Tuning the Electrocaloric Efficiency in Lead Free Ceramics

• Applicant: Privatdozent Dr. Vladimir V. Shvartsman
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448662623

Solid-state cooling based on the electrocaloric effect (ECE) is a potential solution for cooling computers and for industrial refrigeration. It is inspired by the discovery of a giant EC temperature change in antiferroelectric PbZr0.95Ti0.05O3 films. The electrocaloric effect is the change of entropy in a system due to the application of an electric field. It is largest at phase transitions but can also be enhanced by chemical disorder in the respective material like in relaxor ferroelectrics. A further handle on improving performance is the material microstructure which can modify the domain based disorder in ferroelectrics. In this proposal we will use the combination of all these contributions to entropy changes in order to hopefully maximize the resulting electrocaloric output.Lead-free ferroelectric materials have been developed as an alternative to commercial Pb-based ceramics due to growing concerns on the toxicity of lead and for environmental protection. However, the results are often contradictory. There is a lack of reliable data and systematic focused search for materials relevant for applications with a maximal ECE in the vicinity of room temperature. This is also related to the fact that most groups only determine the ECE indirectly and few systems show significant phase transitions at or near room temperature. We here propose to investigate the ECE in lead free systems by combining a morphotropic phase boundary with relaxor features. Such materials are known to show very large electrostriction related to the field induced transformation from a disordered relaxor phase with multiple polarization states to an ordered ferroelectric state. Such a transformation is accompanied by a large entropy change, which is important to reach a large ECE. In the first stage of the proposal we will screen several promising Na0.5Bi0.5TiO3 and K0.5Na0.5NbO3 based systems for identifying materials with largest effects. In the second stage, we will address technologically relevant aspects like the effect of grain size, defect concentration and fatigue on the ECE performance.For the success of the project we combine the expertise of the Russian team in sintering lead-free piezoelectric ceramics and of the German team, who has developed setups for the direct electrocaloric measurements.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Foundation for Basic Research, until 3/2022
• Co-Investigator: Professor Dr. Doru Constantin Lupascu
• Cooperation Partner: Dr. Ekaterina Dmitrievna Politova, until 3/2022