Sihcon carbide

Other forms of carbon-carbon composites have been or are being developed for space shutde leading edges, nuclear fuel containers for sateUites, aircraft engine adjustable exhaust nozzles, and the main stmcture for the proposed National Aerospace plane (34). For reusable appHcations, a siHcon carbide [409-21 -2] based coating is added to retard oxidation (35,36), with a boron [7440-42-8] h Lsed sublayer to seal any cracks that may form in the coating. 

Euture appHcations may involve use of SiC as substrates for siHcon chips, making use of the high thermal conductivity of SiC and its close thermal expansion match to siHcon. The low density and high stiffness of siHcon carbides may also result in appHcations in space. One such appHcation is for space-based mirrors, making use of the high degree of surface poHsh possible on dense SiC. 

Exposure limits for siHcon carbide and powders of zirconium compounds (including zirconium dioxide) have been estabHshed by ACGIH. TLV—TWA s are 10 mg/m and 5 mg/m, respectively. OSHA guidelines for zirconium compounds call for a PEL of 5 mg/m. There are no exposure limits for siHcon nitride powder, but pmdent practice suggests a TLV—TWA of 0.1 mg/m. The soHd ceramics present no apparent health hazard. In machining such ceramics, however, care should be taken to prevent inhalation of respirable particles in amounts in excess of estabHshed limits. Disposal should be in approved landfills the materials are inert and should pose no danger to the environment. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Manufacture of P-Silicon Carbide. A commercially utilized appHcation of polysdanes is the conversion of some homopolymers and copolymers to siHcon carbide (130). For example, polydimethyl silane is converted to the ceramic in a series of thermal processing steps. SiHcon carbide fibers is commercialized by the Nippon Carbon Co. under the trade name Nicalon (see Refractory fibers). 

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). 

Fig. 2. Fuel for high temperature gas-cooled reactor. Fissile material is coated with carbon and siHcon carbide, fertile material with carbon. Particles mixed...

Coated abrasives, such as sheets, disks, and dmms, are used for polishing and finishing. Here, too, the abrasives, such as aluminum oxide and siHcon carbide, have replaced the flint and garnet of common sandpaper. These industrial coated abrasives are manufactured from cloth or tough paper base. 

Fixed-Bed Vapor-Phase Oxidation of Naphthalene. A sihca gel or sihcon carbide support is used for catalyst involved in the oxidation of naphthalene. The typical naphthalene oxidation catalyst is a mixture of vanadium oxide and alkali metal sulfate on the siUca support. Some changes, such as the introduction of feed vaporizers, are needed to handle a naphthalene feed (14), but otherwise the equipment is the same. 

Silicon Carbide. Sihcon carbide is made by the electrofusion of siUca sand and carbon. SiUcon carbide is hard, abrasion resistant, and has a high thermal conductivity. It is relatively stable but has a tendency to oxidize above 1400°C. The siUca thus formed affords some protection against further oxidation (see Carbides). 

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. 

Besides the chemical industry, sihcon is used as a powder in the ceramics (qv) industry for the production of sihcon carbide and sihcon nitride parts (see Advanced ceramics). Sihcon powder is also used as an explosive for defense apphcations and in the refractory industry for plasma spraying with other oxide mixtures (see Refractory coatings). 

At equihbrium the vapors are predominantly hydrogen and sihcon tetrachlorides. However, these can be easily removed from the trichlorosilane and recycled. A once-common commercial manufacturing procedure for sihcon tetrachloride was the reaction of chlorine gas with sihcon carbide. 

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