Pyroelectricity is the ability of certain materials to
generate a temporary voltage when they are heated or cooled.The change in
temperature modifies the positions of the atoms slightly within the crystal
structure, such that the polarization of the material changes. This
polarization change gives rise to a voltage across the crystal. If the
temperature stays constant at its new value, the pyroelectric voltage gradually
disappears due to leakage current (the leakage can be due to electrons moving
through the crystal, ions moving through the air, current leaking through a
voltmeter attached across the crystal, etc.).
Pyroelectricity should not be confused with
thermoelectricity: In a typical demonstration of pyroelectricity, the whole
crystal is changed from one temperature to another, and the result is a
temporary voltage across the crystal. In a typical demonstration of
thermoelectricity, one side of the material is kept at one temperature and the
other side at a different temperature, and the result is a permanent voltage
across the crystal.
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,effect was first discovered in minerals such as tourmaline. 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.
These materials are said to exhibit ferroelectricity. All pyroelectric
materials are also piezoelectric, the two properties being closely related.
However, note that some piezoelectric materials have a crystal symmetry that
does not allow pyroelectricity.
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.
The pyroelectric coefficient may be described as the change
in the spontaneous polarization vector with 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).
Thermal expansion is due to heating and cooling.
Artificial pyroelectric materials, usually in the form of a
thin film, are of gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl
fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine.