Piezoelectricity

In 1880 the Curie brothers, Pierre and Jacques, discovered the piezoelectric effect, which is one of the basic properties of crystals, ceramics, polymers and liquid crystals. There are many different ways to describe this phenomenon. Perhaps the most commonly used definition states that a dielectric material is piezoelectric, if under mechanical stress an internal dielectric displacement is caused. This displacement is manifested as an internal electric polarization or an external electric charge. Conversely, a change in dimension will be obtained if an electric field is applied to a piezoelectric material. This effect is called the reverse piezoelectric effect or piezostriction.

The figure shows a SiO2 tetrahedron. The application of stress results in a finite displacement of the cation charge relative to the center of anion charges; hence this structural unit is piezoelectric. (Note that it is not possible to calculate piezoelectric effects based on simple considerations of symmetry, it is only possible to say that piezoelectricity is not excluded by the crystal symmetry).

There is one important parameter that has to be fulfilled for piezoelectricity: the axis, where pressure is applied has to be polar. This is an axis of rotation where the two orientations along the axis are not equal. Because of that requirement gases and liquids cannot be piezoelectric as well as many different crystals, such as NaCl, where the axis is not polar.

Gerthsen and Vogel have given a simple illustration for polar axes: the axis of a beer-bottle within a stack of boxes is truly tetragonal. Four neighboring bottles positioned at 90° angular distance surround it. The top and the bottom of a bottle are not identical; therefore it is polar.

-	In a piezoelectric material, the application of a stress along a polar axis produces an electric field between the two opposing faces.
-	Conversely, an application of an electric field causes a deformation of the material.
-	The effect is reversible; a change in the sign of the stress changes the sign of the potential difference.

Pyroelectricity

Many piezoelectric materials, such as tourmaline, have the tendency to exhibit a change in internal electrical polarization as a response to temperature changes. Two effects contribute to that phenomenon: the primary pyroelectric effect, caused by a change in existing dipole moments in the crystal and the secondary, caused by a change in charge density upon thermal expansion of the material.

This effect produces confusing aberrations for many industrial applications since pyroelectric "artefacts", that have to be carefully separated, often superimpose the piezoelectric signals of interest.

-	An electric field develops in a piezo/pyroelectric crystal as a result of temperature change.
-	Pyroelectricity requires the presence of permanent electric dipoles (whose magnitude is affected as a result of temperature changes).

The figure shows the dependence of piezoelectric, pyroelectric and ferroelectric materials. As a consequence of symmetry all ferroelelectric and pyroelectric materials are also piezoelectric.