Under- and over-doped bulk (TlaPb0.5)(Sr0.95Ba0.05)2(Ca0.8Gd0.2)Cu2Oz 1212 superconductors prepared via solution gel synthesis

The influence of over- and under-doping of (Tl,Pb)-1212 superconductors by thallium was investigated. A two step approach for the preparation of bulk superconductors with the composition (TlaPb0.5)(Sr0.95Ba0.05)2(Ca0.8Gd0.2)Cu2Oz (a = 0.45, 0.5, 0.6, 0.7) was selected. Tl-free precursors were prepared via the wet malic acid gel technique. The respective amount of Tl2O3 was then co-milled to the precursor. The influence of the sintering temperatures and times on the superconducting properties and on the microstructure of the compounds was investigated. The samples were characterized by X-ray diffraction, by scanning electron microscopy, by energy dispersive X-ray fluorescence, by electrical resistance and critical current measurements. Samples with a Tl-stoichiometry between 0.6 and 0.45 were nearly phase-pure. For this thallium stoichiometry very similar superconducting properties were observed; Tc(0)-values between 99 and 101 K were obtained.