Updated on 2024/05/14

写真a

 
Keisuke Hirata
 
Organization
Graduate School of Engineering Department of Advanced Science and Technology Materials Science and Engineering Energy Materials Laboratory Assistant Professor   
Degree
博士(工学) ( 2023.9   豊田工業大学 )

Professional Memberships

  • The Japan Society of Applied Physics

    2019

  • The Thermoelectrics Society of Japan

    2018.9

Papers

  • Composition, time, temperature, and annealing-process dependences of crystalline and amorphous phases in ductile semiconductors Ag2S1−xTex with x = 0.3–0.6 Reviewed

    Kosuke Sato, Keisuke Hirata, Masaharu Matsunami, Tsunehiro Takeuchi

    AIP Advances   14 ( 1 )   2024.1 (     eISSN:2158-3226 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    We investigate composition, time, temperature, and annealing-process dependences of crystalline and amorphous phases in ductile semiconductors Ag2S1−xTex with x = 0.3–0.6. We reveal that a metastable amorphous phase containing no secondary phases is obtainable at x = 0.6 even with furnace cooling and possesses ductility in the same manner as the end compound of Ag2S, while the high-temperature phase (HTP) of Ag2S precipitates in the amorphous phase at x = 0.3–0.5 by keeping the good ductility. During the crystallization process of the amorphous phase by annealing a sample at 373–503 K for 4–14 days and cooling it down slowly to room temperature, HTP of Ag2S disappears and the low-temperature phase (LTP) of Ag2S and the Ag5−dTe3 phase appear, while the amorphous phase remains. The ductility is observed for the samples containing the LTP of Ag2S but not for those containing the Ag5−dTe3 phase. Based on the obtained results, the possible phase diagram of Ag2S1−xTex with x = 0.3–0.6 is proposed, and the origin of the ductility in the LTP and HTP of Ag2S and amorphous phase is discussed. We believe that our study is helpful for properly predicting mechanical and transport properties of this material and developing this material as a component of bendable/wearable electronic devices for long-term use.

    DOI: 10.1063/5.0180950

  • Improvement of thermoelectric performance of flexible compound Ag2S0.55Se0.45 by means of partial V-substitution for Ag Reviewed

    Kosuke Sato, Saurabh Singh, Itsuki Yamazaki, Keisuke Hirata, Artoni Kevin R. Ang, Masaharu Matsunami, Tsunehiro Takeuchi

    AIP Advances   13 ( 12 )   2023.12 (     eISSN:2158-3226 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    The effects of the partial V-substitution for Ag on the thermoelectric (TE) properties are investigated for a flexible semiconducting compound Ag2S0.55Se0.45. Density functional theory calculations predict that such a partial V-substitution constructively modifies the electronic structure near the bottom of the conduction band to improve the TE performance. The synthesized Ag1.97V0.03S0.55Se0.45 is found to possess a TE dimensionless figure-of-merit (ZT) of 0.71 at 350 K with maintaining its flexible nature. This ZT value is relatively high in comparison with those reported for flexible TE materials below 360 K. The increase in the ZT value is caused by the enhanced absolute value of the Seebeck coefficient with less significant variation in electrical resistivity. The high ZT value with the flexible nature naturally allows us to employ the Ag1.97V0.03S0.55Se0.45 as a component of flexible TE generators.

    DOI: 10.1063/5.0171888

  • Huge Peltier conductivity in valence fluctuating material Yb3Si5 Reviewed

    Kentaro Kuga, Keisuke Hirata, Masaharu Matsunami, Tsunehiro Takeuchi

    Applied Physics Letters   123 ( 20 )   2023.11 (   ISSN:0003-6951   eISSN:1077-3118 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    We report the low temperature thermoelectric properties in the valence fluctuating system Yb3Si5 using high-quality single crystals. The coexistence of the tiny residual resistivity of 0.06 μΩ cm and the considerable Seebeck coefficient of 9 μV K−1 at 7 K leads to a huge Peltier conductivity of 40 A cm−1 K−1 and a power factor of 40 mW m−1 K−2. These pronounced characteristics are possibly attributed to the uncompensated semimetallic state in addition to the valence fluctuation of Yb ions. The large size of Yb3Si5 single crystal, which is typically ϕ 0.5–1 mm in diameter and 10 mm in length, enables the realistic application to the cryogenic thermoelectric device.

