Effect of rare-earth (Eu, Yb and Ag) substitutions on superconducting properties of the Bi1.7Pb0.3Sr2Ca2-R-x(x)(R = Eu, Yb, and Ag)Cu3Oy system

The superconducting properties of Bi1.7Pb0.3Sr2Ca2-xRx(R = Eu, Yb and Ag)Cu 3OY have been investigated by X-ray diffraction, electrical resistance and the peritectic transition of these superconductors was studied kinetically under different atmospheres and temperature gradients. X-ray diffraction re sults show that the volume fraction of the high-T-c (2 2 2 3) phase decreas es and that of the low-T-c (2 2 1 2) phase increases as Eu, Yb and Ag conce ntrations increase. The resistivity measurements reveal that the T-c onset decreases down to 100 K for Eu, 85 K for Yb and 106 K for Ag concentrations . These results are explained on the basis of possible variations of hole c oncentration with trivalent rare earth ion substitutions. Activation energi es and frequency factors for crystallisation were determined by non-isother mal differential thermal analysis (DTA), employing different models. It was found that both peritectic transition and reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres reveale d that the thermal stability of Bi-2 2 1 2 phase was greatly enhanced under oxygen atmosphere. (C) 1999 Kluwer Academic Publishers.