Thermoelectric Properties of Al&amp;ndash;Ga&amp;ndash;Pd&amp;ndash;Re Icosahedral Quasicrystals

Science and Technology of Advanced Materials

2014

In this article, we review the characteristic features of icosahedral cluster solids, metallic–covalent bonding conversion (MCBC), and the thermoelectric properties of Al-based icosahedral quasicrystals and approximants. MCBC is clearly distinguishable from and closely related to the well-known metal–insulator transition. This unique bonding conversion has been experimentally verified in 1/1-AlReSi and 1/0-Al12Re approximants by the maximum entropy method and Rietveld refinement for powder x-ray diffraction data, and is caused by a central atom inside the icosahedral clusters. This helps to understand pseudogap formation in the vicinity of the Fermi energy and establish a guiding principle for tuning the thermoelectric properties. From the electron density distribution analysis, rigid heavy clusters weakly bonded with glue atoms are observed in the 1/1-AlReSi approximant crystal, whose physical properties are close to icosahedral Al–Pd–TM (TM: Re, Mn) quasicrystals. They are considered to be an intermediate state among the three typical solids: metals, covalently bonded networks (semiconductor), and molecular solids. Using the above picture and detailed effective mass analysis, we propose a guiding principle of weakly bonded rigid heavy clusters to increase the thermoelectric figure of merit (ZT) by optimizing the bond strengths of intra- and inter-icosahedral clusters. Through element substitutions that mainly weaken the inter-cluster bonds, a dramatic increase of ZT from less than 0.01 to 0.26 was achieved. To further increase ZT, materials should form a real gap to obtain a higher Seebeck coefficient.

Section: Thermoelectric Properties Of Al–pd–(re Mn) Icosahedral Quasmentioning

confidence: 99%

Metallic–covalent bonding conversion and thermoelectric properties of Al-based icosahedral quasicrystals and approximants

Takagiwa

Science and Technology of Advanced Materials

2014

“…[ 6 ] Another example of enhancing zT max involved the synthesis of Al–Pd–Re‐based QCs via substituting Ru [ 14 ] and Fe [ 15 ] for Re, along with that of Ga for Al. [ 16 ]…”

Section: Introductionmentioning

confidence: 99%

Reinvestigation of the Thermoelectric Properties of Fe‐Substituted Icosahedral Al–Pd–Re Quasicrystals

Takagiwa

Physica Status Solidi (a)

2022

Icosahedral quasicrystals (QCs) have a glass‐like, low thermal conductivity because of their complex crystal structures, and a moderate power factor because of deep pseudogap formation in their electronic density of states (DOS) at the Fermi level originating from icosahedral symmetry and covalent bonding nature. These features are beneficial for high‐performance thermoelectric materials. Densified bulk samples of icosahedral Al–Pd–Re–Fe QCs are synthesized by arc‐melting, annealing, and spark plasma sintering methods. It is found that a 2/1‐approximant crystal (AC) can be produced by higher Fe substitution [Al71Pd20(Re1–xFex)9 (x≥0.75)]. The thermoelectric properties of Al71Pd20(Re1–xFex)9 (where x = 0, 0.2, 0.4, 0.65, 0.7, and 0.75) are evaluated between 373 and 973 K. Fe substitution enhances the power factor and reduces the phonon thermal conductivity by a concept of “weakly bonded rigid heavy clusters” (WBRHCs) and an increase in the carrier concentration, enhancing the dimensionless figure of merit (zT). The 2/1‐AC phase has a much higher electrical conductivity and a moderate Seebeck coefficient in the mid‐temperature region due to a considerable increase in carrier concentration; this results in the highest power factor of 900 μW m−1 K−2, and zT value for the 2/1‐Al−Pd−Re−Fe AC is 0.26 at 573 K.

“…5(b), σ increases exponentially with increasing T , which is a typical trend in intrinsic semiconductors. The σ at 350 K is approximately an order of magnitude lower than the corresponding value of the Al-Pd-Re QC [6,[32][33][34] which was regarded as being closest to the semiconducting state to date. Figure 5(c) shows the T dependences of κ and κ el (≡Lσ T , where L is the effective Lorenz number), and κ lat (≡κ − κ el ), where L is obtained via DFT calculations using the same conditions that were used for S. κ increases with increasing T up to 800 K because of the increasing κ el .…”

Section: Al Irmentioning

confidence: 71%

Experimental realization of a semiconducting quasicrystalline approximant in Al-Si-Ru system by band engineering

Iwasaki

Kitahara

2019

Phys. Rev. Materials

We found that an Al-Si-Ru cubic quasicrystalline approximant has a semiconducting band structure by performing an orbital analysis based on density functional theory. These semiconducting transport properties have been confirmed in an experimentally synthesized sample. The temperature dependences of the electrical conductivity and the Seebeck coefficient were consistent with the trends of an intrinsic semiconductor with a band gap of 0.15 eV above 350 K. The lattice thermal conductivity had a low value of approximately 1.0 W m −1 K −1 above 400 K, which is close to the theoretical minimum.Since the discovery of the quasicrystal (QC) in 1984 [1], many of the unique properties of QCs have been revealed, including their crystal structures [2] and electrical properties [3]. However, the question of whether or not semiconducting or insulating QCs exist remains one of the fundamental problems to be solved in solid state physics, although the QC concept has been established as one of the categories of solid-state structures.Semiconducting QCs have attracted attention, not only from the perspective of academic interest, but also from the viewpoint of application to thermoelectric (TE) materials to realize a direct conversion between the thermal and electrical energies. The performance of a TE material can be evaluated using the dimensionless figure of merit zT = S 2 σ T /(κ el + κ lat ), where S, σ , T , κ el , and κ lat are the Seebeck coefficient, the electrical conductivity, the absolute temperature, the electronic thermal conductivity, and the lattice thermal conductivity, respectively. The highest zT between QCs achieved to date is 0.26, which is only approximately one quarter of the general target value of unity, at 500 K in an Al-Ga-Pd-Mn QC [4]. The main problem is that the value of S (≈100 μV K −1 ) is only approximately one half of that of typical practical materials. To obtain a sufficiently large S at a target temperature T , a semiconductor with a band gap of 6-10k B T , where k B is the Boltzmann constant, is generally required [5]. Therefore, the discovery of a semiconducting QC is necessary to enable the breakthrough of QCs for use as TE materials.While semiconductorlike properties that were attributed to a combination of the pseudogap in the density of states (DOS) and electronic weak localization were previously reported in some aluminum-transition metal (Al-TM) QCs [6-10] and quasicrystalline approximants (QCAs) [10,11], a finite band gap has yet to be observed experimentally in these materials to date. There have been several theoretical works on semiconducting QCAs based on density functional theory (DFT) that have thus far contributed to the understanding of the problem of complex semiconducting QCs. Krajčí and Hafner investigated the electronic structures of hypothetical QCAs that were constructed using a model of Al-TM icosahedral QCs [12][13][14]. While they predicted the existence of some QCAs with semiconducting electronic structures and indicated the presence of an energy gap in QCs, these typ...