    DOI: 10.1063/5.0172472

  • Reduction of working temperature for the large magnitude in thermoelectric dimensionless figure of merit of Ag2−xCuxS Reviewed

    Kosuke Sato, Gareoung Kim, Koki Murase, Saurabh Singh, Keisuke Hirata, Dogyun Byeon, Masaharu Matsunami, Tsunehiro Takeuchi

    Japanese Journal of Applied Physics   62 ( 11 )   111002 - 111002   2023.11 (   ISSN:0021-4922   eISSN:1347-4065 )

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    Publishing type:Research paper (scientific journal)   Publisher:IOP Publishing  

    Abstract

    In this study, we try to reduce the temperature range for the large magnitude of dimensionless figure of merit ZT = 20 that was observed for the Ag2S composite consisting of low- and high-temperature phases under a unique temperature gradient at around 400 K. It reveals that partial substitution of Cu for Ag sites in Ag2S reduces the phase transition temperature, and subsequently the temperature range for this high ZT down to a temperature of 373 K. This result strongly suggests that our developed Cu-substituted Ag2S could be one of the best thermoelectric component materials in the generators capable of effectively recovering electric power from heat exchangers using hot water as a working liquid.

    DOI: 10.35848/1347-4065/ad0579

    Other Link: https://iopscience.iop.org/article/10.35848/1347-4065/ad0579/pdf

  • Development of Cu2Se/Ag2(S,Se)-Based Monolithic Thermoelectric Generators for Low-Grade Waste Heat Energy Harvesting Reviewed

    Artoni Kevin R. Ang, Itsuki Yamazaki, Keisuke Hirata, Saurabh Singh, Masaharu Matsunami, Tsunehiro Takeuchi

    ACS Applied Materials & Interfaces   15 ( 40 )   46962 - 46970   2023.9 (   ISSN:1944-8244   eISSN:1944-8252 )

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    Publishing type:Research paper (scientific journal)   Publisher:American Chemical Society (ACS)  

    DOI: 10.1021/acsami.3c09823

  • Significantly reduced lattice thermal conductivity with anharmonic lattice vibrations and band edge effect in electronic thermal conductivity in Ag2S1-xSex (0 ≤ x ≤ 0.6) Reviewed

    Keisuke Hirata, Saurabh Singh, Tsunehiro Takeuchi

    AIP Advances   13 ( 3 )   2023.3 (     eISSN:2158-3226 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    The composition dependence of the unusual behaviors in electronic and lattice thermal conductivity, κele and κlat, in Ag2S1-xSex (x = 0–0.6 in steps of 0.1, 300 K) is investigated in detail by means of precise electron and heat transport properties measurement, synchrotron X-ray crystal structure and electron density distribution analyses, and sound velocity measurement. We reveal that the κele of Ag2S1-xSex is strongly affected by the fine electronic structure of the conduction band edge near the chemical potential and the thermoelectric motive force; therefore, these effects make the κele of Ag2S1-xSe far different from that calculated by the Wiedemann–Franz law, κele = L0σT, with the Lorentz number L0 = π2kB2/(3e2). It is also clearly demonstrated that the κlat of Ag2S1-xSex is greatly reduced by anharmonic lattice vibrations and that the magnitude of κlat is quantitatively reproduced by an equation representing the thermal conductivity under the strongest scattering limit. The κlat decreases with increasing x and saturates at 0.4 W m−1 K−1 at x ≥ 0.4. This is caused by the increasing anharmonic lattice vibrations with x, and its saturating behavior is determined by the strongest scattering limit. On the other hand, a negligibly small κele at x = 0 turns out to be non-trivial at x ≥ 0.4 owing to the increasing carrier density with x, most likely contributed by the increasing interstitial Ag defects. Consequently, the total thermal conductivity of Ag2S1−xSex becomes minimum not at x = 0.5 (composition of the maximum structure entropy) but at x = 0.3.

