Sintered alumina oxides

Chaipin et al. [C3] and Massignon [MS] have described several types of diffusion banier developed in France and given examples of their characteristics. Materials from which these barriers were made include sintered alumina, oxidized aluminum, Teflon, and nickel. Pore radii were in the range of 0.01 to O.OS fan. Barriers developed in Sweden have been described by Martensson et al. [M3]. 

Sol—Gel Sintered Aluminum Oxide. A new and much more versatile sintered alumina abrasive is now produced from aluminum monohydrate, with or without small additions of modifiers such as magnesia, by the sol—gel process (see Sol-gel technology). The first modified sol—gel abrasive on the market, Cubitron, was patented (27) and produced by the 3M Corporation for products such as coated belts and disks. The success of this material promoted intensive research into sol—gel abrasives. 

Manufactured abrasives include silicon carbide, fused aluminum oxide, sintered aluminum oxide, sol-gel sintered aluminum oxide, fused zirco-nia-alumina, synthetic diamond, cubic boron nitride, boron carbide, slags, steel shot, and grit. 

During the late 1970s, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO, after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering as oxidation, followed by reduction at moderate temperatures, restored the adsorption properties of the metal in full. The suppression of adsorption after high-temperature reduction was attributed to a strong metal support interaction (SMSI) [2]. 

The typical and most widely employed representative of oxide ceramics is sintered alumina on which it is convenient to demonstrate the characteristic features of this group of materials. 

The industrially most important oxide-ceramic material is sintered aluminum oxide. The raw materials used are so-called calcined alumina and melted corundum. 

Venkatachalam and Kuriacose and Uma et al have observed that in the presence of AI2O3, those properties are lost and it becomes a dehydration catalyst, especially if the catalyst is prepared by co-precipitation as opposed to impregnation. The oxides appeared to interact, and this view is strongly supported by the fact that sintered Fe203-Al203 catalyst retains and improves its dehydration activity, whereas on sintering, alumina usually loses dehydration activity. 

As heated alkalies readily attack stainless steel, it was found necessary to use liners or crucibles in the above bombs. Borosilicate glass was found satisfactory up to about 450 °C. for short periods of time, but it could not be used above this temperature. Various ceramic materials were tried for possible use as containers for the sodium peroxide-superoxide mixtures. Sintered aluminum oxide and magnesium oxide were satisfactory up to 450 °C., but above this temperature the sodium oxides penetrated the sintered material and corroded the stainless steel bomb. Commercial ceramic coatings containing (as the principal components) alumina, magnesia, and titania were also tried without success at a temperature above 450 C. 

Sintered oxides with high m.p., single phase materials AljOj, MgO, BeO, ZrOz, ThOz Porous Sintered alumina, magnesia, beryllia, zirconia and thoria... 

A small sintered alumina vessel containii IifeOb is fluorinated in a quartz tube (in an apparatus similar to that used for the preparation of TIF3). After gentle initial heating, the reaction proceeds (occasionally with incandescence) without additional supply of heat. The progress of the conversion may be checked since the yellow oxide becomes colorless and an increase in volume takes place simviltaneously. To obtain a completely oxide-free preparation the product must be kept for several hours at 500°C in a nickel tube, while a stream of fluorine is passed over it. 

I. Strontium oxide can be obtained by heating pure SrCOs in a stream of Hg at 1300°C for several hours, by dehydration of Sr(N03)g 4H20 with subsequent calcining at 1100°C for 1.5 hours, or by dehydration of Sr(OH)2 above 850°C. If a high-purityproduct is desired the heating (as in the case of BaO) is done in a stream of Na or H2, free of Og and CO3. The vessels must be made of nickel or sintered alumina. 

The heating of the fiber up to 1000 - 1200 °C decreased the value of specific surface because of sintering of oxide particles. The fibrous alumina-based materials annealed above 1400 °C become more sintered, their porosity did not exceed 30 %, the specific surface decreased down to 5 m2/g, and the grain sizes were 100-150 nm. At the same time, the strength of this material was enhanced. 

RBAO involves a similar production route, but in this case the aluminum metal powder is a minor component in a powder mix that contains a substantial fraction of alumina. Oxidation of the aluminum metal by gas percolation yields a metastable oxide phase, which is replaced at higher temperatures by the stable corundum phase. Again, the large specific volume of the oxide product reduces the volume fraction of porosity, but in this case the thermal cycle actually sinters the product, and for the optimized powder formulation, expansion during oxidation of the metal at a moderate temperature is followed by the sintering contraction at a higher temperature to yield a dense alumina product whose final shape and volume is still very close to that of the green powder preform. 

Recently, Vonau et. al [27] have suggested an all-solid reference electrode consisting of a sintered Ag/AgCl mixture, which is embedded in a solid melt of KCl inside a cylindrical hollow body of porous alumina oxide ceramics. The outer surface of the ceramics is coated with a chemical-resistant insulating layer. At the bottom end, a circular area is held free from the coating and acts as diaphragm. 

Thorpe s Ratio. A formula suggested by Prof. T. E. Thorpe in 1901 for assessing the probable solubility of a lead frit the sum of the bases expressed as PbO divided by the sum of the acid oxides expressed as Si02 should not exceed 2. Thread Guide. Porcelain thread guides are satisfactory for use with cotton, wool, or silk man-made fibres, e.g. rayon and nylon, are more abrasive and sintered alumina or synthetic sapphire thread... 

Shimohira T., Makishima A., Kotani K., Wakakuwa M. Sintering of monodispersed amorphous silica particles. Proceedings of the International Symposium on Factors in Densification and Sintering of Oxide and Non-oxide Ceramics, October 3-5, 1978, Hakone, Japan Smith J.P., Messing G.L. Sintering of bimodaUy distributed alumina powders. J. Am. Ceram. Soc. 1984 67 238-242... 

Improvements in this procedure have been reported. The procedure proposed by Karpov and Natansov (26) is as follows a nickel-iron alloy containing 85 to 90 % nickel (e.g. 10 g) is fused in a sintered alumina crucible and saturated with oxygen. Then the apparatus is evacuated and the sample put into the melt. Thereby the carbon present is oxidized to carbon dioxide, which, in the case of titanium, leaves the melt for 95 % in 2 min, and is measured by gas chromatography. 

Schaper H, Amesz DJ, Doesburg EBM, et al The influence of high partial steam pressures on the sintering oflanthanum oxide doped gamma alumina, Appl Catal 9(1) 129—132,1984. 

We may lastly underline that thanks to the availability of alumina powder on the market with varied characteristics and the relative ease of sintering, alumina constitutes the first choice for ceramics with mechanical usages. It is only subsequently that insufficient performances for a given appUcation compel us to consider another oxide, even a non-oxide, whose preparation and use have the aimoying tendency of reminding us that we Uve in an oxidizing atmosphere ... 

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