A Historical Perspective on the Occurrence of Piezoelectricity in Materials
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ity" was coined from the Greek verb "piezen," to press, by Pierre and Jacques Curie in the 1880s during an investigation of the symmetry in crystals. In this work it was found that certain crystals, lacking a center of symmetry, produced an electrical charge when mechanically deformed. The converse effect was also found to occur, whereby applying an electric field caused the crystal to change its shape.1-2 This phenomenon was attributed to a deformation of the net internal polarization in the crystal. When no external forces are present, the centers of positive and negative charges will coincide, and there is no net polarization. The application of a stress, be it mechanical (pressure) or electrical (applied field), causes a displacement of the centers of gravity of the positive and negative charges. In the absence of a center of symmetry, the charge displacement will be nonsymmetrical and thereby produce an induced dipole moment.2 This dipole moment, if produced by a mechanical stress, will cause the surfaces to develop an effective charge. If an external field displaces the charges, by electrostatic attraction or repulsion, it produces a mechanical strain which causes the material to deform. The mathematical History of Piezoelectric Materials relations describing this effect were and Their Applications developed in the few years following Piezoelectricity or "pressure electrictheir discovery, making use of tensor notation to describe the directionality of the applied stress and the resultant *An earlier version of this article was published in Journal of Materials Educa- strain. The piezoelectric effect was found to tion, Vol. 9, No. 3. Reprinted with permission. 22
be the most significant in highly polar single crystals of quartz, Rochelle salt (sodium potassium tartrate) and tourmaline. An extremely large effect was found in Rochelle salt, owing to the presence of a. property that was later termed ferroelectricity. This, like piezoelectricity, is manifest in asymmetrical crystals but produces domains of spontaneous polarization, the direction of which can be reversed in an electric field. A ferroelectric domain is a region comprised of many millions of unit cells which contain the same electric orientation. For a crystal such as Rochelle salt, a net electric moment, or polarization, is present if a single polar domain constitutes a large part if not the entirety of the crystal. This net electric moment is a requisite for piezoelectric activity, but can also be produced in nonferroelectric crystals such as quartz and tourmaline, by the application of an electrical or mechanical stress as p r e v i o u s l y described. That is to say, ferroelectricity is not a requirement for a piezoelectric effect in single crystals. It is, however, found to enhance the piezoelectric effect by the high degree of dipole ordering which occurs.3 The magnitude of the piezoelectric activity is a function of the crystal symmetry elements that exist, and these symmetries are temperature sensitive in ferroelectric materials.3 At a part
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