Nanoparticles and Fabrication of Transparent Polycrystalline Yttria Ceramics

In this thesis, we report on a process to obtain yttrium oxide nanosized particles doped with ytterbium and a sintering method that leads to transparent polycrystalline ceramics. First, nano-powder of yttria was fabricated from a precursor with transient morphology, i.d. yttrium hydroxynitrate platelets that decompose 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 with 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. This latter was shown to allow a better distribution of ytterbium in the yttrium oxide matrix after calcination of the corresponding precursor. This was explained in terms of the good cation mixing in the amorphous particles, while formation of the hydroxynitrate platelets resulted in segregation of ytterbium, probably because a second phase with different ytterbium/yttrium composition precipitates in the first stages of the synthesis. Finally, a method combining pre-sintering in vacuum followed by hot isostatic pressing was shown to be successful to produce transparent yttria ceramics from agglomerated powders with high purity. Pre-sintering in vacuum agglomerates of closelypacked particles enables differential sintering, which is responsible for the complete elimination of porosity in the agglomerates and for the formation of intergranular porosity only. Then, hot isostatic pressing treatment of the pre-sintered samples using the glasscanning technique reactivates sintering and coarsening, which causes almost complete pore elimination. Furthermore, a liquid phase formed during sintering because of pollution from the furnace. This liquid phase is thought of as to have helped densification to some extent. The best transparent ceramic showed a transmittance of 43% at 400 nm.