Hydrodynamic cavitation as a tool to control macro-, micro-, and nano-properties of inorganic materials
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Hydrodynamic cavitation was shown to be a powerful tool for the synthesis of nanostructured catalysts, ceramics, and piezoelectrics in high phase purities. The macro-, micro-, and nano- properties of solid-state materials could be controlled through adjusting the cavitational regime during synthesis by simple mechanical adjustment. The synthesis of nanostructured titania, piezoelectrics, perovskites, supported and unsupported cobalt molybdates, and Pd and Ag supported on alumina illustrate changes in morphology and size of crystals, growth in a preferred orientation of crystallites, and control of crystallographic strain and size compared to classically prepared materials. The high shear and cavitational forces during synthesis induce micro-strain into the materials and are a function of the Reynolds and cavitation numbers.
I. INTRODUCTION
II. EXPERIMENTAL
In recent years high shear hydrodynamic cavitation has been reported as a synthesis pathway1–4 for nanophase materials in large quantity and high phase purity compared to classical coprecipitation methods. It is also well known that cavitation has an effect on the grain size of materials. The materials synthesized with a cavitational device have consistently revealed smaller grains than those prepared by a classical route. In most cases, the high local temperatures resulting from cavitation, ranging from 2000 to 5000 K, as reported by Suslick, who used ultrasound,5–7 should result in an in situ calcination of the formed metal oxides or hydroxides. However, the occurrence of cavitation itself is strongly dependent on the design and the geometry of the device used and the experimental parameters. Furthermore, the temperature is a function of the gas atmosphere and the solvents used. Therefore, the laboratory models CaviMax™ and CaviPro 300™ of Five Star Technologies™8,9 were used, which are devices optimized to generate a maximum intensity of cavitation events. The advantages of these devices are the combination of both cavitation and high flow conditions. The flow rate, pressure, and orifice size are parameters that permit a wide range of mechanical control of the synthesized products.
A. Experimental setup
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 12, Dec 2001
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Two cavitational devices manufactured by Five Star Technologies™ (Cleveland, OH) were used for synthesis: the models CaviMax™ and CaviPro 300™. The mechanical details are given in Table I. In this paper the use of capital letters refer to experiments conducted in the CaviMax™ (A to G). Experiments conducted in the CaviPro 300™ are noted with numbers (0 to 11). A series combination of two orifices in the latter is expressed in the form “first orifice–second orifice” (e.g., 9-11). A schematic drawing of the experimental setup is given in Fig. 1. Although the details of a synthesis is given in Sec. II. B, below, the experiments were designed so that all of the salts were precipitated at the bo
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