Papers by Author: Hideki Kita

Abstract: SiC substrates were successfully bonded by tape-cast SiC-Al2O3-Y2O3 interlayers in the temperature range of 1800-1900 °C for 60 min at 15 MPa pressure in flowing Ar. Based on microstructure observations, we find that the α-SiC interlayers result in equiaxed microstructure at all joining temperatures while the tape-cast β-SiC leads to a microstructure varying from a mixed characteristic to an elongated one with joining temperature. The detailed image analysis suggests the close interlayer thickness, the quicker grain growth rate of β-SiC relative to α-SiC one and the good agreement between aspect ratio and morphology in all joints. Moreover, the SiC grains of interlayer near to interface develop preferentially from the external SiC grains of substrate, which is possibly related to the morphology difference between substrate and interlayer and the grain growth behaviors. Finally, the SiC joined with tapes exhibits an average bending strength over 359 MPa and usually fracture within SiC bases.

Abstract: In order to lower environmental impact in manufacturing, it is necessary to know the processes of consumption and emission of resource and energy with bird’s eyes view point. In this paper, exergy analysis was conducted in case when heater tube used in the molten aluminum was made of steel and ceramics. As results, it is revealed that ceramics have effectiveness in the reduction of environmental burden through the lifecycle process, while ceramics consumes much higher exergy than the steel one in production stage.

Abstract: Exergy is a measure which can commonly deal with the quantification of the variety of resources, products and energy coming in and going out the manufacturing systems. In this study, exergy analysis was conducted on ceramics and steel heater protection tube used in aluminum casting, and the amount of exergies consumed through their life cycle were calculated. In the production stage, ceramic heater tube consumes much higher exergy than the steel one does, however, analysis throughout the life cycle including production, operation and waste in seven years shows that exergy consumption for the ceramic tube is less than that of the steel tube.

Abstract: Post-reaction sintering is one of the fabrication processes of Si3N4 ceramics, which has received considerable attention as a cost-effective process due to the use inexpensive Si powder as a raw material. So far, many researches on the development of this method have been performed in order to improve their properties; however, the sintering shrinkage behavior, which is valuable for the optimization of the firing conditions, has not been well clarified. In this study, we focus on the post-reaction sintering of the Si-Y2O3-Al2O3 system, and investigate its sintering shrinkage behavior by dilatometery. It was found that there is no shrinkage from 1400 to 1600 °C due to grain rearrangements in the green body of the reaction-bonded Si3N4. Furthermore, the shrinkage of the reaction-bonded Si3N4 commenced at approximately 1750 °C, which is higher than the shrinkage temperature of the green body of conventional Si3N4 powder. The restriction of the shrinkage appears to result from the neck growth and strong aggregation among the reacted Si3N4 particles.

Abstract: Silicon nitrides are often used as ladles, stalks, heater element protection tubes, etc., in the metal casting industry. A low wettability for molten metals is required for these purposes since wetting by molten metals leads to adhesion of solidified metals, which causes several problems. Surface structure is known to affect wettability. Thus, the present study attempts to fabricate silicon nitrides with controlled surface structures. Silicon nitrides, whose surfaces were covered with ordered hemispherical protrusions, were fabricated by the slip-casting technique. The cast bodies were sintered, and subjected to wettability tests using molten metals. For comparison, silicon nitrides with as-sintered and polished surfaces were also prepared. The surface with protrusions exhibited a lower contact angle compared to the as-sintered and polished surfaces. The contact angle depended on the diameter of the hemisphere; it was the largest at a diameter of 0.3 mm.

Abstract: Post-reaction sintering as a technique for the fabrication of Si3N4 ceramics has received much attention as a cost-effective process due to the use of cheap Si powder as a raw material. In this method, the rapid exothermic nitridation of Si results in local melting of Si to cause its agglomeration, which is expected to be a flaw after densification. Therefore, control of the exothermic reaction is needed to improve the reliability of post-reaction sintered Si3N4 ceramics. In this study, Si3N4 ceramics were fabricated by post-reaction sintering with Si3N4 or SiO2 powders in order to control the exothermic reaction. As a result, the microstructure and bending strength of Si3N4 ceramics was changed by adding these additives. In particular, the addition of SiO2 resulted in the high strength of Si3N4 ceramics. Consequently, it was found that Si3N4 and SiO2 particles played the role of diluents, and SiO2 was effective in post-reaction sintering as an oxygen donor.

Abstract: Graphite powder was treated with lanthanum, aluminum and magnesium phosphate solution, and oxidation resistance of the obtained graphite powder was evaluated. Oxidation starting temperature and oxidation completion temperature of graphite powder treated with various phosphates were 50-100oC higher than those of as-received graphite powder. Graphite powder treated with small amount of lanthanum phosphate exhibited the higher oxidation starting temperature than graphite powder treated with aluminum and magnesium phosphates. LaP5O14 would partially exited on graphite powder, and protect the edge carbon atoms of graphite and reduce the reactivity of carbon atoms toward oxygen, resulting in improving the oxidation resistance.

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: Lu2O3 and SiO2 were used as sintering additives and rod-like β-Si3N4 seeds were added to enhance β-Si3N4 grain unidirectional growth. Silicon nitride ceramics were prepared by tape casting and gas pressure sintering
at 1950oC in 10 atm nitrogen atmosphere for 6 h. Compare to the no-seeded Si3N4, the seeded and tape-cast Si3N4 ceramics have obvious anisotropic microstructure and anisotropic properties. When a stress applied along with the grain alignment direction, the bending strength of the seeded and tape-cast Si3N4 at 1500oC was 738 MPa, which was almost the same as its room temperature bending strength. However, the bending strength of the seeded and tape-cast at room temperature was 556 MPa (perpendicular direction); and their thermal conductivity were 67 W/m·K (perpendicular direction) and 83 W/m·K (parallel direction), respectively. The anisotropic properties of the seeded and tape-cast Si3N4 were attributable to the elongated Si3N4 grain alignment.

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.