Refractories oxide-based

Imanaka, N., Oda, A., Tamura, S. and Adachi, G.-Y. (2004) Total nitrogen oxides gas sensor based on solid electrolytes with refractory oxide-based auxiliary electrode. J. Electrochem. Soc., 151 (5), H113-16. 

The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems (47). The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

Other important properties of Cr02 when used as a magnetic pigment are its black color, electrical conductivity (2.5-400 O-1 cm-1 [5.27]) and relatively high crystal hardness (Mohs hardness 8-9 [5.20]). Therefore, coating formulations based on Cr02 require less or even no additives such as carbon black (good conductivity, black color) or refractory oxides such as alumina. 

The protonic concept of acid and bases is applicable to many of these high lemperatures solvent systems such as the fused ammonium salts which possess the oniunt ion or solvated proton, and the fused anionic acids which arc salts possessing a metallic ion and a hydrogen containing anion. One of the most useful of the anionic acids is KHF which is used to dissolve ore minerals containing silica, tilania and other refractory oxides. 

Several years ago, Taylor et al. (2,3) showed that the oxygen plasma-etch resistance of carbon-based polymers could be markedly enhanced by incorporating certain atoms into the polymer chain. Particularly effective were those elements such as silicon or titanium that form a refractory oxide during oxygen RIE. The oxide is formed at the surface of the resist and greatly retards the subsequent etching rate of the remaining resist. 

Vaporization rates based on weight loss at T were repoted by Noguchi (8) for MgO and six other refractory oxides. Vaporization of MgO near 2000 K has been studied by Langmuir-torsion (IJ ), Langmuir-weight-loss (, 1 ), Knudsen-mass-spectrometric (14), Knudsen-effusion (15) and transpiration (1, 16) methods. Vaporization near 2000 K is primarily to the elements (14). Hg(g) and presumably 0(g) and 02(g), rather than to MgO(g). Quantitative interpretation of the vaporization species is uncertain. The adopted tables (1 ) predict that MgO(g) is insignificant near 2000 K but should become one of the significant components of the vapor near... 

The highest melting points of all the known compounds are exhibited by carbides, nitrides, borides and oxides. Since the first three groups of substances are unstable in an oxidizing atmosphere at high temperatures, most industrial refractories are based on oxides. 

Refractory oxides are an important class of materials that enable processes to exploit extreme environments. A wide variety of unary, binary, and ternary oxides can be considered refractory, based on their melting temperatures. Refractory oxides are generally prepared from powdered precursors using standard ceramic forming techniques such as casting, pressing, or extrusion, and subsequently sintered to achieve final density. In addition to chemical compatibility, the physical properties of refractory oxides such as thermal expansion coefficient, thermal conductivity, modulus of elasticity, and heat capacity must be considered when selecting an oxide for a specific application. 

One early change was the substitution of cerium oxide by mixed oxides based on the latter. Most interesting for this are solid solutions of cerium and zirconium dioxides (with typically Zr cation fractions between 0.3 and 0.5), which have two main advantages firstly, they are more refractory than pure ceria (have higher melting point) and consequently lose surface... 

The material constituting the trays is produced by the Carborundum company (Saint Gobain group). The material is a SiC-based refractory (around 85% SiC), with an oxide-based intergranular phase. In this study, only specimens cut from actual trays, being more representative, have been considered. However, it is important to note that all the analyses presented here have been obtained on small pieces of material because of the large amount of material involved in a column, some variability in the composition may be observed. 

G. Emig, R. Wirth and R. Zimmermann-Chopin, Sol-gel based precursors for manufacturing refractory oxides, J. Mater. Sci., 29,45594566 (1994). 

Refractory ceramic materials are classified as brittle for x = 1 and relatively brittle for < 1- Additional a-e relations for nitride- and oxide-based ceramics are shown in Figs. 4.2 and 4.3. In Fig. 4.2, the relations of two values of x are shown, one of them for y = 1. 

Table 3.2-9 Physical properties of oxides and oxide-based high-temperature refractories [2.4]...

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