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.

Y₂O₃-coated MgO: Ce⁺³ particles with different precipitants were prepared by the co-precipitation method; the phase and morphology of the sample were characterized by XRD, TEM, and DTA-TG, and the apparent activation energy of the coated particles was studied by thermal analysis kinetics. The results showed that the precursors synthesized by single-phase and multi-phase precipitants were calcined at 1000 °C for 1.5 h to obtain Y₂O₃-coated MgO: Ce⁺³ particles with coating thicknesses of about 2.5 nm and 5 nm. The apparent activation energies of the precursor phase change in three stages were calculated using the Doyle–Ozawa method and the Kissinger method. The average values for single-phase samples were 95.61, 74.90, and 275.27 kJ/mol, while those for multi-phase samples were 74.90, 56.06, and 240.14 kJ/mol. The activation energies for the grain growth of the two samples were 30.56 kJ/mol and 26.27 kJ/mol. Due to the differences in activation energies at each reaction stage, the reason for the influence on the thickness of the coating layer of the two precipitants is that the smaller the activation energy, the lower the required synthesis energy. An increase in coating thickness indicates an improvement in the surface activity of the coated particles. Moreover, the luminescence intensity of the composite sample is significantly higher than that of the single-phase sample, and the luminescence performance is optimal when the Ce⁺³ ion in the composite sample is 0.3 mol%.

Low temperature combined with high application pressure sintering technology is one of effective approaches to fabricate ceramics with fine microstructure. We reported that the Gd₂O₃–MgO nanocomposite ceramics were obtained by hot-pressing sintering using nanopowders derived from sol–gel combustion method in this paper. The high optical transmittance (80.1%–83.5%) over 2-6 μm and the high values for Vickers hardness (11.1 ± 0.3 GPa) and bending strength (192.6 ± 8.1 MPa) were obtained by hot-pressing sintering at low sintering temperature (1300 μC) and high application pressure (65 MPa).