Thermoelectric properties of icosahedral cluster solids - Metallic-Covalent Bonding Conversion and Weakly Bonded Rigid H
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Thermoelectric properties of icosahedral cluster solids - Metallic-Covalent Bonding Conversion and Weakly Bonded Rigid Heavy Clusters Kaoru Kimura1,3,4, Junpei T. Okada2, Hongki Kim1, Takehito Hamamatsu1, Tomohiro Nagata3, Kazuhiro Kirihara4 1 Department of Advanced Materials Science, The University of Tokyo, Kiban-toh 502, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan 2 Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 3 Department of Materials Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 4 Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, Central 5, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT Boron- or Aluminum-rich icosahedral cluster solids (ICS) consist mainly of B12 or Al12 icosahedral clusters. In the ICS, a slight change of the structure or environment of icosahedral cluster can cause metallic-covalent bonding conversion, which can cause that the electrical conductivity σ and the Seebeck coefficient S can be as high as those of metals and semiconductors, respectively. Five-fold symmetry of the icosahedral cluster does not match with the translational symmetry of a crystal, consequently makes lower thermal conductivity κ with complex structure. For these reasons, ICS are promising candidates for thermoelectric materials. In α-AlReSi approximant crystal, the bond strength distributes widely from weak metallic to strong covalent bond, and the intra-cluster bonds are stronger than the inter-cluster ones. This means that α-AlReSi is located at the intermediate state of molecular, metallic- and covalent-bonded solids. Composition dependences of atomic density and quasi-lattice constant for AlPdRe icosahedral quasicrystals show the above situation is the same in the quasicrystals. The thermoelectric figure of merit Z and the effective mass m* of AlPdRe quasicrystals can be increased by strengthening the intra- and weakening the inter-cluster bonds. According to this guiding principle (Weakly Bonded Rigid Heavy Clusters), Z was improved by a factor of 1.5 and 2.0 by substitution of Ru and Fe for Re, respectively. In β-rhombohedral boron, several interstitial sites, which have space large enough to accommodate foreign atoms, are known. For the V doped sample, in which V atoms mainly occupy A1 site, the metallic-covalent bonding conversion may occur, σ is increased very much, S is decreased even to negative value and κ is decreased. The maximum and n-type ZT value is obtained and is approaching to that of B4C, which is considered to have the largest and p-type ZT value in boron-rich ICS.
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INTRODUCTION The potential of thermoelectric materials is determined by the dimensionless figure of merit, ZT = S2σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the temperature. In order to obtain a high ZT value, S and σ should be large while κ is low. Some Al-based icosahedral quasicryst
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