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Itsuo Ohnaka, Kaoru Kimura
1999 Volume 63 Issue 11 Pages
1367
Published: 1999
Released on J-STAGE: April 24, 2008
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Sunao Sugihara, Seiji Kawashima, Hiroaki Suzuki, Rika Sekine
1999 Volume 63 Issue 11 Pages
1368-1371
Published: 1999
Released on J-STAGE: April 24, 2008
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The effects of the method of fabricating thermoelecteric materials, such as sintering or direct fabrication, have been the focus of recent attention. The low-dimensional materials sush as two-dimensional film or skutterudite structure has been made use of recently in the higher performance. A computer simulation by DV-Xα calculation using the molecular orbital method was carried out to investigate the electronic structures related to the thermoelectric properties such as electrical conductivity and the Seebeck coefficient of FeSi
2. As regards electrical conductivity, the energy gap was estimated to be 0.53 eV when conventional HOMO and LUMO levels were considered. On the other hand, a new definition was provided for the Seebeck coefficient; it is the energy difference between HOMO and LUMO levels which have the same symmetry at the twofolded- and threefolded-degenerated energy structures. Furthermore, computer simulation and experiment gave the same trends concerning the thermoelectric properties of metal-added materials.
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Yukihiro Isoda, Isao Albart Nishida, Osamu Ohashi
1999 Volume 63 Issue 11 Pages
1372-1376
Published: 1999
Released on J-STAGE: April 24, 2008
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Sintered materials of (1−
x)FeSi
2+
xBSi
2 in the composition range of 0≤
x≤0.08 were prepared by the hot-pressing technique. The solid solutions of Fe
1−xB
xSi
2, which were a β-FeSi
2 single-phase, were obtained in the range of
x≤0.03. The electrical resistivity (ρ) and Seebeck coefficient (α) were measured as a function of temperature over the range from 77 to 1241 K. It was found that the solid solutions possess semiconducting properties below the semiconductor-to-metal transition temperature (
Tc). Logarithmic ρ versus temperature 1⁄
T curves showed an S-shaped variation in the temperature range of 200 to 600 K for the β-FeSi
2 containing boron. The curves could be explained in terms of a small polaron model and a impurity conduction in which boron atoms formed a localized state as a donor in the β-FeSi
2. The energy gap at 0 K (
Eg0) decreased with increasing the boron concentration. The energy gap (
Eg) estimated around
Tc was smaller than the
Eg0 and decreased with increasing the boron concentration.
Tc was of 1241 K and independent of the boron concentration. From these facts it was found that the transition from semiconductor to metal phases was mainly caused by expanding the width of a narrow valence band with increasing hole concentrations. The α of β-FeSi
2 with borons showed a remarked peak larger than that of un-doped, and the peak temperature shifted to lower temperatures with increasing boron concentrations.
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Nobuhiro Sadatomi, Osamu Yamashita
1999 Volume 63 Issue 11 Pages
1377-1385
Published: 1999
Released on J-STAGE: April 24, 2008
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We studied whether or not Si-based materials, which were prepared by arc melting Si with Ge and a small amount of Group 3B or 5B elements, could be produced with high performance thermoelectric properties. The present materials have not only strong alloy segregation but also strong dopant segregation along the grain boundaries at high carrier concentration
n of above 10
19 cm
−3. The combination of alloying with 5 at%Ge and doping with 0.3 at%B or 0.4 at%P decreased the thermal conductivity κ to 5.7% or 6.7% of pure Si at 298 K, respectively, in spite of strong segregation. The electrical resistivities ρ of doped Si and Si
0.97Ge
0.03 decrease linearly with increasing
n. However the Seebeck coefficient
S of doped Si have a local maximum at (3∼4)×10
19 cm
−3, while no
S maximum was observed in doped Si
0.97Ge
0.03. At
n\fallingdotseq2×10
20 cm
−3 the
S values of Si
0.97Ge
0.03 and Si
0.95Ge
0.05 with strong segregation are about 50% higher than those measured previously on Si
0.95Ge
0.05 and Si
0.7Ge
0.3 with little segregation. Such interesting phenomena of
S may probably result from the grain boundary segregation of dopants and/or Ge. The thermoelectric figures of merit (
ZT=
S2T⁄κρ) of
n- and
p-type Si
0.95Ge
0.05 increase linearly with temperature
T, and were 0.90 and 0.57 at 1073 K, respectively. These values correspond to 92% and 81% of those obtained by Dismukes
et al. for Si
0.7Ge
0.3 alloys with little segregation.
