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
Thursday, AM Room: B7
February 8, 1996 Location: Anaheim Convention Center
Session Chairpersons: M. D. Sumption, The Ohio State University, Columbus, OH 43210; S. Abell, University of Birmingham, Edgbaston, Birmingham B152TT, UK
INFLUENCE OF Li DOPING ON FLUX PINNING IN BULK Bi:2212: M. D. Sumption, E. W. Collings, Department of Materials Science, The Ohio State University, Columbus, OH 43210; P. Krishnaraj, N. G. Eror, Department of Materials Science, University of Pittsburgh, PA 15261
Magnetic Jc and Hr were measured for Bi:2212 sintered samples containing varying amounts of Li. The first set of samples consisted of: (1) Bi2Sr2CaCu1.9Li0.1Ox, (2) Bi2Sr2CaCu1.9Li0.2Ox, (3) Bi2Sr2CaCu2Li0.2Ox, and (4) a control sample. It was found that Li generally depressed Tc but enhanced the pinning over a wide range of temperatures. This was found to be true even when Tc suppression was taken into account. For low concentrations of Li (up to Li = 0.4) the increase in pinning is proportional to the concentration. For example, results at 57.5 K (without Tc normalization) show that Hr (0-4 Li) = 2.8 kOe; Hr (0.2 Li) = 2.6 kOe; and Hr (control = 1.8 kOe. For Li = 0-4, Tc is depressed to 88.2 K (down from 91.2 K). Sample microstructure and the role of CuO precipitates in flux pinning are also addressed.
MODIFIED IODODMETRIC TITRATION TECHNIQUE FOR HIGH SENSITIVITY OXYGEN DETERMINATION IN CUPRATE SUPERCONDUCTORS: P. S. Whitfield, J. Bhakta, G. Yang, I. Gameson, J. S. Abell, School of Metallurgy and Materials, University of Birmingham, Birmingham, B15 2TT, UK
The importance of oxygen content on the structure and properties of high Tc materials has long been known. The number of techniques available to routinely determine accurate oxygen contents of these materials is limited, especially where sample size is limited as with crystal growth. The oxygen contents of crystalline and amorphous material have been determined by modified iodometric titration, neutron data refinement, and TGA. For titrations samples of 20-40 mg were dissolved in HBr and the evolved Br3-analyzed iodometrically using citrate complexation of Cu(II). The results from each of the methods will be compared and the suitability of the iodometric titration technique for accurate oxygen content determination of small samples assessed.
ENHANCED PROPERTIES IN SCREEN PRINTED YBa2Cu3O7-d THICK FILMS WITH DOPING ADDITIONS OF PLATINUM AND RHODIUM: J. Langhorn, Y. J. Bi, J. S. Abell, Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT
Platinum group metals have been observed to react within the YBCO system to form phases which may act as heterogeneous nucleation sites for the deposition of refined Y2BaCuO5 (211) precipitates. Additions have been made to thick films of YBCO, and have induced significant improvements in the physical properties of the processed films, in particular critical current densities (Jc). Controlled concentration additions have yielded Jc values in excess of 6 x 103 A/cm-2 at 77 K and zero applied field, this is with respect to Jcs of 2 x 103 Acm-2 for undoped specimens. Optical and transmission electron microscopy has shown a homogeneous distribution of sub-micron sized, and larger highly anisotropic 211. An increased density of dislocations associated with the 123/211 interface with this refinement suggesting that smaller 211 precipitates may act as heterogeneous nucleation sites for flux pinning defects. Additions have also yielded increased homogeneity, and optimized Tcs in the system by aiding oxygen diffusion through the system.
YTTRIA STABILIZED ZIRCONIA BARRIER LAYERS BY ATMOSPHERIC PLASMA SPRAYING: S. C. Watcham, J. S. Abell, School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
Thick films of YBa2Cu3O7-x (YBCO) high temperature superconductor have potential use in microwave device and magnetic screening applications. Deposition onto a substrate to support the film and/or be an integral part of a microwave device is necessary. YBCO films deposited onto alumina substrates (the preferred dielectric material) show a large degree of interaction, unlike films on yttria stabilized zirconia (YSZ). Atmospheric plasma spraying (APS) is used commercially to produce high density ceramic coatings in thicknesses from several microns to a few millimeters. In the present work the production of YSZ barrier layers by APS onto alumina substrates has been studied. The results of the subsequent evaluation of these layers by optical and electron microscopy, and x-ray diffraction are presented. A comparison is made between the properties of plasma sprayed and screen printed barrier layers. Results of the physical and electrical behaviour of screen printed YBCO films prepared on these barrier layers will also be presented.
