Papers by Author: Soichiro Sameshima

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: Cell performance was measured for four types of Ni (40 vol%)-Gd-doped ceria (GDC) anode-supported solid oxide fuel cells with GDC electrolyte (40-120 μm thickness) of Ce_1-xGd_xO_2-x/2 compositions (x = 0.05, 0.1, 0.15 and 0.2) at 773-1073 K using a H₂ fuel. (La_0.8Sr_0.2)(Co_0.8Fe_0.2)O₃ cathode was printed on the GDC films. The open circuit voltage and maximum power density at 873-1073 K showed a maximum at x = 0.1. The maximum power density at x = 0.1 was 166 and 506 mW/cm² at 873 and 1073 K, respectively. The excess oxygen vacancy at x = 0.1-0.2, which does not contribute to the oxide ion conductivity, reacts with a H₂ fuel to form electrons (H₂ + V_O 2H⁺ + V_O^×, V_O^× V_O + 2e^-). This reaction reduces the cell performance.

Abstract: This paper reports the influence of sintering additives (RE2O3, Al2O3RE2O3, RE = Yb, Y and Gd, 13 vol%) and mixing effect of 30 nm SiC powder with 800 nm SiC powder on phases of grain boundaries, grain size of SiC, fracture toughness and strength of SiC hot-pressed at 1950°C under 39 MPa of applied pressure. Rare earth ions were uniformly adsorbed on negatively charged SiC particles with 150 nm Al2O3 particles in aqueous suspensions at pH 5. A rapid densification of SiC with one component RE2O3 occurred above 1700°C when a liquid of SiO2 (formed on SiC particles)RE2O3 system was formed. The Al2O3RE2O3 additives lowered a liquid formation temperature to 14001500°C and enhanced the densification rate of SiC. An increased solubility of 30 nm SiC in a liquid during dissolution-precipitation process provided an amorphous phase of SiCSiO2Al2O3RE2O3 system at grain boundaries and suppressed the grain growth of SiC. The fracture toughness of dense SiC was dominated by the grain boundary thickness controlled by grain size of SiC and amount of oxide additives. Mixing of 30 nm SiC with 800 nm SiC improved greatly the strength of SiC with two component oxides and the mean flexural strengths reached 740810 MPa.

Abstract: The compressive stress-strain relation (room temperature) of SiC compact (75 vol% 800 nm SiC- 25 vol% 30 nm SiC) hot-pressed with 1.6 vol% Al2O3- 0.83 vol% Gd2O3 at 1950 °C was examined at a crosshead speed of 0.05 mm/min. The dense SiC (97.8 ± 1.5 % theoretical density) possessed 796 MPa of average flexural strength, 5.27 MPa・m1/2 of fracture toughness, 8.1 of Weibull modulus, and 475 GPa of average flexural Young’s modulus. The strains of SiC compacts along directions of height and width changed nonlinearly with applied compressive stress. The apparent Young’s modulus and Poisson’s ratio decreased with increasing strain along the direction of height and reached constant values of 275 ± 59 GPa and 0.214 ± 0.05, respectively. The steady-state compressive Young’s modulus was independent of the flexural strength.

Abstract: The stability of dispersed and flocculated colloidal particles under 1 atm and applied pressure was discussed thermodynamically with the activity and chemical potential defined by Henry’s law and Raoult’s law. The calculated result under 1 atm is represented by a colloidal phase diagram as functions of surface potential and solid content of particles. Application of pressure accelerates the phase transition from dispersed to flocculated suspension. The phase transition pressure, which is observed in the applied pressure-suspension height relation during pressure filtration at a constant crosshead speed of piston, is affected by (1) particle concentration, (2) particle size, (3) surface potential, (4) degree of dissociation of polyelectrolyte dispersant and (5) applied electric field (DC and AC). The influence of above factors was discussed theoretically and experimentally.

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.