| 1. |
Sarthak Nag, Yoko Tomo, Koji Takahashi, Masamichi Kohno, Mechanistic Insights into Nanobubble Merging Studied Using In Situ Liquid-Phase Electron Microscopy, LANGMUIR, 10.1021/acs.langmuir.0c03208, 37, 2, 874-881, 2021.01, 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.. |
| 2. |
Haidong Wang, Masahiro Narasaki, Zhongwei Zhang, Koji Takahashi, Jie Chen, Xing Zhang, Ultra-strong stability of double-sided fluorinated monolayer graphene and its electrical property characterization, SCIENTIFIC REPORTS, 10.1038/s41598-020-74618-4, 10, 1, 2020.10, 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.. |
| 3. |
Biao Shen, Jiewei Liu, Gustav Amberg, Minh Do-Quang, Junichiro Shiomi, Koji Takahashi, Yasuyuki Takata, Contact-line behavior in boiling on a heterogeneous surface
Physical insights from diffuse-interface modeling, Physical Review Fluids, 10.1103/PhysRevFluids.5.033603, 5, 3, 2020.03, 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.. |
| 4. |
Masahiro Narasaki, Qin Yi Li, Tatsuya Ikuta, Jin Miyawaki, Koji Takahashi, Modification of thermal transport in an individual carbon nanofiber by focused ion beam irradiation, Carbon, 10.1016/j.carbon.2019.07.056, 153, 539-544, 2019.11, © 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..  |
| 5. |
Hideaki Teshima, Yasuyuki Takata, Koji Takahashi, Adsorbed gas layers limit the mobility of micropancakes, Applied Physics Letters, 10.1063/1.5113810, 115, 7, 2019.08, 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.. |
| 6. |
Qin Yi Li, Tianli Feng, Wakana Okita, Yohei Komori, Hiroo Suzuki, Toshiaki Kato, Toshiro Kaneko, Tatsuya Ikuta, Xiulin Ruan, Koji Takahashi, Enhanced Thermoelectric Performance of As-Grown Suspended Graphene Nanoribbons, ACS nano, 10.1021/acsnano.9b03521, 13, 8, 9182-9189, 2019.08, 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.. |
| 7. |
Qin Yi Li, Tianli Feng, Wakana Okita, Yohei Komori, Hiroo Suzuki, Toshiaki Kato, Toshiro Kaneko, Tatsuya Ikuta, Xiulin Ruan, Koji Takahashi, Enhanced Thermoelectric Performance of As-Grown Suspended Graphene Nanoribbons, ACS Nano, 10.1021/acsnano.9b03521, 13, 8, 9182-9189, 2019.08, © 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.. |
| 8. |
Qin Yi Li, Koji Takahashi, Xing Zhang, Frequency-domain Raman method to measure thermal diffusivity of one-dimensional microfibers and nanowires, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2019.01.057, 134, 539-546, 2019.05, © 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.. |
| 9. |
Biao Shen, Takeshi Hamazaki, Wei Ma, Naoki Iwata, Sumitomo Hidaka, Atsushi Takahara, Koji Takahashi, Yasuyuki Takata, Enhanced pool boiling of ethanol on wettability-patterned surfaces, Applied Thermal Engineering, 10.1016/j.applthermaleng.2018.12.049, 149, 325-331, 2019.02, © 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.. |
| 10. |
Qin Yi Li, Ryo Matsushita, Yoko Tomo, Tatsuya Ikuta, Koji Takahashi, Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance, Journal of Physical Chemistry Letters, 10.1021/acs.jpclett.9b00718, 10, 13, 3744-3749, 2019.01, 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.. |
| 11. |
Qin Yi Li, Koki Katakami, Tatsuya Ikuta, Masamichi Kohno, Xing Zhang, Koji Takahashi, Measurement of thermal contact resistance between individual carbon fibers using a laser-flash Raman mapping method, Carbon, 10.1016/j.carbon.2018.09.034, 141, 92-98, 2019.01, © 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.. |
| 12. |
Qin Yi Li, Xing Zhang, Koji Takahashi, Variable-spot-size laser-flash Raman method to measure in-plane and interfacial thermal properties of 2D van der Waals heterostructures, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2018.05.011, 125, 1230-1239, 2018.10, © 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.. |
| 13. |
Yutaka Yamada, Kanoko Taguchi, Tatsuya Ikuta, Akihiko Horibe, Koji Takahashi, Meniscus Motion and Void Generation Inside Carbon Nanotubes, Journal of Physical Chemistry C, 10.1021/acs.jpcc.8b06406, 122, 38, 21910-21918, 2018.09, 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.. |
| 14. |
Hongbin He, Biao Shen, Sumitomo Hidaka, Koji Takahashi, Yasuyuki Takata, A loop thermosyphon with hydrophobic spots evaporator surface, ASME 2018 16th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2018, 10.1115/icnmm2018-7623, 2018.08, 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.. |
| 15. |
Hongbin He, Biao Shen, Liangyu Chen, Sumitomo Hidaka, Koji Takahashi, Masamichi Kohno, Yasuyuki Takata, Heat transfer enhancement of a loop thermosyphon with a hydrophobic spot-coated surface, Journal of Thermal Science and Technology, 10.1299/jtst.2018jtst0011, 13, 1, 2018.04, © 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.. |
| 16. |
Hideaki Teshima, Koji Takahashi, Yasuyuki Takata, Takashi Nishiyama, Wettability of AFM tip influences the profile of interfacial nanobubbles, Journal of Applied Physics, 10.1063/1.5010131, 123, 5, 054303, 2018.02, © 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.. |
| 17. |
Koji Takahashi, Bubbles and droplets at the initial stage of nucleation
Recent advances in experimental techniques, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 2018-August, 359-367, 2018.01, 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.. |
