Optimization of the room-temperature electrocaloric properties of poly(vinylidene fluoride-trifluoroethylene) copolymers (55/45 mol. %) by means of electron irradiation

Abstract

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.