Fabrication of YBCO coated conductor on Cu substrates by MOCVD

Abstract

The subject of this thesis is divided into two parts. One is the influence of doping concentration and oxygen partial pressure on superconducting properties of metalorganic chemical vapor deposition (MOCVD)-processed YBCO films with Ce doping. The other is fabrication of c-axis oriented YBCO films on biaxially textured Cu substrates using Cu-free MOCVD process. The influence of nanosized secondary particles, such as those produced via rare-earth doping for example, on magnetic field induced flux motion in $YBa_2Cu_3O_{7-\delta}$ (YBCO) coated conductors (CC) has been investigated. In chapter 2-1, we investigated the influence of Ce doping on nanosized secondary phase formation in YBCO films. Ce doped YBCO films on single $SrTiO_3$ crystals were prepared via the MOCVD process. For the first time, a $Y_2BaCuO_5$ secondary phase was observed in the Ce doped YBCO film, while the pure YBCO film contained only a $Y_2O_3$ secondary phase. As Ce doping concentration increased to 3 wt%, a $BaCeO_3$ secondary phase developed. This was due to the low solubility limit of Ce for Y in the MOCVD YBCO film. Additionally, outgrowths on the Ce doped YBCO film surfaces and defect structures, such as $\alpha$-axis grains were examined. Oxygen partial pressure is known to be a key parameter in terms of affecting the critical current density $(\It{J_c})$ of YBCO films. In chapter 2-2, the effects of oxygen partial pressure on the microstructure and $\It{J_c}$ of a 1 wt% Ce doped YBCO film were examined. Ce doped YBCO films were deposited at oxygen partial pressures of 1.9, 2.5, 5.0, and 10.0 Torr using the MOCVD method. Due to the enhanced migration of surface adatoms under a reduced oxygen partial pressure, the 1 wt% Ce doped YBCO film had a stoichiometric, dense surface. In addition, the zero-field $\It{J_c}$ (at 77K) of the 1 wt% Ce doped YBCO film deposited at a reduced oxygen partial pressure increased. Irrespective of the amount of Ce, the Ce doped YBCO film showed an incr...