Zirconia and Zirconates

Zirconia-based sol-gel precursor chemistry is driven by hydrolysis and condensation reactions of Zr(IV) alkoxide-related and inorganic Zr(IV) precursors. In 

The cylindrical shape of zirconia clusters and the constancy of their radius were demonstrated with SAXS by Jutson et [ 103]. Shortly thereafter, Toth etaL inves- 

2 Further Stages of Sol-Gel Processing of Zirconia-Related Materials 

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Other types of refractory that find use are forsterite, zirconia, and zircon. Acid-resisting bricks consisting of a dense body like stoneware... 

Other types of refractory that find use are forsterite, zirconia, and zircon. Acid-resisting bricks consisting of a dense body like stoneware are used for lining tanks and conduits in the chemical industry. Carbon blocks are used as linings for the crucibles of blast furnaces, very extensively in a number of countries and to a limited extent in the United States. Fusion-cast bricks of mullite or alumina are largely used to line glass tanks. 

Zirconia and zirconate NP-5-I-NP-9 and cyclohexane To produce zirconia, lead zirconate and lead zirconate titanate at much lower calcination temperatures [158-161]... 

Mixed zircon, coke, iron oxide, and lime reduced together produce zirconium ferrosiUcon [71503-20-3] 15 wt % Zr, which is an alloy agent. Fused zirconia [1314-23-4] has been made from zircon but baddeleyite is now the preferred feed for the production of fused zirconia and fused alumina—zirconia by electric-arc-fumace processing. 

This dissociated zircon is amenable to hot aqueous caustic leaching to remove the siHca in the form of soluble sodium siHcate. The remaining skeletal stmcture of zirconia is readily washed to remove residual caustic. Purity of this zirconia is direcdy related to the purity of the starting zircon since only siHca, phosphate, and trace alkaHes and alkaline earth are removed during the leach. This zirconia, and the untreated dissociated zircon, are both proposed for use in ceramic color glazes (36) (see Colorants for ceramics). 

Zirconium oxide is used in the production of ceramic colors or stains for ceramic tile and sanitary wares. Zirconia and siHca are fired together to form zircon in the presence of small amounts of other elements which are trapped in the zircon lattice to form colors such as tin—vanadium yellow, praseodymium—zircon yellow [68187-15-5] vanadium—zircon blue [12067-91 -3] iron—zircon pink [68412-79-3] indium—vanadium orange (105—108). 

It is found in the ores baddeleyite (also known as zirconia) and in the oxides of zircons, elpidite, and eudialyte. 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Zirconia. Zircon (zirconium silicate), the most widely occurring zirconium-bearing mineral, is dispersed in various igneous rocks and in zircon sands. The main deposits are in New South Wales, Australia Travancore, India and Florida in the United States. Zircon can be used as such in zircon refractories or as a raw material to produce zirconia. The zircon structure becomes unstable after about 1650°C, depending on its purity, and decomposes into Zr02 and Si02 rather than melting (see Zirconiumand zirconium compounds). 

Zirconia, Lead Zirconate and Lead Zirconate Titanate. 289... 

Figure 12 Conductivities of selected oxides as a function of temperature. Doped ceria and zirconia, and lanthanum gallate are oxide ion conductors, while hariiun zirconate is a proton conductor (From S.M. Haile Materials for fuel cells. Materials Today 6 24-29 (March 2003)). (Reprinted from Ref. 148. 2003, with permission from Elsevier)...

In 1789, Klaproth isolated zirconia from zircon, and in the same year he discovered uranium in the ore pitchblende. He confirmed the existence of strontia (discovered by Thomas Charles Hope [1766-1844]) in 1795 and discovered titanium that same year. In 1797, he isolated chromium, while in 1789 he announced the discovery of tellurium. Klaproth shares a three-way discovery of cerium with Wilhelm Hisinger (1766-1852) and Jons Jakob Berzelius (1779-1848). 

Among other ion-conducting phases with fluorite-like structures, note should be taken of Y4Nl)Ox s, (Y,Nb,Zr)O2 8 solid solutions, and their derivatives (see Refs. [89-91] and references cited therein). The total conductivity of Y4NbO8.5-8 is essentially independent of the oxygen partial pressure, which may suggest a dominant ionic transport [90]. However, the conductivity level in this system is rather low, and similar to that in pyrochlore-type titanates and zirconates, although some improvements can be achieved by the addition of zirconia. 

The investigations of SOEs with oxygen ion conductivity (mainly based on zirconia, ceria and gallates) and with protonic conductivity (mainly cerates and zirconates) included detailed studies of their ceramic and transport properties, contact resistance of grains in SOE, ageing processes depending on the mode of fabrication, temperature, composition of material, impurities, gas atmosphere, and other factors. 

A properly chosen support may stabilize a catalyst against sintering. In the case of NijS2 catalysts such supports are commonly used. Supports of this type include refractory oxides which do not react appreciably with hydrogen sulfide, for example alumina, silica, thoria, zirconia, and titania, as well as certain silicates, aluminates, thorates, zirconates, and titanates. Active carbon can be used as a support for NiS or NiaS2 catalysts for reactions which do not involve oxygen. 

T. Koyama, S. Hayashi, A. Yasumori, K. Okada, M. Schmiicker, H. Schneider, Microstructure and mechaiiical properties of mullite/zirconia composites prepared from alumina and zircon under various firing conditions, J. Europ.Ceram. Soc. 16 231—237 (1996). 

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