Advanced Energy Materials and Research

Biography

Biography: Klaus D. Becker

Abstract

Although air is usually used for fuel combustion, it is well known that oxygen enrichment of combustion air enhances the combustion efficiency. Cryogenic oxygen separation is well established for oxygen production at large scale but its costs are relatively high compared to the economic benefit caused by an improved combustion efficiency. Therefore, the development of alternative oxygen separation processes is still an issue. One of the most promising ceramic materials for oxygen separation membranes is the mixed ionic electronic conducting (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3-δ (BSCF) with its extremely high oxygen vacancy concentration. For 900°C, for example, a value of δ = 0.8 has been reported at an oxygen partial pressure of 10⁻³ bar [1,2].

We report on ⁵⁷Fe Mössbauer in situ studies of the mixed ionic electronic conducting (MIEC) oxide functional materials (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3-δ, BSCF, conducted between room temperature and 1000 °C in atmospheres of variable oxygen content in order to obtain insight into local coordination and valence of iron at working conditions and into the distribution of oxygen vacancies on their different sites. The magnetically-split room-temperature Mössbauer spectra of BSCF reveal the presence of two inequivalent iron species [3]. Evaluation of signal intensities confirms results from theoretical computations on vacancy formation in BSCF which indicate that formation energies of the various types of oxygen vacancies differ by the order of 0.1 eV only [4,5]. The analysis also shows that the distribution of vacancies is far from random [3]. In the paramagnetic high-temperature phase (T ≥ 315 °C), the quadrupole-split signals demonstrate that local symmetry at iron sites is lower than cubic. At 700, 850, and 1000 °C, Mössbauer centre shifts as well as quadrupole splittings are discussed in respect to stoichiometry-related changes in valence and local coordination of the iron probes.