MICROSTRUCTURAL DEVELOPMENT IN YBA_2CU_3O_7 COATED CONDUCTORS BASED ON EX-SITU YBCO CONVERSION PROCESSES

摘要

Ex-situ conversion of YBajCujO,, precursors is an economically attractive process for adding the superconducting layer to a coated conductor substrate. Understanding the phase conversion process and final microstructural assemblage is crucial to maximizing the superconducting properties. Each process used to build a YBe^Cu^Oy (YBCO) coated conductor creates an interface along which defects or interfacial reactions may result. These defects can be additive and propagate through the entire film structure to affect the growth and properties of the superconducting film. The effects of substrate features, buffer layers, and conversion processes of the ex-situ Y-123 films on the resulting microstructures are discussed. Interfacial reactions between the substrate, buffer layers, and Y-123 films are examined. Y-123 films produced from BaF2-based precursors deposited by e-beam evaporation or metal organic deposition of trifluroacetate-based precursors are compared in terms of their final microstructures, phase assemblage, and superconducting properties. The initial stages of processing during which phase segregation can occur, Y-123 and secondary phase development, and the degree of conversion completion in fully processed coated conductors are also examined. Implications for thick film (t > 1 μm) development and structural refinements are discussed.