Pyroelectrics

crystal dielectrics that have spontaneous polarization, that is, polarization in the absence of external perturbations. The spontaneous polarization of pyroelectrics is usually not appreciable, since the electric field thus generated is compensated by the field of the free electric charges that “flow” to the surface of a pyroelectric from within and from the surrounding air. Upon a change in temperature, the magnitude of spontaneous polarization changes. This causes the appearance of an electric field that can be observed until the free charges compensate for it. This phenomenon is called pyroelectricity, or the pyroelectric effect.

A typical pyroelectric is tourmaline, in which a temperature change of 1°C induces a field E ≈ 400 volts per cm. A change in spontaneous polarization and the appearance of an electric field in a pyroelectric can occur not only upon a change in temperature but also upon deformation of the pyroelectric. Thus, all pyroelectrics are piezoelectrics, but not conversely (see Figure 1).

Figure 1

The existence of spontaneous polarization—that is, noncoinci-dence of the “centers of gravity” of the positive and negative charges—is due to the rather low crystal symmetry.

Ferroelectrics are a special group of pyroelectrics. If a ferroelectric is heated, then at a certain temperature its spontaneous polarization will disappear and the crystal will enter a nonpyro-electric state (phase transition). At temperatures close to the phase transition temperature, the magnitude of spontaneous polarization varies sharply with a change in temperature, so that the pyroelectric effect is particularly great in this region.

An effect inverse to the pyroelectric effect also exists: if a pyroelectric is placed in an electric field, its polarization changes, accompanied by heating or cooling of the crystal. The temperature change here is directly proportional to the electric field intensity: ΔT≈E. This phenomenon is called the linear electro-caloric effect. There is also a quadratic electrocaloric effect, when the temperature change is directly proportional to E².

Pyroelectrics are used in engineering as radiation indicators and detectors. Their operation is based on the recording of electric signals that arise in pyroelectrics upon a change in temperature caused by radiation.