Doping Characteristics of Silver in Mg 2 Si 1-x Ge x Prepared by Plasma Activated Sintering
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1044-U06-14
Doping Characteristics of Silver in Mg2Si1-xGex Prepared by Plasma Activated Sintering Takashi Nemoto1, Junichi Sato1, Tsutomu Iida2, Masayasu Akasaka2, Atsunobu Matsumoto2, Tadao Nakajima1, Keishi Nishio2, and Yoshifumi Takanashi2 1 Nippon Thermostat Co., Ltd., 6-59-2 Nakazato, Kiyose-shi, Tokyo, 204-0003, Japan 2 Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan ABSTRACT Silver (Ag) doped Mg2Si1-xGex ( x=0.1 to 0.4) samples were fabricated using a plasma activated sintering (PAS) method. The doping concentration of Ag was varied from 1 to 5 at.%. Undoped Mg2Si1-xGex exhibits n-type conductivity due to residual impurities in the Mg source material used and unintentionally process-induced impurities. The observed unstable behavior of the Seebeck coefficient of Ag-doped p-type Mg2Si1-xGex ( x ≤ 0.3) in the region of 550 to 650 K, exhibiting a considerable drop in the value and occasional conduction type conversion, was correlated with the specific contaminants. For x~0.4, the observed Seebeck coefficient varied from 0.2 mV/K at 823 K to 0.4 mV/K at room temperature, with no remarkable drop in the value with increasing temperature. An estimated ZT value of 5 at.% Ag doped Mg2Si0.6Ge0.4 was 0.18 at 844 K. It was found that both specific residual impurities and process-induced impurities affected the characteristics of the Seebeck coefficient of Mg2Si1-xGex. INTRODUCTION As one of relevant solutions to remedy the Greenhouse effect, a demands to reduce the usage of fossil fuels have been increasing. Since an immediate abstaining from the use of fossil fuels is impossible to attain in the near future, an alternate solution is believed to be the drastic curtailment of fossil fuel consumption by a remarkable increase in the energy conversion efficiency of power generation by combustion. As alternative power sources, Mg2Si and Mg2Si1xGex are promising candidates for a thermal-to-electric energy-conversion material at operating temperatures ranging from 500 to 800 K [1-7]. Feasible forthcoming applications of these thermoelectric (TE) conversion materials include power generation from waste heat sources such as automobile exhaust and solid oxide fuel cell (SOFC) cogeneration systems in the corresponding temperature range. One important aspect of both Mg2Si and Mg2Si1-xGex is the non-toxicity of source materials and their processing by-products, which provides for safe handling and safe device operation in practical applications, giving no concern regarding potential extended restrictions on hazardous substances. With Bi-doped n-type Mg2Si, we have achieved the maximum value of dimensionless figure-of-merit, ZT, of ~1.0 at ~ 850 K [8]. In order to realize a Mg2Si TE power generator, a combination structure, consisting of n- and p-type Mg2Si , seems to be more efficient in terms of space utility and output power density. However, stable doping parameters to produce p-type conductivity in Mg2Si have not been achieved successfully. On the othe
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