Dense silicon carbide

Properties of Dense Silicon Carbide. Properties of the SiC stmctural ceramics are shown in Table 1. These properties are for representative materials. Variations can exist within a given form depending on the manufacturer. Figure 2 shows the flexure strength of the SiC as a function of temperature. Sintered or sinter/HIP SiC is the preferred material for appHcations at temperatures over 1400°C and the Hquid-phase densified materials show best performance at low temperatures. The reaction-bonded form is utilized primarily for its ease of manufacture and not for superior mechanical properties. 

K.M. Taylor, Cold Molded Dense Silicon Carbide Articles and Methods of Making the Same," U.S. Pat. No. 3 205 043. Sept 7,1965. 

Monolithic refractories are used in sections of cement kilns, typically at the outlet, where wear rates are very high. At these zones, dense silicon-carbide-based castables are often used. The cyclones in the preheater section of cement plants are lined with dense castables due to the complexity of their shape and the wear resistance required. The shell of the exhaust gas duct is made from both dense and insulating castable materials. In the cooler section of the plant, monolithic refractories are used to line the ceiling. 

High -performance engineering ceramics Diamond Dense alumina Silicon carbide Silicon nitride Zirconia Sialons... 

Silicon carbide, hot pressed Alumina, dense sintered Boron nitride, hot pressed Silicon nitride, hot pressed Boron carbide, hot pressed... 

Silicon 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 silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate 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 silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron 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 silicon nitride-bonded type exhibits improved resistance to cryolite. 

Silicon carbide s relatively low neutron cross section and good resistance to radiation damage make it useful in some of its new forms in nuclear reactors (qv). Silicon carbide temperature-sensing devices and structural shapes fabricated from the new dense types are expected to have increased stability. Silicon carbide coatings (qv) may be applied to nuclear fuel elements, especially those of pebble-bed reactors, or silicon carbide may be incorporated as a matrix in these elements (153,154). 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Carrillo-Heian, E.M., Carpenter, R.D., Paulino, G., Gibeling, J.C., Munir, Z. (2001), Dense layered molybdenum disilicide-silicon carbide functionally graded composites formed by field-activated synthesis , J. Am. Ceram. Soc., 84, 962-968. 

Future applications may involve use of SiC as substrates for silicon cliips, making use of the high thermal conductivity of SiC and its close thermal expansion match to silicon. The low density7 and high stiffness of silicon carbides may also result in applications in space. One such application is for space-based mirrors, making use of the high degree of surface polish possible on dense SiC. 

Only highly dense ceramics with high thermal shock resistance can be utilized for such strongly mechanically and thermally stressed components. These demands are met by hot-pressed silicon carbide produced at pressures of 350 bar and sintering temperatures of 1900 to 2000°C. The necessary, but very expensive, mechanical finishing is disadvantageous for industrial manufacture of hot-pressed silicon carbide. 

Silicon carbide has been manufactured commercially since 1891 and the current world market is about 500 000 tons. This material is dense and crystalline. It is only recently, however, that a porous form has been reported. These two forms can be regarded as the analogues of quartz (dense, crystalline silicon oxide) and silica gel (porous, amorphous silicon oxide). We were interested in the properties of the porous silicon carbide, and in particular its stability. It is not improbable that this be higher than that of silica in view of the four-fold coordination of carbon compared to the two—fold coordination of oxygen. Although data on the stabilities of dense forms are ayailable, the information is not necessarily relevant to the properties of porous forms. 

Dense SiC platelet reinforced silicon carbide composites were recently fabricated [52]. The highest sintered densities of 97-98% TD were achieved with 20% platelet contents using hot molding as the thermoplastic forming technique [53]. 

This opened up an inexpensive method of producing dense and complex parts consisting of pure silicon carbide (SSiC). The powders (see Fig. 8) can be molded into a green body by any of the methods used in ceramic molding [93,94], depending on the shape required and the number of pieces involved (see Table 5). Sintering is... 

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