Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijft.2024.100634

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Society for Food Science and Technology  

<p>In contrast to the freeze-drying process, the rehydration process of freeze-dried foods remains unclear. This study investigated the rehydration of freeze-dried soybean curd, also known as tofu, and elucidated its breakage mechanism during rehydration, which impairs texture. The rehydration of freeze-dried tofu was observed at different water temperatures (20, 40, 70, and 100 °C), with the required rehydration time found to decrease with increasing water temperature. Furthermore, tofu was found to absorb water faster in cracks formed during production than in the porous body. Crack expansion was observed only in high-temperature water, leading to breakage of the tofu. Environmental scanning electron microscopy revealed that tofu expanded when a sufficient amount of water was absorbed. Accordingly, crack expansion in high-temperature water is attributed to the stress concentration at the tip of the crack, which is caused by differences in the rehydration rate and resulting stiffness between the porous body and cracks.</p>

DOI： 10.3136/fstr.fstr-d-23-00230

Scopus

CiNii Research

researchmap

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.surfin.2024.103923

Language：Others   Publishing type：Research paper (scientific journal)  

Nanoporous films have potential applications in thermoelectric cooling on a chip, sensors, solar cells, and desalination. For phonon transport, amorphization and other pore-edge defects introduced by the nanofabrication processes can eliminate wave effects by diffusively scattering short-wavelength phonons and thus destroying the phonon phase coherence. As a result, phononic effects can only be observed at 10 K or below, when long-wavelength phonons become dominant for thermal transport. In this work, a 70-nm-thick silicon thin film with approximately 100-nm-diameter nanopores was annealed under a high vacuum, and the change of pore-edge defects was observed with in situ transmission electron microscopy. It was found that the pore-edge defects can be minimized to a sub-1-nm layer by annealing between 773 and 873 K for 30 min, without changing the pore sizes. The largely reduced pore-edge defects are critical to the desired phonon wave effects within a periodic nanoporous structure.

DOI： 10.1063/5.0181143

Authorship：Last author   Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.jpclett.3c00428

Authorship：Last author,　Corresponding author   Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ultramic.2023.113747

Repository Public URL： https://hdl.handle.net/2324/6786359

Language：Others  

DOI： 10.1016/j.ijheatmasstransfer.2022.123418

Language：Others  

DOI： 10.1007/s11630-022-1668-8

Language：Others  

DOI： 10.1007/s11630-022-1594-9

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.carbon.2021.12.048

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0079153

Language：Others   Publishing type：Research paper (scientific journal)  

A dynamics study of surface nanobubbles using liquid phase electron microscopy showcases their unique push–push behavior.

DOI： 10.1039/d1cp03451k

Language：Others  

The response of nanoscopic gas phases at solid-liquid interfaces to temperature changes remains unclear. We investigated the interactions between surface nanobubbles and underlying micropancakes upon heat- ing. By atomic force microscopy imaging of the same area before and after heating, we found that the surface nanobubbles exhibited various behaviors upon heating: nucleation, growth, and disappearance. The differences in behavior are attributable to the existence of underlying gas phases, such as micropan- cakes and adsorbed layers. The nucleation sites of the nanobubbles depend on the positions of the mi- cropancakes. The size of the underlying micropancakes is central to the manner of gas transport between the micropancakes and overlying nanobubbles. We propose that the strongly adsorbed gas layers attract dissolved gas molecules and thereby lead to irreversible growth before and after heating.

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.langmuir.1c01589

Language：Others  

Language：Others  

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.langmuir.1c00145

Language：Others   Publishing type：Research paper (scientific journal)  

© 2021 Expanded graphite (EG) is a well-known carbon derivative and widely used as the thermally conductive enhancer for thermal management composites. However, the investigation on thermal conductivity measurement of an individual EG particle is still unexploited, which prevents the exploration of the coupling mechanism between EG and matrices and further measures for thermal conductivity enhancement. Herein, using a variable-length T-type method, we measure the thermal conductivity of an individual expanded graphite ribbon (EGR) obtained by mechanically compressing a separate EG particle. The EGR has a micrometer-sized thickness and a millimeter-sized length. By changing the sample length while maintaining the contact junction, we simultaneously obtained the thermal conductivity of the EGR and the thermal contact resistance between the sample and the probe. The longitudinal thermal conductivity of the EGR reaches up to 335.6±27.4 W m−1 K−1 at room temperature and decreases to 254.8±20.8 W m−1 K−1 as the temperature rises from 300 K to 380 K. With a higher thermal conductivity than most graphene paper products and some carbon fibers, this low-cost nanocarbon-based material exhibits a great advantage in the development of thermally conductive composites, and the presented accurate thermal conductivity provides indispensable data for the rational design of composites.

DOI： 10.1016/j.ijheatmasstransfer.2021.121115

Language：English   Publishing type：Research paper (scientific journal)  

Nanobubbles have attracted great interest in recent times because of their application in water treatment, surface cleaning, and targeted drug delivery, yet the challenge remains to gain thorough understanding of their unique behavior and dynamics for their utilization in numerous potential applications. In this work, we have used a liquid-phase electron microscopy technique to gain insights into the quasistatic merging of surface nanobubbles. The electron beam environment was controlled in order to suppress any new nucleation and slow down the merging process. The transmission electron microscopy study reveals that merging of closely positioned surface nanobubbles is initiated by gradual localized changes in the physical properties of the region between the adjoining nanobubble boundary. The observed phenomenon is then analyzed and discussed based on the different perceptions: localized liquid density gradient and bridge formation for gas exchange. In this study, it is estimated that the merging of the stable nanobubbles is initiated by the formation of a thin gas layer. This work not only enhances our understanding of the merging process of stable surface nanobubbles but will also lead to exploration of new domains for nanobubble applications.

DOI： 10.1021/acs.langmuir.0c03208

Language：Others   Publishing type：Research paper (scientific journal)  

© The Royal Society of Chemistry. We report for the first time a zigzag-shaped gas phase at a highly-ordered pyrolytic graphite/water interface. The novel shape of the gaseous domain is triggered by the holes of the underlying solid-like layers, which are composed of air molecules. Specifically, many holes were created by heating in the thin solid-like layers, which roughened them. The gas domains that formed on these layers deformed from circular to zigzag-shaped as the contact lines expanded while avoiding the holes of the underlying layers. We explained the formation and growth processes of these gas structures in terms of thin film growth, which varies with the mobility of the constituent molecules. This journal is

DOI： 10.1039/d0ra08861g

Language：English   Publishing type：Research paper (scientific journal)  

Fluorinated graphene has a tunable band gap that is useful in making flexible graphene electronics. But the carbon-fluorine (C-F) bonds in fluorinated graphene can be easily broken by increased temperature or electron beam irradiation. Here, we demonstrate that the stability of fluorinated graphene is mainly determined by its C-F configuration. The double-sided fluorinated graphene has a much stronger stability than the single-sided fluorinated graphene under the same irradiation dose. Density functional theory calculations show that the configuration of double-sided fluorinated graphene has a negative and low formation energy, indicating to be an energetically stable structure. On the contrary, the formation energy of single-sided fluorinated graphene is positive, leading to an unstable C-F bonding that is easily broken by the irradiation. Our findings make a new step towards a more stable and efficient design of graphene electronic devices.

DOI： 10.1038/s41598-020-74618-4

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.30919/es8d1136

Language：Others   Publishing type：Research paper (scientific journal)  

© the Owner Societies. Hydration structures are ubiquitous at solid/liquid interfaces and play a key role in various physicochemical and biological phenomena. Recently, it has been reported that dissolved gas molecules attracted to hydrophobic surfaces form adsorbed gas layers. Although a hydration structure and adsorbed gas layers coexist on the surface, the relationships between them remain unknown. In this study, we investigated a highly ordered pyrolytic graphite/pure water interface with and without adsorbed gas layers using frequency-modulation atomic force microscopy. We penetrated the adsorbed gas layers with the strong load force of the AFM tip and thereby obtained the frequency shift curves inside them. By comparing the curves with those measured on a bare HOPG surface, we found that the adsorbed gas layers were located at regions where the molecular density of water was low and were sandwiched between hydration layers with high water density. Moreover, the distance between adjacent hydration layers was larger than that predicted by simulations and was the same with and without the adsorbed gas layers. We propose that gas molecules on the hydrophobic surface interact with the hydration structure before forming the adsorbed gas layers, and extend the distance between hydration layers. This journal is

DOI： 10.1039/d0cp01719a

Language：Others   Publishing type：Research paper (scientific journal)  

© 2019 Elsevier Inc. Surface wettability engineering has attracted growing attention in recent years as an effective tool to enhance boiling heat transfer. On wettability-patterned (so-called biphilic) surfaces in particular, subatmospheric boiling has been shown to be nearly free of the severe degradation of heat transfer rate that tends otherwise to prevail on plain surfaces. The surprisingly consistent performance under reduced-pressure conditions can be attributed to the rather strong pinning of the three-phase contact line (TPCL) at the border between the hydrophobic and hydrophilic surfaces, which essentially eliminates the waiting period between bubble cycles. Only when the pressure is decreased sufficiently low does the transition to the undesired mode of intermittent boiling eventually occur on the biphilic surface. The purpose of the present study is to investigate the physical mechanism for the heat transfer deterioration on a mixed-wettability surface at very low pressures. To that end, we performed high-speed visualization experiments of the process of bubble nucleation and growth on a smooth copper surface coated with a single hydrophobic polytetrafluoroethylene (PTFE) spot, under different surface superheats and system pressures. The results show an interesting correlation between the TPCL behavior and the bubble growth dynamics. Specifically, under some certain threshold of pressure, it would become increasingly likely for the TPCL to be dislodged from its pinned position on the biphilic surface under a particularly rapid bubble expansion. As a result, full flooding of the hydrophobic surface might ensue, which is deemed responsible for temporary deactivation of the hydrophobic spot as a viable nucleation site. Furthermore, based on the diffuse-interface simulations of TPCL propagation across heterogeneous wettabilities, a comparison of cases with different bubble expansion rates offered qualitative evidence supporting the critical role of such accelerated bubble growth rate in driving the TPCL to overcome the energy barrier raised at the wettability divide.

DOI： 10.1016/j.expthermflusci.2019.110026

Language：Others   Publishing type：Research paper (scientific journal)  

© 2020 American Chemical Society. In contrast to microdroplet condensation with high contact angles, the one with low contact angles remains unclear. In this study, we investigated dynamics of microdroplet condensation of low-surface-tension liquids on two flat substrate surfaces by using reflection interference confocal microscopy. Spontaneous migration toward relatively larger droplets was first observed for the microdroplets nucleated on the hydrophilic quartz surface. The moving microdroplets showed a contact angle hysteresis of a0.5°, which is much lower than the values observed on typical flat substrates and is within the range observed on slippery lubricant-infused porous surfaces. Because the microdroplets on the hydrophobic polydimethylsiloxane surface did not move, we concluded that the ultrathin precursor film is formed only on the hydrophilic surface, which reduces a resistive force to migration. Also, reduced size of droplets promotes the thermocapillary motion, which is induced by a gradient in local temperature inside a small microdroplet arising due to the difference in size of adjacent droplets.

DOI： 10.1021/acs.langmuir.0c00148

Language：Others   Publishing type：Research paper (scientific journal)  

© 2020 American Chemical Society. Graphene liquid cells provide the highest possible spatial resolution for liquid-phase transmission electron microscopy. Here, in graphene liquid cells (GLCs), we studied the nanoscale dynamics of bubbles induced by controllable damage in graphene. The extent of damage depended on the electron dose rate and the presence of bubbles in the cell. After graphene was damaged, air leaked from the bubbles into the water. We also observed the unexpected directional nucleation of new bubbles, which is beyond the explanation of conventional diffusion theory. We attributed this to the effect of nanoscale confinement. These findings provide new insights into complex fluid phenomena under nanoscale confinement.

DOI： 10.1021/acsomega.0c01207

Language：English   Publishing type：Research paper (scientific journal)  

Enhancement of boiling heat transfer on biphilic (mixed-wettability) surfaces faces a sudden reversal at low pressures, which is brought about by excessive contact-line spreading across the wetting heterogeneities. We employ the diffuse-interface approach to numerically study bubble expansion on a heating surface that consists of opposing wettabilities. The results show a dramatic shift in the dynamics of a traversing contact line across the wettability divide under different gravities, which correspond to variable bubble growth rates. Specifically, it is found that the contact-line propagation tends to follow closely the rapidly expanding bubble at low gravity, with only a brief interruption at the border between the hydrophobic and hydrophilic sections of the surface. Only when the bubble growth becomes sufficiently weakened at high gravity does the contact line get slowed down drastically to the point of being nearly immobilized at the edge of the hydrophilic surface. The following bubble expansion, which faces strong limitations in the direction parallel to the surface, features a consistent apparent contact angle at around 66.4°, regardless of the wettability combination. A simple theoretical model based on the force-balance analysis is proposed to describe the physical mechanism behind such a dramatic transition in the contact-line behavior.

DOI： 10.1103/PhysRevFluids.5.033603

Language：Others   Publishing type：Research paper (scientific journal)  

© 2020 American Physical Society. Enhancement of boiling heat transfer on biphilic (mixed-wettability) surfaces faces a sudden reversal at low pressures, which is brought about by excessive contact-line spreading across the wetting heterogeneities. We employ the diffuse-interface approach to numerically study bubble expansion on a heating surface that consists of opposing wettabilities. The results show a dramatic shift in the dynamics of a traversing contact line across the wettability divide under different gravities, which correspond to variable bubble growth rates. Specifically, it is found that the contact-line propagation tends to follow closely the rapidly expanding bubble at low gravity, with only a brief interruption at the border between the hydrophobic and hydrophilic sections of the surface. Only when the bubble growth becomes sufficiently weakened at high gravity does the contact line get slowed down drastically to the point of being nearly immobilized at the edge of the hydrophilic surface. The following bubble expansion, which faces strong limitations in the direction parallel to the surface, features a consistent apparent contact angle at around 66.4°, regardless of the wettability combination. A simple theoretical model based on the force-balance analysis is proposed to describe the physical mechanism behind such a dramatic transition in the contact-line behavior.

DOI： 10.1103/PhysRevFluids.5.033603

Language：English   Publishing type：Research paper (scientific journal)  

© 2019 We report on the in situ thermal measurement of a carbon nanofiber (CNF) modified by focused ion beam (FIB) irradiation. The FIB irradiation led to local amorphization of the crystalline structure of the CNF. The in situ measurement was improved by correcting for the effect of the scattered ions on the sensor. The low effective thermal conductivity of the pristine CNF (∼39 W/mK) resulted from the anisotropic structure made of many individual graphitic fibers. The first FIB irradiation decreased the thermal conductivity by approximately 3.2%. This relatively small decrease is attributed to the structure of the CNF consisting of many individual fibers, with some fibers remaining pristine even after the FIB irradiation. Analysis using a thermal-circuit model suggested that the thermal transport in the CNF could include a ballistic feature of phonons in the micrometer range. Our proposed in situ thermal measurement method can be extended to the study of thermal transport in various structurally modified nanomaterials.

DOI： 10.1016/j.carbon.2019.07.056

Repository Public URL： https://hdl.handle.net/2324/7162464

Language：English   Publishing type：Research paper (scientific journal)  

In contrast to surface nanobubbles, the properties of atomically flat gas phases such as micropancakes remain unclear. In this study, we investigated nanoscopic gas phases existing at the interface between highly ordered pyrolytic graphite and air-supersaturated pure water using high-sensitivity frequency-modulation atomic force microscopy (AFM). Micropancakes appeared on a disordered gas layer overlying an ordered gas layer and moved in the direction of AFM scanning. Their movement stopped at the edge of the disordered gas layers, whereas the two gas layers did not move at all. The limited mobility of micropancakes is explained by assuming that the disordered and ordered gas layers, which are composed of strongly adsorbed gas molecules, behave like solid surfaces, and that the surface heterogeneity between them results in a pinning effect.

DOI： 10.1063/1.5113810

Language：English   Publishing type：Research paper (scientific journal)  

© 2019 American Chemical Society. Conventionally, graphene is a poor thermoelectric material with a low figure of merit (ZT) of 10-4-10-3. Although nanostructuring was proposed to improve the thermoelectric performance of graphene, little experimental progress has been accomplished. Here, we carefully fabricated as-grown suspended graphene nanoribbons with quarter-micron length and â40 nm width. The ratio of electrical to thermal conductivity was enhanced by 1-2 orders of magnitude, and the Seebeck coefficient was several times larger than bulk graphene, which yielded record-high ZT values up to â0.1. Moreover, we observed a record-high electronic contribution of â20% to the total thermal conductivity in the nanoribbon. Concurrent phonon Boltzmann transport simulations reveal that the reduction of lattice thermal conductivity is mainly attributed to quasi-ballistic phonon transport. The record-high ratio of electrical to thermal conductivity was enabled by the disparate electron and phonon mean free paths as well as the clean samples, and the enhanced Seebeck coefficient was attributed to the band gap opening. Our work not only demonstrates that electron and phonon transport can be fundamentally tuned and decoupled in graphene but also indicates that graphene with appropriate nanostructures can be very promising thermoelectric materials.

DOI： 10.1021/acsnano.9b03521

Language：Others   Publishing type：Research paper (scientific journal)  

© 2019 Author(s). In contrast to surface nanobubbles, the properties of atomically flat gas phases such as micropancakes remain unclear. In this study, we investigated nanoscopic gas phases existing at the interface between highly ordered pyrolytic graphite and air-supersaturated pure water using high-sensitivity frequency-modulation atomic force microscopy (AFM). Micropancakes appeared on a disordered gas layer overlying an ordered gas layer and moved in the direction of AFM scanning. Their movement stopped at the edge of the disordered gas layers, whereas the two gas layers did not move at all. The limited mobility of micropancakes is explained by assuming that the disordered and ordered gas layers, which are composed of strongly adsorbed gas molecules, behave like solid surfaces, and that the surface heterogeneity between them results in a pinning effect.

DOI： 10.1063/1.5113810

Language：English   Publishing type：Research paper (scientific journal)  

Conventionally, graphene is a poor thermoelectric material with a low figure of merit (ZT) of 10^-4-10^-3. Although nanostructuring was proposed to improve the thermoelectric performance of graphene, little experimental progress has been accomplished. Here, we carefully fabricated as-grown suspended graphene nanoribbons with quarter-micron length and â40 nm width. The ratio of electrical to thermal conductivity was enhanced by 1-2 orders of magnitude, and the Seebeck coefficient was several times larger than bulk graphene, which yielded record-high ZT values up to â0.1. Moreover, we observed a record-high electronic contribution of â20% to the total thermal conductivity in the nanoribbon. Concurrent phonon Boltzmann transport simulations reveal that the reduction of lattice thermal conductivity is mainly attributed to quasi-ballistic phonon transport. The record-high ratio of electrical to thermal conductivity was enabled by the disparate electron and phonon mean free paths as well as the clean samples, and the enhanced Seebeck coefficient was attributed to the band gap opening. Our work not only demonstrates that electron and phonon transport can be fundamentally tuned and decoupled in graphene but also indicates that graphene with appropriate nanostructures can be very promising thermoelectric materials.

DOI： 10.1021/acsnano.9b03521

Language：Others   Publishing type：Research paper (scientific journal)  

© 2019 American Chemical Society. Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid-gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68-104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid-gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement.

DOI： 10.1021/acs.jpclett.9b00718

Language：English   Publishing type：Research paper (scientific journal)  

© 2019 Thermal property measurement of individual micro- and nano-materials has been very challenging and the development of measurement methods is crucial for the experimental investigation of microscale and nanoscale heat transfer. Here we present a noncontact frequency-domain Raman method to measure thermal diffusivity of individual 1D microfibers and nanowires without the need of knowing laser absorptivity. Cosine-wave modulated laser is used to heat the sample, while the laser-intensity-weighted spatiotemporal average temperature is simultaneously detected from the sample's Raman band shift. Transient heat conduction models under periodic heating are established and analytically solved in the frequency domain with considerations of the Gaussian laser distribution and thermal contact resistance. By varying the laser modulation frequency as well as the laser spot size, we can eliminate the laser absorptivity by a normalization technique and extract the thermal diffusivity with high sensitivity. Typically, if the thermal diffusivity is on the order of 10−4 m2/s, we need to use the modulation frequencies on the order of 10 Hz to measure millimeter long microfibers, and ∼MHz frequencies to measure micrometer long nanowires. We also demonstrate that any kind of periodic laser modulation can be decomposed to a series of cosine modes and readily analyzed by this frequency-domain approach, which can greatly broaden the applications of transient Raman techniques.

DOI： 10.1016/j.ijheatmasstransfer.2019.01.057

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Due to the considerably reduced boiling point, organic fluids such as ethanol provide an attractive alternative to water as the working fluid in two-phase thermal management systems for high-heat-flux applications. The state-of-the-art enhancement methods for ethanol boiling normally involve surface structure engineering. Here we report, for the first time, enhancement of nucleate boiling of ethanol using wettability-patterned surfaces. By depositing onto a polished copper surface an array of circular spots of superamphiphobic coating of modified halloysite nanotubes (HNT) with fluoropolymer, which was shown to repel low-surface-tension fluids, we managed to create a meaningful biphilic pattern of alternating hydrophobicity (with ethanol contact angle exceeding 100°) and hydrophilicity (with contact angle close to 0°) on the surface. Boiling heat transfer was found to be improved dramatically on the coated surface. Specifically, the onset of nucleate boiling was found to drop by more than 35%. Moreover, at 20 K surface superheat (above the boiling point), a maximum heat transfer enhancement over 300% compared with a plain copper surface occurred on the surface with a pitch-to-spot ratio close to 2.5. The significantly increased heat transfer rate of the biphilic surfaces could be attributed to facilitated bubble nucleation and stronger agitation effect.

