Electronic conductivity, oxygen permeability and thermal expansion of Sr 0.7Ce 0.3Mn 1− xAl xO 3− δ

Abstract

The maximum solubility of aluminum cations in the perovskite lattice of Sr 0.7Ce 0.3Mn 1− x Al x O 3− δ is approximately 15%. The incorporation of Al 3+ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr 0.7Ce 0.3Mn 1− x Al x O 3− δ ( x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr 0.7Ce 0.3Mn 0.9Al 0.1O 3− δ , measured in the oxygen partial pressure range from 10 −18 to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O 2) below 10 −4 to 10 −2 atm. The oxygen permeability of dense Sr 0.7Ce 0.3Mn 1− x Al x O 3− δ ( x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO 3-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr 0.7Ce 0.3Mn 1− x Al x O 3− δ ceramics in air are (10.8–11.8) × 10 −6 K −1.