Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions |
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| Institution: | 1. Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal;2. Chemistry Department, ITN/CFMC-UL, Estrada Nacional 10, P-2686-953 Sacavém, Portugal;1. Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan;2. Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan;1. Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600036, India;2. Crystal Growth Centre, Anna University, Chennai 600025, India;1. Department of Materials and Mineral Resources Engineering National Taipei University of Technology, Taipei 106, Taiwan, ROC;2. Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, ROC;1. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China;2. Departments of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA;3. College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310023, China;4. Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA;5. Mount Holyoke College, Department of Earth & Environment, South Hadley, MA 01075, USA |
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| Abstract: | The thermogravimetric and Mössbauer 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+δ, but lower compared to the oxygen-deficient perovskite phases based on SrFeO3?δ where the diffusion pathways form a three-dimensional network. The average TECs of Ca2Fe2O5 ceramics, calculated from dilatometric data in air, are 13.1 × 10? 6 K? 1 at 370–950 K and 11.3 × 10? 6 K? 1 at 970–1270 K. |
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