Doctor of Philosophy
Department of Materials Engineering
Zeimetz, Bernhard, High temperature superconducting tapes and current leads, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1998. http://ro.uow.edu.au/theses/1503
The scope of this thesis was (i) to contribute to the optimization of Bi2223 precursor powder and tape processing, and (ii) to design and test HTS current leads using Bi2223 tapes. Various steps of powder and tape processing, and their influence on final properties of tapes, were investigated, including powder stoichiometry, powder calcinations and grinding, mechanical deformation and heat treatment of tapes. The Bi2223 grain size varied considerably between tapes made from different powders, and critical currents were found to increase with increasing grain size. An inverse relationship between the Bi2223 formation rate and the grain size was found. Silver, and especially silver addition to core, were found to have a strong influence on Bi2223 texture and critical currents. Silver additions were also used as a 'tracer' to investigate powder flow during mechanical deformation. The formation of 'sausaging' during mechanical deformation was investigated in a parameter study, and found to be closely related to formation of shear bands and necking. It was demonstrated that Bi2201, forming at Bi2223 grain boundaries, caused depression of critical currents, but could be removed by appropriate processing. Various Bi2223 formation mechanisms were identified. Two sets of current leads were developed and tested, both allowing vapour cooling of the HTS tapes, tapering of the leads, and maximum flexibility of current capacity and heat leak by allowing exchange of individual tapes. The first pair of current leads was developed for a Superconducting Magnetic Energy Storage test facility at Monash University. The leads were successfully tested up 200 A in a stray magnetic field of the SMES coil of 400 mT. In the second lead design, aimed at a current capacity of 2000 A, the HTS tapes were held by steel clamps in a circular arrangement. The leads were successfully tested using applied currents up to 900 A DC, but their current capacity was estimated to be several kA. An analytical formula for optimizing the length of HTS tapes in a tapered current lead was derived.