Papers by Author: Kazuya Kusaka

Abstract: Abaca, also known as Manila hemp, is native to the Philippines, where it is grown as a commercial crop. It belongs to the Musasea family and is cultivated for ropes and fibers. The abaca fiber is a natural fiber with highest cellulose content. In this study, the effects of alkali treatments on mechanical properties and microstructure of abaca fibers are discussed. Abaca fibers were soaked in the aqueous solution of sodium hydroxide at the concentration from 3 to 15wt% for 5 minutes, and subsequently subjected to tensile tests and observations by scanning electron microscopy (SEM) to assess morphological changes caused by the alkali treatment. Fourier transform-infrared spectroscopy analysis demonstrated that the treatment led to the gradual removal of lignin and hemicelluloses from the abaca fibers. The cellulose crystallinity of abaca fibers was analyzed by an X-ray diffraction method. SEM images revealed that the lumen size decreased and shrunk with increasing alkali concentration. The tensile strength of the alkali-treated abaca fibers was higher than that of the untreated ones. The Young’s modulus increased with increasing alkali concentration up to 7wt% and then decreased. However, the strain at break decreased below 7wt% and then increased. The lumen size started to decrease from 7wt% alkali concentration.

Abstract: The structures of Cr-N films deposited by arc ion plating on steel substrates were investigated using a synchrotron radiation system that emits ultraintense X-rays. The Cr-N films were found to be mainly composed of {110} oriented CrN crystals, but they also had a small component of randomly oriented Cr2N crystals. The CrN220 diffraction shifts to a high diffraction angle as the annealing temperature increases. In contrast, the peak position of the Cr2N211 diffraction hardly changes with an increase in the annealing temperature up to 873 K. The ratio of nitrogen and oxygen to chromium at the film surface and inside in the film was estimated by Auger electron spectroscopy. After annealing at 973 K, the surface layer was oxidized, but the composition inside the Cr-N films (N/Cr = 0.83) remained unchanged. The residual stress in a 1600-nm-thick as-deposited CrN layer was found to be -11.0 GPa. The residual stresses of Cr-N films relaxed to thermal stress levels on annealing. However, the residual stress in the Cr2N layer could not be evaluated.

Abstract: Residual stresses near the grain boundary of a bicrystal were measured by synchrotron radiation of SPring-8 at Japan Synchrotron Radiation Research Institute. A copper bicrystal specimen with a 90-degree tilt boundary was deformed 30% in tension. After the plastic extension, kink bands developed in a deformed matrix along the grain boundary. In this study, we focused on the residual stresses in the deformed matrix and the kink band. Residual stresses were evaluated by the X-ray single crystal measurement method. Stereographic projections were used to determine crystal orientations of deformed regions. Our observation showed that crystal orientations were different between the deformed matrix and the kink band. Residual stresses in the direction along the grain boundary in the deformed matrix and kink band were compressive. Residual stresses in the direction vertical to the grain boundary were seen opposite between the deformed matrix and the kink band.

Abstract: Stress-assisted atomic migration occurs in thin films due to thermal stress development, followed by hillock and void formation on a film surface. Relation between thermal stresses and hillock formation was investigated on copper films with and without passivation layer. Copper films with a thickness of 10, 50 and 100 nm on oxidized silicon wafer were prepared for investigating thermal stress and hillock formation. In-situ thermal stress observation by X-ray measurement revealed that compressive stresses develop in an early stage of heating followed by a sudden decrease in the temperature region between 100 and 200 deg. In a cooling stage, stresses in a film linearly changed with decreasing temperature to form a tensile residual stress state. Surface morphology is observed by optical microscope and SEM after the heat cycle as well as at elevated temperatures in a vacuum chamber. Dome-like swells were formed on an AlN passivation layer. Almost of all of the swells on 100 nm thick film collapsed after the heat treatment up to 350 deg whereas the swells on 10 nm thick film had no collapse excepting a few case. Comparing with the film without passivation, the swell is considered to be the result of atomic migration of copper film to form hillocks in the interface between copper film and AlN passivation film during heating. Atoms are considered to migrate reversibly into the copper film in the cooling stage, resulting to make vacant hall in the swell of AlN film and then collapse due to tensile stress development.

Abstract: In this study, evaluation of residual stress in nano-TiO2 film on ITO glass is carried out. The films with thickness less than 30 nm are prepared by the dual-arcs magnetron sputtering with gas pressure 10 and 20 Pa. The surface microstructure and grain morphology of the nano-TiO2 films are observed by the atomic force microscopy (AFM). In order to accurately evaluate the residual stress in the film, the Young’s modulus of the film is determined by the nanoindentation with three point bending method at first, then the internal residual stress in the film is measured by high energy X-ray diffraction with the synchrotron radiation facility Spring-8. The measured residual stresses of nano-TiO2 films prepared with gas pressure 10 and 20 Pa are -11.6 and -9.1 GPa, respectively. It is shown that the residual stress of TiO2 films decrease with the increasing of gas pressure.

Abstract: In-situ observation of thermal stress in thin films deposited on a silicon substrate was made by synchrotron radiations. The specimens prepared in this experiment were nano-size thin aluminum films with SiO2 passivation. The thickness of the films was 10 nm, 20 nm and 50 nm. Residual stress in the as-deposited state was tensile. Compressive stress was developed in the heating cycle up to 300 oC and tensile stresses developed in the cooling cycle. The thermal stresses in the 50 nm film showed non-linear behavior in the first heating cycle from the room temperature to 300 oC. However, they linearly behaved in the first cooling cycle and the second thermal cycle. On the other hand, the thermal stresses in the 10 nm film behaved almost linearly without any hysteresis in the first and the second thermal cycles. The mechanism of thermal stress behavior of thin films is discussed.

Abstract: The crystal orientation and residual stress in gallium nitride (GaN) films deposited on a single-crystal (0001) sapphire substrate using a new sputtering system are examined through x-ray diffraction measurements as part of a study of low-temperature sputtering techniques for GaN. The new rf sputtering system has an isolated deposition chamber to prevent contamination with impurities, and is expected to produce high-purity nitride films. GaN films are deposited at various substrate temperatures and constant gas pressure and input power. This new system is found to produce GaN films with good crystal orientation, with the c-axes of GaN crystals oriented normal to the substrate surface. The crystal size of films deposited at high temperature is larger than that deposited at low Ts. All films except that deposited at 700oC exhibit compressive residual stress, and this residual stress is found to decrease with increasing temperature. Finally, the film deposited at 700 oC was tinged with white, and the surface contained numerous micro-cracks.