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Atsushi Yamamoto, Toshitaka Ohta, Kazushi Miki, Kunihiro Sakamoto, Hid ...
1999 Volume 63 Issue 11 Pages
1386-1392
Published: 1999
Released on J-STAGE: April 24, 2008
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We produced Si
0.8Ge
0.2 multiple quantum wells using MBE and evaluated the in-plane electrical properties. Si
0.8Ge
0.2 quantum wells with various well widths separated by 20 nm Si barrier layers and other samples with the same well/barrier ratio were grown on high resistive Si(100) substrates with a 20 nm buffer layer. Both well and barrier layers were uniformly boron-doped. The electrical conductivity σ and Seebeck coefficient
S were measured at room temperature and showed a size effect in power factor σ
S2, depending on the quantum well period.
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Hiroshi Nakatsugawa, Eisuke Iguchi
1999 Volume 63 Issue 11 Pages
1393-1399
Published: 1999
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Electrical transport and magnetic properties of polycrystalline ceramic specimens of the system La
0.9(Sr
1−xCa
x)
1.1CoO
4 have been investigated as a function of temperature by means of complex-plane impedance analysis which distinguishes the bulk resistivity from the resistivity across grain boundaries, four-probe dc resistivities, Seebeck coefficients and magnetic susceptibilities below room temperature. The temperature dependencies of the magnetic susceptibilities indicate that the ratio of intermediate-spin Co
iii ions (
S=1) is dominant in a specimen with
x=0.1. This suggests that La
0.9(Sr
1−xCa
x)
1.1CoO
4 at
x=0.1 has a high potential as a thermoelectric material. Therefore, it is necessary to search for thermoelectric materials in transition-metal perovskites using a spin-state control.
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Takahiro Nakayama, Kaoru Kimura
1999 Volume 63 Issue 11 Pages
1400-1406
Published: 1999
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β-rhombohedral boron (β-boron) is expected to be one of the candidates for high-temperature thermoelectric materials because of its hopping conduction mechanism. To reveal the controllability of the thermoelectric properties of boron-rich semiconductors which consisted mainly of B
12 icosahedral clusters, β-boron was doped with V, Cr, Fe, Co or Zr, and the composition and temperature dependence of the electrical conductivity and the Seebeck coefficient were investigated. V or Cr dopant which preferentially occupies the A
1 sites in β-boron structure causes an increase of the electrical conductivity and a negative Seebeck coefficient. On the other hand, Zr dopant which does not occupy the A
1 sites causes a large positive Seebeck coefficient. The electrical conductivity and Seebeck coefficient of Co- and Zr-doubly-doped p-type β-boron are nearly determined by the Co content. These results have been discussed with the concept of metallic-covalent bonding conversion for the B
12 cluster. The temperature dependence of the electrical conductivity of metal-doped β-boron is well explained as the variable range hopping conduction. The Seebeck coefficient as well as the electrical conductivity increases with increasing temperatures up to room temperature, contrary to the cases of ordinary metals and semiconductors. Consequently, the power factors of both Co-doped p-type β-boron and V- or Cr-doped n-type β-boron increase with increasing temperature, and materials whose power factor is 3-4 orders of magnitude larger than that of pure β-boron at room temperature were obtained by doping with about 1 at% metal atoms.
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Hiroaki Anno, Kakuei Matsubara
1999 Volume 63 Issue 11 Pages
1407-1411
Published: 1999
Released on J-STAGE: April 24, 2008
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CoSb
3 has been doped with Ni, Pd, and Pt as donor impurities, and the effects on the electronic structure and the electrical and thermoelectrical properties have been investigated so as to improve the thermoelectric figure of merit. It was found that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on the impurities. Our theoretical analysis of these results suggests that the electron effective mass and the conduction band deformation potential are significantly affected by doping. These doping effects in CoSb
3 can be attributed to (1) the influence of doping on the electronic structure and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band, which can be explained in terms of a Kane model. On the other hand, the lattice thermal conductivity decreased to about 4 Wm
−1K
−1 with increasing carrier concentration, almost independently of the impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity of heavily doped
n-type CoSb
3. A large value of the dimensionless thermoelectric figure of merit
ZT=about 0.85 (at 800 K) could be obtained for Pd and Pt double doping. Therefore, the double doping with Pd and Pt is considered to be effective in optimizing the electronic properties and the carrier concentration of CoSb
3.
