Mechanical Properties and Fracture Behavior of Mg 2 Si after Heat Exposure
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Mechanical Properties and Fracture Behavior of Mg2Si after Heat Exposure Takashi Nakamura1, Ryo Inoue1, Shuhei Hasegawa1, Yauso Kogo1 and Tsutomu Iida1 1 Department of Materials Science and Technology, Tokyo University of Science, 6-3-1, Niijyuku, Katsushika-ku, Tokyo, Japan. 125-8585 ABSTRACT The mechanical properties of as-sintered and heat-exposed polycrystalline Mg2Si were investigated. The strength of the as-fabricated specimen depends on the surface flaws. After heat exposure in Ar atmosphere, oxidized products formed, mainly consisting of MgO. The strength increased after the heat exposure, and the surface oxidation product filled the surface cracks. After the surface oxidation products were removed, the strength and TE properties were the same as those of the as-fabricated specimens. 1. INTRODUCTION In order to capture energy from waste heat sources e.g., automotive exhaust [1, 2], various thermoelectric (hereafter denoted as TE) materials have been developed. The efficiency of TE materials is characterized by a dimensionless figure of merit, ZT = S2σ/k, where S represents the Seebeck coefficient, T is the absolute temperature, σ is the electrical conductivity, and k is the thermal conductivity. Among TE materials, the intermetallic compound magnesium silicide (Mg2Si) has been investigated for high-performance TE generators because Mg2Si is a narrow-band-gap semiconductor (Eg*=0.77 eV [3]). The ZT values of various Mg2Si-based materials have been experimentally determined, and these materials exhibit excellent thermoelectric properties at moderate temperatures. For example, the ZT values of Sb-doped and Ni-particle-dispersed Mg2Si are ~0.75 at ~550°C [4] and ~0.97 at 600°C [5], respectively. For practical applications of TE materials, a low density (below ~1.94 g/cm3) and high melting temperature (~1085±1°C [6]) are also attractive. Mg2Si can attain greater ZT values because of its high charge mobilities, large effective mass, and the comparatively low lattice component of its thermal conductivity. The oxidation of Mg2Si is a critical problem for practical applications because its TE properties are strongly influenced by MgO, which is formed by the oxidation of Mg2Si at 300°C. MgO has a higher thermal conductivity than Mg2Si and thus can significantly influence its electrical transport properties [4,7]. Boor et al. [4] reported a reduction in the electrical transport efficiency of ~30% with only a few volume percent of MgO. In order to improve the oxidation resistance of Mg2Si, surface coatings composed of β-FeSi2 [7] and various oxides [8] have been developed. While many studies have investigated the degradation of TE properties at the operation temperature, mechanical degradation during operation has rarely been studied. Some mechanical properties have been reported [9-12]; however, the change in mechanical properties of Mg2Si after heat exposure has been reported. To understand the service life of TE modules, the degradation of the mechanical properties of Mg2Si during operation should be examined. The ob
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