Tetsuhiko Onda

Preparation and Characterization of Bi0.4Sb1.6Te3 Bulk Thermoelectric Materials

This work focused on the preparation of p-type Bi0.4Sb1.6Te3 bulk materials by combining mechanical alloying (MA) and hot extrusion, with emphasis on grain refinement and preferred grain orientation. Pure Bi, Sb, and Te powders were mechanically alloyed then hot extruded in the temperature range 360–450°C. Bi0.4Sb1.6Te3 bulk materials were successfully prepared by MA and hot extrusion. All the samples had sound appearance, with single phases and high densities. The hot-extruded samples had small grain sizes, and the lower the extrusion temperature, the smaller the grain sizes. The results indicated that the extrudates had preferred orientation. The basal plane was predominantly oriented parallel to the direction of extrusion. Similar Seebeck coefficients were obtained when extrusion temperature was in the range 380–420°C. Electrical resistivity decreased with increasing extrusion temperature. Thermal conductivity was relatively low, even if the extrusion temperature was 450°C. As a result, a ZT value of 1.2 was obtained at room temperature for the sample extruded at 400°C. Therefore, combination of MA and hot extrusion results in significant improvement of both the thermoelectric and mechanical performance of Bi0.4Sb1.6Te3 bulk materials.

Isothermal and athermal martensitic transformations in a zirconia–yttria alloy

Abstract The M s (martensite start temperature) of the tetragonal to monoclinic ( t – m ) transformation in the ZrO 2 –Y 2 O 3 system has been measured for bulk specimens as a function of Y 2 O 3 composition. It decreased nearly linearly with increasing Y 2 O 3 content up to 1.75 mol.%, but the transformation suddenly ceased to exist beyond 2.0 mol.% of Y 2 O 3 . The M s showed a tendency to be suppressed by rapid cooling. In fact with a small specimen containing 1.6 mol.% of Y 2 O 3 , the transformation was completely suppressed by water quenching. The highly unstable tetragonal specimen prepared by such a method was found to exhibit both an isothermal transformation at elevated temperature and an athermal transformation during subzero cooling to liquid nitrogen temperature. The C-curve for the isothermal transformation and the M s temperature for athermal transformations were obtained.

Peak effect in critical current density induced by oxygen non-stoichiometry in cation-stoichiometric SmBa2Cu3O7-δ superconductor

Abstract Cation-stoichiometric SmBa 2 Cu 3 O 7− δ superconductor (Sm-123) samples were synthesized by a conventional (without any liquid phases involved) solid-state-reaction technique, followed by annealing in flowing O 2 gas at 350°C for 40–200 h. A “peak effect” was observed in the critical current density ( J c ) vs. applied magnetic field ( H ) characteristics measured at 77 K. Pronounced peak effect was observed in a sample annealed at 350°C for 40 h, while the peak was suppressed in a sample annealed at 350°C for 200 h. Since the average oxygen content determined by a coulometric titration method increased with decreasing J c value at the peak, certain oxygen-deficient regions induced by oxygen non-stoichiometry might have formed. They could work as the magnetic fluxon pinning centers, since transmission electron microscopy (TEM) observations revealed the presence of both the tetragonal phase and a twin-free orthorhombic phase with width of ca. 200 nm in the sample annealed in O 2 gas at 350°C for 40 h which would have contained a significant amount of oxygen deficiency. The presence of the oxygen-deficient Sm-123 phase especially close to the boundary between either or both of the orthorhombic and tetragonal phases and an ordinary orthorhombic phase containing twins could be working for magnetic flux pinning to yield the peak effect in the J c vs. H characteristics.

Effect of Processing Conditions of Dolomite on the Antiviral Activity

The effect of the processing conditions of dolomite powders on the antiviral activity was studied against H5N3 avian influenza virus. When dolomite was fired in the ambient atmosphere, it decomposed into MgO and CaO by two stages at different temperatures, namely into MgO+CaCO3 at ca.1010 K and MgO+CaO at ca. 1070 K. Strong antiviral activity was observed after the complete decomposition into CaO and MgO. However, when the firing temperature exceeded 1673 K, the antiviral activity was degraded. It was also found that the degraded antiviral activity was recovered by sever ball mill grinding.

Effect of CoO Doping on the Sintering Ability and Mechanical Properties of Y-TZP

The effect of CoO addition into Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals) was studied on the evolution of its sintering ability, grain size, grain boundary structure and mechanical properties. The doping of a small amount of CoO effectively reduced the sintering temperature. A small amount of CoO up to ~ 0.3 mol% was effective for the suppression of grain growth, but the addition of 1.0 mole % resulted in an enhanced grain growth. The hardness and toughness of the CoO doped TZP were about the same as those of undoped TZP. Furthermore, despite the grain refinement, CoO doped TZP did not exhibit improved mechanical properties. This may be suggesting that CoO dopant had weakened the grain boundary strength.

