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Gajum, Naima Ramadan
Languages: English
Types: Doctoral thesis
Subjects: QC
The objectives of this research were to investigate the possibility of controlling the '/' phase ratio and morphology in Sialon ceramics. These objectives have been sought by the control of the starting composition, and by post sintering heat treatment. The main emphasis has been on the production of a series of ' and (+') Sialon ceramics with a minimum amount of the glass phase by the pressureless sintering technique and using ytterbium (Yb) as an ' stabilising element. The Yb additions were made via the oxide or the alumino-silicate presynthesised glass; the latter was found to improve the density. The XRD analysis of the as sintered materials revealed ' to be the dominant phase with minor contributions from ' sialon and/or or 12H AIN polytype.\ud \ud Additions of SiO2 or -Si3N4 were made to various materials to assess potential mechanisms for obtaining control over the microstructural development of '/' sialon materials. The addition of silica (SiO2) to sialon with high ' content in Yb system significantly improves the densification and increased the amount of ' phase. The incorporation of -Si3N4 as a seeding agent had a very small effect on the '/' phase ratio and the phase morphology.\ud \ud Further experiments were aimed at optimizing sinterability and sialon microstructure through the introduction of two ' stabilizing cations. Compositions were prepared that contained a combination of light and heavy rare-earth (Yb-Nd and Gd-Nd), and then pressureless sintered and compared with the single cation materials. Materials in the as sintered state were composed of a high ' sialon content with a minor amount of ' sialon and 12H AIN polytype indicating that the heavy rare-earth (which is the principal ' stabilizer) has a dominant effect although EDAX analysis confirmed the presence of both cations (light and heavy) within the ' structure.\ud \ud The research also compared, and developed an understanding of, the thermal stability of '-sialon using single Yb or mixed cations. The Yb single cation '/' materials exhibited excellent stability over a range of temperature (1200 - 1600°C) and for different periods of time up to 168 hrs. The heat treatments result in the crystallisation of the residual phase as a Yb garnet phase which formed at ˜1300°C. The mixed cation '/' materials showed some '-' transformation. The transformation was accompanied by dissolution of RA1O3 (normally crystallized with R=Nd,Gd) and crystallization of melilite.
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