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THE SCIENTIFIC CHALLENGES OF COATED CONDUCTORS
A coordinated, multi-institutional research proposal that addresses the key underlying scientific and engineering issues of Generation II Coated Conductors based on YBa2Cu3Ox (or alternatives such as YBa2Cu4O8, Tl2Ba2CaCu3Ox, or HgBa2CaCu2Ox) superconductor is described. A team composed of researchers from the University of Wisconsin, Stanford University, the University of Kansas and the University of Illinois will fabricate (and acquire through collaboration) forefront samples, develop new tools for their understanding and characterize them so as to resolve their key performance issues. Areas of concentration include substrates, buffer layers, the superconducting overlayer, and their complex interactions, and the underlying physical mechanisms that determine conductor performance. We pay special attention to those factors that control the simultaneous development of high critical current and high critical current density in conductor forms. Our initial thrusts in years 1-3 cover the fabrication of coated conductor samples, the isolation of key grain boundaries, electromagnetic characterization, model samples relevant to coated conductors, nanoscale characterization by scanning probe and scanning transmission electron microscopies, and the formulation of detailed models of how and why low angle grain boundaries and other macroscopic obstacles become barriers to current flow. We consider the way in which epitaxy develops through from an IBAD or deformation-textured substrate, the choice of oxide buffer layer and the influence of the buffer layer formation method. We pursue both chemical and physical vapor deposition methods. Strong interactions with leading companies, the DOE laboratories and other universities will ensure that the scientific, processing and characterization tools and their resulting insights will be applied to the industrial manufacture of coated conductors. As longer lengths of conductor are fabricated, we will add measurement and analysis of important conductor properties such as stability, quench performance and AC loss. Many Air Force applications of superconductors have already been demonstrated as prototypes; however generally with too great a system complexity, especially for airborne use, given the need for helium-temperature refrigerators when Nb-based conductors are used. Coated conductors using much simpler cryoplants will make devices such as airborne gyrotrons, traveling microwave tubes and superconducting generators possible. Many civilian applications of superconductors will be enabled by the development of coated conductors, since they offer the first opportunity to make widespread replacement of copper and iron in the electric utility network economically feasible.