THERMAL-ANALYSIS OF THE OXYGEN ACTIVITY AND ELECTRICAL-RESISTANCE IN THE 2212-COMPOSITION AND 2223-COMPOSITION OF THE (BI,PB)(2)SR2CAN-1CUNOX ISOPLETH

Phase transformations in Pb-2212 and Pb-2223 compositions contained in the isopleth defined by the formula (Bi, Pb)(2)Sr-2 Ca-n-1, Cu-n O-2n +4+delta were investigated by thermal analysis of the oxygen activity and of the electrical resistance, inside an electrolytic cell of CaO-s tabilized zirconia. These methods of thermal analysis allow us to not only determine the main known phase transformations in these compositi ons but also new transformations in the system close to 750 degrees C and 500 degrees C under p(O-2) = 10(5) Pa pure oxygen atmosphere. Our e.m.f. thermal analysis shows the existence of a strong redox equilibr ium spread between 300 and 550 degrees C which was also detected by th e resistance analysis and cannot be interpreted by a phase transformat ion. X-ray diffraction analysis has proved that no structural transfor mation appears below 550 degrees C, The results obtained in both parts of the 750 degrees C transformation by X-ray diffraction of quenched powders are still too incomplete to conclude that the Majewski eutecto id transition: 2212<->2201 + 1/2 Ca2CuO3 + 1/2 CuO occurs at this temp erature but this transformation might correspond to (Ca, Sr)(2)PbO4 pr ecipitation. Both the new redox transformations observed arround 750 a nd 500 degrees C might also be valence transitions, inside the high-T- c superconducting phases.