| 18. |
Hongbin He, Biao Shen, Sumitomo Hidaka, Koji Takahashi, Yasuyuki Takata, A loop thermosyphon with hydrophobic spots evaporator surface, ASME 2018 16th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2018
ASME 2018 16th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2018, 2018.01, 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.. |
| 19. |
Qin Yi Li, Koji Takahashi, Xing Zhang, Heater assisted raman method to measure interfacial thermal conductance in van der waals heterostructures, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 10.1615/ihtc16.nmt.021756, 2018-August, 7137-7143, 2018.01, 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.. |
| 20. |
Yoko Tomo, Alexandros Askounis, Khellil Sefiane, Yasuyuki Takata, Koji Takahashi, Study on liquid-gas interface at nanoscale using transmission electron microscopy, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 2018-August, 1169-1174, 2018.01, 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.. |
| 21. |
Qinyi Li, Xing Zhang, Koji Takahashi, Variable-spot-size laser-flash Raman method to measure in-plane and interfacial thermal properties of 2D van der Waals heterostructures, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2018.05.011, 125, 1230-1239, 2018, 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.. |
| 22. |
Haidong Wang, Xing Zhang, Hiroshi Takamatsu, Koji Takahashi, Effective thermal rectification in suspended monolayer graphene, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 10.1615/ihtc16.nmt.022118, 2018-August, 6903-6908, 2018, 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.. |
| 23. |
Biao Shen, Masayuki Yamada, Tomosuke Mine, Sumitomo Hidaka, Junichiro Shiomi, Gustav Amberg, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Enhanced boiling heat transfer on surfaces patterned with mixed wettability, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 10.1615/ihtc16.bae.023482, 2018-August, 1379-1386, 2018, 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.. |
| 24. |
Hideaki Teshima, Takashi Nishiyama, Yasuyuki Takata, Koji Takahashi, Influence of surface wettability and nanostructure on the generation of interfacial nanobubbles, 16th International Heat Transfer Conference, IHTC 2018
International Heat Transfer Conference, 10.1615/ihtc16.nmt.022210, 2018-August, 6909-6914, 2018, 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.. |
| 25. |
Biao Shen, Masayuki Yamada, Sumitomo Hidaka, Jiewei Liu, Junichiro Shiomi, Gustav Amberg, Minh Do-Quang, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Early Onset of Nucleate Boiling on Gas-covered Biphilic Surfaces, Scientific Reports, 10.1038/s41598-017-02163-8, 7, 1, 2017.12. |
| 26. |
Biao Shen, Masayuki Yamada, Sumitomo Hidaka, Jiewei Liu, Junichiro Shiomi, Gustav Amberg, Minh Do-Quang, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Early Onset of Nucleate Boiling on Gas-covered Biphilic Surfaces, Scientific Reports, 10.1038/s41598-017-02163-8, 7, 1, 2036, 2017.12, © 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.. |
| 27. |
Qin Yi Li, Koji Takahashi, Xing Zhang, Comment on "divergent and Ultrahigh Thermal Conductivity in Millimeter-Long Nanotubes", Physical Review Letters, 10.1103/PhysRevLett.119.179601, 119, 17, 2017.10. |
| 28. |
Masahiro Narasaki, Haidong Wang, Takashi Nishiyama, Tatsuya Ikuta, Koji Takahashi, Experimental study on thermal conductivity of free-standing fluorinated single-layer graphene, Applied Physics Letters, 10.1063/1.5001169, 111, 9, 093103, 2017.08, © 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.. |
| 29. |
Qin Yi Li, Kailun Xia, Ji Zhang, Yingying Zhang, Qunyang Li, Koji Takahashi, Xing Zhang, Measurement of specific heat and thermal conductivity of supported and suspended graphene by a comprehensive Raman optothermal method, Nanoscale, 10.1039/c7nr01695f, 9, 30, 10784-10793, 2017.08, © 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.. |
| 30. |
Hongbin He, Kento Furusato, Masayuki Yamada, Biao Shen, Sumitomo Hidaka, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Efficiency enhancement of a loop thermosyphon on a mixed-wettability evaporator surface, Applied Thermal Engineering, 10.1016/j.applthermaleng.2017.05.145, 123, 1245-1254, 2017.08, © 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.. |
| 31. |
Haidong Wang, Shiqian Hu, Koji Takahashi, Xing Zhang, Hiroshi Takamatsu, Jie Chen, Experimental study of thermal rectification in suspended monolayer graphene, Nature Communications, 10.1038/ncomms15843, 8, 15843, 2017.06, © 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.. |
| 32. |
Yoko Tomo, Koji Takahashi, Takashi Nishiyama, Tatsuya Ikuta, Yasuyuki Takata, Nanobubble nucleation studied using Fresnel fringes in liquid cell electron microscopy, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2017.01.013, 108, 1460-1465, 2017.05, © 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.. |
| 33. |
Hiroshi Takamatsu, Haidong Wang, Koji Takahashi, Zhang Xing, Highly sensitive charge mobility of suspended monolayer graphene, 6th International Symposium on Micro and Nano Technology, 2017.03. |
