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Paper Title Page
Abstract: The discovery of sialons in 1971 was a significant step in the history of nitrogen ceramics, because it broadened the field into a wider range of chemistry, and simultaneously increased the flexibility to modify microstructure and properties. During the last 35 years this has resulted in the development of a spectrum of materials, mainly based on the α- and β- structural forms. However, the subject has remained broadly within the scope of structural ceramics. During the 1990s, a range of new sialon derivatives were prepared with a more varied starting chemistry, and the corresponding final materials demonstrated a correspondingly wider variety of structural complexities. In response, many sialon researchers have started to broaden their interests beyond the limiting horizon of structural applications, and considerable progress has been made in the development of transparent and coloured materials, and also derivatives with useful electronic properties.This enlargement of the sialons field is still in its infancy, but promises to generate a much wider spectrum of materials, which can be tailored to meet the increasingly multifunctional requirements of modern day engineering applications.
Abstract: Silicon nitride (Si3N4) is one of the most attractive materials for wear applications because it has excellent wear resistance and offers advantages such as light weight, higher strength and toughness, and good corrosion resistance. In 1984, Materials Div., Toshiba Corp. (today, Toshiba Materials Co., Ltd.) and Koyo Seiko Co. Ltd. (today JTEKT Corp.) successfully utilized high-strength silicon nitride for anti-friction bearings for the first time in the world.1-3 This ceramic bearing was a most successful product and has expanded in area and volume through key innovations such as pioneered compositions, further improvement of durability against a steel ball and the development of a conventional fabrication process. Since 1989, Yokohama National University group has investigated new materials development in silicon nitride ceramics, densification/strengthening mechanisms in an optimized sintering aids system, powder processing for reliable components and tribological evaluation for bearing applications. Subsequently it was confirmed that the addition of TiO2 and AlN to an Si3N4-Y2O3-Al2O3 system promoted densification at low temperatures.4 During firing, the TiO2 changed into TiN at the grain boundary, causing grain boundary strengthening.5,6 Most recently, it has developed a carbon nanotube (CNT) dispersed silicon nitride with high strength and high electrical conductivity that is expected to open up new applications as a new functional silicon nitride.7 However, there are many items to be overcome toward the future, which are the development of cost reduction processes with higher material reliability, and the opening up of new applications supported by validated evaluation techniques including tribology, flaw detection and life prediction, raw powder problems related to cost and production volume, and the classification of silicon nitride bearings for various graded applications.
Abstract: An important aspect of previous sialon research in NZ has been the development of new synthesis methods, including refinements in carbothermal reduction and nitridation (CRN) methods and the use of mechanochemical activation of sialon precursors (either Al and Si nitrides and oxides or CRN mixtures). Mechanochemical activation of CRN mixtures of clay and carbon heated in N2 formed β-sialon (z = 2) at 1300oC (100oC lower than in unground mixtures) but 21R polytypoid and corundum were also formed. More recently, our attention has focussed on the technique of silicothermal reduction and nitridation (SRN) to synthesise other sialons, including the AlN polytypoids and Na and Li α-sialons. The interest in the polytypoids springs from their expected physical properties (thermal conductivity and good electrical insulation similar to AlN), their covalent bonding and relatively light weight arising from their high Al and N contents and their elongated crystal morphology which may improve the crack resistance of polytypoid composites with α-sialon. This paper describes the development of SRN single-step synthesis of high-purity dense 15R sialon from clay, Si and AlN, and the effect of additives on the synthesis and sintering of the product. A method is also described for SRN synthesis of Na and Li α-sialons from clay, Si and AlN using fluoride additives. Fluorides have the advantage of small size, high electronegativity, leading to their known facilitation of AlN synthesis. Furthermore, they do not readily enter the sialon structure but may toughen it by formation of glassy phases. Fluorides allow use of clay in this SRN synthesis by introducing M+ without additional oxygen, but have the disadvantage of generating SiF4 as a byproduct. The reaction using LiF proceeds readily at the very low temperature of 1200oC via an O-sialon intermediate by a mechanism which probably involves Si migration assisted by the formation of SiF4.The effect of mechanochemical activation (high energy grinding) on the SRN formation and sintering of Na and Li α-sialons, O and β-sialon has also been studied.Grinding the SRN O-sialon precursor promotes O-sialon formation in powders but not in pellets due to pre-reaction sintering, which is facilitated by the smaller particle size. Grinding Na and Li α-sialon SRN precursors forms a mixture of sialons rather than the target monophase product, while sintering of all the sialons is assisted by grinding their SRN precursors.
