer rates. The increased energy transfer rate corresponded to higher radiant energy emission owing to condensation of nano-oxide particles. This study justifies condense-luminescence as a means for increasing the energy release rate of aluminum particles. By strategically altering metal fuels to control a formation of nano-oxide particles upon combustion, appreciable increases in the radiant energy flux can transform energy release rates.

Preparation and Characterization of Silicon-Metal Fluoride Reactive Composites

Fuel-rich composite powders combining elemental Si with the metal fluoride oxidizers BiF3 and CoF2 were prepared by arrested reactive milling. Reactivity of the composite powders was assessed using thermoanalytical measurements in both inert (Ar) and oxidizing (Ar/O2) environments. Powders were ignited using an electrically heated filament; particle combustion experiments were performed in room air using a CO2 laser as an ignition source. Both composites showed accelerated oxidation of Si when heated in oxidizing environments and ignited readily using the heated filament. Elemental Si, used as a reference, did not exhibit appreciable oxidation when heated under the same conditions and could not be ignited using either a heated filament or laser. Lower-temperature Si fluoride formation and oxidation were observed for the composites with BiF3; respectively, the ignition temperature for these composite powders was also lower. Particle combustion experiments were successful with the Si/BiF3 composite. The statistical distribution of the measured particle burn times was correlated with the measured particle size distribution to establish the effect of particle sizes on their burn times. The measured burn times were close to those measured for similar composites with Al and B serving as fuels.