When a piezoelectric crystal is mechanically stressed, it gets polarized, that is, charges appear on its surface. This polarization leads to a voltage difference between the faces of the crystal. For a crystal to be piezoelectric, it is required that it must NOT have a center of symmetry. Therefore, not all crystals are piezoelectric. If the crystal has a center of symmetry, under an applied mechanical stress, the net polarization remains zero. As a result, there is no voltage difference and such a crystal is non-piezoelectric.
There is a total of thirty-two crystal classes, twenty-one of these do not have a center of symmetry. However, only twenty are piezoelectric.
A piezoelectric crystal is, in effect, an electromechanical transducer since it converts an electric signal to a mechanical signal. In fact, very high voltages correspond to only tiny changes in the size of the crystal.
Piezoelectric crystals are put to various applications:
- To generate ultrasonic waves in crystals, for non-destructive testing.
- In microphones, to detect sound. (through the mechanical stress provided by sound waves, a voltage is produced)
- As high voltage sources.
- As actuators, in scanning tunneling microscopes to position the sensing probe.