Electronic conductivity, oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr0.7Ce0.3Mn1−xAlxO3−δ is approximately 15%. The incorporation of Al3+ 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 Sr0.7Ce0.3Mn1−xAlxO3−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 Sr0.7Ce0.3Mn0.9Al0.1O3−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(O2) below 10−4 to 10−2 atm. The oxygen permeability of dense Sr0.7Ce0.3Mn1−xAlxO3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO3-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr0.7Ce0.3Mn1−xAlxO3−δ ceramics in air are (10.8–11.8) × 10−6 K−1.