Year

1999

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

The work in this thesis concentrated on the processing of high temperature superconducting Bi-2223/Ag (Ag-alloy) tapes. A number of critical issues have been investigated. These include Bi-2223 phase formation kinetics and thermodynamics, microstructure, electromagnetic properties and current limiting mechanism.

An investigation into the effect of the phase composition of the precursor powder on the Bi-2223 phase formation showed that tapes made from a precursor powder containing 72.5% Pb in Bi-2212 phase have the highest Jc and the shortest sintering time.

A study of the relationship between filamentary homogeneity and observed current distribution in multifilamentary tapes was carried out using magneto-optical image analysis and current transport measurements. The results show that the microstructure homogeneity in multifilamentary PIT tapes is strongly dependent on the aspect ratio of the tape, and on the deformation procedure used during processing.

An optimised rolling procedure that gradually increased the reduction rate perpass during rolling has been developed. Using this technique, a uniform stress was achieved within the tapes, which improved both the transverse and longitudinal filament interfacial homogeneity and reduced the sausaging effect.

The mechanism and development of grain-growth texture of Bi-2212 and the corresponding inheritance of texture by Bi-2223 in Ag-sheathed tapes has been investigated. It was found that the sliding of precursor powder crystallites was the of Bi-2212 texturing during the cold-work deformation. Bi-2212 formed via a rapid melting process yielded a highly-aligned melt-textured microstructure during subsequent heat treatment. Bi-2223 was found to form from the Bi-2212 plates, with the Bi-2223 texture being inherited from the Bi-2212 texture.

A new intermediate quenching process was found to reduce the heat treatment time of Bi-2223/Ag tapes. In this process, the tape was quenched at the end of the first thermal cycle, and then pressed (or rolled) and heated rapidly to the original sintering temperature. In so doing, any unnecessary decomposition and recovery of Bi-2223 during cooling and heating was eliminated. Jc and the volume fraction of Bi-2223 for tapes heat-treated for a total 20 to 30 hrs using this technique were comparable with those treated using the normal process for 100 hrs.

The phase transformation and any residual liquid conversion during the final heat treatment of Bi-2223/Ag tapes have been investigated. It was found that the phase stability range of Bi-2223 existed between 823°C~832°C. At higher temperatures, the equilibrium shifts to the form of liquid and other secondary phases. Below this range, any residual liquid phase was found to transform into Bi-2212 (major), and below 810°Bi-2223 begun to decompose into Bi(Pb)-3221. Residual liquid phase is preserved as "frozen liquid strips" on fast cooling from annealing temperature above the Bi-2223 stability range. Also on slow cooling below 810°C, Bi-2223 would decompose into Bi(Pb)-3221 phase. This model is suggests a micro current limiting mechanism, in which, the residual "frozen liquid strips" becomes a barrier of transport current.

Mono and multifilamentary tapes with pure Ag and AgCu0.02, Ag(AgCu0.02), AgAl0.25, Ag(AgAl0.25), AgNi0.25Mg0.25, AgTi0.25Mg0.25, and AgTi0.25 sheathed alloys, have been fabricated. The physical and chemical properties were investigated and the effect on Bi-2223 phase formation determined. The alloys were found to have a significant effect on Bi-2223 phase formation and this was apparently linked to the alloying element's reactivity to oxygen in the following sequence Cu < Ni < Al < Mg < Ti. Multifilamentary tapes, composed of an inner A g sheath and an outer A g alloy sheath, showed no such effects.

A design of multifilamentary tapes was developed by packing powder and composite wires into a silver tube (PWIT) during the second stage of the powder-intube process to form a PWIT tape. The final Jc and bend strain tolerance for PWIT tapes with high filling factor were significantly improved when compared to normal multifilamentary tapes. The enhancement of both Jc and the bend strain tolerance was attributed to the increased interface area between the sheath and superconductor

A hot-pressing technique was used to prepare Ag-sheathed Bi-2223 multifilamentary tapes. The self-field Jc after hot-pressing was significantly enhanced. The maximum increase was more than double that found before. Hot pressing was found to raise the JC(B = 0) from 22,000 A/cm2 (Ic= 18 A) to 56,800 A/cm2 (IC=36.5A) for an 81-filament tape and from 26,000A/cm2 (Ic = 36.2 A) to 56,000 A/cm2 (Ic= 56.6 A) for an 19-filament tape. But the Jc in an applied magnetic field was not improved by hotpressing. Microstructural analysis showed that hot-pressing chiefly improved grain connectivity, increased the core density and reduced secondary phase impurities.

The effect of a controlled intermediate quenching technique on the microstructure and critical current density in 69-filament Bi-2223/Ag tapes was also investigated. found that in the final tape, the impurity particle size was much smaller and the percentage of impurity phases lower than in tapes processed using an intermediate normal cooling technique. The highest observed Jcis 63,000 A/cm2.

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