DOI： 10.1016/j.applthermaleng.2018.12.049

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Ltd Thermal contact resistance (TCR) between individual carbon fibers (CFs) can dominate heat dissipation rates in CF-based composite materials. Here, we develop a totally non-contact “laser-flash Raman mapping” method to simultaneously measure the TCR at the CF-CF junction and their thermal conductivities. Laser power is used to heat the sample and the laser absorptivity is experimentally determined by a transient laser-flash Raman technique. The laser heating positions are changed along two connected CFs, and the change of temperature rise with varying positions is in-situ measured from the temperature dependent Raman band shifts. The high spatial resolution of the micro-Raman mapping allows direct observation of the abrupt jump of thermal resistance at the CF-CF junction, from which we extracted the TCR as well as the thermal conductivity. The laser absorptivity of the 11 μm-diameter CFs is measured to be 0.12 ± 0.03, the thermal conductivities of the individual CFs are around 200 W/mK, and the TCR of the CF-CF junction is (2.98 ± 0.92) × 105 K/W. This work provides indispensable knowledge for the design of CF-based composite for thermal management, and the novel non-contact measurement method can stimulate characterization and manipulation of contact/interface heat conduction between various micro- and nano-materials.

DOI： 10.1016/j.carbon.2018.09.034

Language：English   Publishing type：Research paper (scientific journal)  

Water confined in carbon nanotubes (CNTs) can exhibit distinctly different behaviors from the bulk. We report transmission electron microscopy (TEM) observation of water phases inside hydrophobic cup-stacked CNTs exposed to high vacuum. Unexpectedly, we observed stable water morphologies beyond surface-tension dominance, including nanometer thin free water films, complex water-bubble structures, and zigzag-shaped liquid-gas interface. The menisci of the water phases are complex and inflected, where we measured the contact angles on the CNT inner wall to be 68-104°. The superstability of the suspended ultrathin water films is attributed to the strong hydrogen-bonded network among water molecules and adsorption of water molecules on the cup-structured inner wall. The complex water-bubble structure is a result of the stability of free water films and interfacial nanobubbles, and the zigzag edge of the liquid-gas interface is explained by the pinning effect. These experimental findings provide valuable knowledge for the research on fluids under nanoscale confinement.

DOI： 10.1021/acs.jpclett.9b00718

Language：Others   Publishing type：Research paper (scientific journal)  

© Copyright 2018 American Chemical Society. We report a quasi-three-dimensional observation of electron-beam-induced nanobubbles inside a 1000 nm thick layer of water using the liquid cell electron microscopy. In the early stage of observation, heterogeneous bubble nucleation occurred, and small bubbles coalesced with the adjacent bubbles when they come in contact with each other. However, for the first time, we found that after prolonged electron beam irradiation heterogeneous nucleation did not occur more, and then homogeneous nucleation started even though a solid surface was available for heterogeneous nucleation. We conclude that the Ostwald ripening effect prevents heterogeneous nucleation from occurring and that the lower surface tension due to the generation of ions and radicals boosts the homogeneous nucleation. It was also discovered that the generation sites of homogeneous nucleation are beneath the three-phase contact lines of existing interfacial bubbles.

DOI： 10.1021/acs.jpcc.8b09200

Language：Others   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Ltd In this paper, we numerically study pool boiling of a binary (water and nitrogen) mixture on a surface endowed with a combination of hydrophobicity and hydrophilicity (i.e., the so called biphilic surface). Here we adopt a numerical approach based on the phase field theory, where the vapor-liquid interface is assumed to be of a finite thickness (hence diffusive in nature) and requires no explicit tracking schemes. The theoretical modeling of two-phase heat and mass transfer in water diluted with nitrogen demonstrates the significant impact of impurities on bubble dynamics. The simulations show that locally high concentrations of nitrogen gas within the vapor bubble is essential to weakening the condensation effect, which results in sustained bubble growth and ultimately (partial) departure from the surface under the artificially enlarged gravity. Simply increasing the solubility of nitrogen in water, however, turns out to be counterproductive because possible re-dissolution of the aggregated nitrogen by the bulk water could deprive the bubble of vital gas contents, leading instead to continuous bubble shrinkage and collapse. Additionally, it is found that with the significant accumulation of nitrogen, the bubble interface is increasingly dominated by a strong interfacial thermocapillary flow due to the Marangoni effect.

DOI： 10.1016/j.ijheatmasstransfer.2018.06.043

Language：Others   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Ltd Boiling suffers from inefficient intermittent cycles of bubble generation under subatmospheric conditions. Such deterioration in heat transfer rates can be alleviated but not completely eliminated by use of mixed-wettability (biphilic) surfaces. Here we study bubble dynamics on a single hydrophobic spot in low-pressure pool boiling. The results reveal an interesting transition in bubble departure behavior from the surface-driven mode to the drag-driven mode, which correlates closely with the dynamic state of the three-phase contact line on the surface. Based on the force-balance argument, a simple model is derived to map the contact-line mobility during bubble growth. It is found that below a certain threshold pressure, the bubble base expansion is increasingly likely to overcome the strong pinning of the contact line at the interface between the hydrophobic and hydrophilic regions. That could lead to total removal of vapor residues from the surface and cause deactivation of the nucleation site, which portends the eventual takeover of intermittent boiling on the biphilic surface.

DOI： 10.1016/j.ijheatmasstransfer.2018.06.030

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Ltd Stacked layers of different atomically thin 2D materials is called the van der Waals (vdW) heterostructure, which has become a rapidly developing research field due to its extraordinary and tunable properties. In this paper, we develop a variable-spot-size laser-flash Raman method to in-situ measure the thermal properties as well as the laser absorption in the supported 2D vdW heterostructure with arbitrary layers. The extracted thermal properties include the in-plane thermal conductivity and diffusivity of each layer, and interfacial thermal conductance between every two adjacent layers. A three-dimensional transient heat conduction model is developed and analytically solved to describe the process of pulsed Gaussian laser heating supported n-layer heterostructure. The temperature of each atomic layer can be simultaneously non-contact detected from their distinct Raman peaks whose positions are temperature dependent. The laser spot sizes and pulse durations are varied to generate multiple temperature curves. The multiple thermal properties as well as the laser absorption can be extracted by simultaneously fitting these temperature curves into the analytical solutions at multiple spot sizes or/and pulse durations. We also establish the approach of sensitivity and uncertainty analysis for the multi-response multi-parameter least-square fitting in our proposed measurement methods. Case studies show that the transient temperature curves are generally more sensitive to the thermal properties than the steady-state temperatures at variable spot sizes. All the unknown thermal properties and laser absorption can be extracted with sufficiently high accuracy if multiple transient temperature curves at multiple spot sizes are simultaneously fitted into the analytical solution. The measurement method and uncertainty analysis approach presented here are useful for investigating the thermal transport in the emerging 2D materials and vdW heterostructures.

DOI： 10.1016/j.ijheatmasstransfer.2018.05.011

Language：English   Publishing type：Research paper (scientific journal)  

Copyright © 2018 American Chemical Society. The hollow inside of a carbon nanotube (CNT) has great potential not only for flow rate enhancement of nanocapillary, but also for a material container which can be applied for drug delivery and nanoparticle infusion. However, these applications focus on after liquid infusion into CNTs, whereas the understanding of the filling process is still limited. We conducted capillary filling experiments using individual open-ended CNTs, which were stuck into an ionic liquid and visualized by scanning transmission electron microscopy. The results showed that the meniscus stopped inside the CNT, which is not predicted by the Lucas-Washburn equation. To explain this discrepancy, the intermolecular force between the liquid and CNT inner wall was proposed to provide an additional friction force. In addition, voids were observed in the liquid inside the CNT. The generation mechanism of voids was proposed to be induced by the instability of the thin liquid layer along the CNT inner surface caused by the advance of the three-phase contact line. The results of the present study increase our understanding of nanoscale capillary action.

DOI： 10.1021/acs.jpcc.8b06406

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Control for the bubble nucleation at the onset of nucleate boiling (ONB) ensures the stable start of boiling heat transfer. However, the bubble nucleation mechanism at the ONB remains unclear, because of the difficulty of in-situ observation, which is due to the small size of nucleation. Thus, in order to break through the current technological barrier of boiling heat transfer, a new experimental technique enabling the investigation of the dynamics of bubbles near the solid-liquid interface is highly desirable. Liquid cell electron microscopy is the most useful method for the in-situ observation of liquid samples at the nanoscale. We prepared a closed liquid cell fabricated using MEMS technology and observed the generation and growth of bubbles at the nanoscale and in real time using transmission electron microscopy (TEM). In the growing process, the water meniscus between smaller bubbles becomes thinner and thinner and eventually ruptures. However, when the bubbles grow, the meniscus between larger bubbles do not rupture and the bubble overlaps with others, suggesting that thin meniscus can be stable only in the case of larger bubbles because of the difference of the curvature of their liquid-gas interfaces between smaller bubbles and larger bubbles. Our experimental results lead to the insight of the mechanism of the stability and the phase change phenomena at the liquid-gas interface at the nanoscale.

DOI： 10.1615/ihtc16.bae.023136

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Interfacial nanobubbles were first speculated in 1994 and experimentally confirmed in 2000 by atomic force microscopy (AFM) measurements. It was recently proposed that the onset of boiling with the very low superheat on hydrophobic surface could be explained by assuming the existence of interfacial nanobubbles. To reduce the superheat and enhance the reliability of boiling heat transfer, the control of the generation of interfacial nanobubbles is indispensable. In this study, we measured the interfacial nanobubbles by AFM and examined the influence of surface wettability and nanostructure on their generation. First, we measured the spherical-cap shaped nanobubbles generated on the HOPG surface. It was observed that the nanobubbles only generate on the hydrophobic terraced area and do not cross the nanosized hydrophilic steps. Next, we prepared the hydrophilic-hydrophobic hybrid surfaces and generated the nanobubbles on those. As a result, the range of nanobubble generation clearly changed by the difference of wettability between adjacent surfaces. These results show that the generation of interfacial nanobubbles can be controlled by the surface processing of the substrate and can be expected to be applied to boiling heat transfer.

DOI： 10.1615/ihtc16.nmt.022210

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Two-dimensional (2D) layered materials have been the focus of materials research for more than a decade. Stacking different 2D nanosheets and 3D substrates with van der Waals interactions in between has opened up new ways to sophisticated design of novel device functionalities and platforms for new physics. The performance of van der Waals heterostructure (vdWH) devices can be most often limited by the heat dissipation issue, but the thermal transport in vdWHs has rarely been studied yet. This paper presents a novel heater assisted Raman method to accurately measure interfacial thermal conductance between every two layers in the vdWH, which is suitable to detect interfacial thermal transport between all kinds of nanosheets no matter whether the neighboring layer is electrical conductor or insulator. In this method, a transparent insulating thin layer and a patterned transparent conducting indium-tin-oxide (ITO) layer are successively sputtered on top of the vdWH. The ITO layer is electrically heated to provide vertical heat flux, while the temperatures of each atomic layer and the substrate surface are simultaneously detected from their temperature dependent Raman band shifts, thus the interfacial thermal conductance between every two layers can be accurately determined from the ITO's Joule heating power and the Raman-measured temperatures. With high sensitivity and accuracy, the measurement method proposed here provides a general way to experimentally investigate interfacial thermal transport across both electrically conducting and insulating van der Waals interfaces in 2D-material-based devices.

DOI： 10.1615/ihtc16.nmt.021756

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Amongst an extensive collection of surface characteristics that could affect boiling performance, surface wettability (as measured by the contact angle with water) proves to play a unique role in potentially manipulating bubble behavior to the advantage of higher heat transfer rates. In this study, we show experimentally that controlled bubble behavior be realized under the surface design incorporating these two characteristics (namely, by coating an array of hydrophobic spots on a hydrophilic substrate), which leads to a great enhancement in boiling heat transfer under various conditions. In reduced-pressure pool boiling, the strong pinning of the bubble contact line at the border between the hydrophilic and hydrophobic regions manages to prevent total deactivation of nucleation sites. As a result, the deleterious transition to intermittent boiling is effectively delayed, whereby no heat transfer deterioration occurs until a very low pressure of about 8 kPa is reached. Moreover, in subcooled boiling, bubble growth on a patterned surface is found to be facilitated by a pronounced presence of dissolved gas in defiance of exhaustive degassing efforts through continuous boiling, thanks to an unusually strong retention of gas contents by the hydrophobic surface. As experimental and numerical evidence show, only bubbles with sufficiently high concentrations of gas components (i.e., causing weakened condensation) are able to grow large enough on the hydrophobic surface such that periodic pinch-offs might take place, which is responsible for most of the initial heat transfer enhancement before large-scale bubble nucleation starts on the hydrophilic surface as well.

DOI： 10.1615/ihtc16.bae.023482

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Thermal rectification is a phenomenon that the heat flow changes by reversing the direction of temperature gradient. This is a fundamental behavior of the thermal rectifiers, which can be used for the active heat flow control, thermally driven computer, efficient energy harvesting, etc. The key challenge is how to increase the thermal rectification ratio, which is defined as the relative change of thermal conductivities in different heat flow directions. Due to the significant size effect and unique heat transfer mechanisms, nanomaterials (such as carbon nanotubes, nanowires, graphene, etc.) are suggested to have high thermal rectification ratio. However, the experiment result showed that the ratio of the single carbon nanotube thermal rectifier was only 7%. In the past decade, many theoretical researches and molecular dynamics simulations have shown that the monolayer graphene may have high thermal rectification ratio due to its unique two-dimensional heat transfer mechanism. But the experimental work is still a blank because of the difficult fabrication process of suspended graphene electronic device. In this work, we report the experimental demonstration of a suspended monolayer graphene thermal rectifier. Three different types of graphene thermal rectifiers have been fabricated with different asymmetric nanostructures. The focused ion beam manufacturing, electron beam induced deposition and precise electron beam lithography were used to design and create asymmetric nanostructures on the monolayer graphene. The thermal rectification ratios were measured by using a precise H-type sensor method. The highest rectification ratio reaches 28% for the graphene with asymmetric nanopores. The asymmetric dependence of thermal conductivity on temperature and space is known to be the physical reason. For the other two kinds of thermal rectifiers, the rectification ratios are about 10%. The asymmetric phonon scattering is known to be the physical reason, which has been proved by using large-scale molecular dynamics simulation.

DOI： 10.1615/ihtc16.nmt.022118

Language：Others   Publishing type：Research paper (other academic)  

© 2018 International Heat Transfer Conference. All rights reserved. Recent microfabrication techniques have exhibited tremendous opportunities to improve the phase-change heat transfer by tailoring the surface structure and wettability, which indicates that microscopic understanding of liquid-gas phase change is vital for further improvement of heat transfer devices. Boiling and condensation have been studied by numerous researchers for more than a half century and are known to be a successive process of nucleation, growth and departure of bubbles and droplets. Fluid dynamical modeling has been extensively developed for their growth and departure but the nucleation is still incompletely understood because of the lack of imaging techniques of two-phase phenomena smaller than the resolution limit of optical microscopy. This paper introduces new trends to investigate nanoscale bubbles and droplets experimentally, using AFM, SEM and TEM. AFM is of the highest spatial resolution and its feedback control of tip tapping enables us to obtain the accurate shape of nanobubbles at the solid-liquid interface. A new mode of AFM gives force data of approaching and retracting tips, which unveils the strong interaction between nanobubble and AFM tip. Environmental SEM is a useful tool for observing water condensation with droplets of micrometer-order diameter but there are several concerns including the contamination due to the electron beam irradiation. TEM requires ultra-high vacuum environment but utilization of nano liquid cell enables us to image the liquid-gas interface in nanoscale. By using these techniques, some key issues for generation and stability of interfacial nanobubbles and condensed nanodroplets have been understood, which should result in novel techniques to control the initial stage of phase change heat transfer.

DOI： 10.1615/ihtc16.kn.000017

Language：English   Publishing type：Research paper (other academic)  

Copyright © 2018 ASME. Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling improves heat transfer coefficient (HTC) a lot compared to single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circle spots (non-electroplating Ni-PTFE, 0.5~2 mm in diameter and 1.5-3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and lower the incipience temperature overshoot using water as the working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests with heat loads (30 W to 260 W) reveals the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation sites density. Hydrophobic circle spots coated surface with diameter 1 mm, pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. The comparison of three evaporator surfaces with same spot parameters but different coating materials is carried out experimentally. Ni-PTFE coated surface with immersion method performs the optimal performance of the thermosyphon.

DOI： 10.1115/icnmm2018-7623

Language：Others   Publishing type：Research paper (scientific journal)  

© 2018 The Japan Society of Applied Physics. Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) and UNCD/non-hydrogenated amorphous carbon (a-C) composite (UNCD/a-C) films were prepared via coaxial arc plasma deposition, and their thermal conductivity and interfacial conductance in grain boundaries were measured using a time-domain thermoreflectance method. The interfacial conductance was estimated to be 1,010 and 4,892MW/(m2&K) for UNCD/a-C:H and UNCD/a-C films, respectively. The reasons for the hydrogenated film having lower interfacial conductance than the non-hydrogenated film are 1) the reduced number of carriers that contribute to heat transport and 2) the hydrogen atoms, which are preferentially located at the grain boundaries and enhance phonon scattering.

DOI： 10.7567/APEX.11.065101

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 The Japan Society of Mechanical Engineers. Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling is widely used in application of thermal engineering and considerably more efficient than single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circular spots (0.5 - 2 mm in diameter and 1.5 - 3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and low incipience temperature overshoot with water as working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests under heat loads 30 W to 260 W reveal the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation-site density. The surface with hydrophobic spots with diameter 1 mm and pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. A comparison of three evaporator surfaces with identical wettability patterns but with different surface topographies and coating thicknesses is carried out experimentally. The results show superior heat transfer rates and wear resistance on the surface coated with HNTs spots thanks to the large contact angle, great thickness, and durability of the coating layer.

DOI： 10.1299/jtst.2018jtst0011

Language：Others   Publishing type：Research paper (scientific journal)  

© 2018 American Chemical Society. Fluids confined in a nanoscale space behave differently than in the bulk due to strong interactions between fluid molecules and solid atoms. Here, we observed water confined inside "open" hydrophilized carbon nanotubes (CNT), with diameter of tens of nanometers, using transmission electron microscopy (TEM). A 1-7 nm water film adhering to most of the inner wall surface was observed and remained stable in the high vacuum (order of 10-5 Pa) of the TEM. The superstability of this film was attributed to a combination of curvature, nanoroughness, and confinement resulting in a lower vapor pressure for water and hence inhibiting its vaporization. Occasional, suspended ultrathin water film with thickness of 3-20 nm were found and remained stable inside the CNT. This film thickness is 1 order of magnitude smaller than the critical film thickness (about 40 nm) reported by the Derjaguin-Landau-Verwey-Overbeek theory and previous experimental investigations. The stability of the suspended ultrathin water film is attributed to the additional molecular interactions due to the extended water meniscus, which balances the rest of the disjoining pressures.

DOI： 10.1021/acs.nanolett.7b05169

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Author(s). To accurately characterize the shape of interfacial nanobubbles using atomic force microscopy (AFM), we investigated the effect of wettability of the AFM tip while operating in the peak force tapping (PFT) mode. The AFM tips were made hydrophobic and hydrophilic by Teflon AF coating and oxygen plasma treatment, respectively. It was found that the measured base radius of nanobubbles differed between AFM height images and adhesion images, and that this difference depended on the tip wettability. The force curves obtained during the measurements were also different depending on the wettability, especially in the range of the tip/nanobubble interaction and in the magnitude of the maximum attractive force in the retraction period. The difference suggests that hydrophobic tips penetrate the gas/liquid interface of the nanobubbles, with the three phase contact line being pinned on the tip surface; hydrophilic tips on the other hand do not penetrate the interface. We then quantitatively estimated the pinning position and recalculated the true profiles of the nanobubbles by comparing the height images and adhesion images. As the AFM tip was made more hydrophilic, the penetration depth decreased and eventually approached zero. This result suggests that the PFT measurement using a hydrophilic tip is vital for the acquisition of reliable nanobubble profiles.

DOI： 10.1063/1.5010131

Language：English   Publishing type：Research paper (scientific journal)  

Recent microfabrication techniques have exhibited tremendous opportunities to improve the phase-change heat transfer by tailoring the surface structure and wettability, which indicates that microscopic understanding of liquid-gas phase change is vital for further improvement of heat transfer devices. Boiling and condensation have been studied by numerous researchers for more than a half century and are known to be a successive process of nucleation, growth and departure of bubbles and droplets. Fluid dynamical modeling has been extensively developed for their growth and departure but the nucleation is still incompletely understood because of the lack of imaging techniques of two-phase phenomena smaller than the resolution limit of optical microscopy. This paper introduces new trends to investigate nanoscale bubbles and droplets experimentally, using AFM, SEM and TEM. AFM is of the highest spatial resolution and its feedback control of tip tapping enables us to obtain the accurate shape of nanobubbles at the solid-liquid interface. A new mode of AFM gives force data of approaching and retracting tips, which unveils the strong interaction between nanobubble and AFM tip. Environmental SEM is a useful tool for observing water condensation with droplets of micrometer-order diameter but there are several concerns including the contamination due to the electron beam irradiation. TEM requires ultra-high vacuum environment but utilization of nano liquid cell enables us to image the liquid-gas interface in nanoscale. By using these techniques, some key issues for generation and stability of interfacial nanobubbles and condensed nanodroplets have been understood, which should result in novel techniques to control the initial stage of phase change heat transfer.

Language：English   Publishing type：Research paper (scientific journal)  

Control for the bubble nucleation at the onset of nucleate boiling (ONB) ensures the stable start of boiling heat transfer. However, the bubble nucleation mechanism at the ONB remains unclear, because of the difficulty of in-situ observation, which is due to the small size of nucleation. Thus, in order to break through the current technological barrier of boiling heat transfer, a new experimental technique enabling the investigation of the dynamics of bubbles near the solid-liquid interface is highly desirable. Liquid cell electron microscopy is the most useful method for the in-situ observation of liquid samples at the nanoscale. We prepared a closed liquid cell fabricated using MEMS technology and observed the generation and growth of bubbles at the nanoscale and in real time using transmission electron microscopy (TEM). In the growing process, the water meniscus between smaller bubbles becomes thinner and thinner and eventually ruptures. However, when the bubbles grow, the meniscus between larger bubbles do not rupture and the bubble overlaps with others, suggesting that thin meniscus can be stable only in the case of larger bubbles because of the difference of the curvature of their liquid-gas interfaces between smaller bubbles and larger bubbles. Our experimental results lead to the insight of the mechanism of the stability and the phase change phenomena at the liquid-gas interface at the nanoscale.

