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

Abstract

Superconducting Bi‐Pb‐Sr‐Ca‐Cu‐O ceramics have been synthesized by the polymerized complex method at the Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O_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)₂Sr₂Ca₂Cu₃O_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)₂Sr₂Ca₂Cu₃O_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_0.4Sr₂Ca₂Cu₃O_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.