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
Tuesday, PM Room: B7
February 6, 1996 Location: Anaheim Convention Center
Session Chairpersons: P. J. McGinn, University of Notre Dame, Notre Dame, IN 46556; A. Roshko, National Institute of Standards and Technology, Boulder, CO 80803
2:00 pm Invited
FLUX PINNING AND DISSIPATION IN HIGH Tc SUPERCONDUCTORS: C. S. Pande, D. U. Gubser, Naval Research Laboratory, Washington, DC 20375-5343
Many phenomena in high Tc superconductors such as magnetic relaxation, thermally activated resistance, and flux noise can be interpreted quantitatively by applying the concept of flux motion under pinning energy barriers. We show that such vortex dynamics must assume flux motion under a well-distributed spectrum of potential wells. We also show analytically that under some plausible assumptions that such a distribution of potential wells must be log normal. Implication of this result in explaining the transport properties of these materials will be discussed. Review of NRL work in high Tc conductor development will also be presented.
2:20 pm Invited
THE USE OF PHASE DIAGRAMS FOR THE ENGINEERING OF FLUX PINNING CENTRES IN BSCCO CERAMICS: P. Majewsi, Helsenbergstr. 5, D-70569 Stuttgart, Germany
From considerations of the phase equilibrium diagram of the system Bi2O3-PbOSrO-CaO-CuO, different annealing procedures were developed to precipitate various second phases in high-temperature superconducting Bi2Sr2CaCu2O3 and (Bi,Pb)2Sr2Ca2Cu3O10 ceramics with different chemical compositions. In each case, the precipitation results in an increase of the critical current density of the samples which is believed to express improved pinning properties of the superconducting ceramics, in particular an increased pinning energy, which reduces the probability for thermally activated depinning.
2:40 pm Invited
FLUX PINNING ENHANCEMENT IN MELT PROCESSED YBa2Cu3O7-THROUGH RARE EARTH ION SUBSTITUTIONS: P. J. McGinn, C. Varanasi, Dept. of Chemical Engr., Univ. of Notre Dame, Notre Dame, IN 46556
Stoichiometric YBa2Cu307-(Y123) samples doped with excess Nd2O3 and La2O3 additions were melt processed in air. Because decomposition and reformation of 123 occurs during melt processing, light rare earth ion substitution in 123 takes place. As the light rare earth ion sizes (Nd, La) are comparable to that of Ba2+, partial substitution into both the Ba2+ sites and Y ion sites is anticipated. The light rare earth oxide doped, melt processed 123 shows significant improvement in magnetization compared to undoped samples, likely due to point defects created by substitutions of the light rare earth ions.
3:00 pm Invited
MICROSTRUCTURE-PROPERTY RELATIONSHIPS IN EPITAXIAL YBa2Cu3O7-d THIN FILMS: A. Roshko, F. J. B. Stork, D. A. Rudman, L. F. Goodrich, National Institute of Standards and Technology, Boulder, CO 80803
The excellent high temperature properties of YBa2Cu3O7-d thin films make them of interest for many active and passive devices. Understanding the role of microstructure in determining film properties will allow optimization of films for electronic applications, and may also aid in the development of bulk materials with specific properties. We have investigated the influence of microstructure on the superconducting properties of YBa2Cu3O7-d films. The zero resistance transition temperature, critical current density, and surface resistance have all been found to be relatively insensitive to the film morphology. However, small systematic trends have been observed which indicate the importance of surfaces and defects in determining the properties of these materials.
3:20 pm BREAK
3:30 pm Invited
DISTINCTIVE MICROSTRUCTURE OF YBa2Cu3O7-d/Pt OR CeO2 SUPERCONDUCTOR PRODUCED BY NEW POLYCRYSTALLINE SEEDING METHOD: G. Kozlowski, Wright State University, Physics Department, 3640 Col. Glenn Hwy., Dayton, OH 45436; C. E. Oberly, SLPOOX-2, 2645 Fifth St. Ste 13, WPAFB, OH 45433
A new polycrystalline seeding method for providing melt-processed YBa2Cu3O7-d(123) large pellets (up to 2 inches in diameter) or bars (up to 5 inches long) placed on high density Y2BaCuO5 (211) substrate is proposed and discussed. The 123 bars can carry a transport current of up to 1,500 A under criterion of 1 v/cm. Furthermore, a distinctive microstructure of the unusual defects or gaps, with the accumulated Ba-Cu-O liquid, running parallel to (100) and (010) planes in the melt-textured 123/Pt or CeO2 superconductors was discovered. These defects may be interpreted as an instability of the planar front in the presence of fine 211 particles. A theoretical model of planar interface stability confirms a strong dependence of the stability region on the size of 211 particles in the melt.
