Y₂O₃ transparent ceramics are increasingly demanded in advanced technological applications, particularly in high-power laser systems, infrared optical windows, and semiconductor devices. These applications require materials that maintain both exceptional optical transmittance (i.e., >80% at 1064 nm) and mechanical strength (i.e., flexural strength >200 MPa), even when fabricated into complex geometries. Gel casting offers a promising route for producing large-scale, complex-shaped Y₂O₃ components. However, process-induced defects such as delamination, microcracks, and density gradients often compromise the microstructural integrity of green bodies, leading to a reduced performance in the ceramics. These defects primarily stem from excessive internal stresses generated during gelation, thus highlighting a need for precise control over gelation kinetics. This study was to investigate the synergistic effects of catalyst concentration and reaction temperature on the microstructural homogeneity of Y₂O₃ green bodies. The highly uniform green bodies were obtained via optimizing the parameters, thus enabling the fabrication of Y₂O₃ transparent ceramics with a high optical performance through vacuum sintering.

A certain amount of commercial Y₂O₃ powder was mixed with ethanol as a dispersion medium. The slurry was ground in a ball mill with ZrO₂ beads of 2 mm in diameter for 8 h. Ammoniumpolymethacrylate (A40) was added as a dispersant, concentrated H₂SO₄ as an additive and ZrO(NO₃)₂ as a sintering aid. The ground slurry was placed in an oven to dry at 120 ℃ for 6 h. After drying, the powder was sifted through a 200-mesh screen. The sifted powder was then calcined in air at a certain temperature for 4 h to obtain a Y₂O₃ nano-powder.

Acrylamide (AM) and N,N'-Methylenebisacrylamide (MBAM) were dissolved in deionized water to prepare a 15% AM aqueous solution in a mass ratio of AM to MBAM of 88∶10. After the organic compounds were completely dissolved, 5 mm ZrO₂ beads were added in a mass ratio of ball-to-powder of 5∶2. Tetramethylammonium Hydroxide (TMAH), ammonium citrate (TAC), A40, and Y₂O₃ powder were then added sequentially to prepare Y₂O₃ slurry with a solid content of 50%. The suspension was ground at 170 r/min for 6 h, then degassed under a vacuum of 1×10^-1Pa for 30 min. Catalyst and initiator were added sequentially, and followed via degassing the slurry again under a vacuum of 1×10^-1Pa for 3 min. Finally, the well-mixed slurry was poured into polytetrafluoroethylene or silicone molds heated at different temperatures (i.e., 25, 40, 60, and 80 ℃). The obtained green bodies were dried in air.

The gelation rate is evaluated via monitoring temperature variations during the process. The results show that increasing catalyst content accelerates gelation, but exceeding 0.08% causes a rapid reaction, leading to a green body cracking. The SEM images reveal a stratified microstructure, with a dense surface layer and a porous interior. The dense layer thickness decreases as catalyst content increases. Elevated reaction temperature also enhances gelation rate, and, stratification disappears at 60 ℃, yielding uniformly porous structures.

The infrared spectra of green bodies (i.e., 0.02% catalyst, 25 ℃) indicate an incomplete AM polymerization on the surface, while a well-developed PAM network in the interior impeded particle movement during drying. This structural disparity results in a dense surface and porous interior.

Pre-sintered green bodies exhibit heterogeneous microstructures that persist during densification. Higher catalyst content and temperature reduce the surface dense layer thickness and internal porosity.

The final vacuum-sintered samples further demonstrate that lower gelation reaction temperatures correspond to thicker surface dense layers under identical vacuum sintering temperatures, significantly hindering the expulsion of internal pores in the ceramics. This results in the center of the ceramic exhibiting an opaque condition.

Y₂O₃ transparent ceramics with excellent properties were prepared via gel casting and vacuum sintering with Y₂O₃ nano-powder as a raw material. During the gel casting process, the rate of gelation significantly affected the uniformity of the green body microstructure. A complete and uniformly structured Y₂O₃ green body was obtained via controlling the factors influencing the gelation rate at a catalyst addition of 0.04% and a reaction temperature of 60 ℃, while ensuring that the slurry met the filling requirements. After vacuum sintering at 1850 ℃ for 6 h, the obtained Y₂O₃ transparent ceramic (with a thickness of 2 mm) achieved a transmittance of 82% at 800 nm. This study demonstrated that during the gel casting process, the rate of gelation could affect the uniformity of the internal microstructure of the green body, and this internal microstructural non-uniformity further influence the subsequent densification process of the ceramic during sintering.