All materials exhibit a small change in dimensions when subjected to an electric field. However, some materials also show the reverse effect, with the development of electric polarisation on application of stress and resultant strain. These materials are said to be piezoelectric.
The piezoelectric effect was discovered in the 19th Century, but remained something of a curiosity until the 1940s. Piezoelectric materials only really became of use once amplifiers of sufficient power were developed to produce useable signals from them.
Piezoelectric materials exhibit intrinsic (spontaneous) polarisation. Most of the piezoelectric materials are ceramic in nature, barium titanate for example, but there are some polymeric materials that are used for specialist applications. Piezoelectric ceramics are usually polycrystalline materials that are divided up into regions of similar polarisation (domains). Once aligned, these domains produce a net polarisation. If an electric field is applied, the dipoles within the domains either contract or expand (resulting in a change in the volume). If a strain is applied, the dipoles are again forced to contract or expand, this time producing a potential difference.
Piezoelectric materials have found applications as gas igniters, displacement transducer/accelerometers, actuators, delay lines, wave filters, and as generators of ultrasonic energy. Arrays of piezoelectric elements have been used to produce ultrasonic imaging equipment.