Piezoelectric and Pyroelectric Materials and Their Applications

Abstract

The phenomenon of piezoelectricity, the release of electric charge under the application of mechanical stress, occurs in all noncentrosymmetric materials. Pyroelectricity, the release of charge due to a material’s change of temperature, occurs in all materials that belong to a polar crystal symmetry class. It should be noted that, as not all noncentrosymmetric classes are polar (222, 4, 422, $\overline 4 2m$, 32, 6, 622, $\bar 6m2$, 23, and $\bar 43m$ are the nonpolar, noncentrosymmetric classes), not all piezoelectric crystals are pyroelectric. However, all pyroelectric crystals are piezoelectric. (As a final note, crystals belonging to the noncentrosymmetric cubic class 432 are nonpiezoelectric because all the piezoelectric moduli vanish under the operation of the symmetry elements.) Ferroelectrics form a subset of the set of pyroelectrics because they are polar materials in which the direction of the polar axis can be changed by the application of an electric field. As a consequence they are both pyroelectric and piezoelectric. As many of the largest pyroelectric and piezoelectric effects occur in ferroelectric materials, they have become very important technologically. Furthermore, the fact that the polar axis can be reoriented by the application of a field means that polycrystalline ferroelectrics in which the crystallographic axes of the component crystallites are randomly oriented can be made to show polar properties by applying a sufficiently large electric field (the process of “poling”).