Papers by Author: Yoshihiro Hirata

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Authors: Kenji Higashi, K. Sonoda, Haruhiko Ono, Soichiro Sameshima, Yoshihiro Hirata
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Authors: Shuhei Tabata, Yoshihiro Hirata, Soichiro Sameshima, Yoshimitsu Uemura
Abstract: The SiC powder of median size 0.8 µm was mixed with polyacrylic acid (PAA, dispersant) in a 0.3 M-R(NO3)3 solution (R=Yb, Y, Gd, Sm, Nd and La) at pH 5 to adsorb uniformly the sintering additive (R3+ ion) on the SiC surface. The addition of PAA to the SiC suspension with R3+ ion increased the amount of R3+ ion fixed to SiC particles. The aqueous 30 vol% SiC suspension with 0.52 mass% PAA and 1.50 mass% R2O3 (as R3+ ion) against the mass of SiC, was consolidated by filtration through a gypsum mold to form green compacts of 50-52 % of theoretical density. The consolidated green compacts were hot–pressed under a pressure of 39 MPa at 1950°C for 2 h in an Ar flow. The green compacts were densified with grain growth to 76 - 99 % relative density. The addition of smaller R3+ ion was effective to enhance the sinterability of SiC and also to achieve smaller grain size of SiC. The sintering mechanisms were discussed based on the analysis of the shrinkage curves of SiC/R2O3 compacts during the hot-pressing.
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Authors: Tomoyuki Maeda, Yoshihiro Hirata, Joe Sugimoto, Soichiro Sameshima, Toshifumi Yoshidome, Masaki Shibuya
Abstract: A polytitanocarbosilane (20-30 mass%)-xylene solution was infiltrated into a porous laminated composite with 35-40 vol% Si-Ti-C-O fabric of 11 diameter fiber and 15-25 vol% mullite filler, and decomposed at 1000°C in an Ar atmosphere. This polymer impregnation and pyrolysis method was repeated 8 times to produce the composites of 76-82 % theoretical density. The yarn (662-765 filament / yarn), fabric and composite provided the following average strengths : 1240 MPa for the yarn; 768 MPa for the fabric; 117 MPa for the composite. The fracture probability of the yarn, fabric and composite was well fitted by the normal distribution function. The tensile strength of the composite was interpreted by the product of the effective fiber content, the Young's modulus of the fiber and elongation of the composite.
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Authors: Yoshihiro Hirata, N. Numaguchi, W.-H. Shih
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Authors: Taro Shimonosono, Go Hiramatsu, Yoshihiro Hirata, Soichiro Sameshima, Naoki Matsunaga, Toshiya Doi, Teruhisa Horita
Abstract: Electrochemical properties (terminal voltage, ohmic resistance and overpotential) were measured for the cell of indium tin oxide cathode (ITO, 90 mass% In2O3-10 mass% SnO2) or perovskite-type oxide cathode La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) / Gd-doped ceria electrolyte (Ce0.8Gd0.2O1.9, GDC, 600-700 μm thick)/Ni-GDC anode using 3 vol% H2O-containing H2 fuel at 873 and 1073 K. The maximum power densities for the cell with ITO cathode at 873 and 1073 K were 21 and 71 mW/cm2, respectively. Similarly, the maximum power density with LSCF was 12 and 113 mW/cm2 at 873 and 1073 K, respectively. The voltage drop was larger for the cathode than for the electrolyte or anode. The overpotential of the LSCF cathode was comparable to the ohmic resistance.
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Authors: Taro Shimonosono, Go Hiramatsu, Yoshihiro Hirata, Soichiro Sameshima, Naoki Matsunaga, Toshiya Doi, Teruhisa Horita
Abstract: Electrochemical properties (terminal voltage, ohmic resistance and overpotential) were measured for the cells of indium tin oxide (ITO, 90 mass% In2O3-10 mass% SnO2), perovskite-type oxide La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) or SrRuO3 cathode / Gd-doped ceria electrolyte (Ce0.8Gd0.2O1.9, GDC, 600-700 μm thick) / Ni-GDC anode using 3 vol% H2O-containing H2 fuel at 873 and 1073 K. The highest power density was obtained for the cell with SrRuO3 cathode, and was 36 and 328 mW/cm2 at 873 and 1073 K, respectively. The voltage drop was larger for the cathode than for the anode. Both of the ohmic resistance and overpotential were lowest for the SrRuO3 cathode among the investigated cathodes.
