Background

Simply stated, piezoelectric materials produce a voltage in response to an applied force, usually a uniaxial compressive force. Similarly, a change in dimensions can be induced by the application of a voltage to a piezoelectric material. In this way they are very similar to electro-strictive materials.

These materials are usually ceramics with a perovskite structure (see figure 1). The perovskite structure exists in two crystallographic forms. Below the Curie temperature they have a tetragonal structure and above the Curie temperature they transform into a cubic structure. In the tetragonal state, each unit cell has an electric dipole, i.e. there is a small charge differential between each end of the unit cell.

Figure 1. Shows the (a) tetragonal perovskite structure below the Curie temperature and the (b) cubic structure above the Curie temperature.

A mechanical deformation (such as a compressive force) can decrease the separation between the cations and anions which produces an internal field or voltage.

Some examples of piezoelectric materials are given in table 1.

Table 1. Piezoelectric constants of materials

Material


Piezoelectric Constant
(x1012 m/V)

Quartz

Barium titanate

Lead niobate

Lead zirconate titanate


2.3

100-149

80-85

250-365

Key Properties

· The ability to produce a voltage output in response to an applied stress

· The ability to produce a strain output (or deformation) in response to an applied voltage.
Applications
Transducers

Piezoelectric materials are used in electromechanical devices. In the case of a microphone transducer, sound of a particular frequency results in a strain in the material, which in turn induces an electric field. Similarly in speakers, a voltage input into the piezoelectric material can be converted into a mechanical strain, such as in a speaker transducer.
Other Applications

In radios, piezoelectric devices can be used in tuners, where the correct strain in the crystal will amplify only the desired frequency.

They are also employed in fine watch circuits, ones with “quartz movements”.