Year

2001

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

This thesis mainly concentrates on (1) developing a novel process for fabricating Bi-2223 bulk current leads, which has big potential to improve the critical current density Jc and contact resistivity of two end terminals, (2) in particular, developing a technique of combining cold isostatic pressing (CIP) and sinter forging which was first used to prepare Bi-2223 bulks and evaluating it with respect to electro-magnetic properties microstructure features, (3) preparing a prototype of Bi-2223 bar currents. Properties related to technical applications, such as Jc performance and contact resistivity were studied. Grain texture, which has consistently been considered one of the important factors affecting superconducting properties, was quantitatively studied.

A novel technique of combining CIP and sinter forging was applied to the fabrication of Bi-2223 bulks, and pre-sintering was carried out after CIP (route 1) and before CIP (route 2). The effects of the sinter forging deformation rate and pre-sintering were investigated. When the pre-sintering was done after CIP (route 1), the Jc of bulks at K was much higher than that of bulks made by route 2 (pre-sintering was carried out before CIP). Measurements of the field dependence of the Jc revealed that optimised sinter forging could improve Jc behaviour in external fields, particularly in the low field region below 50 mT, for the samples where CIP was carried out before pre-sintering. A higher sinter-forging deformation rate can improve the flux pinning force when the external field is parallel to the sample surface, but degrade the pinning force density when the external force is perpendicular to the sample surface (H//C, 77 K).

The influence of the sinter-forging rate on Jc behaviour (Jc) in an external field and contact resistance Rc for prototype Bi-2223 current leads with two silver contact ends was investigated. The results revealed that Jc was strongly affected by the sinter forging deformation rate and reached a maximum of 725 A/cm2 at a deformation rate of 80% without post annealing. From the measurements of external magnetic field dependence on Jc, it was determined that sinter forging could improve Jc behavior in external fields, particularly in the regime below 50 mT. Results showed that the contact resistance approximately ~ IO-6 Ω. cm2 and for the samples with higher deformation rates it became less dependent on the transport current over a range from 0.5 A to 50 A.

The electro-magnetic properties of Bi-2223 current leads after post annealing, with a Jc of 1000 A/cm2 at 77K and in a self-generated magnetic field, were investigated. The texture and weak link behavior were studied by pole figures and AC susceptibility, respectively. The results showed that the matrix density, grain connectivity and texture of Bi-2223 current leads were improved significantly by sinter forging.

Critical currents Ic were measured at 77 K by varying an external DC magnetic field (B), which was applied parallel and perpendicular to the bulk sample surface, i.e., (B//bar surface) and Ic (Bロbar surface), respectively. The Ic and Jc were 119 A and 300 A/cm2, respectively, for the current lead bar of 0.4 cm thick, 1.0 cm wide, and 5.4cm long. The effective grain misalignment angle, モeff, was around 10°, calculated from its Ic (B//bar surface) and Ic (Bロbar surface) curves by using a 2D model. For comparison, the average grain misalignment angle, モav, found in the microstructure using SEM, was measured by pole figure. The results obtained were in agreement with the effective grain misalignment angle モeff.

However, this is just the beginning of research and development of this proposed new technique for fabricating Bi-2223 bar current leads. Further study and development are definitely necessary; this is because there is much room for improvement, particularly for the processing parameters for each step of this technique.

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