Enhancement in the high-T-c phase of BSCCO superconductors by Nb addition

Pure and Nb2O5 added Bi1.6Pb0.4Nbx Sr2Ca2Cu3O delta superconductors ( x varies up to 0.30) were synthesized by the solid state reaction method. Structural and superconducting properties of the samples were investigated. The fraction of the high-T-c phase ( 2223) increases with Nb addition up to x = 0.10, for which a nearly single 2223 phase was obtained. The critical temperature of the samples increases from 96 to 104 K with doping up to x = 0.20 and a further increase in Nb decreases the Tc sharply. Similarly, there is a considerable enhancement in the critical current density ( jc) of the samples in the same doping range. Both AC susceptibility and transport measurements showed that the optimum Nb concentration is x = 0.20. The out- of- phase component of the AC susceptibility measurements showed that inter- grain coupling was enhanced up to the same doping level. Nb additions above x = 0.30 decrease the fraction of the high- Tc phase and degrade the superconducting properties. Present measurements indicate that x = 0.20 is the optimum Nb concentration in the BSCCO superconductor to enhance phase purity, inter- grain coupling and superconducting parameters ( Tc and j(c)).