    DOI: 10.1063/5.0142241

  • Magneto-thermal conductivity effect and enhanced thermoelectric figure of merit in Ag2Te Reviewed

    Keisuke Hirata, Kentaro Kuga, Masaharu Matsunami, Minyue Zhu, Joseph P. Heremans, Tsunehiro Takeuchi

    AIP Advances   13 ( 1 )   2023.1 (     eISSN:2158-3226 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    In this study, we report a large magneto-thermal conductivity effect, potentially usable in heat flow switches and thermoelectric devices, in Ag2Te over a wide temperature range, including room temperature. When a magnetic field of μ0H = 9 T is applied to Ag2Te at 300 K along the direction perpendicular to the heat and electric currents, the thermal conductivity κ decreases by a remarkable 61%. This effect is mainly caused by the suppressed electronic thermal conductivity in association with a significant magnetoresistance effect, but the suppression of the thermal conductivity is larger than that of the electrical conductivity, presumably due to a field-induced decrease in the Lorenz ratio. Its very low lattice thermal conductivity, as low as 0.5 W m−1 K−1, also greatly contributes to the large relative magneto-thermal conductivity effect. The significant decrease in thermal conductivity and the 18% increase in the Seebeck coefficient S lead to a nearly 100% increase in the thermoelectric figure of merit zT = S2σTκ−1 despite the 43% decrease in electrical conductivity σ.

    DOI: 10.1063/5.0131326

  • Enhanced Performance of Monolithic Chalcogenide Thermoelectric Modules for Energy Harvesting via Co-optimization of Experiment and Simulation Reviewed

    Huajun Lai, Saurabh Singh, Ying Peng, Keisuke Hirata, Masahiro Ryu, Artoni Kevin R. Ang, Lei Miao, Tsunehiro Takeuchi

    ACS Applied Materials & Interfaces   14 ( 34 )   38642 - 38650   2022.8 (   ISSN:1944-8244   eISSN:1944-8252 )

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    Publishing type:Research paper (scientific journal)   Publisher:American Chemical Society (ACS)  

    DOI: 10.1021/acsami.2c06349

  • Electronic structure of silver chalcogenides investigated by hard x-ray photoemission spectroscopy and density functional theory calculations Reviewed

    K. Sato, K. Hirata, S. Singh, K. Kuga, F. Ogawa, M. Matsunami, T. Takeuchi

    Journal of Applied Physics   132 ( 4 )   2022.7 (   ISSN:0021-8979   eISSN:1089-7550 )

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    Publishing type:Research paper (scientific journal)   Publisher:AIP Publishing  

    We have investigated the electronic structure of silver chalcogenides Ag2X (X = S, Se, Te) and their solid solutions using hard x-ray photoemission spectroscopy in combination with density functional theory calculations using generalized gradient approximation (GGA). By including the corrections for on-site Coulomb interactions (GGA + U), we successfully reproduced the valence band photoemission spectra, which consist mainly of the Ag 4d band, by calculation. The estimated values for U = 4–6 eV are slightly high for Ag 4d electrons but are consistent with those used in previously reported structural studies. On the other hand, the magnitude of the energy gap is virtually independent of U. These results suggest the strong correlation between Ag 4d electrons in Ag2X compounds to have surprisingly little impact on their electron transport properties.

