Microstructural analysis of the compatibility of solution deposited buffer layers with the TFA process for YBCO
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Microstructural analysis of the compatibility of solution deposited buffer layers with the TFA process for YBCO K. Salamaa, S. Sathyamurthyb, and M. Mironovaa a
Texas Center for Superconductivity, University of Houston, Houston, TX 77204. b
Oak Ridge National Laboratory, Oak Ridge, TN 37831
Abstract In this paper, the feasibility of applying solution deposition processes for the fabrication of coated conductors has been explored. The crystal and chemical compatibility of the buffer layers processed using metalorganic decomposition with the Y123 deposition using the trifluoroacetate process has been studied. Two buffer layer materials have been used, namely, barium zirconate and strontium titanate. The measured superconducting properties of these conductors were correlated with the microstructure observed on these samples using SEM and cross-sectional TEM. In case of barium zirconate buffer layers, though there exists a very good structural and chemical compatibility between the buffer layer and the Y123, the presence of surface defects in the buffer layer causes compositional heterogeneity and randomly oriented grains in the Y123 film. This leads to poor superconducting properties. In case of strontium titanate buffer layers, due to the excellent crystal and chemical compatibility, and the absence of surface defects, high critical current densities (of the order of 106A/cm2 at 77K and self field) were obtained. However, TEM cross section studies reveals the presence of a significant portions of a-oriented Y123 crystallites which could lead to lower critical current densities. Further studies of the TFA process is required to eliminate the occurrence of a-oriented Y123 in the microstructure. This could lead to further improvements in the properties. 1
Introduction Enormous progress has been made in conductor development for the application of high temperature superconductors (HTS) in electrical utilities. In recent years, the second generation HTS conductors, also known as ‘coated conductors’ has received extensive attention from the scientific community. In these materials, a metallic ribbon, usually a nickel alloy, is covered with a thin buffer layer such as MgO, SrTiO3, CeO2, etc., and a relatively thick film of superconducting YBa2Cu3Ox (Y123). Currently, two competing methods are being explored for the fabrication of coated conductors. They are the Rolling Assisted Biaxially Textured Substrates (RABiTS) approach and the Ion Beam Assisted Deposition (IBAD) approach. In the IBAD technique [1], a biaxially textured buffer layer is deposited on polycrystalline superalloy substrates. The texturing is obtained by sputtering off all orientations that are not favorable using a secondary ion gun. The RABiTS process [2], on the other hand, involves cube texturing of face centered cubic metals by rolling and recrystallization to create a biaxially textured template. Over this template buffer layers and Y123 films are deposited. At the current juncture, both these approaches use conventional thin film process routes to
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