DEVELOPMENT OF HIGH-TC SUPERCONDUCTOR WIRES FOR MAGNET APPLICATIONS

In order to be able to use high-T(c) superconductor wires for magnet and energy technology they have to fulfill the requirement of carrying sufficient current at a critical current density of the order of 10(5) A/cm2 at the operating temperature and magnetic field. At 77 K these requirements have not yet been fulfilled mainly due to weak link and flux creep effects. Progress made so far and remaining problems are discussed on the basis of results obtained on single core untextured YBa2Cu307-x/Ag and Bi2Sr2Ca1Cu2O8+x/Ag round wires and highly textured silver sheathed thin tapes of composition Bi1.80Pb0.33Sr1.93Ca1.93Cu3O9.89+x. In YBa2Cu3O7-x the critical current density at 77 K is limited due to weak links. In Bi2Sr2Ca1Cu2O8+x on the other hand flux creep seems to be the main limiting mechanism at higher temperatures. At 4.2 K however flux creep becomes less significant and technically interesting critical current densities of more than 10(4) A/cm2 could be achieved in Bi2Sr2Ca1Cu2O8+x/Ag wires up to fields above 20 T. Some steps towards conductor development for very high field applications are discussed. Special emphasis is placed on microstructural evolution during the annealing treatment of Bi2Sr2Ca1Cu2O8+x taking into account results of detailed metallographic investigations and in situ x-ray diffraction. The textured Bi1.80Pb0.33Sr1.93Ca1.93Cu3O9.89+x/Ag thin tapes yield appreciably higher critical current densities j(c) of 2.6x10(4) A/cm2 at 77 K and 0 T, j(c) at 4.2 K being slightly higher than in the untextured Bi2Sr2Ca1Cu2O8+x/Ag round wires. The critical current density in such tapes of course is anisotropic with respect to the orientation of the magnetic field. Based on results of temperature and magnetic field dependent critical current measurements flux creep properties of both Bi-based materials are compared.