Synthesis and High Temperature Thermoelectric Properties of Alkaline-Earth Metal Hexaborides MB 6 (M=Ca, Sr, Ba)
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Synthesis and High Temperature Thermoelectric Properties of Alkaline-Earth Metal Hexaborides MB6 (M=Ca, Sr, Ba) Masatoshi Takeda1, Yosuke Kurita1, Keisuke Yokoyama1, Takahiro Miura1, Tsuneo Suzuki2, Hisayuki Suematsu2, Weihua Jiang2, and Kiyoshi Yatsui2 1 Department of Mechanical Engineering, Nagaoka University of Technology, Kamitomioka, Nagaoka, 940-2188, Japan 2 Extreme Energy-Density Research Institute, Nagaoka University of Technology, Kamitomioka, Nagaoka, 940-2188, Japan ABSTRACT Polycrystalline alkaline-earth hexaborides (MB6: M=Ca, Sr, Ba) were synthesized and their thermoelectric and transport properties were examined to discuss their possibility as high temperature thermoelectric materials. Hall measurements showed that carrier concentration of the BaB6 was the highest among the three hexaborides and that of CaB6 was the lowest. Substitution of part of the alkaline earth metals with one of the others changed the carrier concentration of the hexaboride. As the carrier concentration increased, Seebeck coefficient increased and electrical conductivity decreased. These results suggest that the thermoelectric properties of the divalent hexaborides depend largely on the carrier concentration, and optimum carrier concentration which gives maximum power factor was estimated to be approximately 2x1026 m-3. Consequently, such a substitution enables us to control Seebeck coefficient and electrical conductivity of the hexaborides, and will also be effective to reduce the lattice heat conduction due to the alloying effect. A thermoelectric device was fabricated using SrB6 and boron carbide thin films as n-type and p-type elements, respectively. To the best of our knowledge, this is the first demonstration of a thermoelectric device composed of only boron-rich solids. INTRODUCTION Thermoelectric (TE) effects have received renewed attention in recent years, because the effects can be applicable to electric power generation from waste heat and also to cooling device without using any refrigeration medium. The efficiency of a TE device is fundamentally limited by the material properties of the n- and p-type materials composing the TE device. The inherent efficiency of any TE materials is determined by a dimensionless parameter ZT, given by ZT=α2σT/κ, where α, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. For the electric power generation using TE devices, high-temperature TE materials are desired, because higher operating temperature and larger temperature difference lead to higher conversion efficiency. Boron-rich semiconductors such as boron carbide and β-rhombohedral boron (β-boron) have been studied as candidate materials for high-temperature TE applications because of their high Seebeck coefficient, low thermal conductivity and stability at high temperature [1-6]. Most of them are p-type materials, and they possess relatively high TE performance at high temperatures. Some n-type ones were found in metal-doped boron carbide and β-
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