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Authors: Go Hiramatsu, Yoshihiro Hirata, Soichiro Sameshima, Naoki Matsunaga
Abstract: Gd-doped ceria electrolyte (Ce0.8Gd0.2O1.9, GDC, 700 μm thick), 30 vol% Ni-GDC cermet anode and perovskite cathode La0.6Sr0.4CoO3 (LSC) or La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) were used to evaluate the electric power of the cell using 3 vol%-H2O containing H2 fuel at 873 and 1073 K. Terminal voltage, ohmic resistance and overpotential were analyzed during the operation of the cell. The maximum power density with LSC and LSCF cathode was 53 and 113 mW/cm2 at 1073 K, respectively. The ohmic resistance and overpotential at the cathode was smaller for LSCF than for LSC.
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Authors: Naoki Furukawa, Yoshihiro Hirata, Soichiro Sameshima, Naoki Matsunaga
Abstract: Biogas of about 60 % CH4 -40% CO2 composition is produced from waste food or drainage. Electrochemical reforming of CH4 with CO2 using a porous gadolinium-doped ceria (GDC) cell is an attractive process to produce a H2-CO fuel used in solid oxide fuel cell. The supplied CO2 is converted to CO and O2- ions by the reaction with electrons at cathode (CO2 + 2e- → CO + O2-). The produced CO and O2- ions are transported to the anode through a porous mixed conductor GDC electrolyte. In the anode CH4 reacts with O2- ions to produce CO, H2 and electrons (CH4 + O2- → CO + 2H2 + 2e-). This process suppresses the carbon deposition from CH4. The formed H2 and CO fuels were supplied to a solid oxide fuel cell with dense GDC electrolyte (Ce0.8Gd0.2O1.9). The open circuit voltage and maximum power density were measured for the reformed gas and for a pure H2 fuel. Little difference in the electric power was measured at 1073 K for both the fuels.
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Authors: Yoshihiro Hirata, Naoki Matsunaga, Nobuhiro Hidaka, Tomoyuki Maeda, Takashi Arima, Soichiro Sameshima
Abstract: This paper reports the significant effects of addition of 30 nm SiC, polytitanocarbosilane and SiC fabric to enhance the mechanical reliability of SiC. The flexural strengths of dense SiC hot-pressed with 800 nm particles (average strength 565 MPa for Y2O3-Al2O3 additives and 640 MPa for Yb2O3-Al2O3 additives) were enhanced to average strength 735-820 MPa by the addition of 30 nm SiC particles (25 vol%). Addition of polytitanocarbosilane (3 vol%, precursor of SiC fiber) to the bimodal SiC powder compact with Y2O3-Al2O3 additives provided more excellent mechanical properties of average strength 910 MPa, fracture toughness 5.2 MPa·m1/2 and Weibull modulus 11.3. SiC fabric and SiC (60 vol%) - Al2O3 (40 vol%) sheet of 60 micrometer thick were alternatively laminated and bonded to the surfaces of dense SiC under the pressure of 5 MPa. The SiC fabric prevented the propagation of the cracks formed by Vickers indentor and showed a significant nonlinear stress-strain curve. As a result, no change in the strength was measured before and after the introduction of cracks.
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Authors: Takashi Arima, Yoshihiro Hirata, Naoki Matsunaga, Masaki Shibuya
Abstract: SiC fabric including SiC (60 vol%)-Al2O3 (40 vol%) mixed powders and SiC (60 vol%)-Al2O3 (40 vol%) sheet of 60 μm thick were alternatively laminated and hot-pressed to 79 - 83 % of theoretical density under a pressure of 39 MPa in an Ar atmosphere at 1600 °C. Four cracks were formed on the polished laminates with 30 ± 3 and 43 ± 2 vol% SiC fabric along the center line perpendicular to the direction of length by Vickers indentor at the load of 98 N and healed in air at 1100 °C for 24 h. The strength (253 MPa) of the as-hot-pressed laminate without SiC fabric decreased to 119 MPa after the introduction of the cracks but was recovered to 336 MPa after the healing. The crack size decreased because of the oxidation of SiC particles during the healing. However, the laminates with SiC fabric had a high damage tolerance and gave no change in the strength (~100 MPa) after the introduction of the cracks. The SiC fabric prevented the propagation of the cracks. After the healing, the porosity of the laminates decreased owing to the volume increase during the oxidation of SiC particles and SiC fabric. The strength of the healed laminates was comparable to that of as-hot-pressed laminates.
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