The electrocaloric efficiency of ceramic and polymer films

Voltage-driven thermal changes known as electrocaloric (EC) effects are large in ferroelectric thin films near the Curie temperature, where entropic electrical phase transitions may be reversibly driven by electric fields DE that approach the high breakdown fields generically associated with thin films. The thermal changes in a single film are small, but macroscopic assemblies of ferroelectric films in the multilayer capacitor geometry have been proposed for cheap, environmentally friendly and energy efficient cooling applications. However, candidate EC materials have hitherto only been analysed in terms of EC performance, i.e. the change in isothermal entropy DS, the isothermal heat Q, and the change in adiabatic temperature DT. Surprisingly, the corresponding electrical work W that is done when charging and discharging the host EC capacitors has been neglected. Therefore we introduce here electrocaloric efficiency h to describe the ratio of reversible electrocaloric heat to reversible electrical work under isothermal conditions. This figure of merit permits a comparison of EC materials that does not depend on details of any refrigeration cycle, i.e. the type of cycle, the hot and cold temperatures of the EC material, and the sink and load temperatures. We investigate in detail 350 nm-thick films of the prototypical electrocaloric ceramic PbZr_0.95Ti_0.05O₃ (PZT) with Pt electrodes, and 0.4 - 2.0 µm-thick films of the prototypical electrocaloric polymer poly(vinylidene fluoride-trifluoroethylene) (55/45 mol%) [P(VDF TrFE)] with Al electrodes. For these films near their respective Curie temperatures, the electrical data that were used to predict large and nominally reversible EC effects, are used here to calculate the nominally reversible electrical work, yielding h = 3.0 for the ceramic and h = 7.5 for the polymer. We subsequently use the Landau theory of phase transitions to reveal the presence of non electrocaloric edge layers that increase the work, and to reveal the role of a previously observed complexity in the polymer phase transition of ref. 3. In a wider study, we find that ceramic and polymer films possess overlapping efficiencies, and therefore both classes of material may be attractive for applications despite previous indications. More generally, optimising h should in future guide the selection of electrocaloric, elastocaloric and magnetocaloric materials for novel cooling devices that are energy efficient.