Year

2012

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Practical superconducting wires and tapes have significant potential for use in electrical engineering applications. The work presented in this thesis is focused on developing new measurement and theoretical techniques to more accurately model and measure ac losses in superconducting tapes in order to contribute to the skills, tools and understandings needed to assist in their utilization. Investigations of both BSCCO and MgB2 superconducting tape and wire are described.

A review of the literature with special emphasis on the mechanism of AC losses and theoretical approaches is presented. Some of the widely used analytical expressions for losses are presented and existing experimental techniques are reviewed and discussed in detail. Major instrumentation methods generally used in the measurement of ac losses are described.

An experimental chamber for low temperature measurements has been assembled comprising cryogenic parts, electronic and electrical instrumentation and software written to provide a comprehensive data acquisition system. Detailed measurements are made on the effects of magnetic fields, temperature, current and mechanical strain on superconducting tape performance.

A new method for AC loss measurement of BSCCO and MgB2 superconducting tape at low temperatures (< 50 K) using a calorimeter has been investigated and demonstrated. Calorimeter techniques have been widely accepted and have the advantage of being able to measure the total losses in a superconducting wire. The calorimeter presented here allows the measurement to be performed at low temperatures from 10 K to 50K as compared with previously reported calorimeters that only measure at liquid nitrogen temperature at 77 K. This temperature range is of interest since the rapid development of cryogen-free chambers now makes this operating range important for practical applications of superconducting materials. In addition, the calorimeter was built to include a superconducting solenoid coil so that measurements on the effect of both external DC and AC magnetic fields on the superconducting sample can be undertaken. This method has been used to measure losses of BSCCO tape exposed perpendicularly to both DC and AC magnetic fields at various temperatures. Comparison between measurement results and theoretical computations verified the accuracy and reliability of the method. Moreover, AC loss measurement has also been performed on MgB2 tape but with different external conditions in order to measure transport AC current with applied DC transverse field. The tape is mounted distinctively with a sharp bending edge and several calibration and stability tests carried out to assure the validity of measurement results. Losses were found to be higher than the theoretical predictions because the metallic parts of the tape contribute quite significantly to the total losses.

Transport critical current measurement for MgB2 wire and tape has been investigated with two different techniques, the DC four-probe arrangement and pulsed current with varying rate. The former method suffers from inevitable heating effect when measuring wire with high critical current value. This effect is more pronounced for the commonly used measurement of short samples. The pulsed method on the other hand has no significant heating effect but the critical current can depend on the rate of the current change (dI/dt) in the pulse. This method is particularly useful at low field regions which are often inaccessible using conventional DC methods. A finite element method (FEM) analysis is also performed to provide further evidence of the limitation of the DC method in obtaining high transport critical current. It is shown that the best way to accurately measure Ic at low fields is to extrapolate the values of Ic measured at different dI/dt to dI/dt = 0 This overcomes problems caused by the effects of heating (introduced in DC measurements) or effects of vortex dynamics (introduced in pulsed measurements).

A new direct analytical method of solving the nonlinear integral equation that gives the current as a function of the applied external field or of the transport current is also presented. This method solves the singularity problem in thin superconductor and provides analytical expressions for both the sheet current and field distribution under self and external field conditions. In addition, a new numerical scheme is introduced using a two dimensional nonlinear magnetic diffusion equation model to find AC losses with finite thickness.

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