Nitrides and Oxynitrides III

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Authors: A. Ceylan, E. Suvaci, Hasan Mandal
Abstract: Aqueous dispersions of SiAlON forming powders (Si3N4, AlN and Y2O3) were obtained. Dispersibility of powders was found to be improved by addition of sodium tripolyphosphate. Stable aqueous suspensions of SiAlON forming powders were prepared by using a phosphate ester, sodium tripolyphosphate (STPP, Na5P3O10). Effect of the STPP on rheological properties of the powder mixture was investigated by zeta potential measurements, sedimentation and milling studies. It has been shown that addition of the STPP to the aqueous SiAlON forming powder mixture improves the dispersion behavior significantly. These results suggest that STPP can be utilized as a dispersant in such ceramic systems.
Authors: S.R. Kushan, N. Tamsu, N.C. Acikbas, E. Suvaci, Hasan Mandal
Abstract: In this study, textured α-SiAlON ceramic was produced by templated grain growth process. In order to enhance the alignment of the seeds, gated tape casting arrangement was utilized. Y-Ce codoped seeds were synthesized to induce the preferred orientation. For the production of the seeds, Ce3+ was used as a sintering additive due to its well-known effect on the formation of elongated α-SiAlON grains. In addition, Y3+ was used to stabilize Ce3+ in α-SiAlON structure. Gated tape casting arrangement was also used to enhance the alignment of the seeds in the green tape. After the lamination and sintering, preferentially aligned α-SiAlON grains were observed by the microstructural investigation. In addition, XRD results obtained from the planes parallel and perpendicular to the casting direction indicated the alignment of grains in α-SiAlON ceramics. Hardness measurements also confirmed that the alignment of the grains results in 8% increase in hardness value of the plane perpendicular to the casting direction with respect to that of the parallel one.
Authors: Adem Demir, F. Caliskan
Abstract: In this study, the production of β-Si6-zAlzOzN8-z (z =3) powders by the carbothermal reduction and nitridation of kaolin (Al2O3.2SiO2.2H2O) of Turkish origin (Can-Canakkale) was carried out with different processing parameters. It was found that the morphology of the produced SiAlON powder was mixture of irregular and whisker like grains. Therefore this morphology is suitable for liquid metal infiltration since the powders are already high porous. The kaolin powder containing stoichiometric rate carbon black and 30 % charcoal were pressed and reacted under nitrogen flow (2 lt/min) between 1400-1475°C for 4 h. After a carbothermal reduction and nitridation (CRN) process, porous β- SiAlON ceramic was produced from natural kaolin. Residual carbon and charcoal in the produced ß-SiAlON ceramic were fired at 1000°C giving extra porosity. The porous ß-SiAlON ceramics were sintered under N2 atmosphere at 1550°C for 2 h to make preform for infiltration process. SEM image analyses were carried out to determine preform and pore morphology and XRD analysis were performed for phase transformation.
Authors: S. Kurama, Mathias Herrmann
Abstract: At temperature above 1200°C, the thermal stability of α-SiAlON phases has been debated since 1992; however, it has been discussed if any α-SiAlON phase can be formed in Ce, La, Eu and Sr-doped SiAlON systems. In our previous studies it was shown that the use of Mg-Ce and Mg-Sr elements as dopants SiAlON compositions, in which all elements just have very low or no stability in the α-SiAlON structure, would promote the stability of Mg-Ce elements in the α- SiAlON phase [1, 2]. However, in Mg-Sr systems, it was obtained that Mg2+ is predominantly incorporated in α-SiAlON structure whereas Sr2+ mainly remains in the grain boundaries [2]. In this study, by applying spark plasma sintering (SPS) (at 1400-1700°C) and post-sintering thermal heat treatment (at 1500°C for 5 hrs and 1700°C for 2hrs) Mg or Mg-Sr doped SiAlON (50:50 mole ratios) ceramics were prepared. The results were compared with GPS sintered samples data. The effect of sintering temperature on densification process, phase transformation, microstructure and mechanical properties of samples were investigated. The results showed that by using SPS, Sr-Mg doped samples can be sintered at lower temperature (at 1600°C) than at GPS (at 1800°C) and it has no Sr-doped grain boundary phases.
Authors: S.R. Kushan, N.C. Acikbas, J. Křest'an, Pavol Šajgalík, Hasan Mandal
Abstract: In this study, effect of seed addition on the microstructural development of Y-Sm α-SiAlON system has been investigated. Besides using pre-produced rod-like α-SiAlON seeds, growth in the glassy phase and harvested by chemical treatment, powdered Y-Sm/α-SiAlON was also used as nuclei. Microstructural results of both systems were compared with un-seeded samples. It was found that the powdered α-SiAlON addition has also caused the acicular α-SiAlON grain formation, which indicates the dominant effect of thermodynamic stability on the preferential growth of α-SiAlON grains in the c direction.
Authors: V. Demir, Derek P. Thompson
Abstract: Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4x10-4 mbar, and vacuum heat treated at different temperatures and times to remove grainboundary glass. The results showed that this was substantially achieved at 1575oC for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.
Authors: L.K.L. Falk, N. Schneider, Yvonne Menke, Stuart Hampshire
Abstract: The intergranular microstructure in a liquid phase sintered silicon nitride based ceramic may be viewed as an oxynitride glass-ceramic. This work is concerned with the incorporation of yttrium B-phase, which is a five-component phase, into the intergranular regions of silicon nitride ceramics. The silicon nitride materials were fabricated with the addition of a powdered B-phase parent glass with composition (e/o) 35Y:45Si:20Al:83O:17N, or the addition of a mixture of Y2O3, SiO2 and Al2O3 with cation composition (e/o) 35Y:45Si:20Al. The starting powder mixtures contained 10 wt% of sintering additives. Sintering for 2 h at 1800°C was followed by a two-step post-densification heat treatment in order to promote nucleation and growth of yttrium B-phase. Detailed imaging and elemental analysis of the intergranular regions was carried out by EDX in a FEGTEM.
Authors: N.C. Acikbas, Alpagut Kara, Servet Turan, Ferhat Kara, Hasan Mandal, Bernd Bitterlich
Abstract: 25α:75β SiAlON composition was designed with different cations and at different molar ratios. Effect of the type of cations both on the composition and the type of intergranular phase investigated after gas pressure sintering and further post sintering heat treatment.
Authors: Csaba Balázsi, Zsolt Czigány, Ferenc Wéber, Zoltán Kónya, Zófia Vértesy, László Péter Biró, Imre Kiricsi, Péter Arató
Abstract: Multiwall carbon nanotube reinforced silicon nitride composites have been prepared by hot isostatic pressing. A manufacturing process has been worked out to avoid the damage of nanotubes during sintering. This method provides their preservation even in severe circumstances at temperature 1700°C and gas pressure 20 MPa. As shown by scanning and transmission electron microscopy after low and high pressure processing, carbon nanotubes have good adherence to the silicon nitride grains. Moreover, carbon nanotubes have been found to be located not only at grain surfaces, but in several cases they are well integrated with the silicon nitride grains. Composites with higher strengths can be obtained by increasing the nitrogen gas pressure.

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