Silicon polycrystalline ceramic materials

Silicon nitride-based polycrystalline ceramic materials have the potential for structural applications at both room temperature and elevated temperatures, despite containing silicate/silicate-rich oxynitride grain boundary phases. These are generally much weaker compared to the silicon nitride single crystal grains of which the... 

Figure 14.8 Defects at failure origin of ceramic materials, (a) Surface grinding defect in silicon (from O. Borerro-Lopez, unpublished results) (b) Internal pore in polycrystalline alumina ...

A study of Vickers hardness of polycrystalline ceramics revealed that cracking may cause critical transition points in the Vickers ISE trends. The transition point was associated with extensive cracking in and around the indentation and a shift in the energy balance during indentation. Different ratios of the indentation work are expended on volumetric deformation and surface fracture processes above and below the transition point. The transition point was very distinct for brittle materials such as silicon carbide. The Vickers hardness transition point was related to a new index of ceramic brittleness defined as ... 

The priority in this chapter lies in the description of small organosubstituted silanes and of small heterocycles that might be precursors for the production of new materials for high tech applications (e. g. polycrystalline silicon for photovoltaic purposes, thin ceramic coatings from CVD processes). 

Lipowitz, J.. Structure and properties of Sylramic silicon carbide fiber a polycrystalline, stoichiometric p-SiC composition. Presentation to the Committee on Advanced Fibers for High-Temperature Ceramic Composites, National Materials Advisory Board, National Research Council, Cocoa Beach, Florida, January 16, 1997. 

Among other topics, his research interests include the micro- and nanostructure of ceramic fibers derived from organosilicon precursors and polycrystalline silicon carbide fibers derived from organosilicon polymers. Dr. Lipowitz has nearly 20 U.S. patents and more than 50 publications to his credit. He is a member of several professional and honorary societies, including the American Ceramic Society, the Materials Research Society, Sigma Xi, and the New York Academy of Science. 

From the point of view of the volume of production, polycrystalline alumina is the material most frequently used as ceramics for structural applications. However, in comparison with for example, silicon nitride, where the influence of various additives on microstructure and properties has been well characterized and understood, and despite several decades of lasting research effort, alumina remains a material with many unknown factors yet to be revealed. Alumina-based materials can be divided roughly into three groups ... 

Silicon nitride-based ceramics are polycrystalline materials that consist at least of two phases, the silicon nitride grains and the grain boundary. In this chapter, an attempt will be made to describe, separately, the properties of both the grain and grain boundary phases. The effect of combining these phases, with regards to the mechanical properties of the materials, will also be discussed. 

Aluminum nitride (AlN) has interesting properties, such as a high thermal conductivity (70-210 W m for the polycrystalline material, and up to 285 W m for single crystals), a high volume resistance, and moderate dielectric properties. The thermal expansion coefficient of AlN is close to that of silicon, and it is one of the most mechanically strong and thermally stable ceramics. These excellent attributes make AlN a useful material for many applications [160, 161]. 

Materials that are classified as fibers are either polycrystalline or amorphous and have small diameters fibrous materials are generally either polymers or ceramics (e.g., the polymer aramids, glass, carbon, boron, aluminum oxide, and silicon carbide). Table 16.4 also presents some data on a few materials that are used in fiber form. 

Man-made fibers ean be amorphous, polycrystalline, or ciystalline. The amorphous fibers inelude the man-made vitreous fibers typified by the insulation wools, which form the bulk of man-made inorganic fibers. Polycrystalline fibers include continuous carbon fibers used in composite material and specialty fibers such as Saffil. Crystalline man-made fibers include ceramic whiskers, such as silicon carbide and silicon nitride, which are used in reinforce metals and other composite materials. 

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