Siddhartha Bandyopadhyay

Effect of different rare earth cations on the densification behaviour of oxygen rich α-SiAlON composition

Abstract Five different stabilizing cations (Y, Yb, Dy, Sm, Nd) were investigated on an α-SiAlON composition, deliberately shifted to the Al 2 O 3 rich area corresponding to M 0.445 Si 9.998 Al 2.003 O 0.668 N1 5.333, with respect to the reaction sequences, densification behaviour and mechanical properties. The analysis of the reaction sequences show that the formation of α-SiAlON involves the intermediate formation of aluminate, garnet, melilite and a nitrogen rich liquid containing the respective cation. The sintering rate is influenced by volume fraction of the garnet and melilite phase which varies for different cations. Dy 2 O 3 shows the best densification behaviour which is followed by Sm 2 O 3 and Nd 2 O 3 . The fracture toughness for all are comparable while the hardness value seems to decrease with the increasing ionic size of the cations due to the lower proportions of α-SiAlON phase. The present composition favours the removal of melilite from the as cooled product as well as good densification.

Formation of multiphase SiAlON ceramic

Abstract The formation of different in situ formed composite SiAlON phases along the concentration plane Si3N4–Al2O3·AlN–Y2O3·9AlN in the system Y–Si–Al–O–N has been studied. The appearances of other phases are also described.

The comparative behaviour and some parameters on the carbothermal reduction and nitridation of aluminosilicates

Abstract A few natural aluminosilicates with variable Al:Si ratios of 1:2, 1:1 and 2:1 have been subjected to carbothermal reduction and nitridation process. β ′-Sialon with different z -values and in different amounts are formed associated with α -Al 2 O 3 . The degree of substitution in β ′-sialon is always lower than that predicted theoretically. Among all the aluminosilicates under study, kaolin is the best producer of sialon. The reactivity of the reduction and nitridation process increases with increasing Al:Si ratio of the starting raw aluminosilicates. The pressnce of iron is essential, which possibly acts as nitrogen carrier.

Nitride & Oxy-Nitride Ceramics for High Temperature and Engineering Applications

Nitrides and oxynitrides of silicon and aluminum occupy prominent positions among other non-oxide members of ceramics on account of their inherent unique combination of properties arising out of their directional covalent bonding in the condensed state. Since many reviews could be found in the literature on the materials either individually or combining some of them, the present article is devoted to the key areas of the present research on the developments of these materials. For example, α’ – SiAlON is now attracting significant attention and the article highlights how compositional variations could lead to easy densification and improvement of its mechanical properties. Experimental results from various authors show that the mechanism of stabilization of α’ – SiAlON is very complex and cannot be explained on the basis of the size of the metal cations and solubility in the transient glassy phase. Y, Yb, Dy and to some extent Ca are good for stabilization of the phase. Evolution of different phases during temperature rise and how these phases affect sintering of the α’ material has been presented in this article. In-situ formation of elongated β’ grains have been shown to increase the toughness of the material. Simultaneously, the development of AlN ceramics in regard to application of this ceramic in different areas apart from that of microelectronics has been discussed. +Improved mechanical and electrical properties of AlN and its composites have shown their promise in diverse applications such as armors, electron tube components and dielectric devices.

The effect of nitrogen content on the sintering behaviour and properties of sialon prepared from kaolin

The sintering of powders with variable nitrogen content prepared from the carbothermal reduction and nitridation of kaolin has been studied. With increasing nitrogen uptake in the starting powder, the refractoriness and the fusion point increase. The coefficient of linear thermal expansion decreases linearly with the nitrogen content of the sintered product. The product with 80% theoretical nitrogen has good oxidation resistance. The oxidation rate is parabolic, and the product contains mullite, cristobalite, and a glass. The X-sialon phase oxidises preferentially to -sialon.

Nano preparation of Dy3+ substituted ceria via urea-formaldehyde gel combustion route

Abstract Nanocrystalline ceria powders have been synthesised via the gel combustion route, using for the first time urea formaldehyde as fuel for doping of dysprosium oxide. This synthesis route can produce well-crystallised powder within a one step combustion process, eliminating the need for further calcinations. The formation sequences have been described through differential scanning calorimetry-thermo gravimetric analysis study and the crystallinity of the powder was examined using X-ray diffraction, selected area electron diffraction and high resolution patterns. Lattice parameter was found to increase with doping concentration. Very fine grains in the size range of 3–5 nm are found to occur in the form of large and soft agglomerates (50–130 nm). The optical band gap suggests that those powders with low dopant concentration may have useful applications in UV-shielding and in transparent conducting film.