Papers by Author: Yoshihiro Hirata

Abstract: Perovskite solid solution of Sr(Zr_1-xAl_x)O_3-x/2 was prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders at x = 0-0.5, heated at 1000°C for 4 h in air, were identified to be an orthorhombic SrZrO₃ solid solution. The highest electrical conductivity of Sr(Zr_1-xAl_x)O_3-x/2 system sintered at 1400°C was measured at x = 0.2 (3.80×10^-4 S/cm at 800°C, activation energy 109 kJ/mol ). The activation energy also showed a maximum value at x = 0.25 .

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 O^2- ions by the reaction with electrons at cathode (CO₂ + 2e^- → CO + O^2-). The produced CO and O^2- ions are transported to the anode through a porous mixed conductor GDC electrolyte. In the anode CH₄ reacts with O^2- ions to produce CO, H₂ and electrons (CH₄ + O^2- → 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 (Ce_0.8Gd_0.2O_1.9). 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.

Abstract: Liquid phase sintering based on the dissolution-precipitation mechanism was applied to densify a 0.8 μm SiC powder with alumina (1.2 vol%)-yttria (0.9-3.3 vol%) additives. To uniformly distribute the sintering additives around the SiC particles, a heterocoagulated particle network was formed among negatively charged SiC particles, positively charged 0.2 μm alumina and yttrium ions in an aqueous suspension at pH 5. Yttrium ions were electrostatically adsorbed on the negatively charged SiC surfaces. The consolidated green compacts were highly sintered to 97-99 % of theoretical density by hot-pressing at 1950 °C. Four-point strength, fracture toughness and Weibull modulus were highly enhanced when a bimodal particle size system of SiC (75 vol% 0.8 micrometer-25 vol% 30 nanometer SiC) was sintered. The maximum strength reached 1.1 GPa. The fracture toughness was about 6 MPa•m1/2 and the Weibull modulus was 5.9. When a small amount of SiC precursor polymer was infiltrated in the green compact, the strength and Weibull modulus were further improved.

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.

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.

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.

Abstract: A bimodal powder system of 800 nm SiC (75 vol%) - 30 nm SiC (25 vol%) was dispersed at 20 vol% solid in a 0.3 M Y(NO3)3 solution containing 0.2 μm Al2O3 and 1.0 mg/m2 polyacrylic acid (PAA: dispersant). The SiC (97.6 vol%)-Al2O3 (1.2 vol%)-Y2O3 (1.2 vol%)-PAA system suspension was consolidated by casting in a gysum mold. Polytitanocarbosilane (PTC) of 3 vol% was infiltrated into the SiC compact calcined at 800 °C to increase the mechanical properties and Weibull modulus. Both the calcined powder compacts with and without PTC were hot-pressed to relative density above 97 % at 1950 °C. The hot-pressed SiC with or without PTC provided the following excellent mechanical properties: average four-point flexural strength of 911 and 812 MPa, fracture toughness of 5.2 and 6.0 MPa·m1/2, and Weibull modulus 11.3 and 5.8 for PTC addition and no addition, respectively. The PTC addition was effective to decrease the shape factor of flaw and increased the strength and Weibull modulus.

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.

Abstract: Terminal voltage, electric power density and overpotential were measured for the solid oxide fuel cell with gadolinium-doped ceria electrolyte (Ce0.8Gd0.2O1.9, GDC), 30 vol% Ni-GDC anode and Pt cathode using a H2 fuel or biogas (CH4 47, CO2 31, H2 19 vol %) at 1073 K. Addition of 1 ppm H2S in the 3vol % H2O-containing H2 fuel gave no change in the open circuit voltage (0.79 - 0.80 V) and the maximum power density (65 - 72 mW/cm2). Furthermore, no reaction between H2S and Ni in the anode was suggested by the thermodynamic calculation. On the other hand, the terminal voltage and electric power density decreased when 1 ppm H2S gas was mixed with the biogas. After the biogas with 1 ppm H2S flowed into the anode for 8 h, the electric power density decreased from 125 to 90 mW/cm2. The reduced electric power density was also recovered by passing 3 vol % H2O-containing H2 fuel for 2 h.

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.