In-Situ Control of BaZrO₃ and BaSnO₃ Nanorod Alignment and Microstructure in YBa₂Cu₃O_7-x Thin Films by Strain Modulated Growth

Abstract

With the ability to carry very high electrical currents per unit area in kilometer length wires, high temperature superconductors (HTS) are especially promising candidates for applications where size and weight constraints are priorities. From military aircraft and naval applications to energy production by wind power, many types of power generation applications may operate under strenuous conditions, requiring current densities on the order of 105 A/cm2 while subjected to magnetic fields of 3 - 5 T. In the absence of a magnetic field, this current density requirement is well within the intrinsic limits of YBa2Cu3O7-x (YBCO), but operation in high magnetic fields makes the problem of vortex motion a limiting factor to the critical current density, Jc. Vortex pinning by the insertion of non-superconducting oxides like BaZrO3 (BZO) or BaSnO3 (BSO) into the YBCO matrix is an effective means of addressing this problem since these defects self-assemble into columnar structures (nanorods) that provide strong pinning along the length of the flux-line. However, only limited control of nanorod geometry is possible by current growth methods. To meet the requirements of applications that operate in magnetic fields of varying intensity or orientation, this thesis aims to produce a defect landscape that may be designed to meet these demands, as the thin film is grown. Achieving this represents a major challenge in the development of HTS cables and power devices, requiring correlation of material synthesis and characterization on a nanometer scale. The microstructure of BZO- and BSO-doped YBCO thin films was studied using Transmission Electron Microscopy and the findings indicate that it is possible to produce a controllable defect landscape by manipulation of the strain relationships using vicinal substrates, as well as through controlled growth dynamics by varying growth temperature.