Pyroelectricity GLY 4200 – Lecture 4 –Fall, 2016

Pyroelectricity Effect Diagram Image: pyro.jpg Certain crystals, when subject to an increase in temperature, will become polarized along a unique polar axis. Of the twenty classes that possess polar axes, only 10 have a unique polar axis. The degree of polarization is proportional to the magnitude of the temperature change, as shown in equation 1:

Polarization Pi = pi ΔT where Pi is the polarization vector, pi are the pyroelectric coefficients, and ΔT is the temperature change The effect is used in infra-red heat detectors. Quartz is the mineral usually employed. Both pyroelectricity and piezoelectricity can be used as an aid in determining which crystal class an unknown mineral belongs to. If the X-ray data is ambivalent, the ability to show that a crystal is either pyroelectric or piezoelectric would limit the possible crystal classes to which it could belong. Pyroelectricity was probably first observed in tourmaline by ancient Greeks, but quantitatively investigated only in the eighteenth century, during the early studies of electrostatics. Sir David Brewster, a Scottish scientist, was the first to use the term pyro (fire) electricity in 1824 when describing this phenomena in one of his numerous and famous contributions to the Encyclopedia Britannica.

PVDF Polymers - such as Polyvinylidenefluoride (PVDF) - consist of long chains of  molecules, as is shown in the image above

Pyroelectricity in Polymers Certain molecular groups act as dipoles and can be oriented in an electric field When the poled polymer is exposed to heat, the sample expands, causing the dipole spacing to change, and generating a charge Pyroelectric materials have a spontaneous polarization whose amplitude changes under the influence of temperature gradients. The discovery of PZT triggered many applications based on this phenomenon, such as infrared detection, thermal imaging (absorption of energy resulting in polarization changes) and dielectric bolometers.