The inhomogeneities of (Pb,Bi)2223 superconducting tapes and their detection

Van Leeuwen, Suzanne (1999) The inhomogeneities of (Pb,Bi)2223 superconducting tapes and their detection. University of Southampton, Doctoral Thesis.

Abstract

An investigation was carried out into the variation of core density within cross sections and along lengths of (Pb,Bi)2223 tapes during fabrication. It was observed that rolling and thermal treatment brought about a non-uniform core density in both these aspects of the tape.

This was followed by an investigation into the effect of core density on the formation of the (Pb,Bi)2223 phase. It was shown that a high core density formed the (Pb,Bi)2223 phase at a slower rate than a lower core density under the thermal treatment. A high core density and a slow heating rate produced smaller 2212 grains at the end of the incubation period. Smaller 2212 grains were thought to be linked to the faster formation of the (Pb,Bi)2223 phase. The highest Jc was from a high core density tape which had the smaller 2212 grains at the end of incubation period. Smaller 2212 grains were thought to aid a more homogeneous conversion to the (Pb,Bi)2223 phase.

Alloy-sheathed (Pb,Bi)2223 superconducting tapes were produced in order to fabricate a more homogeneous core density. It was found that the alloy sheath (with an addition of 15% wt Ag in the precursor powder) changed the characteristics of the core in several ways: the formation of the (Pb,Bi)2223 phase was homogeneous across the thickness of the core, a smaller 2212 grain size was formed at the end of the incubation period and a higher core density during thermal treatment. All these factors together produced a higher Jc and a more homogeneous tape with reproducible results.

A rig was designed and built to detect inhomogeneities in a non-destructive manner. The rig incorporated the use of an array of hall probes to measure the magnetic field above a high temperature superconducting tape. The method was successful in detecting macroscopic inhomogeneities such as cracks caused by bending the tape and holes drilled through the core.

Finally, a rig was designed and built to measure the transport current as a function of V along consecutive 1cms of length of high temperature superconducting tapes at liquid neon temperatures. The inferior sections of the tape appeared to be more significant at 27K compared to 77K.

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