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Xinfeng Tang, Lidong Chen, Takashi Goto, Toshio Hirai
1999 Volume 63 Issue 11 Pages
1412-1415
Published: 1999
Released on J-STAGE: April 24, 2008
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The filled skutterudite compounds, Ce
yFe
xCo
4−xSb
12(
x=0-1.0,
y=0-0.15), were prepared from powders of Co, Fe, Sb and CeCl
3·6H
2O by solid state reaction, and their thermoelectric properties were investigated. Ce
yFe
xCo
4−xSb
12(
x=0-1.0,
y=0-0.15) compounds showed p-type conductivity. Carrier concentration (
p) increased with increasing substitution Fe for Co site, and decreased with increasing Ce content. Electrical conductivity (σ) increased with increasing Fe content. The seebeck coefficient (α) increased with increasing Fe content for CeFe
xCo
4−xSb
12, but decreased with increasing Fe content for Fe
xCo
4−xSb
12. Thermal conductivity (κ) decreased significantly with the substitution of Fe for Co and filling of Ce into Sb dodecahedron voids. The thermoelectric figure of merit was improved by the combination of Fe substitution and Ce filling. The maximum
ZT value of 0.8 was obtained for Ce
0.12Fe
0.71Co
3.29Sb
12 at 750 K.
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Satoru Matsuo, Mari Yonetsu, Hirofumi Tashiro, Kazunori Nakano, Ken-ic ...
1999 Volume 63 Issue 11 Pages
1416-1422
Published: 1999
Released on J-STAGE: April 24, 2008
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Influence of oxygen content and grain size on the thermoelectric properties of hot pressed bismuth-tellurium based compounds has been investigated. Bismuth, tellurium, antimony and selenium powders were mixed to the stoichiometric compositions of (Bi, Sb)
2Te
3 for p-type and Bi
2(Te, Se)
3 for n-type, respectively. The powder mixtures were ball-milled in acetone and consolidated by hot pressing at 733 K for the p-type and 773 K for the n-type compound in an argon or hydrogen atmosphere. Sintering in a hydrogen atmosphere provided a lower oxygen content, particularly for the p-type compound. Coarser grain size (2-10 μm) was observed for the p-type sintered samples while the n-type samples exhibited a finer grained microstructure (about 1 μm). Seebeck coefficient, electrical resistivity and thermal conductivity of the sintered samples were measured at room temperature. The p-type compound exhibited dependence of the figure of merit (Z value) on grain size while the figure of merit of the n-type compound could be expressed as a function of oxygen content. The obtained maximum Z values were 2.5×10
−3 K
−1 for both of the p-type and n-type compounds.
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Masaki Orihashi, Yasutoshi Noda, Lidong Chen, Toshio Hirai
1999 Volume 63 Issue 11 Pages
1423-1428
Published: 1999
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P-type Pb
1−xSn
xTe single crystals were prepared by a Bridgman method with various Sn content (
x). Electrical conductivity (σ), Hall coefficient (
RH) and thermal conductivity (κ) were measured in the temperature range of 300 to 700 K. Hole concentration at 77 K changed from 2.0×10
24 to 4.9×10
26 m
−3 as
x varied from 0 to 1.0. The σ value increased monotonously with increasing
x. The minimum of κ found at
x=0.25 were resulted from that of the lattice thermal conductivity (κ
lattice), since the phonon-phonon scattering in the solid solution may have decreased the lattice thermal conductivity. The ratio of electrical conductivity to thermal conductivity (σ⁄κ), to which the figure of merit
Z is proportional, increased with increasing
x and then reached its maximum at
x=0.5. It was revealed that the thermoelectric performance of Pb
1−xSn
xTe can be improved by controlling its composition.
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Daisuke Kusano, Yasuhiko Hori, Kunikazu Izumi, Takenobu Kajikawa, Keis ...
1999 Volume 63 Issue 11 Pages
1429-1434
Published: 1999
Released on J-STAGE: April 24, 2008
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Thermoelectric properties of ZnO and (Zn
0.98Al
0.02)O alloys sintered under different pressures, from ultra fine powder have been investigated. The samples are with a grain size under 10 μm. Electrical conductivity, Seebeck coefficient and thermal conductivity of the samples are measured at temperatures ranging from 337 to 973 K. Electrical conductivity of the samples decreases with increasing temperature. Seebeck coefficient of the samples showed negative and the absolute value increased with increasing temperature. Thermal conductivity of the samples decreased with increasing temperature. The electrical conductivity and Seebeck coefficient appeared relatively insensitive to grain size, while the thermal conductivity did.
