The discovery of high temperature superconductors led to a tremendous boom in the development of new applications based on this material. Due to the significant anisotropy and the dependence of the critical current density on the misorientation of grains, the so-called coated conductor technology was developed for these materials to realize long wires. These conductors are applied at liquid nitrogen temperature for cables or motors as well as in liquid helium for high-field applications, such as in magnets for particle accelerators or future fusion reactors. One of the main aspects of using superconducting materials in the above-mentioned areas is their high current-carrying capacity, which decreases for a number of reasons. Therefore, studying the superconducting current flow in such conductors remains a priority to understand the main mechanisms and to increase the critical current density in a wide range of temperatures and magnetic fields.
The major goal of this thesis was to study the correlation between the local microstructure and the superconducting properties for Ag-doped YBa2Cu3O7−δ (YBCO), (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7−δ (NEG) and the iron-based superconductor Ba(Fe1−xNix)2As2 (Ba122:Ni). Therefore, epitaxial films were grown of these materials by pulsed laser deposition on single crystals and two different commercial coated conductor templates having a different degree of granularity. Experimental techniques such as electron backscattering diffraction (EBSD) and scanning Hall probe microscopy (SHPM) allow to investigate both the local microstructure and local distribution of superconducting current in these films.
Ag-doped YBCO films with different thickness were deposited on single crystalline SrTiO3 substrates as well as on RABiTS and IBAD-MgO-based templates. It is expected, that silver as dopant improves the growth of the films, and has a beneficial influence on the current transport across grain boundaries, which is of considerable interest for metal-based templates due to their granular structure. EBSD studies on the local microstructure revealed only minor changes with silver concentration. Nevertheless, an improvement in transport properties was observed for thicker YBCO:Ag layers on SrTiO3 and thin films on both metal-based templates. SHPM measurements show an improvement of the local current distribution, which is probably due to the improvement of the current transport between the grains.
NEG films were grown with different thicknesses on RABiTS and IBAD-MgO-based templates for the first time. Structural studies revealed an epitaxial growth of all samples on both metal-based templates. Whereas NEG layers on SrTiO3 showed broad superconducting transitions due to film inhomogeneities, a narrow transition at about 89 K was measured for films grown on the metal templates. However, the critical current density is still inferior to YBCO films of similar thickness. This might be improved by further optimization of the growth and oxygen loading conditions.
Finally, the Ba122:Ni films were studied on single crystalline CaF2 substrates and commercial metal-based templates. This material might be interesting for applications due to a low anisotropy, high upper critical fields and critical currents as well as a reduced sensitivity to grain boundaries. Structural studies showed an epitaxial growth on RABiTS templates, whereas no epitaxy was found on IBAD-MgO based tapes. Simultaneously, a broad superconducting transition was observed on the metallic templates, which requires a further optimization of the growth process. Detailed studies of the superconducting and electronic properties for Ba122:Ni films on CaF2 substrates revealed similar properties as for single crystals, which opens the prospects to use such films for different applied and fundamental tasks.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:87019 |
| Date | 05 September 2023 |
| Creators | Shipulin, Ilya |
| Contributors | Nielsch, Kornelius, Holzapfel, Bernhard, 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 |
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