Papers by Author: Junichi Hojo

Abstract: ZnO nanorod/TiO₂ nanoparticle composite films were prepared by a sol-gel method, in which the slurry of TiO₂ nanoparticles (NPs) were penetrated into ZnO nanorod arrays. The agglomerate size of TiO₂ NPs was controlled with tetraethylammonium hydroxide. Lowering ZnO nanorod array density was an effective way to fill TiO₂ NPs in the space of ZnO nanorod array. The presence of ZnO and TiO₂ was confirmed by scanning electron microscopy and X-ray diffraction. Triblock copolymer (P123) was added as a template agent to obtain high specific surface area, and the solar cell performance was improved. However, the viscosity of the TiO₂ slurry was increased by addition of P123, and TiO₂ content in the composite film was reduced. Low concentration TiO₂ slurry was preferable to be penetrated into ZnO nanorod array and solar cell performance was further improved.

Abstract: Translucent β-Si₃N₄ sintered ceramics have been fabricated by using AlN-MgO sintering additives. In the present study, the authors employed AlN-MgO as a standard sintering aid, and investigated the effects of sintering conditions on the translucency of Si₃N₄. Furthermore, various oxides such as HfO₂, Sm₂O₃, Y₂O₃, Sc₂O₃, La₂O₃, Nd₂O₃, CeO₂, CaO, ZrO₂ etc. were used as the sintering aids of Si₃N₄, and the sintered β-Si₃N₄ ceramics exhibited different transmittances in the visible region. It was found that the transmittance of sintered ceramics was mainly affected by the sintering additives.

Abstract: SiC ceramics were fabricated from submicron- and nano-sized starting powders with Y2O3 and AlN additives by SPS process. The SPS process and the use of AlN additive were found to be effective for achieving the nano-grained microstructure with retarded grain growth. The nanoporous SiC ceramics were also obtained under the similar conditions. The optimum heating schedule and additive composition were proposed.

Abstract: In this work, porous alumina ceramics were obtained by controlled sintering of alumina-carbon black powder mixtures. In order to develop the porous alumina ceramics with high strength, as the amount of carbon black increased, the number of small pores increased because the pore characteristics and relative density the influence of SPS condition and carbon black content on was studied.

Abstract: The effect of calcination on the mechanical properties of hydroxyapatite and zirconia composite (HAp:ZrO2= 30:70, 50:50, 70:30 mass%) was investigated. The calcination of ball-milled mixture in air at 900°C for 0, 2 and 4 hours increased the crystallinity. Then, it was assigned that the particles form of hydroxyapatite was changed from needle-like to sphere-like by calcination. The calcined mixture was sintered by spark plasma sintering (SPS) at 1200°C with a pressure of 40MPa for 5 minutes. The calcination process caused the enhancement of flexural strength of the composite.

Abstract: SiC and AlN form a solid solution in the wide compositional range, expectantly leading to control of the semiconductive property. In the present work, the SiC-AlN composites were fabricated by sintering process, and evaluated with emphasis on the distribution of SiC and AlN and electrical property. SiC and AlN powders were mixed at a molar ratio between 90:10 and 10:90, and sintered at 1900-2100 °C for 30 min under 50 MPa in Ar atmosphere by spark plasma sintering technique. The sintered bodies reached high densities over 95 % of theoretical, and the grain size increased with an increase in the sintering temperature and the AlN content. The SiC-AlN composites had 3C and 2H phases in SiC-rich composition, while 2H phase only in AlN-rich composition, and the mutual dissolution between SiC and AlN was enhanced at high temperatures. The electrical conductivity decreased with dissolution of AlN into SiC because of the increase in band gap.

Abstract: Small amounts of Al2O3 were added to fine zirconia powder by different ways: powder mixing, hydrolysis of alkoxide, and homogeneous precipitation. During a constant rate heating process, the Al2O3 addition slightly raised the starting temperature of densification of powder compact, and the densification was remarkably stimulated by Al2O3 at temperatures above about 1100oC. According to an isothermal analysis of densification, the densification rate was retarded by Al2O3 addition just after the start of sintering and then the densification rate increased significantly during sintering compared to Al2O3-free powder. These results mean that Al2O3 particles pinned the shrinkage of zirconia powder compact at the initial stage, and diffuse toward zirconia surface to enhance the sintering. The sintering mechanism was explained by the grain-boundary diffusion for the Al2O3-free powder and the volume diffusion for Al2O3-added powder. When the Al2O3 was added to zirconia powder by homogeneous precipitation and alkoxide methods, the densification rate was more stimulated compared to powder mixing method. The sintering mechanism did not change by the way for Al2O3 addition. The Al2O3 addition by the chemical process tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al2O3 by these chemical processes.

Abstract: High-density SiC-AlN composites were fabricated from powder mixtures (50:50 in mol) in 1900oC-2100oC temperature range by SPS process. SiC(0.3μm or 0.03μm) and AlN(1.1μm) were used as starting materials. The density of composite increased with increasing firing temperature. From the identification of crystal phase and the change of lattice constant, mixed phases of 3C(β-SiC)ss and 2H(α-SiC/AlN)ss were found at 1900oC and 2000oC, and only 2Hss was found at 2100oC. The OM and EPMA observation indicated that SiC-rich regions (size:10-50μm) existed throughout SiC(0.3μm)-AlN composite because of aggregation of SiC powder. In SiC(0.03μm)-AlN composite, on the other hand, SiC-rich regions (size:submicron) and AlN-rich regions (size: approximately 1μm) existed on a microscopic level at 1900oC, whereras, it was confirmed from EPMA and SEM observation that homogeneous 2H(ss) formed with large grain-growth at 2100oC. The microstructure of SiC(0.03μm)-AlN composite at 2000oC was analyzed to investigate more detailed compositional variation of solid solution. SEM-EDS observation indicated that 3C(ss), SiC-rich 2H(ss) and AlN-rich 2H(ss) existed in SiC(0.03μm)-AlN composite at 2000oC.

Abstract: TiO2-coated SiO2 particles were prepared using industrially-wasted amorphous SiO2 powder by an alkoxide method, in which the mean particle size of SiO2 was approximately 100 nm. The SiO2 powder and titanium tetraisopropoxide were mixed in ethanol and an appropriate amount of distilled water was added to the suspension for hydrolysis of the alkoxide. The product was calcined in the temperature range from 600 to 1300°C to crystallize the deposited TiO2. The homogeneous deposition of TiO2 on the SiO2 surface was observed by SEM. It was found that the SiO2-support tended to retard the transformation from anatese to rutile. The maximum photocatalytic activity for decomposition of methyleneblue was obtained on the TiO2-coated SiO2 powder calcined at 1000°C, and was higher than that of a commercial TiO2 powder.

Abstract: Si3N4-based composite powders have been synthesized by vapor phase and liquid phase processes. Nano-sized Si3N4-TiN composite particles were formed by the vapor phase method, in which TiN nanocrystallites were included in amorphous Si3N4 particles. The composite powder was also formed by the liquid phase method, where Si and Ti complex imide powders were prepared in an organic solvent and decomposed by heating. Si3N4-TiN nanocomposites were fabricated by hot pressing of the composite powders. The Si3N4-TiN composites were also fabricated by in-situ process from amorphous Si3N4 and Y2O3-TiO2-AlN additive. In all processes, rod-like Si3N4 grain growth was stimulated by TiN inclusion.