Influence of yttria content on the preparation of nanocrystalline yttria-doped zirconia
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Nanocrystalline zirconia doped with 0-10 mol% Y 2 O 3 has been prepared by a combustion synthesis process, followed by a rapid densification process. The concentration of Y2O3 in the as-reacted zirconia appeared to have a significant influence on the reduction of the crystallite size, in the combustion temperature range studied (450 "C-550 °C), as well as on the stabilization of the tetragonal and/or cubic phases. The green compacts were densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final average grain size and the final density of the article. On the other hand, the ranges of heating rates and dwell times that were used in this study were shown to have a much less significant effect on the article's final density and final average grain size. The yttria content had the largest influence on the final density and final average grain size. The densification took place much more rapidly in the 4 mol% Y 2 O 3 -ZrO2 samples than in the 10 mol% Y 2 O 3 -ZrO 2 samples. In particular, the difference in densification rates between the samples with different Y 2 O 3 content was attributed to the influence and magnitude of the associated grain-growth process. It was determined, however, that a high final density (>99% p t h) and a very fine final average grain size (100 °C/min) and brief sintering times (typically, several minutes) in order to prepare a ceramic article with high density and fine grain size, thus establishing its advantage over conventional sintering approaches.4 Further, recent observations have shown that the average grain size in yttria-doped zirconia depends to 762
http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 3, Mar 1995
Downloaded: 26 Nov 2014
a large extent on the Y 2 O 3 content.5 For example, at a processing temperature of 1400 °C, the grain size in samples, whose composition fell within the two-phase region was less than one-fifth of the grain size of samples whose composition fell within the single (tetragonal or cubic) phase fields: 0.3 /xm and 2 /xra, respectively. On the other hand, an interesting variety of properties can be achieved by carefully controlling the Y 2 O 3 content in the yttria-doped zirconia system. Samples that possess a low Y 2 O 3 content ( 4 - 5 wt. %) can be properly fabricated as structural materials that possess very high fracture tougheness and high strength values.6'7 Samples with a higher Y 2 O 3 content (~8 wt. %) can be used as thermal barrier coatings because of their low thermal conductivity,89 or as solid electrolytes10-11 (for example, for use in oxygen sensors, pH meters, etc.) because of their high ionic conductivity.12'13 Samples that possess an even higher Y 2 O 3 content have a cubic structure and can be manufactured as imitation diamonds for use in costume jewelry.14^16 In most yttira-doped zirconia ceramic materials, grain growth behavior varies with composition. Different mechanisms have been proposed to explain this difference in grain growth. Lange5 attributed this difference to t
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