記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

We made composite films of Bismuth Telluride and Halide Perovskite at different sintering temperatures and characterized their thermoelectric properties. We aimed to improve thermoelectric properties through the reduction of effective thermal conductivity by using the interfacial thermal resistance between Halide Perovskite and Bismuth Telluride particles. Halide perovskite has been well investigated for a solar cell as one of the printable semiconductor materials. Here, we utilized Halide Perovskite as a thermoelectric material. The samples were prepared by printing and sintering the ink consisting of Bismuth Telluride powder produced by mechanical grinding and a precursor solution of Halide Perovskite. The thermoelectric performance was evaluated by a dimensionless figure-of-merit ZT at room temperature. The power factor of the composite films was higher than that of the pure Bismuth Telluride printed film, and the maximum value of power factor 205 μW/(m·K2) was obtained annealed at 350 °C. The thermal conductivity of the film was measured by bi-directional 3ω method. Compared the measured thermal conductivity of the composite film with the calculated thermal conductivity by simple models, we found that the thermal conductivity was greatly reduced by the high interfacial thermal resistance between Bismuth Telluride and Halide Perovskite. The proposed composite technique will be a promising method for improving both the thermoelectric properties and production cost performance.

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

One-step solution processed Cesium Tin halide perovskites (CsSnI3) known as lead-free halide perovskite were explored for thermoelectric applications. We report thermoelectric properties of CsSnI3 with some dopants, such as In, Sb, Ni, Mg and Cu in different concentrations (50, 100 ppm) to control the carrier concentration. The fabricated thin film shows p-Type characteristics and their power factors were around 100μW/(m2· K) with any dopants in different concentrations. Their properties agreed with ab-initio calculation for CsSnI3 under the constant relaxation time assumption (τ=2×10-14 s). The high electrical conductivity is realized by Sn4+ increase (Sn2+ oxidation) was confirmed by XPS measurements. Thermal conductivity was also measured to be around 0.3 W/(m· K) by differential 3ω method. The printable materials will will help for further development of direct thermal energy harvesting devices near room temperature.

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The elastocaloric effect, a temperature change induced by the external tensile or compressive strain, is of crucial interest for next-generation thermal management technologies because of its environmental friendliness and economic benefits. Toward applications of the elastocaloric effect, many efforts have been made to determine highly efficient elastocaloric materials. Here, another way of modulating the elastocaloric temperature change is reported by applying a process inspired by the Japanese art of paper-cutting, named kirigami. Simply, by stretching elastocaloric materials with kirigami structure, it is demonstrated that focused heat absorption and release can be generated simultaneously owing to nonuniform internal stress by cutting patterns. Owing to the ultrahigh stretchability of kirigami, the input stress required to generate the elastocaloric temperature change is dramatically reduced, resulting in an improvement in the local cooling/heating performance. The concept demonstrated here can be applied to any elastocaloric material and pave the way for constructing versatile solid-state heat pumps for flexible electronics.

DOI： 10.1002/adfm.202201116

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The Seebeck and Peltier effects have been widely studied and used in various thermoelectric technologies, including thermal energy harvesting and solid-state heat pumps. However, basic and applied studies on the Thomson effect, another fundamental thermoelectric effect in conductors, are limited despite the fact that the Thomson effect allows electronic cooling through the application of a temperature gradient bias rather than the construction of junction structures. In this article, we report the observation of a giant Thomson effect that appears owing to magnetic phase transitions. The Thomson coefficient of FeRh-based alloys reaches large values approaching -1000 μV K-1 around room temperature because of the steep temperature dependence of the Seebeck coefficient associated with the antiferromagnetic-ferromagnetic phase transition. The Thomson coefficient is several orders of magnitude larger than the Seebeck coefficient of the alloys. Using the active thermography technique, we demonstrate that the Thomson cooling can be much larger than Joule heating in the same material even in a nearly steady state. The operation temperature of the giant Thomson effect in the FeRh-based alloys can be tuned over a wide range by applying an external magnetic field or by slightly changing the composition. Our findings provide a new direction in the materials science of thermoelectrics and pave the way for thermal management applications using the Thomson effect.

