Thermoelectric Properties of Al-doped Mg 2 Si Compounds Prepared via Three Kinds of Process for Grain Refinement
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Thermoelectric Properties of Al-doped Mg2Si Compounds Prepared via Three Kinds of Process for Grain Refinement Takashi Itoh1 and Akira Tominaga2 1 Materials Science and Engineering Course, Materials, Physics and Energy Engineering, Nagoya University, Furo-Cho, Chikusa-Ku, Nagoya 464-8603, Japan 2 Nagoya University, [Present Address: Daido Steel Co., Ltd., 2-30 Daido-Cho, Minami-Ku, 4578545, Nagoya, Japan] ABSTRACT Dimagnesium silicide is an eco-friendly thermoelectric compound whose constituent elements of both Mg and Si are non-toxic and exist in abundance on the earth. In this study, we attempted to control the thermal conductivity of Al-doped Mg2Si by grain refinement. Three types of Si powders, i.e., commercial coarse powder, its pulverized powder and commercial fine powder were prepared for synthesizing the Al-doped Mg2Si. Mg powder and one of the Si powders were weighed with Mg/Si mole ratio of 67/33, and mixed with Al powder with amount of 0.1 at.%. The Al-doped Mg2Si compounds were synthesized using three different Si powders via a liquid-solid phase reaction process under unified synthesizing conditions. A part of the synthesized Mg2Si powder using the coarse Si powder was pulverized. Four kinds of Mg2Si powders were sintered by pulse discharge sintering method under unified sintering conditions. The sintered samples of the synthesized Mg2Si powders made from the fine and the milled Si powders and of the milled Mg2Si powder had the grain-refined microstructure. Especially, the sintered sample of the milled Mg2Si powder was effective for grain refinement and for reduction of thermal conductivity, and had the best thermoelectric performance of ZT = 1.15 at 873 K. INTRODUCTION Dimagnesium silicide (Mg2Si) is one of promising candidates among the n-type thermoelectric semiconductors available for thermoelectric power generation using waste heat of temperature range from 600 to 900 K. Advantages of this material are of low density below half of the conventional thermoelectric materials, of abundance of its constituent elements in the Earth’s crust and of harmlessness of its processing by-products. Many researchers have been studied about the dopants for Mg2Si compound theoretically and experimentally for improving the thermoelectric performance of Mg2Si by controlling the carrier concentration and the carrier mobility [1-9]. Aluminum is known as a promising dopant that has a possibility for improving the thermoelectric performance of Mg2Si. Several experimental researches [6-9] about the Al-doped Mg2Si compound prepared with different manufacturing processes have been reported. In our previous work [9], we synthesized the Aldoped Mg2Si compound via a liquid-solid phase reaction (LSPR) process and sintered the compound powder using a pulse discharge sintering (PDS) method. The 1.0 at.% Al-added sample had the maximum dimensionless figure of merit, ZT of 0.86 at 873 K.
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