Yoshiki Takagiwa

Tuning bands of PbSe for better thermoelectric efficiency

Improving the thermoelectric performance of PbSe over its previously reported maximum zT can be achieved by engineering its electronic band structure. We demonstrate here, using optical absorption spectra, first principles calculations, and temperature dependent transport measurements, that alloying PbSe with SrSe leads to a dramatic change of the band structure that increases the thermoelectric figure of merit, zT. The temperature where the two valence bands converge decreases with Sr addition. The zT value, when the carrier density is optimized, increases with Sr addition in Pb1−xSrxSe and when x = 0.08 a maximum zT of 1.5 at 900 K is achieved. The net benefit in zT comes from the band structure tuning even though in other thermoelectric solid solutions it is the thermal conductivity reduction from disorder that leads to net zT improvement.

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

Abstract 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.

Higher mobility in bulk semiconductors by separating the dopants from the charge-conducting band – a case study of thermoelectric PbSe

In the rigid band approximation dopants in semiconductors only change the Fermi level and carrier concentration such that different dopants are thought equivalent when fully ionized. In this work we examine the small but significant difference in mobility due to the type of dopant in heavily doped PbSe by studying n-type samples doped with Br, In and Bi. We propose that cation and anion dopants lead to a difference in mobility at high concentrations. This can be understood considering the predominance of cation states to the conduction band and anion states to the valence band. For higher mobility and better performance for most applications of heavily doped semiconductors, dopants should be on the site that is of less influence on the charge-conducting band. This concept can be viewed as an analog of modulation doping on the atomic level. Its physical origin is the random potential due to disorder that perturbs carriers, which is also the origin of Anderson localization at low temperature, a well-studied topic in theoretical physics. In thermoelectric PbSe, the selection of dopant can lead to 10% difference in mobility and in zT.

Thermoelectric properties of polygrained icosahedral Al71−xGaxPd20Mn9 (x=0,2,3,4) quasicrystals

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.

Thermoelectric properties and electronic structure of the Zintl phase Sr5Al2Sb6

The Zintl phase Sr5Al2Sb6 has a large, complex unit cell and is composed of relatively earth-abundant and non-toxic elements, making it an attractive candidate for thermoelectric applications. The structure of Sr5Al2Sb6 is characterized by infinite oscillating chains of AlSb4 tetrahedra. It is distinct from the structure type of the previously studied Ca5M2Sb6 compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr5Al2Sb6 (~0.55 W mK(-1) at 1000 K) was found to be lower than that of the related Ca5M2Sb6 compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr5Al2Sb6, in agreement with the experimentally determined band gap of E(g) ~ 0.5 eV. High temperature electronic transport measurements reveal high resistivity and high Seebeck coefficients in Sr5Al2Sb6, consistent with the large band gap and valence-precise structure. Doping with Zn(2+) on the Al(3+) site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr5Al2Sb6 suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca5Al2Sb6 could be achieved in this system.

Effect of Carrier-Doping on the Thermoelectric Properties of Narrow-Bandgap (Fe,Ru)Ga3 Intermetallic Compounds

We have focused on the binary narrow-bandgap intermetallic compounds FeGa3 and RuGa3 as thermoelectric materials. Their crystal structure is FeGa3-type (tetragonal, P42/mnm) with 16 atoms per unit cell. Despite their simple crystal structure, their room temperature thermal conductivity is in the range 4–5–W–m−1–K−1. Both compounds have narrow-bandgaps of approximately 0.3–eV near the Fermi level. Because their Seebeck coefficients are quite large negative values in the range 350–<–|S373K|–<–550–μV–K−1 for undoped samples, it should be possible to obtain highly efficient thermoelectric materials both by adjusting the carrier concentration and by reducing the thermal conductivity. Here, we report the effects of doping on the thermoelectric properties of FeGa3 and RuGa3 as n and p-type materials. The dimensionless figure of merit, ZT, was significantly improved by substitution of Sn for Ga in FeGa3 (electron-doping) and by substitution of Zn for Ga in RuGa3 (hole-doping), mainly as a result of optimization of the electronic part, S2σ.

Thermoelectric performance of Al–Pd–Mn quasicrystals: comparison with (1/1, 2/1-)AlPdMnSi approximants and improvement by Ga substitution for Al

Abstract We report the thermoelectric properties of polygrain icosahedral Al–Pd–Mn quasicrystal and compare with those of (1/1, 2/1-)AlPdMnSi approximant crystals. To improve the dimensionless figure of merit (ZT), we substitute Ga for Al atoms in Al–Pd–Mn quasicrystal in accordance with the guiding principle of “weakly bonded rigid heavy clusters” (WBRHCs). Although the thermoelectric properties do not obey the WBRHCs principle, the maximum ZT increased by a factor of 1.4 from 0.18 for Al71Pd20Mn9 to 0.26 for Al68Ga3Pd20Mn9 quasicrystal.

Thermoelectric properties of Al–Pd–Re quasicrystal sintered by Spark Plasma Sintering (SPS): effect of improvement of microstructure

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.

Semimetallic Band Structure and Cluster-Based Description of a Cubic Quasicrystalline Approximant in the Al–Cu–Ir System

Density functional calculations were performed for a cubic quasicrystalline approximant in the Al–Cu–Ir system. A semimetallic band structure was developed and analyzed on the basis of Wannier functions constructed from the valence and a part of the conduction band manifold. The Wannier functions were s- and p-like orbitals centered on either the centers of conventional clusters or the icosahedron-like vertices of pseudo-Mackay clusters, and d-like orbitals centered on the transition metals. Grouping the orbitals according to their center, we considered a small cluster for each group of the orbitals. Most of the orbitals contribute to the density of states only within the valence bands, i.e., they are valence states. The exceptions are some of p-like orbitals centered on the icosahedron-like vertices of the pseudo-Mackay clusters, and they contribute to both valence and conduction bands. Each of these p-like orbitals forms a covalent bond with one centered on the neighboring small cluster. The resulting b...

Chemical shifts of metallic and non-metallic Al–Re–Si approximant crystals studied by EELS and SXES

Chemical shifts of approximant crystals of 1/0-Al12Re (1/0-metallic), 1/1-Al73Re15Si12 (1/1-metallic) and 1/1-Al73Re17Si10 (1/1-non-metallic) were examined by using electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Al L-shell excitation EELS spectra of these alloys showed an apparent chemical shift only for the 1/1-non-metallic alloy to the larger binding energy side by 0.2 eV. Al-Kα, Re-Mα and Si-Kα emission SXES spectra also showed a shift to the larger binding energy side only for 1/1-non-metallic alloy. 1/0-metallic and 1/1-metallic alloys did not show any chemical shift in EELS and SXES experiments. Chemical shifts were observed only in larger binding energy side compared with pure materials. This implies the decrease of valence charge at constituent atomic sites of 1/1-non-metallic alloy compared with 1/0-metallic, 1/1-metallic and pure materials. The decreased charges should distribute intermetallic sites, which should be related to a formation of covalent bonding among Al atomic sites reported by maximum-entropy method (MEM)/Rietveld analysis on this material. This relation between chemical shift and covalent bonding nature of this approximant alloy may support the presence of covalent bonding in Al-based quasicrystals.