Page 40 - Materials Science and Engineering An Introduction

12 • Chapter 1 / Introduction

1.5 ADVANCED MATERIALS
Materials utilized in high-technology (or high-tech) applications are sometimes termed
advanced materials. By high technology, we mean a device or product that operates or
functions using relatively intricate and sophisticated principles, including electronic
equipment (camcorders, CD/DVD players), computers, fiber-optic systems, spacecraft,
aircraft, and military rocketry. These advanced materials are typically traditional mate-
rials whose properties have been enhanced and also newly developed, high-performance
materials. Furthermore, they may be of all material types (e.g., metals, ceramics,
polymers) and are normally expensive. Advanced materials include semiconductors,
biomaterials, and what we may term materials of the future (i.e., smart materials and
nanoengineered materials), which we discuss next. The properties and applications of
a number of these advanced materials—for example, materials that are used for lasers,
integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and
fiber optics—are also discussed in subsequent chapters.
Semiconductors

Semiconductors have electrical properties that are intermediate between those of
electrical conductors (i.e., metals and metal alloys) and insulators (i.e., ceramics and
polymers)—see Figure 1.8. Furthermore, the electrical characteristics of these ma-
terials are extremely sensitive to the presence of minute concentrations of impurity
atoms, for which the concentrations may be controlled over very small spatial regions.
Semiconductors have made possible the advent of integrated circuitry that has totally
revolutionized the electronics and computer industries (not to mention our lives) over
the past three decades.

Biomaterials
Biomaterials are employed in components implanted into the human body to replace dis-
eased or damaged body parts. These materials must not produce toxic substances and must
be compatible with body tissues (i.e., must not cause adverse biological reactions). All of
the preceding materials—metals, ceramics, polymers, composites, and semiconductors—
may be used as biomaterials.
Smart Materials
Smart (or intelligent) materials are a group of new and state-of-the-art materials now
being developed that will have a significant influence on many of our technologies. The
adjective smart implies that these materials are able to sense changes in their environ-
ment and then respond to these changes in predetermined manners—traits that are also
found in living organisms. In addition, this smart concept is being extended to rather
sophisticated systems that consist of both smart and traditional materials.
Components of a smart material (or system) include some type of sensor (which
detects an input signal) and an actuator (which performs a responsive and adaptive
function). Actuators may be called upon to change shape, position, natural frequency,
or mechanical characteristics in response to changes in temperature, electric fields,
and/or magnetic fields.
Four types of materials are commonly used for actuators: shape-memory alloys, pi-
ezoelectric ceramics, magnetostrictive materials, and electrorheological/magnetorheo-
logical fluids. Shape-memory alloys are metals that, after having been deformed, revert
to their original shape when temperature is changed (see the Materials of Importance
box following Section 10.9). Piezoelectric ceramics expand and contract in response to
an applied electric field (or voltage); conversely, they also generate an electric field
when their dimensions are altered (see Section 18.25). The behavior of magnetostrictive
materials is analogous to that of the piezoelectrics, except that they are responsive to