Solid-state cooling based on the electrocaloric effect might be a promising alternative to vapor-compressed refrigeration, not only for its increased efficiency but also for its role in preventing the emission of hazardous gases. The electrocaloric effect (ECE) refers to the reversible adiabatic temperature change that occurs in polar materials when an external electric field is applied or varied. In ferroelectric materials, the ECE is particularly pronounced at the transition temperature between the ferroelectric and paraelectric phases. It was shown recently that ferroelectric thin films in general exhibit excellent electrocaloric properties due to their capacity to withstand high electric fields, which typically results in an increase in the adiabatic temperature change. Therefore, the major aim of this thesis was to study environmentally friendly lead-free compounds for their feasibility as electrocaloric active layers in epitaxial film architectures prepared by pulsed laser deposition.
Reports in literature on bulk materials suggest that Na0.5Bi0.5TiO3 (NBTO) compounds may be suitable for electrocaloric cooling. Therefore, the growth of epitaxial NBTO-based thin films was studied, which helps to study the correlation between composition, microstructure, and functional properties of this material. Epitaxial films were deposited on different single crystalline substrates applying a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural investigation by X-ray diffraction revealed an undisturbed epitaxial growth on LaAlO3, whereas a significantly smaller temperature window for epitaxy was found on YAlO3. The differences might be explained by the lattice misfit resulting in a higher defect density of the intermediate buffer layer on YAlO3. For all samples, a columnar structure with additional pores was found leading to substantial surface roughness. Dielectric measurements revealed significantly decreased permittivity values and increased losses at elevated temperatures if compared to bulk samples. While polarization loops at -100 °C indicated a distinct ferroelectric behavior, ambient temperature data revealed significant resistive contributions due to high leakage currents. As a result, it was not possible to determine the electrocaloric properties for all NBTO-based thin films deposited with the indirect method.
In the second part of the thesis, the correlation between structural properties and the electrocaloric effect was investigated in lead-free epitaxial Ba1-xSrxTiO3 (BSTO) thin films. Here, BSTO thin films with Sr contents ranging from x = 0 to x = 0.3 were deposited on SrRuO3 buffered SrTiO3 single crystalline substrates. X-ray diffraction analysis verified a pure epitaxial growth for all Sr concentrations and film thicknesses indicating a larger tetragonal distortion if compared to the bulk material. Dense layers with a low surface roughness were found in microstructural studies. Temperature and frequency-dependent dielectric measurements indicate a diffuse phase transition for all samples, where thicker films showed larger permittivity values. The temperature of maximum permittivity decreases as Sr concentration increases. Polarization curves demonstrate a relaxor-like behavior, particularly above room-temperature. The adiabatic temperature change due to the ECE was determined with the indirect method showing |ΔT| values of up to 2.9 K for an electric field change of 750 kV cm-1.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:83852 |
| Date | 28 February 2023 |
| Creators | Martins Magalhaes, Bruno |
| Contributors | Nielsch, Kornelius, Dörr, Kathrin, Michaelis, Alexander, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | info:eu-repo/grantAgreement/Deutsche Forschungsgemeinschaft/SPP 1599/198599533//„Caloric Effects in Ferroic Materials: New Concepts for Cooling“ |
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