Soichiro Sameshima

Synthesis and sintering of rare-earth-doped ceria powder by the oxalate coprecipitation method

Doped ceria, which has a higher oxygen ion conductivity than yttria-stabilized zirconia, is one of the possible electrolytes for solid oxide fuel cell at low temperatures. This study concerns powder preparation and densification of rare-earth-doped ceria. Rare-earth-doped ceria powders with a composition of Ce 0.8 R 0.2 O 1.9 (R = Yb, Y, Gd, Sm, Nd, and La) were prepared by heating the oxalate coprecipitate when a mixed rare earth/cerium nitrate solution was added to an oxalic solution. The oxalate and derived-oxide powders were characterized by x-ray diffraction (XRD), thermogravimetry differential thermal analysis (TG-DTA), particle size analyzer with laser diffraction, inductively coupled plasma (ICP), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). This method provided the oxalate solid solutions containing Ce and R, which were calcined to form the oxide solid solutions at 600 °C in air. The lattice parameter of oxide powders increased linearly with increasing ionic radius of doped rare earth. The size of platelike particles of oxalates and oxides depended on the concentration of oxalic acid and showed a minimum at 0.4 M oxalic acid. Dry milling of oxide powder with α–Al 2 O 3 ball was effective in reducing the size and aspect ratios of particles with little contamination of Al 2 O 3 . These rare-earth-doped ceria powders with various sizes were formed by uniaxial pressing (49 MPa) followed by cold isostatic pressing (294 MPa), and sintered at 900–1600 °C in air for 4 h. The micrometer-sized-doped CeO2 powders were densified above 95% of the theoretical density at 1200 °C. The grain size of rare-earth-doped ceria after sintering at 1600 °C was larger in the samples with the larger rare-earth element.

Determination of hydrogen solubility in oxide ceramics by using SIMS analyses

Abstract Hydrogen solubility in oxide ceramics was evaluated by annealing in D 2 O-containing atmosphere and subsequent secondary ion mass spectrometry (SIMS). A good proportional relation was obtained between the relative intensity of deuterium ion in SIMS and literature data of hydrogen solubility for YSZ and Sr(Yb)CeO 3− δ . The hydrogen solubility in CeO 2 and rare-earth doped ceria (Ce 0.8 M 0.2 O 1.9 , M=Y, La, Nd, Sm, Gd, Yb) was evaluated. The hydrogen solubility in ceria dramatically increased by rare earth substitution, and it depended on the lattice parameter of Ce 0.8 M 0.2 O 1.9 .

Reaction between SiC surface and aqueous solutions containing Al ions

Abstract The dissolution behavior of α-SiC coated with amorphous SiO2 films 0.67 ± 0.34 nm thick and the adsorption of Al ions onto SiC surface were studied in 1.0 vol.% solid suspensions containing 0.37 or 3.7 mM of Al(NO3)3 at pH 3–10. In the suspension at pH 3–7, the SiO2 films and H2O interacted to form soluble H4SiO4. The amount of SiO2 dissolved in the SiC suspensions was dominated by the solubility limit of surface SiO2 films and increased at pH 10 due to the formation of H3SiO4− and H2SiO4−2 ions. The addition of Al(NO3)3 to the neutral and basic suspensions suppressed the dissolution of the SiO2 films. The neutral SiC surface near the isoelectric point (pH 3.1) adsorbed no Al3+ ions or just a trace. The negatively charged SiC surface adsorbed 1.13 μmol of Al3+ ions m−2 at pH 4. In the pH range 5–8, negatively charged SiC particles coexisted with positively charged Al(OH)3 precipitate was charged negatively at pH 10 because its isoelectric point was pH 9.0 and some Al(OH)3 dissolved as Al(OH)4− ions.

Dielectric properties of barium titanate prepared by hot isostatic pressing

Abstract Densification behavior and grain size of BaTiO 3 as a function of temperature were compared between hot isostatic pressing (HIP) and pressureless-sintering in air. The HIP-treatment under a pressure of 160 MPa lowered the densification temperature by 250 °C compared to pressureless-sintering for the achievement of relative density above 90%. Grain growth occurred faster in the HIP-treatment than in pressureless-sintering at a similar heating temperature. The decrease of grain size from 3 to 0.5 μm resulted in the change of crystal structure from tetragonal to cubic. The dielectric constant at room temperature showed a maximum at a grain size of 1.4 μm, independent of the density of BaTiO 3 . Porosity significantly affected the dielectric loss (tan δ) of BaTiO 3 . Decrease of porosity in the early stage of sintering drastically reduced the dielectric loss. The dielectric loss of BaTiO 3 in the final stage of sintering correlated with the volume of closed pores.

