Large-area superconducting YBCO thin films with high critical currents

Abstract

This study is focused on the deposition, characterization, and optimization of large-area YBCO thin films on CeO₂-buffered sapphire substrates by pulsed laser deposition (PLD) for fault-current-limiter (FCL) applications. Resistive FCL elements rely on the quality of the base material, which in this case is the YBCO superconducting thin film. Significant results have been achieved, namely: 1) A very important parameter for YBCO thin films to be used as FCL element is the current-carrying capacity, known in technical terms as the I_c/w, where I_c = critical current, w = width. For this purpose, YBCO films with very uniform values of I_c/w exceeding 100 A/cm have been successfully deposited. This is a relatively high value compared with those of large-area YBCO thin films commercially available or deposited by other methods such as thermal coevaporation and metalorganic deposition (only ~75−90 A/cm). 2) Microcracking occurs in YBCO thin films on sapphire substrates when the thickness typically exceeds 0.3 μm (critical thickness) This results to a serious limitation in the value of I_c/w. In this study, microcrack­free YBCO films with thickness exceeding 1.9 μm have been successfully grown. This is the very first reported successful growth of micron-thick YBCO films on sapphire without microcracking, where the thermal stress accumulated in the film is relieved due to its porous microstructure. In the presentation, I will discuss in detail the preparation method as well as characterization of the films including inductive measurement of the critical current density (J_c), and investigation of film morphology using advanced microscopy techniques such as atomic force microscopy (AFM) and transmission electron microscopy (TEM).