Pyroelectric Gems

Pyroelectricity

Pyroelectricity is the ability of certain materials to generate an electrical potential when they are heated or cooled. As a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence, an electrical potential is established. The name Pyroelectricity is derived from the Greek word pyr for fire, and electricity.

Pyroelectricity can be visualized as one side of a triangle, where each corner represents energy states in the crystal: kinetic, electrical and thermal energy. The side between electrical and thermal corners represents the pyroelectric effect and produces no kinetic energy. The side between kinetic and electrical corners represents the piezoelectric effect and produces no heat.

Although artificial pyroelectric materials have been engineered, the effect was first discovered in minerals such as Quartz, Tourmaline, Londonite and other ionic crystals. The pyroelectric effect is also present in both bone and tendon.

Pyroelectric charge in minerals develops on the opposite faces of asymmetric crystals. The direction in which the propagation of the charge tends toward is usually constant throughout a pyroelectric material, but in some materials this direction can be changed by a nearby electric field. All pyroelectric materials are also piezoelectric, the two properties being closely related.

Very small changes in temperature can produce an electric potential due to a materials’ pyroelectricity. Passive infrared sensors are often designed around pyroelectric materials, as the heat of a human or animal from several feet away is enough to generate a difference in charge.

History
The first reference to the pyroelectric effect is in writings by Theophrastus in 314 BC, who noted that Tourmalinebecomes charged when heated. Sir David Brewster gave the effect the name it has today in 1824. Both William Thomson in 1878 and Voight in 1897 helped develop a theory for the processes behind pyroelectricity. Pierre Curie and his brother, Jacques Curie, studied pyroelectricity in the 1880s, leading to their discovery of some of the mechanisms behind piezoelectricity.

Crystal Classes
Crystal structures can be divided into 32 classes, or point groups, according to the number of rotational axes and reflection planes they exhibit that leave the crystal structure unchanged. Of the thirty-two crystal classes, twenty-one are non-centrosymmetric (not having a center of symmetry), and of these, twenty exhibit direct piezoelectricity the remaining one being the cubic class 432. Ten of these are polar (i.e. spontaneously polarise), having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric. Twenty of the 32 crystal classes are piezoelectric. All 20 piezoelectric classes lack a center of symmetry. Any material develops a dielectric polarization when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only 10 of the 32 point groups are polar. All polar crystals are pyroelectric, so the 10 polar crystals classes are sometimes referred to as the pyroelectric classes.

The property of pyroelectricity is the measured change in net polarization (a vector) proportional to a change in temperature. The total pyroelectric coefficient measured at constant stress is the sum of the pyroelectric coefficients at constant strain (primary pyroelectric effect) and the piezoelectric contribution from thermal expansion (secondary pyroelectric effect). Under normal circumstances, even polar materials do not display a net dipole moment. As a consequence there are no electric dipole equivalents of bar magnets because the intrinsic dipole moment is neutralized by “free” electric charge that builds up on the surface by internal conduction or from the ambient atmosphere. Polar crystals only reveal their nature when perturbed in some fashion that momentarily upsets the balance with the compensating surface charge.