Flux pinning properties of Yb substituted (Bi,Pb)-2212 superconductor

A systematic study on phase evolution, bulk density, resistivity, T-C, J(C), and J(C)-B characteristics of pure and Yb substituted (Bi,Pb)-2212 samples at different cationic sites (Bi, Ca, Sr) has been carried out. The results show that there is a substantial improvement in the overall superconducting properties, particularly the T-C, J(C), and J(C)-B characteristics of the Yb substituted samples. Further, it is observed that there is a significant shift in the peak value of the bulk pinning force density F-pmax towards higher magnetic fields when Yb is substituted at different cationic sites, indicative of the enhanced flux pinning strength in these samples. All the substituted samples show almost identical microstructure, but are distinctly different from pure (Bi,Pb)-2212. The natural layered growth mechanism of (Bi,Pb)-2212 is disrupted by grain growth, and coarsening in random directions. The substituted Yb atoms enter into the corresponding sites in (Bi,Pb)-2212, which create inhomogeneities in the crystal structure, decrease the hole concentration, and may produce point like defects. These defects act as the pinning centers. There is also the possibility of formation of nanoscale secondary phases, which can act as pinning centers. (C) 2007 Elsevier B.V. All rights reserved.