Taro Shimonosono

Analysis of gas permeability of porous alumina powder compacts

Abstract Gas permeation of alumina powder compacts with different porosity, pore size and grain size was examined at room temperature for Ar, N2, H2 and CO2 gases. The flux of Ar, N2 and CO2 gases with Knudsen numbers of 0.4–1.3 was measured above a threshold pressure difference between inlet and outlet gases, and then linearly increased with an increase in applied pressure. The H2 gas with Knudsen numbers of 1.1–2.6 showed a relatively large flux at near 0 MPa/m of pressure gradient and increased linearly with an increase in the pressure gradient. The measured gas permeability coefficients for Ar, N2 and CO2 gases were the same order as the calculated permeability coefficients based on the modified Poiseuille equation (viscous flow) and Knudsen flow equation, suggesting the mixed flow mechanisms. The slope of flux–pressure gradient plot for H2 gas permeation was also in agreement with the calculated permeability coefficients by the modified Poiseuille equation. The H2 gas flow near 0 MPa/m of pressure gradient was characterized by surface diffusion of H2 molecules adsorbed on the pore walls of alumina compacts. It is possible to separate H2 gas from the other gases at room temperature in the pressure gradient range lower than the threshold pressure for N2, Ar or CO2 gas.

Electrochemical Properties of Cathode for Solid Oxide Fuel Cell with Gd-Doped Ceria Electrolyte

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.

Compressive deformation of liquid phase-sintered porous silicon carbide ceramics

Abstract Porous silicon carbide ceramics were fabricated by liquid phase sintering with 1 wt% Al2O3–1 wt% Y2O3 additives during hot-pressing at 1400–1900 °C. The longitudinal strain at compressive fracture increased at a higher porosity and was larger than the lateral strain. The compressive Youngs modulus and the strain at fracture depended on the measured direction, and increased with the decreased specific surface area due to the formation of grain boundary. However, the compressive strength and the fracture energy were not sensitive to the measured direction. The compressive strength of a porous SiC compact increased with increasing grain boundary area. According to the theoretical modeling of the strength–grain boundary area relation, it is interpreted that the grain boundary of a porous SiC compact is fractured by shear deformation rather than by compressive deformation.

Electrochemical Properties of Cathode for Solid Oxide Fuel Cell

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.

Separation of Hydrogen from Carbon Dioxide through Porous Ceramics

The gas permeability of α-alumina, yttria-stabilized zirconia (YSZ), and silicon carbide porous ceramics toward H2, CO2, and H2–CO2 mixtures were investigated at room temperature. The permeation of H2 and CO2 single gases occurred above a critical pressure gradient, which was smaller for H2 gas than for CO2 gas. When the Knudsen number (λ/r ratio, λ: molecular mean free path, r: pore radius) of a single gas was larger than unity, Knudsen flow became the dominant gas transportation process. The H2 fraction for the mixed gas of (20%–80%) H2–(80%–20%) CO2 through porous Al2O3, YSZ, and SiC approached unity with decreasing pressure gradient. The high fraction of H2 gas was closely related to the difference in the critical pressure gradient values of H2 and CO2 single gas, the inlet mixed gas composition, and the gas flow mechanism of the mixed gas. Moisture in the atmosphere adsorbed easily on the porous ceramics and affected the critical pressure gradient, leading to the increased selectivity of H2 gas.

Formation of hydrogen from the CO–H2O system using porous Gd-doped ceria electrochemical cell with MnO cathode and Fe3O4 anode

Abstract This paper reports the outlet gas composition and phase change of electrodes during the CO–H2O reaction (CO + H2O → H2 + CO2) using an electrochemical cell with MnO–GDC (Gd-doped ceria: Ce0.8Gd0.2O1.9) cathode/porous GDC electrolyte/Fe3O4–GDC anode system. In the cathode, oxidation of MnO by H2O (3MnO + H2O → Mn3O4 + H2) and electrochemical reduction of Mn3O4 occurred (Mn3O4 + 2e− → 3MnO + O2−). In the anode, reduction of Fe3O4 by CO (Fe3O4 + CO → 3FeO + CO2) and electrochemical oxidation of FeO occurred (3FeO + O2− → Fe3O4 + 2e−). H2 and CO2 gases were produced through the above catalytic reactions. The fraction of H2 gas in the outlet gas increased at a high heating temperature and was 30–50% at 700 °C. As a parallel reaction of the CO–H2O reaction, the supplied CO gas was decomposed to CO2 and solid carbon over Fe3O4 in the anode at low temperatures (disproportion of CO, 2CO → CO2 + C).

