Maria Barrera

Electrocaloric materials are currently under investigation because of their potential use in active cooling systems. As electrocaloric transducer materials, electroactive polymers such as PVDF copolymers with trifluoroethylene [P(VDF-TrFE)] and PVDF terpolymers with trifluoroethylene and chlorofluoroethylene play an important role. While the relaxor ferroelectric PVDF terpolymers show significant electrocaloric properties even at room temperature, the ferroelectric PVDF copolymers show high electrocaloric properties mainly at temperatures above the Curie transition, which depend on the copolymer composition and is at least 50 °C. Structural modifications are one possibility for converting the ferroelectric PVDF copolymers into a relaxor ferroelectric state and, thus, their possible electrocaloric use at room temperature. Here, we demonstrate the structural changes of P(VDF-TrFE) copolymers (55/45 mol. %) caused by electron irradiation. The use of irradiation doses of around 400 kGy allows a partial conversion from the ferroelectric phase to the relaxor-ferroelectric phase but does not yet lead to a complete degradation or cross-linking process so that a certain polarization can still be used. This is accompanied by the reduction of the Curie temperature and an increase in the permittivity at, e.g., room temperature. Thus, a maximum non-remanent polarization and electrocaloric activity are observed after irradiation of P(VDF-TrFE) copolymers with doses of around 400 kGy. These irradiated copolymers show at room temperature a temperature change of about 1.8 K and a figure of merit of 0.35 at an electric field of 100 V/μm, in comparison to about 0.7 K and 0.13, respectively, for non-irradiated copolymers.

Pulse-magnetron-sputtered long-term superhydrophilic coatings have been synthesized to functionalize the surfaces of solid-state cooling devices, e.g., electrocaloric heat pumps, where not only a complete wetting of the surface by a fluid is intended, but also fast wetting and dewetting processes are required. The coatings consist of a (Ti,Si)O₂ outer layer that provides lasting hydrophilicity thanks to the mesoporous structure, followed by an intermediate WO₃ film that enables the reactivation of the wettability through visible light irradiation, and a W underlayer which can work as a top electrode of the electrocaloric components thanks to its suitable electrical and thermal conductivity properties. Process parameter optimization for each layer of the stack as well as the influence of the microstructure and composition on the wetting properties are presented. Finally, water contact angle measurements, surface energy evaluations, and a contact line dynamics assessment of evaporating drops on the coatings demonstrate that their enhanced wetting performance is attributed not only to their intrinsic hydrophilic nature but also to their porous microstructure, which promotes wicking and spreading at the nanometric scale.