3:50 pm Invited
MICROSTRUCTURE DEVELOPMENT IN ISOTHERMALLY MELT-TEXTURED Y-Ba-Cu-O SUPERCONDUCTORS: V. Selvamanickam, Intermagnetics General Corp., Latham, NY 12110
Fabrication of melt-textured Y-Ba-Cu-O superconductors is useful for the development of trapped-flux devices that can operate at 77 K. Isothermal melt-texturing by seed-controlled solidification has been found to be the most effective method of fabricating single-grain materials that are capable of trapping high magnetic fields and exhibiting large levitative forces. In an effort to achieve a high yield of high quality melt-textured Y-Ba-Cu-O, we have examined the microstructure development during the solidification process. Microstructure features such as the solidification interface, grain alignment, and Y2BaCuO5 precipitates have been analyzed in samples fabricated at various conditions of superheat, undercooling, and cooling rates. The role of dopants that affect diffusivity and solid-liquid interfacial energy in the microstructure development has also been investigated. The quality of the materials fabricated under various conditions has been examined by trapped field measurements. This research was sponsored by the Air Force Office of Scientific Research.
4:10 pm
CRYSTAL GROWTH MODEL AND MORPHOLOGY OF 123 MELT-TEXTURED MATERIALS DOPED WITH 211 AND (RE)2O3 PARTICLES: R. Cloots, N. Vandewalle, N. Mineur, M. Ausloos, SUPRAS, University of Liège, Sart Tilman, B-4000 Liège, Belgium
Kinetic growth mechanisms of melt-textured (RE)Ba2Cu3O7 (with RE=Dy, Er) materials doped with (RE)2BaCuO5 and (RE)2O3 are extensively discussed. We present optical observations and computer simulations of the kinetic processes. The model is based on the well-known Eden model. Physicochemical interactions between the growth front and the 211 and/or (RE)2O3 particles are taken into account in a Uhlman-Chalmers-Jackson way. The surface morphology is studied through the TLK model. The 211 particles can be trapped at front instabilities leading to the formation of filamentary structures. Quantitative results are presented through Monte-Carlo simulations. Concerning the influence of the (RE)2O3 influence on the crystal growth process, it is shown that modifications of the solid/liquid interfacial energy leads to patterning formation in the grains.
4:30 pm
REDUCED OXYGEN PRESSURE MELT PROCESSING OF YBa2Cu3Ox/Ag-Pd WIRES: M. A. Black, P. J. McGinn, Dept. of Chemical Engr., Univ. of Notre Dame, Notre Dame, IN 46556; M. Tomsic, Plastronics, Inc., Troy, OH
Powder-in tube processing has been utilized to fabricate YBa2Cu3Ox (YBCO)/9OAg-lOPd composite wires. These wires have been melt textured at 960deg.C using reduced oxygen partial pressures (1000 Pa oxygen). Metal alloy/liquid phase interaction results in small inclusions of BaCuO liquid within the alloy and dissolution of some Ag by the liquid. The use of reduced oxygen pressure and small Pd additions have been used to avoid melting of Ag in the YBCO system, so that composite wires can be fabricated which will retain their shape. Processing concerns, microstructural development during texturing in reduced O2 environment and the effects of metal/liquid interactions will be discussed.
4:50 pm
TEM INVESTIGATION OF MELT PROCESSED YBa2Cu3O7-y DOPED WITH Y2BaCuO5 AND PtO2: J. D. Riches, J. C. Barry, Physics Dept., Univ. of Queensland, St. Lucia QLD 4072, Australia; A. Bhargava, Centre for Microscopy and Microanalysis, Univ. of Queensland, St. Lucia QLD 4072, Australia; P. McGinn, Dept. of Chemical Engr., Univ. of Notre Dame, Notre Dame, IN 46556
Melt texturing causes a significant increase in the Jc of bulk YBCO samples. Thus, it is of considerable interest to investigate the features of such textured microstructures and how they depend on processing parameters. To this end, a study was made of the microstructure of melt processed Y123 doped with Y211 and PtO2. The samples were processed via the SLMG route and their microstructures were examined via TEM. The grain boundaries, both between Y211 and Y123 grains and also between adjoining Y123 grains, were examined. Investigations into the structure of the grain boundaries, and the presence of any dislocations, defects or secondary phases, were undertaken. Previous studies of this material have shown the existence of Y211 precipitation of a size less than 100 nm. These small precipitates should be more beneficial to flux pinning than the micron sized Y211 precipitates that are usually found in melt processed samples. In this study the hold time above the peritectic temperature for the samples was varied and the effect on the size and distribution of the nm sized Y211 precipitates was examined.
5:10 pm Invited
GROWTH OF LARGE-DOMAIN YBa2Cu30x AND THE STUDY OF INTERFACE COUPLING OF DOMAIN BOUNDARIES: Donglu Shi, Krishanu Lahiri, Sandhya, Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012
Several seeded-melt texturing (SMT) methods were used to grow large-domain
YBa2Cu3Ox for magnetic levitation applications. With various seed crystals,
these SMT methods resulted in large single-domain pellets of high levitation
forces. A domain growth model is proposed. We developed a novel method to
determine the coupling strength of the domain boundaries. Instead of measuring
the transport critical current density across a boundary where many technical
problems could occur, this new approach employs the magnetization measurements
on a sample that contains a single interface. The coupling strength of any
interfaces can be determined by comparing with a standard obtained from a
sample without the interface. It was found that the natural grown-in domain
boundaries are strongly coupled. However, the coupling strength is less
compared to that of the domain without any interface. It was also found that
the interface coupling strength has a temperature dependence. At 30 K, it
reached a maximum value of 0.85 assuming the standard coupling strength inside
the domain is F = 1.
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