GTBL042-13 GTBL042-Callister-v2 August 29, 2007 8:52
550 • Chapter 13 / Types and Applications of MaterialsMATERIALS OF IMPORTANCE
Piezoelectric Ceramics
A
few ceramic materials (as well as some
polymers) exhibit the unusual phenomenon
of piezoelectricity^1 —electric polarization^2 (i.e.,
an electric field or voltage) is induced in the ce-
ramic crystal when a mechanical strain (dimen-
sional change) is imposed on it. The inverse piezo-
electric effect is also displayed by this group of
materials; that is, a mechanical strain results from
the imposition of an electrical field.
Piezoelectric materials may be utilized as
transducers between electrical and mechanical en-
ergies. One of the early uses of piezoelectric ce-
ramics was in sonar, wherein underwater objects
(e.g., submarines) are detected and their positions
determined using an ultrasonic emitting and re-
ceiving system. A piezoelectric crystal is caused
to oscillate by an electrical signal, which produces
high-frequency mechanical vibrations that are
transmitted through the water. Upon encountering
an object, these signals are reflected back, and an-
other piezoelectric material receives this reflected
vibrational energy, which it then converts back intoan electrical signal. Distance from the ultrasonic
source and reflecting body is determined from the
elapsed time between sending and receiving events.
More recently, the utilization of piezoelec-
tric devices has grown dramatically as a conse-
quence of increases in automatization and con-
sumer attraction to modern sophisticated gadgets.
Applications that employ piezoelectric devices are
found in the automotive, computer, commercial/
consumer, and medical sectors. Some of these ap-
plications are as follows: automotive—wheel bal-
ances, seat belt buzzers, tread-wear indicators, key-
less door entry, and airbag sensors; computer—
microactuators for hard disks and notebook trans-
formers; commercial/consumer—ink-jet printing
heads, strain gauges, ultrasonic welders, and smoke
detectors; medical—insulin pumps, ultrasonic ther-
apy, and ultrasonic cataract-removal devices.
Commonly used piezoelectric ceramics in-
clude barium titanate (BaTiO 3 ), lead titanate
(PbTiO 3 ), lead zirconate–titanate (PZT) [Pb(Zr,
Ti)O 3 ], and potassium niobate (KNbO 3 ).(^1) The piezoelectric phenomenon is described in more detail in Section 12.25.
(^2) Electric polarization (explained in Sections 12.19 and 12.20) is the alignment of electric dipoles (Section 2.7) in a
common direction, which gives rise to an electric field that is oriented in this same direction.
13.11 DIAMOND AND GRAPHITE
Diamond
The physical properties of diamond make it an extremely attractive material. It is
extremely hard (the hardest known material) and has a very low electrical conduc-
tivity; these characteristics are due to its crystal structure and the strong interatomic
covalent bonds. Furthermore, it has an unusually high thermal conductivity for a
nonmetallic material, is optically transparent in the visible and infrared regions of
the electromagnetic spectrum, and has a high index of refraction. Relatively large
diamond single crystals are used as gem stones. Industrially, diamonds are utilized to
grind or cut other softer materials (Section 13.8). Techniques to produce synthetic
diamonds have been developed, beginning in the mid-1950s, and have been refined
to the degree that today a large proportion of the industrial-quality materials are
man-made, in addition to some of those of gem quality.
Over the last several years, diamond in the form of thin films has been produced.
Film growth techniques involve vapor-phase chemical reactions followed by the film
deposition. Maximum film thicknesses are on the order of a millimeter. Furthermore,
none of the films yet produced has the long-range crystalline regularity of natural dia-
mond. The diamond is polycrystalline and may consist of very small and/or relatively