Bulk processing, microstructure and property of a superconductive ceramic-metal microcomposite system: Yttrium barium(2) copper(3) oxygen(7-delta)-silver.

Abstract

A systematic study has been undertaken with a view to achieve a profound understanding of the correlations among processing, chemistry, microstructure and property in a bulk high-temperature superconductive microcomposite system. The material selected for experimental work is a microcomposite of yttrium barium copper oxide and silver, powder metallurgically formed into strong and tough wires by use of a novel processing technique. Micromechanisms that dominate along the processing route are discussed. Emphasis has been placed on distinguishing unique microstructural features and on gaining an insight into the origin and evolution of them. A physical model on the evolution of connectivity in a duplex microstructure has been put forward. This model serves as the basis for understanding the strong experimental correlation between the width of the ceramic-metal interphase boundary and the width of the resistive transition. The role of elemental diffusion followed by lattice substitution in determining typical microstructural constituents, namely, the tweed and the microtwin, has been discussed in detail. The unique current-voltage behavior displayed by the composite is emphasized, and explained in terms of the individual current-voltage characteristics of these two superconductive constituents of the heterogeneous microstructure. It is revealed that the current-voltage characteristic of ceramic superconductors substantially differs from that of the conventional ones, and this difference is accentuated by substitutions in the superconductive lattice. The dissertation also throws some light of understanding on the area of critical current variability in bulk superconductors. Combining a theoretical and an experimental approach, it is revealed that bulk critical current variability has two distinct origins--one in the microstructural distribution and the other in the measurement criteria employed. When combined, the mathematical models developed for these two individual cases present a complete picture of the length dependence of critical current density in bulk ceramic superconductors. Inception of texture and associated enhancement of critical current density of the bulk material obtainable upon appropriate thermal treatment have also been discussed. A thermodynamically and kinetically plausible explanation has been presented on the enhancement of oxygen availability within the bulk by use of silver oxide as a precursor instead of silver.