| 34. |
Yutaka Yamada, ASKOUNIS ALEXANDROS, Tatsuya Ikuta, Koji Takahashi, Yasuyuki Takata, SEFIANE KHELLIL, Thermal Conductivity of Liquid/CNT Core-Shell Nanocomposites, J. Appl. Phys., 10.1063/1.4973488, 121, 015104, 2017.01. |
| 35. |
Biao Shen, Bambang Joko Suroto, Sana Hirabayashi, Masayuki Yamada, Sumitomo Hidaka, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Bubble activation from a hydrophobic spot at “negative“ surface superheats in subcooled boiling, Applied Thermal Engineering, 10.1016/j.applthermaleng.2014.10.054, 88, 2017.01. |
| 36. |
Hongbin He, Kento Furusato, Masayuki Yamada, Biao Shen, Sumitomo Hidaka, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Efficiency enhancement of a loop thermosyphon on a mixed-wettability evaporator surface, Applied Thermal Engineering, 10.1016/j.applthermaleng.2017.05.145, 123, 1245-1254, 2017.01. |
| 37. |
M. Yamada, B. Shen, T. Imamura, S. Hidaka, Masamichi Kohno, Koji Takahashi, Yasuyuki Takata, Enhancement of boiling heat transfer under sub-atmospheric pressures using biphilic surfaces, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2017.08.078, 115, 753-762, 2017.01. |
| 38. |
Hideaki Teshima, Takashi Nishiyama, Koji Takahashi, Nanoscale pinning effect evaluated from deformed nanobubbles, Journal of Chemical Physics, 10.1063/1.4973385, 146, 1, 2017.01, © 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.. |
| 39. |
Yutaka Yamada, Alexandros Askounis, Tatsuya Ikuta, Koji Takahashi, Yasuyuki Takata, Khellil Sefiane, Thermal conductivity of liquid/carbon nanotube core-shell nanocomposites, Journal of Applied Physics, 10.1063/1.4973488, 121, 1, 2017.01, © 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.. |
| 40. |
Takashi Nishiyama, Masamichi Kohno, Koji Takahashi, Tsuyoshi Yoshitake, Thermal Conductivity of Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films Prepared by Coaxial Arc Plasma Deposition, ECS Transactions, 75, 25, 27-32, 2017. |
| 41. |
Qinyi Li, Koji Takahashi, Zhang Xing, Measurement of specific heat and thermal conductivity of supported and suspended graphene by a comprehensive Raman optothermal method, Nanoscale, 9, 10784-10793, 2017. |
| 42. |
Masahiro Narasaki, Haidong Wang, Yasuyuki Takata, Koji Takahashi, Influence of ion beam scattering on the electrical resistivity of platinum hot films, Microelectronic Engineering, 10.1016/j.mee.2016.09.008, 166, 15-18, 2016.12, © 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.. |
| 43. |
Kazuhiko Suga, Koji Fukagata, Kaoru Maruta, Akio Miyara, Koji Takahashi, Special Issue of the First Pacific Rim Thermal Engineering Conference (PRTEC2016) Preface, JOURNAL OF THERMAL SCIENCE AND TECHNOLOGY, 10.1299/jtst.2016jtst0034, 11, 3, 2016.12. |
| 44. |
Alexandros Askounis, Yutaka Yamada, Tatsuya Ikuta, Koji Takahashi, Yasuyuki Takata, Khellil Sefiane, On the linear dependence of a carbon nanofiber thermal conductivity on wall thickness, AIP Advances, 10.1063/1.4968831, 6, 11, 2016.11, © 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.. |
| 45. |
Hiroshi Takamatsu, Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Koji Takahashi, Zhang Xing, Effect of nanohole defect on the thermal conductivity of free-standing single-layer graphene, 11th Asian Thermophysical Properties Conference (ATPC 2016), 2016.10. |
| 46. |
Experimental Study of Gas Phase Generation in Nano Scale
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. . |
| 47. |
Tingting Miao, Shaoyi Shi, Shen Yan, Weigang Ma, Xing Zhang, Koji Takahashi, Tatsuya Ikuta, Integrative characterization of the thermoelectric performance of an individual multiwalled carbon nanotube, Journal of Applied Physics, 10.1063/1.4962942, 120, 12, 2016.09, © 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.. |
| 48. |
Qin Yi Li, Masahiro Narasaki, Koji Takahashi, Tatsuya Ikuta, Takashi Nishiyama, Xing Zhang, Temperature-dependent specific heat of suspended platinum nanofilms at 80-380 K, Chinese Physics B, 10.1088/1674-1056/25/11/114401, 25, 11, 2016.09, © 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.. |
| 49. |
Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroki Ago, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Yasuyuki Takata, A general method of fabricating free-standing, monolayer graphene electronic device and its property characterization, Sensors and Actuators, A: Physical, 10.1016/j.sna.2016.05.002, 247, 24-29, 2016.08, © 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.. |
| 50. |
Koji Takahashi, Takashi Nishiyama, 山田 寛, Yasuyuki Takata, Experimental Study of Nanobubbles and Nanodroplets on Hydrophilic/Hydrophobic Combined Surfaces, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2016), 11 to 13 July 2016, Costa del Sol, Spain, 655-662, 2016.07. |
| 51. |
Yutaka Yamada, Koji Takahashi, Yasuyuki Takata, Khellil Sefiane, Wettability on Inner and Outer Surface of Single Carbon Nanotubes, Langmuir, 10.1021/acs.langmuir.6b01366, 32, 28, 7064-7069, 2016.07, © 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.. |
| 52. |
Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroki Ago, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Yasuyuki Takata, Simultaneous measurement of electrical and thermal conductivities of suspended monolayer graphene, Journal of Applied Physics, 10.1063/1.4954677, 119, 24, 244306, 2016.06, © 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.. |
