The presence of gas phase species in micro- and nano-aluminum combustion

The presence or absence of gas phase species during combustion of aluminum nano- particles (n-Al) is a crucial observable in evaluating competing theories such as a diffusive oxidation mechanism and the melt dispersion mechanism. Absorption spectroscopy is 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 nano-aluminum 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. A comparison with n-Al in an inert environment did not show Al vapor at temperatures below 2300 K, suggesting a nearly pristine oxide coat that inhibits the production of Al vapor in appreciable quantities without reaction. However, at these temperatures, we would have been able to detect Al vapor from equilibrium partial pressures if Al is present at the surface of a particle. 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 regardless of bath gas. These results further indicate a gas phase component of n-Al combustion is observed in temperatures as low as 1500 K.