Phase silicon carbides

Sintered silicon carbides are formed by all the traditional methods as well as standard plastic forming techniques such as injection molding. These compacts are sintered with small amounts of additives at very high temperatures in inert atmospheres to form essentially a single phase silicon carbide structure. 

P.B. Pickar et al [Engineering Sciences Lab., Dover, NJ, Research on Optical Properties of Single Crystal of Beta Phase Silicon Carbide. Summitry Tech. Rept. no. OR8394, May 5, 1965-June 5, 1966. Contract no. DA-28-017-AMC-2002, A, Oct. (1966) 85p. AD-641 198 ]... 

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. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Seal Face Combinations The dynamic of seal faces is better understood today. Seal-face combinations have come a long way in the past 8-10 years. Stellite is being phased out of the petroleum and petrochemical applications. Better grades of ceramic are available, cost of tungsten has come down, and relapping of tungsten are available near most industrial areas. Silicon carbide is being used in abrasive service. 

Silicon carbide (SiC) is a major industrial material with a considerable number of applications. CVD plays a significant role in its development and production, SiC is a covalent carbide with two phases a and [3. The phase of major interest here is pSiC, which has a cubic zinc blend structure. It is the one reported here. 

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). 

Gas-phase process, of ethylene-propylene polymer manufacture, 10 711 Gas-phase reactions flow mixing for, 14 613 pressure and, 14 623 Gas-phase reactor (GPR), 20 533 Gas-phase sedimentation, 18 142 Gas-phase synthesis, in silicon carbide manufacture and processing, 22 533 Gas pipeline systems, 12 366 Gas pretreatment, 13 841 Gas processing, in petroleum refining, 18 663... 

Silicon carbide is a very hard snbstance with a Young s modulus of 424 GPa [1]. It is chemically inert and reacts poorly (if at all) with any known material at room temperature. The only known efficient etch at moderate temperatures is molten KOH at 400-600°C. It is practically impossible to diffuse anything into SiC. Dopants need to be implanted or grown into the material. Eurthermore, it lacks a liqnid phase and instead sublimes at temperatures above 1,800°C. The vapor constituents during sublimation are mainly Si, SqC, and SiC in specific ratios, depending on the temperature. 

A review article on the CVD processes used to form SiC and Si3N4 by one of the pioneers in this area, Erich Fitzer [Fitzer, E., and D. Hegen, Chemical vapor deposition of silicon carbide and silicon nitride—Chemistry s contribution to modem silicon ceramics, Angew. Chem. Int. Ed. Engl, 18, 295 (1979)], describes the reaction kinetics of the gas-phase formation of these two technical ceramics in various reactor arrangements (hot wall, cold... 

The carriers of anomalous Ne-E (two forms of which were now known) and Xe-S were quickly identified. Neon-E(H), which is released at temperatures above 1200 °C in stepped heating experiments, and Xe-S were found to be carried in presolar silicon carbide (Tang and Anders, 1988). Neon-E(L), which is released below 900 °C, was found to be carried by presolar graphite (Amari et al., 1990). Once these presolar compounds were shown to be present in meteorites, studies were carried out to identify all of the different types of meteorites that carry presolar grains. Concentrated searches for other presolar phases were also initiated, and many new types of presolar grains have been found. This work is just beginning, however, and we cannot yet account for the majority of the presolar components that must have been present in the Sun s parent molecular cloud. 

Most presolar silicon carbide and oxide grains and a significant fraction of presolar silicate grains found in meteorites come from low- to intermediate-mass stars in the asymptotic giant branch (AGB) phase (see Chapter 3). Evidence for this conclusion derives from two sources (1) spectroscopic observations of the envelopes of these stars and (2) comparison... 

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. 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of silicon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-silicon carbide formed by transformation from the p-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

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