Doubling the electrocaloric cooling of poled ferroelectric materials by bipolar cycling

We have investigated the entropy change in the ferroelectric phase of poly(vinylidene fluoride-trifluoroethylene) 70/30 films by direct heat flux calorimetry using Peltier cell heat flux sensors. We find that by applying a negative electric field to a positively poled state, the entropy can be further increased without any significantly change of the remanent polarization or the domain structure. By cycling between positive and negative values of the electric field, the electrocaloric effect (ECE) can be then improved by a factor of 2. As an example, we measured, around the positive remanence Pr = 60 × 10- 3C m- 2, a fully reversible entropy change |Δ s| = 1 J kg- 1K- 1 for a field change from 40 × 106 to - 40 × 106V m- 1 and a maximum of |Δ s| = 3.2 J kg- 1K- 1 for an asymmetric field change from 200 × 106 to - 40 × 106V m- 1. This effect can be exploited to significantly increase the range of operating temperature for ECE materials below their Curie temperature.