Faculty Profiles - ITOH Takashi

Papers - ITOH Takashi

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Synthesis of Higher Manganese Silicide by Pulse Discharge Sintering Method Reviewed

M. Yoshikura, T. Itoh

Abtsracts of R'09 Twin World Congress Nagoya Venue page： 98 2009.10

Language：English

Higher manganese silicide (HMS) is a promising thermoelectric material available to thermoelectric power generation using the disposed heat resources of 500 to 900K. Since both of Mn and Si as constituent elements in the manganese silicide are harmless and they exist in abundance as mineral resources, the manganese silicide can become a low-cost thermoelectric material with low-environmental load. Though combination of mechanical alloying (MA) and pulse discharge sintering (PDS), that is a conventional method, may be an effective method for synthesizing HMS compound, difference of loss ratios of Mn and Si during MA process causes problems of composition shifts and poor reproductivity. In this study, a new synthesizing method of HMS was attempted. Mn and Si powders (75μm, 99.9%) were individually ground by mechanical grinding (MG) with a planetary ball milling equipment at 400 rpm for 1h. The ground powders with particle size of 44μm or less were collected by sieving. The collected Mn and Si powders were mixed at the molar ratio of 1:1.73 (a composition ratio of HMS) with a rotary blender at 75 rpm for 1h. HMS compound was synthesized from the mixed powder and simultaneously sintered by PDS at 1173K for 15 min under 60 MPa. From XRD analysis of the material consolidated by the new method, HMS phase was detected as a main phase, but MnSi phase was also detected as well as the material obtained by the conventional method. The synthesis was carried out under various experimental conditions, and thermoelectric properties were measured.

Influence of Carbon Nanotube Addition on Thermoelectric Properties of Skutterudite-type Cobalt Antimonide Compound Reviewed

M. Tachikawa, T. Itoh

Abstracts of R'09 Twin World Congress Nagoya Venue page： 98 2009.10

Language：English

Skutterudite-type cobalt antimonide compounds are well known as materials with good thermoelectric properties over temperature range of 300-600 oC. However, their thermal conductivity is relatively high, and lowering of the thermal conductivity is required for further improving thermoelectric performance. This study aims at lowering the lattice thermal conductivity by the phonon scattering induced from dispersing uniformly carbon nanotubes (CNTs) into the cobalt tri-antimonide compound. Powders of Co, Ni, Sb and Te were blended with molar ratios of n-type Co0.92Ni0.08Sb2.96Te0.04 compound, and the compound was synthesized through a pulse discharge sintering (PDS) process. After coarsely grinding the synthesized compound, CNTs were added into the compound powder at different fraction, and then the mixture was mechanically ground with a planetary ball milling equipment. The composite powder obtained by the mechanical grinding (MG) was compacted and sintered by PDS. Dispersion condition of CNTs was observed by scanning electron microscopy (SEM). Thermoelectric properties (Seebeck coefficient, electrical resistivity and thermal conductivity) of the sintered samples were measured. It was confirmed by SEM observation that the fibriform CNTs existed uniformly in the Co0.92Ni0.08Sb2.96Te0.04 compound matrix. The addition and dispersion of CNTs into the compound decreased the thermal conductivity, but increased the electrical resistivity.

Synthesis of Thermoelectric Manganese Silicide by Mechanical Alloying and Pulse Discharge Sintering Reviewed

T. Itoh and M. Yamada

J. Electronic Materials Vol. 38 ( 7 ) page： 925-929 2009.7

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Manganese silicide is a candidate for low-cost thermoelectric materials with low-environmental load. In this research, MnSi1.73 compound was studied as a thermoelectric material available for the thermoelectric power generation using waste heat. Manganese and silicon powders were mechanically alloyed under three different conditions (at 200 rpm for 36 ks, at 400 rpm for 3.6 ks and at 400 rpm for 36 ks) with a planetary ball milling equipment. Then, the mechanical alloyed powder was consolidated by a pulse discharge sintering process. Phases of MnSi1.73 (primary phase) and MnSi were synthesized in mechanical alloying and pulse discharge sintering. Thermoelectric properties were dependent on the mechanical alloying condition. The sample mechanically alloyed at 400 rpm for 3.6 ks had the best thermoelectric performance. The maximum dimensionless figure of merit ZT of 0.47 was achieved at 873K.

