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Piezoelectricity

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Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. The word is derived from the Greek piezein, which means to squeeze or press.

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Mechanism

In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. When a stress is applied, this symmetry is destroyed, and the charge asymmetry generates a voltage.

Pizeoelectric materials also show the opposite effect, called converse piezoelectricity, where application of an electrical field creates mechanical stress (distortion) in the crystal. Because the charges inside the crystal are separated, the applied voltage affects different points within the crystal differently, resulting in the distortion.

The bending forces generated by converse piezoelectricity are extremely high, of the order of tens of millions of pounds, and usually cannot be constrained. The only reason the force is usually not noticed is because it causes a displacement of the order of one billionth of an inch (a few nanometres).

History

A related property known as pyroelectricity, the ability of certain mineral crystals to generate electrical charge when heated, was known of as early as the 18th century, and was named by Brewster in 1824. In 1880, the brothers Pierre Curie and Jacques Curie[?] predicted and demonstrated piezoelectricity using tinfoil, glue, wire, magnets, and a jeweler's saw. 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. Twenty natural crystal classes exhibit direct piezoelectricity.

Converse piezoelectricity was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. The Curies immediately confirmed the existence of the "converse effect," and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.

Materials

In addition to the materials listed above, many other materials exhibit the effect, including quartz analogue crystals like berlinite (AlPO4) and gallium orthophosphate (GaPO4), ceramics with perovskite or tungsten-bronze structures (BaTiO3, KNbO3, LiNbO3, LiTaO3, BiFeO3, NaxWO3, Ba2NaNb5O5, Pb2KNb5O15). Polymer materials like rubber, wool, hair, wood fiber, and silk exhibit piezoelectricity to some extent. The polymer polyvinlidene fluoride, (-CH2-CF2-)n, exhibits piezoelectricity several times larger than quartz.

Applications

Converse piezoelectricity has been used to create devices like loudspeakers, where voltages are converted to mechanical movement of a piezoelectric polymer film. The opposite setup is used to make piezoelectric microphones (sound waves create voltages in the piezoelectric material) and piezoelectric pickups for electrically amplified guitars.

Direct piezoelectricity of some substances like quartz can generate thousands of volts. This property is exploited in the portable electrical sparkers used to light gas grills. The effect is being researched by DARPA in the USA in a project called Energy Harvesting, which includes an attempt to power battlefield equipment by piezoelectric generators embedded in soldiers' boots.

Digital watches[?] 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 keep track of the passage of time. The quartz crystal (like any material) has a precisely defined natural frequency at which it prefers to oscillate, and this is used to stabilize the frequency of a periodic voltage applied to the crystal.

Piezoelectric elements are also used in the generation of sonar waves. Piezoelectric microbalances are used as very sensitive chemical and biological sensors.

Atomic force microscopes and scanning tunneling microscopes employ converse piezoelectricity to keep the sensing needle close to the probe.

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