Effect of cobalt on the high T c phase of the Bi1.6Pb0.4Sr2Ca2Cu3Oy system

A.c. susceptibility, X-ray diffraction (XRD), SEM and porosity studies have been performed on Bi_1.6Pb_0.4Sr₂Ca₂Cu_3–xCo_xO_y (x=0, 0.05, 0.1) superconductors. XRD and SEM results have shown that (i) the compound with x=0 contains mixed phases of 2223 and 2212, (ii) the increase in concentration of cobalt (from x=0 to 0.1) helps the compound to attain a structure of strong 2201 and a small amount of Ca₂CuO₃ impurity phase. The onset temperatures of the diamagnetic signal of these superconductor samples with x=0, 0.05, and 0.1 as observed from a.c. susceptibility measurements are 106, 60 and 50 K, respectively. The highest onset temperature, 50 K, observed in the sample with x=0.1 rather than the usually reported value of 20 K associated with 2201 phase in bismuth oxide compounds, may be due to the presence of Ca₂CuO₃ impurity phases. SEM and porosity results show that the cobalt helps to increase the grain and pore sizes.