Sponsored by: Jt. EMPMD/SMD Superconducting Materials Committee, MSD Electronic, Magnetic and Optical Phenomena Committee, and FEMS (Federation of European Materials Societies)
Program Organizers: U. Balachandran, Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439; Paul J. McGinn, University of Notre Dame, Notre Dame, IN 46556; and Stuart Abell, University of Birmingham, Edgbaston, Birmingham B152TT, UK
Wednesday, PM Room: B7
February 7, 1996 Location: Anaheim Convention Center
Session Chairpersons: S. X. Dou, University of Wollongong, Wollongong, NSW, Australia; A. Bhargava, The University of Queensland, Australia
2:00 pm
HOT-ROLLING AND HOT-COMPRESSING TREATMENTS OF Y123 AND Bi2223 BULKS: Q. Y. Hu, D. Yu, H. K. Liu, T. Chandra, S. X. Dou, Centre for Electronic and Superconducting Materials, University of Wollongong, Wollongong, NSW 2500, Australia
YBC0123 and Bi2223 bulks were hot compressed isothermally in a temperature range of 780-850deg.C under constant stress of 15-50 MPa for 60 minutes. The materials have also been hot rolled in a same temperature range by given a small amount of reduction. Hot worked specimens then were examined using scanning electron microscopy and hardness testing. It was found that the mass density of the bulk Y123 and Bi2223 superconductors has been increased by both hot compressing and hot rolling. This was achieved by reducing the enclosed voids in the materials under the pressure at high temperature. In addition of high mass density, texture structure in the superconductors has also improved by hot compressing and hot deformation which in turn improved critical current of both materials. However, the texture extent of the Bi-2223 compound was enhanced by the hot working more significantly compared to compound Y123. Isothermal hot compressing was found more efficient compared to hot rolling in terms of increasing the mass density and texture structure of the superconducting materials.
2:20 pm
SEARCH FOR NEW SUPERCONDUCTING MATERIALS: P. Majewski, Helsenbergstr. 5, D-70569 Stuggart, Germany
A summary of the research efforts, aiming the discovery of new superconducting materials, of the Max-Planck-Institut fur Metalforschung, Pulvermetallurgisches Laboratorium will be given. Results of our investigations of the systems M-Ba-Ca-Cu-O with M = La, Sb, Al, and Bi-Sr-Y-Ca-Cu-O, as well as Y-Ni-B-C will be presented.
2:40 pm Invited
MAGNETO-OPTIC IMAGING OF HIGH-Tc SINGLE CRYSTALS REVEALING FLUX FRONT INSTABILITIES AND MACRO-TURBULENCE: T. H. Johansen, M. Baziljevich, H. Bratsberg, Department of Physics, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway; H. Hauglin and G. P. Lafyatis, Department of Physics, Ohio State University, Columbus, OH 43210-1106
We have used the advanced real-time magneto-optic method, where a doped iron-garnet film with in-plane anisotropy serves as flux indicator, to study the behavior of flux front dynamics in high-Tc single crystals. Flux front instabilities that develop into macro-turbulent behavior of the flux system was observed in plate-shaped YBCO crystals when the field is applied normal to the plate (along the c-axis). The appearance of the instability is seen as a growing meandering of the flux annihilation zone separating the invading flux from an initially frozen-in distribution of opposite polarity. The characteristic fingering structure is seen to develop with a maximum amplitude near the middle of the sides of the sample leaving the corners essentially unperturbed. We report on our observation of this phenomenon, displaying a variety of new effects which include the influence of a transport current on the turbulent behavior.
3:00 pm
Bi-BASED 2223 SUPERCONDUCTING CERAMICS PREPARED BY THE GLASSY "MATRIX" PRECURSOR METHOD: S. Stassen, P. Rouxhet, A. Vanderschueren, R. Cloots, A Rulmont, M. Ausloos, S.U.P.R.A.S.-University of Liège-B6-Sart-Tilman-4000 Liège-Belgium
Glassy "matrix" precursor method has been followed in order to prepare Bi1.7Pb0.3Sr2Ca2Cu3Oy superconducting materials. This method involves the mixture of a glass precursor and a crystalline precursor. Five crystalline precursors with two starting stoichiometries have been studied. We have first analyzed the successive formation of various phases during the thermal treatment which showed that glass compound crystallizes first, 2201 phase is than formed, followed by 2212 and then 2223. Quality of final materials is influenced by the presence of free calcium and copper ions during the process. The role of both crystalline precursors and glassy matrix has been evaluated by X-ray diffraction, EDX analysis, as well as by resistivity versus temperature measurements. SrCuO2 is found to be the best crystalline precursor and 2234 starting stoichiometry leads to 2223 richer materials. Thermal treatment has been modified in order to optimize the 2223 phase formation.
3:20 pm BREAK
3:30 pm
CONTINUOUS PROCESSING OF BSCCO-2212/Ag DIP-COATED CONDUCTORS: J. W. Burgoyne, D. DewHughes, Dept. of Engineering Science; C. J. Eastell, C. G. Morgan, C. R. M. Grovenor, M. J. Goringe, Dept. of Materials; R. G. Jenkins, H. Jones, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK
A continuous process has been developed for the production of unlimited lengths of BSCC0-2212 open-surface conductor using a long multi-zone furnace. Dip-coated tape on an Ag-alloy substrate is pulled through a partial-melting/annealing temperature profile at speeds of the order 1 mh-1. Phase-pure, well-aligned 2212 is formed with Tc-85 K. Initial results of Ic up to 92.5 A at 4.2 K, 0 T demonstrate the viability of this method, and compare well with an Ic of 95 A for identical tape in a conventional heat-treatment. Higher critical currents should be achievable through improved coating and precursor preparation. Highly homogeneous conductors are obtained since every part of the tape receives an identical heat-treatment, avoiding the thermal gradients experienced across wind-and-react coils. Optimisation of the temperature profile and process speed to reduce impurity phases and increase Ic will be discussed, and data on react-and-wind coils from continuously-processed conductor will be presented.
