ch02 - pg020[V G4 7-27060 IRWIN Schaffer A R I T...

I
P A R T
FUNDAMENTALS
2
ATOMIC SCALE
STRUCTURES
3
CRYSTAL STRUCTURES
4
POINT DEFECTS AND
DIFFUSION
5
LINEAR, PLANAR, AND
VOLUME DEFECTS
6
NONCRYSTALLINE AND
SEMICRYSTALLINE
MATERIALS

n Part I we will be concerned with how atoms bond to-
gether in an infinite array to form engineering materials.
These materials possess specific properties that are a direct
result of their atomic scale structures. We will learn that in
some solids, called crystalline materials, the atomic ar-
I
rangement is in the form of a periodic array of atoms. In other
solids, called noncrystalline or amorphous materials, the atoms
are arranged in a near random fashion. Since crystals contain
large numbers of atoms—one gram contains on the order of 10
23
atoms—defects in the atomic arrangement are inevitable. These
defects have a significant impact on the properties of the crystal.
It is important for engineers to understand the atomic scale
structures of crystalline and noncrystalline materials if they are to
use materials effectively in the design and construction of com-
mercial products. As an example, consider the case of a cam-
corder. Camcorders must be light and compact yet sturdy, have
automatic controls to compensate for light intensity and distance,
and have high-quality optics and sound-recording capability, a
pleasing appearance, and a competitive price. Engineers must
satisfy these requirements by using a variety of materials with
specific properties to achieve an optimum design.
Let us examine the major components of a camcorder. The
lenses must transmit light without optical aberrations to achieve
a high-quality picture. Lenses are made from glasses, which owe
their transparency to their noncrystalline structure. The hardness
of glass, which makes it suitable for precision grinding to achieve
the proper lens shape, is a consequence of its three-dimensional
bond network, as discussed in Chapters 2 and 6.
The camcorder housing parts must be light and easily formed
into intricate shapes. These components are made from polymers
composed of molecular chains of light elements that are loosely
packed together resulting in a low-density material. Since the
molecular chains are attached to each other by weak secondary
bonds, which disintegrate easily upon heating, these materials
have low softening points, which permit them to be shaped inex-
pensively. The structure of polymers will be discussed in Chap-
ters 2 and 6.
A camcorder also contains a large number of electronic com-
ponents, including: (1) electrically conducting wires made from
(Courtesy of Sony Electronics Inc.)
metals such as copper or gold, (2) precision resistors fabricated
from carbon or nickel-chromium alloys, (3) semiconductors such as
silicon doped with phosphorus or boron, and (4) ceramic or poly-
mer insulators that isolate electrical signals between active ele-
ments. The tremendous variation in the electrical properties of
these and other materials is a result of their atomic and crystal
structures, as described in Chapters 2 and 3.