Increase in the density of Sr2Fe1.5Mo0.5O6-δ membranes through an excess of iron oxide: The effect of iron oxide on transport and kinetic parameters

Abstract

Due to its widespread application in modern industry, the ongoing development of all the most important technological directions in the field of sensors, oxygen membranes, and reforming is undoubtedly a promising trend in world research and development (R&D). The search for modern highly oxygen-conducting materials used as oxygen separation membranes continues today. In this work, we investigated the following well-proven oxygen-conducting material, such as strontium ferrite-molybdate Sr2Fe1.5Mo0.5O6-δ. As an alternative for obtaining high-density ceramics, we used a co-synthesis method with a sintering additive (Fe2O3). We investigated the influence of sintering additive on the structural characteristics by certifying the phase composition by X-ray phase analysis; matrix continuity and volume distribution of Fe2O3 by secondary electron microscopy using energy dispersive X-Ray analysis; and elemental analysis of ceramic surface by X-ray photoelectron spectroscopy. The influence of Fe2O3 on the electrical conductivity was investigated using the four-probe DC method in the temperature range 100–800 °C in air. The kinetic dependences of 18O isotope-labeled oxygen on time were also obtained in the temperature range 600–800 °C and absolute oxygen pressure of 10−2 atm. The rate of heterogeneous oxygen exchange, the oxygen diffusion coefficient, and the corresponding rates of elementary processes of dissociative adsorption and incorporation of oxygen are calculated. Furthermore, it was revealed that the diffusion coefficient in the near-surface area significantly differs from the bulk one. As a consequence, this paper evaluates the observed effect and carefully analyzes the results obtained. All considered material parameters were analyzed and the regularities of the influence of surface composition on the rate-determining step of oxygen exchange were revealed. This research has made it possible to obtain dense ceramics at low temperatures and to identify a step that determines the rate of the oxygen exchange process. Finally, how strontium molybdate based thin layer on the surface and between the grains of the polycrystalline strontium ferrite-molybdates influence on the oxygen surface exchange and diffusivity is considered. The presence of such a phase in the form of a nanosized layer on the surface was convincingly proven by X-ray photoelectron spectroscopy and a grazing incidence X-Ray Diffraction method. Furthermore, the corresponding layer thickness, as well as the oxygen diffusion coefficients of the near-surface layer and the interface layer, were estimated experimentally and by modeling.