Controlled Synthesis of Nanosized Particles by Aerosol Processes

Solid particles in the 1 nm < d_p < 100 nm size range form in gases as a result of gas phase condensation, particle collision processes, and solid-state processes. The relative rates of sintering and collision determine the size and morphology of the spheroidal primary particles. Rapid sintering is equivalent to the classical theory of coagulation with instantaneous coalescence. When the sintering rate is slow compared with the collision rate, fine primary particles form and aggregate into irregularly shaped agglomerates. The growth of primary particles in an aerosol generator that is cooling at a constant rate was studied theoretically. The most important process parameter determining particle diameter is the maximum gas temperature, because the rate of sintering is a sensitive function of temperature. Aerosol volume loading and cooling rate are important when the rate of particle growth is limited by collision processes. Experiments on the formation of alumina particles were made to study these effects. Predictions of primary particle size did not agree well with experimental measurements, which is attributed to an inadequate understanding of solid-state diffusion processes in nanosized particles. Other experiments showed that low concentrations of sodium and potassium additives reduce the primary particle size of silica.