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

This method consists of a combination of vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The successful key of this method was to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity. However, performing HIP treatment at 1625 °C was found to result in opaque samples. This was attributed to the diffusion of argon inside the capsule. Contamination at different steps of processing was also investigated by inductively coupled plasma mass spectrometry (ICP-MS).

Fabrication of transparent yttria by HIP and the glass-encapsulation method

New relationships between relative density and grain size during solid-state sintering of ceramic powders

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

Sintering mechanisms of Yttria with different additives

Densification of Yttria Ceramics

The influence of several additives (ZrO 2 , CeO 2 , TiO 2 , SiO 2 ) on sintering of conventional yttria powders is investigated. Titanium oxide is the most efficient in promoting densification and in lowering sintering temperatures. Almost full densities can be obtained at 1400–1500°C if a small amount of dopant (0.2 wt% TiO 2 ) is dispersed homogeneously onto the surface of yttria during powder processing.

M. Boncoeur

Papers

Densification of Yttria Ceramics

Enhanced densification of yttria by addition of titanium oxide