Maki Ashida

Fatigue strength of Co-Cr-Mo alloy clasps prepared by selective laser melting.

We aimed to investigate the fatigue strength of Co-Cr-Mo clasps for removable partial dentures prepared by selective laser melting (SLM). The Co-Cr-Mo alloy specimens for tensile tests (dumbbell specimens) and fatigue tests (clasp specimens) were prepared by SLM with varying angles between the building and longitudinal directions (i.e., 0° (TL0, FL0), 45° (TL45, FL45), and 90° (TL90, FL90)). The clasp specimens were subjected to cyclic deformations of 0.25mm and 0.50mm for 10(6) cycles. The SLM specimens showed no obvious mechanical anisotropy in tensile tests and exhibited significantly higher yield strength and ultimate tensile strength than the cast specimens under all conditions. In contrast, a high degree of anisotropy in fatigue performance associated with the build orientation was found. For specimens under the 0.50mm deflection, FL90 exhibited significantly longer fatigue life (205,418 cycles) than the cast specimens (112,770 cycles). In contrast, the fatigue lives of FL0 (28,484 cycles) and FL45 (43,465 cycles) were significantly shorter. The surface roughnesses of FL0 and FL45 were considerably higher than those of the cast specimens, whereas there were no significant differences between FL90 and the cast specimens. Electron backscatter diffraction (EBSD) analysis indicated the grains of FL0 showed preferential close to <001> orientation of the γ phase along the normal direction to the fracture surface. In contrast, the FL45 and FL90 grains showed no significant preferential orientation. Fatigue strength may therefore be affected by a number of factors, including surface roughness and crystal orientation. The SLM process is a promising candidate for preparing tough removable partial denture frameworks, as long as the appropriate build direction is adopted.

Retraction:Fine Measurement of Thermal Conductivity for (Bi0.5Sb1.5)Te3 Compounds

A fine measurement system was constructed for thermal conductivity based on the static comparison method. The thermal conductivity of the sample was 1.397 W/mK. The system has an accuracy of less than 1% for 1.411 W/mK of the reference quartz. The (Bi 0.5 Sb 1.5 )Te 3 samples were prepared by mechanical alloying followed by hot pressing. Grain sizes were controlled in a range from 1 to 10 pm by controlling the sintering temperature in a range from 623 to 773 K. The thermal conductivity was 0.89 W/mK for the sample sintered at 623 K, where the grain size of 1.75μm was measured by scanning electron microscopy (SEM). The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.

Control of Crystallographic Orientation and Grain Refinement in Bi2Te3-Based Thermoelectric Semiconductors by Applying High Pressure Torsion

Prepared were p-type Bi2Te3-based thermoelectric semiconductors, having a grain-refined microstructure and a preferred orientation of anisotropic crystallographic structure. Disks with a nominal composition Bi0.5Sb1.5Te3.0 were cut from an ingot grown by the vertical Bridgman method (VBM) and deformed at 473 K under a pressure of 6.0 GPa by high pressure torsion (HPT). The crystal orientation was characterized with X-ray diffraction. The microstructures were characterized by using optical microscopy and scanning electron microscopy. It was found that the HPT disks had a fine and preferentially oriented grain compared to that of the VBM disks. Further, the power factor of the HPT disks was about twice as large as that of the VBM disks. These results indicate that HPT is effective for improving the thermoelectric properties of Bi2Te3-based thermoelectric semiconductors.

Calcification of MC3T3-E1 cells on titanium and zirconium.

To confirm similarity of hard tissue compatibility between titanium and zirconium, calcification of MC3T3-E1 cells on titanium and zirconium was evaluated in this study. Mirror-polished titanium (Ti) and zirconium (Zr) disks and zirconium-sputter deposited titanium (Zr/Ti) were employed in this study. The surface of specimens were characterized using scanning electron microscopy and X-ray diffraction. Then, the cellular proliferation, differentiation and calcification of MC3T3-E1 cells on specimens were investigated. The surface of Zr/Ti was much smoother and cleaner than those of Ti and Zr. The proliferation of the cell was the same among three specimens, while the differentiation and calcification on Zr/Ti were faster than those on Ti and Zr. Therefore, Ti and Zr showed the identical hard tissue compatibility according to the evaluation with MC3T3-E1 cells. Sputter deposition may improve cytocompatibility.

