Papers by Keyword: ß-SiAlON

Abstract: This paper reviews the most important results by the authors on the processing of textured -Si3N4 and -sialon by slip casting in a strong magnetic filed of 12 T and reaction-sintering. The a, b-axis textured -Si3N4 and -sialon have been obtained using the static magnetic field because of the magnetic susceptibility of ca, b > c c for -Si3N4. However, the c-axis textured -Si3N4 has also been successfully obtained using a rotating magnetic field. The -Si3N4 crystal was found to exhibit substantially stronger orientation ability than the a-Si3N4 crystal regardless of the Si3N4 raw powders. It reveals that the -Si3N4 nuclei play a key role in the texture development in -Si3N4/-sialon.

Abstract: Different compositions of SiAlON materials have been prepared from separate nanopowders and their composites: Si3N4 – AlN, Si3N4, Al2O3 and Y2O3, produced by the method of plasma-chemical synthesis. Compositions have been pressure-less sintered in nitrogen medium up to 1750°C with a heating rate of 10°C/min and holding time for 2 hours or by spark plasma sintering (SPS) in vacuum at 1700°C with a heating rate of 100°C/min and holding time of 5 min. The densification behaviour of the materials at these sintering conditions depends insignificantly on the powder composition. It is possible to obtain dense ceramic materials with relatively fine-grained structure (200-300 nm) from nanosized powders at relatively low temperatures (1400 - 1600°C) with good mechanical properties.

Abstract: Different compositions of α- and α-/β- SiAlON materials have been prepared from separate nanopowders and their composites: Si3N4 – AlN, Si3N4, Al2O3 and Y2O3, produced by the method of plasma-chemical synthesis. Compositions have been sintered by spark plasma sintering (SPS) method in vacuum at 1700 °C with a heating rate of 100 °C/min and holding time of 5 min. The densification behaviour of the materials at these sintering conditions depends insignificantly on the powder composition. It is possible to obtain dense ceramic materials with relatively fine-grained structure (200-400 nm) and good mechanical properties from nanosized powders at relatively low temperatures (1400-1600 °C). The holding time at applied compacting conditions is still too short to ensure the formation of α- SiAlON phase corresponding to the phase diagram. All samples consist of β- SiAlON’s of differing composition and this is the reason for relatively low hardness of samples (HV5 = 15,6-16,9 GPa).

Abstract: In the present work, laboratory experiments were carried out on the sintering of silicone nitride powder without any sintering additives. Silicone nitride powder was produced at own laboratory conditions by carbothermal reduction-nitriding of silica of which was presented elsewhere. It consists of mainly -Si3N4 (83%) and -Si3N4 (17%) phases with a small amount of minor constituents. Before compaction procedure, the powder was ball milled for 2 h and then 2 gr. of the sample was compacted uniaxially at a pressure of 25 MPa. The green compact was placed into furnace in a graphite crucible. The sintering process was carried out in a programmable muffle furnace at 1700 °C and held for 1 h. with a 20 °C/min. heating rate. The sintered compacts were analyzed using X-ray, SEM, SEM-EDS techniques. The results showed that the formation of liquid phase can accelerate the densification of the compact. The densification of the pure SIALON can be increased by adding of sintering aids that accelerates the formation of liquid phases in the sample. The densified sialons give better hardness and thermal properties of the sample. It was well demonstrated that for the densification of sialons large amount of liquid phase forming oxides are needed to add to initial composition. It is also suggested that there is linear relationships between densification and amount of liquid phase formed in the compact.

Abstract: O'-SiAlON/SiC ecomaterials were synthesized by using the Yangtze River sand that containing abundant Si and Al elements as the major raw material with minor additives through the carbothermal reduction nitridation route combined with colloidal process. This study realized the conversion of ecomaterials from non-traditional resources, Yangtze River sand. Orthogonal design was adopted to optimize the colloidal process parameters. The green compact with the largest bulk density was obtained when the solid volume loading is 50%, the addition mass fraction of SL is 0.8%, the addition mass fraction of CMC is 0.05%, the ball milling time is 10 h, the pH value is 9 and particle size distribution is multi-peak in the colloidal process. The effects of reduction agent, flowing rate of N2, sintering temperature during carbothermal reduction nitridation process on the microstructure of as-fabricated SiAlON materials were explored. The XRD results indicated that when sintering temperature is 1450°C, maintaining time is 6 h, flowing rate of N2 is 1.0 L/min and carbon black is selected as reduction agent, O-SiAlON/SiC composites was successfully synthesized. The SEM images showed the densificated microstructure and uniform grains with the short column shape.

Abstract: SiAlON-cubic boron nitride (cBN) composite was prepared by spark plasma sintering (SPS) using -SiAlON and cBN powders as starting materials, and the effect of holding time on densification, phase transformation and hardness of the composite was investigated. The SiAlON-cBN composite containing 20 vol% cBN sintered at 1650oC for 60s was densified to >97% of theoretical density. cBN phase transformed to hexagonal boron nitride (hBN) in the SiAlON-cBN composite with increasing holding time at 1650oC. Vickers hardness of the SiAlON-20vol%cBN composite sintered at 1650oC for 60-300s was 17.7GPa, and the hardness decreased with increasing holding time.

Abstract: The widely used Si3N4-SiC sidewall refractories for aluminum smelting cells, and β SiAlON-SiC composites that can be potentially used for this purpose, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. The formation of the Si3N4 and SiAlON phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. The corrosion resistance of the samples was shown to be dependent on the composition but more importantly on the environment during corrosion, with samples in the gas phase showing higher corrosion.

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: –sialons (Si6-zAlzOzN8-z, z=3) synthesized by mechanically activated combustion synthesis (MA-CS) at a low N2 pressure of 1 MPa, were sintered by Spark Plasma Sintering (SPS) and thermal conductivity was measured at room temperatures. Specimens were fully densified at 1600oC for 10 mins. and showed only –sialon phases confirmed by x-ray diffraction patterns though un-reacted Si was present as impurities after MA-CS. Thermal conductivities increased with sintering temperature and had a maximum value 5.49 W m-1 K-1 for specimens sintered at 1700oC.

Abstract: Mullite film was formed on ß-SiAlON (Si6-zAlzOzN8-z, z = 3) ceramics by reaction of Al2O3 films with silica formed on oxidation to improve a high temperature water vapor corrosion resistance. Sintered ß-SiAlON was fabricated by hot pressing of ß-sialon (z = 3) powder without additive at 1900°C and at 24 MPa for 4 h in N2. The Al2O3 film was deposited on polished ß-SiAlON ceramics by a dip coating process of Al2O3 precursor sols prepared from the alkoxide solution and aluminum sol. The Al2O3 coated sample was heated in Ar/O2 (95/5 vol.%) at 1300°C for 2 h, resulting in the formation of mullite by reaction with silica produced from sialon. The resulting mullite-coated sample was corroded in H2O/Ar/O2 (90/8/2 vol.%) at 1200°C for 2 ~ 100 h. The corroded samples were characterized by XRD and SEM-EDS for the evaluation of the corrosion resistance of mullite-coated sialon.