Effects of Silver and Lead on the Phase Stability of Bi2Sr2Ca1Cu2O8+x and Bi2Sr2Ca2Cu3O10+x above and below the Solidus Temperature

Abstract

A solid‐state ionic technique has been used to determine the effects of Ag on the phase stability of Bi2Sr2Ca1Cu2O8+x(Bi‐2212), and Ag and Pb on the phase stability of Bi2Sr2Ca2Cu3O10+x (Bi‐2223), over a temperature range of 600° to 910°C and oxygen partial pressure range of 10–6 to 10°atm (10–1 to 105 Pa). The addition of Ag to Bi‐2212 leads to a marginal decrease in the stability of the phase above 883°± 3°C and 7 × 10–2 atm (7 kPa) oxygen, resulting in a reduction in the incongruent melting point by 15°C in air. The substitution of Pb for Bi in the (Bi,Pb)‐2223 phase leads to an increase in phase stability below the solidus, but to only a minor change in stability above the solidus. For both phases, several incongruent melting reactions have been identified over the range of reduced oxygen partial pressures studied. In addition, the phase equilibria both above and below the stability line have been determined for certain regions of the stability diagrams. From the thermo‐dynamic information obtained in this study an improved melt‐processing route for Bi‐2212 has been found which minimizes unwanted phases. In addition, the optimum precursors for the formation of more phase‐pure (Bi,Pb)‐2223 during shorter time periods have been found by determining the initial decomposition products of (Bi,Pb)‐2223, both above and below the solidus.