Preparation of Tl-based superconductors for improved flux pinning

Several of the superconductors based on ceramic cuprates show transition temperatures into the superconducting state well above the boiling temperature of liquid nitrogen (77 K). Examples are the rare earth-barium cuprates, the bismuth-barium-calcium cuprates, the thallium-(strontium-barium)-calcium cuprates and, the mercury-(barium- strontium)-calcium cuprates. All of these materials show a relatively rapid decrease at 77 K of the critical current densities with external magnetic field, but the rate of this decrease is governed by the "dimensionality" of the system and the corresponding location of the irreversibility line in the (H,T) phase diagram. Thallium based superconductors, especially the TI-1212 and the TI-1223 phase, exhibit both high critical temperatures and high irreversibility lines. The proposed project aims at improvements of the critical current densities in external fields by improved fabrication methods and by introducing suitable pinning centers into the TI-1223, but also into the TI-1212 phase. Major efforts will be centered on the addition of 235 U to TI-based superconductors, but also to other superconducting compounds for the purpose of comparison. Additional studies will focus on the possibilities to modify the TI-1212 phase by various doping oxides to permit (partial) melt-texturing by lowering the melting point. Another goal of this project is to search for new compounds, which can replace TI in the insulating layer (besides the well known substitution of TI by Pb and Bi) without detrimental effects on the superconducting properties. All of these procedures require improved preparation techniques to produce properly doped single-phase materials. Thus, existing fabrication techniques will have to be modified, in order to obtain high quality material. The 235 U containing samples of either the TI-1223 phase or the (doped) TI-1212 phase will be subjected to thermal neutron irradiation to induce fission tracks, which may act as flux pinning centers in these materials.