Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-12-17
2004-03-09
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C423S608000
Reexamination Certificate
active
06703334
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing zirconium oxide (zirconia) having a stabilized tetragonal or cubic structure.
BACKGROUND ART
Manufacturers use stabilized zirconia powders (SZ), typically zirconia stabilized with yttria (YSZ), to produce zirconia-based bulk ceramics and ceramic coatings. YSZ coatings form particularly effective thermal barrier coatings (TBCs) for gas turbine engines. The critical characteristic of this YSZ powder responsible for performance at high temperatures is homogeneity of the yttria stabilizer distribution throughout the zirconia crystal lattice. Insufficient homogeneity reveals itself as a presence of deleterious monoclinic phase or non-stabilized zirconia. Unfortunately, excessive amounts of monoclinic phase tend to shorten a TBC's life and increase repair frequency for gas turbine engines.
There are two common methods for manufacturing SZ powders. The “wet-chemical” approach relies on mixing of a zirconium salt solution and a solution of metal-stabilizer followed by a separation of a solid, containing both of the metals. Commonly manufacturers separate the solid by co-precipitation and filtering of metal hydroxides; but they can rely on other separation techniques such as, sol-gel, evaporation and spray-pyrolysis. For example, Xu et al. in, “Preparation of Weakly Agglomerate Nanometer ZrO
2
(3 mol % Y
2
O
3
) Ceramic Powder”, Journal of the European Ceramic Society (1993) pp. 157-160, disclose a gel co-precipitation process. The separated solids contain molecularly mixed zirconium and stabilizer ions.
After separating the solids, calcining at elevated temperatures crystallizes the mixture. The calcination temperature required for the formation of SZ is typically lower than 1,000° C. and could be as low as 500° C. The stabilized zirconia can have no monoclinic phase in it, i.e. having stabilizer ions distributed with an atomic scale uniformity. The drawbacks of the chemical approaches however include the rather complicated and time consuming processing steps as well as the formation of large volumes of corrosive and hazardous gaseous or liquid wastes. Furthermore, these “wet-chemical” prepared powders are too expensive for use as starting materials in typical powder consuming technologies such as zirconia-based refractories and stabilized zirconia thermal spray powders.
For example, F. Pitts, in U.S. Pat. No. 3,957,500, describes a co-precipitation process for the preparation of stabilized zirconia by preparing an oxide mixture from zirconia powder and yttria concentrate and calcining the mixture at a temperature from 900 to 1500° C. for a period ranging from 1 to 10 hours. Bickford et al., in U.S. Pat. No. 4,810,680, describe a typical commercial process for preparing high-purity-homogeneous stabilized zirconia powder from zirconium basic carbonate and yttrium carbonate starting materials. First dissolving the starting materials in hydrochloric acid forms a hydroxide solution mixture. Then co-precipitating the hydroxide solution mixture with ammonium or sodium hydroxide solution forms a mixed precipitate. Then the process uses the steps of filtrating the precipitate, washing, drying and then calcination within the range of 680 to 980° C. The by-products of this process are a water solution of ammonium or sodium chloride, i.e. supernatant (680 ml/30 g of product) and a weak-water solution of the same salts, i.e. wash water (500 ml/30 g of product). Umento et al.'s U.S. Pat. No. 6,255,242 describes another precipitation process for the production of zirconium and cerium-based mixed oxide. This process admixes zirconium basic sulfate (insoluble in water) with a solution of cerium salt, preferably nitrate, followed by adding a base (sodium, potassium or ammonium hydroxide or carbonate of sodium or ammonium) to precipitate cerium hydroxide and to convert zirconium basic sulfate into zirconium hydroxide. This method provides high chemical homogeneity of the product—when calcined at 660° C. for 3 hours, it forms the crystal phase of the mixed zirconium-cerium oxide having a cubic phase purity of not less than 95 percent by volume, commonly less than 1 percent monoclinic phase by volume.
Chemical sol-gel processes, such as those described in U.S. Pat. No. 5,750,459 to Marella et al., require exacting separation techniques. For example, Marella et al. describe a sol-gel process for preparing spheres and microspheres of stabilized zirconia powders using zirconium basic carbonate as a starting material. Dissolving the zirconium basic carbonate in nitric acid solution produces the zirconia sol. Then mixing yttrium or cerium nitrate solution with the sol and dripping it in a gelation bath of ammonium hydroxide solution obtains gel spheres or microspheres. After separating these gel spheres from the gelation bath, the gel spheres require rinsing with water, drying and calcination at a temperature higher than 550° C. to form spherical or microspherical stabilized zirconia powder. This powder is useful as either a catalyst or catalyst support. The high filtration rate of gelated particles is a significant benefit of this process in comparison to traditional hydroxide precipitation techniques. But liquid and gaseous wastes stream utilization still remains a major drawback of the process. This process, despite its technological benefits, still has the disadvantages of generation of sodium, potassium or ammonium sulfate- and nitrate-containing waste water streams. In addition, the process requires deep washing of the precipitate to remove by-products. Otherwise, if washing is incomplete, then sulfur oxides or NO
x
off-gases will escape from a furnace during the precipitate calcining.
The other method for manufacturing SZ powders is the solid-state process. In this method, milling a mixture of zirconium oxide powder and an oxide of metal-stabilizer in water forms a blended mixture. After the milling step, filtration, evaporation or spray-drying separates the constituents from the water. Finally, a high temperature calcining of the milled mixture forms the SZ powder. This solid-state approach is technologically simple to perform and not complicated with respect to waste stream treatment; and the typical by-products are recyclable waste water and water steam. The disadvantages of this approach include its high calcination temperature, typically higher than 1300° C., and the low uniformity of the product—content of the monoclinic phase can be as high as 25 to 30 percent by volume. In an attempt to minimize or eliminate the presence of the monoclinic zirconia phase, manufacturers have repeatedly remilled and recalcined the product and used ultra-fine zirconia powders to improve blending. Unfortunately, both of these options result in significant increase of production costs; and thus manufacturers rarely use these techniques in manufacturing SZ powders for refractories or for production of SZ thermal spray powders.
Nakada et al., in U.S. Pat. No. 4,542,110, describe a process for producing zirconium oxide sintering body by ball-milling of a zirconium and yttrium oxide blend, drying the resulting mixture and calcining at a temperature higher than 1300° C., preferably from 1400 to 1500° C. for 10 to 120 min. After this, repeating the blending and calcining steps increases the stabilized cubic phase content to at least 95 percent. In a similar process, Otagiri et al., in U.S. Pat. No. 4,360,598, disclose a method of producing yttrium stabilized zirconia ceramic by mixing a powder of amorphous zirconia or zirconia powder having a crystallite size less than 0.1 &mgr;m with yttrium oxide or any other salt of yttrium including yttrium oxalate—a thermal decomposition of zirconium chloride, zirconium nitrate or zirconium hydroxide at a temperature of 500 to 1050° C. produces the fine zirconia powder. After mixing, a caulking step at a firing temperature between 1000° C. and 1550° C. provides a sintered ceramic having a predominant tetragonal or cubic structure. In addition to this final caulking step, an optional intermediate calcin
Belov Irina
Belov Vladimir
Coon Gerald L.
Marcantoni Paul
Praxair S.T. Technology, Inc.
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