The Effect of Grain Size and Grain Boundary Microstructure on the Oxygen Permeation of Perovskite-type Membranes J. Martynczuk*, H. Wang* † , A. Feldhoff* (*) Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover, Hannover, Germany Abstract The effect of grain size distribution in perovskite- type ceramics on the oxygen permeation behaviour has been investigated by changes of calcination temperature in powder production and sintering time for the ceramics. Decreasing calcination temperature in perovskite powder synthesis delivers fine powders in the nanometer range and smaller grains in the ceramic. We found that grain size distribution in sintered membranes is not constant through membranes since grains in the bulk are smaller compared to those at the surface. With big differences in size, an edge area is observed, where the size differences have to be balanced and which is not favorable for the oxygen permeation of the ceramics. Supplementary, the dwell time during sintering has influence on the microstructure of the ceramic. The longer the dwell time, the further proceeds the grain coarsening, which affects the oxygen permeation in a positive way. Keywords: perovskite, grain size, grain boundary, permeation, microstructure, grain coarsening Introduction Various applications demand ceramic materials that exhibit both high ionic and electronic conductivities. Perovskite-type oxides of the composition ABO 3 can host a lot of different cations on the A and B sites. The introduction of lower valence state ions into the perovskite structure induces oxygen vacancies leading to an improved ionic conductivity benefiting the oxygen permeability. In our group a novel perovskite material with the stoichiometry (Ba 0.5 Sr 0.5 )(Fe 0.8 Zn 0.2 )O 3-δ has been developed that shows high oxygen permeation fluxes (membrane disks: ~ 2.55 mL min -1 cm -2 for the partial catalytic oxidation of methane to syngas (POM)) as well as an excellent phase stability under a 2 % H 2 -Ar atmosphere with an oxygen partial pressure p 02 of less than 1·10 -8 Pa [1, 2]. Doping of the B site of the perovskite structure with a divalent metal like zinc leads to the elimination of non stoichiometric oxygen variations and lattice expansion caused by __________________________________________ the variation of temperature or chemical oxygen potential. For the perovskite synthesis a combined ethylene-diamine-tetraacetic acid (EDTA) and citric acid complexing method has been employed. The fine-scale mixed intermediates appearing during the perovskite synthesis were determined as (Zn 0.2 Fe 0.8 )Fe 2 O 4 spinel and (Ba 0.5 Sr 0.5 )CO 3 carbonate in aragonite modification. The grain sizes of the as-synthesized perovskite are the smaller the lower the applied calcination temperature [3,4]. This profound knowledge of the perovskite formation process opens ways to engage into the microstructure of the underlying ceramic material. Thus, we were able to reduce the common calcination temperature avoiding pre-sintering and therewith facilitating the pressing of the green compact for the ceramic production. Experimental A synthesis method with combined citric acid and EDTA acid as the complexing agents was applied. A given amount of Ba(NO 3 ) 2 powder was dissolved in an aqueous solution of Zn(NO 3 ) 2 , Fe(NO 3 ) 3 , and Sr(NO 3 ) 2 , followed by the addition of EDTA acid. After agitation for a certain time, a proper amount of citric acid was introduced, with the molar ratio of EDTA acid : citric acid : total of metal cations controlled at around 1:1.5:1. After addition of NH 3 ⋅H 2 O, the pH value of the solution was adjusted in the range of 6 to 9 by the addition of supplementary NH 3 ⋅H 2 O. Water was evaporated with stirring in the temperature range of 120 - 150 °C. After evaporation for several hours the transparent solution transformed into a dark purple gel. Further heat treatments were applied at temperatures up to 950 °C. The calcined powders were uniaxially pressed under 140 kN into pellets and sintered pressurelessly at 1150 °C to ceramic discs of 14 mm in diameter and a thickness of 1.15 mm. Scanning electron microscopy (SEM) was employed on a field-emission instrument of the type JEOL JSM-6700F. Secondary electron (SE) micrographs were taken at low excitation voltages of 2 kV. The oxygen permeation was measured in a high- temperature permeation cell [5]. Discs were sealed † Prof. Wang is now at the College of Chemical and Energy Engineering, South China University of Technology, Guangzhou, China.