Research On Fabrication Of Long Length YBCO Coated Conductor And Flux Pinning Mechanism For Thick Film

High temperature superconducting material YBa2Cu3O7-x(YBCO) intrinsically has high critical current density at high magnetic field, which is suitable for electric power applications. The purpose of this thesis is to develop the fabrication processes of meter long coated conductor in order to the high critical current of YBCO. The YBCO was deposited on NiW metal tape with the CeO2/YSZ/Y2O3buffer layer by using pulsed laser deposition (PLD). Furthermore, YBCO thick film and flux pinning mechanism were studied.A home designed reel-to-reel PLD system was set up for continuously depositing meter-long YBCO tape. Our focus was first on the study of the basic deposition process parameters of the YBCO tapes, such as pulsed laser energy, background oxygen pressure, deposition temperature, and target to substrate distance, and their dependence. The images of the laser plasma were analyzed by a frame photography method, and the texture orientation and superconducting properties of the YBCO films at different deposition temperatures were studied. The optimized ranges of these basic parameters were determined.With the optimized deposition parameters established, we studied the technology for reel to reel deposition of YBCO tapes. In order to improve the critical current Ic, we tried to increase YBCO film thickness through two approaches, one is increasing the laser pulse repetition rate, and the other is changing the reel to reel tape speed. It was found that although the YBCO film thickness was increased, the critical current maxed at50A and did not change significantly with increasing thickness. The saturation of Ic with increasing YBCO film happened. The reasons were as follow. The substrate surface temperature decreased as the YBCO film thickness increased, resulting in the formation of a-axis grains, and appearance of micro cracks and holes above certain film thickness. Process parameters affected the superconducting properties of the YBCO tapes in various ways. Minute fluctuations in the growth conditions resulted in deviation of the texture of part of the YBCO film from the preferred c-axis orientation; chemical instability of the substrate and deposition temperature drifting caused interface reaction between the YBCO film and the buffer layer; non-superconducting phases and other defects widened the transition width of the critical temperature Tc of the YBCO films; target surface roughness directly affected the shape of the plasma plume, therefore resulting in changes in the composition of the YBCO films; changes in chamber pressure, small movements of the substrate, and the way of the temperature measured all had an impact the stability of the deposition temperature on the YBCO films. Based on these experimental results, we modified our deposition processes by applying more rigorous control on the above mentioned parameters. A new multi-coating process was developed using layer by layer deposition process, which improved the YBCO texture, and produced in crack free, smooth, high quality YBCO films.Using the multi-coating process and optimized parameters of30Pa oxygen pressure,730℃relative deposition temperature,195mJ laser energy,0.1mm/s reel to reel tape speed,10Hz laser pulse repetition rate, meter long YBCO coated conductor on the buffered tape was successfully fabricated. XRD0-20scan showed a pure c-axis YBCO texture. The best results of YBCO (103) XRD φ-scan and (005) ω-scan values are6.87°and3.60°, respectively. The critical transition temperature of the YBCO tape were typically as90.1-91.7K with0.4-0.7K transition widths. The critical current was over200A/cm-width at77K and OT.An intermitted deposition method was first proposed for fabricating YBCO thick films with and without BaZrO3doped. YBCO films with thicknesses up to2.0μm demonstrated critical current densities in the order of106A/cm2. Analysis showed these films maintained better crystalline. The doping of BaZrO; generated flux pinning centers with cation disorder.Further comparison of the superconducting properties of the doped and undoped YBCO films under magnetic field showed that the BaZrO3doped samples always had a higher critical current density than that of the pure YBCO. The doped YBCO film had a critical current density Jc of3.16x106A/cm2at77K,OT and6.21×105A/cm2at1T which was about twice of those of the undoped samples. The pinning force in the doped YBCO films was7.1GN/m3at77K and1.6T, and47GN/m3at65K and3.7T. The pinning force stayed on the level of45GN/m3under higher magnetic field ranging from3T to5T.We also analyzed the pinning type of single flux pinning in the doped and undoped YBCO samples. It has showed that the pinning type changes with temperature. At low and medium temperature, they are mostly of δl pinning. As the temperature increased the amount of stress pinning increased, and eventually took the majority.