Abstract: Oxynitride glasses are effectively alumino-silicate glasses in which nitrogen substitutes for oxygen in the glass network, resulting in increases in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. Calcium alumino-silicate glasses containing fluorine are known to have useful characteristics as potential bioactive materials. Therefore, the combination of both nitrogen and fluorine additions to these glasses may give useful bioglasses with enhanced mechanical stability. This paper gives a review of oxynitride glasses and reports glass formation and evaluation of glass properties in the Ca-Si-Al-O-N-F system. Within the previously defined glass forming region in the Ca-Si-Al-O-N system, homogeneous, dense glasses are formed. However, addition of fluorine affects glass formation and reactivity of the glass melts and can lead to fluorine loss as SiF4, but also nitrogen loss, and cause bubble formation. At high fluorine and high Ca contents under conditions when Ca- F bonding is favoured, CaF2 crystals precipitate in the glass. It was found that fluorine expands the glass forming region of Ca-Sialon system and facilitates the solution of nitrogen into the melt.
Abstract: Various rare-earth-doped α-SiAlON powders with high purity were prepared to study mechanical and optical properties of SiAlON-based functional materials in connection with ionic radius and electronic structure of rare-earth elements. Single phase rare-earth-doped α-SiAlON powders were obtained at a temperature as low as 1873 K by heating powder mixtures of rare-earth oxide, AlN and highly active ultrafine amorphous Si3N4. Bending strength of highly dense rare-earth-doped α/β-SiAlON-based ceramics was increased with decreasing radii of rare-earth ions, i.e., Yb-SiAlON-based ceramics exhibited excellent high-temperature strength and oxidation resistance caused by the small ionic radius of ytterbium. As for optical application, α-SiAlON is an excellent host lattice with good thermal and chemical stability for doping rare-earth element which activates photoluminescence. Europium-doped Ca-α-SiAlON phosphor formulated as CaxEuy(Si,Al)12(O,N)16 (where 0
Abstract: The results on the effect of nanostructured β-sialon precursor on the sintering and properties of the resultant ceramics are presented. The standard mixture of β-sialon precursor with 0.4z substitution degree was activated for 30 min in a planetary mill with an acceleration of 28g. Activation in the planetary mill resulted in diminution of the crystallite size and significant imperfection of the crystal lattice of the powder particles. The sintering experiments were performed at 1450-1600 °C in a powder bed. The relative density of the sintered bodies achieved the value of 97 %. The nanostructured material was produced after pressureless sintering at 1500 °C. It is concluded that high activation degree of the powders is necessary to obtain fully dense ceramics.
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: A commercial silicon nitride powder with sintering additives was ground by high-energy milling to reduce particle size. Nanometer sized powder was obtained. The powder was densified for short time by spark plasma sintering to prevent grain growth. Nanometer-grained Si3N4 ceramics were obtained. Plastic deformation of the Si3N4 nano-ceramics has been studied in compression over a wide range of strain rates and temperatures. The experimental results revealed that a transition in stress exponent, n, at each temperature. The n value decreased from ~ 2 to ~ 1 with increasing applied stress. Activation energy was also different for the two regions, decreasing from 858.2 kJ/mol in the n ~ 2 region to 571.8 kJ/mol in the n ~ 1 region. Effect of sintering additives on plastic deformation was also discussed.
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.
Abstract: Nano-sized turbostritic-BN (t-BN) was fabricated through chemical process using boric acid and urea in this work. By the same method, the AlN powders coated with nano-BN were prepared too. The results of X-ray diffraction (XRD) and transmission electron microscope (TEM) revealed that nano-sized t-BN was synthesized at about 600°C in nitrogen gas and it surrounded the surface of AlN particles. High-density AlN/BN nano-composites were fabricated spark plasma sintering (SPS). Microstructure and properties of AlN/BN nano-composites (5~30vol% BN) were investigated. The h-BN flake particles were homogenously dispersed at AlN grain boundaries and within grains in the AlN/BN composites. A little nano-BN additions significantly improved the bending strength of the nano-composites. However, the bending strength was decreased with the BN content increasing. The thermal conductivity of AlN/BN nano-composites was investigated too.