Silicon carbide excess carbon

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. ... 

A trivalent hard chromium bath has recently been described . The bath contains potassium formate as a complexing agent, and thicknesses in excess of 20 m can be deposited. Hardnesses of up to l650Hy can be obtained by heat treatment at 700°C. The deposits contain 1.6-4.8% carbon, and the bath is suitable for the deposition of composite deposits containing diamond or silicon carbide powder. 

The ceramic products obtained in the pyrolysis of the "combined" polymers have not been studied in detail, but some of them have been analyzed for C, N, and Si. The compositions of the ceramic materials obtained cover the range 1 Si3N4 + 3.3 to 6.6 SiC + 0.74 to 0.85 C. Thus, as expected, they are rich in silicon carbide and the excess Si which is obtained in the pyrolysis of the [(CH3SiH)x(CH3Si)y]n materials alone is not present, so that objective has been achieved. By proper adjustment of starting material ratios, we find that the excess carbon content can be minimized [11]. 

A considerable amount of subsequent research and process development has been carried out to produce silicon carbide with a reduced level of excess carbon via processes that allow more facile cross-linking.2 -32 Several hundred papers and patents on this topic exist in the literature, and only a few examples will be mentioned here. One process development involves the slurry spinning of fibers in place of melt spinning.33 In this process, silicon carbide powder, made by a conventional industrial process, is dispersed in a solution of carbosilanes in toluene. The syrupy paste is spun into fibers and then pyrolyzed to silicon carbide. These fibers are reported to be stable at 1,500 °C for 120 hours. 

Neckers and coworkers reported a similar polymerization using (chloromethyl)dichloro-methylsilane as a starting material (equation 15)162. Oligomers were obtained in addition to cyclic trimers and tetramers. In the presence of a platinum catalyst and tetravinylsilane, the oligomer crosslinked when irradiated to form a hard, porous material that could be pyrolyzed to a silicon carbide char containing excess carbon. A later report described other platinum photo- and thermal catalysts that could be used in this crosslinking reaction163. 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosUanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

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. 

In the presence of excess carbon concentration or low SiOz, silicon carbide is the major phase present, as shown in the Fig. 6. With decreasing carbon content, the reduction product of Si02 becomes gaseous SiO. In the presence of excess carbon, the SiC is synthesized via a gaseous intermediate according to ... 

What mass of silicon carbide should result when 1.0 kg of pure sand is heated with an excess of carbon ... 

The preparation of a porous silicon carbide has been described by Fox and co-workers (10). The synthesis is based on heating the organosilicon pol3nmer (0 1158iOj 5) at 1600 C under argon (see Fig. 1). The pyrolysis reaction results in an intramolecular carbothermic reduction, i.e. the carbon bonded to silicon is used to remove oxygen and to form the carbide (the commercial manufacture of silicon carbide uses an external source of carbon for example, by mixing quartz sand and petroleum cokes). The product is purified by oxidation to remove excess carbon, followed by treatment with HF to remove silica. 

Artifacts obtained by reaction bonding in a process known for over 15 years as the REPEL process, which is basically still that developed by British technologists. Green products are pressed from silicon carbide powders usually made by the Aecheson process from silica and carbon heated to in excess of 2200°C, with excess graphite present. Molten silicon then infiltrates the connected pores where, on further heat treatment, reaction with the extra graphite produces SiC in the interpore space. This secondary SiC is referred to as reaction sintered. The final body has about 12% free unreacted silicon. 

As with silicon nitride, solid forms of silicon carbide are made by one of three processes sintered, hot-pressed, or reaction-sintered. Sintering requires use of an additive such as alumina, carbon, or boron to promote liquid-phase sintering. Hot pressing is done at 2000°C and results in a very hard, dense material. Reaction sintering occurs when a mixture of SiC powder and carbon is heated in contact with molten silicon. The reaction results in a nearly complete conversion to SiC, although according to Schwartz, most materials made by this process contain an excess of carhon or silicon. ... 

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