By piezoelectricity is meant a linear electromechanical interaction between the mechanical and electrical states of a crystal.
The direct piezoelectric effect is that piezo ceramic generates an electrical charge during mechanical distortion or load. During an inverse piezoelectric effect the piezoceramic body changes under the influence of an electrical field.
Figure: Piezoelectric effect / influence of external forces. In dependence on the direction of the force, electrical charges of the corresponding leading sign are generated (source: CeramTec).
The inverse piezoelectric effect is characterized by a deformation which is proportional to an external electric field generated by applying an electric voltage.
Figure: Inverse piezoelectric effect under the influence of electrical fields. The body changed its dimension along with the change voltage (source: CeramTec).
All the components for the applications as sensors and actuators are derived from the basic oscillation modes. Figure: oscillation modes of piezo ceramic components (source: CeramTec).
In case the piezo ceramic consists out of one layer, one speaks of single-layer technology. As a standard, piezo ceramic discs, plates, strips, rings, spheres, tubes and a variety of special geometries are produced today. If the piezo ceramic component is composed of several active piezo ceramic layers, this is called multilayer technology.
EA further version is the flexural transducer. This is the result, e.g. when piezo ceramic elements are glued with a carrier material. The piezo ceramic responds with change in length while applying electrical voltage. A large deflection of the composite as a function of polarity and voltage is obtained with moderate forces - similar to the bimetal.
The most important piezo electric ceramic materials of today are lead zirconium titanate (PZT). The specific properties of these ceramics depend, on the ratio of lead zirconium to lead titanate, among other influences. This results in manifold possibilities. So-called "piezo electrically hard" materials are used for performance applications (e.g. ultrasonic generation). "Piezo electrically soft" materials with small losses are important for the used in actuator and sensor applications, where it is subject to deflection under voltage or high sensitivity.
Due to EU regulations such as "Restriction of Hazardous Substances" (RoHs) and ELV (End of Life Vehicles Directive), the possibilities of using lead free materials are under investigation.
Despite some progress, a replacement of PZT is currently not foreseeable. For the further use of PZT an exemption is attached to the corresponding directives.
Piezo ceramics has developed a wide range of applications in electronics, the automotive industry, medical technology, mechanical engineering as well as in consumer applications. Piezo ceramic components are used as converters in telecommunications, acoustics, hydro acoustics, material testing, ultrasonic processing, liquid sputtering, flow measurement, distance measurement and in medical technology. As actuators piezo ceramics can be found in micro pumps, optical systems, gas valves, inkjet printers, textile machines and braille modules. Piezo ceramics as sensor react to force, pressure and acceleration and enable the monitoring of a wide range of processes.
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