D.Y. Jeong

AC susceptibility of high Tc superconductor Tl0.8Pb0.2Bi0.2Sr1.6Ba0.4Ca2Cu3Ox using a critical state model

Abstract The Anderson–Kim model is employed to calculate the temperature and AC magnetic field dependence of the complex susceptibility, χ = χ ′ +i χ ′′ , of high T c superconductor Tl 0.8 Pb 0.2 Bi 0.2 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O x . Intergranular Josephson vortices sweep in and out of the weak-link network, while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis loss. The calculated χ ′ and χ ′′ are consistent with experimental data very well by considering the proper volume fraction of superconducting grains and intergranular region. Using field dependence of critical current density obtained from fitting parameter, the property of intergranular region is discussed and the intergranular lower critical field H c1J is determined.

Microstructures in high-Jc Tl-1223/Ag tapes

Abstract High- T c Tl-1223/Ag tapes with a nominal composition of (Tl 0.93 Bi 0.22 )(Sr 1.6 Ba 0.4 )Ca 2 Cu 3 O x were prepared using the powder-in-tube method combined with the two step reaction method. Their microstructures were examined using scanning and transmission electron microscopy. Tape A ( J c = 5 500 A/cm 2 at 77 K under 0 T) prepared using rolling only has a lower J c than tape B ( J c = 14 000 A/cm 2 ) prepared using pressing. In both tapes Tl-1223 forms plate-like grains. Although the size of the plates in tape A is larger than those in tape B, their mutual connection is poorer. Many subgrains are formed in each grain with small angle boundaries, which are composed of tilt-type as well as twist-type components in most cases. The boundaries might be considered as effective pinning sites for magnetic flux as far as the tilt component predominates. Additionally, small holes were found at the subgrain boundaries.

High Jc's observed in just-rolled Tl-1223/Ag tapes prepared using a Tl0.8Pb0.2Bi0.2Sr1.6Ba0.4Ca2Cu3Oz composition

Abstract Tl-1223/Ag tapes with a nominal composition of Tl 0.8 Pb 0.2 Bi 0.2 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O z were prepared using the powder-in-tube method combined with an in-situ reaction method, and their characteristics were mainly compared to those of Tl 0.93 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O z tapes. As a result, the morphology in the present tapes seemed to be almost similar to those in Tl 0.93 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O z tapes, except that it became denser. However, the temperature range for the stabilization of Tl-1223 phase widened a little, and J c s at 77 K and 0 T increased. The enhanced J c s in the tapes were attributed to enhanced effective area for current flow due to enhanced densification, probably resulting from doping of both Pb and Bi. In the present study, a high transport J c of 18,100 A/cm 2 at 77 K and 0 T was obtained.

High-resolution transmission electron microscopy analysis of the interface between a Tl-1223 (001) superconducting film and an untextured Ag substrate

The interface between a screen-printed (Tl0.5Pb0.5)(Sr0.85Ba0.15)2Ca2Cu3Oy (Tl-1223) superconducting film and an untextured Ag substrate is analyzed by transmission electron microscopy (TEM). Preferential orientation between the Tl-1223 grains and the Ag substrate is found. The (001) plane of Tl-1223 grains is parallel to and the {113} plane of Ag is almost parallel to the interface. High-resolution TEM images show that CuO2 planes contact the faceted {113} planes of Ag substrate at the interface. A mechanism for the preferential growth of the Tl-1223 (001) film on untextured Ag substrate is proposed.

Much enhanced Jc by intermediate rolling in just-rolled Tl-1223/Ag tapes

Abstract Tl-1223/Ag tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr 1.8 Ba 0.2 Ca 2.2 Cu 3 O z nominal composition were prepared using the powder-in-tube method incorporating an in situ reaction method. Connectivity and directional alignment of plate-like Tl-1223 grains were much enhanced by incorporating an intermediate rolling during the final heat-treatment process. As a result, high J c values near 2.5−10 4 A/cm 2 at 77 K and 0 T with an excellent reproducibility were obtained in just-rolled tapes. The high J c s seem to result from relatively easy recovery of excellent grain-connectivity during the final heat-treatment after intermediate rolling, probably due to retarded Tl evaporation and excessive Ca content in the present composition. The strong field dependence of J c even in low fields, however, indicated that there still existed significant weak-links and the degree of directional grain-alignment was far from the desired one. The high J c is ascribed to enhanced contact areas between Tl-1223 grains rather than enhanced directional grain-alignment. The intergranular binding in the tapes seemed to be mainly dominated by an SIS junctional type.

