Nonsilicate ceramics

Many useful ceramics exist that are not based on the Si —O bond and the Si04 tetrahedron. They have important nses in electronics, optics, and the chemical in-dnstry. Some of these materials are oxides, but others contain neither silicon nor oxygen. 

Alumina (AI2O3) is the most important nonsilicate ceramic material. It melts at a temperatnre of 2051°C and retains strength even at temperatures of I500°C to 1700°C. Alumina has a large electrical resistivity and withstands both thermal shock and corrosion well. These properties make it a good material for spark ping insnlators, and most spark pings now use a ceramic that is 94% alumina. 

TABLE 22.4 Melting Points of Some Metals and Their Oxides 

FIGURE 22.12 Socket in this artificial hip is made of high-density alumina. 

Metal Melting Point (°C) Oxide Melting Point (°C) 

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Nonsilicate ceramics derive comparable properties from other inorganic structural units. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Almost every inorganic oxide that does not contain silicon, as well as many carbides and nitrides, can be thought of, to some extent, as a nonsilicate ceramic. The bonding across this great variety of... 

The structures of a number of other important nonsilicate ceramics are discussed elsewhere, (Chapters 5, 8-14 and 16). 

Non-SI (unacceptable and obsolete) units, l xxiv-xxv 2-26 xxii-xxiii Nonsilicate crystals, glass-ceramics based on, 12 641-642... 

Wagh and Jeong [4] have reported that, once the metal ions are dissociated and screened in an acid solution that is rich with phosphate anions, the kinetics of transformation to a CBPC is very similar to that of the conventional sol-gel process of fabricating ceramics of nonsilicates [4] with the major difference here being that the acid-base reaction used in forming CBPCs carries the mixture all the way to the formation of ceramics, while in the sol-gel process the sols are ultimately sintered to form superior ceramics. Figure 5.1 illustrates the step-by-step kinetics of the formation of CBPCs. 

The classes of ceramic powder are divided into two categories (silicates and nonsilicates) and are discussed separately in the following sections. 

A few varieties of nonsilicate oxide ceramic powders synthesized through alkoxide processing are presented here. These oxide powders were developed for the use of electronic, optical, and high-temperature structural applications. For each material, we start with a brief description of the synthesis, which is followed by powder characteristics (e.g., particle sizes, morphologies, and size distributions) and densification behavior and some properties of dense material. 

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