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Cubic (paraelectric) and tetragonal (ferroelectric) structure of PZT and BaTiO₃ before and after an electric field has been applied or a mechanical stress taken effect.



Symbolic representation of the electrical reorientation processes in piezoelectric ceramic crystallite and domain structure.

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The Piezoelectric Effect 

Piezoelectric ceramics belong to the group of ferroelectric materials. Ferroelectric materials are crystals which are polar without an electric field being applied. This state is also termed spontaneous polarization. Characteristic of this state is the thermodynamically stable reversibility of the axis of polarization under the influence of an electric field, described graphically by a hysteresis loop. The reversibility of the polarization, and the coupling between mechanical and electrical effects are of crucial significance for the wide technological utilization of piezoceramics. From a crystallographic point of view, these piezoelectric materials exhibit what is called Perovskite crystalline structure. This applies to a series of compounds with three types of atoms with the general formula ABC .

The main piezoceramics in use today, PbTiO₃ - PbZrO₃ are sythesized from the oxides of lead, titanium and zirconium. BaTiO₃ is also used. Special dopings of these leadzirconate- titanate ceramics (PZT) with, for example, Ni, Bi, Sb, Nb ions etc., make it possible to adjust individual piezoelectric and dielectric parameters as required. These materials are not ferroelectric above a characteristic temperature, known as the Curie temperature. They are in a paraelectric state, i.e. no dipoles are present. The relative dielectric constant has a distinct maximum in the vicinity of the Curie temperature. Below the Curie point of the material, the cubic, electrically neutral crystalline form gives way to lattice distortions, resulting in the formation of dipoles and rhombohedral and tetragonal crystallite phases, which are of interest for piezo technology.