H. Kruidhof

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.

Oxygen permeation of La0.3Sr0.7CoO3-δ

The oxygen permeability of dense La0.3Sr0.7CoO3−δ membranes has been measured in the range 750–1100°C under various oxygen partial pressure gradients. A sweep gas method was employed. Results indicate that in the range of thickness 0.057–0.215 cm used in the present study, the oxygen flux is predominantly controlled by bulk diffusion across the membrane. The measured activation energy is 60 kJ mol−1. By fitting the permeation data for various thicknesses to the transport equation obtained upon assuming linear kinetics for the surface exchange reactions and bulk ionic transport, we could derive the oxygen ionic conductivity and the characteristic membrane thickness. The latter quantity determines the transition from predominant control by diffusion to that by surface exchange. The ionic conductivity is about 0.5 S cm−1 at 1000°C. The characteristic thickness is extrapolated at a value of about 80 μm.

Preparation of YBa2Cu3O7- by citrate synthesis and pyrolysis

A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-Tc superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.

Microstructural development, electrical properties and oxygen permeation of zirconia-palladium composites☆

Yttria-stabilized cubic zirconia (YSZ)-palladium dual phase composites have been investigated. The percolative composite containing 40 vol% Pd (ZYPd40) showed a much larger oxygen permeability than that of the non-percolative composite containing 30 vol% Pd (ZYPd30). For a 2.0 mm thick percolative composite, an oxygen flux of 4.3 × 10−8 mol/cm2/s was measured at 1100 °C with oxygen partial pressures at the feed and permeate sides being 0.209 and 0.014 atm, respectively. This value is two orders of magnitude larger than that observed for a 2.0 mm thick non-percolative composite at the same temperature with the oxygen partial pressures at the feed and permeate sides being 0.209 and 1.5 × 10−4 atm, respectively. From the dependence of the oxygen permeation on the temperature and on the oxygen partial pressures, it was concluded that the transport of the oxygen ions through the YSZ phase in the percolative system was the rate limiting step.

Ionic conductivity of perovskite LaCoO3 measured by oxygen permeation technique

Oxygen permeation measurement is demonstrated, not only for a mixed oxide ionic and electronic conductor, but also as a new alternative to determine ambipolar conductivities, which can be usually reduced to be partial conductivities (either ionic or electronic). As a model system and an end member of an important oxygen permeable La1-xSrxCoO3-δ dense membrane system, LaCoO3 dense ceramic was measured with respect to the thickness, temperature and oxygen partial pressure dependencies of its oxygen permeability. Within the thickness range used (down to 0.041cm), the oxygen permeation of LaCoO3 was found to be purely controlled by bulk diffusion with activation energies between 260–300kJmol-1. Its ionic conductivity and oxygen self-diffusion coefficient as functions of oxygen partial pressure were also derived from permeability data. PO2-0.46 and PO2-0.31 relations were found in the high pressure (1.01–10-2bar) and low pressure (10-2.8–10-3.4bar) ranges, respectively.

Preparation of La0.3Sr0.7CoO3–δ perovskite by thermal decomposition of metal-EDTA complexes

Perovskite powders of La0.3Sr0.7CoO3–were prepared by the thermal decomposition of precursor complexes derived from nitrate solutions using ethylenediaminetetraacetic acid (EDTA) as a complexing agent. The calcination temperature is 920 °C. Powders thus obtained have a low carbon contamination. Dense ceramics with a relative density of about 96% have been prepared after sintering at 1150 °C.

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.

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.

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.

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 pressure