abstract
The thermogravimetric and Mossbauer spectroscopy studies showed that, at atmospheric oxygen pressure, the oxygen content in Ca2Fe2O5 brownmillerite is very close to stoichiometric at 300-1270 K. The orthorhombic lattice of calcium ferrite undergoes a transition from primitive (space group Pnma) to body-centered (I2mb) at 950-1000 K, which is accompanied with decreasing thermal expansion coefficient (TEC) and increasing activation energy for the total conductivity, predominantly p-type electronic. The steady-state oxygen permeation through dense Ca2Fe2O5 ceramics is limited by the bulk ionic conduction. The ion transference numbers in air vary in the range 0.002-0.007 at 1123-1273 K, increasing with temperature. Analysis of stereological factors, which may affect oxygen diffusivity, suggests a dominant role of the ion jumps along octahedral and, possibly, tetrahedral layers of the brownmillerite structure. The ionic conductivity of calcium ferrite is higher than that of Ca2FeAlO5+delta, but lower compared to the oxygen-deficient perovskite phases based on SrFeO3-delta where the diffusion pathways form a three-dimensional network. The average TECs of Ca2Fe2O5 ceramics, calculated from dilatometric data in air, are 13.1 x 10(-6) K-1 at 370-950 K and 11.3 x 10(-6) K-1 at 970-1270 K. (c) 2006 Elsevier B.V. All rights reserved.
keywords
ELECTRICAL-CONDUCTIVITY; POWDER DIFFRACTION; OXYGEN DIFFUSION; FUEL-CELLS; PEROVSKITE; TRANSPORT; OXIDES; EXPANSION; CERAMICS
subject category
Chemistry; Physics
authors
Shaula, AL; Pivak, YV; Waerenborgh, JC; Gaczynski, P; Yaremchenko, AA; Kharton, VV
our authors
Aleksey Yaremchenko
Principal Researcher