Thermoelectric Properties of Al-doped Mg₂Si Compounds Prepared by Liquid-Solid Phase Reaction and Pulse Discharge Sintering Reviewed

T. Itoh, M. Matsuno

Abstracts of ICT2009 page： 187 2009.7

Authorship：Lead author Language：English

Dimagnesium silicide is a candidate for thermoelectric materials available for the thermoelectric power generation using waste heat. This material has advantages of the constituent elements in abundance in the earth's crust and of the nontoxicity of its processing by-products as an environmental symbiosis thermoelectric semiconductor. Mg2Si Compounds doped with Al from 0 to 0.2 mol% were synthesized from Al, Mg and Si powders by a liquid-solid phase reaction method. The compounds were sintered through a pulse discharge sintering process. Influence of Al content on thermoelectric properties of Mg2Si compounds was investigated. Although Al-doping decreased the absolute value of Seebeck coefficient and increased the thermal conductivity, it drastically decreased the electrical resistivity. The maximum dimensionless figure of merit of 0.47 at 509 oC was achieved in Mg2Si doped with 0.2 mol% Al.

Syntheses of Ni-doped and Fe-doped CoSb₃ Thermoelectric Nanoparticles through Modified Polyol Process Reviewed

T. Itoh, K. Isogai

Abstracts of 2009 MRS Spring Meeting Vol. CD-ROM page： N3.16 2009.4

Authorship：Lead author Language：English

Skutterudite CoSb3 compounds are of increasing interest as materials with good thermoelectric performance over the temperature range of 600 to 800 K, but the thermal conductivity of the materials is relatively high. Nanostructured materials have been shown to enhance phonon scattering and lower the thermal conductivity of the thermoelectric materials. We have investigated about the synthesis of binary CoSb3 nanoparticles through a modified polyol process. Partial substitution of Ni or Fe on the Co site of CoSb3 is a hopeful route for improving thermoelectric performance of the CoSb3 compounds. In the present work, synthesis of Ni-doped and Fe-doped CoSb3 nanoparticles through the modified polyol process was attempted and the optimum synthesizing condition was investigated. Co(OOCH3)2.4H2O, Ni(OOCH3)2.4H2O, FeCl3 and SbCl3, were prepared as precursors. The precursors were reduced by NaBH4 in tetraethyleneglycol at 513 K in argon gas, for different reaction time (holding time). The reaction products were characterized by the X-ray diffraction, the energy dispersive X-ray spectroscopy, and transmission electron microscopy. The nanoparticles with about 20 to 30 nm in size mainly existed in the reaction products regardless of the chemical composition and the reaction time. The skutterudite phase was identified as a main phase in the sample synthesized for long reaction time, but the other phases of Sb and MSb2 (M=Co, Ni, Fe) were also detected. The lattice parameter of the synthesized skutterudite phase linearly increased with increasing the doping agent concentration, following Vegard's law.

新たな手法による熱電発電用モジュールの作製とその特性評価 Reviewed

生田裕二，伊藤孝至

粉体および粉末冶金 Vol. 56 ( 1 ) page： 30-34 2009.1

圧縮ねじり成形法によるBi-Sb-Te系熱電材料の固化と組織制御 Reviewed

伊藤孝至, 門井　崇, 金武直幸

日本金属学会誌 Vol. 73 ( 1 ) page： 1-6 2009.1

液相－固相反応法によるマグネシウムシリサイド化合物の合成とその熱電特性 Reviewed

松野光晴, 伊藤孝至

粉体および粉末冶金 Vol. 56 ( 1 ) page： 26-29 2009.1

Thermoelectric Properties of Sintered Bodies Made of Synthesized CoSb3 Nanoparticles

