Gas-Phase Reaction in Nanoaluminum Combustion

Abstract

The presence or absence of gas phase species during combustion of aluminum nanoparticles (n-Al) is a crucial observable in evaluating competing theories such as a diffusive oxidation mechanism and the melt dispersion mechanism. Absorption spectroscopy was used to probe the ground state of aluminum monoxide (AlO) and Al vapor in order to quantify the amount of Al and AlO present under conditions where these species were not observed in emission previously. Absorption measurements were made during combustion of nanoaluminum and micron-sized aluminum in a heterogeneous shock tube. AlO was detected in absorption at temperatures as low as 2000 K in n-Al combustion, slightly below the limit seen in micro-Al combustion. Al vapor was detected during n-Al combustion at temperatures as low as 1500 K, significantly lower than in micro-Al combustion, suggestive of a gas phase component. The detection limit for Al vapor was 1 Â 10 12 cm À3 . The gas phase component was much weaker than that seen in 10 lm Al combustion. A comparison with n-Al in an inert environment did not show Al vapor at temperatures below 2300 K, even though the equilibrium concentration of Al from particles at that temperature were several orders of magnitude higher than the detection limit. This suggests a nearly pristine oxide coat that inhibits the production of Al vapor in appreciable quantities without reaction. These results are contrary to predictions of the melt dispersion mechanism, which should result in the generation of aluminum vapor from high-energy Al clusters produced from n-Al particles that spallate from mechanical stresses under rapid heating. This should further be independent of the bath gas.

Effect of surface tension on the temperature of burning metal droplets

“…The effect of surface tension on the boiling point and thus on the rate of evaporation of the burning aluminum particles discussed here is consistent with recent experimental observations on heating different size Al powders in a shock tube [26]. The presence of vaporphase Al was detected optically.…”

Effects of different product phases in aluminum dust detonation modeling

“…However, more evidence of kinetics-controlled combustion was observed in recent experiments [15][16][17][18]. There are two factors inducing the combustion transition from diffusion-controlled to kinetics-controlled.…”

“…Lynch et al [15] demonstrated that the combustion time of particles, whose diameters are less than 10 m, would be dependent on pressure and oxide mole fractions. Furthermore, if the particle diameter is of nanometer scale, the combustion becomes kinetics-controlled from diffusion-controlled [16,17]. Tanguay et al [18,19] found that even for a particle on the scale of 100 m, Al particle combustion is kinetics-controlled due to strong convection induced by detonations.…”

Numerical simulation of one-dimensional aluminum particle–air detonation with realistic heat capacities

“…However, a study on Al combustion by Lynch et al [25] demonstrated that the combustion of particles with diameters less than 10 lm would be dependent on pressure and mole fraction. If the particle diameter is of nanometer scale, the combustion becomes kinetics-controlled rather than diffusion-controlled [26,27]. Tanguay et al [28] found that the even the particle with the diameter 100 lm, the kinetics-controlled combustion may appear due to the high-speed flow behind the leading shock of the detonation.…”