Lower air-crystallization temperature and nanostructured yttria-doped tetragonal zirconia polycrystals ceramics by seedi
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MATERIALS RESEARCH Lower air-crystallization temperature and nanostructured yttria-doped tetragonal zirconia polycrystals ceramics by seeding assisted chemical coprecipitation P. Dur´an, J. Tartaj, J. F. Fern´andez, and C. Moure Electroceramics Department, Instituto de Cer´amica y Vidrio (CSIC), 28500 Arganda del Rey, Madrid, Spain (Received 21 July 1998; accepted 4 February 1999)
The crystallization temperature of yttria-doped tetragonal zirconia polycrystals (Y-TZP) amorphous precursors can be lowered 150 ±C below that currently used (>500 ±C), if “seeding assisted chemical coprecipitation” is used. Completely crystallized Y-TZP nanocrystalline powder was obtained by calcining at 350 ±C in air; the Y-TZP precursors seeded with 10 wt% of nanometric s,8 nmd Y-TZP particles. The seed particles enhanced both the nucleation and the crystallization rates at lower temperatures. From such a powder, 99% dense and nanostructured (grain size ,90 nm) Y-TZP bodies can be prepared by sintering below 1050 ±C.
It is widely recognized that nanostructured ceramic materials present unusual mechanical, electrical, and magnetic properties. Owing to its high strength and toughness, yttria-doped tetragonal zirconia polycrystals (Y-TZP) is one of the most interesting ceramic materials for structural applications. Besides this, submicrongrained Y-TZP ceramics show a superplastic behavior at elevated temperatures.1 To achieve this implies the use of Y-TZP powder with high sinterability and compactability, which are closely related to the strength of the interparticle bonds. Such a strength can be decreased by lowering the crystallization temperature. It is commonly accepted that chemical methods of synthesis lead to powders with no compositional fluctuations and controlled particle size and shape.2,3 However, in many instances metastable compounds are obtained needing a subsequent calcination step to convert the gels to an oxide ceramic powder. In the last decade, the hydrothermal synthesis route has been successfully used in the preparation of many ultrafine oxide powders. This technique, although not needing the calcination step to obtain oxides, involves the use of a closed vessel operating at relatively high temperatures (between 200 and 300 ±C) and elevated pressures s>100 bars). Besides this a further heat treatment at about 300 ±C becomes necessary to eliminate residual organic or inorganic products before using the ceramic powders. Recently, the inert-gas condensation technique is being used for the preparation of nanostructured pure ZrO2 powders.4–6 The process consists of evaporation of zirconium monoxide in an inert-gas atmosphere of 250 to 1500 Pa followed by a postoxidation step of the small ZrO crystals accumulated on the surface of a cold finger kept at 77 K. In order to avoid agglomeration of the highly reactive ZrO nanoparticles, the postoxi1686
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 5, May 1999
Downloaded: 12 Apr 2015
dation step is carried out at a controlled rate, and a stoichiometric ZrO2 is
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