Solubility of ZrO2 in yttrium oxide and its influence on transparent ceramics properties

Defect-free Y2O3 transparent ceramics is an interesting material which is used for IR windows, nozzles or laser host, etc, due to its mechanical and optical properties, high thermal and chemical stability, and high melting point. To achieve full densification and the absence of pores, sintering aids are used. For yttria ceramics, the optimal choice is ZrO2. It has a significant influence not only on transmittance, but on the other characteri-stics of the material as well. For high transparency, ceramics has to remain single-phase, thus forming a solid solution of Y2O3 and ZrO2, but the solubility limit isn't studied in detail. Therefore, the presented work aims to investigate the solubility limit of ZrO2 in yttria ceramics and the dependence of properties on the concentration of sintering aid. Ceramics samples with a concentration of ZrO2 of 0-11 mol% were obtained by uniaxial and cold isostatic pressing (CIP) followed by vacuum sintering at 1735 °C for 22 h. The SEM and XRD analysis, refractive index and transmittance measurements were performed. No secondary phases were observed in analysed samples by SEM, which indicates the formation of a solid solution and full solubility of ZrO2 in the analysed range of concentration. Also, it was found that even 3 mol% of ZrO2 significantly improve the microstructure of samples. It allows for the elimination of residual porosity due to full densification during sintering. Also, the grain size decreased from 14 ?m for pure Y2O3 to 3.8-6.1 ?m for samples with 3-11 mol% of ZrO2. XRD analysis showed that the phase composition did not change between samples with 0 and 11 mol% of sintering aid. Decreasing of lattice parameters with increasing of ZrO2 amount was observed due to the replacement of Y3+ ions in the lattice by the smaller Zr4+. A steady increase of refractive index was observed with increasing Zr content. Obtained results allow us to conclude, that the solubility limit of ZrO2 in Y2O3 was not reached within the studied range and that the presence of ZrO2 improves ceramics properties. In the next step, optical transmittance was analysed. Among all obtained Y2O3:Zr4+ ceramics the highest transmittance (78.60% at 1100 nm at a thickness of 2.11 mm) was achieved for the sample with 7 mol.% of ZrO2.