記述言語：英語   掲載種別：研究論文（学術雑誌）

Nanocellulose is regarded as a green and renewable nanomaterial that has attracted increased attention. In this study, we demonstrate that nanocellulose materials can exhibit high thermal conductivity when their nanofibrils are highly aligned and bonded in the form of filaments. The thermal conductivity of individual filaments, consisting of highly aligned cellulose nanofibrils, fabricated by the flow-focusing method is measured in dried condition using a T-type measurement technique. The maximum thermal conductivity of the nanocellulose filaments obtained is 14.5 W/m-K, which is approximately five times higher than those of cellulose nanopaper and cellulose nanocrystals. Structural investigations suggest that the crystallinity of the filament remarkably influence their thermal conductivity. Smaller diameter filaments with higher crystallinity, that is, more internanofibril hydrogen bonds and less intrananofibril disorder, tend to have higher thermal conductivity. Temperature-dependence measurements also reveal that the filaments exhibit phonon transport at effective dimension between 2D and 3D.

DOI： 10.1021/acs.nanolett.2c02057

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記述言語：英語   掲載種別：研究論文（学術雑誌）

To meet the increasing demand for highly efficient heat dissipation in power electronics, a heat spreader that has significantly greater isotropic thermal conductivity than the commonly used copper (400 W/m·K) should be developed. Although graphite is a promising candidate because of its high basal-plane thermal conductivity, its application is restricted by its low c axis thermal conductivity. This issue can be resolved by transforming graphite into an isotropic thermal conductor by building a structure that can effectively route heat in all three dimensions. Herein, we develop a double-decker structure with differently oriented graphite layers to realize high heat dissipation from a local heat source. The critical issue of bonding the graphite layers is overcome by a high-temperature process using Cu as the binding layer. The graphite/Cu composite efficiently dissipates heat nearly isotropically and performs as well as an isotropic conductor with a thermal conductivity of 900 W/m·K.

DOI： 10.1016/j.xcrp.2021.100621

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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）

Characterizing coefficient of thermal expansion (CTE) for thin films is often challenging as the experimental signal is asymptotically reduced with decreasing thickness. Here, we present a method to measure CTE of thin films by locally confining an active thermal volume using harmonic Joule heating. Importantly, we simultaneously probe the harmonic expansion at atomic-scale thickness resolution using atomic force microscopy. We use a differential method on lithographically patterned thin films to isolate the topographical and harmonic thermal expansion contributions of the thin films. Based on the measured thermal expansion, we use numerical simulations to extract the CTE considering the stress induced from neighboring layers. We demonstrate our method using poly(methyl methacrylate), and the measured CTE of 55.0 × 10-6 ± 6.4 × 10-6 K-1 shows agreement with previous works. This work paves an avenue for investigating thermo-mechanical characterization in numerous materials systems, including both organic and inorganic media.

DOI： 10.1063/5.0049160

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Aiming at developing high thermal conductivity copper/diamond composite, an unconventional approach applying self-assembled monolayer (SAM) prior to the high-temperature sintering of copper/diamond composite was utilized to enhance the thermal boundary conductance (TBC) between copper and diamond. The enhancement was first systematically confirmed on a model interface system by detailed SAM morphology characterization and TBC measurements. TBC significantly depends on the SAM coverage and ordering, and the formation of high-quality SAM promoted the TBC to 73 MW/m2-K from 27 MW/m2-K, the value without SAM. With the help of molecular dynamics simulations, the TBC enhancement was identified to be determined by the number of SAM bridges and the overlap of vibrational density of states. The diamond particles of 210 μm in size were simultaneously functionalized by SAM with the condition giving the highest TBC in the model system and sintered together with the copper to fabricate isotropic copper/diamond composite of 50% volume fraction. The measured thermal conductivity marked 711 W/m-K at room temperature, the highest value among the ones with similar diamond-particles volume fraction and size. This work demonstrates a novel strategy to enhance the thermal conductivity of composite materials by SAM functionalization.

