Journal of the Japan Institute of Metals and Materials
Online ISSN : 1880-6880
Print ISSN : 0021-4876
ISSN-L : 0021-4876
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Volume 79, Issue 11
Displaying 1-19 of 19 articles from this issue
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Special Issue on Progresses in the Development of Thermoelectric Materials: New Analyses and New Materials
Overview
  • Crystal Structure and Thermoelectric Properties of Chimney-Ladder Higher Manganese Silicides
    Yuzuru Miyazaki
    2015 Volume 79 Issue 11 Pages 530-537
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201508
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      Crystal structure, electronic structure and thermoelectric (TE) properties of Nowotny chimney-ladder (NCL) higher manganese silicides (HMSs) are reviewed. According to a (3+1)-dimensional superspace approach, HMSs are revealed to belong to incommensurate composite crystals, wherein [Mn] and [Si] subsystems possess an irrational c-axis ratio γ=cMn/cSi. The γ parameter, also represents a stoichiometry of the HMSs as MnSiγ, dictates the electronic structure, directly relating to a valence electron counting number, VEC. The p-type TE performance can be enhanced by a partial substitution of V, Cr, etc., while n-type HMSs can be obtained by heavily substituting Mn with Fe, etc., to realize VEC>14. A unique thermal expansion has been discovered at T~800 K, which would deteriorate TE performance and hence to be avoided from a practical application point of view.
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  • Hierarchical Structures for High-Performance Chalcogenides: From Tellurides to Sulfides
    Michihiro Ohta
    2015 Volume 79 Issue 11 Pages 538-547
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201513
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      An efficient way to enhance the performance of thermoelectric materials is to tune its structure at all length scales from atomic to microscopic. This review addresses recent developments in all-length-scale hierarchical structuring of the thermoelectric chalcogenides, including PbTe, mineral-based sulfides, and layered sulfides. The inclusion of nanostructures in PbTe significantly reduces the lattice thermal conductivity without affecting the charge carrier mobility, leading to very high thermoelectric figure of merit ZT. The high efficiency in the nanostructured PbTe-based devices was demonstrated. For mineral-based sulfides, the low-energy atomic vibration inhibits heat flow, resulting in a low lattice thermal conductivity and high ZT. In layered sulfides, the lattice thermal conductivity is reduced through the intercalation of guest atoms/layers into host crystal layers which effectively scatters heat-carrying phonons. Furthermore, the highly oriented microtexture of layered systems allows high carrier mobility in the in-plane direction, leading to a high thermoelectric power factor. Among all chalcogenides, sulfides could pave the way for environment-friendly and cost-effective thermoelectric systems.
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  • New Development of Thermoelectric Materials Based on Heusler Compounds
    Yoichi Nishino
    2015 Volume 79 Issue 11 Pages 548-554
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201503
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      The Heusler-type Fe2VAl-based compounds can be a possible candidate for new thermoelectric materials. A significant enhancement in the Seebeck coefficient with both positive and negative sign can be achieved not only by the Fe/V off-stoichiometry combined with doping but also by the V/Al off-stoichiometry even without doping. For the latter alloys, the peak temperature of the Seebeck coefficient increases up to 600 K, and the maximum power factor is 4.3×10−3 for p-type and 6.8×10−3 W/mK2 for n-type, both of which are higher than those of the half-Heusler compound ZrNiSn and Mg2Si compound. A large power factor is required for practical applications of automotive thermoelectric generators. The heavy element Ta doping for the V/Al off-stoichiometric alloys is effective in increasing the dimensionless figure of merit at around 500 K.
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  • Control of Phonon Transport by Phononic Crystals and Application to Thermoelectric Materials
    Masahiro Nomura
    2015 Volume 79 Issue 11 Pages 555-561
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201504
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      Phonon transport and thermodynamic properties of nanostructured materials have been investigated and utilized to improve thermoelectric performance for various materials. In nanostructures, phonon transport is completely different from that in bulk materials and results in dramatic enhancement in the thermoelectric performance. This article reviews the impact of nanostructuring on the phonon transport and mainly focuses on phononic crystal nanostructures, in which the wave nature of phonons also plays an important role. We demonstrate that it is important to efficiently scatter thermal phonons, which distribute to wide range of frequencies, with different phonon scattering mechanisms in the spatial domain. We also demonstrate an enhancement of thermoelectric property of polycrystalline thin films by phononic crystal patterning.
