Determination of the oxygen ion diffusion coefficients in mixed conductors by the volumetric measurement of the reoxidation kinetics

Abstract

Abstract Mixed conducting ceramics (ionic and electronic conducting oxides) have a high application potential for the oxygen separation from oxygen containing gas mixtures (mostly air). Because the ion diffusion controls frequently the rate of the total permeation flux, a simple and fast determination of the oxygen diffusion coefficient is the prerequisite for an efficient material screening. In this work, the volumetric measurement of the reoxidation of chemically reduced mixed conductors was used for the determination of oxygen ion diffusion coefficients. The described method has advantages compared to the frequently used mass relaxation technique, e.g. a higher measuring effect, the suitability for fast processes (by use of large samples) and a high resistance to abrupt pressure changes. The checkup of the measurement method on the reference materials SrFe 1− x Co x O z ( x =0.6) and La 0.79 Sr 0.2 MnO 3+ z indicates a sufficient agreement with previous published values. The experimental results show a high internal consistency, as it is essential for a screening method. Diffusion coefficients are independently on the oxygen concentration over a wide range of the oxygen stoichiometry for all tested materials. Low activation energies in the range 0.10–0.5 eV were found. These latter results are opposed to the majority of publications on mixed conducting oxides. Additionally, new promising perovskite compositions in the system Sr 1− x Ca x Mn 1− y Fe y O 3− z were presented.