6.9: Pyroelectricity
Pyroelectricity is the property presented by certain materials that exhibit an electric polarization P i when a temperature variation δΘ is applied uniformly:
P i = p i T δΘ
where p i T is the pyroelectric coefficient at constant stress. Pyroelectric crystals actually have a spontaneous polarization, but the pyroelectric effect can only be observed during a temperature change. If the polarization can be reversed by the application of an electric field, the crystal is ferroelectric.
If the crystal is also piezoelectric, the polarization due to an applied temperature variation is also partly due to the piezoelectric effect. The coefficient describing the pure pyroelectric effect is the pyroelectric coefficient at constant strain, p i S . The two coefficients are related by:
p i T = c ijkl d kln α jn + p i S
where the c ijkl are the elastic stiffnesses, the d kln the piezoelectric coefficients and the α jn the linear thermal expansion coefficients.
The converse effect is the electrocaloric effect . If a pyroelectric crystal is submitted to an electric field, it will undergo a change of entropy Δσ:
Δσ = p i E i
and will release or absorb a quantity of heat given by Θ V Δσ where Θ is the temperature of the specimen and V its volume.