K. Isogai and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-10 2008.12

Language：English

CoSb3 based skutterudite compounds are the promising thermoelectric materials that have relatively good thermoelectric properties with respect to the Seebeck
coefficient and the electrical resistivity over temperature range of 600-800 K. The high thermal conductivity of the materials, however, prevents from achieving high thermoelectric performance. Superfine
grain structure made by sintering superfine particles is expected to lower the thermal conductivity by phonon scattering. In our previous work, we have reported the CoSb3 nanoparticles synthesis via a modified polyol process. In the present work, we focused at investigating the thermoelectric properties of the sintered bodies made of the synthesized CoSb3 nanoparticles. The CoSb3 nanoparticles were synthesized via the modified polyol process. Two reactants Co(OOCH3)2·4H2O and SbCl3 were mixed
at a stoichiometric mole ratio of 1:3, and were reacted with a reducing agent NaBH4 in a solvent tetraethyleneglycol at 513 K. Then, the reaction products were sintered into bulk samples by a pulse discharge
sintering (PDS) under several operating conditions. The synthesized nanoparticles and the sintered samples were
characterized with respect to the synthesized phases, the particle size and the grain size. The Seebeck coefficient, the electrical resistivity and the thermal conductivity of the sintered samples were also measured. The nanoparticles with size about 20 to 30 nm mainly existed in the reaction products. The CoSb3 phase was identified, but the other phases of Sb and CoSb2 were also detected. It was found
that the synthesis of CoSb3 is promoted by PDS process. In the sample sintered at 853K for 15min under 60MPa, the grains with size about 200nm mainly existed together with several tens nanometer sized grains. The thermal conductivity of the fine grain sized sample was sufficiently reduced compared with that of the sample
synthesized and sintered by only PDS process.

The effect of the sintering conditions and addition of fullerene on thermoelectric properties of Mg₂Si

M. Matsuno and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-9 2008.12

Language：English

Magnesium silicide (Mg2Si) is a promising material for thermoelectric energy conversion at intermediate temperature range. This material has advantages of the abundance of its constituent elements in the earth's crust and of the nontoxicity of its processing by-products as an environmentally friendly thermoelectric semiconductor. However, there are few research reports about this material for improving thermoelectric performance because of difficulty of its synthesis. In this study, Mg2Si was synthesized from Mg and Si by a liquid-solid phase reaction (LSPR) method, which is effective in synthesizing a compound from the two elements having sufficiently different melting points. The powder synthesized by LSPR method was compacted and then sintered by a pulse discharge sintering (PDS) process. All sintered samples indicated the nature of n-type semi-conductor over a measuring temperature range. The power factor of the sintered sample was, however, strongly affected by the sintering temperature and/or the sample density. The sample sintered at 1003 K for 30 min under 60MPa had the maximum power factor of about 0.11×10-2 mWm-1K-2 at 464 K. Mg2Si powder was mixed with fullerene by a planetary ball milling method, and then sintered by PDS process. In the samples sintered under the same sintering condition, the power factor decreased with fullerene addition, because the electrical resistivity considerably increased and/or the sample density decreased. It will be necessary to optimize the sintering conditions for attaining an effective result about the fullerene addition.

Fabrication of Thermoelectric Module by A New Manufacturing Process and Its Performance

