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Session Chairpersons: V. Selvamanickam, Intermagnetics General Corporation, Latham, NY; M.J. Kramer, Ames Laboratory, Ames, IA
CRITICAL CURRENT IN SMALL ANGLE GRAIN BOUNDARIES: C.S. Pande, R.A. Masumura, Naval Research Laboratory, Washington, DC 20375-5343
It is now well known that even small angle grain boundaries (misorientation angle ¾ 10) show drastic reduction in inter-grain critical current. Drawing upon on our previous work, we show that this reduction must be ascribed to the stress field in proximity of the dislocations forming the small angle boundary. The alternate view that the reduction is due to the presence of dislocation cores is investigated by calculating their size as a function of misorientation. We show that the concept though probably valid for large misorientation ( 10) leads to serious disagreement with experiments for small values ( ¾ 10).
2:20 pm INVITED
DIRECT OBSERVATION OF CURRENT DISTRIBUTIONS IN THIN SUPERCONDUCTORS USING MAGNETO-OPTIC IMAGING: T.H. Johansen, M. Baziljevich, H. Bratsberg, Department of Physics, University of Oslo, Box 1048 Blindern, 0316 Oslo, Norway; Y. Shen, P. Vase, NKT Research Center, Sognvej 11, 2605 Brøndby, Denmark
A thin film of YBa2Cu3O7- prepared by laser ablation and shaped by etching as a long strip was studied by magneto-optic imaging. The penetration of a perpendicular magnetic field was investigated in detail, and a model-independent method of determining the space-resolved current distribution was developed. The inverse magnetic problem, i.e., that of using a field map to derive the underlying current distribution, is formulated and solved analytically for the strip geometry. The observed current profile across the strip gives direct evidence for the assumptions made in the Bean model. Also other geometries were investigated, demonstrating the new application of magneto-optics as a tool to determine current flow patterns.
2:40 pm INVITED
FABRICATION OF BIAXIALLY-TEXTURED THICK FILM Y-Ba-Cu-O SUPERCONDUCTOR: V. Selvamanickam, M.S. Walker, P. Haldar, R.S. Sokolowski, Intermagnetics General Corporation, Latham, NY 12110; A. Ivanova, A.E. Kaloyeros, State University of New York at Albany, Albany, NY 12222; D.E. Fenner, Fenner Engineering Associates, Simsbury, CT 06070
High current densities have been recently demonstrated at 77 K in thick film YBCO superconductor deposited on biaxially-textured metallic substrates. The effort at Intermagnetics has been directed towards the development of an industrially scaleable process based on biaxially-textured substrate technology. Biaxially-textured metallic substrates have been fabricated in long lengths with an average in-plane orientation better than 10° and an average out-of-plane orientation better than 7°. Buffer layers that are structurally and chemically compatible with YBCO have been deposited on the metallic substrates with a biaxial texture similar to that of the substrate. Metal-Organic Chemical Vapor Deposition (MOCVD) has been used for deposition of YBCO since this technique enables a high rate of deposition that is not limited by line-of-sight. Stoichiometric, dense, and biaxially-textured films of YBCO superconductor have been successfully deposited by MOCVD on the biaxially textured substrates. This research was partially supported by the Department of Energy.
3:00 pm INVITED
HIGH CRITICAL CURRENT DENSITY TAPES BY EPITAXIAL DEPOSITION OF SUPERCONDUCTING THICK FILMS ON BIAXIALLY TEXTURED METAL SUBSTRATES: A. Goyal, D.P. Norton, M. Paranthaman, E.D. Specht, J.D. Budaj, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, S. Shoup, P.M. Martin, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116
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 scaleable, thermomechanical processes to impart a strong biaxial texture to a base metal like Ni. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces for HTS film deposition. Substrates prepared using this method are referred to as Rolling Assisted Biaxially Textured Substrates (RABITS). Epitaxial YBCO films grown using laser ablation on RABITS have critical current densities exceeding 7 x 105 A/cm2 at 77 K in zero-field and have field dependences similar to epitaxial films on single crystal ceramic substrates. The texture of the base metal has been achieved in lengths over 1 m and scaleable techniques are being pursued to deposit epitaxial multilayers. 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. Research sponsored by U.S. DOE under contract DE-AC05 960R22464.
3:20 pm INVITED
TRANSPORT BEHAVIOR OF GRAIN BOUNDARIES IN YBa2Cu3O7: A COMPARISON BETWEEN THIN FILM AND BULK Bi-CRYSTALS: D.J. Miller, V.R. Todt, M. St. Louis-Weber,* D.G. Steel, X.F. Zhang, K.E. Gray, U. Balachandran*, Materials Science Division and *Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439
The structure and transport properties of grain boundaries in thin film and bulk bi-crystals of YBa2Cu3O7 have been studied in detail. The thin film bi-crystals were prepared by sputter deposition onto SrTiO3 bi-crystal substrates while the bulk bi-crystals were prepared by a dual-seeded melt textured growth process. The structures of these two types of grain boundaries can be very different: thin film boundaries typically exhibit meandering and impurity phases that extend through the thickness of the film while the bulk boundaries tend to be very straight and relatively free of impurity phases. A comparison of the transport behavior between these two types of boundaries will be presented and the implications of the similarities and differences will be discussed. This work was partially supported by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under Contract No. W-31-109-ENG-38 and by the National Science Foundation through the Science and Technology Center for Superconductivity under Contract No. DMR 91-20000.
