Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides

Attention is drawn to the possible involvement of the surface exchange kinetics in limiting the rate of oxygen permeation through mixed-conducting oxide ceramics. A theoretical approach is provided with which it is possible to distinguish between surface exchange- and bulk diffusion controlled kinetics of oxygen permeation. New results on the oxygen permeability of perovskites La0.8Sr0.2CoO3−σ and SrCo0.8Fe0.2O3−σ are presented. The importance of the exchange reaction re to diffusion in limiting overall oxygen transport through (La,Sr)(Co,Fe)O3−σ perovskite-type oxides is emphasized.\u

Thermochemical stability and nonstoichiometry of yttria-stabilized bismuth oxide solid solutions

The thermochemical stability of fast oxygen ion conducting yttria stabilized bismuthoxide (YSB) solid solutions containing 22.0–32.5 mol% of yttria was investigated. It was shown that in the temperature range between 650–740 C the stabilized cubic δ-phase containing less than 31.8 mol% of yttria is not stable during long term annealing treatments (greater-or-equal, slanted 500 hours). During annealing at 650 C a sluggish transformation from the cubic to hexagonal phase appears, while above 740 C this hexagonal phase is converted very fast into the cubic phase again. It was shown, that the oxygen content of YSB solid solutions is a function of temperature and oxygen partial pressur

The determination of major and some minor constituents in lead zirconate-titanate compositions by x-ray fluorescence and atomic absorption spectrometry

An accurate X-ray fluorescence spectrometric method is described for the determination of lead, zirconium and titanium in lead zirconate-titanate ceramics. Careful matching of samples and standards by a borax fusion method resulted in a relative standard deviation of about 0.2% for the major constituents. The determination, after separation, of the unreacted oxide of lead by atomic absorption spectrometry, and of the unreacted oxides of zirconium and titanium by X-ray fluorescence spectrometry is also described. An X-ray fluorescence spectrometric method is proposed for the determination of dope elements (K, La, Sm, Yb) with internal standards (Ca, Ce, Cr, Ni respectively). The magnesium dope is determined by atomic-absorption spectrometry with standard addition

Oxygen permeation through oxygen ion oxide-noble metal dual phase composites

Oxygen permeation behaviour of three composites, yttria-stabilized zirconia-palladium, erbia-stabilized bismuth oxidenoble metal (silver, gold) was studied. Oxygen permeation measurements were performed under controlled oxygen pressure gradients at elevated temperatures. Air was supplied at one side of a dense sintered disk specimen, while helium was fed at the opposite side to sweep away the permeated oxygen. This research has demonstrated that in addition to the presence of percolative metal phase in the oxide matrix, a large ionic conductivity of the oxide phase and a high catalytic activity of the metal phase towards surface oxygen exchange are required for the dual phase composite to possess high oxygen permeability. The bismuth oxide-silver composite fulfils these requirements, hence showing the best oxygen permeability

Preparation of gas-tight strontium-doped lanthanum cobaltate by an aqueous sol-gel process

Gas-tight dense membranes of highly Sr-doped LaCoO3 (such as the composition La0.3Sr0.7CoO3 chosen in this study) are difficult to prepare using usual synthesis processes. This report presents an aqueous sol-gel route using metal acetates as precursors to achieve this goal. Hydrogen peroxide and ammonia are added to the acetates precursor solution to obtain stable sol and xerogel, as well as a homogeneous powder after heat treatment. Thermal analysis and XRD show the presence of SrCO3 and La2O3 when the heat treatment temperature of the xerogel is lower than 950 °C. Single phase of cubic perovskite La0.3Sr0.7CoO3 can be obtained after treatment at above 1000 °C. Sintering the cold pressed green compact at above 1100 °C may result in gas-tight samples. It is found that the sintering temperature has a minor influence on the oxygen permeability of a gas-tight membrane

Oxygen semi-permeability of erbia-stabilized bismuth oxide

The isothermal permeability of oxygen through sintered dense disks of bismuth oxide stabilized with 25 mol% erbia (BE25) has been studied at 610–810°C and oxygen pressures of 0.0001–1 atm. It is concluded that the permeating flux is rate limited both by solid state diffusion of electron holes and by the surface exchange reaction. From the results the p-type electronic conductivity of BE25 and the surface oxygen exchange rate have been evaluated. The value obtained for the latter shows excellent agreement with that obtained from 18O-16O isotope exchange reported previously. Using gold point electrodes it is demonstrated that the oxygen semipermeability flux in case of partial rate control by surface oxygen exchange leads to deviations from Nernst behaviour, though oxygen permeation measurements reveal that under the applied conditions BE25 remains a solid electrolyte with an ionic transference number close to unity.\u

Influence of order-disorder transitions on oxygen permeability through selected nonstoichiometric perovskite-type oxides

New results on the oxygen permeability of perovskite-type oxides SrCo0.8B'0.2O3¿¿ (with B'=Cr, Fe, Co and Cu) and La0.6Sr0.4CoO3¿¿ are presented. The occurrence of order-disorder transitions at elevated temperatures (790¿940°C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction of samples either slowly cooled or quenched from high temperature after annealing in different atmospheres. The oxygen permeability found upon exposing opposite sides of sealed disc specimens to a stream of air and of helium, respectively, increases sharply (between 5¿6 orders of magnitude up to 0.3¿3×10¿7 mol cm¿2s¿1) at the onset of the transition from a low-temperature vacancy-ordered state to defec t perovskite, except for SrCo0.8Fe0.2O3¿¿. In the latter case only a slight anomaly is found in the Arrhenius plot of th e oxygen permeability at 790°C. The comparatively high oxygen flux through SrCo0.8Fe0.2O3¿¿ observed at intermediate tempe ratures is interpreted in terms of a two-phase mixture of a vacancy-ordered state and disordered perovskite, while above 790°C the sample is single-phase of defect perovskite structure

Mixed conducting yttrium-barium-cobalt-oxide for high oxygen permeation

Yttrium-barium-cobalt-oxide (YBC), especially with low Y content, has been prepared. Oxygen permeation in these materials is very high at moderate temperature. The materials Y0.05BaCo0.95O3-δ and Y0.10Ba0.90CoO3-δ consisted of a BaCoO3-δ like main phase and some minor phases. For the Y0.05BaCo0.95O3-δ material these minor phases were not exceeding 10 vol%. Y0.05BaCo0.95O3-δ had the highest oxygen permeation value of 3.9 × 10-7 mol/cm2s at 900°C; the surface exchange reaction may be the rate limiting step here. The material Y0.33Ba0.67CoO3-δ consisted mainly of two unknown cubic phases.\u

Bismuth oxide based ceramics with improved electrical and mechanical properties: Part II. Structural and mechanical properties

Coprecipitation as a method of preparation for bismuth oxides based ceramics yields relatively strong and machineable materials in comparison with the solid state reaction. Compositions within the system (1−x)Bi2O3|xEr2O3 containing up to twenty five mole percent of erbium oxide show a slow transition on heating at 900 K from a cubic to a hexagonal (rhombohedral) phase. The resulting hexagonal phase is stable up to 1000 K where it transforms back to the cubic phase. Long term heating at 775 K causes formation of traces of ∝ - bismuth oxide in these samples, but the material itself remains cubic. Compositions containing thirty mole percent of erbium oxide do not show any such changes and hence are suitable for application purposes