Seramik MühendisliğiCeramic Engineeringhttps://hdl.handle.net/11421/21602024-03-29T08:01:30Z2024-03-29T08:01:30ZThermal stability of rare earth densified ?-sialon ceramicsMandal, HasanThompson, D. P.https://hdl.handle.net/11421/158112020-02-19T20:19:22Z1997-01-01T00:00:00ZThermal stability of rare earth densified ?-sialon ceramics
Mandal, Hasan; Thompson, D. P.
Typical ?-sialon and mixed ?-ß sialon starting compositions were densified by either hot pressing or pressureless sintering using the oxides of Yb, Dy, Sm and Nd. The resulting materials were heat-treated at 1450°C for up to 720 hours (1 month) to observe ? › ß sialon transformation. The results of microstructural characteristics and the phase assembly showed that the high temperature stability of ?-sialon phase depends on the starting composition, the type of rare earth sintering additives, the amount and viscosity of the liquid phase, and the presence or absence of ß-sialon grains in the initial sintered material
1997-01-01T00:00:00ZPreparation of multiple-cation ?-SiAlON ceramics containing lanthanumMandal, HasanHofmann, J. Michaelhttps://hdl.handle.net/11421/158062020-02-19T20:19:21Z1999-01-01T00:00:00ZPreparation of multiple-cation ?-SiAlON ceramics containing lanthanum
Mandal, Hasan; Hofmann, J. Michael
Until recently, it was accepted that Ce 3+ cations, with an ionic radius (r) of 1.03 Å, were too large to form an ?-SiAlON structure. However, more-recent studies have shown that cerium cations can be incorporated into ?-SiAlON via quenching at a rate of 600°C/min, after sintering at 1800°C. Thus far, no ?-SiAlON formation has been observed for La 3+ cations with r = 1.06 Å. In the present work, the possibility of having the La 3+ species as a dopant cation in ?-SiAlON has been investigated by using La 2O 3 alone or in equimolar mixtures with CaO or Yb 2O 3. The resulting materials have been heat-treated at a temperature of 1450°C for up to 720 h to devitrify the grain-boundary glass into crystalline phases and also to observe the ? › ß SiAlON transformation. X-ray diffractometry on samples that were densified with single cations revealed that the La 3+ cation alone does not form an ?-SiAlON; rather, it forms the N-phase (La 3Si 8O 4N 11) with a ß-SiAlON phase. In the case of multiple cations, ?-SiAlON was observed only as a matrix phase. Energy-dispersive X-ray measurements have proven that La 3+ cations can be accommodated into the ?-SiAlON structure and this structure also does not transform to ß-SiAlON at lower temperatures.
1999-01-01T00:00:00Z?ݧ Sialon Transformation in Calcium-containing ? -SiAlON CeramicsMandal, HasanThompson, D. P.https://hdl.handle.net/11421/158082020-02-19T20:19:21Z1999-01-01T00:00:00Z?ݧ Sialon Transformation in Calcium-containing ? -SiAlON Ceramics
Mandal, Hasan; Thompson, D. P.
Recent studies on rare earth densified ?-sialon ceramics have shown that the resulting ?-sialon product, present either as a single phase or in conjunction with ß-sialon, is unstable when heat treated at lower (1350-1600°C) temperatures, and transforms to a mixture of ß-sialon plus other crystalline or liquid metal sialon phases. The present paper describes similar studies carried out on calcium-densified ?-sialon compositions, and shows that for a wide range of starting compositions, with calcium as the sole sintering additive or present with other (Nd, Sr) cations, the resulting calcium stabilised a-sialon products are fully resistant towards ?›ß transformation when heat treated in the temperature range 1450-1550°C. Whereas ?›ß transformation in rare earth stabilised ?-sialons is influenced by the nature of the rare earth cation, the ?-sialon composition, the composition and melting behaviour of the liquid phase and the presence or absence of ß-sialon nuclei, transformation in calcium a-sialons appears to be influenced by none of these parameters. Clearly if ?›ß sialon transformation occurs in this system, the transformation temperature for calcium ?-sialons must be below 1450°C, the heat-treatment temperature which has been most frequently used in current research on rare earth densified ?-sialons.
1999-01-01T00:00:00ZNew developments in ? -SiAlON ceramicsMandal, Hasanhttps://hdl.handle.net/11421/158072020-02-19T20:19:22Z1999-01-01T00:00:00ZNew developments in ? -SiAlON ceramics
Mandal, Hasan
Two different ?-sialon compositions have been densified by either pressureless sintering or capsul-free sinter HIPing using either single cation namely strontium, lanthanum, cerium, neodymium or their equimolar mixtures with calcium/ytterbium as a principal cation. The resulting materials have been heat-treated at 1450°C for a maximum of 720 h to observe the ?›ß sialon transformation. Effect of starting compositions on the phase stabilities, microstructure, room temperature mechanical properties and optical transparent nature of sintered materials have been investigated. SEM observations for oxygen rich starting compositions revealed that ?-sialon grains exist in needle like morphology in all the systems studied after both pressureless sintering and HIPing. Suprisingly, the nitrogen rich starting compositions give rise to coloured, optically transparent ?-sialon ceramics after capsul-free HIPing. Clear evidence of ?-sialon formation with incorporation of large cations namely, La3+, Ce3+, Nd3+ and Sr2+, has been established by XRD, SEM and EDX analysis. The observed phenomenon could possibly extend the usefulness of ?-sialons ceramics to both structural and functional applications
1999-01-01T00:00:00Z