Kei Ishikawa

Thermal Conductivity Measurement of Vertically Aligned Single-Walled Carbon Nanotubes Utilizing Temperature Dependence of Raman Scattering

We present a method for measuring the thermal conductivity and the thermal contact resistance between the film and the substrate of vertically-aligned single-walled carbon nanotubes (VA-SWNTs) grown on Si substrate by ACCVD (Alcohol Catalytic Chemical Vapor Deposition) method, utilizing temperature dependence of the Raman spectrum obtained from SWNTs. The method utilizes the excitation laser of the Raman system to heat the VA-SWNT film and measure the temperature simultaneously. The method finds the thermal conductivity of the VA-SWNT film to be around 1 Wm−1 K−1 and the thermal contact resistance between the substrate and the film to be around 10−5 ∼10−6 m2 KW−1 . The obtained film thermal conductivity is converted into equivalent thermal conductivity of an individual SWNT, whose value is several tens of Wm−1 K−1 , and is more than an order of magnitude smaller than the reports on individual SWNTs.Copyright

Scattering of Monatomic Gas Molecules on Vertically Aligned Single‐Walled Carbon Nanotubes

The scattering process of He molecules on vertically aligned single‐walled carbon nanotubes (VA‐SWNTs) was investigated using the molecular beam technique. The accommodation coefficients for VA‐SWNT films on quartz substrates at room temperature are remarkably high compared to those for bare surfaces, demonstrating the effectiveness of the surface modification technique with VA‐SWNT films for enhancing the energy transfer between gas molecules and surfaces. The enhanced energy transfer suggests that gas molecules can easily penetrate deep into the films because of their high porosity and suffer multiple collisions with bundles of SWNTs. The less effective energy accommodation at elevated temperatures, however, implies that the average number of collisions which gas molecules undergo before leaving the films is not large enough to maintain the perfect accommodation even at high temperature.

Enhanced thermal conductivity of water with surfactant encapsulated and individualized single-walled carbon nanotube dispersions

In the present work, the effective thermal conductivity of single walled carbon nanotube dispersions in water was investigated experimentally. Single-walled carbon nanotubes (SWNTs) were synthesized using the alcohol catalytic chemical vapour deposition method. The diameter distribution of the SWNTs was determined using resonance Raman spectroscopy. Sodium deoxycholate (SDC) was used as the surfactant to prepare the nanofluid dispersions. Photoluminescence excitation spectroscopy (PLE) reveals that majority of the nanotubes were highly individualized when SDC was employed as the surfactant. The nanofluid dispersions were further characterized using transmission electron microscopy, atomic force microscopy (AFM) and optical absorption spectroscopy (OAS). Thermal conductivity measurements were carried out using a transient hot wire technique. Nanotube loading of up to 0.3 vol% was used. Thermal conductivity enhancement was found to be dependent on nanotube volume fraction and temperature. At room temperature the thermal conductivity enhancement was found to be non-linear and a maximum enhancement of 13.8% was measured at 0.3 vol% loading. Effective thermal conductivity was increased to 51% at 333 K when the nanotube loading is 0.3 vol%. Classical macroscopic models fail to predict the measured thermal conductivity enhancement precisely. The possible mechanism for the enhancement observed is attributed to the percolation of nanotubes to form a three-dimensional structure. Indirect effects of Brownian motion may assist the formation of percolating networks at higher temperature thereby leading to further enhancements at higher temperature.Copyright

Energy accommodation of gas molecules with free‐standing films of vertically aligned single‐walled carbon nanotubes

The scattering process of gas molecules on vertically aligned single‐walled carbon nanotubes (VA‐SWNTs) was investigated by the molecular beam technique. To investigate interactions between VA‐SWNT films themselves and helium gas molecules without the presence of substrates, free‐standing films were used. The scattered molecules are divided into three components; reflected molecules, diffusively transmitted molecules, and directly transmitted molecules without interaction with SWNTs. Even with the thin film, most molecules have interacted with the films, which suggests that most molecules have interacted at the randomly oriented layer at the topmost of the films. Because of low accommodation coefficients of helium gas, VA‐SWNTs films are thought to be useful as a surface modification to enhance energy accommodation.

Deposition of Various Metals on Vertically-Aligned Single-Walled Carbon Nanotubes

Evaporation of different metals (Au, Ti, Al and Pd) onto Vertically-Aligned Single-Walled Carbon Nanotubes (VASWNT) has been studied. Observations through Scanning Electron Microscopy (SEM) showed a clear metal-dependence of the deposition layer structure on top of the VASWNT, reflecting the variation of wettability and cohesive energy of each metal. These characteristics also influence the structures of the metal penetrated through the top surface into VASWNT film, where metal forms particles inside VASWNT film except for Ti. A simple annealing technique to remove metals penetrated in the SWNT films is demonstrated. Some peculiar morphologies found during the processes are also presented.Copyright