 Solution

A pyroelectric material has a spontaneous electric polarization that changes with temperature. electrodes collect the resulting change in compensating charge. For a uniform element,Consequently temperature variation in time, rather than a steady temperature difference alone, drives a pyroelectric generator. A cyclic thermal input and a suitable electrical circuit are needed for repeated energy extraction.

The primary pyroelectric effect is the change of electric polarization at fixed strain. The secondary pyroelectric effect arises because thermal expansion strains a piezoelectric crystal and thereby changes its electric polarization. In component notation its contribution iswith piezoelectric coefficients $e_{ij}$ and thermal-expansion coefficients ${\alpha }_j$. Choosing orientation, mechanical constraint and substrate expansion so that the secondary contribution reinforces the primary one can increase the response; the signs can instead oppose one another.

An array of slender nanowires can expand more freely than a film clamped to a substrate and can exchange heat rapidly because of its high surface-to-volume ratio. Reduced clamping can recover a secondary response suppressed in the film. Faster thermal cycling can also increase electric current through $dT/dt$. This is a geometrical opportunity rather than a universal increase of the intrinsic coefficient: screening, leakage, orientation and the actual thermal excursion must be compared. ZnO nanowire generators provide an experimental realization of pyroelectric energy conversion.^[ref]