Piezoelectricity is “pressure electricity”. The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. They found that if certain crystals were subjected to mechanical strain, they became electrically polarized. Also these same materials deformed when exposed to an electric field. This is known as inverse piezoelectric effect.
For a crystal to exhibit the piezoelectric effect, its structure should have no centre of symmetry. A stress (tensile or compressive) applied will alter the separation between the positive and negative charge sites leading to a net polarization at the crystal surface. It's also reciprocal, so if the crystal is exposed to an electric field, it will experience an elastic strain causing its length to change according to the field polarity.
Naturally-occurring crystals, as for instance quartz, tourmaline and sodium potassium tartrate exhibit piezoelectric effecs. Besides crystals an important group of piezoelectric materials are man made piezoelectric ceramics. Most are compounds of lead zirconate and lead titanate, known as PZTs.
PZTs can be considered as a mass of minute crystallites. Above a temperature known as the Curie point, crystallites exhibit simple cubic symmetry. This is centrosymmetric with positive and negative charge sites coinciding, so there are no dipoles present. Below the Curie point the crystallites take on tetragonal symmetry in which the positive and negative charge sites no longer coincide, so each elementary cell then has a built-in electric dipole which may be switched to certain allowed directions by the application of an electric field.
PZT ceramics may be made piezoelectric by a poling treatment, which involves exposing it to a strong electric field at a temperature slightly below the Curie point. When the field is removed and the ceramic is cooled, the dipoles remain locked in approximate alignment, giving the ceramic material a remanent polarization and deformation.
PZT materials main application areas are actuating systems in which the ability to generate high force or large displacements is more important. Other applications are high-performance ultrasonic transducers, where high conversion efficiency and design versatility are important.Here PZT is ideal thanks to the ease with which it can be fashioned into almost any shape, and, in contrast to quartz, can be polarized in any desired direction.
The modern technical world demands the availability of sensors to measure and convert a variety of physical quantities into electrical signals. These signals can then be fed into data processing systems and electronically evaluated and processed.
Sensors can therefore be regarded as links between the non-electrical environment and data-processing electronic systems. Some of the quantities to be measured are mechanical, such as force, acceleration or pressure. PZT transducers are ideal for converting such quantities into electrical signals. Two fundamental types can be distinguished: the axial sensor and the bending or flexure sensor.
In addition to these PZT sensors, which make direct use of the piezoelectric effect for measurements, there is a wide range of acoustic measuring systems that incorporate PZT elements.
A PZT disc can operate as an acceleration sensor if it's clamped firmly between a seismic mass and a base plate. It's also possible to allow the PZT element itself to act as the seismic mass.
Mechanical sensors can also take the form of a flexure element. Two PZT strips or plates, polarized in their thickness direction, are bonded together to form what's known as a bimorph. The two plates can be polarized in opposite directions (series bimorph) or in the same direction (parallel bimorph). One end is fixed to a base plate while the force to be measured acts on the other, free, end (cantilever mounting)
Its favorable cost price and simple construction make the series bimorph the preferred type for sensors. Flexural-mode sensors have a far lower stiffness than axial sensors and therefore a lower resonant frequency. They also have lower mechanical and electrical impedances, so they're better adapted to "soft" mechanical movements and, in general, require simpler amplifiers than axial sensors.
