C.C. Sorrell

VOLUMETRIC MEASUREMENT OF LABILE IONS THROUGH CORROSION OF YBa2Cu3O7−x

The stability of sintered YBa2Cu3O7−x {123} in various aqueous solutions was investigated. It was found that all aqueous solutions have detrimental effects on superconduction in {123}. Based on the evolution of O2 through the reaction of {123} with acid, the volumetric measurement technique has been established for determining the quantity of Cu3+ in {123} with excellent reproducibility. In nitric acid the sintered {123} is much more stable than Ag, Cu or Ni metals, which are sometimes used in processing {123}, for example as cladding in wire drawing. {123} tends to passivate in concentrated acid. Impurity-doped and multiphase specimens generally suffer enhanced corrosion in water due to the formation of micro-electrochemical cells. Material with the tetragonal crystal structure is more stable in water than that with the orthorhombic structure, indicating that the sensitivity of {123} to water can be attributed to the presence of Cu3+ which is the driving force for rapid oxidation and reduction reactions in water.

XPS studies of high-Tc superconducting Bi2Sr2Ca(Cu1−yFey)2Ox single crystals

Abstract Fe-substituted Bi 2 Sr 2 Ca(Cu 1− y Fe y ) 2 O x single crystals have been studied by X-ray photoelectron spectroscopy (XPS) to determine the changes of oxidation states of their constitutes as well as the electronic structure of the valence band of the system. The core level lines of Bi exhibit two main components, which indicates either inequivalent sites or the presence of different oxidation states. The second component shows a systematic increase with Fe-substitution. The Sr lines also exhibit two components in which the lower binding energy component decreases with the dopant content. No significant changes in CuO 2 bilayers and Ca–O layer were observed. The results suggest a dramatic charge transfer between the Bi–O and Sr–O layers which compensates the increase of positive charge near the CuO 2 planes induced by Fe-substitution for Cu, and the impurity scattering of the dopants in the conduction band is one of the key factors for the T c suppression.

High-Tc superconducting current leads

Abstract Metal-sheathed high-Tc superconducting current leads of Jc=1.6–1.8×104 A cm−2 at 77 K under self-field condition were fabricated using a modified short tape processing technique. It was found that section-wise uniaxial pressing can be used to provide the necessary intermediate mechanical deformation between heat treatments of long silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+x superconductor. The overlapping of pressing up to 400 MPa does not impose non-uniformity of transport Jc to the final product. By stacking of single-core tapes, multi-layer current leads can be made. The Ic of this multi-layer current lead is additive, as in normal current in parallel-connected circuits. There is a lateral variation of Jc in the superconducting core of these leads.

EFFECT OF Fe DOPING ON SUPERCONDUCTIVITY IN THE Bi-Pb-Sr-Ca-Cu-O SYSTEM

The effects of Fe substitution for Cu in Bi1.6Pb0.4Sr1.6Ca2Cu3O10−y were investigated. The superconducting transition, phase assemblage, and lattice parameters were studied for samples with up to ~1/4 of the Cu replaced by Fe. With increasing levels of Fe, the superconducting transition gradually degenerated to that of semiconducting behaviour, the phase assemblage underwent the reaction series 2223→2221→2201, and the c-axis of the 2212 phase decreased monotonically when the doped samples were heat treated in air. These results can be explained in terms of structural changes rather than magnetic effects, where the higher valence and smaller size of Fe3+ compared Cu2+ alter the nature of the oxygen co-ordination and bonding. When the Fe-doped samples were treated in nitrogen, the effect of destabilisation of the superconducting phase was minimised. In contrast to the effect of Fe on the structure, the depression in Tc is attributed mainly to the magnetic effects of Fe doping. The critical concentration of the Fe dopant at which superconductivity was completely destroyed, is at ~13% Cu replacement.

CORRELATION OF CRITICAL CURRENT DENSITY WITH Cu3+ CONCENTRATION AND DENSITY IN YBa2Cu3O7−x

Superconducting YBa2Cu3O7 wires and tapes were fabricated by cold drawing, rolling and extrusion processes. It was found that the critical current density, after O2 equilibration, correlates both with density and Cu3+ concentration. Full density was achieved by using a special heat treatment, but the critical current density was low owing to the low Cu3+ concentration present in this heavily twinned material. The best critical current density results were obtained for material with density of 92–95% of the theoretical value. The low critical current density of the porous specimens is attributed not only to a poor connectivity between grains but also to a low Cu3+ concentration due to the instability of Cu3+ at crystallite surfaces which increase in area with specimen porosity.