DOI： 10.1063/5.0077497

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The spin Peltier effect (SPE) in Pt/Fe3O4 hybrid structures with epitaxial Fe3O4 layers synthesized by reactive sputtering using two different process gases, Ar/O2 and Kr/O2, was investigated. The magnitude of the SPE-induced temperature modulation for the Fe3O4 film grown using Kr/O2 was approximately 40% larger than that grown using Ar/O2 despite almost the same crystalline structures and magnetic and electric properties of the films. The enhancement of the SPE signal for the film grown with Kr/O2 can be attributed to an increase in the spin current injected into the Fe3O4 film owing to its large roughness.

DOI： 10.35848/1882-0786/ac44c9

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

<p>冷感マスクの冷却効果をふく射伝熱によって考察するため，それらの垂直入射-半球反射スペクトルを測定した．ポリエステル，ナイロン，綿，ポリプロピレンを主成分とする5種類のマスク，比較のため2種のインナー，1種の日傘で合計8種を測定対象とした．ポリエステルを主成分とする素材が放射伝熱冷却に優れている測定結果が得られ，3μm以下の可視光域で反射率が高く，赤外域で低かった．測定結果に基づき放射冷却に着目した性能指数を導入し，冷却マスクの放射冷却メカニズムを考察した．走査電子顕微鏡で観察し，表面に光学干渉する特徴的構造がなく，材料の持つ光学特性がふく射伝熱冷却性能を決めていると考えられる．</p>

DOI： 10.2963/jjtp.36.132

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

The introduction of spin caloritronics into thermoelectric conversion has paved a new path for versatile energy harvesting and heat sensing technologies. In particular, thermoelectric generation based on the anomalous Nernst effect (ANE) is an appealing approach as it shows considerable potential to realize efficient, large-area, and flexible use of heat energy. To make ANE applications viable, not only the improvement of thermoelectric performance but also the simplification of device structures is essential. Here, we demonstrate the construction of an anomalous Nernst thermopile with a substantially enhanced thermoelectric output and simple structure comprising a single ferromagnetic material. These improvements are achieved by combining the ANE with the magneto-optical recording technique called all-optical helicity-dependent switching of magnetization. Our thermopile consists only of Co/Pt multilayer wires arranged in a zigzag configuration, which simplifies microfabrication processes. When the out-of-plane magnetization of the neighboring wires is reversed alternately by local illumination with circularly polarized light, the ANE-induced voltage in the thermopile shows an order of magnitude enhancement, confirming the concept of a magneto-optically designed anomalous Nernst thermopile. The sign of the enhanced ANE-induced voltage can be controlled reversibly by changing the light polarization. The engineering concept demonstrated here promotes effective utilization of the characteristics of the ANE and will contribute to realizing its thermoelectric applications.

DOI： 10.1038/s41598-021-90865-5

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記述言語：英語   掲載種別：記事・総説・解説・論説等（その他）

line of text are correct below. (Formula presented) where we assumed ZtotT≪1 and the liner temperature gradient. All online versions of the article were corrected on 18 June 2021. AIP Publishing apologizes for this error..

DOI： 10.1063/5.0061893

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記述言語：英語   掲載種別：研究論文（学術雑誌）

This letter reports systematic measurements of the anisotropic magneto-Seebeck effect (AMSE) of ferromagnetic Ni100-x Pt x alloys with different Pt concentration ratios x. By simultaneously measuring an applied temperature difference and generated thermopower at the same positions, the Seebeck coefficient under a magnetic field is quantified with high reliability at the temperatures ranging from 300 to 550 K. The AMSE of the alloys is maximized at around x = 5 at% in the high temperature range and disappears at the temperatures lower than the Curie temperature. The systematic dataset and experimental techniques established here will invigorate spin caloritronics studies.

