Year

2002

Degree Name

Honours Master of Engineering

Department

Institute of Superconducting and Electronic Materials - Faculty of Engineering

Abstract

Bi-2223/Ag tapes are currently the only HTS that can be classified as “conductor”; i.e. long lengths of the tapes can be purchased and used in practical applications. They have started their use primarily in applications with alternating currents (AC), such as transformers, high current/low voltage cables, motors and generators. The energy losses in practical superconductors are negligibly small under suitable working conditions. However, Bi-2223/Ag tapes are composite conductors, consisting of superconducting filaments in a silver matrix. Composite conductors are associated with additional losses when the tape is used in AC applications, commonly referred to AC coupling loss. The susceptibility of different tapes to the coupling losses is usually described by effective transverse resistivity, P⊥. P ⊥differs from resistivity of the silver matrix, and it is a convenient way to describe the susceptibility of a tape to the AC loss. Coupling currents between the filaments flow in a complicated pattern because of the complex geometry of the tapes. The coupling current loss depends not only on the resistivity of the silver matrix, but also on the spatial distribution of the currents. Therefore, the overall shape of the tape, as well as the architecture of the superconducting filaments in the tape are important factors defining the coupling losses. The geometrical factor needs to be known for determination of P ⊥and is for tapes usually taken simply as the aspect ratio of the tapes, a/b, where a is the thickness and b is the width of the tape. The Abstract proposed study is aimed at determining the effective transverse resistivity of 8 filamentary untwisted Bi-2223/Ag tapes, fabricated by the PIT method and heat treated at 837oC. This takes into account the filament configuration, tape thickness and width as well. Three thicknesses of 0.18mm, 0.24mm and 0.33mm tape were studied. Ic, X-ray diffraction, and optical microscopy measurements were performed in order to characterise the tape. The Bi2223/Bi2212 phase ratio in the composite is XBi2223=~87%, and there is no bridging between the filament and no discontinuities. The Ic values for the thicknesses are: Ic(0.18mm)=11.2A, Ic(0.24mm)=17.85A, Ic(0.33mm)=13.8A. The Ic vs. thickness values demonstrate an optimum thickness for Ic. Measurements of the frequency dependence of the AC loss makes it possible to obtain the value of P ⊥&#;for each thickness-length of tape. This measurement was performed using an induction method, with the excitation field applied parallel to the face of the tape. The P ⊥&#;is large, comparing to the tape matrix resistivity, P Ag.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.