Peak effect of critical current density in stoichiometric SmBa2Cu3Oy sample

Abstract A fully oxygenated stoichiometric SmBa 2 Cu 3 O y sample showed a weak but distinct peak effect in the magnetization versus applied magnetic field curve at 40 K. The peak was located at about 2 T. The oxygen content of this sample was determined at y =7.01±0.05. Thus, it is unlikely that the peak observed for this sample was caused by ordering of oxygen atoms. When the sample was pulverized and annealed in oxygen gas at 800°C, part of SmBa 2 Cu 3 O y apparently decomposed into BaCuO 2 and CuO in the part of thin surface layer (less that ∼30 nm thickness) of the powder grains, according to TEM observations and EDX analyses. This would mean that the concentration of Sm became higher than the stoichiometric value in the interior of the grain to form Sm-rich region. The observed weak peak effect may be caused by the Sm-rich region inside the grains, since some Sm-rich Sm 1+ x Ba 2− x Cu 3 O y samples prepared by an oxygen controlled melt growth (OCMG) process were reported to show a peak effect.

Specimen-size dependence of the martensitic transformation temperature of yttria doped zirconia

Small spherical specimens of ZrO2-1.5mol%Y2O3 with diameters ranging from 150 to 900μm were prepared and the burst temperature was measured on an individual specimen under optical microscope with cooling stage. The range of scatter of the burst temperature increased with decreasing specimen’s size. While the upper limit of the scatter remained nearly the same, the lower limit of the scatter markedly decreased with decreasing specimen’s size. The distribution of potential embryos was deduced by fitting the simulated distribution of burst temperatures to the observed result by adjusting the assumed distribution of potential embryos used for the simulation.

Fabrication of Al2O3-ZrO2 (3Y-TZP) Composites with Graded Structures by Centrifugal Slip Casting Method

Centrifugal casting method was used to fabricate composites with graded structures in alumina-zirconia system. The composition profiles of the graded composites showed three distinct regions, with a greater part of the specimen showing nearly linear variation of compositions. The composition profile can be varied by using zirconia powder of different size ranges. The top of the specimens showed nano-scaled microstructures. The composition profiles can also be varied by altering zirconia content in the starting suspension. It was found that the composites with graded structures showed higher bending strength than that of uniform composites. On the other hand, the specimens fractured with the lower zirconia content region being the crack initiate plane showed higher bending strength than that of fractured form higher zirconia content region.

Mechanical Properties of a Graded Alumina‐Zirconia Composite Prepared by Centrifugal Slip Casting

Compositionally graded composite of alumina‐20 vol%zirconia was fabricated by using centrifugal casting incorporated with relatively thin slip. An EPMA analysis exhibited a nearly linear variation of the alumina/zirconia ratio along the centrifugal direction; zirconia tended to accumulate in the bottom section, while alumina in the top section. Such a graded structure exhibited a considerably higher flexural strength when the alumina rich surface was subjected to a tensile stress than compositionally uniform composite of the same average composition. Fracture toughness measurement across the specimen thickness by indentation method revealed that the crack lengths along the vertical and horizontal directions were different. The anisotropy of the fracture toughness was accounted for by the variation of the residual stress across the specimen thicknesss

Effect of Divalent Elements on the Grain Growth of TZP

The effect of divalent elements, Mg, Ca, Sr, and Ba, was investigated on the grain growth suppression during sintering to 12Ce-TZP and 3Y-TZP. All of the dopants were effective in reducing the grain size, but the degree of the effectiveness differs for Ce-TZP and Y-TZP. In case of Ce-TZP, Ca was most effective and reduced the grain size to less than 1/3 of the undoped specimen. However, a small amount of the cubic phase appeared in the specimen of 2mol%Ca. On the basis of empirical equation suggested in the literature, the solubility limits of the dopants were estimated. MgO and CaO are soluble but SrO and BaO are hardly soluble. Within the solubility limits, the smallest grain size was obtained by Ca doped specimen. The resulting relationship between the ionic size of the dopant and the grain size was consistent with Hwang and Chens result. In case of Y-TZP, Sr was most effective and the grain size was reduced to 3/4 of the undoped Y-TZP. The amount of the cubic phase increased with Mg and Ca dopants. The resulting relationship between the ionic size of the dopant and the grain size was consistent with Hwang and Chens result.