Ceramic Oxygen Separation Membranes and Mixed Conductors
A consortium was created to investigate electroceramic materials that show ionic or mixed ionic-electronic conduction (MIEC), for application as oxygen separation membranes and in solid oxide fuel cells (SOFC, electrolytes and cathodes). The focus is on dense structures as supported thin layers (5-20 µm) for reduced temperature operation (600-750°C). Under these conditions grain boundary effects are important. Zirconia, ceria and gallate are considered for SOFC electrolytes. The nature and the amount of the dopants will be varied, and sintering aids added. These parameters will influence the grain size and the grain boundary composition. The effects on microstructure, and conduction and strength properties will be examined by high resolution SEM/TEM, impedance spectroscopy and load testing under fast thermal cycling, respectively. A systematic study on the variation of the crystallite size of starting powders (prepared by ultrasonic spray pyrolysis - USP) and its effect on the grain boundary resistance is planned, for the case of zirconia electrolyte. Of special interest is also the compatibility between ceria and zirconia, when cofired at high temperatures: diffusion couples will be made, and studied by HR-SEM/EDX. Thin layers of these materials will be deposited on anode supports of planar shape, by tape casting, screenprinting or electrostatic spray pyrolysis (ESP). Dip coating and electrophoretic deposition (EPD), for example on tubular supports, is equally envisaged. Special attention is given to the conductivity measurement of such thin layers (in particular that of grain boundaries), and compared with that of bulky samples. Different results due to the thin layer microstructure have been observed. MIEC materials of the perovskite family La1-xSrx(Mn, Fe, Co)O3-x are considered as SOFC cathodes. Thin dense layers will be deposited by ESP on zirconia and ceria electrolytes. The effect of the annealing temperature, and therefore the grain size, on the electrochemical performance and the thermal-cycling performance of such cathode/electrolyte interfaces will be studied. Candidate MIEC Materials for oxygen separation membranes are intended to be fabricated as dense pellets (compaction) or tubes (extrusion). These can be single phase (e.g. La1-xBaxCo1-yFeyO3-d) or composites (e.g. ceria/La-chromite mixtures). They will be subjected to oxygen permeation measurements, i.e. under a large oxygen partial pressure gradient of for example CH4/air. Microstructural effects for permeation stability and mechanical stability will be of first importance: changes (grain size, grain boundaries, microporosity) will be undertaken. In view of their application, it is planned to try and deposit thin dense films of these materials on porous supports (e.g. of ceria): by ESP on porous plates, and by dip coating on porous tubes (made f.ex. by slip casting). Wherever possible, wet chemical synthesis routes are employed to obtain the ceramic starting powders, such as coprecipitation, citrate gel or dyanate complexation, colloidal synthesis or combustion synthesis.
présentation orale (Workshop)
Record created on 2005-08-08, modified on 2016-08-08