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Donor-substituted strontium titanate ceramics demonstrate one of the most promising performances among n-type oxide thermoelectrics. Here we report a marked improvement of the thermoelectric properties in rare-earth substituted titanates Sr0.9R0.1TiO3±δ (R = La, Ce, Pr, Nd, Sm, Gd, Dy, Y) to achieve maximal ZT values of as high as 0.42 at 1190 K < T < 1225 K, prepared via a conventional solid state route followed by sintering under strongly reducing conditions (10%H2-90%N2, 1773 K). As a result of complex defect chemistry, both electrical and thermal properties were found to be dependent on the nature of the rare-earth cation and exhibit an apparent correlation with the unit cell size. High power factors of 1350-1550 μW m(-1) K(-2) at 400-550 K were observed for R = Nd, Sm, Pr and Y, being among the largest reported so far for n-type conducting bulk-ceramic SrTiO3-based materials. Attractive ZT values at high temperatures arise primarily from low thermal conductivity, which, in turn, stem from effective phonon scattering in oxygen-deficient perovskite layers formed upon reduction. The results suggest that highly-reducing conditions are essential and should be employed, whenever possible, in other related micro/nanostructural engineering approaches to suppress the thermal conductivity in target titanate-based ceramics.

The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1 - x M x O4+ δ (M = Fe, Co, Cu; x = 0−0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10-4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (εC) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10-3 to 6 × 10-3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10-6 K-1, does not exceed 5%. The high-temperature X-ray diffraction studies showed that this behavior results from strongly anisotropic expansion of the K2NiF4-type lattice, namely the opposing variations of the unit-cell parameters on changing oxygen stoichiometry.

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