|Previous Session||Next Session|
|Return To Program Contents Page|
Tuesday , PM Room: 315A
Session Chairpersons: K. Togano, National Research Institute for Metals, Tsukuba, Japan; R.D. Blaugher, National Renewable Energy Laboratory, Golden, CO
2:00 pm INVITED
THE EFFECT OF PROCESSING ON THE MICROSTRUCTURE OF Bi2Sr2CaCu2Ox/Ag SUPERCONDUCTOR: K. Togano, H. Kumakura, H. Kitaguchi, H. Fujii, National Research Institute for Metals, Tsukuba 305, Japan; T. Hasegawa, Y. Hikichi, Showa Electric Wire and Cable Co., Ltd., Kawasaki 210, Japan
The Bi2Sr2CaCu2Ox/Ag(2212) conductors with high Jc can be fabricated by applying partial melting process, in which the 2212 phase melts incongruently. However, the presence of nonsuperconducting solid phases in the liquid makes it very difficult to achieve an ideal grain alignment of 2212 phase on cooling. In this paper, we report detailed studies on the distribution of nonsuperconducting phases and its effect on the nucleation and growth behavior of 2212 crystals. As one of the methods to control the distribution, we applied centrifugal gravity fields of up to ~100 g to the samples during melting process. Gravity effect was more significant for thick 2212 conductor, for which interface effect was less dominant. The improvement of grain alignment for the multilayered Ag/2212 conductors and the results of Jc measurement will be presented.
REACT-WIND-AND-SINTER TECHNIQUE FOR THE MANUFACTURE OF POWDER-IN-TUBE Bi2Sr2CaCu2Ox COILS: J. Schwartz, S. Boutemy, National High Magnetic Field Laboratory, 1800 E. Dirac Dr., Tallahassee, FL 32310
Heat treating powder-in-tube Bi2Sr2CaCu2Ox coils such that short sample properties are obtained in large quantities is an important challenge. The wind-and-react technique suffers from poor temperature control due to the coil thermal mass and the sensitivity of the superconductor to the peak temperature and cooling rates. With the react-and-wind approach, however, optimum conductor performance is not obtained since it is degraded by strain during winding. A new technique is being developed at the NHMFL: the react-wind-and-sinter technique. Long lengths of powder-in-tube conductor are reacted by pulling the tape through a furnace with a controlled atmosphere. The temperature profile is controlled to emulate the partial-melt step of the conventional heat treatment. This method allows a more uniform reaction in long lengths of conductor. The tape is then insulated and wound into the desired coil shape and sintered isothermally. This repairs the cracks and achieves high phase purity and grain alignment. Here we report progress on this approach for HTS coils.
2:40 pm INVITED
DEVELOPMENT OF LONG LENGTH Bi-2212 SUPERCONDUCTORS: R. Wesche, EPFL, Centre de Recherches en Physique des Plasmas, CH-5232 Villigen PSI, Switzerland
Superconducting AgNiMg and Ag/Bi-2212 multicore wires of up to 40 m length have been fabricated by the powder-in-tube method. The optimum heat treatment conditions and the critical temperature have been found to depend on the wire diameter and the matrix material. In addition, the cooling rates are of importance for the achievable Tc values. Generally, the AgNiMg-sheathed wires behave like thinner Ag/Bi-2212 wires. Maximum Tc values of 92K have been achieved for both matrix materials. These observations suggest very rapid grain-boundary assisted diffusion of oxygen through the AgNiMg matrix. Short sample jc values (1 mV/cm) of up to 80,000 A/cm2 (4.2 K, B=0) and 3000 A/cm2 (77 K, B=0) have been achieved in Ag/Bi-2212 multicore wires of 1 mm diameter. For AgNiMg/Bi-2212 multicore wires of 1.5 mm diameter maximum zero field jc values of 55,000 and 2000 A/cm2 have been reached at 4.2 and 77K, respectively. First results for the jc values in Bi-2212 wires of long lengths will be presented.
3:00 pm INVITED
HOT-DEFORMATION TREATMENTS OF Y123 AND Bi2223 MATERIALS: 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
Y123 and Bi2223 bulks were hot compressed isothermally in a temperature range of 780-850°C under constant stresses of 15-50 MPa for 60 min. It was found that the mass density of the bulk Y123 and Bi2223 superconductors has been increased. 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 been improved by hot compressing which in turn increased critical current of both materials. Ag-sheathed Bi2223 wires were hot-rolled into thin tapes at about 800°C at a four-high laboratory rolling machine with attachment of a tube furnace. It was found that hot-rolled tapes had higher core mass density but sausaging was severer than cold-rolled samples. Apart from the hot rolling, the Bi2223 tapes were also hot compressed under the condition of uniaxial loading and it was found that the treatment enhanced the core density, resulting in the improvement of the critical current of the samples.
3:20 pm INVITED
EFFECT OF pO2, C, AND Ag ON THE DECOMPOSITION PATHWAYS IN THE Bi-Sr-Ca-Cu-O: R.W. McCallum, L. Margulies, K.W. Dennis, M.J. Kramer, Ames Laboratory, Iowa State University, Ames, IA 50011
The promise of usable forms of high temperature superconducting wires and tapes rests on developing techniques for processing highly textured materials. Understanding the effect of Ag and pO2 on the chain of peritectic reactions initiated during melting is necessary in order to optimize the processing parameters of Bi-Ca-Sr-Cu-O-Ag composites. Previously, work on the solubility of Ag in the Bi2212 melt revealed a large liquid immiscibility gap and a eutectic on each side of this gap. In addition, the sub-liquidus phase reactions in this system were examined in greater detail for 0.01, 0.2, and 1.0 bar pO2. This work showed that the peritectic reactions varied as a function of pO2. The role of Ag was shown only to depress the peritectic decomposition temperatures. C was shown to have only a very minor effect for near stoichiometric materials. This work is currently being extended to the Bi2221 + Ag system. Supported by U.S. Department of Energy, under Contract No. W-7405-Eng-82.
