Pyroelectricity

Pyroelectricity is the generation of electric charge on the surface of certain crystals when their temperature changes. It occurs in materials that have a permanent polarization at a reference temperature, and in which the polarization varies with temperature. The pyroelectric coefficient p = dP/dT describes how the spontaneous polarization P changes as temperature changes. A temperature change ΔT induces a change ΔP = p ΔT in polarization, producing a surface charge density Δσ ≈ ΔP. If the crystal is connected to an electrical circuit, a current i = A p (dT/dt) flows, where A is the crystal’s cross-sectional area.

The effect requires non-centrosymmetric, polar crystal structures; only such materials exhibit pyroelectricity. Many pyroelectrics are ferroelectric, and ferroelectrics are typically pyroelectric, but pyroelectricity can occur without switchable polarization in some cases.

Common pyroelectric materials include natural tourmaline and Rochelle salt, as well as synthetic compounds and polymers such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and polyvinylidene fluoride (PVDF) and related copolymers. The response is anisotropic, depending on crystal orientation and cut.

Applications span infrared sensing and thermal imaging, uncooled detectors, and some energy-harvesting devices. A key characteristic is that the signal requires a temperature change and is limited by the material’s thermal time constants and heat transfer, distinguishing pyroelectric detectors from volume-resistive sensors.