Formation of charged aggregates of Al2O3 nanoparticles by combustion of aluminum droplets in air

The mechanism of Al2O3 nanoparticles aggregation in air was studied under atmospheric pressure conditions. Alumina nanoparticles were generated by combustion of a small sample of solid rocket propellant. Size and morphology of nanoparticles were studied by transmission electron microscopy. It was determined that alumina nanoparticles form aggregates of size about 1 mm composed of primary particles with a diameter of few tens of nm. The aggregates' coagulation was observed directly by a video system. In addition, the aggregate movement in the electric field was recorded by the video system. These observations showed that the majority of aggregates are charged. The typical aggregate charge is about 10 elementary units. Some aggregates are dipoles as indicated by the aggregate rotation when the electric field polarity is changed. The mechanism of aggregate formation has been proposed. The initial stage of alumina nanoparticle synthesis includes the formation of liquid oxide particles in the reaction zone located around the surface of burning Al droplet. When the particles pass to the relatively low-temperature region, solidification of particles occurs followed by clustering of solid spherical Al2O3 particles. Later, an aggregation of clusters formed by different burning Al droplets occurs resulting in formation of final aggregates. Estimations of thermal emission of electrons by the burning Al droplets were carried out via solution of the Poisson-Boltzmann equation. These estimates showed that the negative volume charge in the reaction zone around the Al droplet is 10 exp 8 to 10 exp 9 elementary charges per cu cm.