3:40 pm BREAK
3:50 pm INVITED
THICK-FILM PROCESSING FOR TL-OXIDE WIRE AND TAPE: R.D. Blaugher, R.N. Bhattacharya, D. Ginley, P. Parilla, D. Schulz, National Renewable Energy Laboratory, Golden, CO 80401
Thick-film processing using techniques such as dip coating, ink spraying and electrodeposition represent promising approaches for fabricating HTS wire or tape for the Tl-oxide superconductors. The wire and tape processing for long lengths of the Tl-oxides, in contrast to the Bi-materials, is not as well advanced due primarily to a lower level of effort. The Tl-oxides, moreover, offer the potential for operation at 77K in practical magnetic fields of 3-5 T, which is supported by measurements on the irrreversibility behavior of the Tl-1223 single layer compound with Pb and Sr substitution. Thick-film processing methods such as electrodeposition and ink spraying present a viable approach for producing a cost effective HTS wire or tape with technologically acceptable transport properties. This paper will review the progress in the Tl-oxide wire and tape processing for thick-film techniques. The relative merits of conventional mixed-oxide and highly reactive sub-micron precursor as prepared by electrodeposition will be discussed. The prospects and predicted cost for a "long length" thick-film process using electrodeposition and ink spraying will also be presented.
THE EFFECTS OF CARBON ON TlBa2Ca2Cu3Ox PHASE FORMATION IN THE Tl(F)-Ba-Ca-Cu-O SYSTEM: Y.S. Sung, X.F. Zhang, P.J. Kostic, D.J. Miller, Materials Science Division and Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439
The effects of carbon on TlBa2Ca2Cu3Ox phase formation in the Tl(F)-Ba2Ca2Cu3Oz system have been investigated. Samples of varying carbon content were prepared by controlling the carbon content in the Ba2Ca2Cu3Oz precursor powders that were mixed with TlF and reacted to form the superconducting phase. The samples were characterized by x-ray diffraction, scanning and transmission electron microscopy, and DC magnetization. It was found that carbon induced the formation of the Tl2Ba2Ca2Cu3Oy phase. While TlBa2Ca2Cu3Ox was the primary phase at low carbon contents, Tl2Ba2Ca2Cu3Oy became the dominant phase at high carbon contents, in spite of a 1:2:2:3 (Tl:Ba:Ca:Cu) starting composition. For intermediate carbon levels, there was a strong tendency to form superlattices as a result of intergrowth between TlBa2Ca2Cu3Ox and Tl2Ba2Ca2Cu3Oy. 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.
4:30 pm INVITED
USE OF PARTIAL MELTING IN Tl-1223 COIL PRODUCTION: J.C. Moore, S. Fox, M.J. Naylor, S.K. Wivell, C.R.M. Grovenor, University of Oxford, Dept. of Materials, Parks Road, Oxford OX1 3PH, UK
We have investigated a number of practical aspects in the manufacture of demonstrator coils from Tl-1223 PIT tape. Coils have been fabricated using a wind and react method from 5 m tapes with an insulating MgO coating, giving engineering Jc values of 2500 Acm-2. However, the performance of these sintered tapes in external magnetic field is at present poor. Previous work has suggested that only improved grain alignment can overcome this weak link problem and we have chosen to investigate a melt processing route in order to achieve this. A considerable effort has been put into correlating the effects of process variables (melt T, heating and cooling rates) on the microstructure in PIT samples. We will discuss the choice of precursor powder, composition of 1223 and heating schedule which gives the best properties in small coils.
4:50 pm INVITED
MODIFICATION OF Bi-2223 PRECURSORS BY POWDER ENGINEERING METHODS: O.A. Shlyakhtin, A.L. Vinokurov, V.V. Ischenko, N.N. Oleinikov, Dept. of Chemistry, Moscow State University, 119899 Moscow, Russia
Reactivity of Bi-2223 precursors is rather sensitive to the microstructure of the last ones, i.e., to the size of 2212 particles and spatial distribution of minor phases. The primary way to change precursor microstructure is connected with the change of precursor synthesis technique. Relying on our studies of particle size evolution during synthesis and thermal treatment of oxide powders, we propose a method of fine and directed modification of precursor microstructure in frames of the same synthesis technique. This method is based on the mechanical and ultrasonic treatment of precursor powders at the intermediate stages of their synthesis by chemical methods.
RECENT ADVANCES IN POLYCRYSTALLINE OXIDE SUPERCONDUCTORS: PROCESSING, PROPERTIES, AND APPLICATIONS: M. Sisodia, A. Gupta, R.K. Yadava, Dept. of Metallurgical Engineering, Malaviya Regional Engineering College, Jaipur 302 017, India
This talk will review some of the processing techniques adaptable to high-Tc superconductor fabrication with much emphasis on desirable geometries, improved properties, microstructure control, and its associated practical problems that result in its good performance in magnetic fields. It has been observed that the improvement and optimization of Jc is hindered by the formation of jospehson weak links at grain boundaries, flux creep/flux flow effects, and anisotropic properties in polycrystalline high-Tc superconducting materials. Although the oxide superconductors in polycrystalline form have very poor in-field Jc, it can be improved by studying the current flow. The inhomo geneous current flow in these materials is discussed with the help of results for laser slicing and magnetooptic imaging experiments. Lastly, the advanced applications of each are discussed.
|Previous Session Next Session|
|Search||Technical Program Contents||1997 Annual Meeting Page||TMS Meetings Page||TMS OnLine|