Polymerized complex synthesis and intergranular coupling of Bi-Pb-Sr-Ca-Cu-O superconductors characterized by complex magnetic susceptibility

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics have been synthesized by the polymerized complex method at the Bi_1.8Pb_0.2Sr ₂Ca₂Cu₃O_y, Bi_1.6Pb _0.4Sr₂Ca₂Cu₃O_y and Bi_1.68Pb_0.32Sr_1.75Ca_1.82Cu _2.83O_y compositions. The polymerized complex method includes three important major steps: (1) formation of stable citrate complexes uniformly dispersed into ethyleneglycol and water, (2) fabrication of polymerized complexes which are formed through dehydration (or esterification) between ethyleneglycol and metal-citrate complexes, and (3) pyrolysis of the polymerized complex precursor and the subsequent heat treatments for producing the Bi-Pb-Sr-Ca-Cu-O superconductor. The present technique offers more convenient and easier fabrication of (Bi,Pb)₂Sr₂Ca ₂Cu₃O_y superconductors compared with other more elaborate preparation procedures based on wet-chemical techniques, as it eliminates many steps such as ageing, filtration, washing, and pH control. "X-ray-clean" (Bi,Pb)-2223 superconductors have been obtained for all the three compositions after sintering at 850°C in air for 94-235 h, depending upon the nominal composition of the starting batch. Noticeable is that a small deficiency of Sr, Ca, and Cu relative to the ideal stoichiometric composition of the (Bi,Pb)₂Sr₂Ca₂Cu ₃O_y significantly accelerates the formation of the (Bi,Pb)-2223 phase. Complex magnetic susceptibility measurements show that T_c(onset) of the bulk (Bi,Pb)-2223 phase remains almost constant at around 108 K, independent of the compositions. However, it is stressed that low-level and/or amorphous impurity phases possibly concentrated on grain boundaries can have dramatic effects on the overall magnetic properties even for materials which look like very "clean" by x-ray diffraction. The best superconducting properties were obtained with samples with a nominal composition of Bi_1.6Pb_0.4Sr₂Ca ₂Cu₃O_y after sintering in air at 850°C for 180-280 h. The onset temperature of superconducting transition obtained by complex susceptibility was 108.0 K and the transition width (10%-90%) of full diamagnetism was 5.2 K for a measuring field of 100 mOe.