1 Department of Energy Conversion and Storage, Technical University of Denmark2 Electrofunctional materials, Department of Energy Conversion and Storage, Technical University of Denmark
Due to the zero resistance and no electric energy loss during electricity transmission, the application of superconductivity would provide a considerable benefit, especially when considering the fast development at the expense of high energy cost nowadays. For operation at liquid nitrogen temperature, REBCO (RE= rare earth) has some evident advantages compared to other high-temperature superconductors in retaining high current densities under strong magnetic fields, thus REBCO high temperature superconducto rs have significant potential for high field engineering applications. Compared to Pulsed Laser Deposition (PLD) and Chemical Vapor Deposition (CVD), the trifluoroacetate metal-organic deposition (TFA-MOD) route is more promising for producing REBCO superconducting films, owing to the high-Jc, high reproducibility, and low cost of this technique, which doesn't require any high vacuum equipment. The TFA-MOD has however two intrinsic disadvantages, one being the released hazardous HF gas as a byproduct, while the other is the resulting time-consuming decomposition process, which normally costs more than 10h. In the aim of improving environmental safety, performance and the productivity of the method, the focus of this study is on the production of REBCO films by using fluorine-free MOD (FF-MOD) routes. In Chapter 3, a review was made on the existing CSD methods. According to the different barium intermediate phases after the decomposition of the organic precursors, the main CSD processes are classified into four categories, which are the barium fluoride process, barium hydroxide process, barium nitrate process, and barium carbonate process. In the aim of selecting the most suitable process for producing REBCO films in our research, trial experiments were also done to compare microstructure quality of the film products and the reaction mechanisms of these processes. In Chapter 4 and Chapter 5, a fluorine-free water-based sol-gel process was used to produce YBCO films. The focus of chapter 5 was on the optimization of the pH value in the aim of improving the microstructure and superconducting performance of the YBCO films, and the focus of chapter 4 was on the selection of chelating agents on the basis of the precursor solution stability. In Chapter 6, high-thermal -stability polymerized acrylic acid was used to suppress the grain growth of intermediate phases forming during and after pyrolysis in the aim of producing high quality GdBCO thin films. In Chapter 7, a methanol -based non-fluorine MOD method was used to deposit YBCO-Ag superconducting thin films. A trace amount of silver addition was used to decrease the melting point of YBCO, which could enhance the interconnection between YBCO grains and in consequence increase the Jc of the films. In Chapter 8, based on the effect of BaZrO3-doping in improving the in-field performance of YBCO films, and on the positive influence of Ag-doping in accelerating YBCO grain growth, which has been confirmed in Chapter 6, the effect of BaZrO3/Ag hybrid doping on the microstructure and performance of FF-MOD derived YBCO films was investigated. Chapter 9 is the summary of the thesis.