Papers by Author: Yoshiaki Kinemuchi

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: Microwave sintering is a process in which target materials absorb microwaves and heat themselves from the inside. When microwave energy is effectively absorbed by the material, energy consumption for the sintering can be reduced. Our study is focused on the microwave sintering of BaTiO3 to more rapidly obtain dense ceramics with specific characteristics. For BaTiO3-based electronic components, the sintering temperature is too high for manufacture, so various additives are used to decrease the sintering temperature without undue worsening of the electrical characteristics. In this work, during microwave sintering, BaCO3, H3BO3, BaB2O4 and LiF were added to form a liquid. The effects of the amount of liquid phase on density and dielectric properties were investigated. BaTiO3 sintered with BaCO3 and H3BO3 showed dielectric properties, whereas BaTiO3 sintered with BaB2O4 had semiconducting properties with PTCR characteristics. Also, LiF-added BaTiO3 indicated a dielectric constant in which the peak shifts to lower temperatures with higher LiF concentrations.

Abstract: Wet jet milling is a new technology for the pulverization of powders, in which streams of slurry, higher than 100m/s, collide in the channel made of diamond. The effect of processing parameters was investigated in the system of BN/SiO2 mixed powders. The particle size after milling was evaluated by the measurement of BET surface area. The BET surface area of BN increased with increase in the number of milling, in the mixture ratio of SiO2/BN, in the particle size of SiO2 and in the processing pressure, whereas no significant change in the size of SiO2 particle was found. The surface area of BN increased by about 2.1 times by wet jet milling for only about 20 minutes (5 times).

Abstract: Fine AlN powder doped with Y2O3 and CaO as sintering additives was ground by a ball mill, a planetary ball mill and a super-fine grinding mill in order to obtain fine homogenous powder for low-temperature sintering. The size reduction and the sinterability of ground powders at 1500oC for 6 h were investigated. The size and shape of the agglomeration showed no significant change after the ball mill and planetary mill processes, resulting in poor densification. On the contrary, AlN particles with size of 50~100 nm was pulverized and dispersed by a super-fine grinding mill with very small ZrO2 beads as a mill media. The microstructures of the specimen exhibited equiaxed and homogenous grains with size of 0.3~0.4 μm. Pores in the specimens were eliminated. The thermal conductivity was 70W/mK, which is better than that of Al2O3 ceramics (~20W/mK).

Abstract: Co-firing of multiple materials results in the formation of internal stress due to the difference in shrinkage behavior. This internal stress causes retardation of sintering, crack formation and/or de-lamination at the interface. To reduce internal stress in layered structures, homogeneous pressing via centrifuge has been attempted. The effect of centrifugal pressing was demonstrated by film sintering, in which crack suppression and void removal by the present process were revealed. Owing to the homogeneous microstructure, the thermal stability of the film was significantly improved.

Abstract: Centrifugal sintering is a novel process for the preparation of thick films. In this process, high gravity such as 104g is applied at high temperatures. Because of the distinctive measure of pressing by the centrifugal force, gradient of pressure arises in the specimen, resulting in the graded porous structure during the progress of sintering. After the sintering, highly densified thick films are obtained. Furthermore, crack formation was suppressed by this process.