Shigeyuki Sōmiya

Role of H2O on the degradation process of Y-TZP

Yttrium-doped tetragonal zirconia (Y-TZP) is expected to be a good structural ceramic material because of its high strength, high toughness and wear resistance. However, the instability (degradation) of Y-TZP around 150 to 400° C in air and above 100° C in water during ageing is a severe problem which must be solved before the direct application of this material.

Formation of diffusionlessly transformed tetragonal phases by rapid quenching of melts in ZrO2-RO1.5 systems (R = rare earths)

Rare-earth (Nd, Sm, Er, Yb, Sc)-doped zirconia was melted using an arc-imaging furnace, and rapidly quenched with a hammer and anvil apparatus (cooling rate >105 K sec−1). These ZrO2-RO1.5 samples were investigated by X-ray diffraction and transmission electron microscopy. The existing region of metastable tetragonal zirconia is from 2 to 14 mol% of RO1.5 regardless of the species of dopants, RO1.5. In the lattice parameters of the tetragonal phases, the unit cell volume corresponds to the ionic radii of dopants, whereas the tetragonality (c/a) is independent of the species of dopants, but dependent on the content of dopants.

Crystal structure analyses of the pyrochlore and fluorite-type Zr2Gd2O7 and anti-phase domain structure

Abstract The detailed structure investigations of Zr2Gd2O7 with pyrochlore- and fluorite-type structures have been carried out by X-ray single crystal method at room temperature. The bond distances between cations and oxide ions versus the site population for cations reasonably support that the fluorite phase is made up of the structure with microdomains of the pyrochlore. Sharp fundamental and diffuse superstructure reflections of the pyrochlore are interpreted assuming that anti-phase domain boundaries lie parallel to the &{211} and that domain sizes are not uniform. It should be proposed that the layers composed of the 48f site oxide ions are anti-phase boundaries. The probability density function maps and effective one-particle potentials of the pyrochlore structure reveal that the magnitudes of anharmonic thermal motions for the 48f site oxide ions are large toward the unoccupied 8b site. Compared to the s-pyrochlore phase (with sharp superstructure reflections), the 48f site oxide ions of the d-pyrochlore (with diffuse superstructure reflections) and fluorite phases have larger temperature factors and gentler potential curves. This is due to that the anti-phase domains are coherent each other and that their electron density distributions are the averages of individual domains.

Oxidation of SiC powder by high-temperature, high-pressure H 2 O

The reaction between SiC powder and H2O has been studied at 400°–800 °C under 10 and 100 MPa. Silicon carbide reacted with H2O to yield amorphous SiO2 and CH4 by the reaction SiC + 2H2O→SiO2 + CH4 above 500 °C. Cristobalite and tridymite crystallized from amorphous silica after the almost complete oxidation of SiC above 700 °C. The oxidation rate, as calculated from the weight gain, increased with temperature and pressure. The Arrhenius plotting of the reaction rate based on a Jander-type model gave apparent activation energies of 167–194 kJ/mol. Contrasted with oxidation in oxidative atmosphere, oxidation in H2O is characterized by the diffusion of H2O and CH4 in an amorphous silica layer where the diffusion seemed to be rate determining. Present results suggest that the oxidation of SiC includes the diffusion process of H2O in silica layers when atmospheres contain water vapor.

Synthesis of mullite powder and its characteristics

Abstract Mullite of stoichiometric composition 3Al2O3·2SiO2 was synthesised using γ-aluminium oxide and colloidal silicon dioxide. γ-Aluminium oxide was hydrolysed at 90°C and peptised using concentrated nitric acid at 95° C under refluxing conditions for 3 h. The boehmite sol formed was cloudy. Colloidal silicon dioxide was dispersed in acidified water using a mechanical blender and gelled by evaporation of excess water. On calcination of the gel at temperatures above 1300°C for 1 h mullite was formed. The mullite synthesised can be sintered in 3 h at 1650°C to 98·1% of its theoretical density, and shows high flexural strength of 385 MPa (determined in three-point bend) at room temperature.

Direct preparation of BaTiO3 powders from titanium metal by anodic oxidation under hydrothermal conditions

Elaboration, par une nouvelle technique combinant reaction hydrothermique et oxydation electrochimique, de poudres de titanate de baryum.

Crystallization and crystal growth of CeO2 under hydrothermal conditions

CeO2 crystals have previously been grown by the flux method [1–4] due to their high melting point and high vapour pressure [5] at high temperatures. The flux method requires high temperatures and generally brings about greater contamination of the flux than does the hydrothermal method. This study deals with the effect of several mineralizers on the crystallization and the crystal growth of CeO2 under hydrothermal conditions.

Hydrothermal Preparation of Ultrafine Monoclinic ZrO2 Powder

Ultrafine powder of single-phase monoclinic ZrO2 was prepared by hydrothermal treatments of amorphous hydrated zirconia with 8 wt% KF solution under 100 MPa at 200° to 500°C for 24 h. The process yielded well-crystallized particles 16 nm and 22 nm in size at 200° and 500°C, respectively.

Electrical conductivity of Cr2O3-doped Y2O3-stabilized ZrO2

The electrical conductivity of Cr2O3-doped Y2O3-stabilized ZrO2 (YSZ) has been studied as functions of composition, temperature and oxygen pressure. The specimens have been prepared by hot preoning of co precipitated oxides to yield >99.7% density. The Cr2O3 added above the solubility limit (≈ 0.7 mol %) precipitated as a secondary phase at the grain boundaries. The conductivity of Cr2O3-doped YSZ was almost independent of the oxygen pressure in the range 1018 to 105 Pa, indicating a dominant ionic condition. The electronic conductivity of dopant CR2O3 would be hindered by the higher ionic conductivity in thepO2 ranges studied. The conductivity and the activation energy for conduction decreased slightly with the addition of Cr2O3. These phenomena seemed to be caused by vacancy trapping or polarization at the grain boundaries with the Cr2O3 precipitates. The samples with 1 mol % Cr2O3 addred to zirconia containing various Y2O3 contents showed similar conduction behaviour to those without Cr2O3 addition; that is, the conductivity maxima are observed at around 8 mol % Y2O3 addition to zirconia, and the activation energies increased with tha Y2O3 addition.

Phase changes and lattice distortion in fluorite-related phases of R3TaO7 (3R2O3·Ta2O5, R=rare earth)

Abstract The crystal structuresof the compounds R3TaO7 (3R2O3·Ta2O5, RLaYb, Y) were studied by X-ray diffraction method. According to the powder X-ray patterns, R3TaO7 prepared by solid state reaction at 1700°C for for 4 h showed several polymorphic modifications based on fluorite-related structures. The diffraction patterns are of cubic fluorite-like type for smaller R3+ ions. For larger R3+ ions the superlattice lines appeared and the diffraction peak of the fundamental lines was broadened and/or apparently split, which is of weberite orthorhombic type. The orthorhombic structure has monoclinic distortion of the fluorite unit cell. With the increase of the ionic radii of R3+ ions until Nd3+, the distortion of the fluorite unit cell increased. The compounds of R3TaO7 with much larger radii for RLa3+ and Pr3+, however, have a different distortion type in their unit cells. Therefore, two orthorhombic fluorite related superstructures are formed, of space group Cmcm for RLa and Pr, and C2221 for Nd, Sm, Eu, Gd, Tb and Dy. The smaller ions such as Ho, Er, Yb and Y take the cubic fluorite structure.