Science Note

*Figure 1: The pyroelectric effect*

Pyroelectric effect: Crystals with a unique direction (a direction not repeated by any symmetry element) can maintain a permanent dipole in the absence of an applied field as positive and negative charges will not cancel along the unique direction. When the temperature is constant the charged faces of the crystal attract opposite charges from the surroundings that cancel the charge. But when the temperature changes, the positive and negative charges along the unique direction move relative to one another, leading to a change in polarization. This is known as the pyroelectric effect.

Top

Pyroelectric coefficient (μC/m².K): The change in polarization ΔP caused by a change of temperature ΔT is given by

ΔP=γΔT

where γ is the pyroelectric coefficient. Thus

Two figures-of-merit are used to characterize pyroelectrics: the voltage responsivity (F_v) and the specific detectivity (F_D)

Here γ is the pyro-electric charge coefficient, C_p the specific heat, ε_r the dielectric constant and tan δ the dielectric loss tangent. Above the Curie temperature T_c the polarization is lost completely. These are the parameters used for selection.

Top

Electrocaloric effect: The equivalent reverse effect is known as the electrocaloric effect. It describes the change in temperature caused by a change of electric field across the material.

Top

Applications: Pyroelectric materials are used in temperature sensors, infra-red detectors and thermal imaging. Examples are IR burglar alarms and thermal imaging for photography of wild life at night - in both cases the heat radiated by the causes a change in the polarization at the detector or image-pixel. Compared to alternative semiconductor devices, pyroelectric materials have a strong response across a wide frequency spectrum, are effective at room temperature and have a faster response time.

Top

Further reading

Ashby, Michael, Shercliff, Hugh and Cebon, David. Materials: Engineering, Science, Processing and Design. s.l. : Elsevier, 2014.

Chung, Deborah D.L. Functional Materials: Electrical, Dielectric, Electromagnetic, Optical and Magnetic Applications. s.l. : World Scientific, 2010.

Uchino, Kenji. Ferroelectric Devices. s.l. : Marcel Dekker, Inc., 2000

Haertling, Gene H. Ferroelectric Ceramics: History and Technology. 4, 1999, J. Am. Ceram. Soc., Vol. 82, pp. 797-818.

Top