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; Stuart Abell, University of Birmingham, Edgbaston, Birmingham B152TT, UK
Wednesday, AM Room: B7
February 7, 1996 Location: Anaheim Convention Center
Session Chairpersons: U. Balachandran, Argonne National Laboratory, Argonne, IL 60439; W-K. Chu, Texas Center for Superconductivity at the University of Houston, Houston, TX 77204
8:30 am Invited
DYNAMICS AND FRICTIONAL ENERGY LOSS ON HYBRID SUPERCONDUCTING MAGNETIC BEARINGS: W-K. Chu, Z. Xia, K. B. Ma, Q. Y. Chen, R. Cooley, P. C. Fowler, C. K. McMichael, Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5932
Rotor dynamics and analysis of frictional energy loss have been conducted for a flywheel energy storage (FES) prototype, in which a 42 lb. flywheel is levitated by the hybrid superconducting magnetic bearings (SMB). The flywheel currently can rotate up to 6,000 RPM with kinetic energy of 8 Wh stored. The previous spindown tests indicate an average frictional energy loss <2% per hour in a vacuum of 10-5 torr. To improve overall system performance for a further industrial module design, we installed a sensor system for vibration measurements and did mechanical modifications to minimize the friction loss. In this talk, we will report recent characterization of our bearings and extrapolate its performance onto a larger flywheel system.
8:50 am Invited
A HIGH TEMPERATURE SUPERCONDUCTOR HIGH VOLTAGE GENERATOR: J. X. Jin, S. X. Dou, H. K. Liu, Center for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia; C. Grantham, Dept. of Electric Power, University of NSW, Sydney 2052, Australia
A high voltage generator has been developed based on a R, L, C resonant circuit. The generated high voltages are simulated, and also tested with the built controller. Resistance in the circuit limits the voltages that can be built up. Therefore, using a high-temperature superconducting (HTS) inductor is considered. The (Bi,Pb)2Sr2Ca2Cu3O10+x HTS wires are tested and used to make a HTS coil. The results show that this HTS wire is a good candidate to make a HTS inductor. By replacing a normal copper winding inductor with a HTS inductor, the magnitude of achievable voltages is substantially increased.
9:10 am
APPLICATIONS OF SOIL MECHANICS TO THE EXTRUSION PROCESSING OF SUPERCONDUCTING WIRE: C. H. Schilling, R. W. McCallum, D. Emery, Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50010
Key requirements for extrusion processing of superconducting composite wire are dimensional uniformity of the cross section and high packing densities within the core to enhance the sinter/reaction step. To meet these requirements, the core must exhibit uniform plastic flow, which is in turn regulated by two critical factors: (i) the stress state, which is inherent in the forming operation, and (ii) powder processing variables including the particle morphology, size distribution, and interparticle friction. This presentation will begin with a review of soil mechanics principles that are used to characterize these factors and enhance the predictability of the deformation response of the superconducting core. Soil mechanics tests will be discussed that were used to characterize the compressibility and critical state flow behavior as a function of the particle size distribution of model powder systems.
9:30 am
QUANTITATIVE ANALYSIS OF THE EFFECTS OF STRAIN-STATE ON THE MICROSTRUCTURE AND Jc OF BSCCO TAPES: W. R. Blumenthal, Y. T. Zhu, R. J. Sebring, T. C. Lowe, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545; R. J. Asaro, Dept. of Applied Mechanics and Engineering Sciences, University of California at San Diego, La Jolla, CA 92093
The effect of strain-state on the microstructure and critical current density (Jc) Of silver-sheathed, Bi1.9Pbo 4Pb0.4Sr2Ca2.2Cu3O10+x tape was investigated by rolling tapes embedded within steel blocks. Pure compression and combined compression-shear deformation states were generated in different planes of the blocks. In addition, high hydrostatic loading was investigated by confining the edges of tapes to prevent free lateral deformation. Quantitative image analysis was performed on digital back-scattered electron images of transverse sections of tape using standard gray-level threshold analysis techniques. Image analysis showed highly porous oxide core regions at the edges of unconfined tapes that could explain low Jc values compared to tapes rolled under plane-strain constraint. Image analysis was unable to detect significant differences in the volume of nonsuperconducting phases which could have explained the superior Jc Of confined compression-shear processed tapes compared to pure compression processed tapes.
9:50 am Invited
RECENT DEVELOPMENTS IN FABRICATION AND PROPERTIES OF Ag-CLAD BSCCO CONDUCTORS: U. Balachandran, A. N. Iyer, R. Jammy, J. Y. Huang, Argonne National Laboratory, Argonne, IL 60439; P. Haldar, Intermagnetics General Corporation, Latham, NY 12110
Long lengths of Ag-clad BSCCO superconductors with consistent current transport properties were fabricated by the powder-in-tube technique. A high-Tc magnet, fabricated from 770 m of monofilament conductor, generated a record-high field of 1 T at 4.2 K in a field of 20 T. Strain tolerance of the conductors was evaluated by in-situ tensile and bend tests. Superconducting joints have been developed to connect Ag-clad BSCCO conductors. Typical Ic through a monocore butt joint was 23 A. An analysis of the results will be presented, along with the effects of various parameters and microstructural studies. Work at ANL and part of the work at IGC is supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-31-109-Eng-38.