Imaging of Oxide Ionic Flows at Practical SOFC Cells by Isotope Labeling Technique

Oxygen ionization and diffusion were visualized at the practical flatten tube SOFC stack/cells. Stable isotope oxygen (18O) was applied to label the motion of oxygen/oxide ions with secondary ion mass spectrometry (SIMS) analysis. Surface reactivity and diffusivity of oxygen (oxide ions) were compared between the fuel cell operating condition and non-current flow conditions. A peak of 18O-concentration was observed under fuel cell operating condition at the interlayer between cathode and electrolyte. This suggested that the active sites for 18O ionization and diffusion were at the interlayer. Also, a slow 18O diffusion layer can be formed at the interlayer/electrolyte interface because of the discontinuous 18O distribution profiles.

The Effect of Particle Shape on Sintering Behavior and Compressive Strength of Porous Alumina

Alumina particles with different shapes, such as sphere, rod, and disk, were examined for the sintering behavior and compressive strength of partially sintered porous alumina. While both the spherical and disk-like particles were packed well to the relative density of 61.2–62.3%, the packing density of rod-like particles was only 33.5%. The sintering rate of alumina particles increased in the order of disk < rod < sphere. The compressive strength of sintered porous alumina was higher for the spherical particles than for the rod-like and disk-like particles. The uniform distribution of the applied load over many developed grain boundaries contributed to the increase in the compressive strength for the spherical particles. The applied load concentrated on a few grain boundaries of rod-like or disk-like particles, caused fracture at a low compressive stress.

Thermal Properties of Zeolite-Containing Composites

A zeolite (mordenite)–pore–phenol resin composite and a zeolite–pore–shirasu glass composite were fabricated by hot-pressing. Their thermal conductivities were measured by a laser flash method to determine the thermal conductivity of the monolithic zeolite with the proposed mixing rule. The analysis using composites is useful for a zeolite powder with no sinterability to clarify its thermal properties. At a low porosity <20%, the thermal conductivity of the composite was in excellent agreement with the calculated value for the structure with phenol resin or shirasu glass continuous phase. At a higher porosity above 40%, the measured value approached the calculated value for the structure with pore continuous phase. The thermal conductivity of the monolithic mordenite was evaluated to be 3.63 W/mK and 1.70–2.07 W/mK at room temperature for the zeolite–pore–phenol resin composite and the zeolite–pore–shirasu glass composite, respectively. The analyzed thermal conductivities of monolithic mordenite showed a minimum value of 1.23 W/mK at 400 °C and increased to 2.51 W/mK at 800 °C.

Synthesis and electrical conductivity of (La1−xSrx)(Al1−yMgy)O3−δ perovskite solid solution

Abstract Perovskite solid solution powders with (La1−xSrx)(Al1−yMgy)O3−δ composition (shorten as LSAM) were prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders were heated in air at 1500 °C for 10 h, and subsequently sintered at 1400 °C for 12 h in air. The X-ray diffraction patterns and the lattice parameters for the compositions of x = 0–0.4 at y = 0.1 and y = 0–0.15 at x = 0.2 suggested the formation of rhombohedral LaAlO3 solid solution. The sinterability of LSAM was controlled by the diffusion rate of A site cations and increased by increasing Sr composition at A site and by decreasing Mg composition at B site. The highest electrical conductivity was measured at the composition of (La0.8Sr0.2)(Al0.9Mg0.1)O2.85 (6.79 × 10−3 S/cm at 600 °C, activation energy 98.8 kJ/mol). Although the sintered (La0.8Sr0.2)(Al0.9Mg0.1)O2.85 electrolyte contained 28% porosity, its conductivity was higher than the conductivity of dense 8 mol% yttria-stabilized zirconia electrolyte. The conductivity and activation energy of LSAM greatly vary according to the concentration of available oxygen vacancy and the association of positively charged oxygen vacancy and negatively charged sites or sites. The LSAM composition of (La0.8Sr0.2)(Al0.9Mg0.1)O2.85 provided the perovskite structure of the smaller strain (tolerance factor 1.007). This is another factor for the highest conductivity.