| 53. |
Takashi Nishiyama, Koji Takahashi, Tatsuya Ikuta, Yutaka Yamada, Yasuyuki Takata, Hydrophilic Domains Enhance Nanobubble Stability, ChemPhysChem, 10.1002/cphc.201501181, 17, 10, 1500-1504, 2016.05, © 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.. |
| 54. |
Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Hiroki Ago, Yasuyuki Takata, A simple method for fabricating free-standing large area fluorinated single-layer graphene with size-tunable nanopores, Carbon, 10.1016/j.carbon.2015.12.070, 99, 564-570, 2016.04, © 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.. |
| 55. |
Weigang Ma, Tingting Miao, Xing Zhang, Koji Takahashi, Tatsuya Ikuta, Boping Zhang, Zhenhua Ge, A T-type method for characterization of the thermoelectric performance of an individual free-standing single crystal Bi2S3 nanowire, Nanoscale, 10.1039/c5nr05946a, 8, 5, 2704-2710, 2016.02, © 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.. |
| 56. |
Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Hiroki Ago, Yasuyuki Takata, In-situ measurement of the heat transport in defect-engineered free-standing single-layer graphene, Scientific Reports, 10.1038/srep21823, 6, 21823, 2016.02, 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.. |
| 57. |
Yutaka Yamada, Koji Takahashi, Tatsuya Ikuta, Takashi Nishiyama, Yasuyuki Takata, Wei Ma, Atsushi Takahara, Tuning Surface Wettability at the Submicron-Scale: Effect of Focused Ion Beam Irradiation on a Self-Assembled Monolayer, Journal of Physical Chemistry C, 10.1021/acs.jpcc.5b09019, 120, 1, 274-280, 2016.01, © 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.. |
| 58. |
Koji Takahashi, Kazuma Nomoto, Tatsuya Ikuta, Temperature mapping on a suspended carbon nanotube using electron thermal microscopy, IEEE-NANO 2015 - 15th International Conference on Nanotechnology, 10.1109/NANO.2015.7388726, 781-784, 2016.01, © 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.. |
| 59. |
Koji Takahashi, Yusuke Kuwada, Tatsuya Ikuta, MEASURING THE THERAL CONTACT RESISTANCE WITHOUT SURFACE ROUGHNESS, Proceedings of the 26th International Symposium on Transport Phenomena, 27 Sep. - 1 Oct. 2015, Leoben, Austria, PaperID=16, 2015.09. |
| 60. |
Koji Takahashi, Kazuma Nomoto, Tatsuya Ikuta, Temperature Mapping on a Suspended Carbon Nanotube Using Electron Thermal Microscopy, Proceedings of IEEE NANO 2015, 15th INTERNATIONAL CONFERENCE ON NANOTECHNOLOGY, 27-30 JULY 2015, ROME, ITALY, PaperID=264, 2015.07. |
| 61. |
Takeshi Daio, Thomas Bayer, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Kazunari Sasaki, Stephen Matthew Lyth, In-Situ ESEM and EELS Observation of Water Uptake and Ice Formation in Multilayer Graphene Oxide, SCIENTIFIC REPORTS, 10.1038/srep11807, 5, 2015.07, 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.. |
| 62. |
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.. |
| 63. |
Qin Yi Li, Koji Takahashi, Hiroki Ago, Xing Zhang, Tatsuya Ikuta, Takashi Nishiyama, Kenji Kawahara, Temperature dependent thermal conductivity of a suspended submicron graphene ribbon, Journal of Applied Physics, 10.1063/1.4907699, 117, 6, 065102, 2015.02, © 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.. |
| 64. |
Koji Takahashi, Kazuma Nomoto, Tatsuya Ikuta, Temperature mapping on a suspended carbon nanotube using electron thermal microscopy, 15th IEEE International Conference on Nanotechnology, IEEE-NANO 2015
IEEE-NANO 2015 - 15th International Conference on Nanotechnology, 10.1109/NANO.2015.7388726, 781-784, 2015.01, 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.. |
| 65. |
Takashi Nishiyama, Yutaka Yamada, Tatsuya Ikuta, Koji Takahashi, Yasuyuki Takata, Metastable nanobubbles at the solid-liquid interface due to contact angle hysteresis, Langmuir, 10.1021/la5036322, 31, 3, 982-986, 2015.01, © 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.. |
| 66. |
Yutaka Yamada, Koji Takahashi, Yasuyuki Takata, Study of condensation on hydrophobic surface with nanoscale hydrophilic regions, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 81, 823, 2015, 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.. |
| 67. |
Yutaka Yamada, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Droplet nucleation on a well-defined hydrophilic-hydrophobic surface of 10 nm order resolution, Langmuir, 10.1021/la503615a, 30, 48, 14532-14537, 2014.12, © 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).. |
| 68. |
Koji Takahashi, Hiroyuki Hayashi, Characterization of thermal transport in multiwalled carbon nanotube using FIB irradiation, Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference, 10.1109/ITHERM.2014.6892439, 1375-1378, 2014.09, © 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.. |
| 69. |
Yutaka Yamada, Akira Kusaba, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Wettability-driven water condensation at the micron and submicron scale, Proceedings of the 15th International Heat Transfer Conference, IHTC 2014, 10.1615/ihtc15.cds.009177, 827-834, 2014.08, 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.. |