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional (2D) layered materials have been the focus of materials research for more than a decade. Stacking different 2D nanosheets and 3D substrates with van der Waals interactions in between has opened up new ways to sophisticated design of novel device functionalities and platforms for new physics. The performance of van der Waals heterostructure (vdWH) devices can be most often limited by the heat dissipation issue, but the thermal transport in vdWHs has rarely been studied yet. This paper presents a novel heater assisted Raman method to accurately measure interfacial thermal conductance between every two layers in the vdWH, which is suitable to detect interfacial thermal transport between all kinds of nanosheets no matter whether the neighboring layer is electrical conductor or insulator. In this method, a transparent insulating thin layer and a patterned transparent conducting indium-tin-oxide (ITO) layer are successively sputtered on top of the vdWH. The ITO layer is electrically heated to provide vertical heat flux, while the temperatures of each atomic layer and the substrate surface are simultaneously detected from their temperature dependent Raman band shifts, thus the interfacial thermal conductance between every two layers can be accurately determined from the ITO's Joule heating power and the Raman-measured temperatures. With high sensitivity and accuracy, the measurement method proposed here provides a general way to experimentally investigate interfacial thermal transport across both electrically conducting and insulating van der Waals interfaces in 2D-material-based devices.

DOI： 10.1615/ihtc16.nmt.021756

Language：English   Publishing type：Research paper (other academic)  

Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling improves heat transfer coefficient (HTC) a lot compared to single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circle spots (non-electroplating Ni-PTFE, 0.5~2 mm in diameter and 1.5-3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and lower the incipience temperature overshoot using water as the working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests with heat loads (30 W to 260 W) reveals the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation sites density. Hydrophobic circle spots coated surface with diameter 1 mm, pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. The comparison of three evaporator surfaces with same spot parameters but different coating materials is carried out experimentally. Ni-PTFE coated surface with immersion method performs the optimal performance of the thermosyphon.

Language：English   Publishing type：Research paper (scientific journal)  

Stacked layers of different atomically thin 2D materials is called the van der Waals (vdW) heterostructure, which has become a rapidly developing research field due to its extraordinary and tunable properties. In this paper, we develop a variable-spot-size laser-flash Raman method to in-situ measure the thermal properties as well as the laser absorption in the supported 2D vdW heterostructure with arbitrary layers. The extracted thermal properties include the in-plane thermal conductivity and diffusivity of each layer, and interfacial thermal conductance between every two adjacent layers. A three-dimensional transient heat conduction model is developed and analytically solved to describe the process of pulsed Gaussian laser heating supported n-layer heterostructure. The temperature of each atomic layer can be simultaneously non-contact detected from their distinct Raman peaks whose positions are temperature dependent. The laser spot sizes and pulse durations are varied to generate multiple temperature curves. The multiple thermal properties as well as the laser absorption can be extracted by simultaneously fitting these temperature curves into the analytical solutions at multiple spot sizes or/and pulse durations. We also establish the approach of sensitivity and uncertainty analysis for the multi-response multi-parameter least-square fitting in our proposed measurement methods. Case studies show that the transient temperature curves are generally more sensitive to the thermal properties than the steady-state temperatures at variable spot sizes. All the unknown thermal properties and laser absorption can be extracted with sufficiently high accuracy if multiple transient temperature curves at multiple spot sizes are simultaneously fitted into the analytical solution. The measurement method and uncertainty analysis approach presented here are useful for investigating the thermal transport in the emerging 2D materials and vdW heterostructures.

DOI： 10.1016/j.ijheatmasstransfer.2018.05.011

Language：English   Publishing type：Research paper (scientific journal)  

Interfacial nanobubbles were first speculated in 1994 and experimentally confirmed in 2000 by atomic force microscopy (AFM) measurements. It was recently proposed that the onset of boiling with the very low superheat on hydrophobic surface could be explained by assuming the existence of interfacial nanobubbles. To reduce the superheat and enhance the reliability of boiling heat transfer, the control of the generation of interfacial nanobubbles is indispensable. In this study, we measured the interfacial nanobubbles by AFM and examined the influence of surface wettability and nanostructure on their generation. First, we measured the spherical-cap shaped nanobubbles generated on the HOPG surface. It was observed that the nanobubbles only generate on the hydrophobic terraced area and do not cross the nanosized hydrophilic steps. Next, we prepared the hydrophilic-hydrophobic hybrid surfaces and generated the nanobubbles on those. As a result, the range of nanobubble generation clearly changed by the difference of wettability between adjacent surfaces. These results show that the generation of interfacial nanobubbles can be controlled by the surface processing of the substrate and can be expected to be applied to boiling heat transfer.

DOI： 10.1615/ihtc16.nmt.022210

Language：English   Publishing type：Research paper (scientific journal)  

Amongst an extensive collection of surface characteristics that could affect boiling performance, surface wettability (as measured by the contact angle with water) proves to play a unique role in potentially manipulating bubble behavior to the advantage of higher heat transfer rates. In this study, we show experimentally that controlled bubble behavior be realized under the surface design incorporating these two characteristics (namely, by coating an array of hydrophobic spots on a hydrophilic substrate), which leads to a great enhancement in boiling heat transfer under various conditions. In reduced-pressure pool boiling, the strong pinning of the bubble contact line at the border between the hydrophilic and hydrophobic regions manages to prevent total deactivation of nucleation sites. As a result, the deleterious transition to intermittent boiling is effectively delayed, whereby no heat transfer deterioration occurs until a very low pressure of about 8 kPa is reached. Moreover, in subcooled boiling, bubble growth on a patterned surface is found to be facilitated by a pronounced presence of dissolved gas in defiance of exhaustive degassing efforts through continuous boiling, thanks to an unusually strong retention of gas contents by the hydrophobic surface. As experimental and numerical evidence show, only bubbles with sufficiently high concentrations of gas components (i.e., causing weakened condensation) are able to grow large enough on the hydrophobic surface such that periodic pinch-offs might take place, which is responsible for most of the initial heat transfer enhancement before large-scale bubble nucleation starts on the hydrophilic surface as well.

DOI： 10.1615/ihtc16.bae.023482

Language：English   Publishing type：Research paper (scientific journal)  

Thermal rectification is a phenomenon that the heat flow changes by reversing the direction of temperature gradient. This is a fundamental behavior of the thermal rectifiers, which can be used for the active heat flow control, thermally driven computer, efficient energy harvesting, etc. The key challenge is how to increase the thermal rectification ratio, which is defined as the relative change of thermal conductivities in different heat flow directions. Due to the significant size effect and unique heat transfer mechanisms, nanomaterials (such as carbon nanotubes, nanowires, graphene, etc.) are suggested to have high thermal rectification ratio. However, the experiment result showed that the ratio of the single carbon nanotube thermal rectifier was only 7%. In the past decade, many theoretical researches and molecular dynamics simulations have shown that the monolayer graphene may have high thermal rectification ratio due to its unique two-dimensional heat transfer mechanism. But the experimental work is still a blank because of the difficult fabrication process of suspended graphene electronic device. In this work, we report the experimental demonstration of a suspended monolayer graphene thermal rectifier. Three different types of graphene thermal rectifiers have been fabricated with different asymmetric nanostructures. The focused ion beam manufacturing, electron beam induced deposition and precise electron beam lithography were used to design and create asymmetric nanostructures on the monolayer graphene. The thermal rectification ratios were measured by using a precise H-type sensor method. The highest rectification ratio reaches 28% for the graphene with asymmetric nanopores. The asymmetric dependence of thermal conductivity on temperature and space is known to be the physical reason. For the other two kinds of thermal rectifiers, the rectification ratios are about 10%. The asymmetric phonon scattering is known to be the physical reason, which has been proved by using large-scale molecular dynamics simulation.

DOI： 10.1615/ihtc16.nmt.022118

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-017-02163-8

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 The Author(s). For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures-even below the boiling point-is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.

DOI： 10.1038/s41598-017-02163-8

Language：Others   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd Surface wettability of a heating surface is one of the most important factors affecting boiling performance. While a biphilic surface (with juxtaposed hydrophilic and hydrophobic regions) is known as a promising technique to enhance water pool boiling at the atmospheric pressure, there is no research regarding its potential for sub-atmospheric applications. In the present study, we have investigated the characteristics of pool nucleate boiling on biphilic surfaces at sub-atmospheric pressures. Biphilic surfaces were made by applying Ni-TFEO (tetrafluoroethylene oligomer) electroplating (with a contact angle of about 140°) on a copper surface. The heat transfer performance of various biphilic surfaces (with different hydrophobic spot diameters and pitches) were measured in the pressure range from atmospheric to 6.9 kPa. At a pressure of 14.0 kPa, the wall superheat at the onset of nucleate boiling was reduced by 12 K on a biphilic surface compared with a mirror-finished copper surface. The experiment with three different biphilic patterns revealed that smaller pitch and diameter of the hydrophobic spots were favorable to heat transfer at 14.0 kPa. The enhancement of HTC over Kutateladze's correlation reached 270%. A sharp transition from continuous to intermittent boiling, resulting in large deterioration of HTC, was observed on a biphilic surface at a much lower pressure than that on a copper surface. Boiling performance was less affected by the pressure level above the transition pressure.

DOI： 10.1016/j.ijheatmasstransfer.2017.08.078

Language：Others   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd While wicking or spreading of a liquid through microstructures has been found to be promising for applications such as textiles, microelectronics or heat sinks, the effects of such structured surfaces on condensation phase change has received less attention. On a hydrophilic surface and for a fixed micropillar aspect ratio (height/diameter), the spacing between pillars is found to have a strong impact on the dynamics of condensation and on the final morphology of the condensate. In the case of micropillars with a large spacing between pillars, the condensate grows initially dropwise, and thereafter, as condensation develops, the condensate overcomes the pillars’ height flooding the substrate, and condensation continuous in a filmwise condensation (FWC) fashion. In contrast, filmwise condensation and the continuous nucleation, growth, and departure of drops at the pillars’ tops in a dropwise condensation (DWC) fashion occurs when the spacing between pillars is decreased. In this configuration, the geometry of the microstructures constrains the condensate between the pillars and rise of the condensate interface above the micropillars’ height is not thermodynamically favorable, while the top of the pillars act as nucleation sites. We refer to this latter condensation behavior as simultaneous dropwise/filmwise condensation. These observations were enabled by the excellent spatial and temporal resolution of Environmental Scanning Electron Microscopy. A heat transfer model is proposed to demonstrate the greater heat transfer performance of the simultaneous dropwise/filmwise condensation behavior on these surfaces when compared to solely filmwise condensation. The enhanced heat transfer is realizable due to the ability to maintain a thin film within the microstructures and to the active dropwise condensation at the micropillars’ tops. We report for the first time the occurrence of dropwise condensation on a completely hydrophilic wettability configuration without the assistance of a hydrophobic coating. Our findings pave the way to the development of microstructures for enhanced condensation heat transfer.

DOI： 10.1016/j.ijheatmasstransfer.2017.06.023

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevLett.119.179601

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Author(s). We measured the thermal conductivity of free-standing fluorinated single-layer graphene (FSLG) using a precise T-type method. Pristine graphene was fluorinated and suspended above the substrate using xenon difluoride gas. Compared with the thermal conductivity of pristine single-layer graphene (SLG) (∼2000 W/mK) previously measured by the same T-type method for the same original SLG, the FSLG exhibited a much lower thermal conductivity (∼80 W/mK) and a weak dependence of the thermal conductivity on nanohole defects. The experimental results suggest that the fluorine atoms and sp3 bonding in the FSLG strongly contributed to phonon scattering. The phonon scattering by the fluorine atoms and sp3 bonding has a dominant effect on the thermal conductivity decrease over the phonon scattering by nanohole defects. This study lays a foundation for the thermal measurement of 2D fluorinated materials and benefits future applications of fluorinated graphene.

DOI： 10.1063/1.5001169

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd This study presents an experimental investigation of the heat transfer performance of a two-phase loop thermosyphon with an enhanced mixed-wettability evaporator surface at sub-atmospheric pressures. For central-processing-unit (CPU) cooling applications, a lowering of the saturation temperature (pressure) is essential when water is used as the working fluid. Compared with copper mirror surfaces, up to over 100% enhancement of high heat transfer coefficient (HTC) was observed using surfaces with spotted wettability patterns, which consists of hydrophobic spots with contact angle ranged from 145° to 150°. The results revealed that the boiling behaviors changed drastically with the application of hydrophobic spots coating by artificially increasing the nucleation site density. Parametric tests with a variety of operating conditions, including different filling ratios, condenser temperatures, and heat loads revealed the minimum thermal resistance (i.e., the optimum thermosyphon performance) to be 0.03 K/W on the boiling side.

DOI： 10.1016/j.applthermaleng.2017.05.145

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 The Royal Society of Chemistry. The last decade has seen the rapid growth of research on two-dimensional (2D) materials, represented by graphene, but research on their thermophysical properties is still far from sufficient owing to the experimental challenges. Herein, we report the first measurement of the specific heat of multilayer and monolayer graphene in both supported and suspended geometries. Their thermal conductivities were also simultaneously measured using a comprehensive Raman optothermal method without needing to know the laser absorption. Both continuous-wave (CW) and pulsed lasers were used to heat the samples, based on consideration of the variable laser spot radius and pulse duration as well as the heat conduction within the substrate. The error from the laser absorption was eliminated by comparing the Raman-measured temperature rises for different spot radii and pulse durations. The thermal conductivity and specific heat were extracted by analytically fitting the temperature rise ratios as a function of spot size and pulse duration, respectively. The measured specific heat was about 700 J (kg K)-1 at room temperature, which is in accordance with theoretical predictions, and the measured thermal conductivities were in the range of 0.84-1.5 × 103 W (m K)-1. The measurement method demonstrated here can be used to investigate in situ and comprehensively the thermophysical properties of many other emerging 2D materials.

DOI： 10.1039/c7nr01695f

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 The Author(s). Thermal rectification is a fundamental phenomenon for active heat flow control. Significant thermal rectification is expected to exist in the asymmetric nanostructures, such as nanowires and thin films. As a one-atom-thick membrane, graphene has attracted much attention for realizing thermal rectification as shown by many molecular dynamics simulations. Here, we experimentally demonstrate thermal rectification in various asymmetric monolayer graphene nanostructures. A large thermal rectification factor of 26% is achieved in a defect-engineered monolayer graphene with nanopores on one side. A thermal rectification factor of 10% is achieved in a pristine monolayer graphene with nanoparticles deposited on one side or with a tapered width. The results indicate that the monolayer graphene has great potential to be used for designing high-performance thermal rectifiers for heat flow control and energy harvesting.

DOI： 10.1038/ncomms15843

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd Liquid cell electron microscopy is a useful technique for the observation of chemical, biological, and mechanical processes in liquids at nanometer-scale resolution. This study investigated the generation and growth of nanobubbles using the Fresnel fringe method, which enabled us to determine the location of bubble interface; the nanobubbles were induced in the 600-nm-thick water sample in the cell, by the electron beam. Nucleation occurred first at the solid–liquid interface in the upstream side of electron beam, and this was followed by second-group nucleation at the downstream-side interface; all of the stable nucleations occurred on the solid surfaces. The size of the nucleated bubbles at the moment they became visible depended on the magnification used in the electron microscope, and a higher-energy density in the electron beam induced larger bubbles. The underlying mechanism was also considered in this study.

DOI： 10.1016/j.ijheatmasstransfer.2017.01.013

Language：English   Publishing type：Research paper (other academic)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.4973488

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Author(s). Hollow carbon nanotubes (CNTs) were impregnated with an ionic liquid, resulting in a composite core-shell nanostructure. Liquid infusion was verified by transmission electron microscopy and rigorous observations unveiled that the nanocomposite is stable, i.e., liquid did not evaporate owing to its low vapor pressure. A series of individual nanostructures were attached on T-type heat sensors and their thermal behavior was evaluated. The liquid core was found to reduce the thermal conductivity of the base structure, CNT, from ca. 28 W/mK to ca. 15 W/mK. These findings could contribute to a better understanding of nanoscale thermal science and potentially to applications such as nanodevice thermal management and thermoelectric devices.

DOI： 10.1063/1.4973488

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Author(s). Classical thermodynamics theory predicts that nanosized bubbles should disappear in a few hundred microseconds. The surprisingly long lifetime and stability of nanobubbles are therefore interesting research subjects. It has been proposed that the stability of nanobubbles arises through pinning of the three-phase contact line, which results from intrinsic nanoscale geometrical and chemical heterogeneities of the substrate. However, a definitive explanation of nanobubble stability is still lacking. In this work, we examined the stability mechanism by introducing a "pinning force." We investigated nanobubbles at a highly ordered pyrolytic graphite/pure water interface by peak force quantitative nano-mechanical mapping and estimated the pinning force and determined its maximum value. We then observed the shape of shrinking nanobubbles. Because the diameter of the shrinking nanobubbles was pinned, the height decreased and the contact angle increased. This phenomenon implies that the stability results from the pinning force, which flattens the bubble through the pinned three-phase contact line and prevents the Laplace pressure from increasing. The pinning force can also explain the metastability of coalesced nanobubbles, which have two semispherical parts that are joined to form a dumbbell-like shape. The pinning force of the semispherical parts was stronger than that of the joint region. This result demonstrates that the contact line of the semispherical parts is pinned strongly to keep the dumbbell-like shape. Furthermore, we proposed a nanobubble generation mechanism for the solvent-exchange method and explained why the pinning force of large nanobubbles was not initially at its maximum value, as it was for small nanobubbles.

DOI： 10.1063/1.4973385

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijheatmasstransfer.2017.08.078

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.applthermaleng.2017.05.145

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.applthermaleng.2014.10.054

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Elsevier B.V. Platinum hot films have been used as precise resistance thermometers to measure the thermal conductivities of carbon nanotubes and graphene. Assisted by focused ion beam (FIB) irradiation, the influence of defects on phonon transport have been examined. However, wide lateral ion beam scattering may affect the electrical properties of hot films and cause uncertainty. In this letter, the effect of FIB irradiation on the electrical resistivity of platinum hot films was evaluated. To investigate this effect qualitatively, electrical resistivity measurement and FIB irradiation were alternated while changing irradiation positions and doses. Irradiated ions were found to travel further than 25 μm away from the directly irradiated area, resulting in an increase of electrical resistivity of the film according to total accumulated dose. The number of scattered ions was found to depend on the irradiated surface. An empirical equation describing the relationship between electrical resistivity and assumed ion density in the hot films was proposed. The obtained results enable us to accurately estimate the thermal or electrical properties of nanomaterials using hot-film sensors combined with nanofabrication techniques using FIB.

DOI： 10.1016/j.mee.2016.09.008

Language：English  

DOI： 10.1299/jtst.2016jtst0034

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Author(s). Thermal transport in carbon nanofibers (CNFs) was thoroughly investigated. In particular, individual CNFs were suspended on T-type heat nanosensors and their thermal conductivity was measured over a range of temperatures. Unexpectedly, thermal conductivity was found to be dependent on CNF wall thickness and ranging between ca. 28 and 43 W/(m⋅K). Further investigation of the CNF walls with high resolution electron microscopy allowed us to propose a tentative description of how wall structure affects phonon heat transport inside CNFs. The lower thermal conductivities, compared to other CNTs, was attributed to unique CNF wall structure. Additionally, wall thickness is related to the conducting lattice length of each constituent graphene cone and comparable to the Umklapp length. Hence, as the wall thickness and thus lattice length increases there is a higher probability for phonon scattering to the next layer.

DOI： 10.1063/1.4968831

Language：English   Publishing type：Research paper (other academic)  

Language：Japanese  

Experimental Study of Gas Phase Generation in Nano Scale
<p>Liquid cell electron microscopy (LCEM) is a useful experimental method of bubble nucleation, which is not understood sufficiently because it is difficult to observe nucleation at nanometer scale in real-time. In our TEM observation, when an electron beam irradiated water, interfacial nanobubbles were generated and grew in a nano liquid cell, whose gap was filled with pure water between two silicon nitride (Si₃N₄) membranes. In this paper, we introduce Fresnel fringe method, which enable to understand the position of nucleated nanobubbles and the interaction between nanobubbles on the surface of top Si₃N₄ membrane and the bottom membrane.</p>

DOI： 10.1299/jsmeted.2016.C122

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Author(s). Carbon nanotube-based organic composites and carbon nanotube networks are important flexible and lightweight thermoelectric materials. Characterization of the thermoelectric performance of individual carbon nanotubes is of vital importance for exploring the coupling mechanism between carbon nanotubes and organic composites, and proposing further improvement measures. The thermoelectric performance of an individual multiwalled carbon nanotube with a diameter of 66 nm has been comprehensively studied by applying our T-type method from 260 K to 420 K, using the same measurement configuration. The figure of merit increases from 4.84 × 10-8 to 1.32 × 10-6 on increasing the temperature, which is smaller than previous experimental results on carbon nanotube samples. The thermal conductivity increases from 706 W m-1 K-1 at 260 K to 769.3 W m-1 K-1 at 320 K, and then stays nearly constant until 420 K. The phonons dominate the thermal transport. The electrical conductivity exhibits thermally activated carrier generation and transport with an energy barrier of 194.5 meV. The Seebeck coefficient is in the range of 29.4-41.0 μV K-1 and tends to decrease with temperature.

DOI： 10.1063/1.4962942

Language：Others   Publishing type：Research paper (scientific journal)  

Diamond possesses the highest thermal conductivity (2 × 10³ Wm^-1K^-1) in all bulk materials, owing to low phonon scattering in diamond. Since phonon scattering is drastically enhanced by grain boundaries, the thermal conductivity of polycrystalline diamond films is strongly dependent on the grain size. The thermal conductivity of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films comprising a large number of UNCD grains with diameters of less than 10 nm and an a-C:H matrix have been prepared mainly by chemical vapor deposition (CVD). The highest thermal conductivity reported so far is 12 Wm^-1K^-1, which is comparable with that of sp³-rich non-hydrogenated amorphous carbon (a-C). It has been reported for a-C:H and UNCD/a-C:H films that the thermal conductivity is degraded by the incorporation of H atoms in the films.

