Masatomo Yashima

Metastable-stable phase diagrams in the zirconia-containing systems utilized in solid-oxide fuel cell application

Metastable-stable phase diagrams of ZrO2-RO15 systems (R = Y, Sc and Er) are reviewed, noting the cation diffusionless transformations and reasonable diagrams are proposed. A tetragonal phase with an axial ratio of unity is interpreted to appear in the compositional region around 16 mol.% YO1.5.

Hydroxyapatite ceramics with selected sintering additives

Several sintering additives for hydroxyapatite (HA) have been tested in order to enhance its sinterability without decomposing the HA and/or decreasing bioactivity and biocompatibility, additionally providing a weak interface for HA ceramics. The ion species of sintering additives were selected from those in the mineral constituents of hard tissues and bioactive glasses. After investigation of phase diagrams in the CaO-P2O5-additive systems, and analysis of physiochemical properties of the additives, several sintering aids for HA have been chosen. Subsequently, densification, phase composition, grain growth and fracture behaviour of HA containing 5 wt% of each additive, sintered at 1000-1100 degrees C, have been studied. H3BO3, CaCl2, KCl, KH2PO4, (KPO3)n and Na2Si2O5 did not enhance densification of HA. K2CO2, Na2CO3, KF and sodium phosphates improved the densification significantly. Expect for KCl and some sodium phosphates, all the additives caused formation of large quantities of undesired beta-tricalcium phosphate or CaO; therefore, they are not appropriate for HA. In the case of sodium phosphate additives, it was possible to avoid formation of CaO or beta-tricalcium phosphate by control of the additive quantity and chemical composition. beta-NaCaPO4 has been found to be an effective sintering agent which causes neither decomposition of HA nor formation of other undesired phases.

Determination of tetragonal-cubic phase boundary of Zr1−XRXO2−X2 (R = Nd, Sm, Y, Er and Yb) BY Raman scattering

Abstract Existing phases of arc-melted ZrO2-X mol%RO1.5 specimens were investigated mainly by Raman spectroscopy, which is sensitive to the phase transition induced by the oxygen displacements (0≤X≤24) R = Nd, Sm, Y, Er and Yb). The tetragonal and monoclinic phases coexisted at X = 2 and 4 for R = Nd, Y, Er and Yb, and at X = 2 for R = Sm. Tetragonal single phase is obtained in the specimens with higher contents. The axial ratio c a decreases with an increase of RO1.5 content and becomes unity at 14 or 16mol% RO1.5 content. The Raman peak intensity at about 470 cm−1, which is characteristic of the tetragonal phase, decreases with an increase of RO1.5 content. The cubic-tetragonal phase boundary determined by Raman scattering is located at about X = 18–19mol% regardless of dopant species R. A tetragonal phase whose axial ratio c a is unity appears at about X= 16mol%. The compositional locations of the phase boundaries are almost independent of the dopant species R.

Size effect on the crystal structure of barium titanate nanoparticles

A size effect on crystal structure has been investigated for barium titanate (BaTiO3) nanoparticles of 40-, 140-, and 430-nm sizes, by means of neutron and high-resolution synchrotron x-ray powder-diffraction and Raman-scattering techniques. These samples were prepared by a modified two-step thermal decomposition method from barium titanyl oxalate, resulting in very few lattice impurities. Rietveld analysis of the neutron-diffraction data for the 430-nm- and 140-nm-sized BaTiO3 particles was performed assuming a single phase of tetragonal (P4mm) structure. The axial ratio c∕a of tetragonal BaTiO3 decreases with a decrease in particle size from 430 to 140 nm. Barium titanate particles with a size of 40 nm consist of (1) tetragonal crystals (83 wt %) with a large cell volume and an axial ratio of unity c∕a=1.000(5) and of (2) a hexagonal phase (P63mmc, 17 wt %) with a large unit-cell volume. Rietveld and maximum-entropy method analyses suggest that there exist atomic displacements from the ideal site of a c...

Processing and mechanical properties of hydroxyapatite reinforced with hydroxyapatite whiskers

Hydrothermally synthesized HAp fine crystals/HAp whiskers mixtures have been used for the preparation of HAp/0-30% (whiskers) composites. The composites have been fabricated by pressureless sintering and hot-pressing. The best mechanical properties and the highest densities have been achieved for composites hot pressed at 1000 degrees C (2 h, 30 MPa in flowing Ar). Their density was in the range of 90-97% of the theoretical density. Fracture toughness (Klc) of the composites reflected their microstructure and had the value of 1.4 MPa m1/2 (as compared with Klc = 1.0 MPa m1/2 for the non-reinforced HAp matrix). Compressive prestressing of the HAp matrix and crack deflection (both derived from the residual stress field) contributed to the increase of fracture toughness. Other toughening mechanisms have not been observed. HAp/HAp (whiskers) composites exhibited improved toughness without degradation of biocompatibility, because the HAp whiskers acted both as a reinforcement and as a biocompatible phase. Problems related to biocompatibility and mechanical properties of available HAp-based composites were also discussed.

Size and temperature induced phase transition behaviors of barium titanate nanoparticles

High density, almost impurity-free and defect-free barium titanate (BaTiO3) fine particles with various sizes from 20to1000nm were prepared by the two-step thermal decomposition of barium titanyl oxalate and postheating treatment. The crystal structures of these particles were investigated as a function of the size and the temperature using synchrotron radiation x-ray diffraction (XRD) measurement. As a result, the size-induced ferroelectric (tetragonal-cubic) phase transition observed at around 30nm. Moreover, the temperature dependence of the crystal structures revealed that one phase transition at 135°C separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135°C despite particle sizes while the cell volume expansion temperature shifted to low temperature with decreasing particle sizes. Moreover, the temperature dependence of Raman scattering spectra clarified that the temperature at a discontinuous c...

Biocompatible whiskers with controlled morphology and stoichiometry

Hydroxyapatite whiskers have been prepared by the hydrothermal method. The crystals had diameter, length, and aspect ratio in the range of 1–10 μm, 30–50 μm, and 5–20, respectively. Their Ca/P molar ratio varied from 1.59 to 1.62. The morphology of the crystals can easily be controlled by the concentrations of species in the starting solution, while the Ca/P ratio is almost independent of them. Through the reaction with calcite powder at 600 °C, the Ca/P ratio of the whiskers has been improved even to the stoichiometric value of 1.67. Taking into account morphology and chemical composition of the HAp whiskers, they should not be health hazardous and may find applications as substitutes for asbestos and other fibrous materials which presently have restricted use because of their carcinogenic natures.

Room‐temperature preparation of the highly crystallized luminescent CaWO4 film by an electrochemical method

Highly crystallized polycrystalline film of single‐phase CaWO4 has been prepared on a tungsten substrate at room temperature in an alkaline solution containing calcium ions by an electrochemical method with the current density of 1 mA/cm2. This film showed blue emission (456 nm wavelength) with excitation light (254 nm wavelength) at room temperature.

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.

Internal distortion in ZrO2–CeO2 solid solutions: Neutron and high-resolution synchrotron x-ray diffraction study

Neutron and high-resolution synchrotron x-ray diffraction measurements have indicated a tetragonal zirconia phase with both an axial ratio of unity and internal shear deformation: The oxygen displacements from ideal fluorite positions of 0.088 and 0.078 A are observed for 65 and 70 mol % CeO2–ZrO2 samples, respectively, although both c/a values are 1.000±0.001. The c/a ratio decreases with increasing of CeO2 content and decreases discontinuously to unity around a 60 mol % CeO2 composition, while the oxygen displacement decreases continuously up to about 90 mol % CeO2.