Piezoelectricity
Piezoelectricity is the ability of crystals and certain ceramic materials to generate a voltage in response to applied mechanical
stress.
The word piezo is derived from the Greek word
пιέζω (piezein),
which means to squeeze. The piezoelectric effect is reversible in that
piezoelectric crystals, when subjected to an externally applied voltage, can
change shape by a small amount. (For instance, the deformation is about 0.1% of
the original dimension in PZT.) The effect finds useful
applications such as the production and detection of sound, generation of high
voltages, electronic frequency generation, microbalance, and ultra fine focusing of optical
assemblies.
History
A
related property known as pyroelectricity, the ability
of certain mineral crystals to generate an electircal
charge when heated, was know of as early as the 19th
century, and was named by David Brewster in 1824. The first reference to the pyroelectric effect is in writings by Theophrastus in 314 BC, who noted that Tourmaline becomes charged when
heated. Sir David
Brewster gave the effect the name it has today in 1824. Both William Thomson in 1878 and Voight in 1897 helped develop a theory for the processes behind
pyroelectricity. Pierre
Curie and his brother, Jacques Curie, studied pyroelectricity in the 1880s, leading to their discovery of some of
the mechanisms behind piezoelectricity.
The
first demonstration of the piezoelectric effect
was in 1880 by the Curie brothers using tinfoil, glue, wire, magnets, and a
jeweler's saw. They combined their knowledge of pyroelectricity with their
understanding of the underlying crystal structures that gave rise to
pyroelectricity to predict crystal behavior. They showed that crystals of Tourmaline, Quartz,
Topaz, cane sugar, and Rochelle salt (sodium potassium tartrate
tetrahydrate) generate electrical polarization from mechanical stress. Quartz
and Rochelle salt exhibited the most piezoelectricity.
Frequency standard
The piezoelectrical properties of Quartz are useful as a
standard of
frequency. Quartz clocks employ a
tuning fork made from quartz
that uses a combination of both direct and converse piezoelectricity to generate
a regularly timed series of electrical pulses that is used to mark time. The
quartz crystal (like any elastic
material) has a precisely defined natural frequency (caused by its shape and
size) at which it prefers to oscillate, and this is used to stabilize the
frequency of a periodic voltage applied to the crystal. The same principle is critical in all radio transmitters and receivers, and in computers where it creates a clock pulse. Both of these usually use a frequency
multiplier to reach the megahertz and gigahertz ranges.
Crystal classes
Crystal
structures can be divided into 32 classes, or point groups, according to the
number of rotational axes and reflection planes they
exhibit that leave the crystal structure unchanged. Of the thirty-two crystal
classes, twenty-one are non-centrosymmetric (not having a centre of
symmetry), and of these, twenty exhibit direct piezoelectricity (the 21st is the
cubic class 432). Ten of these are polar (i.e. spontaneously polarize), having a
dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed
by the application of an electric field, the material is said to be ferroelectric.
Piezoelectric Crystal Classes: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3,
32, 3m, 6, -6, 622, 6mm, -62m, 23, -43m
Pyroelectric: 1, 2, m, mm2, 4, 4mm, 3, 3m, 6, 6mm
In a piezoelectric crystal, the positive and negative electrical charges
are separated, but symmetrically distributed, so that the crystal overall is
electrically neutral. Each of these sites forms an electric dipole and dipoles near each other
tend to be aligned in regions called Weiss domains. The domains are usually randomly
oriented, but can be aligned during poling (not the same as magnetic poling), a process
by which a strong electric field is applied across the material, usually at
elevated temperatures.
When a mechanical stress is applied, this symmetry is disturbed, and the
charge asymmetry generates a voltage across the material. For example,
a 1 cm cube of quartz with 500 lbf (2 kN) of correctly applied force upon it, can produce
a voltage of 12,500 V.
Piezoelectric materials also show the opposite effect, called inverse
piezoelectricity, where the application of an electrical field creates
mechanical deformation in the crystal.
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