Effect of High Pressure Torsion on Crystal Orientation to Improve the Thermoelectric Property of a Bi2Te3-Based Thermoelectric Semiconductor

A Bi2Te3-based thermoelectric semiconductor was subjected by high pressure torsion (HPT). Sample disks of p-type Bi0.5Sb1.5Te3.0 were cut from sintered compacts that were made by mechanically alloying (MA) followed by hot pressing. Disks were subjected by HPT with 1, 5 and 10 turns at 473 K under 6.0 GPa of pressure. Crystal orientation was investigated by X-ray diffraction. Microstructures were characterized using scanning electron microscopy. Results indicated that HPT disks after 5 turns had a preferred orientation and a fine grain compared with pre-HPT disks while the orientation factor was decreased after HPT using 10 turns. The power factor had a maximum value at 5 turns as determined by measuring its thermoelectric properties. A maximum power factor of 4.30×10-3 Wm-1K-2 was obtained for HPT disks after 5 turns. This value was larger than that for the pre-HPT disk. The over-HPT of 10 turns was found to have caused a decrease in the preferred orientation leading to a low power factor.

Adhesion and differentiation behaviors of mesenchymal stem cells on titanium with micrometer and nanometer-scale grid patterns produced by femtosecond laser irradiation: ADHESION AND DIFFERENTIATION OF hMSCs ON MICRON AND NANO GRIDS

To clarify the effects of grid topographies with different scales on cell morphology and functionalization, we investigated the adhesion and differentiation of human mesenchymal stem cells (hMSCs) to titanium surfaces with micron, nano, and micron/nano (hybrid) grid topographies created by femtosecond laser irradiation. The results showed that cellular adhesion and differentiation strongly depended on the scales of the grid topography. hMSCs cultured on micron and hybrid grid topographies showed regulation of cellular adhesion plaques following the surface topography and were vinculin-positive, whereas filamentous vinculin was evident at the filopodia of hMSCs cultured on nanogrids. The findings indicate that the micron grid topography was beneficial for cell colonization by anchoring the cells to the substrate surface, whereas the nanogrid topography was beneficial for cell locomotion. With the superposition effect of the micron and nanogrids, micro/nanohybrid grid topography strongly promoted cell adhesion. This differential adhesion induced differences cell differentiation. Nanogrids promoted differentiation of hMSCs, particularly osteogenic differentiation. These findings provide a basis for the design of novel biomaterial surfaces that can regulate specific cellular functions.

Microstructures and Mechanical properties of Al-Al2O3 Composites Processed by Disk-HPT and Ring-HPT

Al-Al2O3 composites were produced through consolidation of Al micrometer powders and Al2O3 nano-powders using high-pressure torsion (HPT) with disk-type samples (Disk-HPT) and ring-type samples (Ring-HPT). Pure Al powders were mixed with a 30% volume fraction of Al2O3 powders by ball milling (BM). HPT was conducted at room temperature with a rotation speed of 1 rpm under a pressure of 6 GPa for disk samples and 3 GPa for ring samples. The composites were also processed by HPT without BM for comparison. Agglomeration of Al2O3 particles became diminished and the dispersion of nano-sized Al2O3 particles became finer and more uniform by combination of BM and Ring-HPT. This combined use is more effective than Disk-HPT with BM to obtain a uniform Al2O3 dispersion in the Al-Al2O3 nanocomposites and thus to improve mechanical properties.

Comparison for Thermoelectric Properties of BiTe Based Semiconductor Processed by the Mechanical Alloying and the High Pressure Torsion after Melt Grown by the Vertical Bridgman Method

Bi 2 Te 3 -related thermoelectric semiconductors were prepared by the mechanical alloying (MA) and the vertical Bridgman (VBM) - High Pressure Torsion (HPT) methods. Structures and electric properties of these alloys were then investigated. These samples had a nominal composition of Bi 0.5 Sb 1.5 Te 3.0 with 0.07 mass% excess Te. The MA sample was sintered into a compact after mechanical alloying by hot-pressing while the VBM - HPT sample was subjected by HPT after being cut into a disk from a melt-grown ingot by the vertical Bridgman method (VBM). The orientation factors of the MA and VBM - HPT samples were 0.054 and 0.525, respectively, indicating that the preferred orientation was formed by VBM - HPT. Average grain sizes of both samples were approximately 2 μm. By estimating the slope of carrier mobility versus temperature the carrier scattering factor of the MA and VBM - HPT samples were estimated to be -2.8 and -1.4. These values are indicative of carrier scatterings by acoustic phonons and by the interaction between acoustic and optical phonons, respectively. The maximum power factor obtained for the isotropic MA sample was 4.1 × 10 -3 W m -1 K -2 at 310 K and this was about twice the 2.1 × 10 -3 W m -1 K -2 at 440K obtained for the anisotropic VBM - HPT sample.