Y. Ikuta and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-8 2008.12

Language：English

In the conventional production of the thermoelectric modules, process automation is difficult since the thermoelectric elements are produced by slicing only one at a time and the assembly of thermoelectric modules requires placement and attachment of the individual, and sometimes very small, thermoelectric elements. This leads to high production cost. Thus, new processing methods will be required for cost-effectively producing compact thermoelectric modules with a multitude of thermoelectric couples. In the present work, a new manufacturing process was attempted for producing the thermoelectric modules, and performances of the modules were evaluated. Powders of the p-type FeCo3Sb12 and the n-type Co0.92Ni0.08Sb2.96Te0.04 compounds were alternately filled into 4x4 square holes of an insulating mold. The powders in the holes were compacted and sintered simultaneously at 900 K in vacuum for 20 min under the isostatic pressure of 60 MPa. In sintering, a pseudo hot isostatic pressing (pseudo HIP) method, in which one-directional load is transmitted isostatically to the sample through the alumina powder filled around the sample, were used to prevent the mold from breaking. The sintered elements in the mold were electrically connected in series using silver paste. Then, a thermoelectric module was constructed by parallel sandwiching the mold with glass plates of 0.5mm in thickness. The hot side of the module was heated up to 796 K, and the cold side was
cooled by water. Under this condition, the thermoelectric performance was evaluated. The maximum open voltage was 561 mV at the maximum temperature difference of 356 K, and the internal resistance in the module was 1.74 Ω. In this case, the output power of 50.1 mW was obtained and the power density of 51.1 mW/cm2 was estimated. The value of output power was less than that of the theoretical calculation. This can be attributed to the high internal resistance due to the low sintering density of each thermoelectric element.

Thermoelectric Properties of Sintered Bodies Made of Synthesized CoSb₃ Nanoparticles

K. Isogai and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-10 2008.12

Language：English

CoSb3 based skutterudite compounds are the promising thermoelectric materials that have relatively good thermoelectric properties with respect to the Seebeck coefficient and the electrical resistivity over temperature range of 600-800 K. The high thermal conductivity of the materials, however, prevents from achieving high thermoelectric performance. Superfine grain structure made by sintering superfine particles is expected to lower the thermal conductivity by phonon scattering. In the present work, we focused at investigating the thermoelectric properties of the sintered bodies made of the synthesized CoSb3 nanoparticles. The CoSb3 nanoparticles were synthesized via the modified polyol process. Two reactants Co(OOCH3)2·4H2O and SbCl3 were mixed at a stoichiometric mole ratio of 1:3, and were reacted with a reducing agent NaBH4 in a solvent tetraethyleneglycol at 513 K. Then, the reaction products were sintered into bulk samples by a pulse discharge sintering (PDS) under several operating conditions. The nanoparticles with size about 20 to 30 nm mainly existed in the reaction products. The CoSb3 phase was identified, but the other phases of Sb and CoSb2 were also detected. It was found that the synthesis of CoSb3 is promoted by PDS process. In the sample sintered at 853K for 15min under 60MPa, the grains with size about 200nm mainly existed together with several tens nanometer sized grains. The thermal conductivity of the finegrain sized sample was sufficiently reduced compared with that of the sample synthesized and sintered by only PDS process.

Fabrication of Thermoelectric Module by A New Manufacturing Process and Its Performance

Y. Ikuta and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-8 2008.12

Language：English

In the conventional production of the thermoelectric modules, process automation is difficult since the thermoelectric elements are produced by slicing only one at a time and the assembly of thermoelectric modules requires placement and attachment of the individual, and sometimes very small, thermoelectric elements. This leads to high production cost. Thus, new processing methods will be required for cost-effectively producing compact thermoelectric modules with a multitude of thermoelectric couples. In the present work, a new manufacturing process was attempted for producing the thermoelectric modules, and performances of the modules were evaluated. Powders of the p-type FeCo3Sb12 and the n-type Co0.92Ni0.08Sb2.96Te0.04 compounds were alternately filled into 4x4 square holes of an insulating mold. The powders in the holes were compacted and sintered simultaneously at 900 K in vacuum for 20 min under the isostatic pressure of 60 MPa. In sintering, a pseudo hot isostatic pressing (pseudo HIP) method were used to prevent the mold from breaking. The sintered elements in the mold were electrically connected in series using silver paste. Then, a thermoelectric module was constructed by parallel sandwiching the mold with glass plates of 0.5mm in thickness. The hot side of the module was heated up to 796 K, and the cold side was cooled by water. The maximum temperature difference between the hot and cold sides was 356 K. The maximum open voltage was 561 mV at the maximum temperature difference of 356 K, and the internal resistance in the module was 1.74 . The output power of 50.1 mW was obtained and the power density of 51.1 mW/cm2 was estimated. The value of output power was less than that of the theoretical calculation. This can be attributed to the high internal resistance due to the low sintering density of each thermoelectric element.