| 70. |
Tingting Miao, Weigang Ma, Xing Zhang, Keisuke Kubo, Masamichi Kohno, Yasuyuki Takata, Tatsuya Ikuta, Koji Takahashi, Study on the cross plane thermal transport of polycrystalline molybdenum nanofilms by applying picosecond laser transient thermoreflectance method, Journal of Nanomaterials, 10.1155/2014/578758, 2014, 2014.06, 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.. |
| 71. |
Hiroyuki Hayashi, Koji Takahashi, Tatsuya Ikuta, Takashi Nishiyama, Yasuyuki Takata, Xing Zhang, Direct evaluation of ballistic phonon transport in a multi-walled carbon nanotube, Applied Physics Letters, 10.1063/1.4869470, 104, 11, 2014.03, 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.. |
| 72. |
Hiroshi Takamatsu, Takanobu Fukunaga, Yuki Tanaka, Kosaku Kurata, Koji Takahashi, Micro-beam sensor for detection of thermal conductivity of gases and liquids, Sensors and Actuators, A: Physical, 10.1016/j.sna.2013.11.019, 206, 10-16, 2014.02, 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.. |
| 73. |
Kazunari Tsuru, Yutaka Yamada, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Experimental study on thermal contact resistance of multiwalled carbon nanotubes, ASME 2013 4th International Conference on Micro/Nanoscale Heat and Mass Transfer, MNHMT 2013, 10.1115/MNHMT2013-22078, 2013.12, 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.. |
| 74. |
Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Nanobubbles on a very flat hydrophobic surface prepared by self-assembled monolayers, ASME 2013 4th International Conference on Micro/Nanoscale Heat and Mass Transfer, MNHMT 2013, 10.1115/MNHMT2013-22077, 2013.12, 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.. |
| 75. |
Haidong Wang, Jinhui Liu, Xing Zhang, Koji Takahashi, Breakdown of Wiedemann-Franz law in individual suspended polycrystalline gold nanofilms down to 3 K, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2013.07.066, 66, 585-591, 2013.11, 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.. |
| 76. |
Yutaka Yamada, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Submicron-scale condensation on hydrophobic and hydrophilic surfaces, ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE), 10.1115/IMECE2013-66186, 15, 2013.11, 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.. |
| 77. |
Qin Yi Li, Jin Hui Liu, Hai Dong Wang, Xing Zhang, Koji Takahashi, Optical absorptance measurement of an individual multiwall carbon nanotube using a T type thermal probe method, Review of Scientific Instruments, 10.1063/1.4824494, 84, 10, 2013.10, 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% 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.. |
| 78. |
Jun Hirotani, Juo Amano, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Carbon nanotube thermal probe for quantitative temperature sensing, Sensors and Actuators, A: Physical, 10.1016/j.sna.2013.04.038, 199, 1-8, 2013.09, 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.. |
| 79. |
Hiroyuki Hayashi, Taysuya Ikuta, Takashi Nishiyama, Koji Takahashi, Enhanced anisotropic heat conduction in multi-walled carbon nanotubes, J. Appl. Phys., DOI: 10.1063/1.4772612, 113, 014301, 2013.03. |
| 80. |
Jun Hirotani, Taysuya Ikuta, Takashi Nishiyama, Koji Takahashi, Measuring the Thermal Boundary Resistance of van der Waals Contacts Using an Individual Carbon Nanotube, Journal of Physics: Condensed Matter, doi:10.1088/0953-8984/25/2/025301, 25, 025301, 2013.01. |
| 81. |
B. A. Cola, H. Daiguji, C. Dames, N. Fang, K. Fushinobu, S. Inoue, G. Kikugawa, M. Kohno, S. Kumar, D. Y. Li, J. R. Lukes, J. A. Malen, A. J.H. McGaughey, O. Nakabeppu, K. Pipe, P. Reddy, S. Shen, L. Shi, M. Shibahara, Y. Taguchi, K. Takahashi, T. Yamamoto, T. Zolotoukhina, Report on the seventh U.S.-Japan Joint seminar on nanoscale transport phenomena-science and engineering, Nanoscale and Microscale Thermophysical Engineering, 10.1080/15567265.2012.745913, 17, 1, 25-49, 2013.01, 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.. |
| 82. |
Yutaka Yamada, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Yasuyuki Takata, Submicron-scale condensation on hydrophobic and hydrophilic surfaces, ASME 2013 International Mechanical Engineering Congress and Exposition, IMECE 2013
Safety, Reliability and Risk; Virtual Podium (Posters), 10.1115/IMECE2013-66186, 2013.01, 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).. |
| 83. |
Hiroyuki Hayashi, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Enhanced anisotropic heat conduction in multi-walled carbon nanotubes, Journal of Applied Physics, 10.1063/1.4772612, 113, 1, 2013.01, 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.. |
| 84. |
Jun Hirotani, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Measuring the thermal boundary resistance of van der Waals contacts using an individual carbon nanotube, Journal of Physics Condensed Matter, 10.1088/0953-8984/25/2/025301, 25, 2, 2013.01, 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.. |
| 85. |
Juo Amano, Jun Hirotani, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Surface temperature measurement using a carbon nanotube probe, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 10.1299/kikaib.79.390, 79, 799, 390-398, 2013, 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.. |
