Year

2007

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Knowing the magnetic flux behavior in a superconducting material is essential for a better understanding of the superconductivity phenomenon. The magneto-optical imaging (MOI) technique is one of the few techniques enabling the real time study of the local magnetic flux behavior in superconductors. This work had several aims: • Firstly, an efficient MOI system enabling the study of superconducting samples at liquid helium temperatures had to be set up to replace the former limited MOI system in ISEM (Institute for Superconducting and Electronic Materials). Indeed, the former MOI equipment was equipped with a cryostat cooled down via a cryocooler in which functioning principle was based on Joule-Thomson cycles of helium gas. This system had two main limitations: it only made it possible to reach temperatures down to 20 K, and it induced strong mechanical vibrations in the sample holder. These limitations had very disadvantageous consequences for the MOI study of superconducting samples: Some phenomenon such as dendritic flux jumps occurring in MgB2 below 10 K could not be studied. In addition, due to the mechanical vibrations, the sample and the magneto-optically active layer had to be strongly pressed on the sample holder in order to maintain them in place. This mounting pressure induced mechanical stress in the magneto-optically active layer, reducing the accuracy and quality of the magneto-optical images obtained with this system. To overcome these limitations, another MOI cryostat was installed. The latest cryostat is cooled down with liquid helium, eliminating the mechanical vibration and enabling the study of superconducting sample at temperatures down to 1.5 K. Due due the numerous differences between the two cryostat, numerous parts of the MOI equipment had to be changed or redesigned. The vacuum system iv was rearranged to enable us to recover the helium gas after its passage through the cryostat. The positioning system was adjusted to the new geometry of the latest cryostat. A new ring magnet was conceived and built to increase the intensity and homogeneity of the supplied magnetic field. The temperature control system was replaced. Finally, extra wires were positioned in the new cryostat to supply current to the sample and allow the study of superconductors under a transport current. All these modifications combined with the advantages of the liquid helium cooled cryostat mentioned earlier have enabled the acquisition of higher quality magneto-optical images and enhanced the reproducibility of the results. • Secondly, two computer programs were designed to enable the calibration of the local magnetic flux density and the quantification of the local current density from the magnetooptical images. The calibration of the grey levels of magneto-optical images into local magnetic flux densities is now possible by using an original user-friendly software. The quantification of flux density can now be done in two different manners: a pixel-by-pixel method or an averaging-pixel-cluster method. In the former, a unique calibration curve is calculated for each pixel of the pictures, while the later uses the averaged light intensity of a cluster of pixels for calibration. Although more rigorous than the averaging-pixel-cluster method, the pixel-by-pixel method is very time-consuming and thus can only be used on small areas of the pictures. In addition, a second computer program has been implemented for the quantification of the current density in a superconducting sample. This program can be used on any thin film samples and provides both current density maps and current flow contour plots (to visualize the flow direction of the current) from the calibrated MOI pictures. • Thirdly, both MOI setups have been used to study samples of the three most promising superconducting materials in terms of potential applications: MgB2, YBCO (YBa2Cu3O7) and Bi-2223 (Bi1.72Pb0.34Sr1.85Ca1.99Cu3Ox). MgB2 thin films prepared by pulsed laser deposition with different processing parameters have been compared to help us understand the influence of the film morphology on the magnetic flux penetration behavior. The microstructure of the film was found to strongly influence the pinning properties and the magnetic flux penetration. This influence is espev cially visible when partial flux jumps invade the films in the form of dendrites at low fields and temperatures. Under these conditions, the formation of abrupt dendritic avalanches appears to be structurally driven. The influence of the magnetic iron sheath on the flux and current distribution in the superconducting core of MgB2/Fe sheathed wires have also been investigated. Global magnetic measurements indicated that the iron sheath shields the superconducting core from the external magnetic field, improving its current carrying capabilities at low fields. A local study of the current distribution in the MgB2 core done with the MOI technique has revealed a redistribution of the supercurrents, although no overcritical currents (currents with a density greater than the critical current density) were observed. YBCO thin films were studied in order to gain more insight into the effects of the film morphology on the magnetic behavior. Samples produced with various techniques and with different microstructures have been visualized with the MOI technique. A key conclusion from this work is that a smooth film surface generally leads to higher critical current density. Furthermore, a comparison of critical current density in mono- and multilayer YBCO films produced by pulsed laser deposition shows the beneficial effect of the multilayer deposition method, especially for the use of YBCO thin films at low temperatures. Finally, by studying a set of Bi-2223 tapes with MOI and various material characterization techniques, optimal sintering conditions for their processing have been identified. The temperature and cooling conditions of the last heat treatment were proved to be two critical factors determining the current carrying capabilities of Bi-2223 multifilamentary tapes though a complex interplay of Bi-2223 phase formation and crack healing processes.

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