3:50 pm
MANUFACTURE OF THICK FILMS AND BULK COMPONENTS OF BSCCO BY SLIP CASTING: Atit Bhargava, I. D. R. Mackinnon, A. Ilushechkin, T. Yamashita, J. A. Alarco, J. C. Barry, Centre for Microscopy and Microanalysis, The University of Queensland, Q 4072, Australia
Room temperature slip-casting is a simple and economical process to fabricate bulk components and thick films of YBCO and BSCCO. When BSCCO precursor powder transforms into HTS phase, such as during sintering, it expands causing cracking and deformation in a fabricated body. We have tailored sintering parameters to convert a slip-cast "green" body into a HTS ceramic of high density, high strength and high current carrying capacity. The process for bulk components has been extended for thick films on polycrystalline substrates. SEM, TEM, XRD, Jct and a.c. magnetic susceptibility are used to characterize the films and components. The process has applications in power, telecommunications, magnetic shielding and imaging industries.
4:10 pm
HOT ROLLING EFFECT ON THE CRITICAL CURRENT DENSITY IMPROVEMENT OF THE Ag-SHEATHED Bi-2223 TAPE: Q. Y. Hu, D. Yu, H. K. Liu, T. Chandra, S. X. Dou, Centre for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia
Ag-sheathed Bi2223 wires were hot-rolled into thin tapes in a temperature range of 500-800deg.C at a four-high laboratory rolling machine with attachment of a tube furnace. It was found that hot-rolled tapes had higher core hardness compared to cold rolled tapes indicating that hot rolling has increased core mass density of the superconductor more significantly than cold-rolling. This was confirmed by scanning electron microscopic observation which has clearly shown that the hot rolled tapes have a lower void density compared to cold rolled specimens. The enhanced core densification has resulted in the improvement of the critical current of the superconductor tapes.
4:30 pm
CHEMICAL COMPOSITION, MICROSTRUCTURE AND PHYSICAL PROPERTIES OF MAGNETICALLY MELT-TEXTURED BULK Be2Sr2Ca0.8Dy0.2Cu2O8-y SUPER-CONDUCTING CERAMICS: S. Stassen, R. Cloots, A. Vanderschueren, Ph. Vanderbemden, M. Ausloos, SUPRAS, Univeristy of Liège, Sart Tilman, B-4000 Liège, Belgium
Dysprosium doped Bi-based 2212 ceramics were synthesized by a melt-texturing growth process in a 1.2 T magnetic field, applied perpendicularly to the main axis of a cylindrical shape sample. Well oriented and (four) chemically defined phases forming successive layers were observed and characterized by X-ray diffraction, scanning electron microscopy and electron dispersive X-ray analysis. The origin and composition of such layers can be described. A novel aspect is the calcium solubility at the strontium site in 2212 phases, and the strontium solubility at the calcium site in 8250 structures. The texturing process was optimized with respect to electrical resistivity and magnetic susceptibilty.
4:50 pm Invited
SIMILAR DEPRESSION OF Tc BY Zn AND Ni SUBSTITUTION AND DEPARTURE FROM THE UNIVERSAL THERMOPOWER BEHAVIOR IN CaLaBaCuO: D. Goldschmidt, Y. Eckstein, Department of Physics, Technion, Haifa 32000, Israel
In the new tetragonal 1-2-3 CaLaBaCuO family, electron concentration becomes constant by Ca2+ and La3+ charge-compensating substitutions and by keeping the oxygen content constant. Nevertheless, Tc and transport properties change substantially indicating that doping (band filling) occurs. These materials have the same Tcmax and a single doping parameter y-YM-I, independent of Ca and La concentration (where Y-YM-I is the oxygen content y measured relative to the metal-insulator transition), suggesting that they have essentially the same band structure. Our results are interpreted within the simple band picture modified to consider the existence of low-mobility states near EF, besides the CuO2 band.
5:10 pm
THE EFFECTS OF PRECURSOR POWDER ON THE TEXTURE FORMATION IN (Bi,Pb)2Sr2Ca2Cu3Ox/Ag TAPES: J. Jiang, J. S. Abell, School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
Three kinds of precursor powders with the same nominal composition were
obtained by co-decomposition method, powder A with Bi-2212 as major phase,
powder B with Bi-2201 as major phase, and powder C with Bi-2223 and 2212 as
major phase. The silver-sheathed Bi-system tapes were made by the
powder-in-tube technique. The texture measurement was made by XRD and SEM. It
was found that the mechanical deformation during fabricating process induced
significant texture of BPSCCO grains in the as-pressed tapes and that this
texture degree depends on the phase content and particle size of the precursor.
The final tapes with 2223 phase made from the three kinds of precursor powders
exhibited almost the same texture degree. The texture degree in the as-pressed
tapes is not the determining factor for the texturing of the Bi-2223 grains in
the final tapes with high Jc. The experiment results indicates that texture
degree was improved significantly during the phase transformation, both in
the process from 2201 to 2212, and from 2212 to 2223. The intermediate
mechanical deformation enhanced the texture degree. The possible mechanisms of
the texture formation are discussed.
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