Mg_2Si熱電発電実用化への取り組み(<特集>熱電発電技術の最新動向)
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Mg_2Si is a promising candidate for practical thermoelectric power generation; however, concerns about it degrading due to oxidation has until now prevented its use in practical applications. Nevertheless, it has several promising features, such as the abundance of its constituent elements, its non-toxicity, and the facts that it is light weight and has the capability of generating power. These features have motivated the development of Mg_2Si for several years now. We have attempted to synthesize stoichiometric and single phase Mg_2Si sources initiated from molten materials in order to reduce the possibility of oxidation owing to residual metal-Mg. This is because the remaining metal Mg in Mg_2Si has been known to disturb the reproducibility of the sintering processes and consequently undermine the aging characteristics of Mg_2Si. In this report we show the present status in the development of residual metal-Mg free Mg_2Si, focusing on the material synthesis, the doping characteristics, the sintering adaptability, the metal electrode compatibility, and the durability at elevated temperatures. Commercially available Mg_2Si is fabricated at 1378 K, beyond the congruent melting point of Mg_2Si at 1358 K. It is initiated from a mixture of pure Mg and Si, while waste Si sludge and recycled Mg-alloys, such as those from the Mg-Al-Zn-Mn system, can also be used as sources. For the dopants, Bi, Al and Sb give n-type conductivity, while Ag gives p-type, and Cu amphoteric characteristics. The atmospheric durability of Mg_2Si, with the hot-side at 873K and the cool-side at 373K, was more than 11,000 h when doped with Sb, while degradation was observed with some of the dopants. The plasma activated sintering (PAS) method allowed sintered pellets as large as 40 mm in diameter to be made. Since sintered Mg_2Si exhibits a lack of residual metal-Mg, wet post-processes even with hot water can be used with no significant degradation.
- 2013-05-20
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