Abstract
Substantial efforts are dedicated worldwide to use lead-free materials for environmentally friendly processes in electrocaloric cooling. Whereas investigations on bulk materials showed that Na0.5Bi0.5TiO3 (NBT)-based compounds might be suitable for such applications, our aim is to clarify the feasibility of epitaxial NBT-based thin films for more detailed investigations on the correlation between the composition, microstructure, and functional properties. Therefore, NBT-based thin films were grown by pulsed laser deposition on different single crystalline substrates using a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural characterization revealed an undisturbed epitaxial growth of NBT on lattice-matching substrates with a columnar microstructure, but high roughness and increasing grain size with larger film thickness. Dielectric measurements indicate a shift of the phase transition to lower temperatures compared to bulk samples as well as a reduced permittivity and increased losses at higher temperatures. Whereas polarization loops taken at −100 °C revealed a distinct ferroelectric behavior, room temperature data showed a significant resistive contribution in these measurements. Leakage current studies confirmed a non-negligible conductivity between the electrodes, thus preventing an indirect characterization of the electrocaloric properties of these films.
Highlights
Materials with distinct ferroelectric properties enable the direct conversion between thermal, mechanical, and electrical energies, which are utilized in devices such as actuators, transducers or sensors [1,2,3]
Whereas investigations on bulk materials showed that Na0.5Bi0.5TiO3 (NBT)-based compounds might be suitable for such applications, our aim is to clarify the feasibility of epitaxial NBT-based thin films for more detailed investigations on the correlation between the composition, microstructure, and functional properties
Due to the breakdown voltage limitation (∼50 kV cm−1) [6,16] of bulk materials, the change in temperature might be significantly higher in thin films, as larger electric fields can be applied, leading to a higher ∆T, especially close to a ferroelectric phase transition [4,17,18]
Summary
Materials with distinct ferroelectric properties enable the direct conversion between thermal, mechanical, and electrical energies, which are utilized in devices such as actuators, transducers or sensors [1,2,3]. The conversion of electrical to thermal energy by using the so-called electrocaloric effect (ECE) received renewed interest due to its potential for realizing energy-efficient solid-state cooling devices [4,5]. In the quest for alternative materials, Na0.5Bi0.5TiO3 (NBT)-based compounds were tested for their ECE, and significant temperature changes of up to 1.5 K (at 50 kV cm−1 and 135 ◦C) were found for bulk NBTbased compound [14,15]. It was our goal to realize such epitaxial thin films of NBT-based materials to enable these detailed studies in the step
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