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Joining of melt-textured YBCO : a direct contact method.

Description: We report a method for making weld joints, capable of transmitting high supercurrent densities, in bulk samples of melt textured YBCO. The joining procedure is carried out in a flowing atmosphere of O{sub 2} to eliminate problems associated with nitrogen gas, which can become trapped in the joint. No filler or fluxing material is used. The method can be used to join large areas (several cm{sup 2}) that are capable of transmitting supercurrent densities exceeding 10{sup 4} A/cm{sup 2}.
Date: December 19, 2001
Creator: Chen, L.; Claus, H.; Paulikas, A. P.; Zheng, H. & Veal, B. W.
Partner: UNT Libraries Government Documents Department

Meissner holes in remagnetized superconductors

Description: Flux distributions on different faces of YBCO single crystals remagnetized by unidirectional and rotating fields are studied using advanced magneto-optical techniques. Unusual structures corresponding to the appearance of strong current concentration along certain fronts are found. At such fronts the vortex lines bend into closed loops which then collapse and form flux free cylinders. Strong magnetization currents along these Meissner holes result in essential flux redistribution and development of dynamical instabilities in the samples.
Date: April 1996
Creator: Vlasko-Vlasov, V. K.; Welp, U. & Crabtree, G. W.
Partner: UNT Libraries Government Documents Department

A{sub 2}Zn{sub 3}As{sub 2}O{sub 2}(A = Ba, Sr): a rare example of square planar zinc

Description: Purpose of this study was to explore synthesis of novel solid-state compounds that could potentially exhibit interesting or useful physical properties. Goal was to extend the classes of compounds A{sub 2}Mn{sub 3}Pn{sub 2}O{sub 2} (A=Sr,Ba; Pn=P,As,Sb,Bi) to include a transition metal other than Mn. High-temperature fluxes were used to produce these new compounds.
Date: June 1994
Creator: Keane, P. M. & Burdett, J. K.
Partner: UNT Libraries Government Documents Department

The nature of twin boundaries in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: Twin boundaries are the most commonly observed lattice defect in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. Furthermore, the region around a twin boundary for which the structure and composition are seriously affected is of a scale comparable to the coherence length for the superconducting order parameter. Thus, twin boundaries can be important in the behavior of magnetic vortices and the critical current density in this material. In this paper the authors review the results of a wide range of investigations of twin boundary structure and composition by advanced transmission electron microscopy methods, both imaging and analytical in nature. A simple Landau model of twin boundary energy and width is proposed.
Date: June 1, 1999
Creator: Zhu, Y. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

Practical superconductor development for electrical power applications: Quarterly report for the period ending December 31, 1999

Description: This is a multiyear experimental research program focused on improving relevant material properties of high-T{sub c} superconductors (HTSS) and on development of fabrication methods that can be transferred to industry for production of commercial conductors. The development of teaming relationships through agreements with industrial partners is a key element of the Argonne (ANL) program. Recent results on substrate deposition for coated conductors, vortex studies, development of hardened Ag-alloy sheaths for powder-in-tube conductors, and sol-gel processing of NdBa{sub 2}Cu{sub 3}O{sub x} (Nd-123) are presented.
Date: February 2, 2000
Partner: UNT Libraries Government Documents Department

Effect of defects on the critical points in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: The upper and lower critical points are investigated in untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals with dilute columnar defects. Dilute columnar defects raise the upper critical point, indicating that the transition near the upper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can be described by a Lindemann-like criterion similar to that for melting. Dilute columnar defects induce non-linear behavior in the I-V curves of the vortex liquid state above the lower critical point, which the authors interpret as a vestige of the critical region associated with the Bose glass transition below the lower critical point.
Date: January 26, 2000
Creator: Kwok, W. K.; Olsson, R. J.; Karapetrov, G.; Paulis, L. M.; Moulton, W. G.; Hofman, D. J. et al.
Partner: UNT Libraries Government Documents Department


Description: The magnetic excitation spectrum of the ideal S=1/2 1-dimensional quantum antiferromagnet (AF), contains no single-mode contributions, and, in fact, can be described as a 2-particle continuum of spinons. Chain-MF theory predicts that a weakly coupled chains system becomes ordered at low temperatures. In the ordered state a sharp magnon excitation is present. The continuum persists, but is separated from the magnon branch by an appreciable energy gap. On the other hand, if weakly coupled chains are described through the 1/S expansion of the conventional Holstein-Primakov spin wave theory, one finds a longitudinal 2-magnon continuum and a transverse 3-magnon continuum that start immediately above the spin wave energy.
Date: March 16, 2000
Partner: UNT Libraries Government Documents Department

In situ real-time studies of oxygen incorporation in complex oxide thin films using spectroscopic ellipsometry and ion scattering and recoil spectrometry