DOI： 10.35848/1882-0786/ac057c

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We phenomenologically calculate the performance of the recently observed Seebeck-driven transverse thermoelectric generation (STTG) for various systems in terms of the thermopower, power factor, and figure of merit to demonstrate the usefulness of STTG. The STTG system consists of a closed circuit comprising thermoelectric and magnetic materials which exhibit the Seebeck and anomalous Hall effects, respectively. When a temperature gradient is applied to the hybrid system, the Seebeck effect in the thermoelectric material layer generates a longitudinal charge current in the closed circuit, and the charge current subsequently drives the anomalous Hall effect in the magnetic material layer. The anomalous Hall voltage driven by the Seebeck effect has a similar symmetry to the transverse thermoelectric conversion based on the anomalous Nernst effect. We find that the thermoelectric properties of STTG can be much better than those of the anomalous Nernst effect by increasing the Seebeck coefficient and anomalous Hall angle of the thermoelectric and magnetic materials, respectively, as well as by optimizing their dimensions. We also formulate the electronic cooling performance in the STTG system, thereby confirming the reciprocal relation for the hybrid transverse thermoelectric conversion.

DOI： 10.1063/5.0055475

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data.

DOI： 10.1016/j.ijheatmasstransfer.2021.121076

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記述言語：英語   掲載種別：研究論文（学術雑誌）

When a temperature gradient is applied to a closed circuit comprising two different conductors, a charge current is generated via the Seebeck effect1. Here, we utilize the Seebeck-effect-induced charge current to drive ‘transverse’ thermoelectric generation, which has great potential for energy harvesting and heat sensing applications owing to the orthogonal geometry of the heat-to-charge-current conversion2–9. We found that, in a closed circuit comprising thermoelectric and magnetic materials, artificial hybridization of the Seebeck effect into the anomalous Hall effect10 enables transverse thermoelectric generation with a similar symmetry to the anomalous Nernst effect11–27. Surprisingly, the Seebeck-effect-driven transverse thermopower can be several orders of magnitude larger than the anomalous-Nernst-effect-driven thermopower, which is clearly demonstrated by our experiments using Co2MnGa/Si hybrid materials. The unconventional approach could be a breakthrough in developing applications of transverse thermoelectric generation.

DOI： 10.1038/s41563-020-00884-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Thermal switching provides an effective way for active heat flow control, which has recently attracted increasing attention in terms of nanoscale thermal management technologies. In magnetic and spintronic materials, the thermal conductivity depends on the magnetization configuration: this is the magnetothermal resistance effect. Here, we show that an epitaxial Cu/Co50Fe50 multilayer film exhibits giant magnetic-field-induced modulation of the cross-plane thermal conductivity. The magnetothermal resistance ratio for the Cu/Co50Fe50 multilayer reaches 150% at room temperature, which is much larger than the previous record high. Although the ratio decreases with increasing the temperature, the giant magnetothermal resistance effect of ∼100% still appears up to 400 K. The magnetic field dependence of the thermal conductivity of the Cu/Co50Fe50 multilayer was observed to be about twice greater than that of the cross-plane electrical conductivity. The observation of the giant magnetothermal resistance effect clarifies the potential of spintronic multilayers as thermal switching devices.

DOI： 10.1063/5.0032531

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We report an enhancement of the anomalous Nernst effect (ANE) in Ni/Pt (001) epitaxial superlattices. The transport and magnetothermoelectric properties were investigated for the Ni/Pt superlattices with various Ni layer thicknesses (t). The anomalous Nernst coefficient was increased up to more than 1μVK-1 for 2.0nm≤t≤4.0nm, which was the remarkable enhancement compared to the bulk Ni. It has been found that the large transverse thermoelectric conductivity (αxy), reaching αxy=4.8AK-1m-1 for t=4.0nm, plays a prime role for the enhanced ANE of the Ni/Pt (001) superlattices.

