A thermal analysis study of the system Bi-Sr-Ca-Cu-O at variable oxygen pressure

Abstract

DTA was carried out as a function of partial oxygen pressure on samples of nominal compositions BiSrCaCu2Ox, Bi1.2SrCaCu2Ox, Bi2Sr2Ca2Cu3Ox, Bi2Sr1.5Ca1.9Cu3.4Ox and Bi4PbSr4Ca5Cu7Ox. Two reversible DTA peaks (endothermic on heating) were observed: a multiple peak I at 745-770°C, decreasing with increasing P(O2), and a single peak II at 835-875°C, increasing in intensity and temperature with increasing P(O2). Quenching at reduced P(O2) showed that peak I mainly consists of the reaction Bi2Sr2Ca1Cu2Ox →Bi2(Sr2-xCax)CuOx with much lower Tc. Peak II is ascribed to the incongruent melting of Bi2Sr2Ca1Cu2Ox. Unlike peak I, the latter is a redox reaction involving gaseous oxygen; its enthalpy is ≈90 kJ/mol (O2). In the Pb-substituted system, peak II is strong at lower P(O2) and clearly split, possibly indicating the range of stabilization of Bi2-xPbxSr 2Ca2Cu3Ox. Pure 110 K transitions were obtained for Bi4PbSr4Ca5Cu7Ox annealed at 835°C and 0.05 atm O2. © 1989.