Studies on aspects of the critical current in high temperature superconductors

Yang, Yifeng (1991) Studies on aspects of the critical current in high temperature superconductors. University of Southampton, Doctoral Thesis.

Abstract

The present project was aimed at the two key problems which have to be solved en route to high current application of the high temperature superconductors: the weak superconducting coupling between grains and the thermally activated relaxation in the material, which bring about a low intergrain critical current and a time dependent decay of both the transport and the shielding currents inside the grains. The extremely short coherence length of the materials and the high application temperature are the origins of these problems. In this study, experiments were focused on the critical current and the flux pinning properties of both the granular samples and the melt-grownsamples with improved microstructure. The intergranular critical state was studied by measuring the transport critical current density J_c and the a.c. susceptibility X_ac of granular samples in a changing magnetic field at different temperatures. Complex but systematic hysteresis was observed in both J_c and X_ac, in regard to the thermal and magnetic histories of the samples. From the experimental results, we pointed out that this hysteresis in J_c and X_ac was not the intrinsic property of the intergranular current-field relation, but the hysteresis of the intergranular local fields, caused by the irreversible intragranular flux exclusion and flux trapping. Such an interpretation was consistent with the observed temperature dependence of the hysteresis and the grain size effects on it. The hysteretic phenomenon provided valuable information about the interplay of inter- and intra-granular properties and about the correlation betwen the intergranular coupling and the microstructures. In addition, the results indicated that the intrinsic intergranular current-field relation would be more accurately represented by the field dependence of the field-cooled virgin critical current, as in this case the intergranular local field was closer to the applied field. A systematic study was also carried out on the preparation and the properties of the melt-grown YBCO samples, which have been indicated to have improved microstructure and critical current by earlier studies. With the resistivity and a.c. susceptibility measurements, we showed that the intergranular coupling in these melt-grown samples was much stronger than in the sintered materials, characterized by the non-granular superconducting transition and the lack of hysteresis in the a.c. susceptibility. The d.c. magnetisation measurements clearly demonstrated that the flux pinning in the melt-grown materials was also enhanced.

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