A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high-temperature superconductors is reported. The 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 chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

In-plane-aligned, c axis-oriented YBa, Cu, O, (YBCO) films with superconducting critical current densities J, as high as 700,000 amperes per square centimeter at 77 kelvin have been grown on thermomechanically rolled-textured nickel (001) tapes by pulsedlaser deposition. Epitaxial growth of oxide buffer layers directly on biaxially textured nickel, formed by recrystallization of cold-rolled pure nickel, made possible the growth of YBCO films 1.5 micrometers thick with superconducting properties that are comparable to those observed for epitaxial films on single-crystal oxide substrates. This result represents a viable approach for the production of long superconducting tapes for high-current, high-field applications at 77 kelvin.Since the discovery of high-temperature superconductivity (HTS) in cuprate materials, substantial efforts have focused on developing a high-current superconducting wire technology for applications at 77 K (1, 2). Early in these efforts it was observed that randomly oriented polycrystalline HTS materials have critical current densities, J,, (500 A/cm2. In contrast, oriented YBCO thin films grown epitaxially on single-crystal oxide substrates, such as SrTiO, (OOl), exhibit J, values >1 MA/cm2 at 77 K (3). This huge difference between randomly oriented HTS ceramics and single crystal-like epitaxial films is directly related to the misorientation angles at the grain boundaries in polycrystalline materials. Values for J, across a grain boundary decrease significantly as the misorientation angle increases, with weak-link behavior observed for misorientation angles at the grain boundaries greater than -10" (4-12). In order to achieve high J, values (-lo5 to lo6 A/cm2, 77 K), the crystallographic orientation of the HTS superconducting wire or tape must have a high degree of both in-plane and out-of-plane grain alignment over the conductor's entire length. Ideally, this would be achieved with YBCO, because the limits for dissipation-free current at 77 K in an applied magnetic field are most favorable for this material (1 3, 14).One approach to producing a high-J, HTS tape is to deposit a thick epitaxial film on a substrate material that has a high degree of in-plane and out-of-plane crystallographic texture and can be produced in long lengths. Epitaxial HTS films on singlecrystal oxides satisfy the requirements for high J,, but it is not feasible to produce long lengths of these substrates. Recent efforts have focused on the use of ion beam-assisted deposition (IBAD) to achieve inplane alignment of oxide buffer layers on polycrystalline metal substrates for subsequent epitaxial growth of . Indeed, a modest degree of in-plane texture for c axis-oriented YBCO films made by IBAD results in a significant increase in J,, with values ranging from lo5 to lo6 A/cm2 at 77 K. However, IBAD techniques have limitations, including the relatively low de~osition rates associated with the IBAD buffer layers as well as difficulties in consistently producing in-plane crystallographic alignment of less than lo0, tha...

Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi, Pb)2Sr2Ca2Cu3Ox oxide-powder-in-tube conductors and (Bi, Pb)2Sr2CaCu2O8 (Bi-2212) and TlBa2Ca2Cu3Ox 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 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'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 YBa2Cu3O7–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.

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The equilibrium resistive transition of YBa2Cu307-~epitaxial films was measured at high pulsed current densities J to promote homogeneous current How and free Aux Aow. The superconducting transition temperature T, was then depressed as a function of J in fixed fields H. The shifts in T"defined near midtransition, scaled following Ginzburg-Landau behavior: hT"(H, J)/T, (H, O) Jo is a fieldand current-independent constant.The formation of the superconducting state is governed by a competition between four energies: condensation, magnetic-field expulsion, thermal, and kinetic. The order parameter, which describes the extent of condensation into the superconducting state, is reduced as T, H, and J are increased.The boundary in T-H-J space that separates the superconducting and normal states is where the three parameters attain their critical values T"(H,J), H"2(T,J), and Jd(T, H), and any one of these functions completely defines the boundary. So far, to our knowledge, the shift in the T, (H, J) boundary caused by the pair-breaking (depairing) action of high currents has not yet been demonstrated in any high-T, superconductor and only the locus T, (H, J =0) has been measured. The commonly measured J,(H, T) boundary that separates p =0 from p40 behavior is a sample-microstructuredependent quantity of lesser fundamental importance, although of pivotal importance to applications.A related but separate motivation for measuring p(T) at high J is to reveal more fully the bulk-equilibrium behavior. The resistive transition shape is altered at low J in two ways. For a finite T, distribution, a weak current is more likely to sample higher T, percolative paths rather than flow uniformly through the entire specimen cross section. Also the tail of the transition is affected by nonequilibrium processes such as flux pinning and flux creep which tend to lower the resistivity below the free-fluxflow value. Both effects tend to make the transition sharper and give an apparently higher T,. High current densities tend to promote uniform current flow as well as free Aux How [I], resulting in a resistive transition that is broader but more intrinsic and representative of the entire sample volume. In addition to these extrinsic effects, high current densities may also alter the transition in various intrinsic ways: e.g., by modifying the nature of fluctuations. Presently a complete theoretical model for p(T) does not exist. The measurement of Jd(T, H) [or equivalently T, (H, J)] is made difficult in high-T, . superconductors because of the large normal-state resistivity p", combined with the large Jd(0, 0)a consequence of the high T, The depairing current density at T=O and H =0 is given by~H "(0) Jd(0, 0) =0.4l Jo = 3 Js~).b(o) c&0 I 2iY~-') . 'b(0)g. , (0) The sample was a c-axis-oriented epitaxial film of Thus to produce a sizable shift hT&, (H, J) =T, (H, J).-T, . (H, O) (distinguishable from flux pinning and inhomogeneity effects) involves high current densities that, in combination with the large p", can cause serious heating probl...