3:40 pm BREAK
3:50 pm INVITED
PROCESS OPTIMIZATION FOR YBCO THICK FILMS: J.S. Abell, T.C. Shields, J. Langhorn, S.C. Watcham, School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
Superconducting thick films are finding technological exploitation in various applications; for example as inductive components in power engineering designs such as fault current limiters, and as microwave devices like filters in cellular communications applications. For efficient performance in these different fields superconducting properties require optimisation with respect to different parameters such as critical current density (Jc) and surface resistance (Rs). The microstructural control to deliver these properties demands careful processing. Flux pinning additions, composition control, doping, and novel processings techniques have been employed to improve the characteristics of the films. Barrier layer technology to enable the exploitation of low dielectric loss and economically attractive alumina substrates has been studied. The relationship between processing, microstructure and superconducting behavior will be discussed.
MULTI-WAFER MANUFACTURING OF LARGE-AREA YBCO THIN FILMS FOR R.F. DEVICE APPLICATIONS: C.N. Soble, V.C. Matijasevic, Z. Lu, T. Kaplan, K. von Dessonneck, Conductus, Inc., Sunnyvale, CA 95123
Scaling up PVD and CVD techniques to reasonable manufacturing levels has been a scientific and engineering challenge for companies developing HTS materials for commercial applications. In this paper, we discuss the process development and optimization work associated with ramping up a multi-wafer reactive co-evaporation deposition system capable of depositing high quality YBCO on 2" to 8" wafers (R-pl sapphire or LaAlO3). YBCO thin films deposited using this technique exhibit good electrical properties with transition temperatures 85-87 K and Jc >2 x 106 A/cm2 at 77K. Microwave surface resistance, R, is <1 m at 10 GHz and 77 K. HTS films on R-pl and LaAlO3 have been fabricated into r.f. devices for applications in NMR spectroscopy, Magnetic Resonance Imaging, and wireless communications.
4:30 pm INVITED
PHASE DIAGRAM AND CATION DISORDER STUDIES IN RE1+xBa2xCu3O7+, RE= Pr, Nd, Sm, Eu, and Gd: M. J. Kramer, H. Wu, K.W. Dennis, R.W. McCallum, Ames Laboratory, Iowa State University, Ames, IA 50011
For the light rare earth elements, the orthorhombic REBa2Cu3O7- structure forms not as a line compound but with varying degrees of RE+3 substitution on the Ba+2 sites and additional oxygen incorporated in the basal plane to balance charge. It has been demonstrated that for RE = Nd, Pr, (Nd + Pr), and (Nd + Gd), the occupation of RE on Ba sites is a function of the oxygen partial pressure (pO2) and temperature. Not only is the minimum value of x effected but for a fixed value of x, large changes in the superconducting transition are observed. By manipulating the pO2 and T profile, phase assemblage can be modified. Under proper processing conditions, these second phases can form pinning sites in these materials, resulting in enhanced high field critical currents. Supported by U.S. Department of Energy, under Contract No. W-7405-Eng-82.
4:50 pm INVITED
THE EFFECT OF PROCESSING PARAMETERS ON Hg-1223 TAPE FABRICATION: R. Meng, Y.Wang, B. Hickey, K. Ross, Y. Xue, C.W. Chu, Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932
We have developed a process to fabricate Hg-1223 tape on a metal substrate with a transition temperature of 130 K and a self-field critical current density of 2 x 104 A/cm2 at 77 K. However, the reproducibility of the tape is poor and weak links exist in the tape so prepared. We have therefore investigated the chemical stabilities, processing parameters, and Ni doping effect on the Hg-1223 phase formation and grain growth. We found that doping can suppress the volatility of Hg in Hg-1223 and speed up the growth of Hg-1223. While Ni was found to be better than the other metal substrates tested it failed to retain its integrity for prolonged exposure to the processing conditions. Only a slight amount of Ni can be incorporated into the Hg-1223 grain, and it usually enters the Cu sites leadiing to a slight suppression of Tc. Proper processing conditions to eliminate the diffusion of Ni to Hg-1223, while favoring the growth and alignment of the Hg-1223 tape, will be discussed.
PREPARATION OF HIGH-TEMPERATURE SUPERCONDUCTING SILVER-SHEATHED HgBa2CuO4+ TAPES: G.B. Peacock,1 M. Khaliq,2 G. Yang,2 T.C. Shields,2 I. Gameson,1 J.S. Abell,2 and P.P. Edwards,11School of Chemistry, 2School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B152TT,UK
We have produced silver-sheathed HgBa2CuO4+ superconducting wires and tapes exhibiting a transition temperature of 96 K. Progress in the synthesis of these tapes was achieved by packing the stoichiometric mixtures of oxides into the silver tubes, air-quenching the samples and the novel use of thermal cycling. Superconducting properties were assessed by SQUID and a.c. susceptibility measurements. Various routes have been adopted to optimize superconductivity in HgBa2Ca2Cu3O8+ tapes: these include packing superconducting Hg-1223 powders in tubes, mixing pre-formed materials with mixtures of oxides in different ratios, using precursors, etc. Phase purity, texture and morphology of samples has been investigated using XRD and SEM.
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