Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

• Autor(a) principal: Kharton, V.V.
• Data de Publicação: 2003
• Outros Autores: Marozau, I.P., Vyshatko, N.P., Shaula, A.L., Viskup, A.P., Naumovich, E.N., Marques, F.M.B.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/21985
• topic: A. Oxides; D. Crystal structure; D. Electrical properties; D. Ionic conductivity; D. Thermal expansion; Activation energy; Crystal structure; Diffusion; Oxygen; Stoichiometry; Thermal expansion; Tetrahedral layers; Calcium compounds

The oxygen permeability of CaAl0.5Fe0.5O2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2 × 10-3 to 5 × 10-3. The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+δ lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+δ ceramics in air are 16.7 × 10-6 and 12.6 × 10-6 K-1 at 370-850 and 930-1300 K, respectively.