Synthesis, microstructural evolution and the role of substantial addition of PbO during the final processing of (Bi,Pb)-2223 superconductors

The evolution of the phase formation and microstructure in (Bi,Pb)-Sr-Ca-Cu -O (BPSCCO) superconducting materials have been studied through a two-cycle annealing process by X-ray diffraction and SEM observations. Samples were Pb-doped (part 0.4 of Bi substituted by Pb) during the first thermal cycle. Yet, the synthesizing efficiency of the (Bi,Pb)-2223 phase was found to be greatly re-enhanced during the second thermal cycle by a second and substa ntial addition of up to 20% excess of PbO in weight. No significant increas e of secondary phases was observed. Pb addition was shown to be more effect ive at the end of a multi-step thermal cycle. Therefore the lack of Pb that arise in BPSCCO system after some heat treatment duration is shown to be o ne of the most important rate-limiting (Bi,Pb)-2223 transformation. The sec ond sintering duration at 835 degreesC was optimized with respect to the we ight percentage of PbO in excess. The present work attempts also to point o ut the complex mechanisms of the (Bi,Pb)-2223 formation reactions. Some int ermediate phases are identified and the liquid phase is suggested to derive from a partial decomposition of the (Bi,Pb)-2212 phase. This decomposition is highly sensitive to Pb addition. (C) 2001 Elsevier Science B.V. All rig hts reserved.