DOI： 10.1016/j.carbon.2021.01.018

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Since solid-state heat transport in a highly porous nanocomposite strongly depends on the thermal boundary conductance (TBC) between constituent nanomaterials, further suppression of the TBC is important for improving performance of thermal insulators. Here, targeting a nanocomposite fabricated by stamping fumed silica nanoparticles, we perform a wide variety of surface functionalizations on fumed silica nanoparticles by a silane coupling method and investigate the impact on the thermal conductivity (Km). The Km of the silica nanocomposite is approximately 20 and 9 mW/m/K under atmospheric and vacuum conditions at the material density of 0.2 g/cm3 without surface functionalization, respectively, and the experimental results indicate that the Km can be modulated depending on the chemical structure of molecules. The surface modification with a linear alkyl chain of optimal length significantly suppresses Km by approximately 30%, and the suppression can be further enhanced to approximately 50% with an infrared opacifier. The magnitude of suppression was found to sensitively depend on the length of the terminal chain. The magnitude is also related to the number of reactive silanol groups in the chemical structure, where the surface modification with fluorocarbon gives the largest suppression. The surface hydrophobization merits thermal insulation through significant suppression of the TBC, presumably by reducing the water molecules that otherwise would serve as heat conduction channels at the interface. On the other hand, when the chain length is long, the suppression is counteracted by the enhanced phonon transmission through the silane coupling molecules that grow with the chain length. This is supported by the analytical model and present simulation results, leading to prediction of the optimal chemical structure for better thermal insulation.

DOI： 10.1021/acsami.0c11066

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Thermal boundary conductance is typically positively correlated with interfacial adhesion at the interface. Here, we demonstrate a counterintuitive experimental result in which a weak van der Waals interface can give a higher thermal boundary conductance than a strong covalently bonded interface. This occurs in a system with highly mis matched vibrational frequencies (copper/diamond) modified by a self-Assembled monolayer. Using finely con trolled fabrication and detailed characterization, complemented by molecular simulation, the effects of bridging the vibrational spectrum mismatch and bonding at the interface are systematically varied and understood from a molecular dynamics viewpoint. The results reveal that the bridging and binding effects have a trade-off relationship and, consequently, that the bridging can overwhelm the binding effect at a highly mismatched interface. This study provides a comprehensive understanding of phonon transport at interfaces, unifying physical and chemical understandings, and allowing interfacial tailoring of the thermal transport in various material systems.

DOI： 10.1126/sciadv.abf8197

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Organic thin film materials with molecular ordering are gaining attention as they exhibit semiconductor characteristics. When using them for electronics, the thermal management becomes important, where heat dissipation is directional owing to the anisotropic thermal conductivity arising from the molecular ordering. However, it is difficult to evaluate the anisotropy by simultaneously measuring in-plane and cross-plane thermal conductivities of the film on a substrate because the film is typically as thin as tens to hundreds of nanometers and its in-plane thermal conductivity is low. Here, we develop a novel bidirectional 3ω system that measures the anisotropic thermal conductivity of thin films by patterning two metal wires with different widths and preparing the films on top and extracting the in-plane and cross-plane thermal conductivities using the difference in their sensitivities to the metal-wire width. Using the developed system, the thermal conductivity of spin-coated poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with thickness of 70 nm was successfully measured. The measured in-plane thermal conductivity of PEDOT:PSS film was as high as 2.9 W m-1 K-1 presumably due to the high structural ordering, giving an anisotropy of 10. The calculations of measurement sensitivity to the film thickness and thermal conductivities suggest that the device can be applied to much thinner films by utilizing metal wires with a smaller width.

DOI： 10.1063/5.0030982

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担当区分：最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Cellulose nanofibers (CNF) are abundant biomaterials that have attracted significant attention in the thermal management field for a wide range of applications including flexible heat dissipation materials and thermal insulators. While thermal transport properties of individual CNFs are significant for the fundamental understanding and design of advanced materials, experimental studies of the thermal transport properties of CNFs are limited to bulk scales and thermal measurement on individual CNFs has not been reported to date. We report here the experimental study on the thermal conductivity (κ) of individual CNFs using the well-established thermal bridge method. The κ of individual CNFs is found to be approximately 2.2 (±1.2) W/m K at 300 K, and the temperature dependent data from 40 to 320 K indicate that the phonon transport of CNFs is dominated by boundary scattering. Theoretical simulation results on κ of individual CNFs and cellulose bulk crystal support the experimental results and suggest that intermolecular interaction also impedes the thermal transport.

