= 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,
$$
\Delta Q=A p\Delta T,\qquad i=A p\frac{dT}{dt}.
$$
Consequently \b[<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 is
$$
p_i^{\rm secondary}=\sum_j e_{ij}\alpha_j,\qquad
p_i^{\rm free}=p_i^{\rm clamped}+p_i^{\rm secondary},
$$
with 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.<Pyroelectric nanogenerators for harvesting thermoelectric energy by Ya Yang and others>[Abstract: vertically aligned ZnO nanowire arrays used for pyroelectric generation]{ref}
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