Preparation, characterization, and sinterability of well-defined silica/yttria powders
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Dispersions of uniform submicron spherical particles consisting of silica cores and yttria coatings, or vice versa, were prepared by a precipitation technique. The overall size of the particles and the thickness of the shells could be varied over a wide range. Such powders were used to form green bodies by sedimentation, centrifugation, or pressure filtration, and the density and the pore size distribution of the resulting solids were evaluated. The green bodies were sintered and the changes in density, phases, and microstructure were followed with temperature. In general, the coated powders exhibited enhanced densification. On processing composite solids at temperatures >1000 °C, the formation of Y 2 Si 2 0 7 took place, which caused a pronounced shrinkage of the samples. Powders of coated particles having the same silica/yttria ratios sintered at lower temperatures when the shell was composed of silica rather than of yttria. When either silica or yttria were in molar excess in the coated particles, the sintered products had a mixed composition of Y 2 Si 2 07 and the component in excess. By terminating the sintering process before the grain growth started, the solids displayed a well-defined microstructure with a uniform distribution of areas of one phase in the matrix of the matter in excess. This property was mainly due to the uniformity of initial powders in terms of the particle size and the coating.
I. INTRODUCTION Rare earth oxides, especially those of yttrium alone or in combination with other metals, are of interest in many ceramic applications. For example, yttrium aluminum garnet (YAG) and yttrium iron garnet (YIG) are useful as artificial diamonds or in the microwave technology. Yttria also stabilizes the high temperature cubic form of zirconium oxide (YSZ),1"5 which has an even higher refractive index and is harder than YAG. Other areas of applications include high temperature and high strength ceramic materials. For example, SiAlON tools or S13N4 engine parts (e.g., valves or turbocharger rotors) are currently produced by sintering a mixture of S13N4, (A1N) A12O3, and Y 2 O 3 powders. In case of silicon nitride (as well as in other systems), yttria acts as a sintering aid (fluxing agent) to achieve solids of full density.6"15 This effect is mainly due to the combination of yttria with silica, which forms low melting yttrium silicates or even lower melting glasses in combination with AI2O3.16 The importance of chemistry in the development of high performance ceramics has been amply documented.17 The major problem in multicomponent a
'Present address: New York State College of Ceramics at Alfred University, Alfred, New York 14802-1296.
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http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 2, Feb 1994
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systems lies in the control of the distribution of their chemical constituents. Inhomogeneities in the starting powder or in the green body result in densification problems and an arising population of "flaws", which limits the strength and the reliability of the final
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