Doped Tl-1223 Superconductors

Abstract

Doped Tl-1223 superconductors of the composition Tl_aBi_bPb_cBa_nSr_2−nCa₂Cu₃O_y (a = 0.50 − 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 − 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl₂O₃, PbO and Bi₂O₃ were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc₍₀₎-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm⁻² at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm⁻² by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.