Recently we have realized the formation of UNCD/a-C films comprising UNCD grains and an a-C matrix, by coaxial arc plasma deposition (CAPD). CAPD does not necessarily require a hydrogen atmosphere during the deposition for the formation of diamond grains. So far, the thermal conductivity of hydrogen-free UNCD/a-C films has never been studied because H atoms are unintentionally incorporated into the films from source gases in CVD. In this study, UNCD/a-C films were deposited under a base pressure, and the thermal conductivity of the films were measured for the first time to our knowledge.

UNCD/a-C films were deposited on n-Si substrates at base pressures of less than 10^-4 Pa and the substrate temperature of 550 °C using coaxial arc plasma gun equipped with a graphite target. Mo films with the thickness of 80 nm were deposited on the UNCD/a-C films by sputtering for the TDTR. The thermal conductivity of the UNCD/a-C films was measured by a time-domain thermoreflactance (TDTR) technique.

The thermal conductivity of the UNCD/a-C films was estimated to be 23.7 Wm^-1K^-1 by TDTR. The thermal conductivity is evidently higher than that of UNCD/a-C:H prepared by CVD. Two reasons for it are supposed. One is that an a-C in UNCD/a-C is probably advantageous for thermal conduction as compared with an a-C:H in UNCD/a-C:H, since it has been reported for a-C:H that the thermal conductivity decreases with increasing hydrogen content in a-C:H. The other is that UNCD/a-C does not contain hydrogen atoms that prevent from phonon scattering, whereas the preferential existence of hydrogen atoms at grain boundaries between UNCD grains and those between UNCD grains and an a-C:H matrix enhances phonon scattering for UNCD/a-C:H films prepared by CVD, which results in a decrease in the thermal conductivity. Further details will be presented in the conference.

DOI： 10.1149/ma2016-02/8/1060

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Chinese Physical Society and IOP Publishing Ltd. Metallic nanofilms are important components of nanoscale electronic circuits and nanoscale sensors. The accurate characterization of the thermophysical properties of nanofilms is very important for nanoscience and nanotechnology. Currently, there is very little specific heat data for metallic nanofilms, and the existing measurements indicate distinct differences according to the nanofilm size. The present work reports the specific heats of 40-nm-thick suspended platinum nanofilms at 80-380 K and ∼5×10-4 Pa using the 3ω method. Over 80-380 K, the specific heats of the Pt nanofilms range from 166-304 J/(kg•K), which are 1.65-2.60 times the bulk values, indicating significant size effects. These results are useful for both scientific research in nanoscale thermophysics and evaluating the transient thermal response of nanoscale devices.

DOI： 10.1088/1674-1056/25/11/114401

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Elsevier B.V. All rights reserved. We demonstrate a general process for fabricating graphene nanoelectronic devices that have next several features: free-standing, micrometer-sized monolayer graphene with high quality, arbitrarily-shaped metallic electrodes or sensors. In contrast to the normal routes, a gas etching process is used to create a deep trench in silicon for suspending the whole graphene device in a much larger area. User-designed electrodes or sensors are fabricated on the suspended graphene at the same time for realizing multiple functions. In this work, a suspended gold nanofilm sensor is designed to measure the intrinsic electrical and thermal properties of graphene on site. The sensor serves as both electrode and precise resistance thermometer at the same time. By simply changing the metallic electrode shape and electrical circuit, the free-standing graphene can be made into different devices, such as single-molecule detector or nano-resonator. In order to test the robustness of graphene device, a high electrical current is applied to heat the graphene in vacuum until it breaks. The breakdown current density is measured to be 1.86 mA/μm. More importantly, this method is not only limited to graphene, but also can be applied to any other two-dimensional materials.

DOI： 10.1016/j.sna.2016.05.002

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 American Chemical Society. The surface wettability of a liquid on the inner and outer surface of single carbon nanotubes (CNTs) was experimentally investigated. Although these contact angles on both surfaces were previously studied separately, the available data are of limited help to elucidate the effect of curvature orientation (concave or convex) on wettability due to the difference in surface structure. Here, we report on the three-phase contact region and wettability on the outer surface of CNT during the dipping and withdrawing experiment of CNT into an ionic liquid. Furthermore, the wettability on the inner surface was measured using a liquid within the same CNT. Our results show that the contact angle on the outer surface of the CNT is larger than that on the flat surface and that on the inner surface is smaller than that on the flat one. These findings suggest that the surface curvature orientation has a noticeable effect on the contact angle at the nanoscale because both inner and outer surfaces expose the same graphite wall structure and the contact line tension will be negligible in this situation. The presented results are rationalized using the free energy balance of liquid on curved surfaces.

DOI： 10.1021/acs.langmuir.6b01366

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Author(s). We measured both in-plane electrical and thermal properties of the same suspended monolayer graphene using a novel T-type sensor method. At room temperature, the values are about 240 000 Ω-1 m-1 and 2100 W m-1 K-1 for the electrical and thermal conductivities, respectively. Based on the Wiedemann-Franz law, the electrons have negligible contribution to the thermal conductivity of graphene, while the in-plane LA and TA modes phonons are the dominant heat carriers. In monolayer graphene, the absence of layer-layer and layer-substrate interactions enhances the contribution of long wave-length phonons to the heat transport and increases the thermal conductivity accordingly. The reported method and experimental data of suspended monolayer graphene are useful for understanding the basic physics and designing the future graphene electronic devices.

DOI： 10.1063/1.4954677

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Highly stable nanoscale gas states at solid/liquid interfaces, referred to as nanobubbles, have been widely studied for over a decade. In this study, nanobubbles generated on a hydrophobic Teflon amorphous fluoroplastic thin film in the presence and absence of hydrophilic carbon domains are investigated by peak force quantitative nanomechanics. On the hydrophobic surface without hydrophilic domains, a small number of nanobubbles are generated and then rapidly decrease in size. On the hydrophobic surface with hydrophilic domains, the hydrophilic domains have a significant effect on the generation and stability of nanobubbles, with bubbles remaining on the surface for up to three days. Bigger, better bubbles: The enhancement of nanobubble generation and stability by the existence of hydrophilic domains on a surface is shown (see picture). Close to the Ti/Si boundary, many nanobubbles are generated on the relatively hydrophobic Si surface. The hydrophilic-hydrophobic combination is one of the key factors for nanobubble generation and stabilization.

DOI： 10.1002/cphc.201501181

Language：English   Publishing type：Research paper (scientific journal)  

© 2015 Elsevier Ltd. All rights reserved. As a solid-state membrane with only one-atom thickness, nano-porous graphene has attracted intense attention in many critical applications. Here, the key challenge is to suspend a single-layer graphene (SLG) and drill nanopores with precise dimensions. Here, we report a simple and reliable route for making suspended fluorinated SLG with size-tunable nanopores. Our method consists of two steps: 1. a free-standing SLG ribbon was created between two gold pads after deep dry etching of silicon substrate by xenon difluoride. The SLG was fluorinated by 5-13%. Superior to the normal wet etching method, the dry etching process is much simpler and results in less hole-defect and edge deformation. A large area fluorinated SLG can be suspended due to the sufficient etch depth. 2. a focused ion beam was introduced to drill nanopores in graphene with an initial diameter around 20 nm. Followed by an electron beam induced carbon deposition, the diameter of nanopore was gradually decreased to sub-10 nm. By changing the deposition time, the size of nanopore can be precisely controlled. High-cost transmission electron microscope is no longer needed. Our method provides a simple and effective way for preparing free-standing fluorinated SLG ribbon suitable for single-molecule detection.

DOI： 10.1016/j.carbon.2015.12.070

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 The Royal Society of Chemistry. A comprehensive method to evaluate the thermoelectric performance of one-dimensional nanostructures, called the T-type method, has been first developed. The thermoelectric properties, including the Seebeck coefficient, thermal conductivity and electrical conductivity, of an individual free-standing single crystal Bi2S3 nanowire have been first characterized by applying the T-type method. The determined figure of merit is far less than the reported values of nanostructured bulk Bi2S3 samples, and the mechanism is that the Seebeck coefficient is nearly zero in the temperature range of 300-420 K and changes its sign at 320 K.

DOI： 10.1039/c5nr05946a

Language：English   Publishing type：Research paper (scientific journal)  

Utilizing nanomachining technologies, it is possible to manipulate the heat transport in graphene by introducing different defects. However, due to the difficulty in suspending large-area single-layer graphene (SLG) and limited temperature sensitivity of the present probing methods, the correlation between the defects and thermal conductivity of SLG is still unclear. In this work, we developed a new method for fabricating micro-sized suspended SLG. Subsequently, a focused ion beam (FIB) was used to create nanohole defects in SLG and tune the heat transport. The thermal conductivity of the same SLG before and after FIB radiation was measured using a novel T-type sensor method on site in a dual-beam system. The nanohole defects decreased the thermal conductivity by about 42%. It was found that the smaller width and edge scrolling also had significant restriction on the thermal conductivity of SLG. Based on the calculation results through a lattice dynamics theory, the increase of edge roughness and stronger scattering on long-wavelength acoustic phonons are the main reasons for the reduction in thermal conductivity. This work provides reliable data for understanding the heat transport in a defective SLG membrane, which could help on the future design of graphene-based electrothermal devices.

DOI： 10.1038/srep21823

Language：English   Publishing type：Research paper (scientific journal)  

© 2015 American Chemical Society. Realizing surface wettability tuning at the submicron-scale resolution is expected to enable the fabrication of micro/nano-structured fluidic devices and is particularly important in nanobiotechnology and high-resolution printing. Herein, we propose an approach to modify the wettability of self-assembled monolayer surfaces using focused ion beam (FIB) irradiation. The contact angle of the irradiated region changed from hydrophobic to hydrophilic by increasing the ion dosage. The chemical composition and associated depth profile of the sample surfaces were analyzed by glow discharge-optical emission spectroscopy. The results indicated that the content of fluorine at the surface decreased after FIB irradiation of the samples. A submicron-scale hydrophobic-hydrophilic hybrid surface was then fabricated by forming hydrophilic dots with diameters of ∼110 nm on a hydrophobic surface by FIB irradiation. The difference in wettability of the hydrophobic and hydrophilic areas on the surface was confirmed by microscale condensation and evaporation experiments. Condensed droplets with diameters of ∼300 nm appeared on the surface according to the fabricated pattern, thus suggesting that condensation preferentially occurred on the hydrophilic dots than on the hydrophobic surface. Furthermore, tiny droplets remained on the hydrophilic dots following evaporation of the larger droplets. The current approach provides a means to control wettability-driven phenomena.

DOI： 10.1021/acs.jpcc.5b09019

Language：English   Publishing type：Research paper (other academic)  

© 2015 IEEE. Nanoscale thermal mapping along an individual multi-walled carbon nanotube suspended between two electrodes/heat-sinks was successfully demonstrated by using the solid/liquid phase transition of indium nanoparticles in a transmission electron microscope. The brightness shift of nanoparticles in the dark-field image was clearly recognized according to the DC heating of the nanotube. It was also found that the indium deposition induces defects in the nanotube, resulting in the decrease of thermal conductivity. The temperature distribution along the nanotube obtained from the dark-field images showed good agreement with the simulated data of a defective nanotube with Joule-heating.

DOI： 10.1109/NANO.2015.7388726

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Others  

© The Royal Society of Chemistry 2016. Carbon nanotubes (CNTs) have many advantages when used as nanoscale thermal probes, for example, their thin diameter, robustness, and low thermal resistance. However, there are some difficulties in both fabrication and analytical steps for CNT thermometry. In this chapter, recently developed techniques for the fabrication of CNT thermal probes and the current understanding of the thermal properties of CNTs are addressed. An example of quantitative temperature sensing at a nanoscale point contact using a CNT bonded to a Pt hot film is also explained.

DOI： 10.1039/9781782622031-00339

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Graphene oxide (GO) is hydrophilic and swells significantly when in contact with water. Here, we investigate the change in thickness of multilayer graphene oxide membranes due to intercalation of water, via humidity-controlled observation in an environmental scanning electron microscope (ESEM). The thickness increases reproducibly with increasing relative humidity. Electron energy loss spectroscopy (EELS) reveals the existence of water ice under cryogenic conditions, even in high vacuum environment. Additionally, we demonstrate that freezing then thawing water trapped in the multilayer graphene oxide membrane leads to the opening up of micron-scale inter-lamellar voids due to the expansion of ice crystals.

DOI： 10.1038/srep11807

Language：Japanese   Publishing type：Research paper (scientific journal)  

Study of condensation on hydrophobic surface with nanoscale hydrophilic regions
Water condensation on a hydrophobic surface with nanoscale hydrophilic regions was investigated to reveal the condensation mechanism of submicron-scale droplets. This feature was found on the graphite step-terrace structured surface; step surfaces are more wettable relative to terrace surfaces, and it was precisely characterized using an atomic force microscope. Condensation experiments were conducted using an environmental scanning electron microscope and droplets were observed to line up on preferentially along the graphite steps. Observed droplets ranged from 150 to 300 nm in diameter and the droplet interval depends on the width of hydrophobic region. The heterogeneous nucleation theory was extended to consider attracted water molecules on hydrophilic step surface, which enable us to explain the current observed result under unsaturated condition. As a result, proposed theory shows qualitatively that narrower hydrophobic region induces short droplet interval. Our suggestion for design the hybrid hydrophilic-hydrophobic surface would enable the development of surface that will perform high heat transfer at dropwise condensation.

DOI： 10.1299/transjsme.14-00495

Language：English   Publishing type：Research paper (scientific journal)  

© 2015 AIP Publishing LLC. Thermophysical characterization of graphene is very important for both fundamental and technological research. While most of the existing thermal conductivity measurements are for graphene sheets with sizes larger than 1 μm, the thermal conductivities for suspended submicron graphene ribbons are still very few, although the thermal conductivity of graphene ribbons at the submicron scale is predicted to be much smaller than large graphene and strongly size dependent for both length and width due to the 2D nature of phonon transport. Here, we report the temperature dependent thermal conductivity of a 169-nm wide and 846-nm long graphene ribbon measured by the electrical self-heating method. The measured thermal conductivities range from (12.7 ± 2.95) W/m/K at 80 K to (932 ± 333) W/m/K at 380 K, being (349 ± 63) W/m/K at 300 K, following a ∼ T2.79 law for the full temperature range of 80 K to 380 K and a ∼ T1.23 law at low temperatures. The comparison of the measured thermal conductance with the ballistic transport limit indicates diffusive transport in this narrow and short ribbon due to phonon-edge as well as phonon-defect scattering. The data were also combined with an empirical model to predict possible width dependence of thermal conductivity for suspended graphene ribbons. These results help understand the 2D phonon transport in suspended submicron graphene ribbons and provide knowledge for controlling thermophysical properties of suspended graphene nanoribbons through size manipulation.

DOI： 10.1063/1.4907699

Language：English   Publishing type：Research paper (other academic)  

Nanoscale thermal mapping along an individual multi-walled carbon nanotube suspended between two electrodes/heat-sinks was successfully demonstrated by using the solid/liquid phase transition of indium nanoparticles in a transmission electron microscope. The brightness shift of nanoparticles in the dark-field image was clearly recognized according to the DC heating of the nanotube. It was also found that the indium deposition induces defects in the nanotube, resulting in the decrease of thermal conductivity. The temperature distribution along the nanotube obtained from the dark-field images showed good agreement with the simulated data of a defective nanotube with Joule-heating.

DOI： 10.1109/NANO.2015.7388726

Language：English   Publishing type：Research paper (scientific journal)  

© 2014 American Chemical Society. Nanobubbles exist at solid-liquid interfaces between pure water and hydrophobic surfaces with very high stability, lasting in certain cases up to several days. Not only semispherical but also other shapes, such as micropancakes, are known to exist at such interfaces. However, doubt has been raised as to whether or not the nanobubbles are gas-phase entities. In this study, surface nanobubbles at a pure water-highly ordered pyrolytic graphite (HOPG) interface were investigated by peak force quantitative nanomechanics (PF-QNM). Multiple isolated nanobubbles generated by the solvent-exchange method were present on the terraced areas, avoiding the steps of the HOPG surface. Adjacent nanobubbles coalesced and formed metastable nanobubbles. Coalescence was enhanced by the PF-QNM measurement. We determined that nanobubbles can exist for a long time because of nanoscale contact angle hysteresis at the water-HOPG interface. Moreover, the hydrophilic steps of HOPG were avoided during coalescence, providing evidence that the nanobubbles are truly gas phase.

DOI： 10.1021/la5036322

Language：English   Publishing type：Research paper (scientific journal)  

Water condensation on a hydrophobic surface with nanoscale hydrophilic regions was investigated to reveal the condensation mechanism of submicron-scale droplets. This feature was found on the graphite step-terrace structured surface; step surfaces are more wettable relative to terrace surfaces, and it was precisely characterized using an atomic force microscope. Condensation experiments were conducted using an environmental scanning electron microscope and droplets were observed to line up on preferentially along the graphite steps. Observed droplets ranged from 150 to 300 nm in diameter and the droplet interval depends on the width of hydrophobic region. The heterogeneous nucleation theory was extended to consider attracted water molecules on hydrophilic step surface, which enable us to explain the current observed result under unsaturated condition. As a result, proposed theory shows qualitatively that narrower hydrophobic region induces short droplet interval. Our suggestion for design the hybrid hydrophilic-hydrophobic surface would enable the development of surface that will perform high heat transfer at dropwise condensation.

Language：English   Publishing type：Research paper (scientific journal)  

© 2014 American Chemical Society. Water condensation on a hybrid hydrophilic-hydrophobic surface was investigated to reveal nucleation mechanisms at the microscale. Focused ion beam (FIB) irradiation was used to change the wettability of the hydrophobic surface with 10 nm order spatial resolution. Condensation experiments were conducted using environmental scanning electron microscopy; droplets, with a minimum diameter of 800 nm, lined up on the FIB-irradiated hydrophilic lines. The heterogeneous nucleation theory was extended to consider the water molecules attracted to the hydrophilic area, thereby enabling explanation of the nucleation mechanism under unsaturated conditions. Our results showed that the effective surface coverage of the water molecules on the hydrophilic region was 0.1-1.1 at 0.0 °C and 560 Pa and was dependent on the width of the FIB-irradiated hydrophilic lines and hydrophobic area. The droplet nucleation mechanism unveiled in this work would enable the design of new surfaces with enhanced dropwise condensation heat transfer. (Figure Presented).

DOI： 10.1021/la503615a

Language：English   Publishing type：Research paper (other academic)  

© 2014 IEEE. Thermal conductance of an individual multiwalled carbon nanotube was measured as a function of its length. Focused ion beam was used to shorten a 4.8 micrometer-long nanotube to 2.4, 1.2, 0.6, and 0.3-micrometer-long specimens on a hot-film sensor. As the nanotube is shorten, the conductance decreases more than expected by the diffusive thermal conduction theory. We treated two nanotubes of 64nm and 85nm diameters, both of which showed quasi-ballistic phonon transport. This is the first experiment to quantitatively investigate the contribution of phonons with long free paths in multiwalled carbon nanotube. The principle of thermal measurement, amorphous carbon induced by the ion beam, and considerable errors are also explained.

DOI： 10.1109/ITHERM.2014.6892439

Language：Others   Publishing type：Research paper (other academic)  

With the continued size reduction and operation frequency increase in micro/nanoelectromechanical systems, heat accumulation has become more critical. It is crucial to carry out an in-depth investigation on the thermal transport properties of thin metal films, i.e., electron-phonon relaxation, thermal diffusivity and interfacial thermal resistance. In this paper, series study of the thermal transport properties of thin metal films have been performed by applying transient thermoreflectance method, including femtosecond and picosecond laser thermoreflectance systems. The electron-phonon coupling factor of thin gold films with different thickness have been measured by applying femtosecond laser thermoreflectance system. The results show that the electronphonon relaxation is nearly the same as that of bulk gold and independent of film thickness. The cross-plane thermal diffusivity of 95.3 and 200-nm-thick molybdenum film has been studied by applying the picosecond laser thermoreflectance system. The measurement results show that the cross-plane thermal diffusivity of molybdenum thin film decrease significantly compared to the corresponding bulk value and tends to increase as films become thicker.

DOI： 10.1615/ihtc15.nmm.009179

Language：English   Publishing type：Research paper (international conference proceedings)  

Water condensation on a graphite surface was investigated at the micron and submicron scale by environmental scanning electron microscopy. The graphite comprised a hydrophobic terrace and hydrophilic step edges, of which the nanoscale structure was precisely measured by atomic force microscopy prior to the condensation experiments. The condensed droplets were preferentially aligned parallel to the step edges with a step height of 1 nm. The droplets featured a diameter of 150-300 nm at intervals greater than 150 nm. Shorter droplet intervals were realized by narrower terraces and higher steps. The current findings extend beyond the nucleation theory, whereby the effect of adsorbed water molecules on hydrophilic step edges was considered. The contact angle (i.e., 10°) of the nucleated droplet at its initial stage (with diameter in the nanoscale) was determined from the extended theory, and was consistent with direct observation of slightly grown droplets. The growth mechanism of the submicrometer-sized droplets was also investigated; under this scale regime, the three-phase contact line does not recede during coalescence.

DOI： 10.1615/ihtc15.cds.009177

Language：Others   Publishing type：Research paper (scientific journal)  

© 2014 Elsevier Ltd. We present experimental results on the controlled bubble generation from a single PTFE (polytetrafluoroethylene) spot-with diameter varying from 2mm to 6mm-deposited on a flat polished copper surface that was submersed in subcooled pure water. The static contact angle of the PTFE coating was measured to be over 120°, which conveniently produced a clear contrast with the copper substrate in terms of wettability that ensured controlled bubble nucleation. By making use of a high-speed camera, statistical details about the bubble formation that include the departure frequency and diameter have been obtained at various surface temperatures. An interesting observation was made of repeated cycles of bubble nucleation and detachment at nominally negative surface superheats (i.e., the wall temperature being below the saturation temperature at the system pressure), which featured particularly long bubble growth time and seemingly no waiting time. The vertical temperature distribution inside the bubble, which was measured by a micro-thermocouple of about 250μm in diameter, suggests a relatively stable bubble composition of water vapor and dissolved air. A heat-pipe analogy was drawn to describe the internal heat transfer mechanism of bubble growth on a mixed wettability surface under subcooled conditions.

