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Thermoelectric properties of icosahedral cluster solids – Metallic-Covalent Bonding Conversion and Weakly Bonded Rigid Heavy Clusters

Published online by Cambridge University Press:  01 February 2011

Kaoru Kimura
Affiliation:
[email protected], The University of Tokyo, Department of Advanced Materials Science, 502, Kiban-toh, 5-1-5 Kashiwanoha, Kashiwa-shi,, Chiba 277-8561, N/A, N/A, Japan, +81-4-7136-5456, +81-4-7136-3758
Junpei Tamura Okada
Affiliation:
[email protected], The University of Tokyo, Department of Applied Physics, Japan
Hongki Kim
Affiliation:
[email protected], The University of Tokyo, Department of Advanced Material Science, Japan
Takehito Hamamatsu
Affiliation:
[email protected], The University of Tokyo, Department of Advancec Materials Sceince, Japan
Tomohiro Nagata
Affiliation:
[email protected], The University of Tokyo, Department of Materials Science, Japan
Kazuhiro Kirihara
Affiliation:
[email protected], National Institute of Advanced Industrial Science and Technology, Nanoarchitectonics Research Center
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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 with complex structure. For these reasons, ICS are promising candidates for thermoelectric materials.

Using MEM/Rietvelt method, we successfully obtained the clear image of the electron density distribution for alpha-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 alpha-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 scenario, Z was improved by a factor of 1.5 by substitution of Ru for Re.

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 kappa 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.

Type
Research Article
Copyright
Copyright © Materials Research Society 2006

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References

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