DOI： 10.1103/PhysRevB.103.L020402

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記述言語：英語   掲載種別：研究論文（学術雑誌）

For any thermoelectric effects to be achieved, a thermoelectric material must have hot and cold sides. Typically, the hot side can be easily obtained by excess heat. However, the passive cooling method is often limited to convective heat transfer to the surroundings. Since thermoelectric voltage is proportional to the temperature difference between the hot and cold sides, efficient passive cooling to increase the temperature gradient is of critical importance. Here, we report simultaneous harvesting of radiative cooling at the top and solar heating at the bottom to enhance the temperature gradient for a transverse thermoelectric effect which generates thermoelectric voltage perpendicular to the temperature gradient. We demonstrate this concept by using the spin Seebeck effect and confirm that the spin Seebeck device shows the highest thermoelectric voltage when both radiative cooling and solar heating are utilized. Furthermore, the device generates thermoelectric voltage even at night through radiative cooling which enables continuous energy harvesting throughout a day. Planar geometry and scalable fabrication process are advantageous for energy harvesting applications.

DOI： 10.1080/14686996.2021.1920820

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

In this paper, we design, develop, and test rotary freestanding triboelectric nanogenerators (TENGs) for harvesting energy from mechanical rotation. The objective is characterizing the fundamental processes involved in energy harvesting and obtaining a thorough understanding of the development process for TENG devices. An experimental testbed was built for mounting, testing, and analyzing the TENG electrical characteristics. To test the electrical performance, several versions of TENGs were created with varying design and measurement parameters. We measured the effects of changing the diameter (50, 75, 100mm) and changing the number of blades (4, 8 and 12-blade). We also analyzed the effect of changing RPM, applied torque and electrode materials on the output performance produced. Additionally, we also measured and characterized the performance of thin-film MEMS deposition techniques by depositing an Aluminum Oxide (Al2O3) layer and Aluminum (Al) electrode layer for the TENGs. The 12-blade 100mm TENG produced an open circuit voltage of 180VOC and a short circuit current of 10μA at 1000RPM and 0.04Nm applied torque. The Al2O3 thin-film blocking layer exhibited a two-fold increase in current and a three-fold increase in voltage output for the tested TENG devices.

DOI： 10.1109/PowerMEMS54003.2021.9658403

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

<p>The triboelectric generator consists of triboelectric charging and electrostatic induction. The generator is characterized by its light weight, flexibility to use a wide range of materials, and the ability to achieve miniaturization through microfabrication technology. Extracting the static electricity generated in our daily lives as electric power can be considered as a sustainable energy source for the future. In this study, we will measure the amount of power generated when the number of blades, rotational speed, torque, material, and size of the generator are varied, and discuss the power generation mechanism in terms of capacitance and external load.</p>

DOI： 10.1299/jsmeted.2021.0116

CiNii Research

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We report systematic measurements of thermal transport properties of a Ni-Co-based superalloy at temperatures ranging from room temperature to around 800 K. In this temperature range, the thermal diffusivity and conductivity of the superalloy monotonically increase with an increase in the temperature, while the specific heat capacity is almost temperature-independent. We found that the observed thermal conductivity of the Ni-Co-based superalloy is higher than the electronic thermal conductivity expected from the Wiedemann-Franz law, indicating the substantial contribution of phonon thermal conductivity. The temperature dependence of the phonon thermal conductivity for the Ni-Co-based superalloy was observed to be weaker than that for other Ni-based alloys, which can be qualitatively explained by enhanced phonon scattering due to small γ′-phase precipitates in the superalloy. This result suggests possible tuning of the thermal conductivity of the superalloy based on phonon transport engineering.

DOI： 10.1063/5.0030847

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We report the observation of the higher-order thermoelectric conversion based on a magneto-Thomson effect. By means of thermoelectric imaging techniques, we directly observed the temperature change induced by the Thomson effect in a polycrystalline Bi88Sb12 alloy under a magnetic field and found that the magnetically enhanced Thomson coefficient can be comparable to or even larger than the Seebeck coefficient. Our experiments reveal the significant contribution of the higher-order magnetothermoelectric conversion, opening the door to "nonlinear spin caloritronics."

DOI： 10.1103/PhysRevLett.125.106601

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Lightweight magnesium alloys with good thermal conductance are in demand for structural components in portable devices. This work presents a strategy to achieve excellent thermal conductivity, room temperature (RT) formability and high strength in magnesium sheet alloys. We found that a Mg-1.6Zn-0.5Ca-0.4Zr alloy shows a substantial increase in thermal conductivity to over 130 W/(m·K) due to the reduced supersaturation of solute atoms in the matrix by the precipitation of Guinier-Preston (G.P.) zones while exhibiting excellent RT formability with a large Index Erichsen value of 8.1 mm, and a high yield strength of 227 MPa by peak aging treatment.

