STRUCTURE AND SUPERCONDUCTING PROPERTIES OF TL-BASE HIGH-T-C OXIDES PREPARED BY A DIFFUSION PROCESS

The Tl-2223 phase, about 150 mu m in thickness, is synthesized through a diffusion process in a short reaction time. The scanning electron m icroscope observation reveals the existence of an appreciable amount o f a second phase and voids in the 2223 phase. The fluorine (F) added t o the coating layer of the diffusion composite acts like an effective flux to transform the 2223 phase into the 1223 phase, and the resultin g 1223 phase shows a dense and homogeneous structure. The c-axis latti ce parameter of the 1223 phase formed in the specimen with F addition reaches a slightly shorter value than the reported one after the react ion. The F addition significantly increases the transport I-c at 77 K, and reduces its degradation under magnetic field. Furthermore, the sl ow cooling after the reaction improves the I-c-B performance in the sp ecimen with F addition. The resistive superconducting transition under magnetic field in the specimen with F addition is much less broad tha n that in the specimen without F addition. The irreversibility field i s apparently shifted to higher temperatures by the F addition, which m ay originate from the phase transformation mentioned above. The anneal ing in O-2 after the reaction increases the I-c and reduces the normal state resistivity of specimens. A J(c) at 77 K of about 1.5 x 10(8) A /m(2) has been obtained in the Tl-1223 diffusion layer of about 150 mu m in thickness through the F addition.