CRUCIBLES FOR CRYSTAL GROWTH BY ATMOSPHERIC PLASMA SPRAYING: S. C. Watcham, D. Holdom, J. S. Abell, School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
High quality crucibles for crystal growth are conventionally produced by the slip casting of alumina or zirconia. The success of slip casting is limited by the sintering characteristics of the material used. Potentially useful materials such as BaZrO3 do not sinter easily, and so do not lend themselves to forming in this way. Recent studies have used hot isostatic pressing to achieve the required properties. Atmospheric plasma spraying (APS) is used to form near net shape artifacts from a number of materials including metals, ceramics, and plastics. The chosen material is sprayed to the required thickness onto a prepared mandrel. Subsequent removal of the mandrel leaves the free standing artifact. The fabrication of crucibles from yttria stabilized zirconia, alumina and barium zirconate by APS has been studied. These have been evaluated by optical and electron microscopy, and density measurements and compared to those commercially available. Characterization of cuprate superconducting crystals grown in these crucibles will be presented.
10:10 am BREAK
MAGNETIC CHARACTERIZATION OF BULK MELT TEXTURED YBCO: LIMITS OF LEVITATION FORCES: W. Gawalek, T. Habisreuther, T. Straer, M. Wu, D. Litzkendorf, and P. Görnert, Institut fur Physikalische Hochtechnologie e.V., Helmholtzweg 4, D-07743 Jena, Germany
Bulk cylindrical blocks of YBCO, 30 mm in diameter and 17 mm in thickness, are prepared by a modified melt textured growth batch process. Blocks are consisting of weak textured 1-2 cm3 grains. By vibration sample magnetometry intra-grain critical current densities up to 5.104 A/cm2 at 77 K are measured. Integral levitation force measurements are performed at 77 K and 18 K in the zero field cooled (zfc) and field cooled (fc) state. Measured values are compared with calculated theoretical limits for an ideal hard pinning superconductor material interacting with a given strong permanent magnet. Using a 0.41 T SmCo magnet, 25 mm in diameter and 15 mm in thickness a zfc maximum levitation force of 36 N at 77 K was measured 0.5 mm ahove the block. This is about 41% of the theoretical limit in case of perfect shielding. At 18 K about 57% of the zfc limit is reached. In the fc state the multi grain material is more efficient. We found about 70% of the fc limit at 77 K and about 93% of the fc limit at 18 K.
PROGRESS IN QUASI-CONTINUOUS DIRECTIONAL SOLIDIFICATION OF BULK YBa2Cu3O7 CERAMICS WITH CeO2 ADDITIVES: X. Obradors, S. Piñol, V. Gomis, F. Sandiumenge, B. Martínez, N. Vilalta, Institute de Ciencia de Materials de Barcelona, C.S.I.C. Campus U. A. Barcelona, 80193 Bellaterra, Catalunya, Spain
Directional solidification of bulk 123/211 ceramic composites has been carried out by means of a vertical Bridgman technique allowing to reach a detailed control of the growth parameters. The use of small concentrations of CeO2 additives (<1% wt) has been shown to have two different beneficial effects. First, the conditions under which the semisolid melt is stable at high temperature are largely widened. Second, these additives further contribute to the refinement of 211 precipitates in the composite thus allowing to increase the critical currents above 105 A/cm2 at 77 K. The Bridgman solidification technique has been shown to have a great potential for quasi-continous production of bulk rods of superconducting textured YBCO. Samples with diameters between 1 mm and 20 mm and lengths up to 120 mm have been fabricated with our methodology and the microstructure and superconducting properties have been investigated by means of several experimental techniques. A detailed discussion of our processing optimization activity will be reported. Finally, the applicability of these materials for several devices (current leads, levitators, etc.) will be stressed.
A COMPARATIVE STUDY OF MAGNETIC AND TRANSPORT PROPERTIES OF YBa2Cu3Oy SINGLE CRYSTALS WITH A HIGH DENSITY OF CRITICAL CURRENT: S. N. Gordeev, Physics Department, University of Southampton, Southampton SO17 lBJ, UK; W. Jahn, A. A. Zhukov, H. Kupfer, T. Wolf, Forschungszentrum Karlsruhe, Institut fur Technische Physik, 76021 Karlsruhe, Germany
Sr-doped YBa2Cu3Oy single crystals were studied. Due to a high density of
point-like defects such samples have high critical current density J
~105 A/cm2 at T = 77 K in magnetic field 1.5 T. Both
magnetic and pulsed transport measurements result in a nonmonotonous J(B)
dependence with a typical for peak-effect maximum at intermediate field Bp. It
was found that the shape of current-voltage curves changes in the region of
J(B)-peak. At low magnetic fields (B<Bp) the log E(logJ) dependence has a
pronounced negative curvature. Good agreement with experimental data is
obtained from relation E=C(J-Jo)m with field independent parameters c and m.
Above Bp the E-J characteristics show a power-like behavior given by E~Jn(B).
At high current densities an effective activation barrier U(J), extracted from
E-J curves, strongly increases with J decrease. Below J(Bp the U(J) dependence
becomes more weak and can be approximated by U~logJ. The change of U(J)
behavior in the region of J(B)-maximum may be connected with a transition from
elastic to plastic creep mechanism.
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