DOI： 10.1080/21663831.2020.1759718

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The anomalous Ettingshausen effect (AEE) in SmCo5-type permanent magnets has been investigated in the high-temperature range from room temperature to around 600 K. The anomalous Ettingshausen coefficient of the SmCo5 and (SmGd)Co5 magnets monotonically increases with increasing the temperature and shows a similar temperature dependence, while the coefficient of SmCo5 is slightly larger than that of (SmGd)Co5 at high temperatures. The dimensionless figure of merit for the AEE in SmCo5 at high temperatures is much greater than the previous record obtained for the anomalous Nernst effect. The observed high-temperature behavior of the AEE is discussed based on the first-principles calculations of transverse transport coefficients.

DOI： 10.1063/5.0023111

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The second-order conventional and first-order inverse magnetocaloric effects (MCEs) in Ni-Mn-based quaternary Heusler alloys have been systematically investigated by means of the lock-in thermography technique, which enables the direct measurement of the MCE-induced temperature change in a periodic magnetic field. Through systematic measurements of the temperature dependence of the MCE signals, the tuning of the conventional and inverse MCEs with temperature for the same Heusler alloys has been demonstrated, where the phase transitions responsible for the MCEs are clearly distinguished. The lock-in thermography measurements show that some Ni-Mn-based Heusler alloys exhibit much smaller temperature changes due to the inverse MCEs in the periodic field as compared to the conventional MCEs, even though they exhibit a larger magnetic entropy change across the first-order transition responsible for the inverse MCEs. We discuss the origin of this behavior in terms of the field-induced entropy change and thermal hysteresis of the alloys. These findings will be useful not only in accelerating the optimization of inverse MCE materials but also in understanding the mechanism of the MCEs.

DOI： 10.1063/5.0005865

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, Ni95Pt5 exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is ∼12%. In contrast, the magnitude of the AEE signals is moderate in Ni95Pt5 but largest in Ni75Pt25 and Ni50Fe50. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.

DOI： 10.1103/PhysRevMaterials.4.034409

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo5-type magnet exhibits a large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo5-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that the large AEE originates from the intrinsic transverse thermoelectric conductivity of SmCo5. This finding makes a connection between permanent magnets and thermal energy engineering, providing the basis for creating "thermoelectric permanent magnets".

DOI： 10.1063/1.5131001

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We have investigated the anomalous Nernst effect (ANE) in Fe1-xGax alloy films with different Ga atomic compositions (x=0-0.44) deposited on MgO(001) single-crystalline substrates. We found that the magnitude of the ANE increases with increasing x up to x=0.32 even though the saturation magnetization decreases with increasing x, suggesting a dominant contribution of the intrinsic mechanism on the observed ANE. The magnitude of the ANE reaches 2.4μV/K in bcc Fe0.68Ga0.32 film, which is two orders of magnitude greater than that of pure Fe film. The magnetotransport measurements and the first-principles calculations revealed that the large ANE in bcc Fe1-xGax is caused by the large transverse Peltier coefficient. The drastic enhancement of the transverse Peltier coefficient with increasing x can be attributed to small Fermi level tuning through the electron doping effect. Thus, we anticipate that our finding will provide a crucial piece of information to enhance the thermopower through the ANE.

DOI： 10.1103/PhysRevMaterials.3.114412

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Thermoelectric conversion is capable of converting heat directly to electricity. However, actual implementations of thermoelectric devices are still limited with the leading bottleneck being the costs of materials and device integration, which calls for thermoelectric materials based on standard semiconductors with sufficient figure of merit (ZT) at room temperature. Bulk silicon crystal comes on the top of the list; however, the ZT even with nanostructuring has been limited due to its high thermal conductivity. Here, we have realized nanostructured silicon material with ZT larger than 0.3 at room temperature by a scalable process consisting of metal-assisted chemical etching and plasma-activated sintering. The material structure is highly complex being composed of randomly distributed nanograins, nanopores, and metal nanoprecipitates with hierarchical sizes, which significantly reduces thermal conductivity without appreciably sacrificing electrical conductivity. It is further identified by detailed experimental and theoretical investigations that the key contribution to the reduction comes from the softening of grain boundaries significantly limiting the interfacial phonon transmission. The developed high-performance silicon nanocomposite is expected to greatly enhance the application of thermoelectrics by lowering the material and process costs.

