Structure and Superconducting Properties of Tl-Base High-<I>T<SUB>c</SUB></I> Oxides Prepared by a Diffusion Process

The Tl-2223 phase, about 150 μm in thickness, is synthesized through a diffusion process in a short reaction time. The scanning electron microscope observation reveals the existence of an appreciable amount of a second phase and voids in the 2223 phase. The fluorine (F) added to the coating layer of the diffusion composite acts like an effective flux to transform the 2223 phase into the 1223 phase, and the resulting 1223 phase shows a dense and homogeneous structure. The c-axis lattice parameter of the 1223 phase formed in the specimen with F addition reaches a slightly shorter value than the reported one after the reaction.The F addition significantly increases the transport Ic at 77 K, and reduces its degradation under magnetic field. Furthermore, the slow cooling after the reaction improves the Ic-B performance in the specimen with F addition. The resistive superconducting transition under magnetic field in the specimen with F addition is much less broad than that in the specimen without F addition. The irreversibility field is apparently shifted to higher temperatures by the F addition, which may originate from the phase transformation mentioned above. The annealing in O2 after the reaction increases the Ic and reduces the normal state resistivity of specimens. A Jc at 77 K of about 1.5×108 A/m2 has been obtained in the Tl-1223 diffusion layer of about 150 μm in thickness through the F addition.