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Takayuki Tsuji, Ryosuke O. Suzuki, Katsutoshi Ono
1999 Volume 63 Issue 11 Pages
1435-1442
Published: 1999
Released on J-STAGE: April 24, 2008
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Thermoelectric properties of Fe
3Si with the DO
3 structure where some Fe were replaced with Mn and V were examined. By adding 5 mol%Mn or V, the Seebeck coefficients, α, reach the maximum values of about+22.5 μV/K, while they decreased to a large negative value by the further replacement of Fe by these transition elements. The minimum values of α were −36 μV/K and −45 μV/K at Fe-30 mol%Mn-25 mol%Si and Fe-20 mol%V-25 mol%Si, respectively. These values are smaller than the minimum value reported for the iron-based alloys. The compositional dependency of α corresponds to the fact that the added transition metal atoms replaced iron atoms at a particular site in the DO
3 lattice. By combining an approximate model approximation suitable for simple metals and the density of state reported, however, the obtained compositional dependency of thermoelectricity could not be explained. The absolute values of α in the Fe-Mn-Si system were small, both in the samples with the α-Fe solid solution and with the γ-Fe solid solution.
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Yoichi Okamoto, Hiroshi Inai, Atsushi Aruga, Jun Morimoto
1999 Volume 63 Issue 11 Pages
1443-1447
Published: 1999
Released on J-STAGE: April 24, 2008
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Thermoelectric properties and electrical transport parameters of the Si and Ni co-doped SiC have been studied as functions of both Si and Ni doping concentration and temperature. It is intended to increase the figure of merit easily by addition of isoelectric element Si and transition metal element Ni. The samples were fabricated by conventional wet mixing and powder sintering process. Measurements of electrical resistivity and thermoelectric power were made in the temperature range from room temperature to around 1000 K in He atmosphere. Thermal conductivity were measured in the temperature range from room temperature to around 600 K in air. Measurements of Hall coefficient, X-ray crystallography and EPMA were also made. The thermoelectric power increases drastically by addition of Si and Ni. The maximum value of thermoelectric power reached to about 600 μV. Thermal conductivity reached to the minimum value of 8.0 W/(mK) at 40.0 mass% of Si concentration and 1.0 mass% of Ni concentration. Enhancement of the figure of merit
Z was achieved by co-doping of Si and Ni. The figure of merit
Z reached to the maximum value of 2.5×10
−4 K
−1.
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Yasutoshi Noda, Kaoru Mizuno, Yan-Sheng Kang, Masayuki Niino, Isao A. ...
1999 Volume 63 Issue 11 Pages
1448-1453
Published: 1999
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The thermoelectric materials, n-PbTe, p-Pb
1−xSn
xTe (
x=0.00-0.50) and p-AgSbTe
2 were prepared by melt-growth and plasma-activated sintering(PAS). Compositional analysis and thermoelectric measurements were carried out on as-grown ingots. The electron concentration in the n-PbTe ingots was controlled in the range 3.0×10
23∼5.0×10
25 m
−3 by PbI
2-doping, while the concentration and Hall mobility of the sintered materials were lower than those of the melt-grown samples. For both the Pb
1−xSn
xTe and AgSbTe
2 ingots, compositional uniformity was confirmed over a region of 80% of the total length of the ingots, while some fluctuations in electronic properties were observed in AgSbTe
2 ingot. The carrier concentration of the Pb
1−xSn
xTe changed by two orders of magnitude from 10
24 to 10
26 m
−3 with an increase of
x, whilst that of AgSbTe
2 was 10
25 m
−3. The properties of the AgSbTe
2 were characterized by larger thermoelectric power and lower Hall mobility than those of the Pb
1−xSn
xTe. By using available thermal conductivity data, the estimated figure of merit for PAS-AgSbTe
2 at 300 K was 9.72×10
−4 K
−1, which is 2-7 times larger than that of PAS-Pb
1−xSn
xTe
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Hiroshi Uchida, Vincent Crnko, Hisao Tanaka, Akio Kasama, Yoshio Itsum ...