DOI： 10.1016/j.applthermaleng.2014.10.054

Language：English   Publishing type：Research paper (scientific journal)  

Thin metal films are widely used as interconnecting wires and coatings in electronic devices and optical components. Reliable thermophysical properties of the films are required from the viewpoint of thermal management. The cross plane thermal transport of four polycrystalline molybdenum nanofilms with different thickness deposited on glass substrates has been studied by applying the picosecond laser transient thermoreflectance technique. The measurement is performed by applying both front pump-front probe and rear pump-front probe configurations with high quality signal. The determined cross plane thermal diffusivity of the Mo films greatly decreases compared to the corresponding bulk value and tends to increase as films become thicker, exhibiting significant size effect. The main mechanism responsible for the thermal diffusivity decrease of the present polycrystalline Mo nanofilms is the grain boundary scattering on the free electrons. Comparing the cross plane thermal diffusivity and inplane electrical conductivity indicates the anisotropy of the transport properties of the Mo films. © 2014 Tingting Miao et al.

DOI： 10.1155/2014/578758

Language：English   Publishing type：Research paper (scientific journal)  

Phonon confinement and in situ thermal conductance measurements in an individual multi-walled carbon nanotube (MWNT) are reported. Focused ion beam (FIB) irradiation was used to successively shorten a 4.8 μm long MWNT, eventually yielding a 0.3 μm long MWNT. After the first FIB irradiation, a 41% reduction in conductance was achieved, compared with that of the pristine MWNT. This was because the contributions from phonons with long free paths were excluded by scattering at FIB-induced defects. Phonon transport in linked multiple-length nanotubes was also investigated. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4869470

Language：English   Publishing type：Research paper (scientific journal)  

A prototype of "micro-beam" MEMS sensor that is made of a thin metallic film suspended across a trench on a silicon substrate was fabricated for examination of the feasibility of detecting thermal conductivity of gases and liquids. Heating the sensor in a sample demonstrated the potential measurement at a steady state because no natural convection took place during heating. The measured temperature rise of the sensor agreed fairly well with the temperature rise estimated by a numerical analysis of heat conduction to a sample fluid from the sensor with given measured dimensions. The temperature of the sensor was significantly higher in the air than FC-72 as well, indicating the feasibility of detecting thermal conductivity by the proposed method. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.sna.2013.11.019

Language：Others   Publishing type：Research paper (scientific journal)  

A carbon nanotube (CNT) has a very high intrinsic thermal conductivity and is expected to be used in a variety of thermal applications. However, the thermal contact resistance (TCR) between a CNT and ambient material still remains unclear. Some analytical and molecular dynamics studies have been reported, but there is no reliable experimental method to quantitatively investigate the interface issues. This article reports on a new technique for measuring the TCR at the end of an individual CNT by using a platinum hot film sensor. Two methods are introduced to obtain the TCR between a multi-walled CNT and a SiO2 surface, and both methods were confirmed to give an identical TCR. © 2011 Springer Science+Business Media, LLC.

DOI： 10.1007/s10765-011-1137-1

Language：English   Publishing type：Research paper (other academic)  

Boiling is one of the most effective heat transfer methods due to its high heat transfer coefficient. Therefore, boiling heat transfer plays a very important role for various applications in many technological and industrial areas. However, a very complex mechanism of boiling, especially bubble nucleation, is still not sufficiently understood. On the other hand, numerous experiments have revealed the existence of soft domains that called nanobubbles at the solid-liquid interface. In this study, to investigate the influence of the solid-liquid interface nanobubbles on the bubble nucleation, an atomic force microscope (AFM) is used to characterize the morphology of nanobubbles. In order to separate the effect of wettability of a solid surface from that of surface structure, a very flat hydrophobic surface was prepared. 1H,1H,2H,2H-Perfluoro-noctylphosphonic acid (FOPA) formed the interface of hydrophobic self-assembled monolayers (SAMs). As the result of AFM measurement, many nanobubbles about 100 nm in diameter and 30 nm thick are observed at the interface of the FOPA surface and the pure water. In addition, to prove the existence of gaseous phase, the heat conductance measurement by time-domain thermoreflectance method (TDTR) was introduced. TDTR is an ultrafast optical pump probe technique well suited for thermal measurement of thin films. It enables to resolve the thermal conductivity of the thin film and the thermal conductance of the interface. If nanobubbles are the gaseous phase, the big change of interface heat thermal resistance will be seen and the TDTR signal should also change. The effectiveness of a TDTR to confirm the existence of nanobubbles is shown by the model simulation of TDTR. A clear difference is seen in TDTR signal by the existence of only 1 nm gaseous phase. After confirming the existence of nanobubbles by AFM measurement, it can be proved that the nanobubbles are truly gaseous phase of the TDTR measurement. Copyright © 2013 by ASME.

DOI： 10.1115/MNHMT2013-22077

Language：English   Publishing type：Research paper (other academic)  

Heat transfer at solid-solid interface is very fascinating where no one knows the full mechanism which has a huge impact in many applications in engineering and science. In many kinds of interfaces, we treat a van der Waals contact of perfectly-smooth surfaces by using multiwalled carbon nanotubes (CNTs). Their thermal contact resistance (TCR) is estimated by comparing measured thermal conductivity of CNT specimen and numerical simulation result. The TCR per unit area is estimated as 1.58∼3.33×10-8 m 2K/W at room temperature in vacuum, which is much higher than our previous result in air. It was also found that TCR is inversely proportional with the temperature to the 1.92th power different from the simple phonon model represented by the diffuse mismatch model. Copyright © 2013 by ASME.

DOI： 10.1115/MNHMT2013-22078

Language：Others   Publishing type：Research paper (other academic)  

The effects of hydrophobic circle spot size and subcooling on local film boiling phenomenon from the copper surface with single PTFE (Polytetrafluoroethylene) hydrophobic circle spot at low heat flux has been investigated. The experiments were performed using pure water as the working fluid and subcooling ranging from 0 and 10K. The heat transfer surfaces are used polished copper block with single PTFE hydrophobic circle spot of diameters 2, 4 and 6 mm, respectively. A high-speed camera was used to capture bubble dynamics and disclosed the sequence of the process leading to local film boiling. The result shows that local films boiling occurs on the PTFE circle spot at low heat flux and was triggered by the merging of neighboring bubbles. The study also showed that transition time required for change from nucleate boiling regime to local film boiling regime depends on the diameter of the hydrophobic circle spot and the subcooling. A stable local film boiling occurs at the smallest diameter of hydrophobic spot. Subcooling cause the local film boiling occur at negative superheat and oscillation of bubble dome. Copyright © 2013 by ASME.

DOI： 10.1115/ICNMM2013-73069

Language：English   Publishing type：Research paper (scientific journal)  

Metallic nanofilms are of great importance in integrated circuit design and electronic devices. Understanding energy dissipation and transport in metallic nanofilms is essential to practical thermal management. The Wiedemann-Franz (WF) law states a precisely fixed ratio by which the electrons transport heat and charge, providing a basic rule to determine the thermal properties. Hitherto no bulk material has been known to violate the WF law. We report compelling evidence for the breakdown of the WF law in polycrystalline gold nanofilms at low temperatures, the Lorenz number increases notably with decreasing temperature. Our results show that the electrons dominate in heat transport at high temperatures, leading to a constant Lorenz number. While below 40 K, inelastic electron scattering at grain boundaries becomes significant and part of the electron energy is transferred to phonons. Correspondingly, the phonon thermal conductivity is increased and the WF law is violated. A detailed kinetic theoretical model has been developed to investigate several phonon scattering mechanisms in depth and matches well with the experimental results. © 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2013.07.066

Language：English   Publishing type：Research paper (other academic)  

Condensation heat transfer is a widely-used technique for industrial applications represented by heat exchanger because of its high heat transfer coefficient. To enhance its performance, a suitable surface is required, where both condensation and droplet removal smoothly occur. In this study, we compared wettability of a graphene surface and an amorphous carbon surface. The result shows that an amorphous carbon surface is more hydrophilic. Then we prepared a graphite surface which has nanoscale hydrophilic regions in large hydrophobic area. We observed the submicron-scale droplet condensation occurs preferentially on the hydrophilic graphite step by using environmental scanning electron microscope (ESEM). Copyright © 2013 by ASME.

DOI： 10.1115/IMECE2013-66186

Language：English   Publishing type：Research paper (scientific journal)  

Optical absorptance is an important property of carbon nanotubes for practical applications but has rarely been accurately measured. We developed a T type thermal probe method to measure the optical absorptance of an individual multiwall carbon nanotube. In this method, one end of the carbon nanotube (CNT) is attached to the center of a platinum nanofilm in a T shape and the Pt nanofilm acts as a thermometer. A laser beam irradiates at the CNT and the absorbed laser power can be determined by measuring the average temperature rise of the Pt nanofilm based on the temperature dependence of the electric resistance. Experimental results showed that a 100-nm-diameter multiwall CNT could absorb 13.2&#37; of the 514-nm-wavelength laser power with the laser spot diameter being 1 μm. This method is useful for determining the optical absorptance of CNTs and other one-dimensional nanostructures such as Si/Ge nanowires for various optical wavelengths in their photovoltaic, photoelectrolysis and other optical applications. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4824494

Language：English   Publishing type：Research paper (scientific journal)  

Quantitative temperature sensing at the nanoscale point contact is developed using a platinum hot film sensor with a carbon nanotube (CNT) as a thermal probe. High spatial resolution and robustness is achieved because of the small tip radius and high stiffness of the CNT. The quantitative local temperature at the CNT probe contact point is determined by bringing the probe in and out of contact and controlling the amount of heat of the Pt hot film in high vacuum environment. Using this method, we overcome the problems of thermal contact resistance (TCR) between the CNT and sample surface. Sensor sensitivity for TCR and thermal conductivity measurement of a CNT is analyzed and the sensor configuration is optimized. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.sna.2013.04.038

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Others   Publishing type：Research paper (scientific journal)  

A new technique of micro/nanoscale temperature measurement is developed using an individual carbon nanotube (CNT) on a platinum hot-film, which can control the heat flow through the CNT probe and sense its own average temperature. A feedback control to extinct the heat flow enables us to neglect the effect of contact thermal resistance and to know the real surface temperature. Spatial resolution of 70 nm, temperature uncertainty of less than 0.5 K and enough robustness are achieved. Using this method, quantitative temperature profiles are obtained around a line heater of 604 nm-width and 9.73 μm-length. ©2013 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.79.390

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1063/1.4772612

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1088/0953-8984/25/2/025301

Language：Others   Publishing type：Research paper (scientific journal)  

The seventh U.S.-Japan Joint Seminar on Nanoscale Transport Phenomena was held in Shima, Japan, from December 11 to 14, 2011. The goals of this joint seminar were to provide a critical assessment of the state of the art and future directions in the field of nanoscale transport phenomena and energy conversion processes, to foster U.S.-Japan collaborations, and to provide international exposure to a new generation of scientists in this field. Issues discussed in the joint seminar were organized in 10 topical sessions, including (1) nanoscale thermophysical measurements; (2) optical characterization; (3) thermal and molecular transport; (4) phonon transport modeling; (5) energy storage and conversion; (6) nanoscale fluidics and phase change phenomena; (7) biological and organic systems; (8) interfacial thermal transport; (9) novel thermoelectric and thermal management materials; and (10) nanocarbon materials and devices. In addition to these topical sessions, the joint seminar featured an opening plenary session and a closing plenary session as well as an expert panel, where leading experts provided critical assessment of the past progress and addressed future directions in the field. In addition, an evening poster session provided opportunities for graduate and postdoc students to present their latest research results. About 35 researchers from Japan and 31 researchers from the United States participated in the meeting. The meeting was organized by S. Maruyama, K. Fushinobu, L. Shi, and J. Lukes together with about 20 other participants who served as session chairs. Summaries of different sessions of the seminar were prepared by the session and conference chairs and are collected into this report. © 2013 Taylor & Francis Group, LLC.

DOI： 10.1080/15567265.2012.745913

Language：English   Publishing type：Research paper (scientific journal)  

Interfacial thermal transport via van der Waals interaction is quantitatively evaluated using an individual multi-walled carbon nanotube bonded on a platinum hot-film sensor. The thermal boundary resistance per unit contact area was obtained at the interface between the closed end or sidewall of the nanotube and platinum, gold, or a silicon dioxide surface. When taking into consideration the surface roughness, the thermal boundary resistance at the sidewall is found to coincide with that at the closed end. A new finding is that the thermal boundary resistance between a carbon nanotube and a solid surface is independent of the materials within the experimental errors, which is inconsistent with a traditional phonon mismatch model, which shows a clear material dependence of the thermal boundary resistance. Our data indicate the inapplicability of existing phonon models when weak van der Waals forces are dominant at the interfaces. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0953-8984/25/2/025301

Language：English   Publishing type：Research paper (scientific journal)  

Anisotropy of heat conduction in multi-walled carbon nanotubes (MWNTs) is investigated by measuring heat flows in a pristine MWNT and in a MWNT with defects. The in- and out-of-shell thermal conductivities of each MWNT graphite shell are determined, and differences of more than four orders of magnitude are obtained because of the inter-shell gaps. This enhanced anisotropy reduces the conductance by 74% compared with that of the pristine MWNT because of the presence of outer shell defects, which comprise only 2.8% volume ratio. Furthermore, the anisotropy-assisted length dependence of thermal conductivity is demonstrated, even though there is no ballistic phonon transport. © 2013 American Institute of Physics.

DOI： 10.1063/1.4772612

Language：English   Publishing type：Research paper (other academic)  

Condensation heat transfer is a widely-used technique for industrial applications represented by heat exchanger because of its high heat transfer coefficient. To enhance its performance, a suitable surface is required, where both condensation and droplet removal smoothly occur. In this study, we compared wettability of a graphene surface and an amorphous carbon surface. The result shows that an amorphous carbon surface is more hydrophilic. Then we prepared a graphite surface which has nanoscale hydrophilic regions in large hydrophobic area. We observed the submicron-scale droplet condensation occurs preferentially on the hydrophilic graphite step by using environmental scanning electron microscope (ESEM).

DOI： 10.1115/IMECE2013-66186

Language：English   Publishing type：Research paper (scientific journal)  

The seventh U.S.-Japan Joint Seminar on Nanoscale Transport Phenomena was held in Shima, Japan, from December 11 to 14, 2011. The goals of this joint seminar were to provide a critical assessment of the state of the art and future directions in the field of nanoscale transport phenomena and energy conversion processes, to foster U.S.-Japan collaborations, and to provide international exposure to a new generation of scientists in this field. Issues discussed in the joint seminar were organized in 10 topical sessions, including (1) nanoscale thermophysical measurements; (2) optical characterization; (3) thermal and molecular transport; (4) phonon transport modeling; (5) energy storage and conversion; (6) nanoscale fluidics and phase change phenomena; (7) biological and organic systems; (8) interfacial thermal transport; (9) novel thermoelectric and thermal management materials; and (10) nanocarbon materials and devices. In addition to these topical sessions, the joint seminar featured an opening plenary session and a closing plenary session as well as an expert panel, where leading experts provided critical assessment of the past progress and addressed future directions in the field. In addition, an evening poster session provided opportunities for graduate and postdoc students to present their latest research results. About 35 researchers from Japan and 31 researchers from the United States participated in the meeting. The meeting was organized by S. Maruyama, K. Fushinobu, L. Shi, and J. Lukes together with about 20 other participants who served as session chairs. Summaries of different sessions of the seminar were prepared by the session and conference chairs and are collected into this report.

DOI： 10.1080/15567265.2012.745913

Language：English   Publishing type：Research paper (scientific journal)  

A new technique of micro/nanoscale temperature measurement is developed using an individual carbon nanotube (CNT) on a platinum hot-film, which can control the heat flow through the CNT probe and sense its own average temperature. A feedback control to extinct the heat flow enables us to neglect the effect of contact thermal resistance and to know the real surface temperature. Spatial resolution of 70 nm, temperature uncertainty of less than 0.5 K and enough robustness are achieved. Using this method, quantitative temperature profiles are obtained around a line heater of 604 nm-width and 9.73 μm-length.

DOI： 10.1299/kikaib.79.390

Language：English   Publishing type：Research paper (other academic)  

Platinum hot-film sensors, whose typical size is 500nm wide x 10&mu;m long x 40nm thick, are developed for investigating micro and nanoscale thermal events. This paper reports four kinds of applications with measurement principle, sensitivity analysis, and test results. Thermal conductivity of individual nanowire, represented by carbon nanotube, has been measured by bridging the specimen between the sensor and a heat sink. Here a new device is newly developed, which enables us to measure quantitatively both intrinsic thermal conductivity of the specimen and thermal contact resistance between the specimen and a target material. Focused beam-induced deposition is also measured by comparing the thermal conductance of deposited sensor and pristine one. Flow sensor is another application and we investigate the performance of carbon nanotube (CNT) fins deposited on the sensor by using dielectrophoresis technique to enhance the flow signal. The applicability of this sensor for measuring the thermal conductivity of fluid of very limited volume (femto-liter order) is also analytically studied.

DOI： 10.1115/MNHMT2012-75030

Language：Others   Publishing type：Research paper (scientific journal)  

As a fundamental theory of heat transfer, Fourier's law is valid for most traditional conditions. Research interest in non-Fourier heat conditions is mainly focused on heat wave phenomena in non-steady states. Recently, the thermomass theory posited that, for steady states, non-Fourier heat conduction behavior could also be observed under ultra-high heat flux conditions at low ambient temperatures. Significantly, this is due to thermomass inertia. We report on heat conduction in metallic nanofilms from large currents at low temperatures; heat fluxes of more than 1×1010 W m-2 were used. The measured average temperature of the nanofilm is larger than that based on Fourier's law, with temperature differences increasing as heat flux increased and ambient temperature decreased. Experimental results for different film samples at different ambient temperatures reveal that non-Fourier behavior exists in metallic nanofilms in agreement with predictions from thermomass theory. © 2012 The Author(s).

DOI： 10.1007/s11434-012-5288-7

Language：English   Publishing type：Research paper (other academic)  

Carbon nanotube is a promising material for thermal-management of micro devices because of its high intrinsic thermal conductivity. However, most bulk nanotubes show very low thermal conductivity due to the high thermal contact resistance. There are very few reliable experimental data for the contact issue of nanotubes. This paper uses three kinds of multi-walled carbon nanotubes; pristine, thermally-oxidized, and acidized nanotube. Each has unique nanoscale structure in their outermost surface. We measured thermal conductivity of their pellets and simultaneously conducted computational analysis treating random network model of spherocylinders. By comparing both results, thermal contact resistances between nanotubes are estimated and the effect of defected structure is discussed. The reliability of our method is also successfully confirmed compared with reported data using individual nanotubes. Copyright © 2012 by ASME.

DOI： 10.1115/HT2012-58203

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/jtst.7.405

Language：English   Publishing type：Research paper (scientific journal)  

Interfacial thermal transport of multi-walled carbon nanotubes (MWNTs) is investigated by using bulk pellet specimens. Steady-state conduction method gives thermal conductivity of 1 to 4 W/mK for the pellets with mass density from 0.2 to 0.35 g/cm3. This low thermal conductivity is due to the thermal boundary conductance between the nanotubes. Computational analysis is conducted for the pellet modeled as a random network of spherocylinders (SCs) and calculated dependency of thermal conductivity on pellet density shows good agreement with experimental data when we treat non-uniform SCs. By comparing the experimental and computational results, the thermal boundary conductance between two MWNTs can be taken as 1.5×10-8 W/K. This result agrees well with the reported data obtained by individual measurement, which suggests this simple method is applicable to probe the interfacial thermal phenomena of nanomaterials. An improved scaling law, k ∝ ρ2.14, for thermal conductivity of MWNTs aggregations is also proposed and discussed. © 2012 by JSME.

DOI： 10.1299/jtst.7.190

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (other academic)  

Platinum hot-film sensors, whose typical size is 500nm wide x 10&mu;m long x 40nm thick, are developed for investigating micro and nanoscale thermal events. This paper reports four kinds of applications with measurement principle, sensitivity analysis, and test results. Thermal conductivity of individual nanowire, represented by carbon nanotube, has been measured by bridging the specimen between the sensor and a heat sink. Here a new device is newly developed, which enables us to measure quantitatively both intrinsic thermal conductivity of the specimen and thermal contact resistance between the specimen and a target material. Focused beam-induced deposition is also measured by comparing the thermal conductance of deposited sensor and pristine one. Flow sensor is another application and we investigate the performance of carbon nanotube (CNT) fins deposited on the sensor by using dielectrophoresis technique to enhance the flow signal. The applicability of this sensor for measuring the thermal conductivity of fluid of very limited volume (femto-liter order) is also analytically studied. Copyright © 2012 by ASME.

DOI： 10.1115/MNHMT2012-75030

Language：English   Publishing type：Research paper (scientific journal)  

Heat conduction in a graphene nanoribbon (GNR) is investigated using nonequilibrium molecular dynamics simulation. GNR shows an intriguing dependence of thermal conductivity on its width, length, and edge shape. For example, thermal conductivity of thin armchair GNR is about three times lower than that of zigzag GNR due to the strong phonon scattering at the armchair edge. The substrate interaction is another critical issue for phonon transport. GNR supported on a substrate is analyzed by using the Lennard-Jones potential, and the thermal conductivity of a zigzag ribbon is found to decrease significantly due to phonon scattering by the substrate. However, under the same conditions, that of armchair ribbon is not affected by the substrate or even increases. This phenomenon is caused by the suppression of edge-localized flexural phonons of armchair GNR, which triggers their smaller thermal conductivity than the zigzag one. This anomalous edge-substrate combined effect on thermal transport in supported GNR is discussed.

DOI： 10.1615/ComputThermalScien.2012004393

Language：English   Publishing type：Research paper (other academic)  

The thermal properties of a half-defective single-walled carbon nanotube, and one-dimensional (1D) nonlinear lattices were investigated to unveil the mechanism of defect-induced thermal rectification. We propose a model of an asymmetrically defective low-dimensional material to explain the underlying mechanism of thermal rectification obtained in a past experiment in which C ₉H ₁₆Pt was asymmetrically deposited on a nanotube. These numerical approaches show the applicability of asymmetrically defective low-dimensional material to solid-state thermal rectification.

