Purity silicon carbides

The treatment assumes that the point defects do not interact with each other. This is not a very good assumption because point defect interactions are important, and it is possible to take such interactions into account in more general formulas. For example, high-purity silicon carbide, SiC, appears to have important populations of carbon and silicon vacancies, and Vsj, which are equivalent to Schottky defects, together with a large population of divacancy pairs. 

High purity silicon carbide as a light yellow powder is made from SiCU hydrolyzed to silica gel in a sugar solution and heating in inert atmosphere at 1800 Cfor4hr(661). 

Polymethylsilane as a Precursor to High Purity Silicon Carbide... 

POLYMETHYLSILANE AS A PRECURSOR TO HIGH PURITY SILICON CARBIDE... 

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. 

Electrical Properties. The electrical properties of silicon carbide are highly sensitive to purity, density, and even to the electrical and thermal... 

V. Venkateswaran, J. M. Halstead, and B. Mehosky, "Synthesis of High-Purity Sinterable Silicon Carbide Powders," in Proceedings of the 23rd... 

U. Schaffer and V. Krivan, Analysis of High Purity Graphite and Silicon Carbide by Direct Solid Sampling Electrothermal Atomic Absorption Spectrometry, Fresenius J. Anal. Chem. 2001,371, 859 R. Nowka and... 

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. 

The values of the creep parameters (p, n and Q) identifying the superplastic behaviour of ceramic-related materials are not unique to such materials, nor to the same type of materials. As shown in the review papers, these parameters are very similar in tension as in compression in zirconia-based materials (probably the most widely studied ceramics in the widest experimental conditions), although that depends strongly on the purity of the ceramics 5,7 however, their behaviour seems to be very different in compression than in tension when an aid-sintering phase is necessary during the processing, as in silicon carbide and silicon nitride ceramics.8... 

By 1980, special applications accounted for approximately 11% of the total coke production in the United States. These uses include titanium pigments, carbon raisers and synthetic graphite (3 ). A specialty use of green coke is as a high purity reactant in the production of calcium and silicon carbide. (4)... 

Among nonoxides, silicon carbide and silicon nitride are two very important ceramics. Both are very hard and abrasive materials, and show excellent resistance to erosion and chemical attack in redudng environments. In oxidizing environments, any free silicon present in a silicon carbide or silicon nitride compact will be oxidized readily. Silicon carbide itself can also be oxidized at very high temperatures, the exact temperatures being a function of purity and... 

Sea sand is primarily comprised of silicon dioxide (silica), which may be converted to elemental silicon (96-99% purity) through reaction with carbon sources such as charcoal and coal (Eq. 2). Use of a slight excess of Si02 prevents silicon carbide (SiC) from forming, which is a stable product at such a high reaction temperature. Scrap iron is often present during this transformation in order to yield silicon-doped steel as a useful by-product. 

Coke is employed for a number of purposes, but the major use is in the manufacture of carbon electrodes for aluminum refining that requires a high-purity carbon— low in ash and sulfur free. In addition, petroleum coke is employed in the manufacture of carbon brushes, silicon carbide abrasives, and structural carbon (pipes, Rashig rings, and so on), as well as calcium carbide manufacture from which acetylene is produced. 

Alloys of chromium and silicon are readily obtained by heating chromium sesquioxide with excess of silicon at 11 white heat, or with silicon carbide, or silicon carbide and carbon, in the electric furnace or by strongly heating chromium sesquioxide, silica, and aluminium. From these alloys several definite silicides have been isolated, which are usually grey in colour, hard and brittle, and very resistant to acids, except hydrofluoric acid, which readily, decomposes them. The silicides, CrjSi, CrjSi, CrgSia, and CrSij, have been obtained in a state of comparative purity by special methods of preparation. ... 

Silicon carbide is produced by reduction of Si02 with carbon (coke) in electric resistance furnaces at temperatures of about 2200 °C, According to the purity of the raw materials, the product is green or black in colour and is composed of intergrown SiC crystals. 

For the preparation of technically important metal carbide and metal nitride materials the application of organosilicon compounds as preceramic precursors is advantageous under certain conditions [1-5]. Compared with the conventional metallurgical powder process, one benefit is the utilization of very low process temperatures for the preparation of individual ceramic materials. Another improvement is the high purity of the ceramics obtained from tailor-made preceramic precursors. Usually, after pyrolysis organosilicon compounds afford silicon-containing ceramic powders Likewise, they can also be used under certain conditions for the production of silicon carbide or silicon nitride fibers. 

Silicon carbide is manufactured industrially by the electrochemical reaction of high purity quartz sand with carbon in an electric resistance furnace (Acheson process) ... 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

NaOH or KOH 318 380 Au, Ag, Ni Powerful basic fluxes for silicates, silicon carbide, and certain minerals (main limitation is purity of reagents)... 

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