# Phasenchemie und Transporteigenschaften co-substituierter, mischleitender Membranenmaterialien auf der Basis von Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3-\delta}$

In the Oxyfuel process, pure oxygen is applied for the coal combustion process instead of air. The carbon dioxide arising from this process is easy to deposit because of its high concentration. In order to produce pure oxygen, gas-tight ceramic membranes with perovskite structure attained a lot of attention. A very high oxygen permeation flux was demonstrated by Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3-\delta}$ (BSCF), but it is only achieved when the temperature is higher than 1123K, otherwise a decrease of oxygen permeation flux appeared because of the phase decomposition into a non-cubic perovskite. The aim of this work is to investigate the effects of different dopants on B-sites of BSCF. Zirconium, titan, and yttrium substitutions with concentrations from 0 mol-% to 20 mol-% were analyzed. 3 mol-% scandium dopants were also examined. The measurement of X ray diffraction diagrams and REM revealed the different maximum solubilities of dopants. For Zr and Ti dopants the solubilities were both 3 mol-%, at the same concentration the maximum oxygen permeation flux was also reached. Considering the high solubility of Yttrium of 15 mol-%, it was very interesting to find out that the maximum permeation flux was reached at 3 mol-%. Possible mechanisms were discussed. Compared with pure BSCF, Zr and Ti dopants with 3 mol-% show no increase in oxygen permeation flux. 3 mol-% Y and Sc dopants made oxygen permeation flux increase slightly. Furthermore, the stability of all doped BSCF was examined at low temperature. Permeation measurements were carried out at 700 °C for 50 hours. By means of XRD the phase transformation of the pouders kept at 800° for three weeks was measured. The results were shown quantitatively and compared with the literature values. Compared with pure BSCF, an increased phase stability was found for 3 mol-% Zr and Ti dopants, the effects with Y and Sc dopants were very limited. All surfaces with 3 mol-% dopants exhibited a strong decrease of decomposition products compared with pure BSCF. The high bond energy between dopant ions and oxygen ions led to a more stable cubic structure, but the large radius of doped ions was also helpful for the stability of the perovskite phase.