Compositions: ceramic – Ceramic compositions – Refractory
Patent
1993-11-03
1995-05-30
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
423266, 423608, C04B 3548
Patent
active
054200865
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a method for producing stabilized zirconium oxide powder with a particle size of at least 100 um, preferably between 100 um and 1000 um, preferably intended for high-density ceramic materials.
2. Description of the Prior Art
Trends in ceramics development have recently been moving toward increasingly high-quality ceramic materials, with the goal of utilizing the advantageous characteristics of these materials--including good temperature resistance, great hardness and wear resistance, and good resistance to chemicals--for engineering purposes.
The first priorities in this effort are to achieve the highest possible density and strength in the shaped elements, to lower the sintering temperature, and in many applications also to increase substantially the purity of the ceramic materials.
To achieve these goals, homogeneous, easily pressed powders of the greatest possible purity, with high sinterability, must be available on an industrial scale; and it must be possible to produce them with simple methods and as cost-effectively as possible.
One promising material that can meet these requirements is partially stabilized zirconium oxide, called PSZ. In this material the tetragonal crystal structure that, in pure zirconium oxide, is stable only at temperatures above 1000.degree. C. is stabilized by the addition of suitable additives. Metal oxides such as CaO, MgO, Y.sub.2 O.sub.3, CeO.sub.2, etc. are used as stabilizing additives. An important factor here is the homogeneity of the powder mixture, which with the usual dry synthesis methods can be achieved only with difficulty and by means of laborious procedures involving several stages, such as generating precursor products by mixing the starting powder, pressing, heat treating, and lastly grinding to a fine powder mixture. Variants aimed at improving homogeneity, such as melting eutectic mixtures, have also been disclosed.
"Wet synthesis" methods have been developed to improve the homogeneity and sinterability of powder mixtures. These involve either precipitating or otherwise consolidating salt solutions which contain the components that form the ceramic, or consolidating hydrosols of these substances by forming gels. Recently, alcoholates of the metals forming the ceramic have also been used to form aels.
In coprecipitation methods, finely dispersed powders are generated--in most cases from chlorides or nitrates of the metals--by hydroxide precipitation using alkaline substances (usually ammonia), or by precipitation of easily decomposed salts of organic acids (such as oxalic acid).
DE-A 34 08 096 describes a method in which a starting powder for a CeO.sub.2 -stabilized sintered zirconium product is produced by precipitating zirconium and cerium hydroxide with aqueous ammonia. Experience shows, however, that hydroxide precipitations of this kind produce voluminous precipitates that often have poor filtration characteristics and are difficult to wash; in addition they are difficult to handle, and after drying and calcining yield large blocks that must first be laboriously sieved to produce a fine, pourable powder.
Particular attention has been paid recently to sol-gel methods, since they produce powders with high sintering activity and large surface areas. In the simplest case, sols can be produced by hydrolysis of suitable salt solutions at boiling heat (DE-A 34 08 096). When the sol is dried and then calcined, however, the result is once again coarse blocks of gel that take a great deal of extra time to grind into powder.
Conventional methods of sol-gel technology in general and for the production of Y.sub.2 O.sub.3 -stabilized zirconium oxide in particular are-described by J. L. Woodhead and D. L. Segal in "Sol-gel processing," Chemistry in Britain, April 1984, pp. 310-313. Considerable space therein is devoted to sol production, and the authors explicitly note that production costs must be commensurate with the value of the ceramic product that is generated. In reality, the procedu
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Brandau Egbert
Brauneis Edwin
Kadner Martin
Group Karl
Nukem GmbH
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