Yutaka Kagawa

Phonons with long mean free paths in a-Si and a-Ge

We investigated phonons with long mean free paths (MFPs) in amorphous Si (a-Si) and amorphous Ge (a-Ge). The thermal conductivity of a-Si and a-Ge thin films prepared by magnetron sputtering was found to depend on film thickness and deposition temperature. From the film thickness dependence, we conclude that phonons with MFPs longer than 100u2009nm contribute to heat transport in a-Si and a-Ge. Also, as deposition temperature was increased, phonons with MFPs ranging from 100 to 250u2009nm in a-Si and from 15 to 250u2009nm in a-Ge increased.

Thermal conductivity of sputtered amorphous Ge films

We measured the thermal conductivity of amorphous Ge films prepared by magnetron sputtering. The thermal conductivity was significantly higher than the value predicted by the minimum thermal conductivity model and increased with deposition temperature. We found that variations in sound velocity and Ge film density were not the main factors in the high thermal conductivity. Fast Fourier transform patterns of transmission electron micrographs revealed that short-range order in the Ge films was responsible for their high thermal conductivity. The results provide experimental evidences to understand the underlying nature of the variation of phonon mean free path in amorphous solids.

Fabrication of micromodel grid for various moiré methods by femtosecond laser exposure

The femtosecond laser exposure system was used to fabricate model grids for the charge-coupled device (CCD) moiré method, scanning laser moiré method, and electron moiré method for microstrain deformation measurements. The femtosecond laser exposure produces mesoscopic variation patterns on the surface. These variation patterns make the grid in the scanning laser microscope and CCD images darker and make the grid in the scanning electron microscope image brighter. The CCD moiré fringe, scanning laser moiré fringe, and electron moiré fringe consisting of bright and dark lines were generated. As a demonstration, microstrain distribution of the three-point bending tested specimen was measured.

Thermal boundary resistance at Si/Ge interfaces by molecular dynamics simulation

In this study, we investigated the temperature dependence and size effect of the thermal boundary resistance at Si/Ge interfaces by non-equilibrium molecular dynamics (MD) simulations using the direct method with the Stillinger-Weber potential. The simulations were performed at four temperatures for two simulation cells of different sizes. The resulting thermal boundary resistance decreased with increasing temperature. The thermal boundary resistance was smaller for the large cell than for the small cell. Furthermore, the MD-predicted values were lower than the diffusion mismatch model (DMM)-predicted values. The phonon density of states (DOS) was calculated for all the cases to examine the underlying nature of the temperature dependence and size effect of thermal boundary resistance. We found that the phonon DOS was modified in the interface regions. The phonon DOS better matched between Si and Ge in the interface region than in the bulk region. Furthermore, in interface Si, the population of low-frequen...

Porous Al2O3/Al catalyst supports fabricated by an Al(OH)3/Al mixture and the effect of agglomerates

Porous Al2O3/Al catalyst supports were fabricated using a mixture of Al(OH)3 and Al powders, followed by pressureless sintering at a temperature of 600 °C in vacuum. Different pressures were used to prepare green compacts. High compaction pressure led to a high surface area and good mechanical and electrical properties for the sintered specimens. However, when the Al content in the sintered specimen exceeded a definite value, high compaction pressure decreased the surface area abruptly. Scanning electron microscopy observations revealed that agglomeration in the starting mixture has a significant effect on the microstructure of the sintered specimens. High compaction pressure greatly eliminated the agglomerates and led to a uniform microstructure for the sintered specimens. However, when the Al content in the starting mixture was too high, Al particles in the compacts prepared by the high pressure were largely sintered due to the high compact density so that most of the pores were closed. The present study indicates that a suitable compaction pressure is critical to obtaining superior Al2O3/Al supports.

Deformation measurement of carbon fiber reinforced plastics using phase-shifting scanning electron microscope Moiré method after Fourier transform

The deformation distributions of carbon fiber reinforced plastics (CFRP) under a three-point bending load were nondestructively investigated using the phase shifting scanning electron microscope (SEM) moiré method. The complex fast Fourier transform (FFT) and the discrete Fourier transform (DFT) were used to filter the useless moiré fringes in the case of bidirectional moiré fringes. The SEM moiré fringes under different magnifications and the deformation results measured by the direct, complex FFT- and the DFT- phase shifting moiré methods as well as the moiré fringe centering method were compared and analyzed. Experiments demonstrate that the deformation measurement is a bit influenced by the useless moiré fringes in the phase shifting moiré methods and complex FFT processing works better for nondense moiré fringes. The relative strain changes gradually and the specimen grating pitch increases gradually from top to bottom along the loading direction, suggesting that the real compressive strain is greater in the upper side. The micro/nano-scale deformation distribution characteristic is helpful for better understanding of the mechanical properties of the CFRP specimen.

Measurement of Strain Distribution of Composite Materials by Electron Moiré Method

It is very important to measure the local strain and stress distributions for understanding the mechanical properties of structural materials. Therefore, there are many techniques to measure the strain or stress distribution such as the strain gage method, optical elasticity method, optical Moire method, etc. In these methods, the optical Moire method (Weller & Shepard, 1948; Morse et al., 1960; Sciammarela & Durelli, 1961; Durelli & Parks, 1970; Theocaris, 1969; Post et al, 1994; Chiang, 1982; Post, 1988) is one of the convenient methods to measure the deformation of the materials. However, these methods are difficult to apply for deformation measurements from a microscopic aspect. To measure the micro-deformation in a very small area, the authors have developed an electron Moire method (Kishimoto et al., 1991, 1993) , and J.W. Dally, D. T. Read (Read & Dally, 1994; Dally &.Read, 1993) and H. Xie (Xie et al., 2007) advocated it. This method keeps the main advantages of the moire and laser moire interferometry methods, and has the additional ability of measuring deformation in a micro-area with a high sensitivity. Besides, the electron moire method also uses a wide range for measuring the deformation. The range of the measurable deformation is from 25microns to 0.1micron using a model grid with different pitches. To measure the micro-deformation i.e. sliding and slip lines in a smaller area, micro-grid method is very useful. Compare these two methods, electron moire method is easy to understand the strain distribution and the large sliding (Kishimoto et al., 1991, 1993). In this study, In order to pursue the application of the electron moire method, some typical experiments were performed. The strain distribution at the interface of the laminated steel, strain distribution of the fiber and the matrix in the fiber reinforced plastic, the thermal strain in or around the metallic fiber in Al alloy were observed.

Measurement of strain distribution in smart materials by electron Moiré method

A method for measuring the stress and strain distribution in composite materials and the residual stress near the interface in smart composite has been developed. The strains are measured using electron Moiré method. In this method a very fine model grid is fabricated using the optical and electron lithography techniques on the surface of the specimen and an electron beam scan of which the spaces are almost same as that of the model grid used for the master-grid. The difference in the amount of secondary electrons per a primary electron produces the Moiré fringes that consist of bright and dark parts. The residual strain and stress around the fibers of the smart composite materials and thermal expansion ratio of a fiber and Al matrix were measured by this method.