Papers by Author: Naoya Enomoto

Paper TitlePage

Authors: Hui Tong, Miki Inada, Yumi Tanaka, Naoya Enomoto, 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.
Authors: Koji Matsui, N. Etoh, M. Ohgai, Masato Uehara, Naoya Enomoto, Junichi Hojo
Authors: Byung Kyu Moon, Kai Kamada, Naoya Enomoto, Junichi Hojo, Soo Wohn Lee
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.
Authors: Wen Wu Yang, Miki Inada, Yumi Tanaka, Naoya Enomoto, Junichi Hojo
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.
Authors: Junichi Hojo, Yuki Nonaka, Kai Kamada, Naoya Enomoto, Mikinori Hotta, Keita Shirouzu
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.
Authors: Dae Ho Choi, Kai Kamada, Naoya Enomoto, Junichi Hojo, Soo Wohn Lee
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.
Authors: Junichi Hojo, Hironori Kugimoto, Masato Uehara, Naoya Enomoto
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.
Authors: Mikinori Hotta, Naoya Enomoto, Junichi Hojo
Abstract: SiC was sintered with AlN and Y2O3 as sintering additives by spark plasma sintering (SPS). Using nano-sized β-SiC powder as the starting material, the sintered density reached about 95% of theoretical at 1800-2000oC for 10min. The β to α phase transformation of SiC was not found by XRD. The secondary phase such as Y2O3 decreased as the firing temperature was elevated, and β-SiC monophase was identified at 2000oC. It seems that the residual intergranular glassy phase is present between the SiC grains. Significant SiC grain growth was observed at 1800-1900oC by SEM. The grain size decreased with increasing amount of AlN additive. The maximum value of flexural strength of the sample reached approximately 800MPa. These results are discussed on the basis of the liquid-phase sintering mechanism in AlN-Y2O3 and Al2O3-Y2O3 systems.
Authors: Koji Matsui, Takanori Yamakawa, Masato Uehara, Naoya Enomoto, Junichi Hojo
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.
Authors: Keita Shirouzu, Yuki Nonaka, Mikinori Hotta, Naoya Enomoto, Junichi Hojo
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.
Showing 1 to 10 of 10 Paper Titles