Papers by Author: Koichi Niihara

Abstract: The nitrogenated diamond-like carbon films (a-C:H:N) were deposited on Si-wafer by the rf-PECVD method with the addition of nitrogen to the mixture gas of methane and hydrogen. We were investigating the effect of the additive nitrogen gases and annealing in relationship between bonding structure and electrical properties of the deposited films. The electrical conductivity of films increased with the flow rate of nitrogen increasing up to 10 sccm. Also as annealing temperature was increased, the electrical conductivity of films increased. The structure analysis results show that an increase of the flow rate of nitrogen and annealing temperature favor the formation of sp2 bonding in the films. Therefore, we confirmed that the increase of the electrical conductivity is due to structure change by graphitization of the films.

Abstract: The TiN/DLC nanocomposite coatings were grown on Si wafers using Ar/CH4/TDMAT (Ti[(CH3)2N]4N2) gas mixtures by r.f. plasma enhanced chemical vapor deposition. The sliding friction tests were carried out using a ball-on-flat type tribometer. The different test parameters such as applied loads, counterpart materials and environment were applied to understand the tribological behavior in terms of friction and wear. The coatings provided a low friction coefficient and high wear resistance depending on the friction test conditions.

Abstract: The microstructure and mechanical properties including wear resistance of Al2O3-based nanocomposites with 5 vol% of Cu and Ni-Co dispersions were investigated. Al2O3/Cu and Al2O3/ Ni-Co nanocomposites were fabricated by hydrogen reduction and sintering process using metal oxide and metal nitrates. The composites showed homogeneous microstructures with nano-sized metal dispersions and enhanced fracture strength and toughness compared with monolithic Al2O3. In particular, high toughness and hardness were measured for the Al2O3/Ni-Co nanocomposite consolidated by PECS. A minimum wear coefficient of 2.33 x 10-5 mm3/Nm was obtained for the Al2O3/Ni-Co nanocomposite, while the monolithic Al2O3 showed a value of 2.0 x 10-5 mm3/Nm. Wear behavior is discussed in terms of microstructure and mechanical properties of the nanocomposites.

Abstract: 6061 aluminum alloy composite reinforced with α-alumina and aluminum borate is fabricated by casting method. The above mentioned composite has been compared to a similar composite without aluminum borate reinforcement for their machinability. The aluminum borate filler was controlled up to (2.5, 5.0 and 30vol %) on the surface of α-alumina. The reaction zone between alumina and the matrix in both the systems are analyzed with the help of TEM micrographs as well as X-ray diffraction profiles. At the reaction zone between alumina and the aluminum alloys, some spinel-like compounds (MgAl2O4) are identified. Finally, machinability was analyzed with the single point tool machining.

Abstract: A novel transparent polycrystalline silicon nitride was fabricated by hot-press sintering with MgO and AlN as additives. The mixed powder with 3 wt.% MgO and 9 wt.% AlN was sintered at 1900oC for 1 hour under 30 MPa pressure in a nitrogen gas atmosphere. Transparent polycrystalline silicon nitride was successfully fabricated. The mechanical properties such as density, hardness, young’s modulus, fracture strength and fracture toughness were evaluated. The effect of α/β phase on the mechanical properties of transparent polycrystalline silicon nitride was investigated. The properties were changed depending on the amount of α/β phase. The hardness and Young's modulus increased with increasing the volume fraction of α-phase fraction as a reflection of the higher hardness of α-phase Si3N4. The fracture toughness and fracture strength decreased with decreasing the volume fraction of β-phase Si3N4.

Abstract: We have investigated a synthesis of metal (Nb, V, Cr, Mn, Co) -doped titania nanotubes using a solution chemical processing in order to control optical and electrical properties. Titania nanotubes doped with a small amount of cations up to 1 wt% exhibited similar morphology and XRD pattern as the pure titania nanotubes, however, color of nanotubes was changed depending on the dopants. It was found that Cr, Mn and Co doped titania nanotubes formed new absorption bands in UV spectra. On the other hand, electrical resistivity of doped titania nanotubes was lower than that of pure titania nanotubes.

Abstract: Nano-sized metal particles have recently attracted considerable interest owing to their application potential. Such particles can be synthesized using physical and chemical methods. In this study, nano-sized noble metals were synthesized through the reaction of metal oxides by ultrasonic. This means that the chemical reactions which take place under conventional conditions can be accelerated by ultrasonic cavitations. In general, the chemical effects of ultrasonic irradiation fall into three areas: homogeneous sonochemical efffects of liquids, heterogeneous sonochemical effects of liquid-liquid or liquid-solid systems, and sonocatalytic effects. It has been proposed that liquid-liquid systems are used for the fabrication of nano-metal particles in the past. In this study, the fabrication of nano-metal particles and supported composites was investigated for the liquid-solid system from a viewpoint of economy and ecology. By choosing suitable conditions, it is reasonable to expect that these simple ultrasonic processes can be extended to obtain nano-sized metal particles. Thus applications by using these reactions were investigated to prepare the nano-sized metal particle supported materials, and mechanisms were investigated.

Abstract: Copper nanosized powders were prepared by pulsed wire discharge (PWD) in N2 gas at low pressure (130 Pa) with high-speed gas puff. The pressure around one of the electrodes was changed to investigate the effect of electrical discharge prevention by gas puff. The peak pressure was changed from 22 kPa to 10 kPa in the gas puff apparatus by changing the distance from an acrylic resin tube and a bottom plate. The energy deposition in the copper wire at 22, 15 and 10 kPa was about 40, 37 and 33 J, respectively. The energy deposition in the wire increased with the increase in pressure around the electrode.

Abstract: Chromium oxynitride (Cr(N,O)) thin film have been successfully prepared by using pulsed laser deposition. The composition of the thin film was determined to be Cr0.50N0.23O0.28 by Ruthreford backscattering spectroscopy (RBS). The structural analysis was carried out by using X-ray diffraction (XRD), and out-of-plane and in-plane measurements were used to clarify the axial ratio (c/a) of the Cr(N,O) phase. The lattice constants of a and c axes in the Cr(N,O) phase were found to be 0.414 and 0.419 nm, respectively. From these results, the cubic to tetragonal phase change by substitution of the oxygen atoms for nitrogen atoms was confirmed for the crystal Cr(N,O) compounds.

Abstract: Nanosized MgAl2O4 powder was prepared following a polymerized complex oxide method. Size of the synthesized powders was 7 to 45 nm depending on the calcination temperature. Powder synthesized at lower temperature showed excellence sinterability. The relative density of the sintering body using the powder synthesized at 700°C reached up to 99.8 % at 1550°C for 3 h in air without any sintering additive.