Solid materials have been conveniently grouped into three basic classifications:met-
als, ceramics, and polymers.This scheme is based primarily on chemical makeup and
atomic structure, and most materials fall into one distinct grouping or another,
although there are some intermediates. In addition, there are the composites, com-
binations of two or more of the above three basic material classes.A brief explana-
tion of these material types and representative characteristics is offered next.Another
classification is advanced materials—those used in high-technology applications—
viz. semiconductors, biomaterials, smart materials, and nanoengineered materials;

Metals
Materials in this group are composed of one or more metallic elements (such as iron,
aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for
example, carbon, nitrogen, and oxygen) in relatively small amounts. Atoms in metals
and their alloys are arranged in a very orderly manner and in comparison to the ceramics and polymers, are relatively dense.






regard to mechanical characteristics, these materials are relatively stiff (Figure 1.4)
and strong yet are ductile (i.e., capable of large amounts of deformation
without fracture), and are resistant to fracture , which accounts for their
widespread use in structural applications. Metallic materials have large numbers of
nonlocalized electrons; that is, these electrons are not bound to particular atoms.Many
properties of metals are directly attributable to these electrons. For example, metals
are extremely good conductors of electricity and heat, and are not trans-
parent to visible light; a polished metal surface has a lustrous appearance. In addi-
tion, some of the metals (viz., Fe, Co, and Ni) have desirable magnetic properties.
Figure 1.8 is a photograph that shows several common and familiar objects that
are made of metallic materials. Furthermore, the types and applications of metals


Ceramics
Ceramics are compounds between metallic and nonmetallic elements; they are most
frequently oxides, nitrides, and carbides. For example, some of the common ceramic







materials include aluminum oxide (or alumina,Al2O3), silicon dioxide (or silica, SiO2),
silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as
the traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as
cement, and glass. With regard to mechanical behavior, ceramic materials are rela-
tively stiff and strong—stiffnesses and strengths are comparable to those of the met-
als (Figures 1.4 and 1.5). In addition, ceramics are typically very hard. On the other
hand, they are extremely brittle (lack ductility), and are highly susceptible to fracture
(Figure 1.6). These materials are typically insulative to the passage of heat and elec-
tricity (i.e., have low electrical conductivities, Figure 1.7), and are more resistant to
high temperatures and harsh environments than metals and polymers.With regard to
optical characteristics, ceramics may be transparent, translucent, or opaque (Figure
1.2), and some of the oxide ceramics (e.g., Fe O ) exhibit magnetic behavior.






Polymers


Polymers include the familiar plastic and rubber materials.Many of them are organic
compounds that are chemically based on carbon, hydrogen, and other nonmetallic
elements (viz.O,N, and Si). Furthermore, they have very large molecular structures,
often chain-like in nature that have a backbone of carbon atoms. Some of the com-
mon and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride)
(PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials
typically have low densities (Figure 1.3), whereas their mechanical characteristics
are generally dissimilar to the metallic and ceramic materials—they are not as stiff
nor as strong as these other material types (Figures 1.4 and 1.5). However, on the
basis of their low densities,many times their stiffnesses and strengths on a per mass



basis are comparable to the metals and ceramics. In addition,many of the polymers
are extremely ductile and pliable (i.e., plastic), which means they are easily formed
into complex shapes. In general, they are relatively inert chemically and unreactive
in a large number of environments. One major drawback to the polymers is their
tendency to soften and/or decompose at modest temperatures, which, in some in-
stances, limits their use. Furthermore, they have low electrical conductivities (Fig-
ure 1.7) and are nonmagnetic.
The photograph in Figure 1.10 shows several articles made of polymers that are
familiar to the reader. Chapters 14 and 15 are devoted to discussions of the struc-
tures, properties, applications, and processing of polymeric materials.



Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbrevi-
ated “GFRP.”