Description: The surface termination of c-axis oriented YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) and the oxygen incorporation mechanism has been investigated using a unique combination of spectroscopic ellipsometry (SE) and time of flight ion scattering and recoil spectrometry (ToF-ISARS). The high surface sensitivity of the ToF-ISARS technique combined with the bulk oxygen sensitivity of SE are shown to yield complimentary information. The SE provided the film orientation and quality, while ToF-ISARS supplied surface compositional and structural information and enabled isotopic {sup 18}O tracer studies. It was determined that the O content of the film had little effect on the surface termination of the film, indicating a lack of labile Cu(1) sites at the c-axis oriented YBCO surface. Also, strong evidence for a Ba or BaO terminated structure is shown. The data related to the {sup 18}O tracer studies indicate that O from the reaction ambient incorporates only into the labile Cu(1) sites during both deposition and annealing, while stable O sites were populated with O from the sputtered target, indicating either the need for sputtered atomic O or sputtered YCuO complexes to occupy the stable Cu(2) sites.
Date: May 25, 2000
Creator: Mueller, A. H.; Gao, Y.; Irene, E. A.; Auciello, O.; Krauss, A. R. & Achultz, J. A.
Partner: UNT Libraries Government Documents Department

Critical current density and dissipation in sintered YBCO filaments

Description: Critical current density and dissipation of four sintered YBCO filaments were measured using a continuous DC power supply. Each sample exhibited somewhat different characteristics from the others even though 3 of the 4 samples are from the same batch. The data suggest that it is possible for the YBCO filament to be in a state of stable equilibrium when part of the filament is in the flux-flow state and part of the filament is in the normal state (local hot spots). This behavior may be advantageous for resistive fault current limiters because the intermediate state (partly flux-flow and partly normal) has an overall resistance much higher than that of the flux-flow state but it does not lead to thermal runaway (burnout) or excessive heating compared to a YBCO filament driven into normal state over its entire length. Two of the samples were broken as a result of local burnout.
Date: September 1, 1997
Creator: Cha, Y.S.; Hull, J.R. & Seol, S.Y.
Partner: UNT Libraries Government Documents Department

Comment on quantitative model for the superconductivity suppression in R{sub 1-x}Pr{sub x}Ba{sub 2}Cu{sub 3}O{sub 7} with different rare earths

Description: In a recent letter, a model for the electronic structure of R{sub 1-x}Pr{sub x}Ba{sub 2}Cu{sub 3}O{sub 7}, based on the LDA + U scheme was proposed. The T{sub c} suppression by Pr substitution was argued to be due to the transfer of holes from the superconducting CuO pd{sigma} to a dispersive pf{pi} band formed by O2p{sub {pi}} and Pr 4f orbitals. The insulating nature of PrBa{sub 2}Cu{sub 3}O{sub 7} was ascribed to disorder from Ba atoms on the Pr sites. The ac conductivity of the above compound is discussed.
Date: July 1, 1995
Creator: Fehrenbacher, R. & Rice, T.M.
Partner: UNT Libraries Government Documents Department

High J{sub c} YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} Films via Rapid, Low pO{sub 2} Pyrolysis

Description: In this investigation, YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) films were fabricated via a metal acetate, trifluoroacetic acid based sol-gel route, and spin-coat deposited on (100) LaAlO{sub 3} with a focus on maximizing J{sub c}, while minimizing processing time. We demonstrate that the use of a low pO{sub 2} atmosphere during the pyrolysis stage can lead to at least a tetiold reduction in pyrolysis time, compared to a 1 atm. O{sub 2} ambient. High-quality YBCO films on LaAlO{sub 3}, with J{sub c} values up to 3 MA/cm{sup 2} at 77 K, can be routinely crystallized from these rapidly pyrolyzed films.
Date: September 21, 2000
Partner: UNT Libraries Government Documents Department

Pulsed laser deposition of thick YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} films on flexible substrates

Description: As an alternative to bulk processing of high-{Tc} materials for conductor applications, we have been studying the applicability of physical vapor deposition to the problem of developing flexible superconducting elements with high I{sub c}. Specifically, we have used a two-step process consisting of ion-assisted sputtering of textured buffer layers on nickel-based substrates, followed by pulsed laser deposition of YBaCu{sub 3}O{sub 7-{delta}} (YBCO) films. The films have been analyzed by measuring the J{sub c} of microbridges, I{sub c} for 1 cm-wide strips, and field dependence up to 9 T. In addition tests have been conducted to determine minimum bending radius of the films.
Date: July 1995
Creator: Foltyn, S. R.; Arendt, P. N. & Wu, X. D.
Partner: UNT Libraries Government Documents Department