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 Bi1.8Pb0.2Sr2Ca2Cu3Oy, Bi1.6Pb0.4Sr2Ca2Cu3Oy and Bi1.68Pb0.32Sr1.75Ca1.82Cu2.83Oy 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)2Sr2Ca2Cu3Oy 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)2Sr2Ca2Cu3Oy significantly accelerates the formation of the (Bi,Pb)-2223 phase. Complex magnetic susceptibility measurements show that Tc(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 Bi1.6Pb0.4Sr2Ca2Cu3Oy 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.