Silicon carbide 1114 Subject

Zinc oxide in solid or fine particle form is kept in a reactor cavity that is subjected to irradiation from solar concentrators [92], The dissociation products are zinc (vapor) and oxygen for this first reaction AG=0 at about 2235K [91], The reactor is made of materials like inconel steel, zirconia, silicon carbide or graphite [68,89,92], The graphite is used in special designs to avoid direct contact with chemical species [68], The dissociation products are then cooled rapidly to separate zinc and oxygen, transporting the... 

An important application of polydimethylsilane is as a source of silicon carbide (SiC) fibres, which are manufactured under the trade-name Nicalon by Nippon Carbon in Japan. Heating in an autoclave under pressure converts polydimethylsilane to spinnable polycarbosilane (-Me2Si-CH2-) with elimination of methane. The spun fibres are then subjected to temperatures of 1200-1400 °C to produce silicon carbide fibres with very high tensile strengths and elastic moduli." As a result of their conductivity, polysilanes have also been used as hole transport layers in electroluminescent devices. In addition, the photoconductivity of polymethylphenylsilane doped with Cgo has been found to be particularly impressive. ... 

When a composite is subjected to external forces, the energy of the matrix is only transferred to the fibres when there is question of a proper attachment. For that reason fibres are some-times provided with a layer of another material. An example boron fibres in an aluminium matrix are provided with a silicon carbide coating and as a result the fibres are called borsic fibres. The thermal expansion coefficient of a fibre and its matrix must correspond. Figure 14.9 is a representation of what takes place when a crack in a fibre-reinforced matrix grows. 

Silicon carbides are generally synthesized by the pyrolysis of precursors, prepared by liquid phase methods. One possible way for precursor synthesis is the addition of carbon black or sucrose, to a gelling silica.8 In this method, the carbon is introduced from an external source. A more intimate contact between the carbon and silicon in the precursor is assured with the use of organometallic polymer precursors. The use of silane polymers for silicon carbide production was initiated by Yajima.9,10 Polymers having a -[Si-C]- backbone are crosslinked and pyrolysed to yield SiC." In the initial work, dimethyldichlorosilane was used as a starting monomer, which was subjected to a sodium catalyzed polymerization (reaction (C)). 

From these results it has become apparent that both polymerized and monolayer aminosilane coatings can be converted to a silicon carbide layer. A further assessment and fine-tuning of carbide layer thickness can be effectuated from controlled variation of the modification parameters. This is subject to further studies. 

The form that silicon carbide takes depends on many factors including thermal history, impurity type and level, and environment. The p form is generally felt to be the stable phase at low temperatures, whereas the a form is the high-temperature form. There are many exceptions to the rule, as the conversion to a from /3 and the converse have been reported. The stability and transformations of the various polytypes vary among themselves and constitute a subject that is too broad for this effort. The basic a and p descriptors will be used for the remainder of this section. 

Calcium-silicon, ferro-silicon, silicon, silicon carbide, magnesium, zinc and its alloys, copper, and the rare metals have all been the subjects of patents regarding the production of ellccts similar to those of aluminium. 

Petroleum coke, obtained by subjecting the distillation bottoms to pyrolitic polymerization and thermal decomposition in a delayed or fluid coker, is used in the manufacture of calcium and silicon carbides, electrodes for aluminum manufacturing, and graphite. It also finds use as a utility fuel and as a construction material in foundry and blast furnaces. 

Shaped catalyst bodies with optimized geometries (e.g., wagonwheels, honeycombs) offer lower resistance to gas flow and lower the pressure loss in reactors. The mechanical and thermal stabihty of catalysts and supports is being improved. New support materials such as magnesite, silicon carbide, and zircon (ZrSi04) ceramics with modified pore structures offer new possibilities. Meso- and macropores can be incorporated into solids to accelerate transport processes, and the question of porosity will increasingly be the subject of interest. 

The samples generated for this paper were all produced in the following fashion. Firstly, a slip was developed with silicon carbide powder and a carbon based binder. The SiC powder type, 6H or 3C, and carbon content were systematically varied in the slip formulation. Secondly, a preform was cast from this slip. Finally, the preform was subjected to a reactive infiltration step in which the preform was brought into contact with a molten silicon bath under vacuum. 

Composites frequently consist of a material with embedded fibers, which in the example just given is epoxy plastic with embedded carbon fibers. The epoxy plastic is strong but subject to fracture. The embedded fibers impede a fracture that might start, thus enhancing the stability of the structure. Another example of a composite is silicon carbide ceramic with embedded silicon carbide fibers. These fibers are made by pyrolysis of a polymer of dimethylsilane, (CH3)2SiH2. By itself, silicon carbide ceramic is brittle, like most ceramics. The silicon carbide fibers, however, break up any fracture lines that may form, making the composite much less brittle. 

---