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Session Chairpersons: P.J. McGinn, University of Notre Dame, Notre Dame, IN; J.L. MacManus-Driscoll, Imperial College of Science, Technology and Medicine, London, UK
8:30 am INVITED
EFFECT OF COMPOSITION ON GROWTH MECHANISM IN SEEDED-MELT PROCESSED YBa2Cu3Ox: D. Shi, K. Lahiri, S. Sagar, Dept. of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012
Single- and multiple-domain levitators of YBa2Cu3Ox have been processed using the seeded melt texturing method. It has been found that the growth rate has varied depending upon the undercooling, the temperature gradient, and composition. A systematic study has been carried out to establish the relationship between the growth front velocity and composition, in particular the 211 concentration. For a given undercooling and temperature gradient, the growth rate of the domain can be strongly affected by the 211 density in the matrix. Evidences will be provided to show the important contributions of compositions and 211 morphology to the growth rates of center domain during the seeded melt texturing.
8:50 am INVITED
EFFECTS OF Ce-BASED ADDITIONS ON THE MICROSTRUC-TURE AND MAGNETIC PROPERTIES OF MELT TEXTURED YBa2Cu3O7-: P.J. McGinn, A. Banerjee, T. Meignan, Dept. of Chemical Engr., Univ. of Notre Dame, Notre Dame, IN 46556
The effects of CeO2 and BaCeO3 additions on the microstructure and magnetic properties of melt textured YBa2Cu3O7- (Y-123) have been investigated. Both types of additions are effective in reducing the coarsening of Y2BaCuO5 particles in the melt during texturing. Both additions also lead to improvements in the magnetic properties of Y-123 compared to samples without Ce-based additions. The Ce-based additions are found to produce a "peak effect" in the magnetic hysteresis loop. The dependence of microstructure and magnetic flux pinning properties on the amount of Ce-based addition will be discussed.
GROWTH KINETICS OF MELT TEXTURED YBCO SUPERCONDUCTOR: V Selvamanickam, R.S. Sokolowski, Intermagnetics General Corporation, Latham, NY 12110; C.E. Oberly, Wright Patterson Air Force Base, Dayton, OH 45433; K. Salama, Y. Zhang, S. Salib, Texas Center for Superconductivity, Houston, TX 77204
The growth kinetics of YBCO superconductor have been studied by quench experiments during seeded isothermal melt-texturing. An undercooling of 15°C is found to be necessary to initiate nucleation of YBa2Cu3Ox following peritectic decomposition. A smooth transition is observed in the growth anisotropy with increasing undercooling, with the growth being faster along the c-axis at low undercoolings (15 to 25°C) and faster along the a-b plane at high undercoolings (>25°C). Based on these findings, a modified melt-texturing process was developed, where, instead of slow cooling following melting, an isothermal hold was employed in the temperature range where the growth is isotropic. By this modification, the time required to texture 1" diameter disks was reduced to 15 hours which is about 4 to 5 times faster than a typical slow cooling melt-texturing process. Correspondingly, the same magnitude of levitation force was achieved in a much shorter time in samples quenched during the modified process. Further, this process modification enabled a single do main growth over the entire sample thickness of 0.5", which could not be achieved by the slow cooling process. This research was sponsored by the Air Force Office of Scientific Research.
9:30 am INVITED
HIGH CRITICAL CURRENTS IN HIGH-ANGLE GRAIN BOUNDARIES IN YBCO SUPERCONDUCTOR: K. Salama, S. Sathyamurthy, A.S. Parikh, Texas Center for Superconductivity, 3201 Cullen Blvd., University of Houston, Houston, TX 77204-5769
A serious limitation to applications of YBCO superconductors is the presence of weak links in the form of grain boundaries. The high resistivity of YBCO grain boundaries is mainly attributed to the presence of solidified liquid phase and cracks at these boundaries. Using the liquid phase removal method, YBCO melt textured bars, over 10 cm in length and 0.5 cm2 in cross-section, are processed with "clean" grain boundaries free of secondary phases and cracks. Bars processed by this method are found to consist of several large domains separated by clean grain boundaries. The bars are polycrystalline in nature with an average of 5 grain boundaries for every one cm in length. Using transport current measurements, the bars are found to carry currents in excess of 1000 Amps (current limit of source available) at 77 K and self field with very small voltage drop (20 to 50 µV). Also, the same effect persists in the presence of external magnetic field up to 1.5 T at 77K. Contrary to previous findings, the misorientation angles of grain boundaries in these samples are found to be significantly high, ranging between 30° and 80°. These results indicate that the high angle grain boundaries in these bars are capable of sustaining very large currents and therefore make them suitable for applications such as current leads and current limiters.
9:50 am INVITED
SEARCH FOR NEW SUPERCONDUCTING SYSTEMS RELATED TO THE INFINITE LAYER FAMILY COMPOUNDS: NORMAL PRESSURE SYNTHESIS AND THE ROLE OF DOPING: R. Cloots, S.U.P.R.A.S., University of Liège, Chemical Institute B6, Sart-Tilman, B-4000 Liège, Belgium; H. Bougrine and M. Ausloos, S.U.P.R.A.S., University of Liège, Physics Institute B5, Sart-Tilman, B-4000 Liège, Belgium
The aim of this paper is to report on investigations on the influence of alkali and rare-earth ions doping in Ca1-xSrxCuO2 and Ba2Cu3O4Cl2 (and related compounds), in particular on their synthesis conditions and crystallographic structures. We concentrate the attention on the possibility to induce a charge transfer mechanism between constitutive layers by impurity doping, leading to a possible insulating-metal (superconductor) transition. Electrical resistivity and thermoelectric power measurements are presented in order to verify a possible superconducting behavior.