| 86. |
Koji Takahashi, Jun Hirotani, Takashi Nishiyama, Tatsuya Ikuta, Hiroshi Takamatsu, Applications of nano hot-film sensor for micro/nanoscale thermal measurement, ASME 2012 3rd International Conference on Micro/Nanoscale Heat and Mass Transfer, MNHMT 2012
ASME 2012 3rd International Conference on Micro/Nanoscale Heat and Mass Transfer, MNHMT 2012, 10.1115/MNHMT2012-75030, 677-681, 2012.12, Platinum hot-film sensors, whose typical size is 500nm wide x 10μm 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.. |
| 87. |
Yutaka Yamada, Takashi Nishiyama, Tatsuya Ikuta, Koji Takahashi, Effect of nanoscale structure on thermal contact resistance of carbon nanotubes, ASME 2012 Heat Transfer Summer Conf. Collocated with the ASME 2012 Fluids Engineering Div. Summer Meeting and the ASME 2012 10th Int. Conf. on Nanochannels, Microchannels and Minichannels, HT 2012, 10.1115/HT2012-58203, 1, 173-177, 2012.07, 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.. |
| 88. |
Yutaka Yamada, Takashi Nishiyama, Takahiro Yasuhara, Koji Takahashi, Thermal boundary conductance between multi-walled carbon nanotubes, Journal of Thermal Science and Technology, 10.1299/jtst.7.190, 7, 1, 190-198, 2012.06, 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.. |
| 89. |
Koji Takahashi, Jun Hirotani, Takashi Nishiyama, Tatsuya Ikuta, Hiroshi Takamatsu, APPLICATIONS OF NANO HOT-FILM SENSOR FOR MICRO/NANOSCALE THERMAL MEASURMENT, Proceedings of the ASME 2012 3rd Micro/Nanoscale Heat & Mass Transfer International Conference, March 3-6, 2012, Atlanta, Georgia, USA, MNHMT2012-75030, 2012.03. |
| 90. |
Koji Takahashi, Jun Hirotani, Takashi Nishiyama, Tatsuya Ikuta, Hiroshi Takamatsu, Applications of nano hot-film sensor for micro/nanoscale thermal measurement, ASME 2012 3rd International Conference on Micro/Nanoscale Heat and Mass Transfer, MNHMT 2012, 10.1115/MNHMT2012-75030, 677-681, 2012.03, Platinum hot-film sensors, whose typical size is 500nm wide x 10μm 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.. |
| 91. |
Masanari Kimura, Takafumi Matsuzaki, Koji Takahashi, Edge effect on phonon transport in suspended and supported graphene nanoribbons, Computational Thermal Sciences, 10.1615/ComputThermalScien.2012004393, 4, 3, 193-199, 2012, 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.. |
| 92. |
Hiroyuki Hayashi, Koji Takahashi, Defect-induced thermal rectification
Numerical study on carbon nanotube and FPU-beta lattice, ASME/JSME 2011 8th Thermal Engineering Joint Conference, AJTEC 2011
ASME/JSME 2011 8th Thermal Engineering Joint Conference, AJTEC 2011, 2011.12, 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.. |
| 93. |
Jun Hirotani, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Thermal boundary resistance between the end of an individual carbon nanotube and a Au surface, Nanotechnology, 10.1088/0957-4484/22/31/315702, 22, 31, 2011.08, 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.. |
| 94. |
Hai Dong Wang, Jin Hui Liu, Xing Zhang, Zeng Yuan Guo, Koji Takahashi, Experimental study on the influences of grain boundary scattering on the charge and heat transport in gold and platinum nanofilms, Heat and Mass Transfer/Waerme- und Stoffuebertragung, 10.1007/s00231-011-0825-5, 47, 8, 893-898, 2011.07, 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.. |
| 95. |
Hiroyuki Hayashi, Koji Takahashi, Defect-induced thermal rectification: Numerical study on carbon nanotube and FPU-beta lattice, ASME/JSME 2011 8th Thermal Engineering Joint Conference, AJTEC 2011, 10.1115/ajtec2011-44071, 283-288, 2011.03, 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.. |
| 96. |
Hiroyuki Hayashi, Yohei Ito, Koji Takahashi, Thermal rectification of asymmetrically-defective materials, Journal of Mechanical Science and Technology, 10.1007/s12206-010-1008-x, 25, 1, 27-32, 2011.01, 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.. |
| 97. |
Jun Hirotani, Tatsuya Ikuta, Takashi Nishiyama, Koji Takahashi, Experimental study on the thermal boundary resistance between an individual carbon nanotube end and a au surface, ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011
Heat and Mass Transport Processes, 10.1115/imece2011-62844, 301-302, 2011. |
| 98. |
Koji Takahashi, Masato Inoue, Yohei Ito, Defective carbon nanotube for use as a thermal rectifier, Japanese journal of applied physics, 10.1143/JJAP.49.02BD12, 49, 2 PART 2, 2010.12, 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.. |
| 99. |
Koji Takahashi, Yohei Ito, Tatsuya Ikuta, A graphene chain acts as a long-distance ballistic heat conductor, 2010 14th International Heat Transfer Conference, IHTC 14
2010 14th International Heat Transfer Conference, IHTC 14, 10.1115/IHTC14-22289, 331-336, 2010.12, 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.. |
| 100. |
Yohei Ito, Taku Higuchi, Koji Takahashi, Submicroscale flow sensor employing suspended hot film with carbon nanotube fins, Journal of Thermal Science and Technology, 10.1299/jtst.5.51, 5, 1, 51-60, 2010.10, 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.. |
| 101. |
