Sihcides

Siliconizing is yet another process used especially for coating of the refractory metals Ti, Nb, Ta, Cr, Mo, and W (see Refractories). These metals form siHcides which have a surface oxidation protection layer of Si02. Siliconizing is especially effective on molybdenum against air oxidation up to 1700°C. 

In most of the nonmetaHurgical uses of molybdenum compounds the metal is coordinated by oxygen or sulfur ligands. Molybdenum nitrides, carbides, and sihcides are, however, coming under increasing study for various appHcations. Roughly 75% of all molybdenum compounds are used as catalysts in the petroleum and chemicals industries. 

Advanced Structural and Heating Materials. Molybdenum siHcide [12136-78-6] and composites of MoSi2 and siHcon carbide, SiC, have properties that allow use as high temperature stmctural materials that are stable in oxidizing environments (see Composite materials Metal-matrix composites). Molybdenum disiHcide also finds use in resistance heating elements (87,88). 

There is great interest in the use of thin-film platinum siHcide in siHcon-integrated circuit technology. Platinum siHcide has low resistance and high... 

FdSe[12137-76-7], PdSe2 [60672-19-7], PdTe [12037-94-4], and PdTe2 [12037-95-5], Borides, sihcides, phosphides, and arsenides are also known (23,24). 

By far the most common iadustrial refractories are those composed of single or mixed oxides of Al, Ca, Cr, Mg, Si, and Zr (see Tables 1, 4, and 6). These oxides exhibit relatively high degrees of stabiHty under both reduciag and oxidizing conditions. Carbon, graphite, and siHcon carbide have been used both alone and ia combination with the oxides. Refractories made from these materials are used ia toa-lot quantities, whereas siHcides are used ia relatively small quantities for specialty appHcation ia the auclear, electronic, and aerospace iadustries. 

Sihcide coatings of refractory metals may contain as much as three to five coating components other than siUcon. A mixture of haUde carriers is selected containing the best carrier for each component. 

Diffusion alurninide and sihcide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 10 /yr in North America and about 50 x 10 worldwide. 

Rare-Earth Silicides. Rare-earth sihcides, in the form of a ferroalloy that contain up to 33% rare earths, are used increasingly by the iron and steel industries. Whereas the term sihcides is no longer used for alloys of this type, it is stih in common usage for these materials. Eor nodular iron, addition... 

Dkect synthesis is the preparative method that ultimately accounts for most of the commercial siUcon hydride production. This is the synthesis of halosilanes by the dkect reaction of a halogen or haUde with siUcon metal, siUcon dioxide, siUcon carbide, or metal sihcide without an intervening chemical step or reagent. Trichlorosilane is produced by the reaction of hydrogen chloride and siUcon, ferrosiUcon, or calcium sihcide with or without a copper catalyst (82,83). Standard purity is produced in a static bed at 400—900°C. 

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

For the binary alkaline earth sihcide SiSr, two different structures have been reported. One contains one-dimensionally extended zigzag chains beside isolated Si" atoms [73]. Schafer et al. prepared a modification with the same composition, which instead contained isolated sUicide units of ten atoms. In these units, planar hexagons are substituted in the 1-, 2-, 4-, and 5-ring positions by four additional Si atoms. An isostructural compound was found for germanium as well, but showing defects in this unit in the positions 1, 2, 4, and 5. Both materials could not be obtained from stoichiometric approaches, and their formation obviously is coupled to strontium excess [69] (Fig. 3). 

This section presents a brief overview of a few other compounds that have not been described in previous sections. Because it can function as a nonmetal, silicon forms sihcides with several metals. These materials are often considered as alloys in which the metal and silicon atoms surround each other in a pattern that may lead to unusual stoichiometry. Examples of this type are Mo3Si and TiSi2. In some sihcides, the Si-Si distance is about 235 pm, a distance that is quite close to the value of 234 pm found in the diamond-type structure of elemental silicon. This indicates that the structure contains Si22-, and CaSi2 is a compound of this type. This compound is analogous to calcium carbide, CaC2 (actually an acetylide that contains C22- ions (see Chapter 10)). 

The silicides of the actinides have not been widely characterized. A uranium sihcide of the form U3Si2 has received the maximum amount of study owing to its nse as a dispersion fuel in research reactors. Another common phase observed for actinide silicides is AnSi2. One of the common synthetic routes to the silicides is the reaction of actinide fluorides, for example, Anp4, with elemental Si at high temperatures. One of the thermodynamic driving forces for this reaction is the release of Sip4. 

The formation of silane from magnesium siHcide by acidic hydrolysis is thought to involve one of two paths after formation of a dihydroxymagnesiosilane intermediate (56). 

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