2, TU Dresden, Dresden, , Germany
3, Fraunhofer IKTS, Dresden, , Germany
Electrocaloric (EC) materials show reversible thermal changes in response to the variation of an applied electric field. This EC effect got a renewed interest within the last decade due to the quest for energy-efficient cooling technologies and recent discoveries of large adiabatic temperature changes ΔT in various ferroelectric thin films during the application or removal of an electric field. Among them, lead-containing oxides exhibit strong caloric effects but contain hazardous elements. BaTiO3 (BT) based materials might be a more environment-friendly alternative to these compounds. Therefore, we have chosen BaZrxTi1-xO3 (BZT) and BaHfxTi1-xO3 (BHT) as model systems for our studies in order to investigate the correlation between the composition dependent phase transitions, the dielectric and ferroelectric properties as well as the EC effect of such BT based thin films. Moreover, we use epitaxial films, which additionally enable a detailed microstructural analysis as well as a study of orientation dependent properties.
Accordingly, epitaxial BZT and BHT films were grown by pulsed laser deposition on single crystalline substrates utilizing a conducting oxide buffer layer and additional top electrodes to obtain capacitor like structures. The grown films were studied by X-ray diffraction as well as scanning electron and atomic force microscopy. Depending on the specific growth parameters, a twin-free epitaxial growth and a smooth surface morphology is observed. Temperature depended measurements of the relative permittivity suggest diffuse phase transitions, where the transition temperature clearly varies with the film composition. The EC properties of the thin films were determined by an indirect method from temperature dependent polarization measurements showing values of up to 0.3 K for a ΔE of 170 kV/cm. We assume that the clamping of the thin films to the rigid substrate reduces the magnitude of the EC effect significantly compared to the respective bulk materials. Finally, we will present our approaches for the direct measurement of the EC effect in our epitaxial thin films.
This work is supported by DFG priority program 1599 “Ferroic cooling”.