1999 Volume 63 Issue 11 Pages
1454-1460
Published: 1999
Released on J-STAGE: April 24, 2008
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The development of mass production process of thermoelectric materials is necessary to practical use. The most important thing is how to make large amounts of high quality thermoelectric materials with high productivity and low cost. Skutterudite structure materials are new kinds of thermoelectric materials which is recently paid much attention. From these view points, Skutterudite structure materials were produced by gas atomizing and sintering process. The materials examined in this study are CoSb
3 of the typical Skutterudite structure material, (CoPdPt)Sb
3 of the substituted structure
n-typed material and CeFe
3CoSb
12 of the filled Skutterudite structure
p-typed material. Sintering was tried with hot pressing process or spark plasma sintering process. Each gas atomized powder contains every necessary element with fine structure almost the same stoichiometric ratio of aimed thermoelectric material. Accordingly gas atomized powders could be sintered into thermoelectric materials in a short time without mixing and grinding process by both sintering methods. These lead to high productivity and low cost in manufacturing. Although there is no remarkable difference of thermoelectric properties between hot press sintered and spark plasma sintered materials, it was confirmed that sintering time required can be shorter for spark plasma sintering than for hot press sintering. CeFe
3CoSb
12 has some problems to attain high
ZT value. On the other hand, (CoPdPt)Sb
3 produced by gas atomizing and sintering process had almost same value of
ZT which was obtained in laboratory scale. The combination process of gas atomizing and sintering methods is useful for mass production of thermoelectric materials.
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Seiji Yoneda, Eiji Ohta, Hiromasa T. Kaibe, Isao J. Ohsugi, Ichiro Shi ...
1999 Volume 63 Issue 11 Pages
1461-1467
Published: 1999
Released on J-STAGE: April 24, 2008
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Sintered PbTe materials with average grain sizes of 28-309 μm were prepared by spark plasma sintering technique. The apparent densities of the sintered PbTe were 8.17-8.23 Mg/m
3, which were higher than 99% of the theoretical one. Thermoelectric properties of the sintered materials and the as-grown boule by Bridgman method were measured in the temperature range from 77 to 350 K. Resistivity, ρ, of the sintered materials increased with decreasing grain size. Temperature dependence of ρ of the sintered PbTe was remarkably different from that of the as-grown boule below 250 K because of potential barriers at grain boundaries. Hall coefficient,
RH, of the sintered materials at room temperature increased from 1.4×10
−6 to 3.4×10
−6 m
3/C, as the average grain size increased from 28 to 309 μm, which suggested that oxidation in the crystal grains was caused in the sintering process. Temperature dependence of the thermoelectric power, α, of the sintered PbTe below 250 K reflected the effect of carrier scattering at grain boundaries. Lattice thermal conductivity of the sintered PbTe decreased with decreasing grain size below 250 K, while it was independent of the grain size above that temperature. Above 250 K, thermoelectric properties of the sintered PbTe except those in the 28 μm grain case were consistent with those of the as-grown boule. The values of α and
RH at 295 K were calculated by using a two-valence-band model involving a non-parabolic and a parabolic valence band. The calculation results were in good agreement with the experimental data.
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Hideyuki Yasuda, Itsuo Ohnaka, Tetsuya Yano, Hideaki Kaziura
1999 Volume 63 Issue 11 Pages
1468-1474
Published: 1999
Released on J-STAGE: April 24, 2008
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A novel sintering process has been developed for porous thermoelectric devices such as gas-combustion-type and heat-exchange-type devices. Mixture of PbTe and KCl particles with a certain particle diameter and volume fraction of KCl was sintered at 973 K, 7.2 ks, 35 MPa. The KCl particles were removed by water and porous PbTe was obtained. Porosity ratio, specific surface area and permeability of gas were controlled by changing volume fraction of KCl and diameter of PbTe particles. Addition of KCl during sintering did not affect the Seebeck coefficient of PbTe. Therefore, this process can be applied to production of porous PbTe thermoelectric devices.
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Keiko Ikoma, Masayuki Munekiyo, Kenji Furuya, Masakazu Kobayashi, Hiro ...
1999 Volume 63 Issue 11 Pages
1475-1478
Published: 1999
Released on J-STAGE: April 24, 2008
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A thermoelectric generator has been made consisting of an inner shell as a exhaust pipe, an outer shell as a water jacket and 16 Bi
2Te
3 modules working at low temperature established between the inner shell and the outer shell. Heat-exchanging fins were formed inside the inner shell with heat transfer area ratios of 0.92, 1.21, 1.65, and 1.99 along the exhaust gas flow, in order to reduce the temperature distribution at the hot side of the modules without over overheating of the module. When the exhaust gas was introduced to the inner shell of the generator under conditions corresponding to a 2-3 litre gasoline engine vehicle running at 60 km/h up a 3-5% hill, the electric power generated by the generator was 193 W. The heat change conversion efficiency of the generator was estimated at to be 37% of the primary exhaust gas energy flux. The generated power was 2.9% of the heat flux through the generator.
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