Language：English   Publishing type：Research paper (scientific journal)  

The thermal boundary resistance between an individual carbon nanotube and a Au surface was measured using a microfabricated hot-film sensor. We used both closed and open-ended multi-walled carbon nanotubes and obtained thermal boundary resistance values of 0.947-1.22 × 107KW- 1 and 1.43-1.76 × 107KW- 1, respectively. Considering all uncertainties, including the contact area, the thermal boundary conductances per unit area were calculated to be 8.6 × 107-2.2 × 108Wm- 2K- 1 for c-axis orientation and 4.2 × 108-1.2 × 109Wm- 2K- 1 for the a-axis. The trend in these values agrees with the predicted conductance dependence on the interface orientation of anisotropic carbon-based materials. However, the measured thermal boundary conductances are found to be much larger than the reported results. © 2011 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/22/31/315702

Language：English   Publishing type：Research paper (scientific journal)  

The electrical and thermal conductivities of polycrystalline gold and platinum nanofilms have been measured simultaneously using a direct current heating method from 60 to 300 K. The measured electrical and thermal conductivities are greatly decreased from the corresponding bulk values. And it is found that the reduction increases as the temperature decreases. The deviation from the bulk value is due to the effect of grain boundary scattering. Furthermore, the experimental results indicate that the grain boundary scattering effect imposes greater influence to the charge transport than to the heat transport. Consequentially, the Lorentz number is several times larger than that of bulk materials, leading to the violation of the Wiedemann-Franz law. The reflection coefficient R (0.86 in platinum, 0.42 in gold) at grain boundaries is obtained based on the Mayadas-Shatzkes theory and Matthiessen's rule, which agrees well with the previous experiments. © 2011 Springer-Verlag.

DOI： 10.1007/s00231-011-0825-5

Language：Others   Publishing type：Research paper (scientific journal)  

Electron-beam-induced deposition (EBID) is a simple and versatile technique of processing materials for three-dimensional nanoscale structures. A variety of precursor molecules can be used to build a localized solid deposition by the exposure of a substrate to an electron beam. This paper reports nano carbon deposition using solid n-tetracosane as a precursor, because this paraffin-based EBID can be introduced into existing scanning electron microscope (SEM) systems without difficulty. The paraffin is prepared on an aluminum film and operated by a manipulator in the SEM. The effects of the accelerating voltage, beam current, magnification, distance from the paraffin to the exposure point, the amount of paraffin, and the working distance are measured and discussed. It is found that the electron-beam-induced etching and the beam diameter are sometimes the dominant factors influencing the deposition rate, and that the thickness of the paraffin also affects the deposition distribution. Electron-beam bending, which is a critical factor causing degradation of nanofabrication, is treated carefully in order to understand the declination of carbon pillars. The obtained carbon pillar configurations suggest that electrically charged paraffin produced during preliminary irradiation of the electron beam causes a Coulomb force and results in the movement effect of deposition. Copyright © 2011 Wiley Periodicals, Inc.

DOI： 10.1002/ecj.10192

Language：English   Publishing type：Research paper (other academic)  

The thermal properties of a half-defective single-walled carbon nanotube, and one-dimensional (1D) nonlinear lattices were investigated to unveil the mechanism of defect-induced thermal rectification. We propose a model of an asymmetrically defective low-dimensional material to explain the underlying mechanism of thermal rectification obtained in a past experiment in which C 9H16Pt was asymmetrically deposited on a nanotube. These numerical approaches show the applicability of asymmetrically defective low-dimensional material to solid-state thermal rectification. Copyright © 2011 by ASME.

DOI： 10.1115/ajtec2011-44071

Language：English   Publishing type：Research paper (scientific journal)  

We conducted numerical simulations of heat conduction in one-dimensional (1D) nonlinear lattices to reveal the mechanism of thermal rectification of asymmetrically-defective materials. A decreased spring constant simulates the defective lattice and the obtained temperature profile suggests a thermal resistance existing at the interface of two linked segments with different spring constants. Our numerical results suggest that the thermal rectification of two-segment system is dependent on the spring constant and temperature gradient. Introducing the estimated phonon band, most of the rectification mechanisms are clearly explained and performance limit as a thermal rectifier is found for the defective/pristine materials. © 2011 The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

DOI： 10.1007/s12206-010-1008-x

Language：English   Publishing type：Research paper (other academic)  

DOI： 10.1115/imece2011-62844

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the thermal properties of a single-walled carbon nanotube with vacancy defects to determine its applicability to solid-state thermal rectification. Nonequilibrium molecular dynamics simulation of a nanotube with randomly located defects only along half the length revealed asymmetric heat conduction at room temperature. The direction of rectification is in good agreement with that obtained in a past experiment in which C9H16Pt was asymmetrically deposited on a nanotube, as far as the local deposition is supposed to cause defects in the nanotube lattice. The mechanism underlying the current thermal rectification effect is discussed considering the temperature dependence of the local thermal conductivity and the phonon filtering effect. The calculated phonon density of states shows larger overlapping when heat flows from the defective part to the pristine part and intermediate-frequency phonons are mainly responsible for rectification.

DOI： 10.1143/JJAP.49.02BD12

Language：English   Publishing type：Research paper (other academic)  

A carbon nanofiber material, consisting of bottomless graphene cups inside on each other in a line, like a set of soft-drink cups, has been discovered to have the potential to conduct heat ballistically over a long distance. Its longitudinal heat transport ability had been forecast to be extremely poor due to the weak van der Waals force operating between the graphene cups, but our measurements using nano thermal sensor showed that its thermal conductivity is much higher than that along the c-axis of bulk graphite. This unexpected result can be understood by its similarity to a one-dimensional (1D) harmonic-chain where no phonon is scattered even for an infinite length. The current graphene-based nanofiber resembles this type of "superconductive" chain due to the huge difference between the stiff covalent bonding in each cup and the weak inter-cup interaction. A non-equilibrium molecular dynamics simulation is conducted to explore the phonon transport in this fiber. The simulation results show that the thermal conductivity varies with the fiber length in a power law fashion with an exponent as large as 0.7. The calculated phonon density of states and atomic motions indicate that a low-frequency quasi-1D oscillation occurs there. Our investigations show that treating the current nanofiber as a 1D chain with three-dimensional oscillations explains well why this material has the most effective ballistic phonon transport ever observed.

DOI： 10.1115/IHTC14-22289

Language：Others   Publishing type：Research paper (scientific journal)  

The temperature-dependent electrical properties of four suspended polycrystalline thin gold films with thicknesses of 20.0, 23.0, 36.0 and 54.0 nm have been measured in the temperature range 100-310 K. The measured results show that the electrical resistivity of the films significantly increases while the corresponding temperature dependence decreases compared with bulk gold. The significantly increased electrical resistivity indicates that grain boundary scattering dominates over surface scattering in the studied films. However, fixing the Debye temperature to the bulk value will lead to an erroneous temperature dependence of resistivity. Taking into account the reduced characteristic Debye temperature along with the surface and grain boundary scattering, the electrical properties of the films can be well described in the whole temperature range. The extracted grain boundary reflection coefficient is 0.3 ± 0.03, within the range of the previous reported values, 0.1-0.45. The films' characteristic Debye temperatures decrease from the bulk value of 165 K to between 83 and 121K and tend to increase with increasing film thickness. This tendency coincides with the previous studies on thin gold, copper, platinum, silver films or wires, and cobalt/nickel superlattices. The possible mechanism responsible for the reduced Debye temperature is phonon softening at the surfaces, grain boundaries, disorder, defects and impurities, part of which has been demonstrated in other studies. © 2010 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/43/46/465301

Language：English   Publishing type：Research paper (scientific journal)  

A submicroscale flow sensor has been developed that consists of a suspended hot film and carbon nanotube (CNT) fins. Flow measurement experiments, together with a theoretical model, revealed the advantages of the use of CNT fins. The suspended metal film reduces heat loss and the CNT fins enhance the heat transfer to the fluid flow. Herein, the working principle of the CNT fins is presented in detail, together with a description of the micro electro mechanical systems (MEMS)/nano electro mechanical systems (NEMS) techniques used to fabricate the sensor. The CNTs were deposited by a manipulation method that is based on dielectrophoresis.

DOI： 10.1299/jtst.5.51

Language：Japanese   Publishing type：Research paper (scientific journal)  

固体間の熱伝導に関して界面熱抵抗を深く理解するにはナノスケールでの現象の把握が必要不可欠となる．そこで我々は，CNTと固体表面の間の熱抵抗を正確に計測することを目指し，サブミクロンスケールの微細な白金ホットフィルムを用いた独自のセンサを開発した．これを用いることで，CNT 先端とSiO2面の間の界面熱抵抗値を実験的に得ることに初めて成功した．また，CNT 先端での界面熱抵抗は接触圧力に依存しないことも確認することができた．

Language：Others   Publishing type：Research paper (scientific journal)  

Measurement method of the interfacial thermal resistance (ITR) between a multi-walled carbon nanotube (MWCNT) and solid surface is developed by using a sub-micrometer Pt hot-film. Thermal application of CNTs is promising due to their very high intrinsic thermal conductivity. However ITR could be dominant in the total thermal resistance when CNT is used for heat dissipation devices. Therefore measuring ITR between CNT and solid material is important in order to determine the total thermal resistance. Though it is fundamental to understand the ITR in nano or atomic scale, there have been reported very few experimental approaches to measure the ITR due to technical difficulties. By using a MWCNT as a probe on the Pt hot-film, it is concluded that the ITR between MWCNT tip and SiO2 surface is 1 × 105 K/W, which is confirmed to be independent of contact pressure.

DOI： 10.1299/kikaib.76.769_1412

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：Others   Publishing type：Research paper (scientific journal)  

Direction-controlled growth of horizontally aligned single-walled carbon nanotubes (SWNTs) on r-plane sapphire substrates and their alignment mechanisms are demonstrated. On a flat r-plane substrate, anisotropic nanotube-substrate interaction is known to align SWNTs parallel to the [11̄01̄] direction of the sapphire. We find that the introduction of a slight miscut (-1° inclined to the [11̄01̄] direction) on the substrate changed the SWNT growth direction by 90°, aligning perpendicular to the [11̄01̄] direction. This dramatic change of the growth direction is explained by the contribution of newly proposed one-dimensional surface atomic rows and/or atomic steps appeared on the r-plane. Annealing the substrate in hydrogen atmosphere prior to SWNT growth recovers the original nanotube growth direction, while annealing in air deteriorates the alignment. The direct growth of an orthogonally aligned SWNT array is achieved through optimized surface treatment. Site-selective directional control of aligned SWNTs is also demonstrated by applying hydrogen annealing to the miscut substrate whose surface is partially covered with SiO2. Our study gives insights into the alignment mechanism on single crystal substrates and offers a new means to assemble SWNTs for advanced integrated structures. © 2010 American Chemical Society.

DOI： 10.1021/jp1032993

Language：Others   Publishing type：Research paper (scientific journal)  

We studied patterning of metal nanoparticle catalyst on sapphire (α-Al 2O 3) substrates to grow horizontally-aligned single-walled carbon nanotubes (SWNTs) from defined positions. Various modifications of sapphire surface were investigated for effective adsorption of iron oxide nanoparticles. We found that phosphoric acids bind to sapphire surface and enhance the adsorption of the nanoparticles. This surface modification enabled effective patterning of nanoparticle via electron beam lithography. The small nanoparticles with a mean diameter of 5 nm tended to diffuse at high temperature during the nanotube growth, while the large particles with 12 nm mean diameter mostly stayed at the same position giving aligned SWNTs grown from the catalyst pattern. Copyright © 2010 American Scientific Publishers All rights reserved.

DOI： 10.1166/jnn.2010.1977

Language：English   Publishing type：Research paper (scientific journal)  

A new method was proposed to measure the thermal conductivity of liquids with infinitesimal samples, which are much smaller than those required in conventional methods. The method utilizes a micro-beam-type MEMS sensor fabricated across a trench on a silicon substrate. Numerical analysis of heat conduction within and around a uniformly heated sensor showed that the temperature of a 10 μm long sensor reached a steady state within approximately 0.1 ms, after the start of heating. It was also revealed that the average temperature of the sensor at the steady state was higher in liquids with lower thermal conductivity. These results demonstrate a new idea of measuring the thermal conductivity of liquids within an extremely short time at a steady state before the onset of natural convection.

DOI： 10.1007/s10765-009-0700-5

Language：English   Publishing type：Research paper (other academic)  

A new method was proposed to measure the thermal conductivity of liquids with infinitesimal samples, which are much smaller than those required in conventional methods. The method utilizes a micro-beam-type MEMS sensor fabricated across a trench on a silicon substrate. Numerical analysis of heat conduction within and around a uniformly heated sensor showed that the temperature of a 10 μm long sensor reached a steady state within approximately 0.1 ms, after the start of heating. It was also revealed that the average temperature of the sensor at the steady state was higher in liquids with lower thermal conductivity. These results demonstrate a new idea of measuring the thermal conductivity of liquids within an extremely short time at a steady state before the onset of natural convection. © Springer Science+Business Media, LLC 2010.

DOI： 10.1007/s10765-009-0700-5

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Measurement method of the interfacial thermal resistance (ITR) between a multi-walled carbon nanotube (MWCNT) and solid surface is developed by using a sub-micrometer Pt hot-film. Thermal application of CNTs is promising due to their very high intrinsic thermal conductivity. However ITR could be dominant in the total thermal resistance when CNT is used for heat dissipation devices. Therefore measuring ITR between CNT and solid material is important in order to determine the total thermal resistance. Though it is fundamental to understand the ITR in nano or atomic scale, there have been reported very few experimental approaches to measure the ITR due to technical difficulties. By using a MWCNT as a probe on the Pt hot-film, it is concluded that the ITR between MWCNT tip and SiO₂ surface is 1 × 10⁵ K/W, which is confirmed to be independent of contact pressure.

DOI： 10.1299/kikaib.76.769_1412

Language：English   Publishing type：Research paper (scientific journal)  

We treat the ballistic heat conduction of cup-stacked carbon nanofibers (CSCNF) by a nonequilibrium molecular dynamics simulation. The CSCNF consist of numerous tiny graphene cups linked in line by weak intermolecular forces. The simulation results show that the thermal conductivity varies with the fiber length in a power law fashion with an exponent as large as 0.7. The calculated phonon density of states revealed that a low frequency oscillation in the radial and axial directions dominates the heat conduction in CSCNF. The atomic motions indicate that these low frequency oscillations are quasi-one-dimensional (1D) where each cup moves axially like a rigid body and radially with a breathing motion. This quasi-1D oscillation occurs due to the unique structure of a CSCNF that resembles a 1D harmonic chain. Our investigations show that treating a CSCNF as a 1D chain with three-dimensional oscillations explains why this material has the highest ballistic phonon transport ever observed.

DOI： 10.1088/0953-8984/22/6/065403

Language：English   Publishing type：Research paper (other academic)  

This paper reports on a thermal probe using a carbon nanotube (CNT) on a platinum hot-film. CNT probe is expected to breakthrough the limitations of the existing ones owing to its unique characteristics but no practical thermal device has been fabricated yet. In order to explore the mechanisms of heating and measuring the smaller region than 10nm, we applied our recently developed sensor coupled with CNT, which consists of a suspended platinum film of 40nm × 500nm × 10micrometer. The principle of this probe as heater and sensor is explained, based on one dimensional heat conduction. Fabrication process using MEMS technique is also introduced, especially for a couple of critical techniques. One is to fabricate the nano sensor on an edge of the sensor substrate. The other is to bond a CNT on the suspended hot-film sensor. A CNT thermal probe using a multi-walled CNT of 70nm diameter and ca. 10 micrometers length is successfully fabricated. Its performances are tested in vacuum environment as to eliminate the presence of in-air conduction effect and water absorption effect around the contact point, which work for heat transport dominantly in atmospheric condition and degrade the spatial resolution. Our CNT probe showed a clear and reliable signal in vacuum and its sensitivity available for nanoscale thermal sensing and heating is confirmed.

DOI： 10.1115/MNHMT2009-18356

Language：English   Publishing type：Research paper (scientific journal)  

The small size of nanomaterials deposited by either focused ions or electron beams has prevented the determination of reliable thermal property data by existing methods. A new method is described that uses a suspended platinum hot film to measure the thermal conductivity of a nanoscale deposition. The cross section of the Pt film needs to be as small as 50 nm × 500 nm to have sufficient sensitivity to detect the effect of the beam-induced nanodeposition. A direct current heating method is used before and after the deposition, and the change in the average temperature increase of the Pt hot film gives the thermal conductivity of the additional deposited material. In order to estimate the error introduced by the one-dimensional analytical model employed, a two-dimensional numerical simulation was conducted. It confirmed the reliability of this method for situations where the deposit extends onto the terminals by (1 μm or more. Measurements of amorphous carbon (a-C) films fabricated by electron beam induced deposition (EBID) produced thermal conductivities of 0.61 W • m -1 • K -1 to 0.73 W • m -1 • K -1 at 100 K to 340 K, values in good agreement with those of a-C thin films reported in the past. © 2009 Springer Science+Business Media, LLC.

DOI： 10.1007/s10765-009-0666-3

Language：English   Publishing type：Research paper (scientific journal)  

A carbon nanofiber material, consisting of a stacked graphene cups, with the potential to conduct heat ballistically has been discovered and tested. Its unexpected high thermal conductivity can be understood by the similarity to a one-dimensional harmonic chain where no phonon is scattered even for an infinite length. A non-equilibrium molecular dynamics simulation for this fiber validated this hypothesis by revealing a uniform temperature distribution between hot and cold reservoirs. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2009.05.001

Language：Others   Publishing type：Research paper (scientific journal)  

Bending of horizontally aligned single-walled carbon nanotubes (SWNTs) was achieved on surface engineered single-crystal sapphire (α-Al 2O3). The SWNTs grown on the r-plane sapphire are aligned along the specific crystallographic [11̄01̄] direction due to the lattice-oriented growth, and we created artificial step structures perpendicular to this SWNT growth direction. These steps changed the nanotube growth direction from the [11̄01̄] to the step direction with the bending angle of nearly 90°. Effects of the bending structure on electron transport property were studied. Our approach to combine the lattice-oriented growth with the step-templated growth will offer a new route toward the growth of two-dimensionally controlled SWNT architectures for future nanoelectronics. © 2009 American Chemical Society.

DOI： 10.1021/jp902409w

Language：Others   Publishing type：Research paper (scientific journal)  

We discuss the measurement error caused by fabrication and measurement of a T-type nanosensor with a suspended sub-micrometer Pt hot film that was developed to measure the thermal properties of individual nanowire materials. Comparison of numerical simulation and one-dimensional analysis revealed that the thermal conductivity of nanowire material such as a carbon nanotube is calculated to be 17% lower. As an example, the thermal conductivity measurement result for a SiC nanowire is reported. The error caused by contact thermal resistance is found to depend on the contact length and can be as great as 20%. It can be said that future measuring can have higher reliability by correcting the estimated measurement error. © 2009 Wiley Periodicals, Inc.

DOI： 10.1002/htj.20228

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

For the integration with modern Si-based electronics, it is important to organize single-walled carbon nanotubes (SWNTs) into a rational structure on a Si wafer with a SiO2 oxide layer. In this study, the aligned growth of SWNTs was achieved on the SiO2/Si substrate whose surface was pretreated with CF4 plasma. The plasma treatment gave the radially extended steps which guided the SWNT growth. Back-gate field effect transistors were demonstrated with the aligned SWNTs. Our work presents the possibility of assembling SWNTs on SiO2/Si substrate through the formation of artificial step structures, which is a great step toward fully functional SWNT-on-Si devices. © 2009 American Chemical Society.

DOI： 10.1021/jp810036t

Language：English   Publishing type：Research paper (scientific journal)  

The measurement of the in-plane thermal conductivity of a nanofilm by the direct-current heating method is examined by a numerical heat transfer simulation to obtain reliable data for nanosensor applications. A platinum film of 500 nm in width and 10 μm in length is fabricated to be suspended between two terminals. An underetched part always exists on the edge of the terminals owing to the isotropic etching process, which causes a temperature jump at the end of the suspended film. As a result, the thermal conductivity measured by the direct- current heating method is found to be underestimated from the intrinsic properties of the suspended nanofilm. Numerical simulations are conducted to calculate the temperature jump and the necessary correction of thermal conductivity is derived, which critically depends on the width of the underetched part. The corrected thermal conductivity is discussed with the simultaneously obtained electrical conductivity in comparison with the bulk data. © 2009 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.48.05EB01

Language：English   Publishing type：Research paper (scientific journal)  

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thickness and measured in a temperature range between the boiling point of liquid nitrogen (77 K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann-Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadsa & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann-Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.

DOI： 10.1088/1674-1056/18/5/051

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (other academic)  

Heat transport characteristics of micro oscillation heat pipe have been investigated. A single winding flow path consists of 28 turns microchannels fabricated on a silicon wafer the size of which was 31mmx27mm. We used heat pipe with non-uniformed cross section. Equivalent diameters of channels were 0.19 and 0.10mm. Test fluid was R141b and liquid fractions were 0, 75, 85%. It was found that steady pulsating flow occurred by increasing the number of turns and the frequency of vibration has an effect on heat transfer performance.

DOI： 10.1115/ICNMM2008-62082

Language：Others   Publishing type：Research paper (scientific journal)  

Unidirectional growth of single-walled carbon nanotubes (SWNTs) was achieved using the patterned Co-Mo salt catalyst on the r-plane sapphire substrate. This is in marked contrast with the SWNTs grown on an a-plane sapphire and ST-cut quartz, on which the SWNTs grew bidirectionally. This new growth mode is not dependent on the gas flow and attributed to the asymmetric surface atomic arrangement of the sapphire surface. Copyright © 2008 American Chemical Society.

