Oxygen Transport Ceramic Membranes

Abstract

The present quarterly report describes some of the investigations on the structural properties of dense OTM bars provided by Praxair and studies on newer composition of Ti doped LSF.In this report, in situ neutron diffraction was used to characterize the chemical and structural properties of La 0.2 Sr 0.8 Fe 0.55 Ti 0.45 O 3-δ (here after as L2SF55T) specimen, which was subject to measurements of neutron diffraction from room temperature to 900ºC. It was found that space group of R3c yielded a better refinement than a cubic structure of Pm3m. Oxygen occupancy was nearly 3 in the region from room temperature to 700ºC, above which the occupancy decreased due to oxygen loss.Dense OTM bars provided by Praxair were loaded to fracture at varying stress rates. Studies were done at room temperature in air and at 1000ºC in a specified environment to evaluate slow crack growth behavior. The X-Ray data and fracture mechanisms points to non-equilibrium decomposition of the LSFCO OTM membrane. The non-equilibrium conditions could probably be due to the nature of the applied stress field (stressing rates) and leads to transition in crystal structures and increased kinetics of decomposition. The formations of a Brownmillerite or Sr2Fe2O5 type structures, which are orthorhombic are attributed to the ordering of oxygen vacancies. The cubic to orthorhombic transitions leads to 2.6% increase in strains and thus residual stresses generated could influence the fracture behavior of the OTM membrane.Continued investigations on the thermodynamic properties (stability and phase-separation behavior) and total conductivity of prototype membrane materials were carried out. The data are needed together with the kinetic information to develop a complete model for the membrane transport. Previously characterization, stoichiometry and conductivity measurements for samples of La 0.2 Sr 0.8 Fe 0.55 Ti 0.45 O 3-δ were reported. In this report, measurements of the chemical and thermal expansion as a function of temperature and p O2 are described.iv