Fabrication of Mg 2 Si bulk by spark plasma sintering method with Mg 2 Si nano-powder
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Fabrication of Mg2Si bulk by spark plasma sintering method with Mg2Si nano-powder Koya Arai, Keishi Nishio, Norifumi Miyamoto, Kota Sunohara, Tatsuya Sakamoto, Hiroshi Hyodo, Naomi Hirayama, Yasuo Kogo and Tsutomu Iida Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 287-8510, Japan ABSTRACT Mg2Si bulk was fabricated by spark plasma sintering (SPS) nano-powder, and the thermoelectric characteristics of the bulk sample were evaluated at temperatures up to 873 K. A pre-synthesized all-molten commercial polycrystalline Mg2Si source (un-doped n-type semiconductor) was pulverized into powder of 75 ȝm or less. To obtain nano-sized fine powder, the powder was milled using planetary ball mill equipment under an inert atmosphere. Fine Mg2Si nano-powder with a mean grain size of about 500 nm was obtained. XRD analysis confirmed that no MgO existed in the nano-powder. The fine powder was put in a graphite die to obtain a sintering body of Mg2Si and treated by SPS under vacuum conditions. The resulting Mg2Si bulk had high density and did not crack. However, the XRD analysis revealed a small amount of MgO in it. The thermoelectric properties (electrical conductivity, Seebeck coefficient, and thermal conductivity) were measured from room temperature to 873 K. The microstructure of the sintered body was observed by scanning electron microscopy. The maximum dimensionless figure of merit of a sample made from Mg2Si nano-powder was ZT = 0.67 at 873 K. INTRODUCTION Thermoelectric (TE) power generation has recently attracted much interest as a technology for the conversion of waste heat into electric power. The performance of TE materials is usually denoted as dimensionless figure of merit, ZT = S2 ı T / ț, where S is the Seebeck coefficient, ı is electrical conductivity, T is temperature, and ț is thermal conductivity. n-type magnesium silicide (Mg2Si) has been studied by many researchers because of its nontoxicity, environmental friendliness, lightness, and comparatively abundance compared with other TE systems. Moreover, Mg2Si has high electrical conductivity, low thermal conductivity, and a high Seebeck coefficient. Mg2Si is a candidate TE material operating in the temperature range from 600 K to 900 K, a range that corresponds to the operating temperatures of industrial furnaces, automobile exhausts, and incinerators [1-5]. Our group has reported that Mg2Si bulk material, consisting of micron-order grains and prepared by spark plasma sintering, had high thermoelectric performance (ZT=1.08) [6]. A large number of studies have been reported about synthesis method, TE performance of Mg2Si [7-12]. However, it has been reported that nanostructuring can give rise to large reductions in lattice thermal conductivity due to an increase in phonon scattering at interfaces between grains. As a result, this may raise the ZT of thermoelectric materials [13,14]. In this study, Mg2Si nano-powder was fabricated by a planetary ball milling and Mg2Si bulk was fabricated from the nano-powder
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