    DOI: 10.1063/5.0095987

  • Synergetic Enhancement of the Power Factor and Suppression of Lattice Thermal Conductivity via Electronic Structure Modification and Nanostructuring on a Ni- and B-Codoped p-Type Si–Ge Alloy for Thermoelectric Application Reviewed

    Omprakash Muthusamy, Saurabh Singh, Keisuke Hirata, Kentaro Kuga, Santhana Krishnan Harish, Masaru Shimomura, Masahiro Adachi, Yoshiyuki Yamamoto, Masaharu Matsunami, Tsunehiro Takeuchi

    ACS Applied Electronic Materials   3 ( 12 )   5621 - 5631   2021.12 (   ISSN:2637-6113   eISSN:2637-6113 )

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    Publishing type:Research paper (scientific journal)   Publisher:American Chemical Society (ACS)  

    DOI: 10.1021/acsaelm.1c01075

  • Capacitor-type thin-film heat flow switching device Reviewed

    Keisuke Hirata, Takuya Matsunaga, Saurabh Singh, Masaharu Matsunami, Tsunehiro Takeuchi

    Japanese Journal of Applied Physics   60 ( 12 )   124004 - 124004   2021.12 (   ISSN:0021-4922   eISSN:1347-4065 )

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    Publishing type:Research paper (scientific journal)   Publisher:IOP Publishing  

    Abstract

    We developed a capacitor-type heat flow switching device, in which electron thermal conductivity of the electrodes is actively controlled through the carrier concentration varied by an applied bias voltage. The device consisted of an amorphous p-type Si–Ge–Au alloy layer, an amorphous SiO2 as the dielectric layer, and an n-type Si substrate. Both amorphous materials are characterized by very low lattice thermal conductivity, ≤1 W m–1 K–1. The Si–Ge–Au amorphous layer with 40 nm in thickness was deposited by means of molecular beam deposition technique on the 100 nm thick SiO2 layer formed at the top surface of Si substrate. Bias voltage-dependent heat flow density through the fabricated device was evaluated by a time-domain thermoreflectance method at room temperature. Consequently, we observed a 55% increase in the heat flow density at the maximum.

    DOI: 10.35848/1347-4065/ac3723

    Other Link: https://iopscience.iop.org/article/10.35848/1347-4065/ac3723/pdf

  • Mixed-phase effect of a high Seebeck coefficient and low electrical resistivity in Ag2S Reviewed

    Gareoung Kim, Dogyun Byeon, Saurabh Singh, Keisuke Hirata, Seongho Choi, Masaharu Matsunami, Tsunehiro Takeuchi

    Journal of Physics D: Applied Physics   54 ( 11 )   115503 - 115503   2021.1 (   ISSN:0022-3727   eISSN:1361-6463 )

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    Publishing type:Research paper (scientific journal)   Publisher:IOP Publishing  

    Abstract

    The improvement of thermoelectric performance is very challenging, because the thermoelectric properties, the Seebeck coefficient, electrical resistivity, and thermal conductivity are generally correlated with each other via a common physical quantity known as carrier concentration. In this study, we successfully achieved separate control of the thermoelectric properties of polycrystalline Ag2S using spatial-phase separation into low- and high-temperature phases using a configuration based on bottom heating and top measurement. We experimentally confirmed that the Seebeck coefficient was determined by the low-temperature phase at the top surface but the electrical resistivity was dominated by the high-temperature phase lying below the low-temperature phase. As a result, a high Seebeck coefficient ($\sim\! - 650\, \mu {\text{V } }{ { \text{K } }^{ - 1 } }$) and low electrical resistivity ($\sim\! 2\,{\text{m}\Omega\ \text{cm } }$) were simultaneously observed over a broad temperature range (390 ∼ 440 K). This experimental discovery suggests a new concept for thermoelectric materials and devices.