Koji Takahashi, Yohei Ito, Tatsuya Ikuta, A GRAPHENE CHAIN ACTS AS A LONG-DISTANCE BALLISTIC HEAT CONDUCTOR, Proceedings of the International Heat Transfer Conference IHTC14, August 8-13, 2010, Washington, DC, USA,, IHTC14-22289, 2010.08. |
| 102. |
Hiroshi Takamatsu, Kyosuke Inada, Satoru Uchida, Koji Takahashi, Motoo Fujii, Feasibility study of a novel technique for measurement of liquid thermal conductivity with a micro-beam sensor, International Journal of Thermophysics, 10.1007/s10765-009-0700-5, 31, 4-5, 888-899, 2010.05, 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.. |
| 103. |
Hiroshi Takamatsu, Kyosuke Inada, Satoru Uchida, Koji Takahashi, Motoo Fujii, Feasibility study of a novel technique for measurement of liquid thermal conductivity with a micro-beam sensor, International Journal of Thermophysics, 10.1007/s10765-009-0700-5, 31, 4-5, 888-899, 2010.05, 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.. |
| 104. |
Koji Takahashi, Masato Inoue, and Yohei Ito, Defective Carbon Nanotube for Use as a Thermal Rectifier, Japanese Journal of Applied Physics, 49, 02BD12, 2010.04. |
| 105. |
Yohei Ito, Taku Higuchi and Koji Takahashi, Submicroscale Flow Sensor Employing Suspended Hot Film with Carbon Nanotube Fins, Journal of Thermal Science and Technology, Vol. 5, No. 1, pp.51-60, 2010.01. |
| 106. |
Jun Hirotani, Satoshi Kai, Tatsuya Ikuta, Koji Takahashi, Experimental study on interfacial thermal resistance between carbon nanotube and solid material, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 10.1299/kikaib.76.769_1412, 76, 769, 1412-1419, 2010.01, 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.. |
| 107. |
Y. Ito, M. Inoue, Koji Takahashi, One-dimensionality of phonon transport in cup-stacked carbon nanofibers, Journal of Physics Condensed Matter, 10.1088/0953-8984/22/6/065403, 22, 6, 2010, 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.. |
| 108. |
Koji Takahashi, Jun Hirotani, Satoshi Kai, Tatsuya Ikuta, Carbon nanotube thermal probe using platinum nano hot-film, ASME 2009 Micro/Nanoscale Heat and Mass Transfer International Conference 2009, MNHMT2009
Proceedings of the ASME Micro/Nanoscale Heat and Mass Transfer International Conference 2009, MNHMT2009, 10.1115/MNHMT2009-18356, 2, 457-462, 2010, 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.. |
| 109. |
Koji Takahashi, Norsyazwan Hilmi, Yohei Ito, Tatsuya Ikuta, Xing Zhang, Measurement of the thermal conductivity of nanodeposited material, International Journal of Thermophysics, 10.1007/s10765-009-0666-3, 30, 6, 1864-1874, 2009.12, 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.. |
| 110. |
Koji Takahashi, Yohei Ito, Tatsuya Ikuta, Xing Zhang, Motoo Fujii, Experimental and numerical studies on ballistic phonon transport of cup-stacked carbon nanofiber, Physica B: Condensed Matter, 10.1016/j.physb.2009.05.001, 404, 16, 2431-2434, 2009.08, 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.. |
| 111. |
Yohei Ito, Koji Takahashi, Motoo Fujii and Xing Zhang, Estimating Error in Measuring Thermal Conductivity Using a T-type Nanosensor, Heat Transfer - Asian Research, Vol. 38, No.5, pp. 297-312, 2009, 2009.05. |
| 112. |
Ma Wei-Gang, Wang Hai-Dong, Zhang Xing, Takahashi Koji, Different effects of grain boundary scattering on charge and heat transport in polycrystalline platinum and gold nanofilms, CHINESE PHYSICS B, 10.1088/1674-1056/18/5/051, 18, 5, 2035-2040, 2009.05, 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.. |
| 113. |
Yohei Ito, Koji Takahashi, Tatsuya Ikuta, Xing Zhang, Effect of underetching on thermal conductivity measurement of suspended nanofilm, Japanese Journal of Applied Physics, 10.1143/JJAP.48.05EB01, 48, 5 PART 3, 2009.05, 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.. |
| 114. |
Naoki Yoshihara, Hiroki Ago, Kenta Imamoto, Masaharu Tsuji, Tatsuya Ikuta, Koji Takahashi, Horizontally aligned growth of single-walled carbon nanotubes on a surface-modified silicon wafer, Journal of Physical Chemistry C, 10.1021/jp810036t, 113, 19, 8030-8034, 2009.05, 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.. |
| 115. |
Masamichi Kohno, Takashi Nishizono, Yasunori Onaka, Sumitomo Hidaka, Koji Takahashi, Yasuyuki Takata, Micro oscillation heat pipe fabricated on silicon wafer, 6th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM2008
Proceedings of the 6th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM2008, 10.1115/ICNMM2008-62082, 1343-1346, 2008.12, 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.. |
| 116. |
Xing Zhang, Koji Takahashi, Motoo Fujii, Charge and heat transport in polycrystalline metallic nanostructures, Chinese Physics Letters, 10.1088/0256-307X/25/9/071, 25, 9, 3360-3363, 2008.09, 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.. |
| 117. |
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⁄(m·K) to 0.73 W⁄(m·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 μm 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.. |
| 118. |
Deposition Rate and Position Shifting of Paraffin-Based EBID. |
| 119. |
Koji Takahashi, Yohei Ito, Tatsuya Ikuta, Takashi Nishiyama, Motoo Fujii, Xing Zhang and Andrzej Huczko, , Thermal conductivity of SiC nanowire formed by combustion synthesis, High Temperatures-High Pressures, Vol. 37, Number 2, pp.119-125, 2008, 2008.04. |