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  • Crystal Structure and Thermoelectric Properties of Ge-Sb-Te Homologous Structure
    Atsuko Kosuga, Hiroki Ishibashi, Yoshiki Kubota, Kouichi Kifune
    2015 Volume 79 Issue 11 Pages 562-568
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201506
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      We studied the effects of different preparation method and In-substitution on its crystal structure and thermoelectric (TE) properties of polycrystalline GeSb6Te10. Firstly, the effects of spark plasma sintering (SPS) on the crystal structure and elemental distribution of GeSb6Te10 were discussed. GeSb6Te10 consolidated using SPS consisted of a mixture of GeSb6Te10-type homologous and Sb2Te3-type tetradymite structures, whereas the sample prepared by melting had a single homologous structure. The elemental compositional deviation from the nominal composition of the sample prepared by SPS was wider than that prepared by melting. This implies that SPS promoted atomic diffusion and rearrangement of elements, leading to a substantial change in the crystal structure and elemental distribution of GeSb6Te10. Secondly, the effects of In-substitution on the crystal structure and TE properties of GeInxSb6−xTe10 (x=0, 0.18, 0.3, and 0.6) prepared by melting were discussed. Rietveld and Le Bail analyses showed that all compositions crystallized in trigonal structures with a 51-layer period. Substituting In decreased both the lattice and electronic thermal conductivity, as well as markedly increased the Seebeck coefficient. We ascribed this increase to increases in the effective mass of the carriers, likely caused by the formation of additional energy states near the Fermi level. In GeIn0.6Sb5.4Te10, we found a maximum dimensionless figure of merit: ZTmax of 0.75 at 710 K, 1.9 times higher than that of GeSb6Te10.
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  • Enhancement of Thermoelectric Properties of Silicon by Nanoscale Structure Control
    Ken Kurosaki, Aikebaier Yusufu, Yoshinobu Miyazaki, Yuji Ohishi, Hiroa ...
    2015 Volume 79 Issue 11 Pages 569-572
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201501
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      The effectiveness of thermoelectric (TE) materials which can convert heat gradients into electricity and vice versa is quantified by the dimensionless figure of merit (ZT). Current TE materials such as Bi2Te3 and PbTe whose ZT values are around unity contain highly toxic and/or rare elements, limiting their widespread application. Silicon (Si) is a non-toxic, inexpensive, and earth-abundant element. Although bulk Si exhibits good electrical properties, its lattice thermal conductivity (κlat) is high (>100 W m−1 K−1), leading to the ZT value of around 0.01 at room temperature. If its κlat could be lowered while maintaining good electrical properties, Si could be an ideal TE material. These changes can be realized in Si by nanostructuring. Here we review recent results on the enhancement of TE efficiency of Si by nanoscale structure control. Based on the achievements of nanostructured Si in TEs, we point out some ideas for further enhancement of TE efficiency of Si.
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Regular Article
 
  • Microstructure and Thermal Conductivity of RuAl2 Prepared by a Single-Roll Melt-Spinning Method
    Ken Kurosaki, Akira Umeda, Masaya Kumagai, Yuji Ohishi, Hiroaki Muta, ...
    2015 Volume 79 Issue 11 Pages 573-576
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201502
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      The performance of thermoelectric (TE) materials is characterized by a dimensionless figure of merit (ZT). To achieve high ZT, it is important to reduce the thermal conductivity (κ). One known method for reducing the κ value of bulk materials is by controlling the microstructure. RuAl2 is known as a good TE material, although it has a relatively high κ value. In the present study, microstructurally controlled RuAl2 was prepared by a single-roll melt-spinning (MS) method. The obtained ribbons were crushed into powders and subjected to spark plasma sintering (SPS) to obtain high-density bulk samples. We measured the κ value of the obtained bulk samples and compared the data with that of a sample prepared by a traditional arc-melting method. The κ value of the MS-SPS sample was 13.5 W m−1 K−1 at room temperature, which was approximately 25% lower than that of the arc-melted sample. It can be concluded that the MS-SPS method is effective for making a RuAl2 bulk sample with moderately reduced κ value.