The effect of the sintering conditions and addition of fullerene on thermoelectric properties of Mg2Si

M. Matsuno and T. Itoh

Abstracts of The IUMRS International Coference in Asia 2008 page： CD-ROM UP-9 2008.12

Language：English

Magnesium silicide (Mg2Si) is a promising material for thermoelectric energy conversion at intermediate temperature range. This material has advantages of the
abundance of its constituent elements in the earth's crust and of the nontoxicity of its processing by-products as an environmentally friendly thermoelectric semiconductor. However, there are few research reports about this material for improving thermoelectric performance because of difficulty of its synthesis. In this study, Mg2Si was synthesized from Mg and Si by a liquid-solid phase reaction (LSPR) method, which is effective in synthesizing a compound from the two elements having sufficiently different melting points. Firstly, the influence of sintering conditions on the thermoelectric properties of Mg2Si was investigated. The powder synthesized by LSPR method was compacted and then sintered by a pulse discharge sintering (PDS) process. Thermoelectric properties, that is, Seebeck coefficient, electrical resistivity and thermal conductivity of each sintered sample were measured at temperature of 300 to 900 K. All sintered samples indicated the nature of n-type semiconductor over a measuring temperature range. The power factor of the sintered sample was, however, strongly affected by the sintering temperature and/or the sample density. The sample sintered at 1003 K for 30 min under 60MPa had the maximum power factor of about 0.11×10-2 mWm-1K-2 at 464 K. This value is higher than the previous report. Secondly, we attempted to add and disperse the fullerene nanoparticles into Mg2Si compound for lowering the thermal conductivity according to the knowledge obtained in our previous work. Mg2Si powder was mixed with fullerene by a planetary ball milling method, and then sintered by PDS process. In the samples sintered under the
same sintering condition, the power factor decreased with fullerene addition, because the electrical resistivity considerably increased and/or the sample density decreased.

Synthesis of Co_1-xNi_xSb₃ Thermoelectric Nanoparticles

K. Isogai, T. Itoh and K. Kitagawa

Abstracts of The 14th International Symposium on Advanced Technology and Applications (ATA) page： 42 2008.11

Language：English

Cobalt-antimony skutterudite compounds have a relatively high power factor over a temperature range of 600 to 800 K, but its thermal conductivity is relatively high. Nanostructuring is expected to lower the lattice thermal conductivity by grain boundary phonon scattering, improving the thermoelectric performance of the materials. Partial substitution of Ni on the Co site of CoSb3 is a hopeful route for obtaining n-type CoSb3-based skutterudite. In the present work, synthesis of Co1-xNixSb3 thermoelectric nanoparticles by the modified polyol process was attempted and the optimum synthesizing condition was investigated. Co(OOCH3)2·4H2O, Ni(OOCH3)2·4H2O and SbCl3 were mixed at the desired composition of x<0.1, and they were reduced by NaBH4 in tetraethyleneglycol at 513 K in an atmosphere of argon gas, for different reaction time (holding time). The nanoparticles of sizes ranging from about 20 to 30 nm mainly existed in the reaction products regardless of the composition and the reaction time. The skutterdite phase is identified as the main phase in the sample synthesized for the longer reaction time, but the other phases of Sb and MSb2 (M=Co, Ni) are also detected. The lattice constant of the synthesized skutterudite phase was almost linearly increased with Ni content. This is in good agreement with the Vegard's law.

