Refractory Silicon Carbide Products

Despite its comparatively high price, silicon carbide is a significant refractory product due to its exceptional properties, such as its high thermal conductivity, high hardness and mechanical strength. It is used in zinc distillation kilns and in the manufacture of muffles, capsules and kiln furniture for the clay ceramic industry (see Section 5.5.4.7.2). In recent years silicon carbide has also been used in the refractory linings of blast furnaces and utilization in other sectors of the steel producing industry is in evaluation. 

A temperature resistant binder is necessary to bond SiC-bricks (carborundum bricks). Clays or other silicates are usually used, the particles being bonded by a glass phase. The bricks must be fired at ca. 1500°C in an oxidizing atmo.sphere to reduce the reduction of bonding clay to silicon and thereby prevent the bricks becoming brittle. The resulting oxidation of silicon carbide is limited by the formation of a passivation layer of Si02 on the SiC particles. 

Clay-bonded bricks have a SiC-content of 40 to 90%. Bricks without a glassy binder are obtained by bonding with silicon nitride. Since these SiC-rich bricks exhibit thermal shock resistance, Si3N4-bonding is of increasing importance. 

Coarse SiC products arc bonded with clay minerals or silicon nitride 

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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. 

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... 

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. 

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. 

According to the data in Table 25.5 and to Eq. (25.6) the compressive strength of filaments of refractory materials such as carbon and silicon carbide have compressive strengths about 10 times as large as those of organic fibres. This would seem to be a serious restriction to the use of organic polymers such as aramids in their application in composites. For most of the applications this restriction is of minor importance, however, since long before ac max is reached, instability in the construction will occur. The resistance of a column or a panel under pressure is proportional to the product of a load coefficient and a material efficiency criterion ... 

Impermeable silicon carbides of both types, sintered and reaction bonded, perform generally better than the permeable refractories as shown in Table 19-3 Both reaction bonded and sintered products can be exposed to higher temperature for longer periods of time with lower weight loss than the oxide, Si3N4 or Si20N2 bonded refractories. This is due to the lower surface area available for reaction and to the greater relative inertness of their bond phases. 

Silicon carbide refractories are employed in considerable quantities in kiln furniture and in muffles in the ceramic industry, where use is made of their thermal conductivity. The life depends on conditions it is impaired by access of air and water vapour. Oxidation results in volume expansion, embrittlement and cracking of the product. 

Calcines are products obtained by removing the volatile components of the waste, i.e., water and nitrate, at temperatures between 400 and 900° C. The result is a mixture of oxides of fission products, actinides, and corrosion products in particulate form with a specific surface of 0.1 to 5 ra /g. The plain calcine is not very stable chemically because of its large surface area and the chemical properties of some of the oxides, and it is highly friable. To improve the properties of calcines, advanced forms are developed. One such product is the so-called multibarrier waste form, a composite consisting of calcine particles with inert coatings, such as pyrocarbon, silicon carbide, or aluminum, embedded in a metal matrix. Another advanced calcine is the so-called supercalcine. This is essentially a ceramic obtained by adding appropriate chemicals to the HLW to form refractory compounds of fission products and actinides when fired at 1200°C. Supercalcine requires consolidation by embedding in a matrix but does not need to be coated, as the material is supposed to have inherent chemical stability. 

Lime is also used as a bonding and stabilising agent in the production of silicon carbide and zirconia refractories [32.4]. 

The chemical shift differences between reactants and products permit NMR to be used to follow the course of a reaction and to choose the optimum reaction conditions. In Fig. 3.53, the Si NMR spectra show that NMR can follow the process of making fS-SiC, a refractory ceramic, from polymethylvinylsilane and silicon metal. The NMR spectmm of the product silicon carbide (top spectmm) is clearly different from the spectmm of the starting mixture (bottom spectmm). In the bottom spectmm, the resonance at —18 ppm is due to the organo-silane the resonance at — 82 ppm is the elemental silicon signal. All reactant and product signals are well separated in chemical shift, so any unreacted starting material can be measured in the product and the production process can be optimized. 

Refractory bricks and other materials are used, both with and without a lining between the steel and bricks, to enclose reaction chambers. Depending on the conditions to be withstood, the products used include basic and neutral materials with high silicon carbide bricks. The structure of a chemically resistant brick lining is shown schematically in Figure 20.80. 

For applications that require a particularly high grade of silicon carbide, such as industrial ceramics, the refractory industry, and ceramic-bound abrasives, the granular product must be washed with alkali or acid to remove adhering traces of elemental silicon, metals, metal compounds, graphite, dust, and silica. 

Ultrafme refractory carbide and nitride powders can be produced in an inductively coupled radio-frequency (rf) plasma torch.a schematic drawing of a plasma torch for the production of silicon carbide powder is shown in Fig. 14.1. 

Different refractory ceramics are used for the filter matrix. Most common are cordierite-, aluminum titanate-, and silicon carbide-based products [15, 17-21]. Typical physical properties for these materials are provided in Table 20.1. AU materials can withstand very high temperatures. Key differences between these... 

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