Critical currents in neutron irradiated Bi- and Tl-high temperature superconductors

High critical current densities are the key requirement for practical applications of high temperature superconductors. The oxides, being ceramic materials, are more difficult to fabricate in long lengths than conventional superconductors. Their microstructural features, especially their layered structure, allow to employ special processing techniques, which have greatly improved their critical current up to the present level. This work presents tapes and wires of high temperature superconductors, based on Bi and Tl. The processing and properties of the phases 2212 and 2223 of the Bi superconductors and the 1223 phase of the Tl superconductors will be described in detail. A main part of the structural investigation was to find out the role of silver during the phase formation. Short samples of silver-sheathed Bi-2212 wires were quenched from different stages of the sintering process. Microstructural studies of the phase formation have been performed. It is observed, that silver agglomerates between grains all over the ceramic core. These studies were used to optimize the preparation of Bi-2212 wires of medium length. Experimental results on the magnetic field behavior, including the angular dependence, of the critical current densities Jc in silver-sheathed tapes are presented. The experiments consist of transport measurement in a wide temperature range and in external magnetic fields up to 6 T. In Bi-2223 enhancements of the transport critical current densities are observed at higher magnetic fields after irradiation with fast neutrons, in particular for H//c. This is attributed to an improvement of the flux pinning capability by the neutron induced defects, but the weak link structure is somewhat damaged as evidenced by a small degradation of Jc at low fields. After irradiation no enhancement of Jc was found in Tl-1223, because the grains in these tapes are not well aligned and not well connected. In this work a new model for the transport critical current was developed. The so called 3-current model separates the critical current into intergrain and intergrain current. The intergrain current is the sum of a weak link and a strong link current. This model can explain the limitation of the critical currents and it is now possible to clarify the increase and decrease of Jc after irradiation with fast neutrons. This model also clears up phenomena, which were observed on rotation data, providing strong evidence for the correctness of this current separation. (author)