Single phase, cubic perovskites of composition La 0.2 Sr 0.8 Fe 0.8 B 0.2 O 3−δ , B = Al, Ga, Cr, Ti, Ta and Nb, were prepared by spray pyrolysis from aqueous precursor solutions. The effect of B-site substitution on the stability in a H 2 containing atmosphere was investigated using temperature programmed reduction (TPR). High temperature X-ray diffraction (HT-XRD) was performed under conditions similar to those in the TPR experiments. La 0.2 Sr 0.8 FeO 3−δ forms a brownmillerite-type anion-deficient ordered perovskite upon reduction at T ≈ 500• C. B-site substitution suppresses oxygen vacancy ordering, maintaining a perovskite structure under the above reducing conditions up to T ≈ 700• C. Trends in the kinetics of decomposition of B-site substituted La 0.2 Sr 0.8 FeO 3−δ are discussed.

In this work, the validity of electrical conductivity relaxation (ECR) as a method for the assessment of chemical surface exchange, k chem , and bulk diffusion, D chem , coefficients is investigated with respect to mass transport limitations in the gas phase. A model encompassing both the oxygen surface exchange, mass transport in the bulk sample and the gas phase was set up and solved under different conditions using finite element software. It is found that the transport of oxygen in the gas phase is insufficient at low oxygen partial pressures, causing a concentration boundary layer at the sample surface to develop. This significantly decreases the driving force for oxygen exchange. The effect of mass transport limitations on the computed apparent transport coefficients is shown to be pronounced and surface exchange coefficients are shown to deviate as much as one order of magnitude from the set values. When mass transport limitations are pronounced, a discrepancy between oxidation and reduction values of the apparent k chem and D chem is evident and modelled apparent activation energies for k chem are shown to decrease significantly. The validity of the apparent transport coefficients can be improved by changing the experimental parameters, however the surface exchange coefficient is extremely sensitive to insufficient transport of oxygen in the gas phase and improvements are in general marginal. A criteria for the validity of D chem is introduced while no such measure could be introduced for k chem . The effect of experimental parameters and material properties on mass transport limitations are presented and general recommendations concerning the assessment of k chem and D chem are given.

The oxygen non-stoichiometry was determined by coulometric titration for the perovskite oxides La 0.2 Sr 0.8 FeO 3−δ and La 0.2 Sr 0.8 Fe 0.8 B 0.2 O 3−δ (B = Al 3+ , Ti 4+ and Ta 5+ ) in the temperature range 600• C and the oxygen partial pressure range: 1 · 10 −15 ≤ p O 2 ≤ 0.209 atm. The non-stoichiometry (δ) is observed to decrease with B-site substitution of Fe. The data can be well fitted with simple defect chemistry models. At low oxygen non-stoichiometry all compositions show a deviation from a localized electrons defect model. The standard and partial molar thermodynamic quantities were obtained and a gradual transition from localized to itinerant electrons with decreasing non-stoichiometry is proposed from the δ-dependency of the configurational entropy. The absolute value of the enthalpy of oxidation decreases upon B-site substitution of Fe proposing a decreased thermodynamic stability for the substituted materials. The electrical conductivity was measured at T = 900• C in the oxygen partial pressure range: 1 · 10 −17 ≤ p O 2 ≤ 0.209 atm. The electrical conductivity and charge carrier mobility decrease upon 20% substitution of Fe roughly by a factor of 2, but do not show a significant dependence on the nature of the B-site dopant.

La0.2Sr0.8Fe0.8Ta0.2O3-(LSFT) is a mixed ionic electronic conductor (MIEC) at elevated temperatures and as such a candidate material for applications both in syn-gas synthesis and as electrodes in solid oxide fuel cells (SOFC). This study addresses the variation in oxygen permeation rates for LSFT symmetric-and asymmetric-membranes at temperatures between 800 and 1000 o C with and without surface modification. The surface was strutured in two different scales, macro (porous LSFT-layer) and micro (acid etching). The asymmetric membranes showed a significant variation in permeation rate with surface treatment with increasing rate in the sequence from non-treated to macro-structured and finally micro structured, corresponding to oxygen permeation being controlled by surface exchange and gas diffusion. It was found that the permeation rate was sensitive to the gas sweep rate when H2-mixtures was introduced on the permeate side, which was rationalized by adsorption of H2O-molucules on the surface hampering the exchange of oxygen.

High temperature dense ceramic membranes made from mixed ionic/electronic conducting perovskite ceramics represent a high potential as a reliable source for oxygen and syngas production. In this work, La 0.2 Sr 0.8 Fe 0.8 Ta 0.2 O 3- based thin film perovskite system was evaluated, addressing the effect of structural surface modification on oxygen permeation rates. Membranes with 20 µm thick dense functional layer and varying surface area on permeate side were fabricated by a dip coating technique. Oxygen permeation was measured at temperatures between 800 and 1000 C by using varying partial pressure of O 2 as a driving force. Maximum O 2 flux values of 5.8 and 8.7 mlcm -2 min -1 were recorded for smooth and structured permeation surfaces, respectively. This indicates that the surface roughness, which corresponds to an increase in surface area at the permeate side; can lead to a significant improvement in oxygen permeation rates reaching 50 % at 1000 o C.