DOI： 10.1063/5.0042463

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Scalability is a common challenge in the structuring of nanoscale particle dispersions, particularly in the drying of these dispersions for producing functional, porous structures such as aerogels. Aerogel production relies on supercritical drying, which exhibits poor scalability. A solution to this scalability limitation is the use of evaporative drying under ambient pressure. However, the evaporative drying of wet gels comprising nanoscale particles is accompanied by a strong capillary force. Therefore, it is challenging to produce evaporative-dried gels or "xerogels"that possess the specific structural profiles of aerogels such as mesoscale pores, high porosity, and high specific surface area (SSA). Herein, we demonstrate a structure of mesoporous xerogels with high porosity (∼80%) and high SSA (>400 m2 g-1) achieved by exploiting cellulose nanofibers (CNFs) as the building blocks with tunable interparticle interactions. CNFs are sustainable, wood-derived materials with high strength. In this study, the few-nanometer-wide CNFs bearing carboxy groups were structured into a stable network via ionic inter-CNF interaction. The outline of the resulting xerogels was then tailored into a regular, millimeter-thick, board-like structure. Several characterization techniques highlighted the multifunctionality of the CNF xerogels combining outstanding strength (compression E = 170 MPa, σ = 10 MPa; tension E = 290 MPa, σ = 8 MPa), moderate light permeability, thermal insulation (0.06-0.07 W m-1 K-1), and flame self-extinction. As a potential application of the xerogels, daylighting yet insulating, load-bearing wall members can be thus proposed.

DOI： 10.1021/acsnano.0c08769

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Individual carbon nanotubes (CNTs) possess extremely high thermal conductivities. However, the thermal conductivities and their anisotropy of macroscopic assemblies of CNTs have so far remained small. Here, we report the results of directional thermal transport measurements on a nearly perfectly aligned CNT film fabricated via controlled vacuum filtration. We found the thermal conductivity to be 43 ± 2.2 W m-1 K-1 with a record-high thermal anisotropy of 500. From the temperature dependence of the thermal conductivity and its agreement with the atomistic phonon transport calculation, we conclude that the effect of intertube thermal resistance on heat conduction in the alignment direction is negligible because of the large contact area between CNTs. These observations thus represent ideal unidirectional thermal transport, i.e., the thermal conductivity of the film is determined solely by that of individual CNTs.

DOI： 10.1063/1.5127209

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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）

Thermal boundary conductance between graphite and metal plays an important role in developing thermally conductive composites and contacts for thermal management. On the basis of the premise that the thermal boundary conductance (TBC) correlates with interfacial bonding strength, we conducted triazine-based molecular-bonding process to improve interfacial adhesion forces between a-axis of highly oriented pyrolytic graphite and aluminum. The surface coverage of molecular bonding at the interface is estimated by the X-ray photoelectron spectroscopy and thermal boundary conductance is measured by the time-domain thermoreflectance method. It is found that the TBC is directly proportional to the surface coverage of covalently bonded triazine linkers, with the proportionality constant for their increment rates being about unity. The experimental finding is supported by the corresponding simulation using the atomic Green's function method, which exhibits the same linear dependence on the surface coverage.

DOI： 10.1021/acsami.9b11951

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Cellulose-nanofibril aerogels have been found to be highly effective thermal insulators, where some reports state that they have lower thermal conductivity than air. To further enhance the performance of the material, it is important to understand the contribution of the three heat-transfer components: solid-heat conduction by the fibrils, gas-heat conduction by the Knudsen gas in pores, and thermal radiation between the fibrils. The overall effective thermal conductivity is measured by an in-house steady-state setup under atmospheric and vacuum conditions. Contributions from each heat transfer component are quantified by constructing a simple open-cell model and fitting it to the experimental measurements, which vary based on the solid-volume fraction. The thermal conductivity values of a single cellulose-nanofibril filament that constitutes the struts of the open-cell structure are well within the range of previous studies, which confirms the validity of the analysis results. The analysis model can also be used to reveal target dimensions when fabricating aerogels with minimum thermal conductivity. All in all, the simple analysis method can be further applied to improve other porous thermal-insulation materials.

