Callister - An introduction - 8th edition

12.3 Silicate Ceramics • 465
Si 4+ O 2– Figure 12.10 The arrangement of silicon and oxygen
atoms in a unit cell of cristobalite, a polymorph of SiO 2 .
for crystalline and noncrystalline silica are compared schematically in Figure 3.23.
Other oxides (e.g., B 2 O 3 and GeO 2 ) may also form glassy structures (and polyhedral
oxide structures similar to that shown in Figure 12.9); these materials, as well as
SiO 2 , are termed network formers.
The common inorganic glasses that are used for containers, windows, and so
on are silica glasses to which have been added other oxides such as CaO and
Na 2 O. These oxides do not form polyhedral networks. Rather, their cations are incorporated
within and modify the SiO 4
4 network; for this reason, these oxide additives
are termed network modifiers. For example, Figure 12.11 is a schematic
representation of the structure of a sodium–silicate glass. Still other oxides, such
as TiO 2 and Al 2 O 3 , though not network formers, substitute for silicon and become
part of and stabilize the network; these are called intermediates. From a practical
perspective, the addition of these modifiers and intermediates lowers the melting
point and viscosity of a glass and makes it easier to form at lower temperatures
(Section 13.9).
Figure 12.11 Schematic representation of
ion positions in a sodium–silicate glass.
Si 4+ O 2– Na +