CONTROLLED SYNTHESIS OF NANOSIZED PARTICLES BY AEROSOL PROCESSES

Solid particles in the 1 nm < d(p) < 100 nm size range form in gases a s a result of gas phase condensation, particle collision processes, an d 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 compa red 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 studie d theoretically. The most important process parameter determining part icle diameter is the maximum gas temperature, because the rate of sint ering is a sensitive function of temperature. Aerosol volume loading a nd cooling rate are important when the rate of particle growth is limi ted by collision processes. Experiments on the formation of alumina pa rticles were made to study these effects. Predictions of primary parti cle size did not agree well with experimental measurements, which is a ttributed to an inadequate understanding of solid-state diffusion proc esses in nanosized particles. Other experiments showed that low concen trations of sodium and potassium additives reduce the primary particle size of silica.