The quality of the powder is crucial for the performance of transparent yttria ceramics. The powder characteristics, such as size, shape, distribution, and agglomeration, directly affect the densification and microstructure of the ceramics. Co-precipitation is a common method for preparing yttria powder, which has the advantages of simplicity, low cost, and scalability. However, it also has some drawbacks, such as uneven particle size distribution, severe agglomeration, and complicated post-treatment processes. The agglomeration of the powder not only reduces its activity, but also impairs its sinterability. Therefore, how to effectively disperse the powder is one of the key techniques for improving the quality of transparent yttria ceramics. Adding dispersant is a common method for dispersing the powder, among which PEG (polyethylene glycol) is a widely used dispersant, which can form strong hydrogen bonds with the surface of hydroxide colloids, and enhance the stability of the colloids by steric hindrance effect. Moreover, since PEG is an organic substance, it can be completely removed during the high-temperature calcination process, without causing any negative impact on the properties of the ceramics. In this study, we use PEG as the dispersant, and investigate its influence on the optical properties of yttria nanocrystalline powder and its sintered ceramics.

Zirconium oxynitrate and yttrium nitrate hexahydrate were dissolved in anhydrous ethanol to prepare dilute solutions as reaction materials. Two salt solutions were mixed according to a stoichiometric ratio of (Y_0.97Zr_0.03)₂O₃. Ammonium hydroxide was dissolved in 100 mL of anhydrous ethanol as a precipitant, and to investigate the effect of the addition amount of polyethylene glycol 4000 as a dispersant on the dispersion of samples, and PEG4000 with different molar amounts of 0, 0.1% (mole fraction), 0.3%, 0.6%, 0.9% of Y³⁺ content was added to the precipitant. The precipitant solution was gradually added to the reactants at 4 mL/min under vigorous stirring until the pH reached 9. The resulting precipitates were aged at room temperature for 4 h, then washed for 4 times with deionized water to remove impurity ions, and collected by centrifugation. The washed precipitates were freeze-dried for 10 h to obtain well-dispersed precursor powders. The precursor powders were calcined in a muffle furnace at 400, 600 ℃ and 850 ℃ for 4 h, respectively. The calcined yttria powders were wrapped with tantalum foil and then loaded into a customized graphite mold. The graphite mold was then placed in a vacuum hot-pressing furnace and sintered under vacuum. Another ceramic sample without tantalum foil shielding was also sintered using the same process for comparison. After the hot-pressing step, the Ta foil was removed, and the samples were annealed at 900 ℃ in air for 2 h and then ground and polished for the coming characterizations.

The precursor was completely decomposed and crystallized into well-crystallized yttria powder with little organic and impurity residues after calcination at 850 ℃, as revealed by TG-DSC, FT-IR and XRD analyses. The average particle size of the yttria powder prepared with different dispersant concentrations was in the range of 20-30 nm, indicating that the addition of PEG had little effect on the grain growth. However, the addition of PEG changed the agglomeration state of the particles in the powder. At low PEG addition, the surface charge of the powder was unbalanced, and the particles tended to agglomerate due to van der Waals force or electrostatic force, because PEG could not form a uniform and thick adsorption layer on the particle surface. This agglomeration reduced the energy demand for high-temperature sintering of the nanopowder, but also resulted in the microstructural inhomogeneity of the green body. At high PEG addition, the particles were effectively isolated from the surface charge by the uniform and thick adsorption layer formed by PEG on the particle surface, which suppressed the agglomeration effect of the particles, and also provided steric hindrance and solvation effects, preventing the particles from contacting and bonding.

Yttria transparent ceramics were obtained after sintering the yttria powder. The use of tantalum foil effectively shielded the carbon contamination. The sintering temperature of 1 500 ℃ was the optimal condition for the sample performance. The density and transmittance of the samples decreased when the PEG addition was low. The transmittance of the samples increased slightly when the PEG addition increased to 0.9% (the content of PEG in ethanol). This was because the addition of a small amount of PEG caused the agglomeration of yttria powder, which affected the forming, sintering and densification processes of the ceramics. With the increase of PEG content, the agglomeration degree of yttria powder decreased, and the ceramic grain size also gradually decreased. When the transmittance of the samples reached the maximum value, the ceramic grain size was 1-2 μm. However, the transmittance of all samples in the visible light range was still not high, which might be related to the oxygen vacancies and carbon impurities in the samples.

Yttria nanopowders were synthesized by a precipitation method in ethanol solvent with ammonia as a precipitant, PEG as a dispersant, and 3% ZrO₂ as a sintering aid. The optimal calcination temperature of the precursor was determined to be 850 ℃ by thermal analysis, at which a high purity cubic yttria phase could be obtained. Y₂O₃ ceramics were prepared by a vacuum hot-pressing sintering technique at 1 450-1 600 ℃ under 30 MPa, and the samples were wrapped with tantalum foil to prevent carbon contamination. The results showed that 1 500 ℃ was the optimal sintering temperature, at which Y₂O₃ ceramic had the maximum optical transparency, with a linear transmittance of 48.4% at 1 100 nm wavelength for samples with the thickness of 2 mm. In addition, the PEG addition also affected the microstructure and optical properties of Y₂O₃ ceramic, and when the PEG content was 0.9%, Y₂O₃ ceramic had uniform and fine grains, which was beneficial to improving the transparency. This work could provide a reference for the effective preparation of a high-performance Y₂O₃ ceramic.