DOI： 10.1021/ja8080549

Language：Others   Publishing type：Research paper (scientific journal)  

Recently, we discovered the horizontally-aligned growth of single-walled carbon nanotubes (SWCNTs) on R- and A-plane sapphire substrates, which we call "atomic arrangement-programmed growth (AAP growth)." This is a unique method because the growth direction of SWCNTs is determined by the crystallographic direction of the sapphire surface. In this paper, we report on the characterization of the aligned SWCNTs by polarized Raman and electron transport measurements, and on the effect of the step/terrace structure formed on sapphire surface. These results may open up a possibility of creating the artificial SWCNT network, which can be applied to high-performance electronics. Copyright © 2008 American Scientific Publishers. All rights reserved.

DOI： 10.1166/jnn.2008.SW01

Language：Others   Publishing type：Research paper (scientific journal)  

The objective of this U.S.-Japan joint seminar series is to provide a cross-disciplinary and international forum for discussing and identifying outstanding science and technology issues in the area of nanoscale thermophysics and energy conversion and to foster collaboration among researchers in these areas. The first of this seminar series, championed by the late Professors Chang-Lin Tien and Kunio Hijikata, was held in Kanazawa, Japan, in June of 1993. Subsequent meetings have been held every three years, alternating venues between the United States and Japan. The Sixth U.S.-Japan Joint Seminar on Nanoscale Transport PhenomenaScience and Engineering was held in Boston, Massachusetts, July 13-16, 2008, and was organized by Professors Gang Chen from MIT, Fushinobu Kazuyoshi from Tokyo Institute of Technology, Shigeo Maruyama from Tokyo University, and Pamela Norris from University of Virginia. Nearly 100 scientists participated in the seminar. (The agenda of the seminar is attached at the end at this report[14].) The seminar included keynote sessions and invited sessions, as well as a dedicated poster session of selected presentations from an open call for papers. All papers presented in the regular sessions, the invited sessions, were upon invitation by the organizers. Invited sessions used a mixed form of communication: each speaker gave a 5-minute summary of his work followed by a 30-minute poster session of just the papers summarized orally, and then these speakers came back to the podium, serving as panelists to answer questions regarding their papers and session themes. This format offered good opportunities for the presenters to discuss their work with the participants. Reports for each session were summarized by session chairs. Following is a brief summary of the sessions.

DOI： 10.1080/15567260802591928

Language：English   Publishing type：Research paper (scientific journal)  

Metals are typically good conductors in which the abilities to transport charge and to transport heat can be related through the Wiedemann-Franz law. Here we report on an abnormal charge and heat transport in polycrystalline metallic nanostructures in which the ability to transport charge is weakened more obviously than that to transport heat. We attribute it to the influence of the internal grain boundaries and have formulated a novel relation to predict the thermal conductivity. The Wiedemann-Franz law is then modified to account for the influence of the grain boundaries on the charge and heat transport with the predictions now agreeing well with the measured results. © 2008 Chinese Physical Society and IOP Publishing Ltd.

DOI： 10.1088/0256-307X/25/9/071

Language：Japanese  

Measuring Thermal Conductivity of Nano Deposition Using Pt Hot Film
A novel method to measure the thermal conductivity of nano scale deposition is developed by using platinum hot film suspended between two terminals. The nano-deposition, which can be built by either focused ion or electron beam, is one of the newest fabrication techniques for three dimensional nano-structure, and can be applied to patterning, repairing, bonding, et al. However, its thermal property is still unknown because the size is too small to measure by the existing thermal sensors. The hot film used here is fabricated by NEMS (Nanoelectromechanical Systems) technology and has enough sensitivity to measure nanoscale materials. Direct current heating method is applied before and after the deposition and the change of averaged temperature increase of the platinum film gives the thermal conductivity of additionally-deposited material by using one-dimensional heat conduction model. As an example, amorphous carbon (a-C) with the thickness of 563 nm is deposited by electron beam induced deposition (EBID) method and thermal conductivity of a-C nano-deposition is obtained to be 0.61 W&frasl;(m&middot;K) to 0.73 W&frasl;(m&middot;K) at 100 K to 340 K. In order to confirm the reliability of this method, two-dimensional numerical simulation is conducted and the measuring uncertainty is calculated exactly. The effect of thermal boundary resistance is also treated and discussed. From the comparison of 1D model and 2D simulation, it is found that the deposition extended on the terminals by 1 &mu;m length can decrease the error of 1D model from 5.5 % to 3.1 % but no more extension has little effect to improve the accuracy of this method.

DOI： 10.11368/tse.16.105

Language：Japanese   Publishing type：Research paper (scientific journal)  

Deposition Rate and Position Shifting of Paraffin-Based EBID

Language：English   Publishing type：Research paper (scientific journal)  

Thermal conductivity of SiC nanowire formed by combustion synthesis

Language：Japanese   Publishing type：Research paper (scientific journal)  

Measuring Thermal Conductivity of Nano Deposition Usinh Pt Hot Film

Language：Japanese   Publishing type：Research paper (scientific journal)  

Estimating Error of Measuring Thermal Conductivity Using T-Type Nano Sensor

Language：Others   Publishing type：Research paper (scientific journal)  

Horizontally aligned growth of single-walled carbon nanotubes (SWNTs) on single-crystal surfaces has attracted great interest in terms of nanoelectronic applications, but their growth mechanism is not fully understood. We report on the 13C/12C isotope-labeled growth of SWNTs on a sapphire surface to visualize their growth process. Switching carbon feedstock from 13CH4 to 12CH4 during SWNT growth induces a gradient distribution of the carbon isotopes along the tube axis. From the Raman mapping analysis, we succeeded to observe the gradual change in the isotope distribution of individual SWNTs. The results indicate the base-growth mode for the horizontally aligned SWNTs, which suggests that nanotube-sapphire interaction is essential to alignment. This method offers a unique technique to analyze the nanotube growth mechanism and kinetics. © 2008 American Chemical Society.

DOI： 10.1021/jp709737q

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

This paper reports on the measurement of the thermal conductivity of an individual silicon carbide (SiC) nanowire of 140 nm diameter. T-type nanosensor and high-resolution transmission electron microscopy (HRTEM) are applied to obtain reliable property data. HRTEM images show that this nanowire has highly-crystalline SiC core of 126 nm diameter and surrounding amorphous silicon-dioxide layer of 7 nm thickness. Thermal contact resistance is estimated by using a simple analysis of the amorphous-carbon nanostructure between nanowire and nanosensor. Obtained apparent thermal conductivity of this nanowire suggests the thermal conductivity of SiC core is over 100W·m ^-1·K^-1 at room temperature, which is much greater than the past-reported data of thin film but less than the pure bulk data. Compared with bulk samples, phonon scattering mechanism is also discussed.

Language：English   Publishing type：Research paper (scientific journal)  

The in-plane thermal conductivity of Au nanofilms with thickness of 23 nm, which are fabricated by the electron beam- physical vapor deposition method and a suspension technology, is experimentally measured at 80-300 K by a one-dimensional steady-state electrical heating method. Strong size effects are found on the measured nanofilm thermal conductivity. The Au nanofilm in-plane thermal conductivity is much less than that of the bulk material. With the increasing temperature, the nanofilm thermal conductivity increases. This is opposite to the temperature dependence of the bulk property. The Lorenz number of the Au nanofilms is about three times larger than the bulk value and decreases with the increasing temperature, which indicates the invalidity of the Wiedemann-Franz law for metallic nanofilms.

DOI： 10.1080/10020070612331343248

Language：Others   Publishing type：Research paper (scientific journal)  

This paper reports on measurements of in-plane thermal conductivities, electrical conductivities, and Lorentz number of two microfabricated, suspended, nanosized thin films with a thickness of 28 nm. The effect of the film thickness on the in-plane thermal conductivity is examined by measuring other nanofilm samples with a thickness of 40 nm. The experimental results show that the electrical conductivity, resistance-temperature coefficient, and in-plane thermal conductivity of the nanofilms are much smaller than the corresponding bulk values from 77 to 330 K. However, the Lorentz number of the nanofilms is about two times that of the bulk value at room temperature, and even up to three times that of the bulk value at 77 K. These results indicate that the relation between the thermal conductivity and electrical conductivity of the nanofilms does not follow the Wiedemann-Franz law for bulk metallic materials. © Springer Science+Business Media, LLC 2007.

DOI： 10.1007/s10765-006-0135-1

Language：English   Publishing type：Research paper (scientific journal)  

Language：Others   Publishing type：Research paper (scientific journal)  

Six Pt films with thicknesses of 15-62 nm have been fabricated by electron beam-physical vapor deposition (EB-PVD). The grain sizes of the Pt nanofilms and its effect on the thermal conductivity have been studied experimentally. It is found that the grain size increases with the nanofilm thickness increasing and goes to a constant about 20 nm. The grain size is nearly comparable with the nanofilm thickness with the thickness less than 30 nm, while becomes much less than the nanofilm thickness with the thickness larger than 30 nm. Influenced by the surface and grain boundary effects, the thermal conductivity of the Pt nanofilms is greatly lower than that of the Pt bulk. It is noted that the thermal conductivity of the studied platinum nanofilms increases with the thickness increasing and runs to 35 W/mK, which is much lower than that of the bulk material.

Language：English   Publishing type：Research paper (other academic)  

This paper describes a novel method to measure the thermal conductivity of individual carbon nanotubes (CNTs) by using a suspended sample-attached T-type nanosensor. The CNTs used were made by an arcdischarge evaporation method. We could successfully measure the thermal conductivity of individual carbon nanotubes in the range of temperature from 77 K to 320 K, and the results show that the thermal conductivity increases with the temperature and approaches an asymptote near 320 K. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/pssb.200669194

Influence of Grain Boundary Scattering on the Electrical and Tthermal Conductivities of Polycrystalline Gold Nanofilms

Language：English   Publishing type：Research paper (scientific journal)  

In this paper, the thermal characteristics of suspended platinum (Pt) nanofilm sensors have been investigated experimentally. The Pt nanofilm sensors with the thickness of 28-40 nm, the width of 260-601 nm, and the length of 5.3-5.7 μm were fabricated by electron beam lithography, electron beam physical vapor deposition and isotropic/anisotropic etching processes. Based on the one-dimensional heat conduction model, the in-plane thermal conductivity of the nanofilm sensors was obtained from the linear relation of the volume-averaged temperature increase and the heating rate measured in vacuum. Furthermore, natural convection heat transfer coefficients of air around the suspended nanofilm sensors at the pressures ranging from 1 × 10-2 Pa to 1 atm were also investigated. The experimental results show that the in-plane thermal conductivities of the nanofilm sensors are much lower than those of the bulk values, the natural convection heat transfer coefficients are, however, very high at the atmospheric pressure. © 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2006.04.016

Language：Others   Publishing type：Research paper (scientific journal)  

The surface and grain-boundary effects on the in-plane thermal conductivity of polycrystalline platinum nanofilms have been investigated. The thicknesses of the nanofilms range from 15.0 to 63.0nm and the mean grain sizes measured by x-ray diffraction vary from 9.5 to 26.4nm. The thermal conductivities of the nanofilms measured by a direct electrical heating method are greatly reduced from the bulk values. The measured results are compared with the values predicted by the Qiu and Tien model and the Kumar and Vradis theory. It is found that the reduction in the thermal conductivity is mainly caused by grain-boundary scattering and the reflection coefficient of electrons striking the grain boundaries is around 0.35. The relaxation time model is also applied to study the size effects to check whether the Matthiessen rule is still valid in predicting the in-plane thermal conductivity of polycrystalline metallic nanofilms. The results indicate that by considering only grain-boundary scattering and background scattering the Matthiessen rule is still valid. If surface scattering, however, is included, deviations of the Matthiessen rule from other theories mentioned above have been found. © 2006 IOP Publishing Ltd.

DOI： 10.1088/0953-8984/18/34/007

Language：Others   Publishing type：Research paper (scientific journal)  

We experimentally studied the in-plane thermal and electrical properties of a suspended platinum nanofilm in thickness of 15nm. The measured results show that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient of the studied nanofilm are much less than those of the bulk material, while the Lorenz number is greater than the bulk value. Comparing with the results reported previously for the platinum nanofilm in thickness of 28nm, we further find that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient decrease with the decreasing thickness of the nanofilm, while the Lorenz number increases with the decreasing thickness of the nanofilm. These results indicate that strong size effects exist on the in-plane thermal and electrical properties of platinum nanofilms. ©2006 Chinese Physical Society and IOP Publishing Ltd.

DOI： 10.1088/0256-307X/23/4/048

Language：English   Publishing type：Research paper (scientific journal)  

The electrical and thermal conductivities of polycrystalline gold nanofilms have been measured simultaneously by a direct current heating method, and the measured results are compared with the Mayadas and Shatzkes theory. It is found that the reduced electrical and thermal conductivities of gold nanofilms are strongly dominated by grain boundary scattering. The reflection coefficient of electrons striking the grain boundaries for charge transport is 0.7, which agrees well with a previous scanning tunneling potentiometry study. The reflection coefficient for thermal transport, however, is only 0.25. The Lorenz numbers for the polycrystalline gold nanofilms, which are calculated from the measured electrical and thermal conductivities, are much greater than the value predicted by the Wiedemann-Franz law for the bulk material. The results indicate that the electron scatterings on the grain boundaries impose different influences on the charge and heat transport in the polycrystalline gold nanofilms. A model of effective density of conduction electrons has been utilized to interpret the violation of the Wiedemann-Franz law in polycrystalline gold nanofilms.

DOI： 10.1103/PhysRevB.74.134109

Language：English   Publishing type：Research paper (scientific journal)  

The electrical conductivity and temperature coefficient of resistance of polycrystalline platinum nanofilms have been investigated experimentally and theoretically. The results show that these electrical properties have been greatly reduced mainly by grain boundary scattering. By applying the theory of Mayadas and co-workers [Appl. Phys. Lett. 14, 345 (1969); Phys. Rev. B 1, 1382 (1970)] to predict the electrical conductivity and temperature coefficient of resistance with the same reflection coefficient, however, obvious discrepancies have been found. These discrepancies indicate that Drude's relation for bulk metals cannot be applied directly in the nanosized grain interior of polycrystalline metallic films.

DOI： 10.1063/1.2338885

Language：Japanese   Publishing type：Research paper (scientific journal)  

Porous silicon as a proton exchange membrane for micro fuel cells

DOI： 10.5796/electrochemistry.73.939

Language：Others   Publishing type：Research paper (scientific journal)  

Although the thermal properties of millimeter-sized carbon nanotube mats and packed carbon nanofibers have been readily measured, measurements for a single nanotube are extremely difficult. Here, we report a novel method that can reliably measure the thermal conductivity of a single carbon nanotube using a suspended sample-attached T-type nanosensor. Our experimental results show that the thermal conductivity of a carbon nanotube at room temperature increases as its diameter decreases, and exceeds 2000W/mK for a diameter of 9.8 nm. The temperature dependence of the thermal conductivity for a carbon nanotube with a diameter of 16.1 nm appears to have an asymptote near 320 K. The present method is, in principle, applicable to any kind of a single nanofiber, nanowire, and even single-walled carbon nanotube. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevLett.95.065502

Language：Others   Publishing type：Research paper (scientific journal)  

This letter reports on the measurements of the in-plane thermal conductivity and the electrical conductivity of a microfabricated, suspended, nanosized platinum thin film with the width of 260 nm, the thickness of 28 nm, and the length of 5.3 μm. The experimental results show that the electrical conductivity, the resistance-temperature coefficient and the in-plane thermal conductivity of the nanofilm are greatly lower than the corresponding bulk values from 77 to 330 K. The comparison results indicate that the relation between the thermal conductivity and the electrical conductivity of this nanofilm might not follow the Wiedemann-Franz law that describes the relation between the thermal conductivity and the electrical conductivity of a bulk metallic material. © 2005 American Institute of Physics.

DOI： 10.1063/1.1921350

Language：Japanese   Publishing type：Research paper (scientific journal)  

MEMS Technology and Space Propulsion

Language：Japanese   Publishing type：Research paper (scientific journal)  

Microfluidics of liquid propellant microthruster for pico-satellites
Yasuhiko Osaki, Koji Takahashi
IEEJ Transactions on Sensors and Micromachines, Vol.123, No.10, pp.436-441, 2003

Language：Others   Publishing type：Research paper (scientific journal)  

An evaporator unit for use in microreactor systems was constructed. The unit consisted of a micropump driven by piezoelectricity and a microevaporator heated electrically by means of a Pt wire. The micropump was comprised of two tightly bonded plates; a silicon (100) plate having three diaphragms and a glass plate having valve and pump chambers. These flow conduits were patterned by photolithography and formed by wet-etching. The diaphragms, as well as the valve and pump chambers, were 7 mm in diameter, and were arranged triangularly. The discharge rates of the pumps were found to be dependent on the voltage and frequency applied to the piezo discs, the actuation pattern, the backpressure at the outlet, and the liquid viscosity. The pump assembled with an epoxy adhesive showed high discharge rates, while a similar one, assembled by anodic bonding, showed high discharge pressures. The flow rate of the former pump reached 60 ml/min for water. The microevaporator channel was wet-etched on a 10 mm × 40 mm silicon wafer, and a channel for a Pt wire heater was formed on the reverse side. Both sides of the silicon plate were covered with glass plates. Benzene, when pumped to the microevaporator, was evaporated to give a vapor flow rate of 6.8 cm3/min, which is sufficiently large as a feed rate for a gas-phase catalytic microreactor.

DOI： 10.1252/jcej.36.7

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Others   Publishing type：Research paper (scientific journal)  

A bubble or droplet can be driven in a microchannel due to the thermocapillary force controlled by microheaters, which comes from the surface tension imbalance. Though practical optical switches based on this principle have been developed by micromachining technique, there has been no contribution of fluid dynamics both experimentally and analytically. In order to treat gas-liquid interface accurately in a microchannel, it is necessary to improve the models of surface tension and contact angle. A new numerical scheme is developed based on C-CUP and Level-Set function method with CSF model and is able to calculate the surface tension distribution precisely. It is the first time to simulate the thermally-driven bubble in a microchannel and the obtained results show good agreements with experimental results qualitatively. Comparisons with the conventional numerical schemes and fundamental mechanisms of the thermally-driven interfaces are also discussed.

DOI： 10.1299/kikaib.68.1344

Publishing type：Research paper (bulletin of university, research institution)  

Language：Japanese  

MEMS Fabrication and Micro thruster

Language：Others   Publishing type：Research paper (scientific journal)  

The violent motion of cavitation bubble near a solid boundary is treated by a newly developed numerical code based on the CIP (Cubic Interpolated Propagation) method. This is the first study to successfully simulate the repetitive collapses and rebounds of bubbles with the penetration of microjets until the second collapse while maintaining toroidal shape. The obtained gas-liquid interfaces show similar tendency to bubble shapes observed in past experimental studies, although phase change, surface tension and viscosity are neglected. The key factor of this flow field is the density difference between gas and liquid. The velocity of microjet and resulting water hammer pressure are calculated successfully. However no shock wave appears from the collapsed bubbles in the present study, which suggests that an appropriate model for highly-compressed gas is important for tempestuous two-phase flow. This study indicates a high possibility of using the Euler-type numerical code to calculate such complicated two-phase problems and also suggests which factor is dominant in the cavitation bubble dynamics.

DOI： 10.1299/jsmeb.44.238

Language：Japanese  

Micro Motor Using Micro Bubbles

DOI： 10.11426/nagare1982.20.92

Language：English   Publishing type：Research paper (other academic)  

Isolation valve system for low-cost liquid tank array is designed and demonstrated. This valve is normally closed and opens only once for liquid injection. This array system can supply precise amount of liquid at each operation as many times as the number of tanks. In order to tolerate the ambient high pressure and to accomplish the perfect injection, a large and thick membrane is fabricated and tested as valve. Without any special structure, our valves are found to open just by applying low AC voltage to a pair of electrodes that are apart from each other by tens of micrometers. Dielectric breakdown and pressure increase inside tank are found to work effectively to form a large hole.

Language：English   Publishing type：Research paper (scientific journal)  

Language：Others   Publishing type：Research paper (scientific journal)  

This work explores the application of the Marangoni effect in micro systems involving small gas or vapor bubbles in a liquid environment subjected to a temperature gradient. The Marangoni effect characterizes the variation of surface tension along the bubble surface resulting from the temperature gradient around the bubble, thus driving the bubble toward the higher temperature region. This phenomenon is more pronounced as the bubble becomes smaller and the temperature gradient becomes steeper, both of which can be achieved in microbubble systems. Potential applications based on the Marangoni effect include linear bubble actuators, dynamic microvalves, and hot-spot locators. The optimum bubble size for these applications is expected to be of the order of 10 μ m. A smaller bubble may be difficult to introduce into the working system and maintain its size. Presented for illustration is a feasibility analysis for both a noncondensable gas bubble and a vapor bubble situated above a microheater. The analysis yields results for the temperature field around the bubble surface and the Marangoni driving pressure, which is a key element of the performance evaluation for all Marangoni-effect-based applications. The findings demonstrate clearly that the Marangoni effect on microbubbles is very significant and shows great promise for applications in microelectromechanical systems (MEMS). © 1999 Taylor … Francis Group, LLC.

DOI： 10.1080/108939599199729

Language：English   Publishing type：Research paper (scientific journal)  

This work explores the application of the Marangoni effect in micro systems involving small gas or vapor bubbles in a liquid environment subjected to a temperature gradient. The Marangoni effect characterizes the variation of surface tension along the bubble surface resulting from the temperature gradient around the bubble, thus driving the bubble toward the higher temperature region. This phenomenon is more pronounced as the bubble becomes smaller and the temperature gradient becomes steeper, both of which can be achieved in microbubble systems. Potential applications based on the Marangoni effect include linear bubble actuators, dynamic microvalves, and hot-spot locators. The optimum bubble size for these applications is expected to be of the order of 10 μ m. A smaller bubble may be difficult to introduce into the working system and maintain its size. Presented for illustration is a feasibility analysis for both a noncondensable gas bubble and a vapor bubble situated above a microheater. The analysis yields results for the temperature field around the bubble surface and the Marangoni driving pressure, which is a key element of the performance evaluation for all Marangoni-effect-based applications. The findings demonstrate clearly that the Marangoni effect on microbubbles is very significant and shows great promise for applications in microelectromechanical systems (MEMS).

