Papers by Author: Toru Wakihara

Abstract: AlN ceramics used in electronic substrates and packages are fabricated by the densification of green bodies with sintering aids such as rare-earth oxides. The homogeneous dispersion of the sintering aids, achieved with the help of the mechanochemical bonding of several types of fine powders, is the key process for obtaining a good sinterability and high performance. In this study, we fabricated the AlN ceramics using AlN and nano-Y2O3 composite particles prepared by mechanical treatment. The AlN powder and nano-Y2O3 powder were ball-milled with Al2O3 balls and a dispersant in ethanol in a plastic pot. The powder mixture of AlN and Y2O3 was composited by mechanical treatment. The composite powder was granulated and pressed to obtain a green body. After dewaxing, the AlN green body was fired at 1800°C in 0.6 MPa N2. The sintered body possessed a fracture toughness of 3.6 Pa•m1/2, higher than that, 3.1 Pa•m1/2, of the AlN ceramics fabricated without the mechanical treatment. An observation of the fractured surface revealed that grain boundary reinforcement enhances the fracture toughness of the AlN ceramics made of composite particles.

Abstract: TiN nanoparticle-dispersed Si3N4 ceramics is one of the typical ceramics used for bearing applications. Because larger TiN particles considerably damage the mating metallic materials, smaller TiN particles must be dispersed in Si3N4 ceramics. In this study, we fabricated TiN nanoparticle-dispersed Si3N4 ceramics from Si3N4–nano TiO2 composite particles prepared by mechanical treatment. The mechanical properties of the fabricated TiN nanoparticle-dispersed Si3N4 ceramics were evaluated. At first, TiO2 nanoparticles were dispersed in ethanol using polyethylene imide as a dispersant with a lower molecular weight. Si3N4 powder was mixed with this slurry to obtain a powder mixture. In this case, the reaggregation of the TiO2 nanoparticles during the drying process is the problem that has to be solved. In this study, TiO2 nanoparticles and Si3N4 particles were mechanically joined by a particle composer to fabricate the composite particles from the powder mixture. TiN nanoparticles were uniformly dispersed in Si3N4 ceramics by using composite powder. The bending strength of the developed Si3N4 ceramics with TiN nanoparticles was improved, and its distribution was narrow due to the homogeneous dispersion of TiN nanoparticles.

Abstract: Rare-earth activated oxynitride or nitride luminescent materials have attracted considerable attention due to their potential applications as phosphors and pigments. Eu2+-doped -sialon has been reported to represent a new class of green phosphors with high efficiency. In this study, -sialon phosphor was synthesized by reduction nitridation of a zeolite. Eu ion-exchanged zeolite was fired at 1400 °C for 1 hour under NH3 gas containing 0.5 vol%C3H8. As a result, formation of -sialon with green emission under UV irradiation was confirmed.

Abstract: AlN–SiC solid solutions with p-type electrical conduction were fabricated with the addition of small amounts of Al and C. Powder mixtures of AlN and SiC with small amounts of Al and C (below 10 mol%) were consolidated by spark plasma sintering (SPS) at 2000°C for 10 min under 1 atm Ar, and then heat-treated at 2200°C for 3 h in an Ar flow to afford 2H AlN–SiC solid solutions. The relative densities of the 50AlN-50SiC-Al4C3 (A50-1AC) and 50AlN-50SiC-3Al4C3 (A50-3AC) samples were about 95%, whereas that of the 75AlN-25SiC-Al4C3 (A75-1AC) sample was about 86%. X-ray diffractometry (XRD) analysis showed that the samples comprised only the 2H phase, and except in the case of the A50-3AC sample, no diffraction peaks of Al and C were observed. Although the samples without the additives (Al and C) were electrical insulators, addition of Al and C introduced p-type semiconduction. The electrical conductivities at 300°C of the A50-1AC and A50-3AC samples were about 30 and 100 S/m, respectively, whereas that of the A75-1AC sample was about 10–1 S/m. It was found that addition of Al and C brought about electrical conduction in AlN–SiC solid solutions.

