Silicon carbide spheres

Figure 12.1 Calculated extinction efficiencies of silicon carbide spheres in air. The wave number denotes the inverse of the wavelength.

Figure 12.2 Calculated extinction efficiencies of a silicon carbide sphere (0.1 /im) in air and in potassium bromide.

Other stacking sequences than these are also possible, for example AaBpAaCy... or statistical sequences without periodic order. More than 70 stacking varieties are known for silicon carbide, and together they are called a-SiC. Structures that can be considered as stacking variants are called polytypes. We deal with them further in the context of closest-sphere packings (Chapter 14). 

Alumina spheres polluted by carbon residues have been also reactivated by use of microwaves [33]. Their regeneration has been performed in a stream of air and in the presence of silicon carbide as an auxiliary microwave absorber. Microwave heat treatment led to full recovery of the catalyst in times varying from a half to a quarter of the conventional treatments. Regeneration of a commercial Ni catalyst (Ni/Al203) deactivated, presumably, by coke formation, by means of a flow of hydrogen or oxygen and water vapor under the action of microwave irradiation was, however, unsuccessful [34]. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

Figure 12.15 Measured infrared extinction by silicon carbide particles (dashed curve) compared with calculations for spheres and a continuous distribution of ellipsoids (CDE).

Fillers used in large quantities to reinforce plastics are alumina (aluminum oxide), calcium carbonate, calcium silicate, cellulose flock, cotton (different forms), short glass fiber, glass beads, glass spheres, graphite, iron oxide powder, mica, quartz, sisal, silicon carbide, dtanium oxide, and tungsten carbide. Choice of filler varies and depends to a great extent upon the requirements of the end item and method of fabrication. 

A temperature resistant binder is necessary to bond SiC-bricks (carborundum bricks). Clays or other silicates are usually used, the particles being bonded by a glass phase. The bricks must be fired at ca. 1500°C in an oxidizing atmo.sphere to reduce the reduction of bonding clay to silicon and thereby prevent the bricks becoming brittle. The resulting oxidation of silicon carbide is limited by the formation of a passivation layer of Si02 on the SiC particles. 

PREPARATION OF SILICON CARBIDE HOLLOW SPHERES BY A TEMPLATE METHOD... 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

Preparation of Silicon Carbide Hollow Spheres by a Template Method... 

Yong Zhang, Er-Wei Shi, Zhi-Zhan Chen, Xiang-Biao Li, and Bing Xiao, Large-scale fabrication of silicon carbide hollow spheres, J. Mater. Chem., 16,4141-4145(2006). 

The sphere arrangements described are only two of an infinite number of ways for spheres to be stacked. It is surprising, for example, that the sequence ABAC is not commonly encountered in metal structures, although this is the structure of the metal lanthanum (La). In addition, a number of more complex arrangements have been found, especially in the compounds silicon carbide (SiC) and zinc sulphide (ZnS). 

The fuel pins used in the core are a new type for OCR s. The vast majority of OCR reactors use a TRISO type fuel embedded in a graphite matrix and are thermal neutron spectrum reactors. TRISO fuels are small UC spheres coated with layers of silicon carbide and pyrolitic carbon. While this results in fuels that can be used to extremely high bumup, the uranium density of the fuel is low. A coolant hole is then drilled through the blocks of fuel and these blocks are put in a prismatic array. 

A A glass sphere is molded into an optical lens by pressing against an optically smooth silicon carbide surface under heat and pressure. In order to avoid surface irregularities and distortions due to friction between the two surfaces, a lubricant must be employed. Under the conditions of the process, what lubricating mechanism would you predict to be most effective What characteristics should the lubricant have Explain your conclusions. 

The micro fuel element, shown in Fig. X-8, was developed for the conditions of fuel operation in a light water cooled and moderated core. It appears as a sphere of 1.8 mm outer diameter and includes the uranium dioxide kernel and a three-layer coating made of high-temperature ceramic materials. The kernel has a diameter of 1.4 mm. The first coating layer is made of porous pyrolythic graphite (PyC) it has a density of 1 g/cm. The thickness of this layer is 100 mkm. The second layer is made of dense PyC (the density is about 1.8 g/cm ) and is 5 mkm thick. The third, outer layer is made of silicon carbide (SiC) and has the thickness of about 95 mkm. 

Both reactors have in total produced about 1 Million of spent fuel elements during their operating time. The typical fuel element is a tennis-ball sized sphere from graphite, containing up to twenty thousand pinhead-sized fuel particles containing oxide or carbide fuel each. The particles are surrounded by a high-porosity buffer layer to limit the internal pressure from swelling and gas production, and coated with a hi -density pyrocarbon layer (BISO) or with a combination of two pyrocarbon layers with a silicon carbide layer in between (TRISO) to retain radionuclides (see FIG. 1). 

The other method of developing fine, spherical particles is through the use of plasma techniques which may involve a phjrsical phenomenon or a chemical reaction [105]. In the former case, a simple spherodizing process takes place when irregular shaped powder is introduced into the plasma which melts it and then vaporizes it to form uniform spheres. When a chemical reaction is involved, the reactants in the solid or gaseous form in contact with the plasma are allowed to react in the vapor phase and the products are allowed to condense as deposits of tine powder on the cooler regions of tire chamber after leaving the plasma. Colloidal spheres of alumimun, copper, aluminum nitride and silicon carbide are made in this way. 

The structure of a typical coated particle is shown in Fig. 5.4. The fuel kernel is a sphere of uranium carbide with a diameter in the range of 100-400 pm. The innermost coatihg consists of a layer of pyrolytic carbon laid down by deposition from hydrocarbon gases in a high-temperature fluidized bed. This inner layer absorbs fission fragment recoils and is made relatively porous to provide voidage for the accommodation of fission gases. The next layer is made up of silicon carbide, which is particularly effective in the... 

It can be observed that Chong s equation approximates to Einstein s equation at very low solid loading where < )c is 0.605 (for monosize spheres under body centered cubic packing). Stedman et al (16) measure the rheology of silicon nitride particulates/silicon carbide whiskers composite mixed with a polypropylene based binder. The relative viscosity data was found to fit Chong s equation well. Zhang and Evans [17] investigated on... 

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