Reciprocal piezoelectric effect

Ultrasonic waves are generated by a piezoelectric crystal and emitted via a sound-conductive medium. These waves are reflected, broken, dispersed and absorbed by boundary layers. The piezoelectric crystal also acts as a sound-wave receiver and registers the modified ultrasonic waves (= reciprocal piezoelectric effect). They are then converted into an electric signal and displayed by means of oscilloscopic imaging. 

Certain crystals, such as quartz, feature a physical relationship between mechanical force and electric charge. When the crystal lattice ions are elastically shifted relative to one another due to an external force, an electric polarization can be detected by means of metallic electrodes on the surface. This so-called piezoelectric effect was first scientifically explained by the brothers Jacques and Pierre Curie in 1880 and forms the basis for piezo sensors (see Sect. 7.3). The effect is reversible and is then called reciprocal or inverse piezoelectric effect. If, for instance, an electric voltage is applied to a disc shaped piezo crystal, the thickness of the crystal changes due to the reciprocal piezoelectric effect. It is this property that is made use of in actuators. 

Figure 53. Schematic representation of Ihe direct piezoelectric effect (A) and the converse (reciprocal) piezoelectric effect (B) for an X-quartz plate (right-handed quartz) [222] F= Applied force F=Polarization =Applied electrical field...

Conversely, if such a crystal is placed in an electrical field it will become deformed, a phenomenon known as the converse or reciprocal piezoelectric effect (Fig. 53 B). The contributions of stress and strain in this case can be expressed ... 

Piezoelectric crystals are transducers that generate an oscillating electrical polarization when subjected to an external oscillating mechanical stress, and vice versa. The brothers Paul-Jacques and Pierre Curie discovered the piezoelectric effect in 1880 when they compressed certain crystals along certain axes (Curie and Curie, 1880). The reciprocal behavior was deduced from thermodynamic principles a year later by Gabriel Lippman... 

Closely related is the converse (reciprocal, inverse) effect, whereby a piezoelectric crystal becomes strained if an external electric field is applied. Both effects are the manifestation of the same fundamental property of the non-centric crystal. Otrly for historical reasons the term direct is used about the first and not the other effect. 

Unfortunately, even today, inverse (or converse) piezoelectricity is still sometimes called electrostriction because the name electrostriction suggests the electromechanical direction (electrical stimulus leads to mechanical response), while piezoelectricity seems to refer only to the opposite mechano-electrical direction. In order to avoid the misleading use of the term, it should be kept in mind that our modem terminology is based on phenomenological thermodynamical relations so that the linear effects of direct and inverse piezoelectricity must be identical due to the mathematically required reciprocity. 

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