In-depth investigation of the origin, evolution and interaction of structural defects in YBCO nanocomposite thin films

Abstract

Since the discovery of high-temperature superconductivity (HTS) in cuprate materials, efforts have focused on developing a high-current superconducting wire technology to fully exploit their fundamental current-carrying capability. Superconducting YBa2Cu3O7-ɗ (YBCO) based tapes, referred to as coated conductors (CCs), have achieved the needed requirements for the power demand and production of second generation (2G) superconducting wires. These wires are based on an architecture where a coating of a superconducting film grows epitaxially onto a biaxially textured substrate. The techniques used in the production of 2G superconducting wires have given new alternatives, like chemical solution deposition (CSD), where the main advantage is a lower economic impact in its fabrication compared to the vapor deposition techniques that needs of sophisticated high vacuum systems. Furthermore, it allows for an easy scalability and thus it is a way to obtain competitive commercial superconducting wires. However, to achieve the maximum potential of applications is good to consider another requirement before facing the large scale deposition processes. This requirement implies the controlled introduction of a density of defects allowing the enhancement of the flux pinning when applying magnetic field. Current-carrying capabilities are strongly enhanced if well-controlled nanometric defects are present inside the epitaxial superconducting matrix acting as vortex pinning centers and preventing resistive loses. Therefore, the investigation of the atomic structure of individual defects is thus critical to the understanding of the flux pinning efficiency of these HTS superconductors. This thesis is focused in the detailed investigation and characterization of the particular defects in the material responsible of the strong enhancement of the superconducting properties, as well as its origin, evolution and its complex interaction, by means of the aberration-corrected Scanning Transmission Electron Microscope (STEM). The particular geometry of this microscopes operated in scanning mode, provides the possibility to combine the structural information together with the chemical analysis of the material at high spatial and energy resolution, permitting to link the atomic and electronic structure of solids to their macroscopic properties. This thesis is divided in six main chapters. The properties of high temperature superconductors and especially of YBCO are reviewed in Chapter 1, including an overview of the chemical solution deposition route and a brief summary of the TFA-YBCO growth process. In Chapter 2 I describe briefly the experimental techniques used for the characterization of the CSD thin films presented in this work. Chapter 3 is a general overview of the microstructure found in YBCO films with the addition of BaZrO3, Ba2YTaO6 and Y2O3 as secondary phase nanoparticles. These nanostructured films are found to feature a high density of artificial defects and a strong enhancement of the superconducting properties compared to pristine YBCO films. The particular defects responsible of this enhancement are characterized in Chapter 4. They mainly consist on the introduction of an extra Cu-O layer in the YBCO unit cell (Y248 intergrowths). The high density of defects found in CSD films give rise to a highly complex defect landscape where they interact with each other and with the non-superconducting nanoparticles. In Chapter 5 I will describe the interactions between the commonest structural defects found in YBCO thin films: Y248 intergrowths and twin boundaries. The formation of a large amount of Cu-rich faults may lead to local off-stoichiometries since such films are prepared from a stoichiometric metalorganic precursor solution that is converted to the YBCO following a two step thermal treatment. In Chapter 6 it will be shown how the system overcome this issue, which involves the generation of Cu vacancies along the planar defects. Finally, I present the general conclusions of this study.

Date of Award	20 Dec 2013
Original language	English
Supervisor	Gazquez Alabart Jaume (Director), Jordi Arbiol (Director) & Alvaro Sanchez Moreno (Tutor)