Production of nanometer-sized metal oxide particles by gas phase reaction in a free jet .2. Particle size and neck formation - Comparison with theory

Experimental measurements of nanosized primary particle diameters were compared with calculated values based on a collision-coalescence model. The method of analysis permits calculation of the primary particle size when growth is collision limited (individual particles colliding), coalescence limited (primary particles coalescing in agglomerates), or in a transition regime (particles coalescing about as fast as they collide). Calculated particle sizes compared well with experimental measurements. Particle characteristics were studied along the jet axis for the following conditions: exit velocity = 27.8 m/s, volume loading = 3.2 X 10(-7), dame gas flow rate = 33 l/min. The growth of niobium oxide particles (largest diffusion coefficient) was collision limited, yielding particles that are large and nonagglomerated. The growth of titania particles (mid-range diffusion coefficient) occurred in the collision limited and coalescence limited regimes to form mid-sized particles in agglomerates. The growth of alumina particles (lowest diffusion coefficient) was coalescence limited forming small, oblong particles necked together in large agglomerates. The extent of necking between particles can be estimated from the collision and coalescence times along the jet axis. When the coalescence time rapidly exceeds the collision time, subsequent collisions form agglomerates which are loosely held together. When the coalescence time slowly becomes longer than the collision time, strong necks form between the particles. (C) 1997 American Association for Aerosol.