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 mode l. 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 me asurements. Particle characteristics were studied along the jet axis f or 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 o xide particles (largest diffusion coefficient) was collision limited, yielding particles that are large and nonagglomerated. The growth of t itania particles (mid-range diffusion coefficient) occurred in the col lision limited and coalescence limited regimes to form mid-sized parti cles in agglomerates. The growth of alumina particles (lowest diffusio n 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 al ong the jet axis. When the coalescence time rapidly exceeds the collis ion time, subsequent collisions form agglomerates which are loosely he ld together. When the coalescence time slowly becomes longer than the collision time, strong necks form between the particles. (C) 1997 Amer ican Association for Aerosol.