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  • Thermoelectric Properties of Al-Mn-Si Based C54-Phase Containing Small Amount of C40-Phase
    Akio Yamamoto, Hidetoshi Miyazaki, Yoichi Nishino, Tsunehiro Takeuchi
    2015 Volume 79 Issue 11 Pages 577-580
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201505
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      In this study, we intended to increase the dimensionless figure of merit, ZT, of Al-Mn-Si based C54-phase using the precipitation of a small amount of Al-Mn-Si based C40-phase that possesses slightly smaller magnitude of both electrical resistivity and Seebeck coefficient. By knowing that both C54-phase and C40-phase in Al-Mn-Si alloy system are line compounds that are stabilized in a very narrow composition range, we prepared a series of samples by simply mixing two alloys consisting solely of C54-phase or C40-phase at [Al37(Mn26Ru3Re3Fe1)Si30]100−x[Al27.5(Mn29W3Fe1)Si39.5]x (x=0, 1, 3, 5, and 100). These initial alloys contained small amount of heavy elements for the reduction of lattice thermal conductivity. We found that the magnitude of Seebeck coefficient was kept essentially unchanged at x<3, while the lattice thermal conductivity and electrical resistivity were effectively decreased. The magnitude of ZT observed was increased by 16% from 0.38 of the single C54-phase at Al37(Mn26Ru3Re3Fe1)Si30 to ZT=0.42 at [Al37(Mn26Ru3Re3Fe1)Si30]97[Al27.5(Mn29W3Fe1)Si39.5]3.
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  • Improvement of Thermoelectric Performance and P-N Control for Metal-Doped β-Rhombohedral Boron
    Yoshiki Takagiwa, Norihide Kuroda, Erika Imai, Ikuzo Kanazawa, Hiroshi ...
    2015 Volume 79 Issue 11 Pages 581-585
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201507
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      The effects of Cu-doping into β-rhombohedral boron on the thermoelectric properties have been investigated. The electrical conductivity increases with increasing Cu concentration up to 5 at%, while a positive Seebeck coefficient monotonically decreases because of an increase in the carrier concentration. Consequently, the power factor enhances by four times from 22 μW m−1 K−2 to 90 μW m−1 K−2 at 973 K. In addition, Cu-doping contributes to lower the thermal conductivity from 4.3 W m−1 K−1 to 2.3 W m−1 K−1 at 973 K due to an increase in phonon scattering events. The dimensionless figure of merit, ZT, enhances from 0.006 to 0.038 at 973 K for Cu4B105 as p-type. The observed ZT is higher than that of conventional thermoelectric boride B4C.
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  • Thermal Conductivity of β-FeSi2–Si Self-Assembled Nanocomposite
    Yuji Ohishi, Shingo Okajima, Yoshinobu Miyazaki, Hiroaki Muta, Ken Kur ...
    2015 Volume 79 Issue 11 Pages 586-590
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201509
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      In the present study, we synthesized composite material composed of Si and β-FeSi2 by self-assembly process. In this process, mesoscale α-FeSi2 and Si crystals were formed by rapidly solidifying Fe-Si alloys with eutectic composition (Fe:Si=26:74) using melt spinning. Nanoscale β-FeSi2 and Si crystals were formed by subsequent heat treatment by eutectoid decomposition from α-FeSi2 to β-FeSi2 and Si. Scanning electron microscope observation showed that two kinds of Si precipitates were formed in the β-FeSi2 matrix: Si particle with a diameter of 100–200 nm and Si nanoscale lamellar plates with a thickness of about 50 nm. The thermal conductivity of the composite was significantly reduced compared to the effective thermal conductivity of Si and β-FeSi2 composite calculated based on effective medium approximation. This result indicates that the thermal conductivity was reduced by enhanced phonon scattering by the formed nanostructure.
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  • First-Principles Based Phonon Calculation and Raman Spectroscopy Measurement of RuGa2 and RuAl2 with High Thermoelectric Power Factor
    N. Sato, Y. Takagiwa, A. Kuwabara, N. Uchida, K. Kimura
    2015 Volume 79 Issue 11 Pages 591-596
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201510
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      TiSi2-type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of ~0.3 eV and a relatively large power factor at 600–900 K. However, the maximum dimensionless figure of merit, ZT, are 0.5 and 0.2 for RuGa2 and RuAl2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds by using a first-principles calculation and Raman spectroscopy to develop a further reduction scheme of the lattice thermal conductivity. Phonon dispersion relations and density of states were obtained from a real-space force constants method based on supercells with finite displacements. The calculated zone-center wavenumbers were comparable to the experimental Raman wavenumbers. Besides, the phonon group velocities of acoustic branches were in accord with the experimental transverse and longitudinal speed of sound. The differences of phonon dispersion between RuGa2 and RuAl2 are explained by the fact that Ga is heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are attributed to Ga site and Ru site, respectively. Heavier atom substitution for these sites can effectively reduce averaged phonon group velocity and lattice thermal conductivity.