DOI： 10.1103/PhysRevApplied.11.024044

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Thermal conduction in complex periodic nanostructures remains a key area of open questions and research, and a particularly provocative and challenging detail is the impact of nanoscale material volumes that do not lie along the optimal line of sight for conduction. Here, we experimentally study thermal transport in silicon nanoladders, which feature two orthogonal heat conduction paths: unobstructed line-of-sight channels in the axial direction and interconnecting bridges between them. The nanoladders feature an array of rectangular holes in a 10 μm long straight beam with a 970 nm wide and 75 nm thick cross-section. We vary the pitch of these holes from 200 nm to 1100 nm to modulate the contribution of bridges to the net transport of heat in the axial direction. The effective thermal conductivity, corresponding to reduced heat flux, decreases from ∼45 W m-1 K-1 to ∼31 W m-1 K-1 with decreasing pitch. By solving the Boltzmann transport equation using phonon mean free paths taken from ab initio calculations, we model thermal transport in the nanoladders, and experimental results show excellent agreement with the predictions to within ∼11%. A combination of experiments and calculations shows that with decreasing pitch, thermal transport in nanoladders approaches the counterpart in a straight beam equivalent to the line-of-sight channels, indicating that the bridges constitute a thermally dead volume. This study suggests that ballistic effects are dictated by the line-of-sight channels, providing key insights into thermal conduction in nanostructured metamaterials.

DOI： 10.1039/c8nr01788c

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The top surface of carbon nanotube (CNT) “forests” produced by thermal chemical vapor deposition (CVD) often has a tangled morphology, owing to the self-organization of the CNTs at the initial stage of the CVD process. Removal of this top “crust” layer, without damaging the intrinsic microstructure of the CNT forest is often a key step for further applications. This paper studies the tailored use of Ar/O2 plasma etching to modify the surface morphology of multi-walled CNT forests. First, we investigate the effects of process parameters including plasma power, flow rate, and gas composition on the etching of CNT forests. As a result, we identify the plasma conditions that successfully remove the top crust yet maintain the structural shape of a CNT forest. Second, we prepare CNT forests having different packing densities and heights to study the influence of the initial characteristics on the etching result. We experimentally confirm that the etching rate depends strongly on the density and morphology of the crust layer. We also compare the material removal rate in vertical and lateral directions. Finally, we explore the enhancement of alignment and chemical uniformity of the top surface of CNT forests by the Ar/O2 plasma

DOI： 10.1016/j.carbon.2021.04.066

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Abstract: Phase separation micromolding (PSμM) is an effective technique for fabricating porous membranes with micropatterned structures. However, reports on procedures to control the size and number of open pores on the patterned surface are scarce, which often limits the use of the surface-patterned membranes. This work presents a systematic study on tailoring open pores on the patterned surface of polyethersulfone (PES) membranes prepared by the PSμM procedure. The composition of the solvent and the concentration of PES in the casting solution were optimized to tune the size and number of pores on the membrane surfaces formed on a flat substrate during the nonsolvent-induced phase separation (NIPS) process. The surface porosity changed significantly and macrovoids appeared when the flat substrate was replaced by a micropatterned substrate. The vapor-induced phase separation process was applied prior to the NIPS process to prevent the formation of macrovoids. The composition of the casting solution was tuned again to prepare micropatterned porous PES membranes with open pores on the patterned surface. We observed that the size and number of pores were different depending on the pore locations on the patterned surface, which was caused by different solvent/nonsolvent demixing dynamics resulting from the physical discontinuity of micro-patterned membranes.

DOI： 10.1038/s41428-019-0298-9

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Wetting phenomena, i.e. the spreading of a liquid over a dry solid surface, are important for understanding how plants and insects imbibe water and moisture and for miniaturization in chemistry and biotechnology, among other examples. They pose fundamental challenges and possibilities, especially in dynamic situations. The surface chemistry and micro-scale roughness may determine the macroscopic spreading flow. The question here is how dynamic wetting depends on the topography of the substrate, i.e. the actual geometry of the roughness elements. To this end, we have formulated a toy model that accounts for the roughness shape, which is tested against a series of spreading experiments made on asymmetric sawtooth surface structures. The spreading speed in different directions relative to the surface pattern is found to be well described by the toy model. The toy model also shows the mechanism by which the shape of the roughness together with the line friction determines the observed slowing down of the spreading.

DOI： 10.1038/s41598-019-44243-x

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