Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Wang, Lin, Chemical solution deposition for YBCO superconducting films and Sm2O3 buffer layers on single crystal and biaxially textured metallic substrates, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3422
Development of YBa2Cu3O7-δ (YBCO) based high-temperature superconductors is going on worldwide, as it enables superconducting applications with low-cost liquid nitrogen as coolant. For most applications, such as microwave devices and coated conductors, YBCO is required to be in an epitaxial film form to minimize the weak-link effect at the grain boundaries, and a multilayer structure of YBCO/buffer layer/substrateis commonly used. Chemical solution deposition (CSD) is of great research interest as it is a low-cost, highly flexible route for the fabrication of high-quality thin films. This thesis reports studies on the fabrication of large-area YBCO films for microwave application by metal-organic deposition using trifluoroacetates (TFA-MOD), on the properties of YBCO based superconducting films, including binary and ternary rare earth element based superconducting films, grown by the TFA-MOD method, and on the growth of Sm2O3 buffer layers by CSD.
Fabrication of large-area YBCO films for microwave application by the TFA-MOD method was investigated, including the influence of processing parameters such as gas flow rate and crystallization duration. With optimized processing conditions, high quality large-area YBCO films can be obtained, and the microwave surface resistance is comparable to values for commercial films prepared by the vacuum technique.
REBa2Cu3O7-δ (RE = rare earth) compounds are attractive candidates for YBCO applications, as they have higher transition temperatures (Tc) and better in-field performance than YBCO. In this thesis, binary compound Y1-xRExBa2Cu3O7-δ((Y,RE)BCO) thin films have been fabricated on LaAlO3 (LAO) single crystal substrates by the TFA-MOD method. It has been found that the improved performance under intermediate field of the (Y,RE) BCO films is associated with the ionic radius difference between Y3+ and RE3+, and Eu substitution results in the most significant enhancement under current processing conditions. Fabrication of ternary REBCO compound Nd1/3Eu1/3Gd1/3Ba2Cu3O7-δ (NEG-123) films on LAO substrates by the TFAMOD method has been studied as well. The processing conditions for NEG-123 films are more critical than those for YBCO films, possibly due to the large lattice mismatch between NEG-123 and LAO substrate. Biaxially textured NEG-123 film with Tc, onset at 90.5 K and critical current density (Jc) of 0.19 MA/cm2 (77 K, self field) was obtainedwith a crystallization temperature of 810 °C under 3 ppm oxygen partial pressure.
Due to health and environmental considerations, large-scale use of the standard TFAMOD process for YBCO film is not a good choice. In my work, a low toxicity TFAMOD method using ethanol as a solvent has been developed to minimize the health risks in industrial scale fabrication of YBCO films. Highly biaxial textured YBCO films on LAO substrates with Tc,onset of 91 K and Jc of 1.47 MA/cm2 (77 K, self field) can be obtained by the ethanol-based TFA-MOD method. To the best of my knowledge, there has been no other report on this kind of research in the world.
Sm2O3 is a promising candidate as a buffer layer for YBCO coated conductor, due to the small lattice mismatch between Sm2O3 and YBCO. In this thesis, a chemical solution deposition route using samarium acetate and propionic acid has been studied for the fabrication of biaxially textured Sm2O3 films on biaxially textured Ni-5%W substrates.
Keywords: YBCO; thin film; coated conductor; buffer layer; Sm2O3; chemical solutiondeposition; TFA-MOD.