Direct and efficient thermoelectric conversion of solar or geothermal waste heat into electricity requires the development of p- and n-type semiconductors with similar materials properties. Perovskite-type transition metal- oxides are investigated as potential candidates for thermoelectric devices operating at high temperatures as they can possess large positive as well as large negative thermopower depending on their composition. The three parameters defining the thermoelectric figure of merit ZT are in most cases interdependent: The thermopower increases with increasing resistivity. The heat conductivity increases with electric conductivity. Therefore an optimum charge carrier concentration and mobility has to be defined, which is depending on e.g. the substitution level, the spin states of the transition metals, the ligand field, i.e. the crystallographic structure, the valence states of the cations, ionic deficiencies, etc. It was further shown that the heat conductivity can be lowered by enhanced boundary scattering in nanostructured oxides without changing the electronic transport properties.
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