Direct measurement of the electrocaloric efficiency in lead-free microstructured film architectures

• Applicants: Dr. Peter Gaal; Dr. Ruben Hühne
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450862010

The thermal stabilization of complex microsystems, e.g., sensors or microelectronic components, is one of the central challenges of current technology development. This includes in particular the cooling of active elements. One option to realize this is integrating electrocaloric materials as thin layers in the device architecture. These materials exhibit a change of entropy or temperature upon application of an external electric field that can be used for energy-efficient cooling. In order to determine the efficiency and performance of the electrocaloric material in a thermodynamic cycle, the direct measurement of the field-induced temperature change is of crucial importance.The aim of the joint project proposal is therefore to develop new, direct methods for measuring the temperature change in electrocaloric thin films in order to compare the results with established indirect characterization routines. The main focus of our work is on epitaxial thin films with a thickness below 1 µm, which might be integrated directly into future device architectures. Additionally, we confine our study to eco-friendly lead-free materials to ensure the environmental compatibility of this new technology. In contrast to the already established direct measurement strategies for bulk samples, we rely on methods with high temporal resolution to determine temperature changes in thin films due to its small volume. These methods employ ultrashort light pulses to detect smallest variations in the samples. The transient structural dynamics is directly probed in time-resolved x-ray diffraction experiments. In combination these methods can also determine the thermoelastic coefficients of the electrocaloric material, which are quite often insufficiently known for thin epitaxial films. In an alternative approach, we determine the temperature change locally in microstructured areas using integrated sensor elements that are either probed electrically or optically. In addition to processing of the corresponding measurement structures, such experiments require a detailed modeling of the thermal transport in these microstructures. Finally, we will combine the time-resolved and local approaches in order to compare the results with each other.In summary, this joint project proposal combines the complementary expertise of the applicants in the area of epitaxial, ferroelectric thin film growth and microstructuring with the extensive experience on time-resolved measurement methods. Our investigations will help to resolve open questions in solid-state cooling by directly measuring temperature changes in thin-film architectures during the cooling cycle. Furthermore, the use of new, lead-free and eco-friendly electrocaloric materials will open new prospects for the future applications.

• DFG Programme: Research Grants