Modelling of microstructural features in Y-Ba-Cu-O superconductors

Abstract

We have investigated characteristics of four important microstructural features, namely, grain boundaries (small-and large-angle), twins, stacking faults and cracks in high-T_c YBa₂Cu₃O₇-_x superconducting oxides using transmission electron microscopy techniques. A detailed analysis of these microstructural features is carried out to understand their effect on the superconducting properties, mechanical properties and their environmental sensitivity. An atomic model of the small-angle and large-angle grain boundaries in these superconducting oxides is provided using a geometric description. Some important differences between the grain-boundary structure of the superconducting oxides and that of metals have been identified. The environmental sensitivity of the superconducting oxide is explained in terms of the atomic structure of dislocations associated with the grain-boundary configurations. The strain energy associated with the twinned regions and oriented domains is determined using discrete dislocation analysis. It is found that oriented domains should be more difficult to form than twins in these superconducting oxides. The nucleation of cracks along stacking faults is analysed to understand the brittle behaviour of these materials. The energy associated with the crack configurations is determined from the discrete dislocation analysis. The formation of moisture-induced stacking faults, followed by further nucleation of cracks along the faulted regions, is found to be an important mechanism for degradation of the mechanical and superconducting properties of this material.