The Piezoelectric Effect – an Indispensable Solid State Effect for Contemporary Actuator and Sensor Technologies

The piezoelectric effect, which was discovered for the first time by the brothers Pierre and Jacques Curie, combines electrical with mechanical quantities and vice versa. If piezoelectric materials (e.g. quartz, turmaline) are subjected to electrical signals along certain crystal orientations, deformations along well-defined crystal orientations appear. Contrary, a mechanical deformation results in a generation of polarization charges. Even if there exist numerous publications on this so-called direct and reciprocal piezoelectric effect, the aim of this paper is to convey a clear and easy understanding of this essential solid body effect in particular for the non-specialist, since a large number of publications is rather superficial and unfortunately sometimes incorrect. A variety of ionic crystals show the direct and reciprocal piezoelectric effect. In this paper, an illustrative representation of both effects is given by the molecular structure of alpha-quartz, a stable modification of the silicon dioxide, the second most common mineral of the earth’s crust. Both effects always involve an important physical quantity, the so-called electrical polarization, which represents an Euclidean vector being defined as the quotient of the total dipole moment resulting from the deformation of the hexagonal unit cell of alpha-quartz and the volume of the unit cell. Based on the physical explanation of the dipole moment, it is shown how the directions of the electrical polarization can be calculated in a simple manner. This finally enables the physical understanding of both effects that are nowadays used in numerous technical applications in the broad field of sensor and actuator technologies.