LIQUID PHASE SINTERING OF Tl-Ba-Ca-Cu-O WITH SUPERCONDUCTING TRANSITION AT 122K

The Tl-Ba-Ca-Cu-O system, when prepared by standard sintering procedure, showed mass loss by volatilization of Tl even at sintering temperature lower than the decomposition temperature of Tl2O3 (875°C). The disappearance is a result of difficulty in preparation and Tl2O3 emission is severely toxic. The use of a two stage sintering technique involves the preparation of Ba-Ca-Cu-O by standard ceramic processing, so that Tl2O3 can be added to the sintered Ba-Ca-Cu-O enclosed in a silver tube, and sintered. This technique ensures reproducible results and safety. Bright field transmission electron microscopy demonstrates a layered structure by absorption contrast, and stacking faults.

REMANENT EFFECTS AND GRANULAR JOSEPHSON TUNNELLING IN 1:2:3 MICRO-BRIDGE JUNCTIONS

A study has been made of supercurrent in a 1:2:3 micro-bridge or slot-junction. It is shown that the maximum supercurrent through a slot-junction is strongly dependent on temperature, applied magnetic field, and magnetic history. In addition, inter-granular Josephson tunnelling has been observed using field-modulation techniques.

Superconducting Properties in Alkali Element Doped Bi2Sr2CaCu2O8

The effect of alkali element doping on the superconducting properties of the Bi-Sr-Ca-Cu-O system has been investigated. It was found that a TC of 94 K and To of 90 K in alkali element doped Bi2Sr2CaCu2O8 were achieved through the use of a melt-processing technique. Alkali elements assist the melting process, promote grain growth and grain alignment, and produce dense dislocation lines. A possible mechanism for the Tc enhancement is proposed in the bulk materials.

Superconductor/Silver Composites and Tapes Based on Bi-Pb-Sr-Ca-Cu-O

High-J c , bulk composites, containing 10–80 vol%Ag, and silver-sheathed tapes, 100 µm thick, were fabricated using the (Bi1.6Pb0.4)Sr2Ca2Cu3O10−y compound. A process of pressing and rolling with intermediate sintering was used to promote grain alignment (up to 90% texture) and improve flux pinning. AC susceptibility measurements on the composites show improvement of flux pinning with 20–30 vol%Ag, giving maximum J c values of 1600 A cm−2 at 77 K. The resistivity of composites decreased in the range 10−3–10−6 ohm cm with increasing silver content. J c in tapes depended on phase purity and the degree of c-axis texture. The best tapes had J c = 6.5 × 103 A cm−2 (77 K) in zero field, decreasing with field to ~103 A cm−2 for H⊥c-axis and to ~30 A cm−2 for H||c-axis at 0.4 T. The temperature dependence of J c was fitted by the flux creep model using pinning energy U(0) ≃ 90 meV.

Comparison of The Stability of Bi 2 Sr 2 CaCu 2 O 8+y with YBa 2 Cu 3 O 6,5+y in Various Solutions

The chemical behaviour of both YBa2Cu3O6 . 5 + y {123} and Bi2Sr2CaCυ2O8+y {2212} in various solvents was studied in a comparative manner. In both compounds superconductivity apparently depends on a high oxidation state of Cu. Electrochemical corrosion rates were measured by a variety of techniques and are related to the concentration of labile Cu3+ ions in the specimens. Generally {2212} is the more stable compound, except in strongly basic solutions owing to the amphoteric nature of Bi. In the presence of acidic or basic solutions Bi3+ is oxidized to Bi4+ or Bi5+ by the high oxidation state of Cu in {2212} (similar to oxidation by Na2O2), demonstrating a common feature of cuprate superconductors. The concentration of labile ions was determined by the volumetric measurement of evolved oxygen from acid solution, and the technique was extended to measure the concentration of undecomposed or re-formed carbonates after sintering.