Structure Characteristics of ISD Coated Conductors
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Structure Characteristics of ISD Coated Conductors Y. L. Tang1,3, D. J. Miller1, B. Ma2, R.E. Koritala2 and U. Balachandran2 1 3
Electron Microscopy Center, Materials Science Division, 2Energy Technology Division,
Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
ABSTRACT The inclined-substrate deposition (ISD) method for growth of biaxially textured MgO is of interest due to its applications in coated conductors based on high-temperature superconductor (HTS). The ISD method is especially attractive since it offers the potential to produce a high– quality biaxially textured layer in a simple and efficient process. In this work, YBa2Cu3O7-x (YBCO) coated conductors based on two-step deposited MgO buffer layers (ISD and homoepitaxial) on Hastelloy tapes were examined by high-resolution electron microscopy (HREM) to study both the structure of individual layers and, especially, the interfaces between them. Special attention was paid to the MgO buffer layer because of its importance to biaxial YBCO formation. TEM investigation shows the MgO [111] direction is not parallel to the substrate normal but tilted slightly toward the deposition direction. The second layer of MgO has a good epitaxial relationship with the first layer, while dislocations were found near the interface area of the MgO layers. Twin boundaries, which are in the (111) plane, were found between the ISD MgO columns. The multi-buffer layers gave better growth of epitaxial YBCO. INTRODUCTION Record superconducting performance has recently been demonstrated with conductors prepared by coating thick films of YBCO onto flexible metallic substrates, and this has stimulated intense worldwide activity and expectations for realizing the full potential for operating superconducting devices at liquid nitrogen temperatures with current densities in the MA/cm2 range. The key ingredient in this new development has been the ability to deposit YBCO films that are biaxially textured onto polycrystalline substrates, thus reducing the number of weak, high-angle grain boundaries that are known to limit current transport. Several techniques have been developed for preparing a biaxially textured template upon which the YBCO film can be subsequently grown epitaxially. The ion beam-assisted deposition (IBAD) process [1] achieves texture by means of a secondary ion gun that orients an oxide-film buffer layer that is being deposited onto the metallic substrate. The rolling-assisted biaxially textured substrate (RABiTS) process [2] achieves texture by mechanical rolling of a face-centered cubic metal and subsequent heat treatment. The ISD method is a simple and unique method, which was first reported by Hasegawa et al. [3,4] using excimer laser ablation. A similar biaxial alignment was observed in CeO2 and MgO by the ISD process [5]. At present, the critical current density (Jc ) values for YBCO films deposited on ISD MgO are less than those of IBAD, but ISD has very high deposition rates, which should be advantageous for scaling up toward pr
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