Silicon carbide refractories

Atomic absorption spectrometry Electrothermal atomic absorption spectrometry (ETAAS) is at present the most commonly used technique for the determination of silicon in biological fluids. Owing to the formation of refractory silicon carbide and volatile silicon oxide compounds during the atomization cycle in the graphite tube. 

Neutral refractories (e.g. bauxite, lime. These are used on areas where slags and atmosphere are basic they are stable to alkaline Artificially used refractories Silicon carbide, zirconium carbide etc. 

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). 

Flameholder - Flameholders are necessary to prevent the flame from "riding" up to the top of the stack. They provide a surface at which burning can take place and also promote better mixing of air and gas by the additional turbulence which they cause above the jets. Construction is simply a solid, 25 mm diameter rod of refractory material (silicon carbide) supported horizontally above each burner line. The bottom of the rod should be 13 mm above the tips of the jets. 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

CVD silicon carbide fibers are a recent development with prom-ising potential which may take over some of the applications of CVD boron fibers or other refractory fibers, providing that the production cost can be reduced. 

In the drying of compound intermediates of refractory and reactive metals, particular attention is given to the environment and to the materials so that the compound does not pick up impurities during the process. A good example is the drying of zirconium hydroxide. After the solvent extraction separation from hafnium, which co-occurs with zirconium in the mineral zircon, the zirconium values are precipitated as zirconium hydroxide. The hydroxide is dried first at 250 °C for 12 h in air in stainless steel trays and then at 850 °C on the silicon carbide hearth of a muffle furnace. 

Refractories such as boron nitride, silicon nitride, silicon carbide, and boron carbide are of great importance for the production or protection of systems which can be operated in very high... 

Silicon carbide (carborundum) Talc Bluish-black, very hard crystals. Used as an abrasive and refractory material. A hydrous magnesium silicate used in ceramics, cosmetics, paint and pharmaceuticals. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Ceramic catalytic unit, 10 40 Ceramic composites systems, 5 552-554 Ceramic decoration, use of gold in, 12 693 Ceramic ferrites, 11 59 Ceramic fibers, 24 614, 618 refractory, 13 388 silicon carbide, 13 386 Ceramic-grade fluorspar, 4 579, 580 analysis, 4 577t Ceramic, high temperature... 

Silicon carbide (SiC), nearly as hard as diamonds, is used as an abrasive in grinding wheels and metal-cutting tools, for lining furnaces, and as a refractory in producing nonferrous metals. 

By far the most common industrial 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 stability under both reducing and oxidizing conditions. Carbon, graphite, and silicon carbide have been used both alone and in combination with the oxides. Refractories made from these materials are used in ton-lot quantities, whereas silicides are used in relatively small quantities for specialty application in the nuclear, electronic, and aerospace industries. 

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

The properties of silicon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal silicon carbide. The pressureless-sintered silicon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded silicon carbide refractories. Table 1 lists the properties of the fully compacted, high purity material. 

Because of high thermal conductivity and low thermal expansion, silicon carbide is very resistant to thermal shock as compared to other refractory materials. 

Resistivity. The temperature coefficient of electrical resistivity of commercial silicon carbide at room temperature is negative. No data are given for refractory materials because resistivity is gready influenced by the manufacturing method and the amount and type of bond. Manufacturers should be consulted for specific product information. 

One of the first important uses of silicon carbide was as an abrasive (see Abrasives). Additional markets in refractories, electrical devices, and metallurgy have since been developed. In 1988, the silicon carbide market was estimated at 55% metallurgical, 43% abrasive, and 2% for refractories and... 

Refractories. Its low coefficient of expansion, high thermal conductivity, and general chemical and physical stability make silicon carbide a valuable material for refractory use. Suitable applications for silicon carbide refractory shapes include boiler furnace walls, checker bricks, mufflers, kiln furniture, furnace skid rails, trays for zinc purification plants, etc (see REFRACTORIES). 

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