R.H.E. van Doorn

Structural aspects of the ionic conductivity of La1xSrxCoO3-d

The crystal structure of La1−xSrxCoO3−δ in the composition range 0≤x≤0.8 was investigated by means of powder XRD. A more detailed study was performed on the x=0.7 composition by neutron diffraction, HRTEM, SAED, image analysis and PEELS. The XRD results indicate a long-range cubic perovskite symmetry (spacegroup Pm m) for x>0.5 and a rhombohedrally distorted perovskite structure (spacegroup R c) for x≤0.5. Neutron diffraction of La0.3Sr0.7CoO3−δ, annealed at 650°C in air showed ideal cubic symmetry. HRTEM and SAED studies, however, revealed microdomains with an ac×ac×2ac superstructure in addition to regions without such a superstructure. The superstructure could be explained by assuming the removal of all the oxygen ions from every other (001) AO4/4 plane. PEELS measurements indicated a lower oxidation state for Co in the superstructure regions than in the non-superstructure regions. The importance of oxygen-vacancy ordering on the magnitude of ionic conductivity is discussed.

Oxygen permeation of La0.3Sr0.7CoO3-δ

The oxygen permeability of dense La0.3Sr0.7CoO3−δ membranes has been measured in the range 750–1100°C under various oxygen partial pressure gradients. A sweep gas method was employed. Results indicate that in the range of thickness 0.057–0.215 cm used in the present study, the oxygen flux is predominantly controlled by bulk diffusion across the membrane. The measured activation energy is 60 kJ mol−1. By fitting the permeation data for various thicknesses to the transport equation obtained upon assuming linear kinetics for the surface exchange reactions and bulk ionic transport, we could derive the oxygen ionic conductivity and the characteristic membrane thickness. The latter quantity determines the transition from predominant control by diffusion to that by surface exchange. The ionic conductivity is about 0.5 S cm−1 at 1000°C. The characteristic thickness is extrapolated at a value of about 80 μm.

Kinetic decomposition of La0.3Sr0.7CoO3−δ perovskite membranes during oxygen permeation

In this paper, a study is presented towards the stability of oxygen permeable membranes of perovskite La0.3Sr0.7CoO3−δ in an oxygen pressure gradient. It is shown that phase separation occurs at the oxygen-lean side of the membrane, at 900°C, when the membrane is exposed to streams of air and inert gas at opposite sides of the membrane. It is absent when the (homogenous) oxide is annealed in either flowing air or nitrogen for several days, indicating that the phase separation is induced by the dynamic forces acting during oxygen permeation.

Surface analysis of doped lanthanide cobalt perovskites by X-ray photoelectron spectroscopy

Laboratory for Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, 7500 AE Enschede, The Netherlands High oxygen fluxes through mixed-conducting oxi- des with fluoride and perovskite structure are well known [1-3]. According to [4] the selective feeding of oxygen into high temperature electrochemical reactors generally comprises: (i) providing an electrochemical cell comprising a first zone and a second zone separated from the first by a solid multi- component membrane; (ii) raising the temperature of the electrochemical cell from about 300 °C to about 1400 °C; (iii) passing the oxygen-containing gas in surface contact with the membrance, and (iv) passing the methane in contact with the membrane surface in the second zone. It is evident that under a high temperamre working regime, the permeability of oxygen, as well as all of the other physical and chemical parameters, are very important for the realization of a highly effective chemical process at a large oxygen partial pressure gradient. The catalytic activity of such an oxidation process, e.g. of CO, methane, etc., is a function of the chemical state of the surface elements and decreases monotonically with the decrease of the surface atomic ratio of the 3-rd elements. In the work reported here the chemical state of the surface of an Sr-doped lanthanide cobalt perovskite (La0.3Sr0.7CoO3_~, 3LSC) was the object of an investigation carried out with the help of electron spectroscopy for chemical analysis (ESCA), includ- ing the X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) methods. In our work the method of preparation of perovskite ceramic samples developed by van Doorn and Bouwmeester [5] was accepted. The materials used were La(NO3)3.6H20,