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  • P-Type Thermoelectric Properties of Pr1−xSrxMnO3 (0.1≦x≦0.3) and La1−xSrxFeO3 (0.1≦x≦0.3)
    Hiroshi Nakatsugawa, Masaki Kubota, Miwa Saito
    2015 Volume 79 Issue 11 Pages 597-606
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201516
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      In this study, polycrystalline samples of Pr1−xSrxMnO3 (0.1≦x≦0.3) and La1−xSrxFeO3 (0.1≦x≦0.3) were synthesized using a conventional solid-state reaction method. We investigated crystal structure, magnetic susceptibility (χ), and P-type thermoelectric properties, such as electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ), as a function of temperature (T) or Sr content (x). The perovskite structure at room temperature showed orthorhombic Pbnm phases for all samples. The samples for Pr1−xSrxMnO3 (0.1≦x≦0.3) showed the ferromagnetic-like ground state below Curie temperature. Conversely, the samples for La1−xSrxFeO3 (0.1≦x≦0.3) showed the antiferromagnetic-like ground state below Néel temperature. Although the samples for Pr1−xSrxMnO3 (x=0.1 and 0.2) showed a large positive S below room temperature, the carrier type changed from hole-like to electron-like behavior above 1000 K and 500 K, respectively. On the other hand, the samples for La1−xSrxFeO3 (0.1≦x≦0.3) showed a large positive S over the whole temperature range. The largest dimensionless figure of merit (ZT) in the specimen for La0.7Sr0.3FeO3 was attaining a maximum value of 0.14 at 1000 K, by a decrease in both ρ and κ, and an increase in S. Recently, the coefficient of linear thermal expansion of La1−xSrxFeO3 has achieved the value which is equivalent to that of N-type CaMnO3. We expect that La0.7Sr0.3FeO3 is one of the candidate P-type materials for the oxide thermoelectric elements consisting of the same type of crystal structure.
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  • Angle-Resolved Photoemission Analysis on Electronic Structures and Thermoelectric Properties of Off-Stoichiometric Fe2−xV1+xAl
    Kazuo Soda, Shota Harada, Toshimitsu Hayashi, Masahiko Kato, Fumihiro ...
    2015 Volume 79 Issue 11 Pages 607-612
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201517
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      The electronic states of Heusler(L21)-type off-stoichiometric Fe2−xV1+xAl have been investigated by soft X-ray angle-resolved photoelectron spectroscopy (ARPES) in order to clarify the origin of their large thermoelectric powers, which cannot be explained in terms of the rigid band model. In off-normal and normal ARPES, Fe2.05V0.95Al shows a weakly dispersive bulk band around the binding energy of 0.3 eV in the Γ-X direction and an almost dispersion-less one around 0.3 eV in a gap of dispersive bulk bands in the Γ-L direction, which is attributed to the anti-site Fe defect. At the Γ point, the bulk band does not appear to cross the Fermi level EF, consistent with the rigid band model for the excess Fe content bringing about the increase in the valence electrons, but no band crossing EF down is found at the X point. The anti-site Fe defect states near EF might push up the band at the X point and cause the p-type thermoelectric properties, unexpected with the rigid band picture. The change in the electronic structures and thermoelectric properties are discussed on the off-stoichiometry and substitution of the forth element.
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  • Fabrication of β-FeSi2-Based Thermoelectric Composite Alloys by Oxidation and Reduction Reactions during Sintering of Eutectoid Si and Iron Oxide Powder
    Yoshisato Kimura, Koichiro Takeno, Ayaka Mori, Yaw-Wang Chai
    2015 Volume 79 Issue 11 Pages 613-620
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201518
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      β-FeSi2 is an ecofriendly thermoelectric material for high-temperature applications. In the present work, we demonstrate the validity of a new proposed fabrication process for composite-type thermoelectric alloys comprising a β-FeSi2 matrix and dispersed SiO2 particles (including Fe2SiO4 particles). The starting materials were single-phase α-FeSi2 alloy powder and Fe2O3 powder. We propose that the following reaction sequence occurs during the sintering process: (1) α-FeSi2 decomposes into β-FeSi2 and Si via the eutectoid reaction, (2) SiO2 is formed by the oxidation of Si, and (3) β-FeSi2 is additionally formed by the solid-phase reaction between eutectoid Si and reduced Fe that is formed by the reduction of Fe2O3. The microstructure of the composite alloys formed by the combined reactions during the sintering process was observed and characterized mainly using scanning transmission electron microscopy in conjunction with energy-dispersive X-ray spectroscopic chemical analyses and X-ray diffraction. The electrical and thermoelectric properties of the composite alloys were measured at temperatures from 300 to 1073 K. High Seebeck coefficient values were observed for n-type Co-doped composite alloys from −150 to −250 μV•K−1 and for p-type Mn-doped alloys from 200 to 500 μV•K−1. The partitioning of the Co and Mn dopants from the α-FeSi2 phase to the β-FeSi2 phase throughout the process is important for controlling the Seebeck coefficient. The electrical resistivity is lowered by the dispersed SiO2 particles that are expected to reduce the lattice thermal conductivity of the composite alloys.