10:20 am
MASS DENSITY EVOLUTION OF (Bi,Pb)+Sr1Ca2Cu3O10/Ag WIRE AND TAPE DURING MECHANICAL DEFORMATION: Q. Y. Hu, H. K. Liu, S. X. Dou, Centre for Electronic and Superconducting Materials, University of Wollongong, Wollongong, NSW 2500, Australia
The mass densities of the Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10 wire and tape varies during the mechanical deformation process, as one of the steps of the oxide-powder-in-tube technique used to fabricate the composite superconductor. Results show that the rolling has more significant effect on densifying tape core. Whereas the drawing process can only densify the core to about 75% of the theoretical density. SEM observations confirm the mass density calculations. SEM observation also show that with increasing the deformation extent, the average grain size is reduced. It is proposed that although the rolling densifies the tape core much significantly, it also destroy the crystallinity of the superconducting phases and results in the formation of amorphous phase. Since the textured Bi-2223 phase forms by the epitaxial growth on the textured Bi-2212 seed crystals, the deformation induced texture is critical. Appropriate deformation extent is necessary, since too high extent deformation may change the well aligned grains into amorphous phase. The formation of the amorphous phase is harmful to the texturing formation of the Bi-2223 phase, which finally leads to a critical current degrading. Microhardness measurement results are also identical to the mass density calculations.
10:40 am
CONTROL OF MECHANICAL DEFORMATION PROCESSING IN FABRICATION OF Bi-BASED SINGLE AND MULTIFILAMENT TAPE: W. G. Wang, H. K. Liu, S. X. Dou, Centre for Electronic and Superconducting Materials, University of Wollongong, Wollongong, NSW 2500, Australia
Different strain between Ag and oxide core is measured to describe interface between Ag and oxide core. The sausage effect is induced by different deforming resistance during mechanical deformation process which comes from difference in mechanical properties between silver or silver alloy and oxide material. The thickness of 0.1 mm without sausage effect is achieved by controlling processing parameters, such as smaller diameter of roller, variable reduction from 30% to 5% depending on density of oxide core from low to high, smaller powder particle size, lower rolling speed, doping pure silver powder, lower and homogenized packing density and reducing annealing times in fabrication of single filament tape. For the multifilament tape, the satisfied packing factor and eliminating inhomogeneity of internal stress are very important. A model is put forward to explain the effect of various parameters of material and mechanical deformation to interface between silver and oxide. The improvement of interface is achieved by processing control according to this model for fabricating Bi-2223 long multifilament tape.
11:00 am
HIGH TEMPERATURE PRESSING OF MONO AND MULTIFILAMENTARY Bi SUPERCONDUCTING TAPES: P. A. Bain, D. Yu, Q. Y. Hu, T. Chandra, H. K. Liu, S. X. Dou, Centre for Electronic and Superconducting Materials, University of Wollongong, Wollongong, NSW 2500, Australia
Silver clad (Bi,Pb)2Sr2Ca2Cu3O10 superconducting monofilamentary and multifilamentary tapes have been uniaxially pressed at various temperatures ranging from 700deg.C to 850deg.C under various pressures ranging from 5 MPa to 20 MPa. The uniaxial pressing was performed at various stages of thermomechanical processing consisting of alternate cold pressings and sinterings. To determine complementary or similar mechanism benefits, high temperature vacuum treatments were also performed at various processing stages. SEM, TEM, Jc behaviour in magnetic field, AC susceptibility and microhardness measurements were performed at each stage of processing to assess induced changes. These were compared with measurements of tape undergoing the same thermomechanical treatment to the exclusion of high temperature pressing and vacuum treatments. All the obtained microstructural and electromagnetic properties of high temperature pressed and vacuum treated tapes show significant improvements over non-treated tapes. In particular, powder texture and consequently Jc show outstanding improvements. A substantial improvement Of Jc behaviour in magnetic field also indicates a beneficial effect of high temperature pressing on powder density and possibly flux pinning properties. All aspects of both treatments have been thoroughly investigated and analyzed.
11:20 am
MICROSCOPIC MECHANISMS OF THE SUPERCURRENT TRANSPORT IN (Bi, Pb)2Sr2Ca2Cu3010 SILVER-SHEATHED TAPES: B. Hensel, G. Grasso, R. Flükiger, Université de Geneve, Departement de Physique de la Matiere Condensee, 24, quai Ernest-Ansermet, 1211 Geneve 4, Switzerland
We have proposed the "railway-switch" model to describe the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes. Primary limitations for the critical current are imposed by the small effective cross-section for the current, by the low intragrain critical current density jcc along the c-axis (compared to jcab) and by the even lower critical current density jc1 across twist boundaries or intergrowths within individual grains and colonies of grains. In magnetic fields a secondary limitation for ic is given by the lack of effective pinning that is omnipresent in Bi-based superconductors. A limitation by weak links is only of minor importance. In this work we give an overview of the experimental evidence in favour of the "railway-switch" model and present ways to overcome the secondary level of limitation for icIrradiation with high energy protons generates splayed columnar defects that serve as effective pinning centers and enhance the irreversibility field by one order of magnitude at T = 77 K.
11:40 am
LOCAL MEASUREMENTS OF CRITICAL CURRENT DENSITY IN BSCCO-PHASE CONDUCTORS: M. Johnston, J. Everett, M. Dhallé, A. D. Caplin, Centre for High Temperature Superconductivity, Blackett Laboratory, Imperial College, London SW7 2BZ, UK
We describe scanning Hall probe studies of the current distribution in
BSCCOphase tape and ribbon conductors. The pattern of current flow is affected
by temperature, applied magnetic field and also the current level itself. We
correlate these local studies with macroscopic information from transport and
magnetization experiments, and also with the microstructure. Preliminary
studies have been made also of the current distribution between filaments in
multi-filamentary BSCCO-phase tapes.
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