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...

Magneto-optical imaging was used to visualize the inhomogeneous penetration of magnetic flux into polycrystalline TlBa2Ca2Cu3Ox films with high critical current densities, to reconstruct the local two-dimensional supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have almost perfect c-axis alignment and considerable local a- and b-axis texture because the grains tend to form colonies with only slightly misaligned a and b axes. Current flows freely over these low-angle grain boundaries but is strongly reduced at intermittent colony boundaries of high misorientation. The local (<10-micrometer scale) critical current density Jc varies widely, being up to 10 times as great as the transport Jc (scale of approximately 1 millimeter), which itself varies by a factor of about 5 in different sections of the film. The combined experiments show that the magnitude of the transport Jc is largely determined by a few high-angle boundaries.

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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.

Surface texture and interior residual stress variation induced by thickness of YBa2Cu3O7-δ thin films J. Appl. Phys. 112, 053903 (2012) New application of temperature-dependent modelling of high temperature superconductors: Quench propagation and pulse magnetization J. Appl. Phys. 112, 043912 (2012) Flux-pinning-induced interfacial shearing and transverse normal stress in a superconducting coated conductor long strip J. Appl. Phys. 112, 043908 (2012) High, magnetic field independent critical currents in (Ba,K)Fe2As2 crystals Appl. Phys. Lett. 101, 012601 (2012) Fabrication of binary FeSe superconducting wires by diffusion processThe semiempirical scaling law for flux pinning, Fp = AB ~2 b 1(1 -b )m has been tested for bronze process Nb 3 Sn conductors under uniaxial tension. The dependences of the bulk pinning force Fp on the upper critical field B e2 and the reduced field, b = B / B e2' were weakly affected by strain.However, the factor A, which depends on the number and strength of the pinning centers, and on the Ginzburg-Landau parameter K decreased by more than a factor of 20 when the compressive prestrain on the Nb 3 Sn was removed by application of external stress. The variations of Te and Be2 with strain suggest that the change in K is not sufficient to account for the change in A.Therefore, it is probable that strain induces microstructural changes which affect the number and/or strength of the pinning centers.

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.

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