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)₂Sr₂Ca₂Cu₃O_x oxide-powder-in-tube conductors and (Bi, Pb)₂Sr₂CaCu₂O₈ (Bi-2212) and TlBa₂Ca₂Cu₃O_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'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 enhancement 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'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 (metal and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa₂Cu₃O_7-δ films grown using laser ablation on such substrates have critical current densities exceeding 10⁶ A/cm² 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.