3: Pyroelectrics and Electro-Optics

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Electrical engineers interested in materials often focus their study on semiconductors or occasionally conductors. However, energy conversion devices are made out of all types of materials. In the last chapter we discussed capacitors and piezoelectric devices. Both are constructed from a layer of insulating material between conductors. The properties of this dielectric layer determine the properties of the resulting devices. This chapter discusses two additional types of devices that involve material polarization of insulators, pyroelectric devices and electro-optic devices. As with other types of energy conversion devices, these can operate two ways. Pyroelectric devices can convert a temperature difference to a material polarization and therefore electricity, or they can convert a material polarization to a temperature difference. Electro-optic devices can convert optical electromagnetic radiation to a material polarization or vice versa. As with the devices studied in the last chapter, these devices are constructed around a dielectric layer, and the choice of material in the dielectric layer determines the behavior of the device. Studying these devices is worthwhile even though they are encountered significantly less often than capacitors and piezoelectric devices because this study illustrates the variety of energy conversion devices that engineers have produced.

If a solid is heated enough, it melts. Some materials have multiple crystal structures that are stable at room temperature. These materials may be converted from one crystal structure to another by heating and cooling. Similar eects can occur if energy is supplied by shining a strong enough laser on the material instead of heating it. This chapter is not concerned with effects involving melting or thermally changing the crystal structure from one phase to another. Instead, we consider the case when a small amount of energy is supplied, by heat or by electromagnetic radiation. The energies involved are enough to change the material polarization and the internal momentum of electrons but not the location of the nuclei of the material, for example.

Thumbnail: Pyroelectrictic device. (CC BY-SA-NC 2.0 UK; via https://www.doitpoms.ac.uk/tlplib/pyroelectricity/infrared.phpTLP)