Papers by Author: Shunkichi Ueno

Abstract: Lotus-type porous metal with many straight pores is attractive as a heat sink because larger heat transfer capacity is obtained due to the small diameter of the pores. The heat transfer capacity of the lotus-type porous copper heat sink was calculated using the model with the pores of uniform diameters. However, actual lotus-type porous metals have a distribution of pore diameter. In the present work, we investigated the lotus-type porous copper fin model by considering size distribution of the lotus-type porous copper fin. Prediction of the heat transfer characteristics for the lotus-type porous copper heat sink shows a good agreement with the experimental data.

Abstract: In this paper, the internal friction behaviors of unidirectionally solidified Al2O3/YAG eutectic that are examined by the authors in passed several years are reviewed. The internal friction of this eutectic increased with increasing temperatures above 1200oC. The apparent activation energy of the eutectic sample in the range from 1100 to 1400oC was same to that of Al2O3 single crystal. Above 1200oC, the internal friction of this material drastically increased with increasing temperatures. The magnitude of the internal friction strongly depended on the elevated temperatures and the cycle numbers of the torsional loading. For the 1400oC test, the internal friction gradually increased with the loading cycles and then saturated by 103 times of the loading cycles.

Abstract: A porous alumina with cylindrical pores was fabricated by unidirectional solidification under hydrogen gas flow (0.1 MPa) using alumina feed rods doped with silica, calcia or sodium oxide. The additives in the feed rods strongly affect the formation of porous structure during the solidification. The porosity increases with increasing silica content. The increase of porosity is enhanced by further addition of sodium oxide. The addition of calcia to the feed rod is effective on the homogenization of pore distribution. The porous alumina with 1mol%CaO and 20mol%SiO2 additives showed 50% porosity and homogeneous pores distribution.

Abstract: A new fabrication method of porous ceramics using unidirectional solidification was proposed in this paper and the role of silica additive on the formation of porous alumina structure was discussed. A porous alumina was formed by unidirectional solidification process that was conducted using floating zone melt method in hydrogen atmosphere at normal pressure. The effect of silica additive on the porosity and the pore size of the solidified samples was examined. The porosity of the solidified samples increased with increase of silica contents. On the other hand, the pore size decreased with increase of silica content. The average pore size of 10% and 20% silica added samples were 0.396 mm and 0.184 mm, respectively.

Abstract: The solubility of manganese ion in layered perovskite La4Ni3O10 phase was examined and the detailed crystal structure of this phase was discussed. The solid solution of La4Ni3-xMnxO10 was prepared by solid state reaction. The crystal structure of La4Ni3-xMnxO10 was analyzed by powder X-ray diffraction and Rietveld methods. The solid solution for x=0.5 had a orthorhombic system with lattice parameter a=0.5485(0), b=0.5433(6) and c=2.8034(9). In detailed observation of the X-ray diffraction data, some superlattice reflections that were forbidden for a face centered orthorhombic cell were identified. A possible space group Pnnn (no.48) was derived from the reflection conditions.

Abstract: In this work, cordierite whiskers were successfully coated on cordierite honeycomb support by in-situ method. Whisker coating was performed on walls of the pore channels of cordierite support from aqueous slurry. Sintering was carried out at 1300 °C for 4 hours. XRD measurements revealed a complete formation of cordierite phase, no traces of other phases were identified. A well adherence of coating on the support was identified from the microstructure. SEM analysis also revealed the formation of three-dimensional network of cordierite whiskers with open pores between them on the cordierite support.

Abstract: The corrosion behaviors of Ln2Si2O7 (Ln=Yb and Lu) and ASiO4 (A=Ti, Zr and Hf) EBC's were examined at 1500oC in water vapor environment. These oxides were coated on silicon nitride specimens by oxidation-bonded reaction sintering technique. Among Ln2Si2O7 system, though the thermal expansion coefficient of Yb2Si2O7 phase is closer to silicon nitride than that of Lu2Si2O7 phase, the corrosion resistance of silicon nitride with Lu2Si2O7 EBC was higher than that of Yb2Si2O7 EBC sample. In these EBC materials, boundary silica phase was easily corroded by water vapor. Among ASiO4 system, though the corrosion rate of HfSiO4 bulk was larger than that of ZrSiO4 and TiSiO4 phases, the oxidation of the silicon nitride substrate for HfSiO4 coated sample was smaller than that of other two. Many cracks were in ASiO4 EBC layer during the corrosion test. The introduced crack length in HfSiO4 EBC layer is shorter than in ZrSiO4 EBC layer due to the thermal expansion mismatch between EBC materials and the silicon nitride substrate.

Abstract: Various coating methods of EBC layer for silicon nitride were discussed. High density EBC layer was successfully coated by different techniques such as sputtering, sol-gel and reaction sintering methods. Water vapor corrosion and recession mechanisms of Lu2Si2O7 which is a potential material for EBC were discussed. The problems in the development of EBC revealed by corrosion tests were summarized. The most important problem addressed here was the corrosion of silica at grain boundary. Due to corrosion of silica at the boundary, formation of porous surface is inevitable, then the silicon nitride substrate gets easily oxidized and/or corroded by water vapor. To resolve this issue, we propose a new EBC material without boundary silica and the corrosion mechanism of this improved EBC material is discussed.