Stabilization and Transformation of the Phases in Nanostructured Irconia Prepared by Wet Chemical Synthesis Route
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conductivity [5-6]. Structurally stabilized zirconia is widely used in thermal barrier coatings for advanced engines where extremely high temperatures are experienced. Other uses of zirconia include milling balls, refractors, oxygen sensors, and fuel cells, as well as electronic ceramics. Zirconia has a monoclinic structure at low temperatures but exists in different forms at elevated temperatures For example, undoped ZrO 2 with the monoclinic structure transforms near 1170 'C to the tetragonal structure and then near 2370 'C to the cubic structure. This transformation is accompanied by a volume change, which can lead to mechanical damages of parts [7]. The presence of lower-valance cations such as Mg 2+,Ca 2÷, y3+, and rare earth cations stabilizes the high temperature phases to lower temperatures so that metastable tetragonal or cubic phases can exist down to ambient temperature. Methods used to produce conventional micro-scale stabilized ZrO 2 includes coprecipitation, microemulsion, and sol-gel synthesis [1-4, 8]. Recently, attention has been focused on reducing zirconia crystallite size from the micrometer to the nanometer dimensions. This produce nanostructured materials, which, especially in high temperature oxides, offer several advantages [9], including low thermal conductivity, reduced sintering temperature, improved mechanical properties and superplasticity. Currently, a number of production techniques are available for the synthesis of nanostructured oxide and non-oxide materials [9]. These include laser ablation, microwave plasma synthesis, spray conversion, flame pyrolysis, inert gas condensation, and sol-gel synthesis. These synthesis techniques are also being employed to prepare ultrafine and nanostructured Y203/ZrO 2 [10]. The currently available techniques for the synthesis of nanostructured zirconia and yttria stabilized zirconia, however, 261
Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
are not viable as commercial processes. The inert gas condensation (IGC), chemical vapor condensation (CVC) methods are inherently slow and thus non cost effective, while nanoparticles produced by sol-gel synthesis are heavily agglomerated. In this paper, we describe a wet chemical approach for the synthesis of high quality nanostructured Y20 3/ZrO2 , that is scalable to large volume manufacturing and anticipated to be low in cost [11]. This is accomplished by utilize a method exploiting extremely high nanoparticle nucleation combined with suppression of nanoparticle growth. This results in a high production of nanostructured powder, without undue nanoparticle agglomeration. EXPERIMENTAL The synthesis of Y2OJ/ZrO 2 powder was obtained by the hydrolysis reaction of an aqueous solution of zirconyl chloride (ZrOCI2 .8H 2 0) and yttrium chloride (YC13 .6H2 0) with ammonium hydroxide (NH4bOH). This reaction was accomplished by spray atomization of the mixed the zirconyl chloride and yttrium chloride into the reaction vessel that contained distilled deionized water with diluted NH{4OH, while
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