Synthesis of Co1-xNixSb3 Thermoelectric Nanoparticles Reviewed

K. Isogai, T. Itoh and K. Kitagawa

Abstracts of The 14th International Symposium on Advanced Technology and Applications (ATA) page： 42 2008.11

Language：English

Cobalt-antimony skutterudite compounds have a relatively high power factor over a temperature range of 600 to 800 K, but its thermal conductivity is relatively high. Nanostructuring is expected to lower the lattice thermal conductivity by grain boundary phonon scattering, improving the thermoelectric performance of the materials. In our previous work, we have reported about the synthesis of p-type CoSb3 nanoparticles via a modified polyol process. Partial substitution of Ni on the Co site of CoSb3 is a hopeful route for obtaining n-type CoSb3-based skutterudite. In the present work, synthesis of Co1-xNixSb3 thermoelectric nanoparticles by the modified polyol process was attempted and the optimum synthesizing condition was investigated. Co(OOCH3)2·4H2O, Ni(OOCH3)2·4H2O and SbCl3 were mixed at the desired composition of x<0.1, and they were reduced by NaBH4 in tetraethyleneglycol at 513 K in an atmosphere of argon gas, for different reaction time (holding time). The reaction products were characterized by the X-ray diffraction, the energy dispersive X-ray spectroscopy, and transmission electron microscopy.The nanoparticles of sizes ranging from about 20 to 30 nm mainly existed in the reaction products regardless of the composition and the reaction time. The skutterdite phase is identified as the main phase in the sample synthesized for the longer reaction time, but the other phases of Sb and MSb2 (M=Co, Ni) are also detected. The lattice constant of the synthesized skutterudite phase was almost linearly increased with Ni content. This is in good agreement with the Vegard's law.

Synthesis of Thermoelectric Manganese Silicide by Mechanical Alloying and Pulse Discharge Sintering

T. Itoh and M. Yamada

Abstracts of ICT2008 (27th International Conference on Thermoelectric) page： 53 2008.8

Authorship：Lead author Language：English

Global energy and environmental issues are problems to be urgently solved. Thermoelectric power generation is a hopeful method to decrease dependence of fossil fuels and to promote the environmental safeguards. In this research, manganese silicide MnSi1.73 was focused as one of the promising thermoelectric materials available to the thermoelectric power generation using the disposed heat resources of 500 to 900K. Since both of Mn and Si as constituent elements in the manganese silicide are harmless and they exist in abundance as mineral resources, the manganese silicide can become a low-cost thermoelectric material with low-environmental load. Raw powders of manganese and silicon were mechanically alloyed through a planetary ball milling under the different milling time and the different rotating speed. Then the mechanical alloyed powder was consolidated through a pulse discharge sintering process (at 1023 K for 20 min in vacuum under a pressure of 100 MPa). The manganese silicide phases of MnSi1.73 (primary phase) and MnSi were synthesized in mechanical alloying and pulse discharge sintering. The thermoelectric properties were dependent on an XRD intensity ratio of MnSi1.73 to MnSi. The maximum dimensionless figure of merit ZT of 0.47 was achieved at 873K.

Effect of Combined Addition of Silicon Carbide Whiskers and Fullerene Nanoparticles on Properties of Thermoelectric Zinc Antimonide Reviewed

T. Itoh, N. Asari, N. Kanetake, A. Okada

Mater. Res. Soc. Symp. Proc. Vol. 1044 page： 265-272 2008.4

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Thermoelectric power generation is a hopeful method harnessing waste thermal energy particularly covering a middle temperature range between 500 and 800 K. A Zn4Sb3 compound is a promising "phonon glass electron crystal" material applicable to thermoelectric power generation around 700 K. This material, however, has a problem of its brittleness. In this research, the silicon carbide whiskers were added into the Zn4Sb3 compound for overcoming the brittleness, and the fullerene nanoparticles were also added for improving the thermoelectric performance. The Zn4Sb3 compound was synthesized from mixture of pure zinc and antimony powders by a liquid-solid phase reactions method. Firstly, the synthesized compound powder was mixed with the fullerene nanoparticles. The planetary ball milling method was used in order to disentangle the fullerene agglomerate and to obtain a uniform mixture. Subsequently, the mixture was uniformly mixed with the SiC whiskers by the planetary ball milling. The final mixture was consolidated by the pulse discharge sintering. The synthesized phases were identified by XRD. The morphology of the whiskers after mixing was observed. The flexural strength and the thermoelectric properties of the sintered samples were measured. The length of SiC whiskers and the flexural strength were decreased with the mixing time. Though the addition of SiC whiskers lowered the thermoelectric performance, the combined addition of SiC whiskers and fullerene nanoparticles restored the performance especially through a decrease of the thermal conductivity owing to the phonon scattering.

