Dang, Van-Son (2011)
Ph.D. thesis, University of Birmingham.
For cost-efficient power applications of superconducting coated conductors based on YBa\(_2\)Cu\(_3\)O\(_7\) (YBCO) films, in applied fields or in self-field, further improvement of critical current by artificial flux pinning centres is required. This project investigated the increase in critical current density (J\(_c\)) and related physical properties of YBCO films by self-assembling nanotechnology of pinning centres, using substrate decoration, quasi-multilayers (using noble metals and Pr\(_2\)Cu\(_3\)O\(_7\) (PBCO) in both cases), and targets containing BaZrO\(_3\) (BZO) nano-inclusions. Samples were prepared by pulsed laser deposition (PLD) on single crystal SrTiO\(_3\) (STO) substrates and on Ni-W Rolling-Assisted Biaxially Textured Substrates (RABiTS). Optical lithography and chemical etching were used to prepare samples for transport measurements. The superconducting properties were characterised by AC susceptibility, magnetisation loops and transport measurements using a Magnetic Property Measurement System (MPMS) and a Physical Properties Measurement System (PPMS). Scanning and Transmission Electron Microscopy (SEM) and (TEM), Atomic Force Microscopy (AFM) and X-ray diffraction were also used to characterise the micro-structure of the films and the structure of artificially-induced pinning centres. The optimum conditions for the growth of YBCO films, Ag and PBCO nano-dots, and BZO nano-columns were investigated. Combinations of all three nana-structuring approaches resulted in a maximum J\(_c\) in applied fields and self-field. The related physical properties such as angular dependence of J\(_c\), vortex melting line, pinning force, frequency dependence of J\(_c\), were also investigated to understand pinning mechanisms in the films. The combination of Ag nano-dots and BZO nano-inclusions in the YBCO target provided the greatest improvement of critical currents of the film in applied fields.
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