High-Temperature Superconducting Ceramics: Processing Related Issues and the Thermodynamics of the Phase Transition
Abstract
Practical applications of high-T_ c ceramic superconductors requires considerable enhancement of their electrical and mechanical properties, as well as thermal stabilization of the superconducting phase. In order to achieve these desirable qualities, various processing techniques have been used to fabricate rm YBa_2Cu_3O_{7 -delta} and rm YBa_2Cu _3O_{7-delta}-Metal composites. In the initial part of this research, superconducting particulate composites were fabricated using powder metallurgy and liquid phase sintering techniques. Composites were fabricated with Ni, Cu and Sn. The processing conditions required to fabricate these composites were investigated in detail. It was found that the contacts between individual ceramic grains controlled the transport behavior of polycrystalline 123. Various methods, including the effect of additives (Ag/Ag_2O) were investigated to reduce this contact or weak-link problem. As the field progressed, it was established that misalignment between adjacent grains in polycrystalline 123, drastically reduces the critical current density J _ c. In view of this, melt-processing techniques were used to fabricate to highly aligned 123 and 123/Ag composites. Melt-textured 123/Ag composites consist of highly aligned, high aspect ratio 123 grains, with silver particles embedded between and within the 123 grains. Processing conditions required to fabricate these textured materials were investigated in detail. The growth behavior of aligned 123 from the melt was studied. The mechanical properties of these materials, including the Young's modulus, hardness and fracture toughness were examined. A detailed microstructural characterization, including the identification of various defects in these materials was conducted. Extensive magnetization characterization of these aligned materials was also performed. In addition to processing studies, a classical thermodynamical analysis across the superconducting transition to study the variation of physical quantities across the transition was conducted. Some of the topics investigated include the determination of the crystallographic thermal expansion coefficient matrix of 123, the jumps in the thermal expansion coefficients and the elastic compliances across the second-order normal to superconducting phase transition, the pressure dependence of the transition temperature and the dependence of superconductivity on oxygen stoichiometry in 123. This dissertation is a compilation of research in the area of high-T_ c superconductors, involving the above mentioned topics.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- February 1991
- Bibcode:
- 1991PhDT........80G
- Keywords:
-
- YTTRIUM BARIUM COPPER OXIDE;
- BARIUM COPPER OXIDE;
- Engineering: Materials Science; Engineering: Chemical; Physics: Condensed Matter