Pores silicon carbide

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Composite samples are sectioned with a diamond saw and mounted in cold curing epoxy resin. Because of their porous nature, the composites are infiltrated under vacuum and subsequently cured under pressure in order to force the mounting resin into the pores. Mounted samples are ground flat on 240 grit silicon carbide paper, finely ground with a 9 pm oil-based diamond slurry and finally polished with a 1 pm diamond slurry and a 50 nm silica suspension. 

Reaction bonded silicon carbides are formed by all of the above techniques. They are then fired in an atmosphere where large amounts of silicon metal is available to react with carbon in the compacted part to form a silicon carbide bond at high temperatures. Residual silicon is left in the pores of these products after firing. 

Zhao[266] demonstrated the successful synthesis of highly ordered mesoporous silicon carbides with unusually high surface areas (430-720 m2/g), uniform pore sizes (<3.5 nm), and extremely high thermal stabilities (up to 1400 °C) replicated by mesoporous silica hard templates via a one-step nanocasting process. Highly ordered 2-D hexagonal (p6m) and bicontinuous cubic (Ia3d) SiC nanowire arrays have been cast from the hard templates, mesoporous silica SBA-15 and KIT-6, respectively. 

Infiltration combines a melt with a porous free-standing solid (the preform ). In the main and defining step of the process, the melt flows into open pores of the preform after solidification a new material results. Composites of all classes (polymer, ceramic and metal) are produced by this process, as are compounds such as reaction bonded silicon carbide. The process can also be adapted to make open-pored foams of carbon, ceramic, polymer or metal. 

Four examples were given in the patent, all using supercritical propane with polysilane and aluminum isopropoxide, so-called precursor ceramic materials. In one instance scanning electron microscopy was used to demonstfate that polysil me was deposited as a smooth surtace film on the alumina fibers and fine silicon nitride whiskers. A silicon carbide material with internal pore openings as small as 10 microns was penetrated by supercritical propane laden with aluminum isopropoxide. Weight gains of up to 43% can be obtained depending on the density of the initial host ceramic. 

Hendricks and Howell [253] measured the spectral normal transmittance and normal hemispherical reflectance of three sample thicknesses each of reticulated partially stabilized zirconia and silicon carbide at pore sizes of 10, 20, and 65 ppi. The measurements covered a spectral range of 400-500 nm. They used an inverse discrete ordinates method to find the spectrally dependent absorption and scattering coefficients as well as the constants appropri-... 

FIGURE 7.30 Extinction coefficient versus pore diameter for alumina, partially stabilized zirconia, and silicon carbide. 

The Crystal FT filters are made of 100% self-bonded, re crystallised silicon carbide wuth multi-layer SiC membranes. The filter carrier is characterised by high porosity and an open three-dimensional pore structure, Crystar says modules with 4CFT and 5CFT filters showed time and energy savings of 36%. 

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. 

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