Page 112 - Materials Science and Engineering An Introduction

84 • Chapter 3 / The Structure of Crystalline Solids

Figure 3.18 (a) Close-packed
stacking sequence for the B
face-centered cubic structure.
(b) A corner has been removed A
to show the relation between the
stacking of close-packed planes C
of atoms and the FCC crystal
structure; the heavy triangle
outlines a (111) plane. B
[Figure (b) from W. G. Moffatt,
G. W. Pearsall, and J. Wulff, The A
Structure and Properties of
Materials, Vol. I, Structure, p. 51.
Copyright © 1964 by John Wiley & C
Sons, New York.]
B

A
(a) (b)

specify crystallographic planes and directions in terms of directional and Miller indi-
ces; furthermore, on occasion it is important to ascertain the atomic and ionic arrange-
ments of particular crystallographic planes. Also, the crystal structures of a number
of ceramic materials may be generated by the stacking of close-packed planes of ions
(Section 12.2).

Crystalline and Noncrystalline Materials

3.13 SINGLE CRYSTALS
For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect
or extends throughout the entirety of the specimen without interruption, the result is
single crystal a single crystal. All unit cells interlock in the same way and have the same orientation.
Single crystals exist in nature, but they can also be produced artificially. They are ordi-
narily difficult to grow because the environment must be carefully controlled.
If the extremities of a single crystal are permitted to grow without any external con-
straint, the crystal assumes a regular geometric shape having flat faces, as with some of
the gemstones; the shape is indicative of the crystal structure. A garnet single crystal is
shown in Figure 3.19. Within the past few years, single crystals have become extremely
important in many modern technologies, in particular electronic microcircuits, which
employ single crystals of silicon and other semiconductors.

3.14 POLYCRYSTALLINE MATERIALS

grain Most crystalline solids are composed of a collection of many small crystals or grains;
polycrystalline such materials are termed polycrystalline. Various stages in the solidification of a
polycrystalline specimen are represented schematically in Figure 3.20. Initially, small