DOI： 10.1080/108939599199729

Language：Others   Publishing type：Research paper (scientific journal)  

Amorphous carbon films and diamond like carbon films were grown by pulsed laser deposition (PLD) using different laser wavelengths (λ=193 nm, 532 nm, 1064 nm) and substrate temperatures (from room temperature to 500°C). The morphology of the film surface was observed using an atomic force microscope (AFM) and a scanning electron microscope (SEM). Mechanisms for film growth are explained qualitatively using the subplantation model. All films showed the occasional incorporation of spherical particles with a diameter of 1-10 μm ejected from the targets due to surface being melted.

DOI： 10.1016/S0169-4332(98)00601-1

Language：Japanese  

Personal Memory of Stay in Chancellor's Laboratory : How to start new topics in USA

Language：Others   Publishing type：Research paper (scientific journal)  

Amorphous carbon films and diamond-like carbon films were grown by pulsed laser deposition (PLD) using different laser wavelengths (λ=193, 532 and 1064 nm) and substrate temperatures (ranging from room temperature to 500 °C). The morphology of the film surface was observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The mechanisms of film growth using laser wavelengths of 1064, 532 and 193 nm are explained qualitatively to be surface growth, subsurface growth accompanied with migration of the penetrating species to the film surface, and subsurface growth, respectively, using the subplantation model.

DOI： 10.1016/s0925-9635(98)00348-3

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1615/AnnualRevHeatTransfer.v10.60

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Cavitation bubbles induced by pulsed laser near a free surface have been investigated experimentally and numerically. The successive pressure waves from them and their vertical migrations and volume change were measured by using hydrophone and high-speed streak camera. The characteristics of the first and second rebound times due to the distance from the free surface enable us to guess the mechanisms of bubble collapse with microjet. Because our new scheme by CIP method is successfully applicable for such two-phase flow, the comparison between experimental and numerical results is also very useful to understand the fundamental factor of the complicated phenomena of cavitation bubbles.

DOI： 10.1299/kikaib.64.2897

Language：Others  

Numerical Analysis on Unsteady Motion of Cavitation Bubble by CIP Method

Language：Others   Publishing type：Research paper (scientific journal)  

Aerodynamic acoustic problems near plate edges are treated by a new method based on the linear theory. For sound radiation and generation problems, we can calculate one of the half acoustic fields divided by semi-infinite plates by determining the distributions of the acoustic monopoles. Some numerical calculations confirm the validity of this distributed monopole method. Applying this method, we can impose the Kutta condition explicitly at the trailing edge and explain the feedback mechanism in terms of sound wave effects on the flow field. The generation of vorticity waves at the trailing edge due to the incident sound wave is calculated and the possibility of self-excited tones between the trailing edge and the leading edge is verified. The relationship obtained is the same as that of the edge tone phenomenon, and the necessary amplification of the vortex during its convection for the sound to become self-excited is quantified.

DOI： 10.1299/jsmeb.39.86

Language：English   Publishing type：Research paper (scientific journal)  

Laser differential interferometry using two laser beams of perpendicular polarization is applied to sound pressure measurements. The weak density difference due to the sound wave no more than 70dB between the two beams is measurable in proportional to the differential output signal of the two photo detecters. Both steady and transient sound wave signals are obtained by our interferometer as clearly as by microphone. Acoustic vibration of the optical systems and the way of sound visualization in future are also discussed.

Language：Others   Publishing type：Research paper (other academic)  

© 1995 SPIE. Air bubbles are observed at the moment of there emergence from a vertical nozzle in water. We can classify them into two types by formation process. The bubbles broken off from a large air bulk always emit sound pulses, but those not being split but keeping their initial volume hardly produce any sound. By comparing their motions and distortions not only by means of the sequential series of photographs but also by the streak photographs of the vertical linear portion of the axisymmetric bubble, we obtain the difference of the subtle distortion of their surface which causes the emission of sound pulse.

DOI： 10.1117/12.209625

Language：Japanese  

Analysis of Aerodynamic Noise by Distributed Monopole Method
Aerodynamic acoustic problems near plate edges are treated by a new method based on the linear theory. For sound radiation and generation problems, we can calculate one of the half acoustic fields divided by semi-infinite plates by determining the distributions of the acoustic monopoles. Some numerical calculations confirm the validity of this distributed monopole method. Applying this method, we can impose the Kutta condition explicitly at the trailing edge and explain the feedback mechanism in terms of sound wave effects on the flow field. The generation of vorticity waves at the trailing edge due to the incident sound wave is calculated and the possibility of self-excited tones between the trailing edge and the leading edge is verified. The relationship obtained is the same as that of the edge tone phenomenon, and the necessary amplification of the vortex during its convection is quantified for the sound to become self-excited.

Language：Others   Publishing type：Research paper (scientific journal)  

The problem of noise generation due to the interaction between flows and plate edges is treated analytically. In uniform flow containing vorticity waves, two semi-infinite flat plates are placed with the trailing edge of one plate and leading edge of the other being tandemly situated a finite distance apart. This flow is considered to be a simplified model for self-excited tones such as edge tone and cavity noise. An approximate solution to the sound pressure is obtained by the Wiener-Hopf technique, and the calculated acoustic field shows the characteristics of the trailing edge noise and the leading edge noise. The sound pressure level varies with peaks and troughs as the wave number increases, especially in the region upstream from both edges, and these peaks show a frequency dependence similar to edge tones. Such a selective response mechanism will be explained by the phase relationship between vortex and sound.

DOI： 10.1299/jsmeb.36.214

Language：Others   Publishing type：Research paper (scientific journal)  

An analysis is performed on the trailing edge noise which is one of the important mechanisms of noise generation in flow machines. An acoustic field is treated where a semi-infinite flat plate is placed parallel to the inviscid uniform flow with incident vorticity waves convected from the upstream direction. Applying the Wiener-Hopf technique, we obtain an exact solution to the sound pressure proportional to the amplitude of the incident vorticity wave without restriction of frequency or velocity. The calculated acoustic field, which varies with flow velocity, exhibits general features of the sound pressure level (SPL) in a cardioid pattern with the constant phase surface distorted by the main flow. The relationship between flow velocity and SPL is ascertained to be dependent on the 5th law at low Mach numbers. However, the results show that such dependence does not hold at higher Mach numbers where the radiated noise level rises progressively as the flow velocity increases.

DOI： 10.1299/jsmeb1988.34.4_431

Language：English   Publishing type：Research paper (scientific journal)  

An analysis is performed on the trailing edge noise which is one of the important mechanisms of noise generation in flow machines. An acoustic field is treated where a semi-infinite flat plate is placed parallel to the inviscid uniform flow with incident vorticity waves convected from the upstream direction. Applying the Wiener-Hopf technique, we obtain an exact solution to the sound pressure proportional to the amplitude of the incident vorticity wave without restriction of frequency or velocity. The calculated acoustic field, which varies with flow velocity, exhibits general features of the sound pressure level (SPL) in a cardioid pattern with the constant phase surface distorted by the main flow. The relationship between flow velocity and SPL is ascertained to be dependent on the 5th law at low Mach numbers. However, the results show that such dependence does not hold at higher Mach numbers where the radiated noise level rises progressively as the flow velocity increases.

DOI： 10.1299/jsmeb1988.34.4_431

Event date： 2023.7

Language：English  

Country：Other  

Event date： 2022.9

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Online   Country：Other  

Nanomaterials are of unique thermophysical properties and have been used in a wide variety of experimental studies aimed at significant enhancement of heat transfer performance. Many successful results have been reported, for example, selective thermal radiation of surface coatings with nanoparticles, high-thermal conductivity composites or coolants with nanoscale fillers. So far, most of their roles are to improve the conduction or radiation heat transfer. On the other hand, their applications to improve mass transfer are still in the stage of trial and error. The most well-known application is to use hydrophilic materials for preventing surface dryout at high heat flux but the underlying mechanisms of capillary motion and evaporation in nanoscale are not clarified yet, thus we have no strategy to precisely control the liquid-vapor phase change by designing artificial nanostructured surfaces.
In this talk, a novel approach to enhance the convection in the evaporating meniscus on a hot surface is introduced, where the atomic-scale flatness of two-dimensional materials represented by graphene is used. Its beneficial role is the large slip length of liquid on the surface. In addition to the reviews of past reports, new experimental and theoretical results are presented for evaluating the slip flow on graphene, together with the relationship between wettability and slip length of an identical surface.
The gap between ideal thermophysical property of nanomaterials and real performance of heat transfer devices is also discussed. At first, the above-mentioned surface of atomic-scale flatness is mostly dirty with contaminants and nanoparticles, which pin the three-phase contact lines and consequently influence the cooling performance. Newest nanoscale findings on the pinned or unpinned contact lines using AFM and TEM are summarized in this talk. Next, the thermal contact resistance between fibrous materials is treated, which is a difficult research target for thermal application of nanomaterials. An experimental method is presented to quantitatively evaluate the contact resistance using Raman spectroscopy. The last concern is the quality of nanomaterials and a STEM-based experimental study clearly demonstrated that the thermal conductivity of an individual nanotube depends on its structural quality. The history of nanotechnology is over 20 years but these kinds of experimental studies are still needed to increase the reliability of heat transfer devices with nanomaterials and nanostructures.

Event date： 2022.5

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：岐阜市   Country：Japan  

Event date： 2021.5

Language：English   Presentation type：Oral presentation (general)  

Venue：Online   Country：Other  

Other Link： https://www.micronanoflows.com/mnf2021

Event date： 2021.1

Language：English   Presentation type：Oral presentation (general)  

Venue：Online   Country：Other  

Event date： 2019.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Maui   Country：United States  

Event date： 2019.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Maui   Country：United States  

Event date： 2019.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Halong   Country：Viet Nam  

Revealing nanoscale dynamics of three-phase contact line promotes practical application such as nano-channels and micro heat transfer devices. However, there are still many unexplained phenomena that are obstacles to establish precise models. Thus, in order to expand knowledge of three-phase contact line, further investigation near the phase interfaces are desirable. In this study, we used liquid cell electron microscopy that enables us to investigate in-situ liquid dynamics with nanoscale spatial resolution. We prepared graphene liquid cells that give the highest resolution in liquid cells and observed three-phase contact line consisting of confined water and nanobubbles in real time using transmission electron microscopy (TEM). In our samples, nanobubbles, smaller than a few hundred nanometers in diameter, scattered around and thin water rings surrounded them. During the observation for several tens of seconds, nanobubbles deformed their shapes, which allowed us to see the movements of three-phase contact line. We elucidated the mechanism of this movement by considering the influences of electron beam irradiation on water and the graphene layers.

Event date： 2019.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Xi'an   Country：China  

Event date： 2019.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Seville   Country：Spain  

Event date： 2019.2

Language：English   Presentation type：Oral presentation (general)  

Venue：Lucca   Country：Italy  

New experimental techniques to investigate nanoscale bubbles and droplets are explained, using TEM and AFM. AFM is of the highest spatial resolution and its feedback control of tip tapping enables us to obtain the accurate shape of nanobubbles at the solid-liquid interface. TEM requires ultra-high vacuum environment but utilization of nano liquid cell enables us to image the liquid-gas interface in nanoscale. By using these techniques, some key issues for generation and growth of interfacial nanobubbles have been understood.

Event date： 2018.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Fukuoka   Country：Japan  

Event date： 2018.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Beijing   Country：China  

Event date： 2018.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Dubrovnik   Country：Croatia  

Event date： 2018.1

Language：English   Presentation type：Oral presentation (general)  

Venue：Fukuoka   Country：Japan  

Event date： 2017.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Tampa   Country：United States  

Event date： 2017.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Boston   Country：United States  

Event date： 2017.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Tokyo   Country：Japan  

Event date： 2017.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Tokyo   Country：Japan  

Event date： 2017.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Foz do Iguacu   Country：Brazil  

Event date： 2016.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Milan   Country：Italy  

Event date： 2016.7

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Costa del Sol   Country：Spain  

Event date： 2016.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Krakow   Country：Poland  

Event date： 2016.3

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.3

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.3

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.3

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2016.3

Language：English   Presentation type：Oral presentation (general)  

Venue：Honolulu, Hawaii   Country：United States  

Event date： 2015.9 - 2015.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Leoben   Country：Austria  

Event date： 2015.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Edinburgh   Country：United Kingdom  

Event date： 2015.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Cancun   Country：Mexico  

Event date： 2015.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Roma   Country：Italy  

Event date： 2015.5

Language：English   Presentation type：Oral presentation (general)  

Venue：Calgary   Country：Canada  

Event date： 2015.4

Language：English   Presentation type：Oral presentation (general)  

Venue：Boulder, Colorado   Country：United States  

Event date： 2015.1

Language：English   Presentation type：Oral presentation (general)  

Venue：Galveston   Country：United States  

Event date： 2014.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Darmstadt   Country：Germany  

Event date： 2014.8 - 2014.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Porto   Country：Portugal  

Event date： 2014.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Kyoto   Country：Japan  

Event date： 2014.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Santa Cruz, CA   Country：United States  

Event date： 2014.5

Language：English   Presentation type：Oral presentation (general)  

Venue：Florida   Country：United States  

Event date： 2014.4

Language：English   Presentation type：Oral presentation (general)  

Venue：Marseille   Country：France  

Event date： 2013.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Hong Kong   Country：China  

Event date： 2013.11

Language：English   Presentation type：Oral presentation (general)  

Venue：San Diego   Country：United States  

Event date： 2013.10

Language：English   Presentation type：Symposium, workshop panel (public)  

Venue：Shanghai   Country：China  

Event date： 2013.9 - 2013.10

Language：English   Presentation type：Symposium, workshop panel (public)  

Venue：Jeju   Country：Korea, Republic of  

Event date： 2013.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Granada   Country：Spain  

Event date： 2013.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Granada   Country：Spain  

Event date： 2013.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Sapporo   Country：Japan  

Event date： 2013.4

Language：English   Presentation type：Oral presentation (general)  

Venue：San Francisco   Country：United States  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Auckland   Country：New Zealand  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Auckland   Country：New Zealand  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagasaki   Country：Japan  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagasaki   Country：Japan  

Event date： 2012.9

Presentation type：Oral presentation (general)  

Venue：Granada   Country：Spain  

Event date： 2012.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Rio Grande, Puerto Rico   Country：United States  

Event date： 2012.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Boulder, CO   Country：United States  

Event date： 2012.3

Presentation type：Oral presentation (general)  

Venue：Atlanta   Country：United States  

Event date： 2011.12

Presentation type：Oral presentation (general)  

Venue：Shima   Country：Japan  

Event date： 2011.11

Presentation type：Oral presentation (general)  

Venue：Denver   Country：United States  

Event date： 2007.11

Country：Japan  

Event date： 2007.11

Country：Japan  

Event date： 2007.10

Venue：札幌   Country：Japan  

Event date： 2007.9

Venue：札幌   Country：Japan  

Event date： 2007.5

Presentation type：Oral presentation (general)  

Venue：長崎   Country：Japan  

Study on NEMS fluidic sensor using carbon nanotubes

Event date： 2007.5

Presentation type：Oral presentation (general)  

Venue：長崎   Country：Japan  

Experimental Study on Amorphous Carbon Nanostructure

Language：Others  

Country：Other  

Language：Others  

Country：Other  

In-situ electron microscopic study of effect of temperature on nanobubble generation

Language：Others  

Country：Other  

Nanobubble dynamics studied using in-situ liquid cell electron microscopy

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Raman optothermal methods to measure interfacial thermal conductance of low-dimensional materials

Language：Others  

Country：Other  

Raman optothermal methods to measure interfacial thermal conductance of low-dimensional materials

Language：Others  

Country：Other  

Non-diffusive molecular transport in graphene liquid cells

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Nanobubble dynamics during merging revealed using in-situ liquid cell electron microscopy

Language：Others  

Country：Other  

Presentation type：Symposium, workshop panel (public)  

Venue：福岡市   Country：Japan  

Research for Ultrasensitive Thermal Sensor with CNT Fins

Presentation type：Oral presentation (general)  

Venue：ハワイ   Country：United States  

SILICON-FREE MICRO RESITOR ARRAY FOR SMART HOT FILM
Koji Takahashi
The 1st International Symposium on Micro & Nano Technology, 14-17 March, 2004, Honolulu, Hawaii, USA, 2004

Presentation type：Oral presentation (general)  

Venue：ハワイ   Country：United States  

HEAT CONDUCTION OF SILICON DIOXIDE THIN FILM LAYERS FOR MEMS THRUSTER
Hideyo Ebisuzaki, Xing Zhang, Motoo Fujii and Koji Takahashi
The 1st International Symposium on Micro & Nano Technology, 14-17 March, 2004, Honolulu, Hawaii, USA, 2004

Presentation type：Oral presentation (general)  

Venue：ハワイ   Country：United States  

POROUS SILICON AS A SUPERHYDROPHOBIC MICROCHANNEL SURFACE
Koji Takahashi
The 1st International Symposium on Micro & Nano Technology, 14-17 March, 2004, Honolulu, Hawaii, USA, 2004

Presentation type：Oral presentation (general)  

Venue：長崎   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：長崎  

Presentation type：Oral presentation (general)  

Venue：東京  

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Presentation type：Symposium, workshop panel (public)  

Venue：Nagano   Country：Japan  

Optical study of the SWNT dispersion after dielectrophoresis

Presentation type：Symposium, workshop panel (public)  

Venue：北九州   Country：Japan  

Presentation type：Symposium, workshop panel (public)  

Venue：北九州   Country：Japan  

Presentation type：Oral presentation (general)  

Venue：Toulouse   Country：France  

CONTEMPORARY TECHNOLOGY AND APPLICATION OF MEMS ROCKET

Presentation type：Symposium, workshop panel (public)  

Venue：熊本市   Country：Japan  

Numerical Study on Nano Thermal Sensor with Ultra Fine Fins

Presentation type：Symposium, workshop panel (public)  

Venue：takamatsu   Country：Japan  

Catalytic Porous Microchannel for hydrogen Peroxide MEMS Thruster

Presentation type：Symposium, workshop panel (public)  

Venue：takamatsu   Country：Japan  

Study on Local Heat Transfer of Nano Fluidic Sensor

Presentation type：Symposium, workshop panel (public)  

Venue：Berkeley   Country：United States  

Development Study of 100?m-order Solid Rocket for Pico Satellite Application

Language：Japanese  

20pm3-PM020 Experimental study on phonon thermal conduction using focused ion beam
We conducted an experiment to investigate phonon thermal conduction in multi-walled carbon nanotube. Length dependence of thermal conductivity of a multi-walled carbon nanotube up to 10 μm is reported by applying quasi-ballistic thermal conduction model for experimentally obtained thermal conductance. We suggest that phonon thermal conduction in this length is still in ballistic or quasi-ballistic region. Experimental equipment used for this experiment are T-type sensor for thermal conductance measurement, focused ion beam (FIB) irradiation to control the sample length and phonon free path, and transmission electron microscopy (TEM) to evaluate the size and uniformity of the defects induced by FIB irradiation.

Language：Japanese  

G114 Formation and growth of nanobubbles at solid-liquid interfaces
Boiling is applied to many industrial machines due to its high heat transfer coefficient. However, a very complex mechanism of boiling, especially bubble nucleation, is still not sufficiently understood. On the other hand, numerous experiments have revealed the existence of soft domains that called nanobubbles at the solid-liquid interface. In this study, to investigate the influence of the solid-liquid interface nanobubbles on the bubble nucleation of boiling, an atomic force microscope is used to characterize the morphology of nanobubbles. The temperature dependence of the nanobubbles and temporal changes are also observed.

Language：Japanese  

C215 Study of droplet condensation process on graphite surface by using ESEM
Heat transfer performance of dropwise condensation is higher than filmwise one due to droplet removal from the condensed surface. In order to further enhancement of its performance, removal of microscale droplet is a key issue. However, condensation mechanism at microscale on hydrophobic-hydrophilic hybrid surface is not understood, although all surfaces are consisted with those combinations. In this study, we conducted condensation experiments on a graphite surface at 0℃ and 550 to 600 Pa by using environmental scanning electron microscope (ESEM). Nanoscale step-terrace structures on graphite surface are obtained by using an atomic force microscope (AFM) before ESEM experiments. It was found that condensed droplets with diameter of 150 to 300 nm are lined up along step edges at over 150 nm intervals. In addition, we found that most droplets are on the steps of height of 1 nm and shorter droplet intervals are induced by higher steps and shorter terrace width. Our results were analyzed by an extended nucleation theory and we found that water molecules attracted on steps play an important role for droplet condensation.

Language：Japanese  

B121 Experimental study on heat transfer between an individual carbon nanotube and surrounding gases
Carbon nanotubes (CNTs) have high intrinsic thermal conductivity and high surface-to-volume ratio, thus CNT-based fins is considered to be promising for hign performance device cooling. In addition, exploring the heat transfer between a CNT and gases is important not only for CNT-based gas sensors but also for scientific interest oi mterfacial heat transport. In this work, we have developed novel technique for measuring heat transfer coefficients between an individual carbon nanotube and surrounding gases (Air, N_2, Ar, He) using platinum hot film sensor. Measured heat transfer coefficients quantitatively show good agreement with estimated values by kinetic gas theory at atmospheric pressure but do not match with theoretical prediction at low pressure regime.

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese  

G33 Observation of ionic liquid interface in a nanotube

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese   Publishing type：Internal/External technical report, pre-print, etc.  

Report on CANEUS2004

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：English  

Measurements of In-Plane Thermal Conductivity and Electrical Conductivity of Suspended Platinum Thin Film

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Trends in MEMS and Dynamics of Microbubble, KONSORYU, Vol. 16, No. 3, PP198-206

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Microfabrication and Application to Thruster

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese  

Carbon Thin Films Prepared by Pulsed Laser Deposition

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Number of participants：200

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Number of participants：400

Type：Competition, symposium, etc. 

Number of participants：150

Type：Competition, symposium, etc. 

Number of participants：200

Type：Competition, symposium, etc. 

Number of participants：150

Type：Competition, symposium, etc. 

Number of participants：500

Type：Competition, symposium, etc. 

Number of participants：150

Type：Competition, symposium, etc. 

Number of participants：500

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Number of participants：300

Type：Competition, symposium, etc. 

Number of participants：500

Type：Competition, symposium, etc. 

Number of participants：100

Type：Competition, symposium, etc. 

Number of participants：100