Fabrication of c -axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties
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Satoshi Tanaka, Atsushi Makiya, Zenji Kato, and Keizo Uematsu Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan (Received 2 April 2007; accepted 29 June 2007)
We report the preparation and thermoelectric properties of oriented higher manganese silicide (HMS) with a composition of MnSi1.73 bulk. The grain alignment and densification were achieved by rotating high magnetic field and spark plasma sintering (SPS) techniques, respectively. The easy magnetization axis of MnSi1.73 was found to be c-axis, and the applied magnetic field of 2 T was strong enough to rotate the powder with a mean grain size of 1 m. The c-axis of grains was oriented when applying the magnetic field, and the degree of orientation was further increased after heat treatment. However, a secondary phase that was mono manganese silicide (MnSi) was observed as a result of oxidation on the surface of synthesized powder. The electrical conductivity of the c-axis oriented specimen along the ab-plane was about 40% larger than that for sample processed only by SPS, while the Seebeck coefficient of oriented and nonoriented specimens showed similar values regardless of existence of the second phase. Consequently, the power factor of the c-axis oriented specimen along the ab-plane was enhanced by about 35% compared to the nonoriented one. The proposed approach is found to be very effective not only in obtaining the oriented materials with nonductility but also in enhancing the thermoelectricity.
I. INTRODUCTION
Higher manganese silicides (HMS) with a MnSi2−x formula (0.250 艋 x 艋 0.273) have been studied as one of the promising p-type thermoelectric materials because of their superior corrosion and oxidation resistance and nontoxic elements that are abundantly available.1–6 It has an energy gap of about 0.6 to 0.8 eV and is suitable to be used at an intermediate temperature region (573 to 873 K).7,8 There are several reports concerning different tetragonal phases within MnSi2−x formula that includes Mn4Si7, Mn11Si19, Mn15Si26, Mn26Si45, and Mn27Si47.9,10 The lattice parameters parallel to c-axis direction of these materials are found to be a function of composition, whereas in the perpendicular direction they are almost independent. It is also well known that HMS has a layered structure and thus exhibits a strong anisotropy in its thermoelectric properties. The anisotropy of electrical conductivity () is quite large depending on the temperature of measurement; ⊥/|| ⯝ 5 to 10 were reported a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0360 J. Mater. Res., Vol. 22, No. 10, Oct 2007
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where notations ⊥ and || represent perpendicular and parallel directions to the c-axis, respectively, while the anisotropy in Seebeck coefficient (S) and thermal conductivity () has been reported as S⊥/S|| ⯝ 1.5 and ⊥/|| ⯝ 2, respectively.8–11 It is noted that mono manganese silicide (MnSi) layers segregate as a sec
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