AC susceptibility measurements: a quality test for high-T(C) superconducting bulk materials

Temperature dependent measurements of high T(c) bulk material reflect the quality of the high T(c) grains and the intergranular weak links. Besides, ac field dependent measurements give access to a quantitative quality label of the bulk material : its critical current density J(c). Herefore critical state models as the Bean and the Kim model are used. Our measurements on long orthorombic samples with different cross sections a x b show that the Kim model is the most realistic one. It leads to a b/a independent average value of J(c), which equals the J(c) value of transport measurements and of magnetic SQUID measurements at low dc fields

Impact of sulphur contamination on the oxygen transport mechanism through Ba0.5Sr0.5Co0.8Fe0.2O3-delta: Relevant issues in the development of capillary and hollow fibre membrane geometry.

Fabrication of dense perovskite membranes in the form of capillaries or hollow fibres is considered attractive for large-scale oxygen separation applications. For the preparation of such membranes by phase-inversion process polysulphone or polyethersulphone are commonly used as a binder. The decomposition of the sulphur-containing binder during the calcination leads to the formation of sulphates, which negatively affect the oxygen permeation through the membrane. The present work focuses on the comparative analysis of the oxygen transport mechanism through sulphur-free and containing Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) membranes. The analysis of the thickness dependence of the oxygen permeation fluxes indicated that sulphates decrease the permeation rate mostly due to the partial blocking of the surface oxygen exchange, whilst the bulk ambipolar conductivity remains essentially unchanged. SEM/EDS studies revealed segregation of BaSO4 at the grain boundaries, which might be responsible for the fast oxygen exchange in phase-pure BSCF. The negative impact of sulphur contamination on oxygen permeation was more pronounced at temperatures below 1123 K. It has been demonstrated, that, by surface activation, the oxygen flux through sulphur-containing BSCF membranes can be increased to the level of that of sulphur-free membranes. (C) 2012 Elsevier B.V. All rights reserved