GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33
Summary • 89Si4+ O2– Na+Figure 3.41 Schematic representation of ion
positions in a sodium–silicate glass.Silica Glasses
Silicon dioxide (or silica, SiO 2 ) in the noncrystalline state is calledfused silica,or
vitreous silica; again, a schematic representation of its structure is shown in Figure
3.40b. 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 3.12); these materials, as
well as SiO 2 , are termednetwork 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^44 −network; for this reason, these oxide additives are
termednetwork modifiers.For example, Figure 3.41 is a schematic representation of
the structure of a sodium–silicate glass. Still other oxides, such as TiO 2 and Al 2 O 3 ,
while not network formers, substitute for silicon and become part of and stabilize the
network; these are calledintermediates.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 14.7).SUMMARY
Fundamental Concepts
Unit Cells
Atoms in crystalline solids are positioned in orderly and repeated patterns that are
in contrast to the random and disordered atomic distribution found in noncrystalline
or amorphous materials. Atoms may be represented as solid spheres, and, for crys-
talline solids, crystal structure is just the spatial arrangement of these spheres. The
various crystal structures are specified in terms of parallelepiped unit cells, which are
characterized by geometry and atom positions within.