Abstract: HfO2-added Si3N4 ceramics are known to exhibit excellent high-temperature strength and excellent damage characteristics because HfO2 assists the crystallization of the grain boundary phase. However, the sintering shrinkage behavior and mechanical properties of HfO2-added Si3N4 have not been well clarified so far, although it has been reported that TiO2, in which Ti is from the same group as Hf in the periodic table, enhances the densification of the Si3N4-Y2O3-Al2O3-AlN system and wear resistance due to TiN formed from TiO2 and AlN in the grain boundary. In the present study, we focus on HfO2 as the sintering aid to investigate the sintering shrinkage behavior and mechanical properties of HfO2-added Si3N4. The powder mixtures are prepared by the addition of HfO2 to the Si3N4-Y2O3-Al2O3 or Si3N4-Y2O3-Al2O3-AlN system. The sintering shrinkage curves of HfO2-added Si3N4 ceramics show rapid shrinkage at 1600°C as compared with those of the Si3N4 ceramics without HfO2.The shrinkage can be explained by the formation of SiO2-Y2O3-HfO2 derived liquid phases. Furthermore, the mechanical properties of HfO2-added Si3N4 were as excellent as those of the Si3N4 ceramics without HfO2.

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: Electrically conductive Si3N4 ceramics were fabricated by dispersion of different characteristics of carbon nanotubes (CNTs). When the sintering aid of Y2O3-Al2O3-TiO2-AlN was used for lower temperature densification, it was confirmed that CNTs existed in Si3N4 ceramics from SEM observation and SiC was not identified in XRD analysis, which means that CNTs did not react with Si3N4. Relative density and electrical conductivity of the CNT dispersed Si3N4 ceramics depended on the characteristics of CNTs. Aggregation of CNTs, which is outstanding in much thinner CNTs, should limit densification of Si3N4. CNTs were well-dispersed by beads milling in ethanol. As a result, beads milling process was confirmed to be effective in unraveling and dispersing CNTs. It was shown that better dispersion of CNTs with higher aspect ratio resulted in higher density and electrical conductivity.

Abstract: Changes in crystal phase and electrical properties of oxides composed of Mn, Co, and Ni with the molar ratios of 5:2:1 and 3:3:1 were investigated. Starting oxides were fired from 250°C to 800°C, then maintained at prescribed temperatures for 3 h in air. In the 5:2:1 specimen, a monophase consisting of a cubic spinel oxide that is important to electrical conductivity was obtained at firing temperature of 800°C. In the 3:3:1 specimen, the monophase was obtained at temperatures ranging from 600°C to 800°C. Electrical resistance decreased exponentially with increasing temperature for all specimens fired at temperatures ranging from 250°C to 800°C, indicating that the oxides have intrinsic thermistor characteristics with negative temperature coefficient (NTC). The temperature dependence of the thermistor constant (B value) necessary for practical application was considered to be related to the existence ratio of cubic spinel-type and ilmenite-type structures and the lattice constant of the cubic spinel-type structure. The electrical conduction was stabilized by annealing at prescribed temperatures for more than 720 minutes.

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: Dense and homogenous Si3N4-TiN composites (5 vol% TiN) were prepared by using in situ synthesis method from Si3N4, AlN and TiO2 mixtures, containing Y2O3 and Al2O3 as sintering aids. In the prepared Si3N4-TiN composites, TiN grains were formed from TiO2 and AlN powders during the sintering process, in which ammonium citrate was used as a dispersant for raw TiO2 powders. The microstructures of the Si3N4-TiN composites with the increase of ammonium citrate were investigated by scanning electron microscopy (SEM). Citrate ions modified on the surface of TiO2 particles and protected the TiO2 particles in the mixed slurry to reduce the aggregations of TiO2 powders, and homogenous Si3N4-TiO2-AlN composite powder was prepared for sintering. The microstuctures of Si3N4-TiN were developed after sintering with the uniform distribution of TiN grains in the Si3N4 ceramics. It was found that the microstructure of Si3N4-TiN composite was improved significantly with 0.20 g ammonium citrate in the system, TiN grains with 0.2-0.3 μm in diameter distributed throughout Si3N4 matrix. It was a practical and useful way to improve the microstructure of Si3N4-TiN composite without the alteration of the preparation procedure.