C. Boulesteix

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

Abstract 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 substantial addition of up to 20% excess of PbO in weight. No significant increase of secondary phases was observed. Pb addition was shown to be more effective 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 one of the most important rate-limiting (Bi,Pb)-2223 transformation. The second sintering duration at 835°C was optimized with respect to the weight percentage of PbO in excess. The present work attempts also to point out the complex mechanisms of the (Bi,Pb)-2223 formation reactions. Some intermediate 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.

Domain structures and plasticity of ferroelastic materials: Case of rare earth sesquioxides and YBa2Cu3O7

Abstract Domain structures and the number of different orientation variants for ferroelastic materials are studied here for different cases and with different degrees of approximation. The special case of YBa2Cu3O7 is discussed. The dissociation into two twins of rotation interfaces occurring in the 3 mF 2m transitions of Sm2O3 or Nd2O3 leading to star patterns is studied in detail. The plasticity due to mechanical twinning of ferroelastic materials is also studied with the problem of twin intersections and of microtwinning. Conditions for twin intersections are given which are more general than the previous conditions given by Cahn, which are shown to be too restrictive. The problems of brittleness and of annealing in these materials are also studied.

Superconducting Wires Obtained by Bi-2223 Coating, or Bi-2223 Synthetization from Precursors, on a Metal Core

uperconducting wires have been prepared by Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10 (Bi-2223) coating of a great number of different metallic cores. The best electrical results (high T c0 and small ΔT c ) were obtained with silver and silver coated iron cores for a sintering temperature T s = 835 °C. For short sintering durations (less than 32 h) the resistance -versus-temperature curves R = f(T) presented several drops revealing the existence of several phases. In most cases one of these phases at least presents a large ΔT c value that can be estimated by a percolation based parametrization method. For very long sintering durations, up to six days, and with addition of an excess of PbO (the percentage of PbO in excess is related to the sintering duration), these curves presented only one drop. In the latter case T c 0 = 108 K, corresponding to 60% Bi-2223, as shown by the percolation based parametrization method. It has been shown also that it is not necessary to use a good Bi-2223 ceramic to obtain these results, even precursors can be used, the Bi-2223 coating being then synthesized on the silver core. The role of the substrate, sintering temperature and of the excess of PbO will be discussed.

Variation with voltage of fine structure in TEM 〈111〉 Pole figure for cubic Dy2O3

Abstract Fine structure caused by the nodes of upper Laue zones can be seen near the centre of a TEM pole figure where the electron beam is parallel to a crystallographic axis. When the high voltage of the TEM is varied continuously, the evolution of such fine structure, observed near the 〈111〉 pole figure of cubic Dy2O3, shows that only nodes outside the Ewald sphere are responsible for it. By measuring the voltage for which the bend contours of the fine structure contain the centre of the pole figure, a shift has been revealed between the geometrical and the real positions of Bragg contours produced by upper Laue zones. This feature has been confirmed by computation.

Corrosion of bismuth-based superconductor wires by some atmospheric agents

Superconducting wires (SWs) were previously prepared in our group by heat treatment of bismuth-based superconductor coatings on a metal core. This paper presents an analysis of the corrosion process by some atmospheric agents on the SWs. SWs with different concentrations of a (Bi, Pb)-2223 phase have been exposed to water vapour saturated air, to pure water vapour or to dry carbon dioxide. Each of these atmospheric agents was found to act differently on the SWs. However, all the studied corrosion processes present a high sensitivity to the concentration of the (Bi, Pb)-2223 phase. For a higher concentration of this phase, the SWs are less sensitive to atmospheric agents.