Papers by Author: Naoki Kondo

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: Silicon nitride ceramics with and without β-Si3N4 seed addition were prepared via tape casting nonaqueous ceramic slurries, laminating the green ceramic tapes, and gas pressure sintering the green bodies in nitrogen atmosphere. Lu2O3 and SiO2 were used as sintering additives. The results showed that the rod-like β-Si3N4 seed was helpful to enhance β-Si3N4 grain growth unidirectionally. The elongated grains grown from seeds were preferentially oriented parallel to the casting direction, resulting in anisotropic microstructure. When a stress was applied with along the grain alignment, the bending strength of the tape-cast Si3N4 with 3 wt% β-Si3N4 seed addition measured at 1500oC was 738 MPa, which was almost the same as the room temperature bending strength 739 MPa, and the fracture energy can be improved from 301 J/m2 at room temperature to 781 J/m2 at 1500oC. The large fracture energy and bending strength at 1500oC were attributable primarily to the unidirectional alignment fibrous grains and a high melting point grain boundary phase.

Abstract: Grain alignment control to make anisotropic microstructure is one of the most promising techniques to achieve superior mechanical properties in specific directions. Anisotropic silicon nitrides, which were fabricated by a forging technique, can show superior mechanical properties at room temperature as well as at elevated temperatures. A sinter-forged silicon nitride with yttria and alumina additives exhibited very high strength of 2.1GPa at room temperature, meanwhile that with lutetia additive showed high strength of 700MPa at 1500oC. Anisotropic silicon nitrides are also advantageous to achieve higher fracture energy. Such silicon nitrides can show 3~5 times higher fracture energy than isotropic ones. Sinter-forging technique is also applicable to fabricate porous anisotropic silicon nitrides. In this paper, fabrication and mechanical properties of anisotropic silicon nitrides are briefly described.

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.