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  • Thermoelectric Properties of the Off-Stoichiometric Heusler Alloys Fe2−yV1+x+yAl1−x
    Manabu Inukai, Hidetoshi Miyazaki, Naoki Ide, Yoichi Nishino
    2015 Volume 79 Issue 11 Pages 621-626
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201519
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      We report the thermoelectric properties of the off-stoichiometric Fe2−yV1+x+yAl1−x alloys in the temperature range from 300 to 750 K. Fe2−yV1+x+yAl1−x alloys with V-rich (1+x+y>1−x, i.e., y>−2x) and Al-rich (y<−2x) composition show the negative and positive sign of Seebeck coefficient, respectively, and the peak temperature of Seebeck coefficient shifts to the higher temperature side with increasing V and Al composition. Electrical resistivity of Fe2−yV1+x+yAl1−x alloys shows the metallic temperature dependence. Due to the combination of the Fe/V and V/Al off-stoichiometric effects, Fe1.97V1.12Al0.91 and Fe2.03V0.89Al1.08 alloys show the highest power factors of 4.03×10−3 W/m K2 for n-type and 2.52×10−3 W/m K2 for p-type at 500 K, respectively.
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  • Thermoelectric Properties of Off-Stoichiometric Fe2V1−xAl1+x Sintered Alloys and Design of Thermoelectric Power Generation Module
    M. Mikami, M. Inukai, H. Miyazaki, Y. Nishino
    2015 Volume 79 Issue 11 Pages 627-632
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201520
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      Heusler-type Fe2V1−xAl1+x alloys having off-stoichiometric composition were fabricated by the powder metallurgical process using mechanical alloying and pulsed current sintering. Depending on the valence electron concentration, a positive (x>0) or negative (x<0) Seebeck coefficient was obtained resulting from a Fermi level shift. The electrical resistivity was reduced via the carrier doping effect, resulting in an increase of the thermoelectric power factor of 2.8 mW/mK2 for p-type and 5.0 mW/mK2 for n-type. In addition, the thermal conductivity was reduced by phonon scattering at crystal lattice defects induced by the off-stoichiometry. Consequently the thermoelectric figure-of-merit, ZT, was enhanced and reached 0.07 for p-type and 0.18 for n-type at around 500 K. Using the measured thermoelectric properties, the power generation ability was estimated. Owing to the enhancement of thermoelectric performance especially at high temperatures, power generation efficiency reaches 2% at temperature difference between 300 K and 773 K.
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  • Material Dependence of Maximum Possible Thermoelectric Figures of Merit
    Yukari Katsura
    2015 Volume 79 Issue 11 Pages 633-637
    Published: 2015
    Released on J-STAGE: November 01, 2015
    DOIhttps://doi.org/10.2320/jinstmet.JA201523
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      The electronic structures and thermoelectric properties of 12 parent compounds for thermoelectric materials were compared, from first-principles calculations and numerical solutions of Boltzmann transport equations. Carrier doping level dependences of the maximum possible thermoelectric figures of merit ZeT were calculated, in the limit of zero phonon thermal conductivity and infinite electron relaxation time. High enough ZeT was only observed in semiconductors with finite bandgaps. Higher ZeT was expected in compounds with steep density of states at the band edge. Such electronic structures were found in transition metal compounds, especially in transition metal oxides. We evaluated the temperature dependence of electron relaxation time, by combining calculation results with experimental transport properties. These analyses reduce the number of experiments to search for new thermoelectric materials, and will reveal the nature of various electron scattering centers within the thermoelectric materials.
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