Year

2011

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

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.

Share

COinS