A model is developed of aerosol synthesis of mixed metal oxide nanocomposite film for sensor applications. The synthesis technique involves atomization of a solution of mixed salts in water, spraying of solution droplets, droplet deposition on a heated substrate, evaporation and chemical reaction to produce the mixed oxides, and film growth. The precise control of oxide nanoparticle size distribution and inter-particle spacing in the film is crucial to achieving high sensitivity. These in turn largely depend on the droplet characteristics prior to impingement on the substrate. This paper focuses on development of a model to describe the atomization and spray processes before the film growth. Specifically, a mathematical model is developed utilizing Computational Fluid Dynamics solution of the equations governing the transport of atomized droplets from the nozzle to predict droplet characteristics in flight. The predictions include spatial distribution of droplet size and concentration, and the effect on these characteristics of swirling inlet flow at the spray nozzle.
The influence of processing parameters is investigated on the structural characteristics of single and mixed oxides produced by spray pyrolysis technique. The films were synthesized by spraying precursor solutions through a nozzle onto a heated alumina substrate. The precursor consisted separately of aqueous solutions of tin chloride for SnO 2 and zinc chloride for ZnO for single oxide cases, and aqueous solutions of tin chloride and indium nitrate for SnO 2 + In 2 O 3 and zinc chloride and indium nitrate solutions for ZnO + In 2 O 3 for mixed oxide cases. The substrate temperature was varied accordingly for each single and mixed case. The films produced were characterized by X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The results indicate that a non-homogenous film is formed at low temperature for both single oxides considered. The temperature has significant effect on the composition of the synthesized films of both single oxides below 450˚C. The results for mixed oxides show that the best homogeneous films are obtained for 80 wt% ZnO + 20 wt% In 2 O 3 , and 80 wt% SnO 2 + 20 wt% In 2 O 3 .
This paper describes mathematical modeling of transport and chemical reaction phenomena in a single droplet on a heated substrate deposited by spray pyrolysis. The droplet contains mixed salt solution which reacts on the heated substrate to produce mixed oxides and water residue. The water is subsequently evaporated, leaving a thin film of the mixed oxides for sensor application. The droplet, containing solvent and precursors is modeled using Computational Fluid Dynamics (CFD) technique. The variety of stages is predicted of the evolution of droplet morphology associated with surface energy and evaporation. The transient distribution is also predicted of the concentration of various species in the droplet.
Chemical Spray Pyrolysis (CSP) of ZnO and SnO2 is of interest for gas sensor applications. The structural properties of the deposited film can be strongly influenced by deposition conditions. In this work, two solutions consisting of Tin Chloride and Zinc Chloride was sprayed on a heated substrate, where temperature was varied from 400° C to 450° C for ZnO, and from 350° C to 500° C for SnO2. X-ray diffraction and scanning electron microscopy, indicating a non-homogenous-structured film formed at low temperature for both oxides. At 450° C, a porous structure is observed for SnO2. This structure becomes homogenous at higher temperature. It was also found that at temperatures lower than 450° C, substrate temperature has significant impact on the composition of the synthesized films.
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