| 120. |
Measuring Thermal Conductivity of Nano Deposition Usinh Pt Hot Film. |
| 121. |
Estimating Error of Measuring Thermal Conductivity Using T-Type Nano Sensor. |
| 122. |
Yohei Ito, Koji Takahashi, Xing Zhang, Measuring Thermal Conductivity of Nano Deposition Usinh Pt Hot Film, Thermal science and engineering, 16, 3, 105-111, 2008. |
| 123. |
Koji Takahashi, Yohei Ito, Tatsuya Ikuta, Takashi Nishiyama, Motoo Fujii, Xing Zhang, Andrzej Huczko, Thermal conductivity of SiC nanowire formed by combustion synthesis, High Temperatures - High Pressures, 37, 2, 119-125, 2008, 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.. |
| 124. |
Bingyang Cao, Qingguang Zhang, Xing Zhang, Takahashi Koji, Ikuta Tatsuya, Wenming Qiao, Fujii Motoo, Experimental study on the in-plane thermal conductivity of Au nanofilms, Progress in Natural Science, 10.1080/10020070612331343248, 17, 2, 212-216, 2007.02, 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.. |
| 125. |
Naoki Ishigami, Hiroki Ago, Koji Takahashi, Masaharu Tsuji, Microreactor utilizing a vertically-aligned carbon nanotube array grown inside the channels, Chemical Communications, 16, 1626-1628, 2007. |
| 126. |
Motoo Fujii, Xing Zhang, Koji Takahashi, Measurements of thermal conductivity of individual carbon nanotubes, Physica Status Solidi (B) Basic Research, 10.1002/pssb.200669194, 243, 13, 3385-3389, 2006.11, 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.. |
| 127. |
Influence of Grain Boundary Scattering on the Electrical and Tthermal Conductivities of Polycrystalline Gold Nanofilms. |
| 128. |
Xing Zhang, Huaqing Xie, Motoo Fujii, Koji Takahashi, Tatsuya Ikuta, Hiroki Ago, Hidekazu Abe, Tetsuo Shimizu, Experimental study on thermal characteristics of suspended platinum nanofilm sensors, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2006.04.016, 49, 21-22, 3879-3883, 2006.10, 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.. |
| 129. |
Q. G. Zhang, B. Y. Cao, X. Zhang, M. Fujii, K. Takahashi, Influence of grain boundary scattering on the electrical and thermal conductivities of polycrystalline gold nanofilms, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.74.134109, 74, 13, 2006, 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.. |
| 130. |
Q. G. Zhang, X. Zhang, B. Y. Cao, M. Fujii, K. Takahashi, T. Ikuta, Influence of grain boundary scattering on the electrical properties of platinum nanofilms, Applied Physics Letters, 10.1063/1.2338885, 89, 11, 2006, 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.. |
| 131. |
Porous silicon as a proton exchange membrane for micro fuel cells. |
| 132. |
MEMS Technology and Space Propulsion. |
| 133. |
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. |
| 134. |
MEMS Fabrication and Micro thruster. |
| 135. |
Micro Motor Using Micro Bubbles. |
| 136. |
Koji Takahashi, Kunihito Nagayama, Tetsuo Yasaka, Tanemasa Asano, Micro-isolation valve for fuel tank array, 2001 ASME International Mechanical Engineering Congress and Exposition
ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2, 2845-2850, 2001, 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.. |
| 137. |
Chou, F. C., J. R. Lukes, X. G. Liang, K. Takahashi, and C. L. Tien, Molecular Dynamics in Microscale Thermophysical Engineering, Annual Review of Heat Transfer, Vol. 10, pp.141-176, 1999 , 1999.08. |
| 138. |
Koji Takahashi, Jian Gang Weng, Chang Lin Tien, Marangoni effect in microbubble systems, Microscale Thermophysical Engineering, 10.1080/108939599199729, 3, 3, 169-182, 1999.08, 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).. |
| 139. |
Personal Memory of Stay in Chancellor's Laboratory : How to start new topics in USA. |
| 140. |
J. R. Lukes, Xingang Liang, Koji Takahashi, Chang Lin Tien, Molecular Dynamics in Microscale Thermophysical Engineering, Annual Review of Heat Transfer, 10.1615/AnnualRevHeatTransfer.v10.60, 10, 141-176, 1999. |
| 141. |
Koji Takahashi, Computational Fluid Dynamics of Collapsing Bubble with Micro-jet, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 64, 625, 2874-2879, 1998.01. |
| 142. |
Satoshi Mohri, Ryoji Doihara, Koji Takahashi, Kunihito Nagayama, Time intervals of the cavitation bubble's collapse near a free surface, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 10.1299/kikaib.64.2897, 64, 625, 2897-2901, 1998, 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.. |
| 143. |
Numerical Analysis on Unsteady Motion of Cavitation Bubble by CIP Method. |
| 144. |
Koji Takahashi, Optical Measurement of Sound Field by Laser Differential Interferometry, 可視化情報学会誌 = Journal of the Visualization Society of Japan, 16, 119-122, 1996, 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.. |
| 145. |
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.. |
| 146. |
Koji Takahashi, Shojiro Kaji, Analytical study on plate edge noise. (Trailing edge noise caused by vorticity waves), JSME International Journal, Series 2: Fluids Engineering, Heat Transfer, Power, Combustion, Thermophysical Properties, 10.1299/jsmeb1988.34.4_431, 34, 4, 431-438, 1991.01, 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.. |
