Fabrication and properties of thallium-based superconductors from tartrate gel precursors

TlxBa2Ca3Cu4Oy bulk superconductors (0.5<x<0.7) containing predominantly the '1234' phase were prepared from tartaric acid gels, yielding homogeneous, amorphous precursor material. Careful calcination at 400 degrees C, grinding and uniaxial pressing of the powder at 7500 bar preceded the sintering process at temperatures between 900 and 915 degrees C under flowing oxygen. The effect of each thermal treatment on the overall thallium content of the samples was monitored by atomic absorption spectrometry (AAS) analysis. Best results were obtained for specimens with a nominal overall composition of Tl0.63Ba2Ca3Cu4Oy, showing an XRD spectrum generally assigned to the 1234 phase, with a Tc(0) value of 122 K and a critical current density for bulk material of 350 A cm-2. Electron microprobe analysis revealed a variable cation stoichiometry for the superconducting phase as well as (Ba, Ca)-Cu-oxides as the secondary phase. (author)