Piezoelectric Gems : ClassicGems.net

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.

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.