Effect of oxide shell growth on nano-aluminum thermite propagation rates

Nanocomposite energetic materials show increased flame propagation rates over their micron scale counterparts. These energetic formulations consist of fuel and oxidizer powder mixtures, commonly referred to as thermites. A theory explaining the faster flame propagation speeds associated with nano-particles is called the melt dispersion mechanism and based on the mechanochemistry of the fuel particle’s core-shell structure. The theory supposes that if the ratio (M) of particle radius to shell thickness exceeds a critical threshold, the melt dispersion mechanism is activated, oxidation is accelerated and flame propagation will increase. This study expands on this theory by growing the oxide shell around aluminum fuel particles in a hot, oxygenated environment to achieve varying M ratios. Flame propagation was examined for untreated and treated aluminum particles in an Al-MoO3 thermite. Experimental setup consisted of a closed end tube shot and high speed photography. In all cases, alumina shell grew and was damaged due to treatment, and flames rates were reduced. Flame speed of several hundred meters per second, reduction in flame rate with damage to oxide shell, and weak dependence of the flame speed on the ratio M of particle radius to shell thickness in the range 6.1<M<13.4 is consistent with the melt dispersion mechanism of reaction of Al nanoparticles. While the melt dispersion mechanism is activated in an increasing number of particles, the mixture is simultaneously diluted with alumina. This produces an observed plateau, where large changes in shell thickness produce only minor changes in burn rate. Unsteady propagation behavior was observed as the samples were increasingly oxidized. Sharp drop in flame rate at further reduction in M down to 5 is consistent with similar drop while adding similar amount of alumina to the reactive mixture. This indicates that diluting effect is present.