Preparation of Bi_<0.7>Pb_<0.3>Sr_<1.0>Ca_<1.0>Cu_<1.8>O_y High-T_c Superconductor by the Citrate Method

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A high-T_c superconductor with a nominal composition of Bi_<0.7>Pb_<0.3>Sr_<1.0>Ca_<1.0>Cu_<1.8>O_y was prepared by the citrate method. The solid precursor produced by the dehydration of the gel at 120℃ for 12 h is not in the amorphous state as expected, but in a crystalline state. X-ray diffraction (XRD) peaks of nearly the same angular position as the peaks of high-T_c phase were observed in the precursor. After pyrolysis at 400℃ and calcination at 840℃ for 4 h, the (002) peak of the high-T_c phase was clearly observed. However, the volume fraction of the high-T_c phase did not increase significantly with sintering time. The zero-resistance temperature, T_c(0), was found to be 90.8, 95.6, and 98.3 K after sintering at 845℃ for 10, 20, and 80 h, respectively. Experimental results suggest that the intermediate phase formed before the formation of the superconducting phases may be the most important factor in determining whether it is easy to form the high-T_c phase or not, because the nucleation barriers of the two superconducting phases may be altered by the variation of the crystal structures of those intermediate phases.
社団法人応用物理学会の論文
1990-04-20