Covalent bonds, Inorganic materials

Ceramics are inorganic solids, usually oxides, which contain ionic and covalent bonds. The material, formed by sintering at high temperatures, ranges from amorphous glass-like material to highly crystalline solids, from insulators to conductors or semiconductors. They include earthenware, which is fired at 1,100-1,300 K and a porosity of about 8% fine china or bone china, fired at 1,400-1,500 K with a porosity of less than 1% stoneware, fired at over 1,500 K with a porosity of about 1% before glazing and porcelain which is fired at over 1,600 K and has a much finer microstructure than either stoneware or bone china. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

Until this point, we have focused on cases in which we could neglect chemical bond formation between the sorbate and materials in the solid phase. However, at least two kinds of surface reactions are known to be important for sorption of some chemicals (referred to as chemisorption). Simply, some organic substances can form covalent bonds with the NOM in a sediment or soil (see Fig. 9.2) other organic sor-bates are able to serve as ligands of metals on the surfaces of inorganic solids (Fig. 11.le). We discuss these processes below. 

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