10:10 am BREAK
INVESTIGATIONS OF THE CHEMICAL COMPATIBILITY OF SUBSTRATES FOR 123 MELT-TEXTURED CERAMICS: F. Auguste, R. Cloots, S.U.P.R.A.S., University of Liège, Chemical Institute B6, Sart-Tilman, B-4000 Liège, Belgium; G. Moortgat, Centre de Recherche de l'Industrie Belge des Céramiques, Avenue Gouverneur Cornez 4, B-7000 Mons, Belgium; H. Bougrine, P. Clippe, M. Ausloos, S.U.P.R.A.S., University of Liège, Physics Institute B5, Sart-Tilman, B-4000 Liège, Belgium
The production of dense superconducting ceramics is affected by the thermal processing and parameters like the oxygenation process and the texture development. One of the serious problems which prevents the formation of useful REBa2Cu3O7-x-like bulk ceramics (RE = rare earth) for practical applications is the existence of weak links at the grain boundaries. Another annoying feature is that all usual containers or substrates for the growth react with the melt leading to a strong contamination of the "in situ" produced materials. The choice of the substrate is also very relevant for film growth. The cases of SrTiO3, MgO, Al2O3, and BaZrO3 crucibles for solution growth of single crystals of REBa2Cu3O7 has been investigated for the production of long length fibers or other complicated shapes of bulk Y or RE-123 materials. The chemical compatibility of the "substrate" towards the Y or RE-123 mixture is reported at various melt-texturing growth stages. For comparison, we looked at three different materials: BaZrO3, SrTiO3, and MgO.
MACROSEGREGATION OF 211 PARTICLES DURING ZONE MELT TEXTURING OF Y123: H. Balwada, P.J. McGinn, Dept. of Chemical Engr., Univ. of Notre Dame, Notre Dame, IN 46556
The phenomenon of 211 particle pushing by the solidification front during zone melt texturing has been studied. In the presence of second phase additions of PtO2 and BaCeO3, 211 particle refinement occurs. This refinement results in macroscopic segregation of 211 particles to the periphery of the sample rod. This phenomenon has been observed in samples textured with additions of PtO2 (.2 wt% and .5 wt%) and BaCeO3 (2 wt%) at speeds of 0.5 mm/hr and 1mm/hr. Without additions to refine the 211, this phenomenon is not observed. It was also found that higher texturization speeds lead to multiple grain nucleation at the solid/liquid interface. Particle pushing continues to take place under these circumstances, as long as the growth mechanism does not become dendritic, in which case this phenomenon is not observed. Effects of various solidification parameters on the extent of 211 particle segregation will be discussed.
11:00 am INVITED
MELT PROCESSING OF SCREEN-PRINTED ReBCO ON SILVER AT REDUCED TEMPERATURES AND OXYGEN PARTIAL PRESSURES: J.L. MacManus-Driscoll, N. Zafar, Centre for High Temperature Superconductivity and Dept. of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, UK
A thermodynamic approach has been applied to the processing of rare earth barium cuprate superconductors (ReBCO). Studies were made on various ReBCO thick films which had been screen-printed onto Ag substrates. Using a coulometric titration technique, thermodynamic studies and reduced temperature and pressure (RTP) processing methods were carried out the thick film samples of ReBCO/Ag. The parameters governing the melt processing of ReBCO/Ag were optimised with a knowledge of phase stabilities and phase relations with respect to both temperature and oxygen partial pressure. Furthermore, accurate control of the oxygen partial pressure enabled the determination of the phase chemistry and the crystal growth rates during processing.
THE FABRICATION AND ANALYSIS OF BULK YBCO FOR USE IN AN INDUCTIVE FAULT CURRENT LIMITER: J.P.G. Price,* S. A. L. Foulds,* T.C. Shields,* D. J. Moule, P. D. Evans, J. S. Abell,* *School of Metallurgy and Materials, School of Electrical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
Fault current limiters (FCL) have the potential of becoming one of the first major applications for high temperature superconductors. To improve commercial confidence in high temperature superconductivity, it is imperative that superconductors are produced and tested in certain applications. In this work, melt processed YBCO rings have been produced for use in an inductive fault current limiter. Various dopants and thermal treatments are utilized to improve the mechanical, electrical, and magnetic properties of the material. Using the FCL, critical current density (Jc) measurements can be made. Several different arrangements of the components of the FCL and the limiting performance are outlined. Characterization of the material is performed using polarized light microscopy, scanning electron microscopy, and pulsed Jc measurements. The final aim of this work is to determine whether bulk YBCO is suitable for use in inductive fault current limiters.
ADVANCES IN THE PROCESSING AND APPLICATIONS OF HIGH TEMPERATURE SUPERCONDUCTING FILMS: A. Gupta, M. Sisodia, R.K. Yadava, Dept. of Metallurgical Engineering, Malaviya Regional Engineering College, Jaipur 302 017, India
High critical temperature (high-Tc) superconductors have captured the interest of engineers and scientists world wide. For its commercial utilization in the field of microelectronics and electronic systems, they must be fabricated into desirable configurations - thin and thick films. During studies, it has been observed that these superconducting thin films operating at liquid N2 temperature offer great possibilities for faster, more sensitive and precise electronics devices. Present paper reviews the fabrication techniques of high-Tc superconducting thin films which primarily includes pulsed laser deposition, molecular ion beam epitaxy, chemical vapor deposition (CVD) by using epitaxial growth on single crystal substrates (YSZ, MgO, Si, etc.) with excellent microstructural control and properties relationship. In addition to it, thick film fabrication is also discussed with major techniques like screen printing and plasma spraying along with associated processing parameters. Furthermore, advanced applications of each are enumerated at length.
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