Piezoelectricity

Piezoelectricity

From the Greek piézein to press, to push.

This is the property possessed by certain bodies to become electrically polarised under the action of a mechanical stress and conversely to deform when an electric field is applied to them.
The two effects are inseparable. The first is called the direct piezoelectric effect; the second the inverse piezoelectric effect.
This property finds a very large number of applications in industry and everyday life.
One of the most familiar applications is the gas lighter. In a gas lighter, the pressure exerted produces an electric voltage which discharges suddenly in the form of sparks: this is an application of the direct effect.
More generally, the direct effect can be used in the making of sensors (pressure sensor etc.) while the inverse effect allows the making of actuators (piezoelectric-controlled injectors in automobiles, nano-manipulator…).
There are many piezoelectric materials. The best known is undoubtedly quartz, still used today in watches to generate clock pulses. But it is synthetic ceramics, the PZT, that are most widely used today in industry.

The first demonstration of the direct piezoelectric effect is due to Pierre and Jacques Curie in 1880. Combining their knowledge of pyroelectricity and crystalline structure, they predicted and verified the existence of piezoelectricity on crystals of quartz, tourmaline, topaz, sugar and Rochelle salt. The existence of the inverse effect was predicted the following year by Gabriel Lippmann on the basis of thermodynamic calculations, and immediately verified by the Curies. It was also in 1881 that Hermann Hankel suggested the use of the term piezoelectricity from the Greek "piézein" meaning to press, to push.
Piezoelectricity remained a laboratory curiosity for about thirty years; it mainly gave rise to theoretical work on crystalline structures exhibiting this property. This work culminated in 1910 with the publication by Woldemar Voigt of the Lehrbuch der Kristallphysik which gives the twenty piezoelectric crystal classes, and rigorously defines the piezoelectric constants in the formalism of tensor analysis.
From a practical point of view, piezoelectricity was only used to make a few laboratory instruments.

First applications:

The first application of piezoelectricity was the sonar developed by Paul Langevin and his collaborators during the First World War. This sonar was composed of quartz plates glued between two steel plates and a hydrophone and allowed, by measuring the time elapsed between the emission of an acoustic wave and the reception of its echo, to calculate the distance to the object. Shortly afterwards, in the early 1920s, the first quartz oscillator was developed by Walter Cady, thus opening the way to frequency control.
The success of these projects aroused great interest in piezoelectricity, revived research and led over the following years to the development of new devices for a wide range of applications in everyday life, industry and research. The improvement of the phonograph or the development of the reflectometer and the acoustic transducer, widely used for hardness or viscosity measurements, are examples.
During this period, the main materials used were quartz, Rochelle salt and potassium dihydrogen phosphate KH2PO4. However, although they can be used, these materials have disadvantages which limit both the possible applications and the development of piezoelectric theories.

Discovery of ferroelectric oxides

During the Second World War, the search for more efficient dielectric materials led different research groups in Japan, the United States and Russia to discover the piezoelectric properties of synthetic ceramics composed of oxides with perovskite structure: barium titanate (BaTiO3) and then a little later lead titanate-zirconate (PbZrxTi1-xO3, abbreviated as PZT). The development of these materials represents a decisive step in the development of piezoelectric devices. Indeed, their properties are overall much better; they have piezoelectric coefficients about 100 times higher than those of natural crystals. But above all, it is possible with these synthetic ceramics to play on different synthesis parameters and thus adjust the material properties for a specific application. In particular, doping with various metal ions allows considerable modification of their dielectric constant, hardness, machinability, etc.
From a theoretical point of view, these materials also constitute the first simple ferroelectrics which will allow the development and validation of microscopic models.
A new leap was made in the early 1980s with the synthesis of PZN-PT and PMN-PT crystals which have the highest piezoelectric coefficients known to date.

Single crystals

A single crystal is a regular and periodic arrangement of atoms. It is in this form that natural piezoelectric materials such as quartz or tourmaline present themselves, and it is also in this form that they were used in first-generation applications before the development of ceramics.
Ferroelectric crystals can have a domain structure. One then distinguishes single-domain and multi-domain single crystals according to whether one or several polarisation directions coexist in the crystal. In a crystallographic description, multi-domain crystals are not strictly single crystals but twinned crystals; however, the usage is to continue to speak of single crystals.
The highest piezoelectric coefficients known to date are obtained for multi-domain single crystals. In practice, they have disadvantages which limit their use in many devices: cost, availability.