FABRICATION AND PROPERTIES OF THALLIUM-BASED SUPERCONDUCTORS FROM TARTRATE GEL PRECURSORS

TlxBa2Ca3Cu4Oy bulk superconductors (0.5 < x < 0.7) containing predomi nantly the '1234' phase were prepared from tartaric acid gels, yieldin g homogeneous, amorphous percursor material. Careful calcination at 40 0-degrees-C, grinding and uniaxial pressing of the powder at 7500 bar preceded the sintering process at temperatures between 900 and 915-deg rees-C under flowing oxygen. The eff ect of each thermal treatment on the overall thallium content of the samples was monitored by atomic ab sorption spectrometry (AAS) analysis. Best results were obtained for s pecimens with a nominal overall composition of Tl0.63Ba2Ca3Cu4Oy showi ng an XRD spectrum generally assigned to the 1234 phase, with a T(c)(0 ) value of 122 K and a critical current density for bulk material of 3 50 A CM-2 . Electron microprobe analysis revealed a variable cation st oichiometry for the superconducting phase as well as (Ba, Ca)-Cu-oxide s as the secondary phase.