Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Jim P. Zheng, Florida State University, College of Engineering, Dept. of Electrical and Computer Engineering. Title and description from dissertation home page (viewed Sept. 21, 2004. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 215-216). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Hong Kong, 2005.

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2006. / Tetali, Prasad, Committee Member ; First, Phillip, Committee Member ; Sa de Melo, Carlos, Committee Chair ; Zangwill, Andrew, Committee Member ; Kennedy, Brian, Committee Member. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xvii, 109 p.; also includes graphics (some col.). Includes bibliographical references (p. 105-109). Available online via OhioLINK's ETD Center

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 43-45). Also available in electronic version.

The work described in this thesis consists of an investigation into the behaviour of the critical current density (Jc) of several different high temperature superconductors (HTSCs) as a function of temperature and applied magnetic field. The focus of this research has been to investigate the discrepancies which generally exist between magnetic and transport measurements on HTSCs. In order to do this a number of systems were selected with different weak link strengths, overall alignment and pinning characteristics. Systematic studies were carried out on these systems using both transport and magnetic techniques. The results obtained were compared to obtain a coherent picture of the relation between critical currents in HTSCs, their granularity and structure, and how this accounts for the differences in magnetic and transport measurements. The Jc of granular bulk sintered YBCO was measured as a function of magnetic field and orientation, and attempts made to fit the results obtained to theory. The results obtained indicate that hysteresis of Jc with field cannot be explained by flux trapping alone. Jc measurements were carried out on the 'hub-and-spoke' (H-S) grains of melt-processed YBCO thick films. These indicated that within each H-S grain the current is constrained to radial paths through the centre of the grain. This has major implications for the analysis of any measurements carried out on these samples, as well as for applications, as only a small fraction of the sample carries the applied current. Silver-clad tapes of Tl:1223 and Tl:2223 were measured in different temperatures, applied magnetic fields and orientations. The variation of Jc along the tape length was also measured to provide an estimate of their homogeneity. It was found that the processing route used in the production of these tapes had not aligned the superconductor within them, and also that the tape properties were not homogenous along their length. This probably arises from the crystal structure of the thallium materials. The Jcs of melt-processed thick films of Bi:2212 on silver substrates were measured as a function of temperature and applied magnetic field, and compared with results obtained from magnetic measurements obtained from a VSM. Attempts were made to fit these results to theory. The results obtained show clear scaling behaviour of Jc with T at constant B and imply that the variation in the form of the Jc versus T curves is due simply to the suppression of Tc with increasing applied field.

The excitement which followed the discovery of high-temperature superconductors in 1986 was short-lived, as it became clear that their current carrying capacity (the critical current density Jc) was limited by grain boundaries (GBs). In order to reduce their detrimental effects coated conductors have been developed, in which a superconducting thin film is deposited on a polycrystalline, textured substrate. Within certain temperature and magnetic field ranges, however, GBs still limit the overall Jc. This fact motivated the present thesis, for which the electrical properties of different types of coated conductors, and in particular their GBs, were investigated. Several GBs and a single grain were isolated in a tape produced by metal-organic deposition (MOD), using a novel approach based on electron backscatter diffraction and a focused ion beam microscope. Measurements of their critical current densities for fields swept in the film plane showed the expected decrease with increasing misorientation angle at low fields. At higher fields an angle dependent crossover was found, from a GB to grain limited Jc. In order to confirm this result and put it into broader perspective, the dependence of Jc on the width of polycrystalline tracks was studied, and then explained in terms of the behaviour of the single GBs. Investigations using low-temperature scanning laser microscopy rounded out the picture, which also showed GB dissipation at certain angles and grain limitation at others. In measurements on samples produced by metal-organic chemical vapour deposition (MOCVD) characteristic differences compared to the MOD film were found. While both conductors exhibited high values of Jc, the variation with in-plane angle was significantly stronger for the MOCVD conductor, which can be explained by its sharper texture. In a track patterned perpendicular to the tape direction the phenomenon of vicinal channelling was observed, which previously was known only from films on single crystal substrates. Finally, an isolated boundary showed very high values of Jc, consistent with its low misorientation. In order to better understand how the substrate influences the superconducting properties, measurements were carried out on otherwise identical samples grown on different substrates. A tape with grains elongated along its rolling direction showed particularly good properties at all examined field orientations. This extends the previously reported result that high aspect ratios are beneficial at fields applied perpendicular to the tape plane.

In this dissertation, the problem of calculating and measuring AC losses in superconducting coils is addressed, with a particular focus on the transport AC loss of coils for electric machines. In order to model the superconducting coil's electromagnetic properties and calculate the AC loss, an existing two dimensional (2D) finite element model that implements a set of equations known as the H formulation, which directly solves the magnetic field components in 2D, is extended to model a superconducting coil, where the cross-section of the coil is modelled as a 2D stack of superconducting coated conductors. The model is also modified to allow the nclusion of a magnetic substrate, which is present in some commercially available HTS wire. The analysis raises a number of interesting points regarding the use of superconductors with magnetic substrates. In particular, the presence of a magnetic substrate affects the penetration of the magnetic flux front within the coil and increases the magnetic flux density within the penetrated region, both of which can increase the AC loss significantly. In order to investigate these findings further, a comprehensive analysis on stacks of tapes with weak and strong magnetic substrates is carried out, using a symmetric model that requires only one quarter of the cross-section to be modelled. In order to validate the modelling results, an extensive experimental setup is designed and built to measure the transport AC loss of a superconducting coil using an electrical method based on inductive compensation by means of a variable mutual inductance. Measurements are carried out on the superconducting racetrack coil and it is found that the experimental results agree with the modelling results for low current, but some phase drift occurs for higher current, which affects the accuracy of the measurement. In order to overcome this problem, a number of improvements are made to the initial setup to improve the lock-in amplifier's phase setting and other aspects of the measurement technique. New measurements are carried out on a single, circular pancake coil and the discrepancies between the experimental and modelling results are described in terms of the assumptions made in the model and aspects of the coil that cannot be modelled. Using the original measured properties of the superconducting tape, there is an order of magnitude difference between the experiment and model. The properties of the superconductor can degrade during the winding and cooling processes, and a critical current measurement of the coil showed that the tape critical current reduced from nearly 300 A, down to around 100 A. Applying this finding to the model, the experimental and modelling results show good agreement, and the difference in the slope of the AC loss curve can be described in terms of the B-dependent critical current dependency Jc(B) used in the model. Finally, methods used to mitigate AC loss in superconducting wires and coils are summarised, and the use of weak and strong magnetic materials as a flux diverter is investigated as a technique to reduce AC loss in superconducting coils. This technique can achieve a significant reduction in AC loss and does not require modification to the conductor itself, which can be detrimental to the superconductor's properties.

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