Reversible melting and equilibrium phase formation of (Bi,Pb)2Sr2Ca2Cu3O10+d

The decomposition and the reformation of the (Bi,Pb)2Sr2Ca2Cu3O10+d (?Bi,Pb(2223)?) phase have been investigated in-situ by means of High-Temperature Neutron Diffraction, both in sintered bulk samples and in Ag-sheathed monofilamentary tapes. Several decomposition experiments were performed at various temperatures and under various annealing atmospheres, under flowing gas as well as in sealed tubes, in order to study the appropriate conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was found to melt incongruently into (Ca,Sr)2CuO3, (Sr,Ca)14Cu24O41 and a Pb,Bi-rich liquid phase. Phase reformation after melting was successfully obtained both in bulk samples and Ag-sheathed tapes. The possibility of crystallising the Bi,Pb(2223) phase from the melt was found to be extremely sensitive to the temperature and strongly dependent on the Pb losses. The study of the mass losses due to Pb evaporation was complemented by thermogravimetric analysis which proved that Pb losses are responsible for moving away from equilibrium and therefore hinder the reformation of the Bi,Pb(2223) phase from the melt. Thanks to the full pattern profile refinement, a quantitative phase analysis was carried out as a function of time and temperature and the role of the secondary phases was investigated. Lattice distortions and/or transitions were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212), (Ca,Sr)2CuO3 and (Sr,Ca)14Cu24O41, due to cation diffusion and stoichiometry changes. The results indicate that it is possible to form the Bi,Pb(2223) phase from a liquid close to equilibrium conditions, like Bi(2212) and Bi(2201), and open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape processing.Comment: 45 pages (including references,figures and captions), 13 figures Submitted to Supercond. Sci. Techno

Diagramme de phases dans le systeme (Bi, Pb)-Sr-Ca-Cu-0 : influence de la pression partielle d'oxygène sur la composition Pb-2212

DTA-DTG study of 2212 compound, with and without lead, shows that the different phase transformations observed on heating and cooling involve a variation of oxygen stoechiometry quantitatively invertable. Influence of constraint oxygen partial pressure on phase transformations has been studied under several $P_{\rm O_2}$, by thermal scanning and isothermal dwell. Lead loss under PbO form has been distinguished from oxygen variations and the existence of an isobaric invariant under $1\,013 \pm 15$ h Pa at $884 \pm 1~^\circ$C for $0,5 < P_{\rm O_2}$ (of constraint) $< 1$ has been shown.L'étude par ATD-ATG de l'espèce 2212, avec et sans plomb, montre que les différentes transformations de phases observées au chauffage et au refroidissement mettent en jeu une variation de stoechiométrie en oxygène renversable quantitativement. L'influence de la pression partielle d'oxygène de contrainte sur les transformations de phases a été étudiée sous différentes $P_{\rm O_2}$, par balayage thermique d'une part et par maintien isotherme d'autre part. La perte de plomb sous forme de PbO a pu être dissociée des variations en oxygène et l'existence d'un invariant isobare (sous $1\,013 \pm 15$ h Pa) à $884 \pm 1~^\circ$C pour $0,5 < P_{\rm O_2}$ (de contrainte) $< 1$ a été mise en évidence