Under WP1, a quasi-adiabatic calorimeter was implemented for direct measurements of the ECE with an operating temperature range 4.2 – 370 K and voltage up to 5 kV. The sample is held in place by the wiring only: a high voltage (HV) coaxial line for the electric (E) field application (parallel-plate condenser geometry, thus E=V/d, where V is the applied voltage and d the sample thickness) and a K-type thermocouple to measure the induced adiabatic temperature change (ΔT). Measurements of the electrocaloric response in reference materials demonstrated the reliability of our setup, which adds to the handful number of instruments for direct ECE measurements worldwide. For enabling the indirect determination of the ECE through temperature dependent polarization versus E field measurements, i.e. P(E,T), and their analysis applying thermodynamics formulations, a sample holder was also developed to couple with a polarimeter and a HV amplifier (up to 10 kV), which permits measurements up to 425 K within a silicone oil bath.
In WP2, several compositions of four- and five-layer bulk Aurivillius phases were synthesized by solid-state chemistry and investigated by X-ray diffraction, electrical properties measurements and ECE experiments. The best electrocaloric response for near room temperature refrigeration applications was found in La/Nb codoped five-layer Sr2Bi4Ti5O18, which features a ferroelectric relaxor behavior and an interesting large E field dispersion of the ECE as shown by the direct measurements. The commonly used indirect estimations of the ECE in current electrocaloric research were also applied, but they failed to quantitatively reproduce the direct measurements. We are currently continuing the work on Aurivillius thin films prepared by pulsed laser deposition. Two first series of c-oriented epitaxial thin films of Sr2Bi4Ti5O18 were grown with high crystal quality and ferroelectric behaviour as confirmed by polarization measurements using interdigitated electrodes. Next step is the growth of La/Nb codoped Sr2Bi4Ti5O18 thin films and the measurement of their ECE, in which considerable gain is expected with respect to the bulk ECE results.
Finally, WP3 applied atomic-selective X-ray absorption spectroscopy (XAS) to probe the local electronic and geometrical structure of the most important cations in the surveyed Aurivillius phases to dig for the microscopic origin of the ECE. A first XAS experiment at the Ti K and Bi L3 edges confirmed Ti4+ and Bi3+ oxidation states for all the codoped compositions and also that La3+ substitutes Bi at the perovskite-like layers as intended, discarding its presence at the Bi2O2 layers. Besides, it revealed that upon La/Nb codoping, the ferroelectric distortion of the TiO6 octahedra is weakened while it does not influence the Bi local structure. Further experiments are planned to investigate the role of the Nb dopant in the ECE enhancement for La/Nb codoped Aurivillius phases and provide a full picture of the contributions from the different ferroactive cations.
At the end of the project, the instruments implemented under WP1 have led to a publication about the ECE in the related family of simple perovskites Ba1-yCayTi1-xHfxO3, [https://doi.org/10.1063/5.0173585] besides a manuscript on the electrocaloric properties of La and Nb codoped bulk Aurivillius compositions currently under review [preprint available at:
https://dx.doi.org/10.2139/ssrn.4618272](odnośnik otworzy się w nowym oknie). Also, the results from the project have been widely disseminated in conferences, including an invited talk (“New electrocaloric oxides for sustainable and efficient refrigeration” at the XII Meeting of the Condensed Matter Physics Division of the Spanish Royal Physics Society in Salamanca, Spain 2023).
