Nanoparticles Synthesis by Air-Assisted Ultrasonic Spray Pyrolysis
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NANOPARTICLES SYNTHESIS BY AIR-ASSISTED ULTRASONIC SPRAY PYROLYSIS Shirley C. Tsai1, Yu L. Song, C.Y. Chen, T. Kuan Tseng, Chen S. Tsai, and Hong M. Lin2 Institute for Applied Science and Engineering Research Academia Sinica, Taipei 115, Taiwan 1 Department of Chemical Engineering, California State University, Long Beach, CA, U.S.A. 2 Department of Material Engineering, Tatung University, Taipei, Taiwan ABSTRACT This paper presents new findings regarding the effects of precursor drop size and concentration on product particle size and morphology in ultrasonic spray pyrolysis. Large precursor drops (diameter >30 µm) generated by ultrasonic atomization at 120kHz yielded particles with holes. Precursor drops 6-9 µm in diameter, generated by an ultrasonic nebulizer at 1.65MHz and 23.5W electric drive power, yielded uniform spherical particles 150nm in diameter under proper control of heating rate and precursor concentration. Moreover, air-assisted ultrasonic spray pyrolysis at 120kHz and 2.3W yielded spherical particles of which nearly half were smaller than those produced by the ultrasonic spray pyrolysis of the 6-9 µm precursor drops, despite the much larger precursor drop sizes (28 µm peak diameter versus 7 µm mean diameter). These particles are much smaller than those predicted by the conventional one particle per drop mechanism, suggesting that a vapor condensation mechanism may also be involved in spray pyrolysis. It may be concluded that through this new mechanism air-assisted ultrasonic spray pyrolysis can become a viable process for mass production of nanoparticles. INTRODUCTION Spray pyrolysis is widely used in industry to produce fine-grained powders because it is inexpensive and versatile. Its chemical flexibility offers numerous opportunities for controlled synthesis of advanced ceramic powders and films [1]. Spray pyrolysis is comprised of two main processes: (1) the generation of drops, and (2) the conversion of these drops to particles. Drops are typically generated through either two-fluid atomization (liquid atomization by high velocity air) or ultrasonic atomization (without air) [2]. Two-fluid atomization has the advantage of high throughput but the disadvantage of broad drop size distribution (which results in broad particle size distribution). Ultrasonic atomization, on the other hand, has the disadvantage of low throughput, but the advantage of narrow drop size distribution (and therefore, narrow particle size distribution). Recently, we reported a new atomization technique---called ultrasound-modulated two-fluid (UMTF) atomization---that has a higher throughput and produces a narrower drop size distribution with a smaller peak drop diameter than the conventional ultrasonic atomization at the same ultrasonic frequency. For example, while the conventional ultrasonic atomization yields a bimodal drop-size distribution (a primary peak at 55 µm and a much weaker secondary peak at 22 µm diameter) at 110 kHz, addition of air at 170 m/s velocity and 5.6 mA/mL (ratio of air-to-liquid mass flow rate) in th
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