Ceramic covalent engineering

The materials of particular interest in this book have already been defined and listed in Section 1.2, and they are now considered in relevant detail in this chapter. Because ceramics encompass all main bonding types—ionic, covalent, and metallic—it is convenient to divide this chapter into sections on covalent engineering ceramics, ionic engineering ceramics, electronic ceramics, and special hard-metal ceramics. 

Silicon nitride (Si3N4), a high-temperature ceramic useful for making engine components, is a covalent network solid in which each Si atom is bonded to four N atoms and each N atom is bonded to three Si atoms. Explain why silicon nitride is more brittle than a metal like copper. 

Silicon nitride has the composition Si3N4 and its chemical bonding is predominantly covalent. Si3N4 represents the backbone of silicon nitride ceramics, a class of ceramic materials which, because of their exceptional profile of properties, are gaining increasing acceptance in engineering applications. 

Forccd flow mode. Invertase, an enzyme, can be chemically immobilized to the surfaces of ceramic membrane pores by the technique of covalent bonding of silane-glutaraldehyde [Nakajima et al., 1989b]. The substrate (reactant), during the sucrose conversion process, enters the membrane reactor in a crossflow mode. Under suction from the other side of the membrane, the substrate flows into the enzyme-immobilized membrane pores where the bioconversion takes place. Both the product and the unreacted substrate indiscriminately pass through the membrane pores. Thus, no permselective properties are utilized in this case. The primary purpose of the membrane is to provide a well-engineered catalytic path for the reactant, sucrose. 

Engineers generally btiild things from a fimited menu of materials— namely, metals, polymers, and ceramics. This menu follows directly from the three types of primary chemical bonding metalfic, covalent, and ionic. 

Ceramic nitrides have many of the same properties as carbides and nitrides are also difficult to fabricate, especially in the pure form, due to strong covalent bonding. Sihcon nitride and boron nitride are the primary materials in the family of nitrides to be developed for engineering applications. ... 

Ceramic materials have a large fraction of ionic or covalent bonds, and this results in some special behavior - and, consequently, some special problems - with their reliable use in engineering. Within the temperature range of technical interest, dislocations are relatively immobile, and dislocation-induced plasticity is almost completely absent in ceramics l-3]. This is the basis for their extreme hardness and inherent brittleness, with typical values for the fracture toughness of ceramics ranging from 1 to lOMPa m, and the total fracture strain being commonly less than a few parts per thousand [4]. 

Interest in these covalent ceramics is almost equally divided between potential engineering applications and their use as electroceramics. Carbon... 

The excellent corrosion resistance of ceramics can be gauged from the following. Household cutlery, pottery, century-old vases, and so on are made of ceramics. Corrosion resistance arises due to their chemical stability and the high covalent bonding. Often hydrofluoric acid (HF), one of the strongest chemicals, is required to etch the microstructure of engineering ceramics. 

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