Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33

88 • Chapter 3 / Structures of Metals and Ceramics

(a) (b)

Silicon atom

Oxygen atom

Figure 3.40 Two-dimensional schemes of the structure of (a) crystalline silicon dioxide and

(b) noncrystalline silicon dioxide.

may exist in both states. Figures 3.40aand 3.40bpresent two-dimensional schematic

diagrams for both structures of SiO 2 , in which the SiO^44 −tetrahedron is the basic

unit (Figure 3.10). Even though each silicon ion bonds to three oxygen ions for

both states, beyond this, the structure is much more disordered and irregular for the

noncrystalline structure.

Whether a crystalline or amorphous solid forms depends on the ease with which a

random atomic structure in the liquid can transform to an ordered state during solid-

ification. Amorphous materials, therefore, are characterized by atomic or molecular

structures that are relatively complex and become ordered only with some difficulty.

Furthermore, rapidly cooling through the freezing temperature favors the formation

of a noncrystalline solid, since little time is allowed for the ordering process.

Metals normally form crystalline solids, but some ceramic materials are crys-

talline, whereas others, the inorganic glasses, are amorphous. Polymers may be com-

pletely noncrystalline or semicrystalline consisting of varying degrees of crystallinity.

More about the structure and properties of amorphous materials is discussed below

and in subsequent chapters.

Concept Check 3.4

Do noncrystalline materials display the phenomenon of allotropy (or polymor-

phism)? Why or why not?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Concept Check 3.5

Do noncrystalline materials have grain boundaries? Why or why not?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]