Synthesis of Boron-Rich Metal Borides and Their Thermoelectric Properties
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Synthesis of Boron-Rich Metal Borides and Their Thermoelectric Properties Masatoshi Takeda, Ferrer Domingo, Takahiro Miura, and Tadahiro Fukuda Department of Mechanical Engineering, Nagaoka University of Technology, Kamitomioka, Nagaoka, 940-2188, Japan ABSTRACT Polycrystalline metal borides AlMgB14, CaB6, and MB2 (M: transition metals) were synthesized and their thermoelectric properties were examined. Single phase of orthorhombic AlMgB14, which contains B12 icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 K and 1823 K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500 (µV/K) and 10-1 (1/Ωm) at room temperature, respectively. These values are comparable to those of metal-doped β- rhombohedral boron. Synthesized AlB2-type diborides (MB2; M=Ti, V, Cr, and Mn) exhibited metallic conduction. The MB2’s α varied with number of valence electrons, and showed a maximum α in VB2. The variation was similar to that predicted for pseudogap system. The CaB6 also possessed metallic conduction, but the α of the phase was as large as 200 (µV/K) with negative sign. INTRODUCTION Boron-rich semiconductors are expected to be promising materials for high-temperature thermoelectric conversion [1]. Intensive studies have been conducted in boron carbides [2] and metal-doped β-rhombohedral boron (β-boron) [3,4] because of their large Seebeck coefficient (α) and low thermal conductivity. They contain 12-atom icosahedral clusters (B12) as building blocks, and their large α and hopping type conduction are attributed to the existence of the clusters [5]. At high temperatures, the boron carbides’ α is temperature independent or slightly increases with temperature, while the electrical conductivity (σ) increases [2]. As a consequence, relatively large figure of merit, Z, can be obtained. However, all boron carbides and doped ones [6-9] studied are p-type semiconductor except for n-type behavior in Ni-doped boron carbide reported by S.Hwang and coworkers [9]. Since the β-boron has interstitial sites, which can accommodate atoms, metal-doped ones are studied to improve thermoelectric properties, particularly σ, and to search for n-type materials. Significant increase in the σ by the doping results in the improvement of total performance, although the absolute value of the α decreases. And the doping of certain metals, such as V and Fe, changes α from positive to negative [3,4]. In the present study, we synthesized and examined some metal borides other than β-boron-type structure, more specifically, AlMgB14, CaB6, and AlB2-type metal diborides. The AlMgB14 is an orthorhombic phase containing B12 icosahedral clusters identical with those in β-boron, but arrangement of the cluster is different. Figure 1 (a) shows the crystal structure of the AlMgB14. As shown in the figure, there are large openings that can accommodate boron, aluminum, or magnesium atoms. The CaB6 has almost the same metal/boron ratio as AlMgB14 phase, but the clusters constituting CaB6 structure are octahed
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