An easy approach is described for the preparation of ZrO₂-coated Y₂O₃ nanopowder from a solution of zirconium nitrate with commercial Y₂O₃ nanopowder. The evolution process of the ZrO₂ coating layer upon calcination, such as the phase and microstructure of the particles’ surface, was studied. Calcination of the powder at 700 °C resulted in ZrO₂-coated Y₂O₃ nanopowder. The rheological properties of the suspensions of ZrO₂-coated Y₂O₃ powders were studied. A well-dispersed suspension with a solid loading of 35.0 vol% using ZrO₂-coated Y₂O₃ nanopowder was obtained. The consolidated green body obtained by the centrifugal casting method showed improved homogeneity with a relative density of 50.2%. Transparent ceramic with high transparency and an average grain size of 1.7 µm was obtained by presintering at 1500 °C for 16 h in air, followed by post-HIP at 1550 °C for 2 h under 200 MPa pressure. The in-line transmittance at the wavelength of 1100 nm (1.0 mm thick) reached 81.4%, close to the theoretical transmittance of Y₂O₃ crystal.

Improved optical and mechanical properties are required for future infrared windows working in harsher mechanical and thermal environments than today. Ameliorating the homogeneity of the phase domain and reducing the size of the phase domain are effective approaches for enhancing the optical transmittance and mechanical hardness of a nanocomposite. In this work, we reported that the Gd₂O₃-MgO nanopowders were prepared by two different processes. The core–shell nanopowders synthesized by urea precipitation have a much lower agglomeration than the nanopowders prepared by sol–gel. Excellent transmittance (70.0%–84.1%) at 3–6 μm mid-infrared wave range and a high Vickers hardness value (10.3 ± 0.6 GPa) were maintained using the nanopowders synthesized by urea precipitation mainly due to its homogenous phase domain distribution.

Fine-grained Yb:Y₂O₃ laser ceramics with excellent transmittance and thermal conductivity were fabricated from commercial powders. The process involved aqueous colloidal forming, additive-free air pre-sintering at 1400 °C, and hot isostatic pressing at 1550 °C. Suspensions were prepared with a deionization process to alleviate the hydrolysis issue, which optimizes the microstructure uniformity and enhances the green compacts’ density after consolidation. The microstructure, in-line transmittance, microhardness, and fracture toughness of the Yb³⁺-doped Y₂O₃ ceramics with different concentrations were measured. The 5.0 at% Yb³⁺-doped Y₂O₃ ceramic yielded a superior transmittance of 80.1% at 1100 nm and 83.0% in the mid-infrared region. The average grain size was 752 nm. The sample exhibited a thermal conductivity of 9.94 W·m⁻¹·K⁻¹ while achieving a 1076 nm laser output with a 42 mW peak power and 4.3% slope efficiency.