Page 41 - Materials Science and Engineering An Introduction

1.5 Advanced Materials • 13

magnetic fields. Also, electrorheological and magnetorheological 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), piezoelec-
tric materials (including some polymers), and microelectromechanical systems (MEMS;
Section 13.9).
For example, one type of smart system is used in helicopters to reduce aerodynamic
cockpit noise 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 that generates noise-canceling antinoise.

Nanomaterials
One new material class that has fascinating properties and tremendous technological
promise is the nanomaterials, which may be any one of the four basic types—metals,
ceramics, polymers, or composites. However, unlike these other materials, they are
not distinguished on the basis of their chemistry but rather their size; the nano prefix
denotes that the dimensions of these structural entities are on the order of a nanometer
(10 -9 m)—as a rule, less than 100 nanometers (nm; equivalent to the diameter of ap-
proximately 500 atoms).
Prior to the advent of nanomaterials, the general procedure scientists used to
understand the chemistry and physics of materials was to begin by studying large and
complex structures and then investigate the fundamental building blocks of these struc-
tures that are smaller and simpler. This approach is sometimes termed top-down science.
However, with the development of scanning probe microscopes (Section 4.10), which
permit observation of individual atoms and molecules, it has become possible to design
and build new structures from their atomic-level constituents, one atom or molecule at
a time (i.e., “materials by design”). This ability to arrange atoms carefully 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. 10
Some of the physical and chemical characteristics exhibited by matter may experi-
ence dramatic changes as particle size approaches atomic dimensions. For example,
materials that are opaque in the macroscopic domain may become transparent on the
nanoscale; some solids become liquids, chemically stable materials become combustible,
and electrical insulators become conductors. Furthermore, properties may depend on
size in this nanoscale domain. Some of these effects are quantum mechanical in origin,
whereas others are related to surface phenomena—the proportion of atoms located on
surface sites of a particle increases dramatically as its size decreases.
Because of these unique and unusual properties, nanomaterials are finding niches
in electronic, biomedical, sporting, energy production, and other industrial applications.
Some are discussed in this text, including the following:
• Catalytic converters for automobiles (Materials of Importance box, Chapter 4)
• Nanocarbons—Fullerenes, carbon nanotubes, and graphene (Section 13.9)
• Particles of carbon black as reinforcement for automobile tires (Section 16.2)
• Nanocomposites (Section 16.16)
• Magnetic nanosize grains that are used for hard disk drives (Section 20.11)
• Magnetic particles that store data on magnetic tapes (Section 20.11)

10 One legendary and prophetic suggestion as to the possibility of nanoengineered materials was offered by Richard
Feynman in his 1959 American Physical Society lecture titled “There’s Plenty of Room at the Bottom.”