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

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

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 mgrm and a zirconia grain size of 0.2 mgrm. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the agr alumina phase stage shifted to lower temperature. During pressureless sintering the gamma to agr alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature

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

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

Improvement of mechanical properties of zirconia-toughened alumina by sinter forging

ZTA powder with a composition of 85 wt% alumina/15 wt% zirconia was prepared by a gel precipitation method. Sinter forging was performed with this powder to enhance the mechanical properties of ZTA materials. The influence of processing flaws on mechanical properties of sinter forged materials and pressureless sintered materials was investigated. Sinter forging at 40 MPa effectively decreases process flaw sizes resulting in a homogeneous microstructure and improves the grain boundary structure because of large shear applied in this process. Sinter forging resulted in an increase in strength and toughness by a factor of 1.5–2 when compared with pressureless sintered compacts. The fracture energy is enhanced by a factor of two. The predominate mechanism for improvement of mechanical properties of these sinter-forged ZTA materials is grain boundary strengthening

Synthesis, characterisation and gas permeation studies on microporous silica and alumina-silica membranes for separation of propane and propylene

Microporous silica membranes are known to exhibit molecular sieving effects. However, separation of nearly equal sized molecules is difficult to carry out by size exclusion. Introducing sorption selectivity and keeping the kinetics favourable to facilitate a good contribution of permeation from sorption is a possible solution to enhance selectivity of adsorbing molecules. Results are presented in this paper on the synthesis of a microporous silica membrane with commendable permselectivity between helium and propylene. Modifications are performed on the membrane to improve its almost non-selective nature to propylene/propane mixtures to give practical separation values. Gas separation results on the modified membranes are presented. Surface selectivity on the newly added alumina surface layer is identified as the helping mechanism in realising this separation

Thermochemical stability and nonstoichiometry of erbia-stabilized bismuth oxide

A phase study has been performed of high oxygen ion conducting erbia-stabilized bismutch oxide (1-x)Bi2O3·xEr2O3 (BE100X) using thermal analysis and X-ray powder diffraction. Investigation of the effect of a long-time (500 h) anneal of samples at 650°C in air revealed that the minimum amount of erbia needed to stabilize the high-temperature cubic ¿-Bi2O3 phase is 27.5 at%. This boundary value is much larger than the one usually reported in literature where the sluggishenss of the transformation from cubic to hexagonal at high Bi contents is not taken into account. Changes in nonstoichiometry of solid solutions Bi2-2xEr2xO3+¿ between 550°C and 850°C upon varying the ambient oxygen partial pressure are minimal for samples with 27.5 at% erbia, increasing with increasing erbia content. The parameter ¿ in pure oxygen increases from 0. 0044 for BE27.5 to 0.022 for BE50 taking the composition in nitrogen (PO2 ¿ 10¿4 atm) as stoichiometric reference (¿ = 0)

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