Papers by Author: Takeshi Ohgai

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Authors: Masayuki Mizumoto, Takeshi Ohgai, Akio Kagawa
Abstract: In order to develop the alumina fiber reinforcements optimized to FRMMCs, the effect of characteristics of alumina fibers on the fabrication process and the characteristics of the alumina fiber reinforced Al alloy composites was investigated. Alumina fibers which have different alumina content were prepared. Alumina content in the fibers was varied from 80% to 100%. Al-4mass%Cu alloy, Al-12mass%Si alloy and Al-10masss%Mg alloy were used as matrix. The FRMMC specimens were fabricated by a low-pressure infiltration process (LPI process). The formability of the preform was improved with increasing alumina content in the fibers. However, broken fibers were observed in the preform when alumina fibers with high alumina content were used. The number of the broken fibers seemed to be increased with increasing alumina content in the fibers. This result could be attributable to a change of fiber strength resulting from a change of alumina content in the fiber. The FRMMC specimens were characterized by using Vickers hardness test. The Vickers hardness of FRMMC specimens depended on the elasticity or the hardness of the fibers. The results obtained suggest that the characteristics of the FRMMCs largely depend on the intrinsic characteristics of the reinforcement fibers.
Authors: Takeshi Ohgai, Keizo Takao, Masayuki Mizumoto, Akio Kagawa, Yoshitomo Tanaka, Shigekazu Sumita
Abstract: Ni-Fe alloy films and nanowires were fabricated using electrodeposition technique. The cylindrical shape of nanowires was precisely transferred from the nanochannels of membrane filters and the aspect ratio reached to around 60. Coercive force in in-plan direction of Ni-Fe alloy films decreased to ca. 1 Oe with increasing Fe content in deposits while, in perpendicular direction, the films were hardly magnetized. Magnetic hysteresis loops revealed that the nanowires were spontaneously magnetized to the long axis direction and the coercive force reached to ca. 200 Oe.
Authors: Takeshi Ohgai, Masayuki Mizumoto, Shigeki Nomura, Akio Kagawa
Abstract: A polycarbonate membrane filter with numerous cylindrical nanopores was used as a template for growing metallic nanowires such as Ni, Co and Fe. The nanoporous template with pore-diameter of 150 nm, pore-length of 6000 nm, and pore-density of 108 pore•cm-2 was modified as a cathode with sputter-deposited gold layer. Inside the nano-pores, the metallic nanowires were electrochemically deposited from an acidic sulfate solution containing metal ions. The growth rate of metallic nanowires depended on the cathode potential during electrodeposition. The diameter of electrodeposited nanowires corresponded to that of nanopores in the template. TEM diffraction pattern suggested that each metallic nanowire composed of a single crystalline structure.
Authors: Takeshi Ohgai, Takashi Ikeda, Yasuyuki Kawanaka, Keizo Takao, Akio Kagawa
Abstract: ZnTe compound semiconductors were synthesized in acidic aqueous solution using a pulsed current electrodeposition technique. The optimum condition to obtain ZnTe deposits was determined by the cathodic polarization curves measured at a wide potential range. During the co-deposition of Zn and Te, under potential deposition (UPD) of Zn was observed. Increasing the solution temperature up to 353 K, UPD of Zn was promoted by the formation of Zn(OH)2. Crystal phase, structure and chemical composition of electrodeposited ZnTe was controlled by the solution composition and electrolysis condition. The band gap energy of ZnTe films annealed at 573 K was close to 2.26 eV.
Authors: Takeshi Ohgai, Keisuke Hashiguchi, Takao Morimura, Keizo Takao, Akio Kagawa
Abstract: Co/Cu multilayered nanowires with 40 nm in diameter were fabricated using a pulsed current deposition technique into a nanoporous template with numerous nanochannels. To determine the optimum electrodeposition condition of Cu and Co into the template, cathodic polarization behavior was examined at a wide range of cathode potential. Time-dependence of deposition current was monitored to determine the growth rate of Co and Cu nanowires. Co layer and Cu layer thicknesses were adjusted to several tens nanometers, by controlling the deposition times. With decreasing the each layer thickness, the coercive force of Co/Cu multilayered nanowires was decreased and the soft magnetic property was improved.