    DOI: 10.1088/1361-6463/abcfe7

    Other Link: https://iopscience.iop.org/article/10.1088/1361-6463/abcfe7/pdf

  • A Field Effect Heat Flow Switching Device Reviewed

    Takuya Matsunaga, Keisuke Hirata, Saurabh Singh, Masaharu Matsunami, Tsunehiro Takeuchi

    MATERIALS TRANSACTIONS   62 ( 1 )   16 - 19   2021.1 (   ISSN:1345-9678   eISSN:1347-5320 )

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    Publishing type:Research paper (scientific journal)   Publisher:Japan Institute of Metals  

    DOI: 10.2320/matertrans.e-m2020844

  • Development of High-Performance Solid-State Thermal Diodes Using Unusual Behavior of Thermal Conductivity Observed for Ag2Ch (Ch = S, Se, Te) Reviewed

    Keisuke Hirata, Takuya Matsunaga, Singh Saurabh, Masaharu Matsunami, Tsunehiro Takeuchi

    MATERIALS TRANSACTIONS   61 ( 12 )   2402 - 2406   2020.12 (   ISSN:1345-9678   eISSN:1347-5320 )

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    Publishing type:Research paper (scientific journal)   Publisher:Japan Institute of Metals  

    DOI: 10.2320/matertrans.e-m2020847

  • Dynamical variation of carrier concentration and colossal Seebeck effect in Cu2S low-temperature phase Reviewed

    Dogyun Byeon, Robert Sobota, Keisuke Hirata, Saurabh Singh, Seongho Choi, Masahiro Adachi, Yoshiyuki Yamamoto, Masaharu Matsunami, Tsunehiro Takeuchi

    Journal of Alloys and Compounds   826   154155 - 154155   2020.6 (   ISSN:0925-8388 )

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    Publishing type:Research paper (scientific journal)   Publisher:Elsevier BV  

    DOI: 10.1016/j.jallcom.2020.154155

  • High-Performance Solid-State Thermal Diode Consisting of Ag2(S,Se,Te) Reviewed

    Keisuke Hirata, Takuya Matsunaga, Saurabh Singh, Masaharu Matsunami, Tsunehiro Takeuchi

    Journal of Electronic Materials   49 ( 5 )   2895 - 2901   2020.2 (   ISSN:0361-5235   eISSN:1543-186X )

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    Publishing type:Research paper (scientific journal)   Publisher:Springer Science and Business Media LLC  

    DOI: 10.1007/s11664-020-07964-8

    Other Link: http://link.springer.com/article/10.1007/s11664-020-07964-8/fulltext.html

  • Investigation of Thermoelectric Properties of Ag2SxSe1−x (x = 0.0, 0.2 and 0.4) Reviewed

    Saurabh Singh, Keisuke Hirata, Dogyun Byeon, Takuya Matsunaga, Omprakash Muthusamy, Swapnil Ghodke, Masahiro Adachi, Yoshiyuki Yamamoto, Masaharu Matsunami, Tsunehiro Takeuchi

    Journal of Electronic Materials   49 ( 5 )   2846 - 2854   2019.12 (   ISSN:0361-5235   eISSN:1543-186X )

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    Publishing type:Research paper (scientific journal)   Publisher:Springer Science and Business Media LLC  

    DOI: 10.1007/s11664-019-07879-z

    Other Link: http://link.springer.com/article/10.1007/s11664-019-07879-z/fulltext.html

  • Discovery of colossal Seebeck effect in metallic Cu2Se Reviewed

    Dogyun Byeon, Robert Sobota, Kévin Delime-Codrin, Seongho Choi, Keisuke Hirata, Masahiro Adachi, Makoto Kiyama, Takashi Matsuura, Yoshiyuki Yamamoto, Masaharu Matsunami, Tsunehiro Takeuchi

    Nature Communications   10 ( 1 )   2019.1 (     eISSN:2041-1723 )

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    Publishing type:Research paper (scientific journal)   Publisher:Springer Science and Business Media LLC  

    Abstract

    Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m−1 K−2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K−1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm−1 leads to a colossal value of S2σ = 2.3 W m–1 K–2. The small thermal conductivity less than 2 W m−1 K−1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

    DOI: 10.1038/s41467-018-07877-5

    Other Link: https://www.nature.com/articles/s41467-018-07877-5

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Awards

  • Toyota scholarship foundation award

    2020.3   Toyota Technological Institute  

  • Outstanding oral presentation award (15th Thermoelectrics Society of Japan)

    2018.9   Thermoelectrics Society of Japan