Date of Award
Doctor of Philosophy
Materials Science and Engineering
Since the discovery of the high-Tc ceramic superconductors there has been a vast amount of work performed with regard to synthetic methods, and toward improving materials properties. Only in the last two to three years has there been significant amounts of work directed toward use of forming technologies to produce conductor shapes;A chemical synthesis route is discussed which results in a low-temperature precursor to Y-Ba-Cu-O ceramics. The synthesis is based on use of molten Ba(OH)[subscript]2·8H[subscript]2O as a molten salt flux. Two different chemical systems have been examined. The first, based on nitrate salts, has been demonstrated to be a viable precursor material for tape casting and extrusion. The second, made from acetate salts, has been used for powder synthesis and extrusion. The rheological properties of the precursor pastes obtained from either system may be controlled by removal of the liquid fraction during processing;Examination of the rheology of the pastes shows that flow characteristics of the pastes may be fit to either Bingham Plastic or Hershel-Bulkley models. Yield stress is controlled in both pastes by the volume fraction solids. Viscosity also follows solids loading in the paste. Shear thinning tendencies are controlled by the colloidal nature of the precursor. Rheological measurements indicate the existence of colloidal microstructure in the paste. Comparison of concentric cylinder rheometry and piston extrusion rheometry shows order of magnitude differences in yield stress. These differences result from the inherent nature of the test methods, where the degree to which paste dilation is constrained has a major influence on the rheology.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Timothy Myron Green
Green, Timothy Myron, "Synthesis, rheology and forming of Y-Ba-Cu-O ceramics " (1993). Retrospective Theses and Dissertations. 10235.