Piezoelectricity

In the direct piezoelectric effect, mechanical stress changes the arrangement of charges in a suitable material, producing an electrical polarization and, under appropriate conditions, a measurable voltage. The converse effect runs in the other direction: an applied electric field changes the material's dimensions. These effects are linked aspects of electromechanical coupling. Their magnitude and direction depend on the material's structure and the orientation of the applied force or field; a given sample does not respond identically in every direction.

Piezoelectric behavior occurs in certain non-centrosymmetric crystals and in engineered materials with suitable structures. Quartz is a familiar natural example; tourmaline and Rochelle salt were among the crystals studied in early experiments. When a crystal is stressed along an appropriate direction, shifts in the positions of positive and negative charges can create a net polarization. The converse response follows from changes in the structure under an electric field. Not every crystal without a centre of symmetry is piezoelectric, and practical devices often use engineered ceramics or polymers as well as crystals.

In 1880, French physicists Pierre and Jacques Curie reported electrical charges produced when pressure was applied to certain crystals. The following year, Gabriel Lippmann deduced the converse effect from thermodynamic principles, and the Curie brothers experimentally confirmed it. The combination of these effects made piezoelectric materials useful both for converting mechanical signals into electrical signals and for producing controlled motion from electrical inputs.

Piezoelectric elements are used as sensors and actuators. In a microphone or instrument pickup, mechanical vibrations produce an electrical signal; a piezoelectric loudspeaker or transducer can use an electrical input to generate vibrations. Devices operating at ultrasonic frequencies use piezoelectric elements to send and receive sound waves, including in imaging and nondestructive testing. Pressure, vibration and acceleration sensors also exploit the direct effect. Some materials can convert small amounts of mechanical energy into electricity, but output depends on material, device design and the available motion; this does not mean that ordinary piezoelectric harvesters can supply large amounts of power.