Dielectric Phenomena in Solids

4.4 Pyroelectric Phenomena

Of 20 noncentrosymmetric crystal classes, 10 contain a unique polar axis under the unstrained condition. This implies that such crystals are already spontaneously polarized in a certain temperature range. Thus, they exhibit both piezoelectric and pyroelectric effects. For the latter, the crystal will develop free electric charges at the surfaces when uniformly heated because the polarization inside the crystal is changed by the heat. It should be noted that if a crystal specimen is heated nonuniformly (for example, with a temperature gradient along the specimen), then the temperature gradient will also develop a mechanical stress. So in this case, the specimen may have mixed piezoelectric and pyroelectric effects. ...