Thermodynamic and transport properties of mixed ionic-electronic conducting perovskite oxides
Prof. Dr. Ir. H. Verweij
Dr. H.J.M. Bouwmeester
Solid state thermodynamics and associated transport properties are studied of mixed ionic-electronic conducting oxides. The chemical potential and the partial energy and entropy of O2 in La1-xSrxCoO3-δ were measured as a function of δ and x by coulometric titration. The observed behaviour is interpreted as increase in the Fermi-level upon filling up states in a broad electron band. The apparent density of states at the Fermi-level is in good agreement with XPS data. At either increasing δ or lowering temperature ordering occurs of oxygen vacancies into microdomains. Chemical diffusion coefficients in La1-xSrxCoO3-δ from transient titration currents are in agreement with theoretical considerations.
The temperature- and pO2-dependence of δ were correlated with electrical conductivity, σel, and Seebeck coefficient, α, of La1-xSrxCoO3-δ (x=0.2, 0.4, 0.7). The data indicate that α in these solids is positive but small, typically below 25 µV/°C, corresponding to a low entropy of p-type charge carriers. At high temperature the α is correlated with δ. The data indicate a decreasing electron hole entropy with increasing charge carrier concentration. For x=0.4 and 0.7, σel decreases with temperature, indicative of metallic-like conduction. At high temperatures σel is nearly independent of temperature, but is found to depend strongly on changes in δ due to its relation with the electron hole concentration. The electron-hole mobility calculated from the slope of the σ-δ plots is about 1 cm2/V s.