JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 3647 – 3657 Ultrasonic spray pyrolysis for nanoparticles synthesis S. C. TSAI Institute for Applied Science and Engineering Research, Academia Sinica, Taipei 115, Taiwan; Department of Chemical Engineering, California State University, Long Beach, CA 90840, USA Y. L. SONG Institute for Applied Science and Engineering Research, Academia Sinica, Taipei 115, Taiwan C. S. TSAI Institute for Applied Science and Engineering Research, Academia Sinica, Taipei 115, Taiwan; Department of Electrical and Computer Engineering, University of California, Irvine, CA 92697, USA E-mail: cstsai@uci.edu C. C. YANG, W. Y. CHIU Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan H. M. LIN Department of Material Engineering, Tatung University, Taipei, Taiwan This article presents new findings regarding the effects of precursor drop size and precursor concentration on product particle size and morphology in ultrasonic spray pyrolysis. Large precursor drops (diameter > 30 μm) generated by ultrasonic atomization at 120 kHz yielded particles with holes due to high solvent evaporation rate, as predicted by the conventional one particle per drop mechanism. Precursor drops 6–9 μm in diameter, generated by an ultrasonic nebulizer at 1.65 MHz and 23.5 W electric drive power, yielded uniform spherical particles 90 nm in diameter with proper control of precursor concentration and residence time. Moreover, air-assisted ultrasonic spray pyrolysis at 120 kHz and 2.3 W yielded spherical particles about 70% of which were smaller than those produced by the ultrasonic spray pyrolysis of the 6–9 μm precursor drops, despite much larger precursor drop size (28 μm peak diameter versus 7 μm mean diameter). These particles are much smaller than predicted by the conventional one particle per drop mechanism, suggesting that a gas-to-particle conversion mechanism may also be involved in spray pyrolysis. C  2004 Kluwer Academic Publishers 1. Introduction Spray pyrolysis is widely used in industry to produce fine-grained (>0.5 μm diameter) powders because it is relatively inexpensive and quite versatile. Spray pyrol- ysis is a continuous flow process that operates at ambi- ent pressure; therefore, it is more economical than other processes (such as sol-gel and chemical vapor deposi- tion) that involve multiple steps or must be conducted under vacuum. Moreover, its chemical flexibility offers numerous opportunities for controlled synthesis of ad- vanced ceramic powders and films [1]. However, the mechanisms of spray pyrolysis are at present not fully understood. Understanding of these mechanisms is es- sential to evaluating the potential of spray pyrolysis for the mass production of uniform nanoparticles of ma- terials such as zirconia and titania, which are used in thermal insulation, solid oxide fuel cells [2], gas sens- ing [3], photo catalysis, and many other applications [3]. Spray pyrolysis involves four major steps: (1) gen- eration of drops from a precursor solution, (2) drop size shrinkage due to evaporation, (3) conversion of precursor into oxides, and (4) solid particle formation. Drops are typically generated through either two-fluid atomization (liquid atomization by high velocity air) or ultrasonic atomization (without air) [4]. Two-fluid atomization has the advantage of high throughput but also has the disadvantage of broad drop size distribu- tion (which results in broad particle size distribution). On the other hand, ultrasonic atomization has the dis- advantage of low throughput, but has the advantage of narrow drop size distribution (and therefore, narrow particle size distribution). Furthermore, by increasing the ultrasonic frequency, one can decrease the drop size 0022–2461 C  2004 Kluwer Academic Publishers 3647