Strukturelle und elektronische Untersuchungen am hoch sauerstoffdefizitären Mischleiter Ba x Sr 1-x Co 0,8 Fe 0,2 O 3-delta (BSCF)
Oxides of the perovskite structure as BSCF are well known for their ability to accommodate large deviations from the ideal stoichiometry of a perovskite ABO3. This leads to many interesting properties of these materials, such as superconductivity and high oxygen permeability, whereas these properties are also highly dependent on the surrounding oxygen partial pressure and temperature. In this family of materials, BSCF stands out as it accommodates an extremely high oxygen nonstoichiometry delta which leads to one of the highest oxygen permeation rate know thus far. This makes BSCF a promising candidate for industrial application as material for oxygen separation membranes. Unfortunately, BSCF tends to decompose into a hexagonal polymorph of the perovskite structure at typical application temperatures of 800°C, which could pose problems for an long term industrial application. In this work, two central questions were tackled: On the one hand, the question how the electronic compensation of the oxygen nonstoichiometry is achieved, on the other hand the nature of the decomposition of BSCF into a hexagonal phase. The electronic compensation was investigated by means of in situ X-Ray Absorption Spectroscopy, where the valence states of Co and Fe were determined as a function of oxygen partial pressure and temperature. It has been found that the charge compensation of the high oxygen nonstoichiometry is mainly achieved by the reduction of Co, and the further dependence of the valence states on temperature and oxygen partial pressure is very complex. Additionally it has been found that the charge compensation of the charge left behind by removed oxygen is not only done be reduction of the B-site cations Co and Fe, but also that the valence of oxygen changes during that process. A semiempiric band structure model has been suggested to describe this behaviour. The decomposition of BSCF has been investigated by means of X-Ray Diffraction and Electron Microscopy. The quantitative evaluation of the X-Ray Diffraction data as a function of annealing time at 800°C yielded the decomposition kinetics which shows that BSCF decomposes into a two phase mixture following and Avrami type of kinetics. The equilibrium composition of the two phase mixture is very close to 1:1. The electron microscopic investigations showed that the cation stoichiometries relative to the parent material changed in the product phases; the resulting cubic phase is depleted of Ba and Co, leading to an enrichment of Ba and Co in the hexagonal phase. This is in accordance with expectations, as Ba(Fe,Co)O3 favours hexagonal polymorphs of the perovskite structure whereas Sr(Fe,Co) mostly crystallizes in the cubic structure. Furthermore the electron microscopic investigations showed that the hexagonal phase is predominantly formed at the grain boundaries which is in accordance with the kinetics found fort the decomposition.
info:eu-repo/classification/ddc/540, Gemischter Leiter, Röntgenabsorption, Perowskit, Röntgenbeugung, Chemie, perovskite, oxygen separation membrane, X-ray absorption, X-ray diffraction
Networked Digital Library of Theses and Dissertations
Publikationsserver der RWTH Aachen University