Process optimization of the production of transparent Y2O3 from commercially available powders

Yttrium oxide (Y2O3) optical ceramics, pure or doped with rare earth ions, are currently investigated as a promising material for transparent windows, solid-state lasers or refractory components. Y2O3 combines high melting point, transparency in a wide wavelength range, good thermal and optical properties and excellent mechanical and chemical stability. There are several approaches to produce transparent yttria ceramics by using hot pressing (HP), hot isostatic pressing (HIP), spark plasma sintering (SPS), microwave sintering, or vacuum sintering. Sintering using high pressures (HP, HIP, SPS) allows decreasing the consolidation temperature and recrystallization rate due to considerable activation of volume diffusion and creep. However, the high cost of the equipment and significant operational expenses restrict the application of sintering under high pressures. An alternative approach to fabricate Y2O3 transparent ceramics is sintering without applying high pressures, such as vacuum sintering. In order to increase sintering activity and reduce consolidation temperatures, very fine powders (submicron- to nanopowders) with high surface energy are required. The powder characteristics (particle size, morphology or specific surface area) determine the sintering behavior and, as a consequence, the possibility and conditions required for the achievement of transparency of ceramics. However, it should be noted that the effect of different starting powders on the processability of Y2O3 transparent ceramics has not been studied in detail in the current literature. This project is focused on the powder treatment of Y2O3 and the production of transparent ceramics: a number of commercial powders with suitable morphology will be selected for the study of the effect of milling parameters on the morphology and sinterability of the powders, eventually with the use of sintering aids. The proposed activities plan includes: 1.Characterization and selection of starting Y2O3 powders (determination of their morphology by SEM, specific surface area, etc.); 2.Milling tests: study of the variation of process parameters (milling speed and time, milling media size); 3.Characterization of the milled powders; 4.Shaping of powders by pressing and sintering in vacuum; 5.Characterization of sintered ceramics (optical properties, microstructure). Depending on the results (transmittance) and the time available, the use of a sintering aid may be considered for one selected powder. The aim of this study is to provide practical information related to the selected powders, and to make more general considerations, linking the microstructure and the type of defects in the sintered ceramics to the process parameters and powder characteristics.