Solid State Thermal Gradient Processing of YTTRIUM(1) BARIUM(2) COPPER(3) OXYGEN(7-X)/SILVER Superconducting Composite Ribbons

The effect of solid state thermal gradient processing on microstructure was examined experimentally and theoretically. The starting material Y_1Ba_2Cu _3Ag_{15} was used to produce Y_1Ba_2Cu_3O_ {7-x}/Ag superconducting composite ribbons. During the low temperature oxidation stage, internal oxidation resulted in the formation of elemental oxides Y_2O_3, BaO, and CuO in Ag. The low temperature oxidation treatment had a very strong influence on the microstructure of the final ribbon, with the tendency for forming Ag bands being stronger in samples subjected to longer oxidation stage treatments. Ag nodules formed at the surfaces due to stress relief and an Ag band formed at the center of samples due to outward solute diffusion in samples that were completely oxidized. Complete oxidation was achieved by heating the samples in flowing oxygen 5^circC/min to 140 ^circC, 0.5^ circC/min to 420^circ C, and holding for 13 hours. The high temperature transformation stage enabled the elemental oxides to transform to the superconducting oxide Y_1Ba_2Cu_3O _{7-x}. During the high temperature transformation treatment, the Ag nodules on the surface spread out to become a surface band of Ag. Coarsening resulted in an increase in the average oxide particle size and a thickening of the Ag bands as the hold time increased, with temperatures in the range of 890^circ C to 900^circC. The superconducting properties of the composite Y_1Ba_2Cu_3O_ {7-x}/Ag ribbons were tested after oxygenation. The samples were found to contain superconducting Y _1Ba_2Cu_3O_{7 -x} with a critical transition temperature in the range of 86 K to 90 K. The samples were found to be unable to support a supercurrent at either 77 K or 4.2 K. The use of solid state thermal gradient processing to produce textured Y_1Ba_2Cu _3O_{7-x}/Ag composite superconducting ribbons during the high temperature transformation stage was investigated experimentally and theoretically. Experimentally, no significant amount of either preferential alignment or elongation of superconductor grains along the length of the ribbon was seen. Using a theoretical approach, nucleation should occur at orientations that are randomly distributed. Since there is no driving force for rotation, a superconductor grain will maintain the orientation of its nucleus and no preferential alignment will result. Anisotropic growth in an isothermal situation will favor the formation of plate-like grains that extend in the basal plane. In the presence of a temperature gradient, surface energy considerations show that growth will usually occur in the c-axis direction; the c-axis may or may not be perpendicular to the traveling direction. Plate-like grains are expected to form and have a wide variety of orientations and an average size that increases according to coarsening predictions. (Abstract shortened by UMI.) (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).