Microstructure and properties of textured bulk phase Tl(Bi)-1223 superconductors

Bi-doped Tl-1223 superconductors of the composition Tln-xBixBa0.2Sr1.8Ca2Cu3Oy (n = 0.96 - 1.12 and x = 0.25, 0.3, 0.35, 0.4) and Tln-0.25Bi0.25Ba0.4Sr1.6Ca2Cu3Oy (n = 0.97 - 1.04) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the metal nitrates, the respective amounts of Tl2O3 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 114 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.8 kA cm-2 at 77 K were measured for the as-sintered samples. Texturing was obtained by grinding, repressing and resintering of the superconducting material followed by annealing at 750 0C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 4.8 kA cm-2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.