Physics of HTS coated conductors: porous films and magnetic decoupling of vortices
Coated conductor (see Fig. 4) research has been a central topic in the HTS research during the past few years. The so-called second-generation HTS tapes are expected to have tremendous potentials for various electrical applications. One of the outstanding problems is the dramatic decrease of Jc with increasing HTS coating thickness. This problem must be solved for coated conductors to carry large currents. Our group has taken two approaches�bottom up by growing HTS films of different thickness and top down by thinning them using ion milling�to understand the physics related. We have also developed new scheme of making porous HTS thick films to tailor the current flow. In collaboration with researchers at the Air Force research laboratory and Oak Ridge national laboratory, we have developed a microstructure-engineering scheme using vicinal growth induced strain plus Y2BaCuO5 nanoparticle insertion and achieved uniformly porous-structured YBCO thick films with much improved Jc.
The relatively slowly decreasing Jc with increasing thickness could be attributed to microstructure degradation for films thicker than 0.8 �m. However the major Jc reduction happens in the thin film regime (film thickness less than 0.8~1.0 �m), where the microstructure degradation is not a concern. Our approach to address this issue is to apply the multilayer structure in which a thick YBCO films is evenly chopped into thin films with insulating spacers. The experiment result on a trilayer YBCO/CeO2/YBCO, where a 0.5-�m-thick YBCO layer was divided into two 0.25-�m-thick YBCO layer by a thin insulator layer of CeO2, shows that the Jc increases with the thickness of CeO2. This observation suggests that, with gradually decoupling the fluxons in two adjacent YBCO layers, the thin film pinning efficiency is achieved which is in line with the 2D collective pinning model.
High critical current density (Jc) is the most critical specification for high temperature superconductor coated conductors as required by numerous electric power-related applications. This has motivated an intensive research effort on the effects of microstructure on Jc. By growing YBa2Cu3O7-? (YBCO) films at a small vicinal angle we have recently obtained a highly porous structure in these films accompanied with a significantly enhanced Jc and smaller Jc reduction at larger film thickness. Furthermore, we tailored the porosity by inserting Y2BaCuO5 (211) nanoparticles in vicinal YBCO thick films to alter the strain at the nanometer scale. A nearly doubled pore density was obtained. A further improved Jc as the consequence of the enhanced pore density in these films suggests a direct correlation between microstructure and Jc and projects an even higher Jc in YBCO films with microstructure engineered optimally at a nanometer scale
