Papers by Author: Koji Watari

Abstract: Thermal properties of SiC at the micrometer-scale were measured quantitatively with a thermal microscope using thermo-reflectance and periodic heating techniques. In this study, SiC single crystal and polycrystal were investigated. The small values of standard deviation suggest that the SiC single crystal had constant thermal conductivities. For the single crystal, the average value of the thermal conductivity at the micrometer-scale was in good agreement with the macro-scale thermal conductivity value obtained by the laser flash technique. On the other hand, thermal conductivity of the polycrystal was heterogeneous at the micrometer-scale. An average thermal conductivity value of 257 Wm-1K-1 was obtained within an area of 50 m ×100 µm. The highest and lowest values of the thermal conductivity from the polycrystal were 300 and 220 Wm-1K-1, respectively.

Abstract: The thermal conductivity at the micrometer-scale of AlN ceramics with eliminated grain boundary phase was measured by the thermoreflectance technique with periodic heating. Thermal conductivities were ranged from 160 to 260 W/m•K and an average value of 201 W/m•K was obtained from a 22 m2 area. The variation in the thermal conductivities was related to the individual AlN grains and grain boundary characteristics.

Abstract: Thermoelectric properties of zinc oxide ceramics are largely influenced by the mobility variation because of the formation of double Schottky barrier at grain boundary. It was demonstrated that magnetic texturing enabled to fabricate highly c-axis oriented ceramics with orientation degree of 100 MRD. This high orientation of grains resulted in periodic grain boundary structures along ab -plane, which was confirmed by edge-on HRTEM images. Hall measurement revealed that the mobility was enhanced at 80% by the magnetic texturing. Furthermore, it was estimated that the effect of magnetic texturing was equivalent to the reduction in the density of trap by 5.9×1012 cm-2.

Abstract: A Bi2-xSbxTe3(x=0-1.5) thick film thermoelectric element was fabricated using centrifugal deposition and its Sb content-dependent thermoelectric properties were investigated. When the Sb content was low (x=0.5), two types of fine structure along the direction of the thickness were observed. Pole figure measurements revealed that the vicinity of the film surface was composed of single crystal layers oriented along the c-axis and the vicinity of the interface with the substrate was composed of randomly orientated layers. As the content of Sb increased, the degree of orientation improved, and at x=1.5 the entire film was close to a single crystal. A Bi0.5Sb1.5Te3 thick film showed p-type thermoelectric properties and a thermoelectric power factor of 3.5 ×10-3W/mK2. It was thus demonstrated that centrifugal deposition can be used to fabricate thermoelectric elements with high efficiency.

Abstract: Hydroxyapatite powders were prepared via mechanochemical synthesis method. The starting reagents for HAP powders were vibration ball milled for 2 h and then calcined at 800°C for 1 h. X-ray diffraction patterns and IR spectrum shown that pure crystalline apatite powder (HAP) was obtained. The as-synthesized powders were gelcasted and then sintered at 1200°C for 2 h. It was observed from SEM micrographs that the prepared green body has uniform microstructure. The bending strength of the dried green body was as high as 18 MPa that is enough to be machined into required form before sintering. The sintered ceramics has a density of 2.5 g/cm3 and a flexural strength of 72 MPa with a homogenous microstructure.

Abstract: WC-Co cermet is widely used in industrial applications such as cutting tools, dies, wear parts and so on. It is of great importance to establish the recycling process for the precious metal resources contained in WC-Co cermet, because all these metals used in Japan are imported. In this paper we reported a hydrothermal oxidation technique using nitric acid for the reclamation of WC and Co. The WC-Co cermet specimens with various WC particle sizes and Co contents were hydrothermally treated in HNO3 aqueous solutions at temperatures of 110-200°C for durations of 6-240 h. The Co was preferentially leached out into the acidic solution, while the WC was oxidized to insoluble WO3 hydrate which was subsequently separated by filtration. The hydrothermal treatment parameters such as solvent concentrations, treatment temperatures, holding time were optimized in respect to different kinds of WC-Co cermets. A hydrothermal oxidation treatment in 3M HNO3 aqueous solution at 150°C for 24 h was capable of fully disintegrating the cermet chip composed of coarse WC grains of 1-5 µm in size with 20 wt% of Co as binder. While the more oxidation resistant specimen composed of fine WC grains of 0.5-1.0 µm in size with 13 wt% of Co, was completely disintegrated by a treatment in 7 M HNO3 aqueous solution at 170°C for 24 h. The filtered solid residues were composed of fine WO3.0.33H2O powder and a small amount of WO3. The recovered WO3.0.33H2O powder can be easily returned to the industrial process for the synthesis of WC powder so that the overall recycling cost can be possibly lowered.

Abstract: The sintering behavior of AlN powders with different particle sizes and specific surface areas was investigated in the present work. 4.5-8.0mass% of the as-synthesized sintering aid from the Li2O-Y2O3-CaO system was added to these AlN powders, and they were then fired at 1400-1650oC for 6h. At the initial stage of sintering the fine AlN powder, it was recognized that sintering of primary particles occurred in agglomerations. With an increase of firing temperature, particles were rearranged and grains grew rapidly at the final stage. On contrary, in sintering of samples using the coarse AlN powder, the densification increased with raising sintering temperatures slowly, and the sintering temperature higher than 1650oC was required to obtain densified specimens.

Abstract: Monodispersed ZrO2 (includes 8mol % Y2O3) precursor nanoparticles, which were well dispersed in aqueous solution, were successfully synthesized. The as-synthesized ZrO2 precursor nanoparticles were homogenously coated on the surface of polystyrene particles. Multilayer coating process was successfully implemented by using polyacrylic acid (PAA) to modify the surface charges of the coated particles, which was characterized by zeta-potential, particles size distribution and scanning electron microscopy (SEM).

Abstract: The drying and sintering processes require large quantities of energy. If this energy could be used more efficiently, less would be consumed. In this research, the effect of the microwave on the manufacturing process of ceramics where heat was needed was examined. We obtained the following results when oxide ceramics were dried and sintered by microwave technique: In drying process, slip cast bodies were dried rapidly by microwave drying and compared with those dried by conventional drying techniques. In sintering process, power consumption and efficiency were lower with the time-control method than with the power-control method. The power-control method gave more precise temperature control under our experimental conditions.

Abstract: The effect of sintering additives on dielectric loss tangent and microstructure of AlN ceramics have been investigated. Different amounts of Y2O3 and La2O3 were added as sintering additives to AlN powder and pressureless sintering was performed at 1900 °C for 2 h in a flowing nitrogen atmosphere. Bulk densities of sintered AlN were found to be 3.27 and 3.32 Mg·m-3 when Y2O3 contents were 0.5 and 1 mol%, respectively. The bulk densities of AlN-Y2O3 increased with increasing La2O3 content. Densities of 3.41 and 3.42 Mg·m-3 were obtained for 0.5 mol%Y2O3-1 mol%La2O3-AlN and 1 mol%Y2O3-1 mol%La2O3-AlN ceramics respectively. A decrease in tan δ was observed with increase in La2O3 content and a minimum value of 1.3 x 10-3 was obtained for 0.5 mol%La2O3-1 mol%Y2O3-AlN ceramic in the present study.