Improved superconducting properties by La addition in (Bi,Pb)-2212 bulk superconductor

The effects of La addition on the structural and superconducting properties of (Bi,Pb)-2212 bulk superconductor prepared by solid state synthesis in the polycrystalline form were studied. The La content of the samples was varied from 0 to 0.5 on a general stoichiometry of Bi1.7Pb0.4Sr2.0Ca1.1Cu2.1LaxOy. Phase analysis by XRD, microstructural examination by SEM, measurements of density and superconducting properties were done to evaluate the relative performance of the samples. The critical temperature (TC) and critical current density, in zero [JC(0)] and applied field [JC(H)], of La added samples were found to be much higher than those of the pure sample for optimum doping levels. A maximum TC-onset of 93.5 K (x = 0.3) and a maximum JC(0) of 676 A/cm2 (x = 0.2) at 64 K were observed for the doped samples as against 76.7 K and 100.5 A/cm2 at 64 K for the pure sample, respectively. At higher doping levels the TC and JC(0) decrease from the maximum values. The increase in TC and JC(0) and the improved JC-H characteristics are explained to be due to the substitution of La3+ in place of Sr2+ with a consequent change in the carrier (hole) concentration in the Cu-O2 planes, and to be due to the flux pinning effect of structural defects and nanoscale precipitates brought about by La addition.