Papers by Author: Yuji Hotta

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: 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: Porous ceramics of hydroxyapatite was fabricated utilizing the crystal growth of thin ice columns parallel to one another in gelatin gel containing hydroxyapatite nanoparticles. The obtained ceramics possessed unidirectional pore channels with a porosity of around 75% and showed compressive strength of up to 13.1 MPa. As control materials, porous hydroxyapatite ceramics with a directionless pore structure were also fabricated by isotropic freezing and compared with the unidirectional samples regarding compressive strength and tissue reaction in vivo. Although the porosity and pore size distribution were similar, the compressive strength and new bone formation ability of the unidirectional samples were significantly greater than those of the random structured porous ceramics.

Abstract: We have developed a new environmentally friendly fabrication process for alumina ceramics using hydraulic alumina (HA) and water. Without the addition of an organic binder, alumina green bodies retained their original shapes by the hydration of HA in the green bodies. In the case of α-Al2O3-HA-H2O system, alumina ceramics with 97.8% theoretical density were obtained. Added water acted as a fugitive material in the hardened green body. In the case of HA-H2O system, porous alumina ceramics with a wide range of open porosity (56.6–69.1%) and narrow pore size distributions were fabricated without the use of organic fugitive materials. The results of evolved gas analysis-mass spectrometry measurements showed that the new fabrication process for alumina ceramics was confirmed to be eco-friendly.

Abstract: Hydrothermal synthesis of nano ZrO2 powders were carried out at a temperature range of 100 to 200°C, using zirconium solution. Formation of ZrO2 formation was accelerated with temperature and precipitation of ZrO2 took place directly from the solution. XRD analysis revealed that monoclinic ZrO2 was formed at all conditions. Crystallite size was found to range from 2.6 to 4 nm. Mean particle size increased from 93 nm at 135°C to 125 nm at 200°C after reaction time of 2 h.

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).