Composites


A composite is composed of two (or more) individual materials, which come from
the categories discussed above—viz.,metals, ceramics, and polymers.The design goal
of a composite is to achieve a combination of properties that is not displayed by
any single material, and also to incorporate the best characteristics of each of the
component materials.A large number of composite types exist that are represented
by different combinations of metals, ceramics, and polymers. Furthermore, some
naturally-occurring materials are also considered to be composites—for example,
wood and bone. However, most of those we consider in our discussions are syn-
thetic (or man-made) composites.
One of the most common and familiar composites is fiberglass, in which small
glass fibers are embedded within a polymeric material (normally an epoxy or
polyester).
The glass fibers are relatively strong and stiff (but also brittle), whereas
the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is
relatively stiff, strong, (Figures 1.4 and 1.5) flexible, and ductile. In addition, it has
a low density (Figure 1.3).
Another of these technologically important materials is the “carbon fiber-
reinforced polymer” (or “CFRP”) composite—carbon fibers that are embedded within
a polymer. These materials are stiffer and stronger than the glass fiber-reinforced
materials (Figures 1.4 and 1.5), yet they are more expensive.The CFRP composites

are used in some aircraft and aerospace applications, as well as high-tech sporting
equipment (e.g., bicycles, golf clubs, tennis rackets, and skis/snowboards). Chapter 16
is devoted to a discussion of these interesting materials.


ADVANCED MATERIALS


Materials that are utilized in high-technology (or high-tech) applications are some-
times termed advanced materials. By high technology we mean a device or product
that operates or functions using relatively intricate and sophisticated principles; ex-
amples include electronic equipment (camcorders, CD/DVD players, etc.), com-
puters, fiber-optic systems, spacecraft, aircraft, and military rocketry.These advanced
materials are typically traditional materials 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” (that is, smart materials and nanoengineered materials),
which we discuss below. 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 the elec-
trical conductors (viz. metals and metal alloys) and insulators (viz. ceramics and
polymers)—Figure 1.7. Furthermore, the electrical characteristics of these materi-
als are extremely sensitive to the presence of minute concentrations of impurity
atoms, for which the concentrations may be controlled over very small spatial re-
gions. 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 for
replacement of diseased 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 above materials—metals, ceramics, poly-
mers, composites, and semiconductors—may be used as biomaterials. For example,
some of the biomaterials that are utilized in artificial hip replacements are dis-
cussed in Section 22.12.


Materials of the Future
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 tech-
nologies.The adjective “smart” implies that these materials are able to sense changes
in their environments and then respond to these changes in predetermined manners—
traits that are also found in living organisms. In addition, this “smart” concept is be-
ing extended to rather sophisticated systems that consist of both smart and tra-
ditional materials.
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,
piezoelectric ceramics, magnetostrictive materials, and electrorheological/magne-
torheological fluids.Shape memory alloys are metals that, after having been deformed,
revert back to their original shapes when temperature is changed (see the Materi-
als of Importance piece 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, ex-
cept that they are responsive to magnetic fields.Also, electrorheological and mag-
netorheological fluids are liquids that experience dramatic changes in viscosity upon
the application of electric and magnetic fields, respectively.
Materials/devices employed as sensors include optical fibers (Section 21.14),
piezoelectric materials (including some polymers), and microelectromechanical
devices (MEMS, Section 13.8).
For example, one type of smart system is used in helicopters to reduce aero-
dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric
sensors inserted into the blades monitor blade stresses and deformations; feedback
signals from these sensors are fed into a computer-controlled adaptive device,which
generates noise-canceling antinoise.
Nanoengineered Materials
Until very recent times the general procedure utilized by scientists to understand
the chemistry and physics of materials has been to begin by studying large and com-
plex structures, and then to investigate the fundamental building blocks of these
structures that are smaller and simpler. This approach is sometimes termed “top-
down” science. However, with the advent of scanning probe microscopes (Sec-
tion 4.10), which permit observation of individual atoms and molecules, it has be-
come possible to manipulate and move atoms and molecules to form new structures
and, thus, design new materials that are built from simple atomic-level constituents
(i.e., “materials by design”). This ability to carefully arrange atoms provides op-
portunities to develop mechanical, electrical, magnetic, and other properties that
are not otherwise possible.We call this the “bottom-up” approach, and the study of
the properties of these materials is termed “nanotechnology”; the “nano” prefix de-
notes that the dimensions of these structural entities are on the order of a nanome-
ter (10 9
m)—as a rule, less than 100 nanometers (equivalent to approximately 500
atom diameters).
One example of a material of this type is the carbon nanotube,
discussed in Section 12.4. In the future we will undoubtedly find that increasingly
more of our technological advances will utilize these nanoengineered materials.