Pyroelectric effects and discharge currents in polyethylene terephthalate (PET)

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Abstract
We have studied the depolarization of polyethylene terephthalate (PET) films under short‐circuit conditions, using white light as a probe. Working at room temperature with times in the range 1–104 sec (i.e., in the minimum between the α and β dielectric loss peaks) we have established that the discharge is almost entirely of dipolar origin, that the enhanced current under illumination is due to warming and not to detrapping, and that the transient pyroelectric response is accounted for quantitatively by thermal expansion of the polarized material in the thickness direction. The observed magnitude of the pyroelectric current falls with thickness and the time constant rises, but these are largely artifacts due to lateral thermal gradients in the material. A representative value for the pyroelectric coefficient is 4 nC m−2 K−1. The frozen‐in polarization is thickness dependent, and this suggests a nonuniform distribution of dipoles across the film. Excess charge injected by using uv light during polarization plays no significant part in the discharge, indicating that any stored charge remains deeply trapped.