Authors: Takeshi Ohgai, Takafumi Fujimaru, Keizo Takao, Akio Kagawa
Abstract: To synthesize an array of numerous ferromagnetic nanowires, iron-group metals such as Ni, Co, Fe, Ni-Fe and Co-Fe alloys were electrodeposited from aqueous solution into a nanoporous template with numerical cylindrical nanochannels . The shape of nanowires was precisely transferred from the nanochannel template and the aspect ratio reached to around 150. Magnetic hysteresis loops revealed that Ni, Co and Fe nanowires were spontaneously magnetized to the long axis direction. Coercive force of the nanowires with 6000 nm in length was increased in decreasing the pore-diameter. The coercive force of Co nanowires with 40 nm in diameter has increased up to 1084 Oe.
Authors: Hayato Sakaki, Masayuki Mizumoto, Takeshi Ohgai, Akio Kagawa
Abstract: In order to develop a new application of cast iron, high niobium cast iron has been developed as a grain refiner for stainless steel. High niobium cast iron was prepared by adding pure niobium to a commercial cast iron. Coarse primary niobium carbide crystals were observed in the microstructure of the cast iron. The effect of the high niobium cast iron as an inoculant on the grain size of austenitic and ferritic stainless steels was examined in various experimental conditions. When the amount of the cast iron inoculant more than 3 mass% was added into the steel melt, fine equiaxed grains were observed and grain size was significantly reduced to 210 μm. The results indicate that the high niobium cast iron is effective as a grain refiner for the austenitic and ferritic stainless steels. From the dissolution rate measurement, the grain refining mechanism was proposed.
Authors: Masayuki Mizumoto, Takeshi Ohgai, Akio Kagawa
Abstract: To develop a novel separation technique of matrix alloys from metal matrix composite, separation experiments for various kinds of particle reinforced metal matrix composites (PRMMCs) were carried out. The Al-4mass%Cu alloy, Al-7mass%Si alloy and cast iron were used as matrix. The SiC particles (particle size: 75μm) and Al2O3 particles (particle size: 120μm) were used as reinforcement. The PRMMC specimen was placed in a silica tube container with a small nozzle (nozzle size: 0.75mm) at the bottom and was melted by H.F. induction heating. Then the molten PRMMC specimen was forced to flow out through the nozzle by applying a certain pressure of Ar gas. Most of the molten matrix alloy flowed out through the nozzle and the remainder in the container consisted of the reinforcements and a part of the matrix alloy. The amount of separated matrix alloy increased with decreasing the volume fraction of reinforcement particles in PRMMC specimens. With decreasing the fabrication temperature from 1273K to 1073K, the amount of matrix alloy separated from SiCP/Al-7mass%Si alloy composites increased. It is considered that a reaction layer formed on the surface of SiC particles at 1273K improves the wettability between the molten matrix alloy and SiC particle, which prevents the separation of molten matrix alloy from reinforcements. On the other hand, the amount of separated matrix alloy from 20vol% Al2O3P/cast iron composites was very high due to no reaction layer formed at interface between Al2O3 particle and cast iron.
Authors: Akio Kagawa, Masayuki Mizumoto, Takeshi Ohgai
Abstract: The influence of process parameters on shape stability and surface smoothness of cylindrical and rectangular castings produced by fused spinning deposition (FSD) method was investigated. The optimum conditions to control the shape and smoothness of side surface of the castings produced by the FSD method without water-cooling were limited to a narrow range of process parameters. The casting with a stable shape and a smooth side surface was obtained with the aid of water-cooling. The tensile strength of the castings produced by the FSD method was 1.4 times higher than those of the specimens cast into metal-mold and sand-mold, due to a finer equiaxed grain structure of the FSD castings. Employing the combination of the deposition patterns for tube and solid castings, some practical metallic components were fabricated by the FSD method.
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