CONDUCTORS WITH CONTROLLED GRAIN BOUNDARIES : AN APPROACH TO THE NEXT GENERATION, HIGH TEMPERATURE SUPERCONDUCTING WIRE

Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} oxide-powder-in-tube conductors and (Bi,Pb){sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (Bi-2212) and TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} thick film conductors. It is demonstrated that in each of these conductors, critical current densities are dictated by the grain boundary misorientation distributions (GBMD{close_quote}s). Percolative networks of low-angle boundaries with fractions consistent with the active cross-sectional area of the conductor exist in each of these conductors. Further enhancements in the properties require increased numbers of small-angle grain boundaries. Given the processing methods used to fabricate these materials, no clear route employing a simple modification of the established processing method is apparent. To address this need, conductors with controlled or predetermined GBMD{close_quote}s are necessary. Development of biaxial texture appears to be the only possible way to increase the number of small-angle boundaries in a practical and controllable manner. We summarize in this paper recent results obtained on epitaxial superconducting films on rolling-assisted-biaxially-textured-substrates (RABiTS). This technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metalmore » and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films grown using laser ablation on such substrates have critical current densities exceeding 10{sup 6} A/cm{sup 2} at 77 K in zero field and have a field dependence similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of the next generation high temperature superconducting (HTS) wire capable of carrying high currents in high magnetic fields and at elevated temperatures. {copyright} {ital 1997 Materials Research Society.}« less