Complex Oxide Materials for Potential Thermoelectric Applications
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Complex Oxide
Materials for Potential Thermoelectric Applications
Kunihito Koumoto, Ichiro Terasaki, and Ryoji Funahashi Abstract Layered CoO2 materials are excellent candidates for potential thermoelectric applications. Their single crystals show good p-type thermoelectric properties at temperatures higher than 800 K in air. Recently, the mechanism of thermoelectric properties was clarified through a discussion of electronic and crystallographic structure. In order to fabricate thermoelectric modules possessing good power-generation properties, thermoelectric materials and metallic electrodes must be connected with low contact resistance and high mechanical strength. It has been found that good junctions can be formed using Ag paste including p- and n-type oxide powders. The role of spin entropy contributions to thermopower will be presented, in connection with strong electron correlation and triangular lattices. Keywords: oxide, thermal conductivity, thermoelectricity.
Introduction Waste heat from automobiles, factories, and similar sources offers a high-quality energy source equal to about 70% of the total primary energy, but it is difficult to reclaim because its sources are small and widely dispersed. Thermoelectric (TE) generation systems offer the only viable method of overcoming these problems by converting heat energy directly into electrical energy irrespective of source size and without the use of moving parts or the production of environmentally deleterious wastes. The requirements placed on materials needed for this task, however, are not easily satisfied. Not only must they possess a high conversion efficiency, but they must also be composed of nontoxic and abundantly available elemental materials having high chemical stability in air, even at temperatures of 800–1000 K. Oxide materials, such as those used in the present
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study, are particularly promising for TE applications because of their stability even at high temperatures in air. The challenge of creating novel TE oxides has recently motivated investigations from various materials viewpoints. As it is difficult to control an electronic system and a phonon system simultaneously in a simple crystalline field, a complex crystal composed of more than two nanoblocks with different compositions and structural symmetries, the so-called hybrid crystal, is considered to be effective in controlling electron and phonon transport separately, thus enhancing the total conversion efficiency. Layered cobalt oxides, such as NaxCoO2 and Ca3Co4O9, can be regarded as consisting of complex crystalline fields.1–4 In these oxides, CoO2 nanosheets possessing a strongly correlated electron system serve
as electronic transport layers, while sodium ion nanoblock layers or calcium cobalt oxide misfit layers serve as phononscattering regions to achieve low thermal conductivity.5,6 This fact inspired us to generate high-performance TE materials from natural superlattices or hybrid crystals that are composed of the periodic arrangement of nanoblocks or nanosheets poss
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