Network atomic solids ceramic

Oxides of Silicon Silicates Silica, Si02, is the only stable oxide of silicon. Silica is a network covalent solid (not a molecular solid, like CO2). In silica, each Si atom is bonded to four O atoms and each O atom to txvo Si atoms. The structure is that of a network covalent solid, as suggested by Figure 21-31 (a). This structure is reminiscent of the diamond structure, and silica has certain properties that resemble those of diamond. For example, quartz, a form of silica, is fairly hard (with a Mohs hardness of 7) but not as hard as diamond (with a Mohs hardness of 10), has a high melting point (about 1700 °C), and is a nonconductor of electricity. Silica is the basic raw material of the glass and ceramics industries. 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

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. 

In the case of PSCSs when x > 0.1, most of the products were found soluble and were converted into the corresponding PCS at 450 °C, under atmospheric pressure. These spinnable PCS were transformed into SiC-based materials with ceramic yields very close to those of Yajima ( 60 %). PSCSs of formula (MeRSi),.j-(HR SiCH2SiR H)j also were prepared in order to appreciate the influence of R and R on the carbon content of the ceramic. Otherwise, multinuclear solid state NMR studies indicated that the network was first built around silicon atoms (formation of SiC4) then, at higher temperature, aroimd carbon atoms (formation of CSi4). 

Most metals are malleable, which means that they can be hammered into thin sheets, and ductile, which means that they can be drawn into wires ( FIGURE 12.10). These properties indicate that the atoms are capable of slipping past one another. Ionic and covalent-network solids do not exhibit such behavior. They are typically brittle and fracture easily. Consider, for example, the difference between dropping a ceramic plate and an aluminum cooking pan onto a concrete floor. 

Silicon nitride (Si3N4) has long been considered a promising material for use in ceramic gas turbines. A hard network solid ( 9 on the Mohs scale), Si3N4 features a structure in which each nitrogen atom is surrounded by three silicon atoms, and each silicon atom is tetrahedrally surrounded by four nitrogen atoms. 

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