Thermoelectric Properties of β-Zn4Sb3 Synthesized by Mechanical Alloying and Pulse Discharge Sintering Reviewed

T. Itoh, J. Shan and K. Kitagawa

J. Propulsion and Power Vol. 24 ( 2 ) page： 353-358 2008.3

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Thermoelectric power generation is a hopeful method for harnessing waste thermal energy, particularly covering a middle temperature range between 500 and 800 K. A β-Zn4Sb3 compound has become the focus of attention as a thermoelectric material applicable to thermoelectric power generation around 700 K. In this research, a method combining the mechanical allying with the pulse discharge sintering was adopted to obtain the sintered compact of β-Zn4Sb3. Pure zinc and antimony powders were used as the starting material for mechanical alloying. These powders were mixed in the stoichiometry ratio of 4 to 3, or more Zn-rich. The mechanical alloying was carried out by planetary ball milling under conditions of fixed revolution speed and variable milling time. The influence of the milling time on the synthesis of β-Zn4Sb3 was investigated. The sintering was executed at two sintering temperatures and under fixed conditions of sintering time, pressing pressure, and atmosphere. The influences of the sintering temperature of pulse discharge sintering on the polycrystalline grain size and the thermoelectric properties were also studied. The following results were clarified. The phases of ZnSb and Zn are easily obtained by mechanical alloying for 50 h or longer time. These phases change to a single phase of β-Zn4Sb3 through pulse discharge sintering under the optimum condition of Zn-rich mixing. The sintered compact of β-Zn4Sb3 with 1.17 of nondimensional figure of merit can be obtained at 673 K with the proposed mechanical alloying with pulse discharge sintering method.

Thermoelectric Properties of Iron-and Lanthanum-Doped CoSb3 Compounds by Pulse Discharge Sintering Reviewed

T. Itoh, E. Hattori and K. Kitagawa

J. Propulsion and Power Vol. 24 ( 2 ) page： 359-364 2008.3

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

More than 60% of thermal energy generated through various combustion systems is discharged as waste heat into the atmosphere. Using the waste heat effectively is among the most important issues from a viewpoint of the depletion
of fossil fuels. In this regard, thermoelectric power generation that can directly convert the thermal energy to
electrical energy is one of the attractive methods. Skutterudite compounds of CoSb3 have been investigated as promising candidate materials for thermoelectric power generation. In this research, CoSb3 compounds were synthesized from raw metal powders by using a pulse discharge sintering process. To improve the thermoelectric properties, iron was doped as an element substituting for cobalt of the CoSb3 compound. The syntheses of FexCo1-xSb3 were confirmed by x-ray diffraction analysis. The Seebeck coefficient, electric resistivity, and thermal conductivity were measured as the thermoelectric properties. Optimum content of iron was decided from the maximum thermoelectric figure of merit. Furthermore, filling of lanthanum into Fe0.1Co0.9Sb3 was attempted to decrease the thermal conductivity. The lanthanum-filled Fe0.1Co0.9Sb3 compounds were synthesized from raw metal powders and LaSb powder by the pulse discharge sintering process. The optimum lanthanum content that leads to the maximum thermoelectric performance is discussed. The substitution of iron for cobalt improved the thermoelectric performance. The lanthanum-filling decreased the thermal conductivity at a low lanthanum content and led to higher thermoelectric performance in spite of low lanthanum content.

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