Chromium metal silicon dioxide

The metal layers frequently consist of chromium (semitransparent absorber metal) or aluminum (opaque reflector metal). Silicon dioxide or magnesium fluoride are the materials mostly used for the dielectric layers. In the case of pigment particles, there is a symmetrical arrangement of the layers, as shown in Figure 5.28, whereas optical coatings can also consist of a system of unsymmetrical layers. All these arrangements are the basis for an optical phenomenon called the Fabry-Perot effect... 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Chromium metal is commercially produced in the United States by the reduction of chromite ore with carbon, aluminum, or silicon, and subsequent purification. Sodium chromate and dichromate are produced by roasting chromite ore with soda ash. Most other chromium compounds are produced from sodium chromate and dichromate (Hartford 1979 Westbrook 1979). For example, basic chromic sulfate (Cr(0H)S04), commonly used in tanning, is commercially produced by the reduction of sodium dichromate with organic compounds (e.g., molasses) in the presence of sulfuric acid or by the reduction of dichromate with sulfur dioxide. Lead chromate, commonly used as a pigment, is produced by the reaction of sodium chromate with lead nitrate or by reaction of lead monoxide with chromic acid solution (IARC 1990). 

Deposition of a metallic film onto the silicon dioxide surface requires a good adhesion between the two layers. An enhancement of the adhesion can be accomplished by first depositing a very thin layer of titanium or chromium, 50-100 A in thickness, then the desired metallic film. 

As the above-mentioned formation of zirconium hydride shows, not only metals but also various compounds can be the end product of a thermitic process. Silicides, such as the compound BaSi from the peroxide and oxide with silicon, have been quoted by Ullmann from German patents. The lower oxide of titanium (TiO) has been described as obtainable from the dioxide and magnesium. A very hard form of artificial corundum (crystalline aluminum oxide) of superior quality for grinding and polishing operations is the byproduct of the above-described production of chromium metal. ... 

Eighty grams of the molybdenum dioxide and 21g of aluminum are used and worked up as under chromium (No. 2). A regulus of about 50g of metal is obtained with a purity of 98%. With smaller quantities the yield drops due to excessive volatilisation of the product. The metal contains traces of aluminum and silicon. 

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