Oxygen self-diffusion in cerium oxide doped with Nd

Polycrystalline Ce 0.77 Nd 0.23 O 1.885 having a relative density in excess of 98% was prepared. Oxygen diffusion experiments were performed for the temperature range from 750 to 1100 °C, in an oxygen partial pressure of 6.6 kPa. The concentration profile of 18 O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). The oxygen self-diffusion coefficient obtained using secondary ion mass spectrometry was expressed by D = 6.31 × 10 −9 exp(−53 kJ mol −1 / RT ) m 2 s −1 and was in the extrinsic region. The oxygen diffusion coefficient of Ce 0.77 Nd 0.23 O 1.885 was larger than that of Ce 0.8 Y 0.2 O 1.90 ; it was close to those of Ce 0.6 Y 0.4 O 1.80 and Ce 0.69 Gd 0.31 O 2−δ . The oxygen diffusion coefficient obtained by the tracer method at 700 °C agreed with that calculated from the electrical conductivity in Ce 0.77 Nd 0.23 O 1.885 . The activation energy of the surface exchange coefficient was 94 kJ mol −1 , and the values of the surface exchange coefficient were similar to those of stoichiometric CeO 2 and ThO 2 .

Colloidal processing and mechanical properties of silicon carbide with alumina

Submicrometer-sized SiC coated with SiO 2 of 0.4–1.8 wt.% and α–Al 2 O 3 powder of median size 0.2 μ m were mixed in aqueous solutions in the pH range 3.0–10.0. The SiC/Al 2 O 3 (4.3–6.9 wt. %) powders were consolidated by filtration through gypsum molds and hot-pressed at 1600°–2040 °C under a pressure of 39 MPa. These compacts were densified to near the theoretical density at 1700°–1800 °C. The sintering mechanisms are discussed based on the analysis of shrinkage curves of SiC/Al 2 O 3 compacts during hot-pressing. The equiaxed SiC grains grew with low aspect ratios below 1800 °C and changed to plate-like grains at 1900 °C. The fracture toughness of SiC as a function of average grain size reached a maximum of 5 Mpa · m 0.5 at 2.5 μ m grains of low aspect ratios of 1–2. The flexural strengths at room temperature were 230–430 MPa in the SiC above 98% of the theoretical density and showed a similar grain size dependence.

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.

Analysis of microstructure of two-component powder compact by electrical conductivity measurement (Part 2)

Particle connection in two-component powder compact was quantitatively evaluated by electrical conductivity measurement. As a model system, an Al 2 O 3 powder (median size 0.33 μm or 0.51 μm, insulator) was mixed with an indium tin oxide powder (ITO, 90 mass% In 2 O 3 -10 mass% SnO 2 , median size 0.20 μm, electronic conductor) in aqueous solutions with and without polyacrylic acid (dispersant) at pH 3.3-10.0. The packing density of the powder compacts consolidated by filtration depended on the pH of the suspension, the size of Al 2 O 3 particles and the volume fraction of ITO. The above green density was well simulated using the fractional collision frequency and packing density of Al 2 O 3 -Al 2 O 3 , Al 2 O 3 -ITO and ITO-ITO clusters in the powder compact. A continuous connection of ITO particles in the consolidated green compact formed at 25 vol% ITO fraction in both the powder compacts with 0.33μm- and 0.51μm-Al 2 O 3 median size, respectively. On the other hand, particle connection of ITO-ITO formed at 95% relative density for 20 vol% ITO fraction after the sintering at 1400°C. At 10 vol% ITO fraction, no ITO-ITO connection was formed in the dense compact with a relative density above 95%. The conduction path in the green and sintered compacts was formed at Df=0.15 (D: relative density of the compact, f: ITO volume fraction), which corresponded to 0.66 cm of the critical length of connecting ITO particles in a 1 cm-cube of the compact. It is possible to interpret that one string of connecting ITO particles is in contact three-dimensionally with another string to make the conduction path.

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.

Evaluation of Electric Power of SOFC Using Reformed Biogas

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.