DOI： 10.1021/acsaem.9b00893

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We report the observation of anomalous Ettingshausen effect (AEE) in ferrimagnetic Co100-xGdx amorphous alloy thin films, which was visualized using lock-in thermography. We found that the sign of AEE is determined by the magnetic moments of Co sublattice, as well as those of the magneto-optical Kerr effect and the anomalous Hall effect, and the magnitude of AEE does not decrease even when x is close to the magnetization compensation composition. Our experimental results suggest that the magnetically-compensated ferrimagnets become candidates for high-performance thermomagnetic materials with tiny magnetization.

DOI： 10.7567/1882-0786/aafb5a

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We have fabricated bulk nanostructured ferrimagnetic materials with different grain sizes by sintering ball-milled Y3Fe5O12 (YIG) nanoparticles and measured the grain-size dependence of the thermal conductivity and spin Seebeck thermopower. The nanostructuring reduces both thermal conductivity and thermopower, but the reduction of the latter was found to be considerably stronger despite the moderate difference in magnetization, which suggests that the length scales of transport of magnons and phonons contributing to the spin Seebeck effect are significantly larger than that of phonons carrying thermal current. This is consistent with the measurements of high-magnetic-field response of the spin Seebeck thermopower and low-temperature thermal conductivity, where the quenching of magnons seen in single-crystalline YIG was not observed in nanostructured YIG due to scattering of long-range low frequency magnons.

DOI： 10.1103/PhysRevMaterials.1.014601

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記述言語：英語   掲載種別：研究論文（学術雑誌）

To solve the problem of lack of thermal percolation in thermal interface materials (TIM), we propose a two-step synthesis method to realize thermally percolated nanofiber network in polymer matrix. First, by packing vapor grown carbon fibers (VGCFs) on top of aluminum heat sink and integrally sintering the whole material, the aluminum partially melts and connects the VGCF network, forming a continuous thermal path, i.e., realizing thermal percolation. Second, the pores in the hybrid network are filled by Silicone oil to obtain a polymer nanocomposite. The direct synthesis of VGCF-aluminum network on the heat sink (onsite synthesis) omits pasting process of the TIM, and thus, removes the restriction on the network morphology. By this onsite synthesis method, we reinforce thermal contact not only between the nanofibers but also between nanofibers and the heat sink. By testing the developed TIM for thermal contact to silicon surface, we demonstrate the potential to significantly reduce thermal contact resistance from what can be achieved by a conventional TIM.

DOI： 10.1063/1.4941275

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Responding to the need for thermoelectric materials with high efficiency in both conversion and cost, we developed a nanostructured bulk silicon thermoelectric materials by sintering silicon crystal quantum dots of several nanometers in diameters synthesized by plasma-enhanced chemical vapor deposition (PECVD). The material consists of hybrid structures of nanograins of crystalline silicon and amorphous silicon oxide. The percolated nanocrystalline region gives rise to high power factor with the high doping concentration realized by PECVD, and the binding amorphous region reduces thermal conductivity. Consequently, the nondimensional figure of merit reaches 0.39 at 600°C, equivalent to the best reported value for silicon thermoelectrics. The thermal conductivity of the densely packed material is as low as 5 W m-1 K-1 in a wide temperature range from room temperature to 1000°C, which is beneficial not only for the conversion efficiency but also for material cost by requiring less material to establish certain temperature gradient. (Figure Presented).

DOI： 10.1021/acsami.5b02537

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記述言語：英語   掲載種別：研究論文（学術雑誌）

DOI： 10.1103/PhysRevX.4.041023

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受賞国：日本国