Synthesis of ytterbium-doped yttrium oxide nanoparticles and transparent ceramics
Abstract: In this thesis, the author reports on a ceramic process leading to transparent ytterbium doped yttrium oxide ceramics. The full production route was investigated, namely: powder preparation, compaction and sintering. First, yttria nano-powder was fabricated from a precursor with transient morphology, i.e. yttrium hydroxynitrate platelets that decomposed into spherical yttria nano-particles during calcination. The influence of different dewatering methods on such precursors was investigated. Water removal by freeze-drying was shown to be optimal for the production of non- agglomerated nano-particles compared to other methods that involve solvent removal by evaporation. This was attributed to the ability of freeze-drying to avoid the formation of solid bridges, since water is directly removed by sublimation. In a second step, doping with ytterbium was performed. Two precipitation routes were compared: precipitation of hydroxynitrate platelets and precipitation of amorphous carbonate. The latter route was shown to allow a better distribution of ytterbium in the yttrium oxide matrix after calcination of the corresponding precursor. This was attributed to good cation mixing within the amorphous particles, while formation of the hydroxynitrate platelets resulted in segregation of ytterbium because of the precipitation of an ytterbium-rich secondary phase. Because of the laborious character of the above techniques, an alternative method for synthesizing yttrium oxide powder was developed. This method not only allows for producing weakly-agglomerated nano-particles with equiaxed morphology, but is also a very time-efficient process. Its characteristic feature lies in the ability to skip two processing steps encountered in common precipitation techniques (i.e. filtration and drying). The method is based on combustion synthesis, sulfation, and calcination at high temperature. Through careful tailoring of the process parameters, evolution of the cellular nanostructure towards individual yttrium oxide nano- particles was obtained during firing. A method combining pre-sintering in vacuum followed by hot isostatic pressing (HIP) using glass encapsulation was shown to be successful to produce transparent yttria ceramics from agglomerated powders. Pre- sintering in vacuum of compact agglomerates enables differential sintering, resulting in only intergranular porosity. Subsequent HIP treatment of the pre-sintered samples caused almost complete pore elimination and achievement of transparency. However, macro-defects were present in the final material. From these results it became obvious that the packing homogeneity must be improved. For comparison, green-bodies were produced by slip-casting or pressing followed by cold isostatic pressing (CIP). Rheology studies were performed in order to tailor the milling conditions and the quantity of dispersant to obtain slurries with minimum viscosity and which were suitable for slip-casting. Although slip-cast pellets exhibited similar densities as pressed samples after pre-sintering, they showed more homogeneous optical properties after HIP. This was attributed to density gradients in the pressed compacts. By using the novel powder synthesis and HIP methods developed in this work, yttrium oxide transparent ceramics doped with 30 at% ytterbium were fabricated. The best sample showed a transmittance of 25% at 400 nm for a thickness of 3mm.
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