Silica high-temperature alloys

In the process of SiCfr preparation the main raw stock is ferrosilicon, an alloy of iron and silicon, made in shaft electric furnaces. Electrothermal ferrosilicon is produced from quartzite and iron chipping it is reduced with charcoal or coke (petroleum or metallurgical). The process is based on the endothermal reaction of silica reduction with carbon, which takes place at high temperature. 

Praliaud and Martin (77) proposed the formation of Ni-Si and Ni-Cr alloys on silica and chromia supports, respectively, under H2 at sufficiently high temperatures. They suggested that hydrogen spilt over from Ni to the Cr203 carrier and partially reduced it to Cr°, which was then alloyed with Ni as indicated by magnetic measurements. The same technique in conjunction with IR spectroscopy and volumetric adsorption of H2 was applied to partially reduced Ni-on-alumina and Ni-on-zeolite catalysts by Dalmon et al. (78). These supported Ni systems contained Ni° and Ni+. H2 was found to be activated only when the couple Ni°/Ni+ was present according to... 

After the room-temperature equilibria are known, a determination of the phases present at high temperatures can be undertaken. Powder specimens are sealed in small evacuated silica tubes, heated to the desired temperature long enough for equilibrium to be attained, and rapidly quenched. Diffraction patterns of the quenched powders are then made at room temperature. This method works very well in many alloy systems, in that the quenched powder retains the structure it had at the elevated temperature. In some alloys, however, phases stable at high temperature will decompose on cooling to room temperature, no matter how rapid... 

High-temperature treatment of silicon- or titanium-containing alloys leads to the formation of crystalline titania or silica surface layers with thicknesses in the micrometer range, as demonstrated by Seo et al., who used these materials for the construction of heat exchangers with catalyti-cally active surfaces [166]. However, the thermal treatment of nickel alloys that also contain iron leads to a layer of amorphous iron oxide, which is not suitable for depositing catalytic materials [167]. 

OTHER COMMENTS Silicon is the second most abundant element on earth, and constitutes about 25% of the earth s crust does not occur freely in nature, but is found as silica (e.g., sand, quartz, sandstone) or as various silicates (e.g., kaolinite, anorthite, etc.) used as a semiconductor in transistors, computer circuitry, rectifiers, etc. used for making alloys such as steel, aluminum, copper, bronze, and iron utilized in the manufacture of silanes and silicones has also been used as a reducing agent in high temperature reactions. 

The process described above in which a solute oxidizes preferentially to the parent element and forms a continuous layer on the surface is referred to as selective oxidation. The selective oxidation of elements which form a slowly growing, protective layer is the basis for the oxidation protection of all alloys and coatings used at high temperature. The only elements which consistently result in protective scales are Cr (chromia scale), Al (alumina scale), and Si (silica scale). Therefore, much research has been directed at finding alloy and coating compositions, which meet other property (e.g., mechanical) requirements and also form one of these scales. 

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