Determination of Oxygen Nonstoichiometry and Diffusivity in Mixed Conducting Oxides by Oxygen Coulometric Titration. II Oxygen Nonstoichiometry and Defect Model for La0.8Sr0.2CoO3-delta

The oxygen nonstoichiometry of La0.8Sr0.2CoO3-delta has been determined as a function of oxygen partial pressure and temperature using a high-temperature coulometric titration cell. For each measured value of the oxygen chemical potential, the oxygen nonstoichiometry is found to be nearly independent of temperature. The equilibrium partial energy and entropy associated with oxygen incorporation have been determined as a function of oxygen nonstoichiometry and temperature. The results are interpreted in terms of a model in which it is assumed that conduction electrons, created during vacancy formation, gradually fill electron states in a wide electron band. A new relation between vacancy concentration, temperature, and oxygen partial pressure has been formulated which does not have the familiar appearance of a mass action type of equation

Determination of oxygen nonstoichiometry and diffusivity in mixed conducting oxides by oxygen Coulometric titration

Oxygen coulometric titration has been applied to measure chemical diffusion in La0.8Sr0.2CoO3-δ between 700 and 1000°C. The transient current response to a potentiostatic step has been transformed from the time domain to the frequency domain. The equivalent circuit used to fit the resulting impedance data contains the element that describes the finite-length diffusion of oxygen into the sample specimen. Other elements included are the gas-phase capacitance and the sum of the gas-phase diffusion resistance and that associated with the limited surface exchange kinetics of the sample. The chemical diffusion coefficient of perovskite La0.8Sr0.2CoO3-δ has been determined as a function of temperature and oxygen partial pressure. Its value can be represented by ~D (cm2/s) = 5.91 x exp [(-135 kJ/mol)/RT], and turns out to be practically independent of oxygen partial pressure in the range 10-2 - 0.209 bar

Oxygen transport through La@1-x]Sr@x]FeO@3-gamma] membranes. I. Permeation in air/He gradients

Oxygen permeation measurements in air/He gradients were performed on dense La1 ¿ xSrxFeO3 ¿ ¿ membranes in the composition range x = 0.1¿0.4 and temperature range 1123¿1323 K. Pretreatment of the lower oxygen partial pressure side of the membranes in a CO-containing atmosphere for several hours at 1273 K led to higher oxygen fluxes, which were in the range of 0.1¿4.5 mmol m¿2 s¿1. After treatment, the observed oxygen fluxes could be described in terms of bulk diffusion-limited permeation behaviour. Experimental evidence for a bulk-diffusion controlled flux was found from thickness dependence measurements on membranes with thicknesses between 0.5 mm and 2.0 mm. Model calculations, based on Wagner theory in conjunction with data of oxygen nonstoichiometry and vacancy diffusion coefficients from literature, were performed. The experimental flux values deviated from the model calculations with factors up to 2.5. Adjustment of the value of the vacancy diffusion coefficient led to good agreement between the experimental data and the model calculations. The calculated vacancy diffusion coefficients Dv0 were virtually independent of composition and were found to be in the range 5.3¿9.3 × 10¿6 cm2 s¿1

Neutron-diffraction and Mossbauer-spectroscopy study of La0.6Sr0.4Fe1-xCoxO3-y (x=0, 0.5) perovskites

The crystal and magnetic structure of La0.6Sr0.4Fe1-xCoxO3-y (x=0,0.5) perovskites was studied by neutron diffraction and Mössbauer spectroscopy. At T=2–300 K the oxides possess an oxygen-deficient, rhombohedrally distorted structure, with space group R\overline{3}c. 57Fe Mössbauer spectra below T=154 K show a regular arrangement of Fe spins, determined as a G-type antiferromagnetic structure by neutron diffraction. The mean magnetic moment at T=40 K is 1.61 μB per B-site for x=0.5, and 2.63 μB for the x=0 composition. An increase of TN and oxygen deficiency accompany the heating to 850 K. The structure of La0.6Sr0.4Fe0.5Co0.5O2.84 at 850 K is cubic with space group Pm3m

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)

Influence of ionic conductivity of the nano-particulate coating phase on oxygen surface exchange of La0.58Sr0.4Co0.2Fe0.8O3-δ

The oxygen surface exchange kinetics of mixed-conducting perovskite La0.58Sr0.4Co0.2Fe0.8O3 d (LSCF) ceramics coated with a porous nano-particulate layer of either gadolinea (Gd2O3), ceria (CeO2) or 20 mol% Gd-doped ceria (GCO) was determined by electrical conductivity relaxation (ECR). The measurements were performed in the temperature range 700–900 C, following pO2-step changes between 0.2 and 0.4 atm. The apparent value of the surface exchange coefficient, kchem, is found to vary with the loading amount and ionic conductivity of the coated phase whilst, as expected, the chemical diffusion coefficient Dchem remains invariant with the applied coating. Partial coverage of the LSCF surface with non-ionic conductive Gd2O3 or CeO2 lowers the value of kchem relative to that observed for bare LSCF, which is attributed to surface blocking effects. In contrast, partial coverage of LSCF with GCO electrolyte particles enhances the apparent value of kchem up to a factor of 6 compared to bare LSCF. The data of pulse isotope exchange (PIE) measurements show that the surface exchange reaction on bare LSCF is predominantly limited by dissociative adsorption of O2. Different mechanisms for the improved oxygen surface exchange kinetics after partially covering the LSCF surface with GCO are discussed

In depth compositional analysis of ceramic (Bi2O3)0.75(Er2O3)0.25 by AES and XPS

The chemical composition of dense ceramics of erbia-stabilized δ-Bi2O3 was analyzed by Auger electron spectroscopy (AES) depth profiling using Ar+ ion sputtering. The relative sensitivity factors (rsf) and sputter rates of bismuth and erbium in this material have been determined by electron probe microanalysis (EPMA) and chemical analysis. These results, supplemented by data from angle resolved X-ray photoelectron spectroscopy (ARXPS), shows a bismuth enrichment at the surface. Evidence has been found for reduction of the bismuth-oxide at the outermost part of the surface layer