A homologous series Pb

The PbO–Nb2O5 phase diagram has been investigated, using electron diffraction and high resolution electron microscopy (HREM), for [Pb]/[Nb] ratios between 1 and 1.25. Apart from the basic pyrochlore structure, different long period superstructures, forming a homologous series Pb2n+1Nb2n−1O7n−1 based on the pyrochlore structure have been found. Members with n = 5, 7, 9, 11 have been observed and their structure determined based on electron diffraction and HREM. Image simulations allow studying the influence of cation and oxygen vacancies in the basic structure. Ordering of oxygen vacancies could be detected while no cation vacancies are observed. Deviations from the perfect structure and ordered stacking defects are discussed

Electrical properties and corrosion of Bi-2223 coatings on silver wires directly synthetized from a Bi-2223 precursor or from a Bi-2223/Bi-2212 mixture

Superconducting wires are prepared by coating of a silver substrate, by sintering of a slurry deposited onto the metal core and containing a grinded (Pb doped) Bi-2223 ceramic, a grinded Bi-2223/Bi-2212 mixed ceramic or a grinded mixture of the Bi-2223 precursors, and of 20% PbO in excess. The resistance-versus-temperature R(T) curves obtained for a 6 days sintering at 835°C presented only one drop, with Tc0 = 108K (107K when the slurry is a mixture of the Bi-2223 precursors). When the coating is obtained from a grinded Bi-2223/Bi-2212 mixed ceramic the R(T) curves are better (higher Tc and narrower ΔTc) than the R(T) curves obtained with the ceramic used for preparing the wires, for which Tc0 = 34K. The role of PbO in excess is discussed. These coatings hold rather well against corrosion by waper vapor at 90°C and the Bi-2223 phase is less corroded than the Bi-2212 phase

Development of Bulk Bi2+xSr3-yCa yCu 2O8+delta Superconductors by Partial-Melting Route for Fault Current Limiters Application

The production of bulk Bi2+xSr3-yCa yCu 2O8+delta (Bi-2212) superconductors for fault current limiter application was developed via a partial-melting route. Aiming high Ic (critical current), which is the essential superconducting characteristic for application of this material in the construction of Fault Current Limiters (FCL), the produced blocks have predominance of Bi-2212 phase (83 wt%), which characterizes with high values of zero and onset transport critical temperature of 92K and 97.5K, respectively. A relatively low transition width, deltaT, from the superconducting to the normal state of 5.5K, revealed a good intergrain connectivity. Consequently, current measurements on the blocks of Bi-2212 show promising Ic values of 230A and 850A for direct and alternate current, respectively. It is expected that further increases in the Ic values will depend on the elimination of an observed amorphous phase and further reduction of amount and grain sizes of secondary phases, still present in the blocks obtained by the proposed partial-melting route. This may be achieved by a further optimization of the partial-melting processing parameters