Microstructure and properties of high temperature superconducting materials

Abstract

The properties of high T c superconducting oxides are correlated with the microstructural features such as point defects, dislocations, and grain boundaries. The magnetic properties of the high T c superconductors, especially the field vs. temperature, are shown to exhibit different behavior from that of conventional superconductors. The irreversibility line and its significance in flux lattice melting are discussed to highlight the important limitations of the current-carrying capacity of the high T c superconductors in an applied magnetic field. The mechanisms of flux pinning by oxygen vacancies, dislocations and grain boundaries are described quantitatively. The pinning energies associated with these defects are shown to be small enough that the critical current density cannot be increased considerably at higher temperatures where flux melting takes place. Grain boundaries are shown to be important weak links in the high T c superconductors. The small coherence length relative to the dimensions of the width of the grain boundary is shown to be responsible for the large reduction in the critical current density associated with the boundaries. The atomic structure of different types of boundary is described in the light of the earlier theories of grain boundaries, and the structural features are shown to be similar. The four important factors contributing to the weak link behavior of the grain boundaries are analyzed and the critical current density associated with the boundaries is derived. The experimental results of the dependence of the critical current density on the misorientation angle, temperature and the magnetic field have been explained based on the grain boundary modelling. More importantly, the spatial dependence of the critical current density around the grain boundary is derived and shown to explain the experimentally observed features.