Large Thermopower in Metallic Oxides: Misfit Cobaltites and Mangano-Ruthenates
Abstract
Two different kinds of metal transition oxides have been studied for their large thermopower values. The first one corresponds to the Tl-based misfit cobaltite which is a hole-doped metal. We demonstrate that the partial Bi-substitution for Tl in this phase induces an increase of the room temperature (RT) thermopower (TEP) value. Same result is obtained with the new Pb1/3SrCoO3+delta misfit corresponding to the Tl complete replacement by lead. Simultaneously, the T dependence of their resistivity exhibits a re-entrance below 70-90K where a large negative magnetoresistance is observed. Magnetic measurements reveal a strong interplay between spins and charges for this class of materials. Electron-doped (n-type) perovskite manganites are a second class of potential candidates for applications. In particular, the Ru4+/5+ substitution for Mn in the CaMnO3 semi-conductor induces a drastic drop of the resistivity values. Metals with large RT TEP values and not too large thermal conductivities are generated. A comparison with best known materials, Bi2Te3 and NaCo2O4 is made.
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