Dependence of superconducting properties and microstructure on heat treatment in Ag-sheathed BiPbSrCaCuO tapes

Abstract Ag-sheathed BiPbSrCaCuO tapes were prepared using three different heat treatments, and studied in terms of T c , J c , χ dc and microstructure. The results suggested that a complicate process using quenching partial melt, cold working, partial melt growth and sintering will be effective for the preparation of highly densified and well grain-aligned Bi-2223 tapes within a short period of time.

A new process to prepare thick bufferless Tl-1223/Ag high temperature superconducting coated conductors using electrodeposition

The present study shows general characteristic features of processes such as electrodeposition, drying, and heat-treatment, involved in the preparation of bufferless Tl-1223/Ag superconducting coated conductors by using an electrodeposition method, the details of which have not been reported in the literature. The importance of the present study, however, lies in a new process to make a thick Tl-1223 superconducting layer by using electrodeposition. In the new method, a relatively thin Tl-1223 precursor film is firstly electrodeposited on an Ag sheet, dried, and then heat-treated for phase transformation to superconducting phases to assign conductivity to the film. Then Tl-1223 precursor film is electrodeposited again on the heat-treated superconducting film, dried, and heat-treated. The process of electrodeposition, drying, and heat-treatment can be repeated many times to prepare a very thick superconducting layer. In the present study, a 2?3??m thick Tl-1223 layer was prepared on a intensified Ag sheet by repeating the process once. Even though Jc values in thick Tl-1223/Ag were quite low mainly due to material loss during heat-treatment, the microstructure analysis indicated that Tl-1223 layers were epitaxially grown on the intensified Ag sheets provided that the Ag surfaces were flat and clean.

The pre-oxidation process of electropolishing for the textured {113} Ag substrate for Tl-1223 superconductor substrate

The surface roughness of a substrate is typically influenced by grain boundaries, grooves and voids. In an effort to improve the surface roughness, a textured {113} <121> Ag substrate was electropolished using a new method that causes the formation of an oxidation layer at the constant current density before electropolishing at a constant voltage of 1.75 V vs. SCE. The constant current density and potential were determined using the anodic polarization method. The pH effect for the optimized electropolishing condition was also investigated by adding phosphoric acid. The edges of the grain boundaries, grooves and voids became smoother when a constant current density was supplied with the pre-oxidation. A RMS (root mean square) value of 5.90-nm regarding the surface roughness of the Ag substrate was obtained without a final surface treatment before electropolishing, while a RMS surface roughness value of 34.12-nm was obtained for an Ag substrate that was given a final surface treatment for mechanical polishing.

Enhanced grain-texturing and grain-connectivity in Tl-1223/Ag tapes prepared by using pretreated precursors

Abstract Phase evolution, interface uniformity, microstructure, J c and junctional characteristic of the inter-granular contacts were investigated in Ag-sheathed Tl-1223 tapes prepared using three kinds of reacted precursors, and compared to those in the tape prepared using an unreacted precursor. The precursors were prepared by heat-treating a mixture of Sr–Ba–Ca–Cu–O, Tl 2 O 3 , PbO and Bi 2 O 3 powders at 805 °C (precursor I), 840 °C (precursor II) and 905 °C (precursor III) for 20 min. Tl-1223 phase content, grain size and J c in the tapes appeared to increase in an order of precursors I, II and III. Compared to tapes prepared using an unreacted precursor, the tapes prepared using precursors II and III revealed much reduced pore density and thus much high core density, in addition to enhanced directional-grain-alignment. As results, junctional characteristic of inter-granular contacts appeared to be shifted to a more SNS type. Also the fraction of strong-links were improved. The improved properties are attributed to enhanced grain-connectivity due to the high core density resulting from using the reacted precursors.

Preparation of Bi-2223 Tapes by Partial-Melt Process

The availability of partial-melt process(PMP) in enhancing the densification and the directional grain-alignment was investigated in Ag-sheathed Bi-Pb-Sr-Ca-Cu-O tapes prepared by the powder-in-tube method. The PMP was employed after the Bi-2223 phase was fully formed. The results indicated that the PMP resulted in a dense, layer-like textured micromorphology in a short range order and little grain-texturing in a long range order, resulting in a low Jc. An attempt to extend the locally developed, dense and layer-like structure to a long range order was made by employing the PMP after obtaining a well-textured morphology by using a repetition of the combined process of rolling and sintering. It seemed, however, that the PMP destroyed the previously-made textured morphology.