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 Tourmaline becomes 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.
- Piezoelectric Crystal Classes: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3,
32, 3m, 6, -6, 622, 6mm, -62m, 23, -43m
- Pyroelectric: 1, 2, m, mm2, 4, 4mm, 3, 3m, 6, 6mm
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. |