The electrical and thermal transport properties of quaternary icosahedral Al71−xGaxPd20Mn9 (x=0,2,3,4) quasicrystals, obtained by replacing Al in icosahedral Al–Pd–Mn quasicrystal with Ga, have been measured in accordance with the guiding principle of “weakly bonded rigid heavy clusters” (WBRHCs). While the electrical conductivity and Seebeck coefficient were not dramatically changed, the thermal conductivity effectively decreased with increasing Ga concentration except for the sample with x=4. Although the thermoelectric properties do not obey the WBRHCs, the dimensionless figure of merit increased by a factor of 1.4 from 0.18 for Al71Pd20Mn9 to 0.26 for Al68Ga3Pd20Mn9 quasicrystal.

Quasicrystals / Thermoelectric properties / Microstructure / Spark Plasma Sintering (SPS) / Al--Pd--Re Abstract. We report the thermoelectric properties of poly-grain Al--Pd--Re icosahedral quasicrystals and discuss an effect of improvement of their microstructure. The improvement of microstructure by using Spark Plasma Sintering (SPS) method resulted into a large increase of the electrical conductivity but less increase of the thermal conductivity. The relative density dramatically increased up to more than 90% by SPS. On the other hand, the microstructure itself does not have critical influence on the Seebeck coefficient, which is found to be strongly correlated with e=a (sample's compositions). Consequently, the dimensionless figure of merit (ZT) increased three times from 0.05 to 0.15.

Single crystals of Ni3(A1, M) were grown by the Bridgman method, where M is Ti, V, Cr, Mn, Fe, Nb, Mo and Ta. The composition was controlled to be about NivsA120M5 so that the alloying element, M, substitutes mainly for A1. With these crystals conventional X-ray structural analysis was performed. The measured static displacements of atoms from the average lattice points depended largely on the alloying elements and varied in the range 0-00-0.13 A for Ni atoms and 0"09-0.18 A for AI atoms. It was found that these atomic displacements correlated well with the atomic radius of the alloying element, M. For example, when the atomic radius of M is larger than that of A1, the static displacements are large for the atoms in the A1 sublattice but small for the atoms in the Ni sublattice. By contrast, when the atomic radius of M is smaller than that of AI, the displacements are more enhanced in the Ni sublattice than in the AI sublattice. Thus, there is an interesting correlation between the atomic displacements in both the AI and Ni sublattices in the presence of alloying elements. This seems to be one of the characteristics of alloyed compounds with several sublattices.

Various compositions of AlGaPdRe icosahedral quasicrystals (QCs) were synthesized using arc-melting and annealing methods, and their thermoelectric properties were investigated. With the same trend in the Seebeck coefficient as AlPdMn and AlPdRe QCs, the AlGa PdRe QC has a similar pseudogap electronic structure near the Fermi level. More particularly, we found that a sample with a nominal composition of Al 66 Ga 4 Pd 21 Re 9 exhibited a higher Seebeck coefficient of 90 µV K ¹1 at 373 K, and is thus a highly efficient thermoelectric material. This dense sample, having neither cracks nor pores, shows a 1.5 times higher dimensionless figure of merit ZT of 0.18 compared with sintered Al 71 Pd 20 Re 9 . ZT enhancement through Ga substitution for Al is enabled through an increase in both electrical conductivity and Seebeck coefficient, and a decrease in phonon thermal conductivity. This behavior is discussed in terms of the precipitation of metallic secondary phases and the "weakly bonded rigid heavy clusters (WBRHCs)" scheme applicable to cluster-based solids including quasicrystals.

We report the thermoelectric properties of polygrain icosahedral Al 71 Pd 20 Mn 9 and Al 71 Pd 20 Re 9 quasicrystals (QCs). The microstructure of polygrain (arcmelted and annealed) Al-Pd-Re QCs is highly porous. We synthesized sintered pellets of the Al 71 Pd 20 Re 9 QC by the spark plasma sintering (SPS) method, resulting in an improvement in their relative density from 65% to more than 99%. This enables direct comparison of Al 71 Pd 20 Mn 9 QCs with Al 71 Pd 20 Re 9 QCs. The maximum dimensionless figures of merit ZT were 0.18 and 0.12 for Al 71 Pd 20 Mn 9 and Al 71 Pd 20 Re 9 QCs, respectively. For further improvement of the ZT value, we substituted Ga for Al atoms in the Al 71 Pd 20 Mn 9 QC in accordance with the guiding principle of weakly bonded rigid heavy clusters (WBRHCs). The sound velocity and Debye temperature beneficially decreased with increasing Ga concentration, indicating weakening of the intercluster bond strength, as predicted for WBRHCs. The maximum ZT value was increased by a factor of 1.4 from 0.18 for the Al 71 Pd 20 Mn 9 QC to 0.26 for the Al 68 Ga 3 Pd 20 Mn 9 QC by the addition of an alloying effect to that of the WBRHCs.