Europium (Eu) was found to act as a solid-state sintering aid in Y2O3 optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ceramic samples of Eu-doped Y2O3, with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 1580°C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering conditions. In order to study the effect of Eu concentration on ceramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were measured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with increasing Eu concentration according to a log–linear relationship. Eu2+ was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y2O3. A sintering model with diffusion rate limited by yttrium interstitial transport and controlled by the incorporation of Eu2+ onto the cation sublattice was found to be in good agreement with experimental diffusivity data.

Effect of Europium Concentration on Densification of Transparent Eu:Y2O3 Scintillator Ceramics Using Hot Pressing

Sintering in air of a pure yttria powder was investigated on green samples shaped by slip casting. The “relative density/grain size” trajectory has been drawn and hypotheses concerning the mechanisms controlling grain growth and densification were formulated. Samples were fully densified by an additional hot isostatic pressing step on pre-sintered samples. After optimal polishing, optical properties were measured in the UV, visible, and infrared ranges.

Sintering Behavior and Optical Properties of Yttria

Densification behaviour and three-dimensional printing of Y2O3 ceramic powder by selective laser sintering

Undoped Y2O3 and Y2O3 doped with either Si4+, Er3+, Yb3+, Al3+ or Ti4+ were fabricated by a two-step sintering method. The concentration of each dopant cation (Si4+, Er3+, Yb3+, Al3+, and Ti4+) was adjusted so that the dopant:yttria molar ratio was 0.01 and the mechanical properties and the microstructures of each were characterized. The Vickers hardness and the bending strength of cation-doped Y2O3 increased in the order Si4+, Er3+, Yb3+, Al3+, and Ti4+ paralleling increases in the relative density. Ti-doped-Y2O3 had the highest density and the highest strength. Transmission electron microscopy analysis revealed that Ti segregated to the grain boundaries of Y2O3. The two-step sintering method was optimized to enhance the mechanical properties of Y2O3. As a consequence, Ti-doped Y2O3 with the Ti4+:yttria molar ratio of 0.01 sintered at the optimized heating schedule was found to have a Vickers hardness and three-point bending strength as high as 874 Hv and 307.0 MPa, respectively.

Hervé Vesteghem

Papers

Densification of Yttria Ceramics