Combustion of aluminum nanoparticle agglomerates: From mild oxidation to microexplosion

While the nano-sized energetic materials are featured with ultra-high energy density, the ubiquitous agglomeration in their combustion is still unexplored. In this paper, the combustion characteristics of aluminum nanoparticle agglomerates in the size range of 4-20 μm are investigated on a modified Hencken burner with different temperature (800-1800 K) and oxygen concentration (0.5-5.5 mol/m³). Due to the heat accumulation effect of the designed porous structures, the nanoparticle agglomerates even maintain the advantages of combustion process of single nanoparticle in terms of a low ignition temperature (∼800 K) and a fast energy release rate. Further, the combustion of agglomerates is numerically studied by a newly-developed model, which accurately predicts both burn time and temperature of agglomerate of the mild combustion process. The microexplosion phenomenon occurs when the oxygen concentration exceeds 3.5 mol/m³. Measurements of particle temperature, burn time, emission spectra and morphologies indicate that this explosion is driven by the vaporization of unreacted aluminum core, which results in huge stresses to tear the Al/Al₂O₃ particle into many smaller, dispersed clusters. Thus a melt/vapor dispersion mechanism (MVDM) based on melt dispersion mechanism is proposed to cover the microexplosion and subsequent accelerated oxidation reactions.