Optimizing Sintering Conditions for Y₂O₃ Ceramics: A Study of Atmosphere-Dependent Microstructural Evolution and Optical Performance

Abstract

This paper systematically investigated the influence of sintering atmospheres, vacuum, and oxygen, on the microstructure and optical properties of Y₂O₃ ceramics. Compared with vacuum sintering, sintering in flowing oxygen atmosphere can effectively inhibit the grain growth of Y₂O₃ ceramics at the final stage of sintering and improve the uniformity of microstructure. After hot isostatic pressing, the samples pre-sintered at oxygen atmosphere showed good in-line transmittance from a visible-to-mid-infrared wavelength range (0.4–6.0 μm) except in the range of 2.8–4.1 μm. Spectral analysis showed that an obvious broadband absorption peak (2.8–4.1 μm) of characteristic hydroxyl groups is detected in the above samples. However, before densification, a low-temperature heat treatment at 600 °C under vacuum can effectively diminish the hydroxyl groups in Y₂O₃ ceramics. However, laser experiments in the ~1 μm wavelength range showed that although the Yb:Y₂O₃ ceramic carrying hydroxyl had obvious absorption in the 2.8–4.1 μm range, it had little effect on its laser oscillation in the ~1 μm wavelength. Yb:Y₂O₃ ceramics pre-sintered in an oxygen atmosphere at 1460 °C followed by hot isostatic pressing at 1440 °C achieved 12.85 W continuous laser output at room temperature, with a laser slope efficiency of 84.4%.

1. Introduction

In recent decades, yttrium oxide (Y₂O₃) ceramics have garnered significant attention due to their superior properties, including low phonon energy, high thermal conductivity, good thermal stability, and broad optical transparency (0.22–8 μm) [1,2,3]. Consequently, Y₂O₃ transparent ceramics are considered ideal candidates for applications in high-power laser systems, high-temperature lens, and infrared windows [4,5]. As a cost-effective alternative to obtain Y₂O₃ materials with inherent high melting temperature (>2400 °C), the ceramic counterparts can be fabricated under much lower sintering temperature (<1800 °C), meanwhile demonstrating the advantages of composite structure design and volume scalability [6,7]. However, achieving high-quality Y₂O₃ ceramics remains challenging, with demands to suppress residual pores, exaggerated grain growth, and impurity incorporation (e.g., hydroxyl groups) that degrade optical transparency. Some defects are closely related to the sintering conditions, particularly sintering atmosphere and temperature, that can greatly influence the microstructure evolution and densification process [8,9].

Conventional sintering methods for Y₂O₃ ceramics typically involve vacuum sintering [10], which can avoid the entrapment of insoluble gas within pores, thereby promoting final full densification. However, during the final-stage sintering of Y₂O₃ ceramics, accelerated grain boundary mobility can lead to pore–boundary separation, resulting in the formation of intragranular pores that are difficult to remove, even further with pressure-assisted hot isostatic pressing (HIP) sintering [11]. Therefore, it is necessary to control microstructure evolution during the vacuum sintering process. Additionally, it is well known that the grain boundary mobility of Y₂O₃ ceramics is rate-controlled by yttrium cation interstitial concentrations, which is inversely proportional to the concentration of oxygen interstitials [9,12]. Under an oxygen-rich sintering atmosphere, the concentration of oxygen interstitials in Y₂O₃ tends to increase, thus inhibiting the abnormal grain growth. Therefore, sintering in an oxygen atmosphere can serve as an effective way to fabricate highly dense Y₂O₃ transparent ceramics.

In this paper, a systematic comparison was conducted on the effects of vacuum and oxygen pre-sintering on the microstructure and densification behaviors of Y₂O₃ ceramics. The optical properties, including transmittance and absorption spectra, were compared with different sintering methods. A laser experiment was also conducted on Yb:Y₂O₃ ceramics, which were pre-sintered under oxygen atmosphere.

2. Experimental Section

2.1. Ceramic Fabrication

Y₂O₃ and 3 at.% Yb:Y₂O₃ transparent ceramics were prepared by the chemical co-precipitation method. Firstly, commercial high-purity Y₂O₃ and Yb₂O₃ powder was used as the raw material. They were dissolved in dilute hydrochloric acid solution. According to the chemical formula of Y₂O₃ and (Yb₀.₀₃Y₀.₉₇)₂O₃, the prepared YCl₃ and YbCl₃ solutions were mixed to make the concentration of Y³⁺ reach 0.2 mol/L. Next, a mixture of NH₄OH and NH₄HCO₃ was used as the precipitant (with molar concentrations of 1.5 mol/L and 2 mol/L, respectively). The precipitant was sprayed into the mother solutions at a rate of 10 mL/min until the pH value reached 8. The obtained precursors were aged at 15 °C for about 5 h, then filtered and washed 4 times with ultrapure water and 3 times with ethanol to remove reaction by-products. Subsequently, the washed precursors were dried in an oven at 80 °C for about 48 h. Then, they were crushed and calcined in a muffle furnace at 1200 °C/5 h. Then, the obtained Y₂O₃ and Yb:Y₂O₃ powders were ball-milled by using a planetary milling machine. Ethanol and ZrO₂ balls were used as the ball-milling medium. The solid contents of the slurries were fixed at 11 vol%. The rotational speed of the machine was set to 140 r/min, with a milling duration of 15 h. The resulting slurries were completely dried and sieved through a 100-mesh sieve. The sieved powder was then calcined at 700 °C for 10 h to remove any by-products. The calcined powder was weighed and poured into a mold with a diameter of 22 mm. A uniaxial pressure of 5 MPa was applied and maintained for 5 min. The resulting green compact, formed via dry pressing, was subsequently subjected to cold isostatic pressing at 200 MPa to obtain ceramic green bodies. The green bodies underwent pre-sintering in either oxygen (oxygen purity 99.9%) or vacuum (vacuum degree < 5 × 10⁻³ Pa) atmospheres. The pre-sintering temperature and duration were 1400–1550 °C and 7 h, respectively. Subsequently, some of the samples were further densified via hot isostatic pressing (HIP) at 198 MPa in an argon atmosphere for 3 h at 1440 °C. After HIP treatment, the samples were annealed in air using a muffle furnace at 1000 °C for 10 h. Finally, the transparent ceramic samples were double-sided polished.

2.2. Characterization

The relative densities of the samples were tested using Archimedes’ method. The microstructures of the sintered ceramics were observed using a scanning electron microscope (SEM, JSM-6510, JEOL, Kariya, Japan). Before observation, the samples were mirror polished on the surface and then thermally etched at 1250 °C for 5 h in a muffle furnace. In-line transmittances in the visible to near-infrared wavelength range were measured using a UV–Vis-NIR spectrophotometer (Lambda 950, Perkin-Elmer, Waltham, MA, USA). The measurement wavelength range was from 200 to 2500 nm with a step size of 1 nm. In-line transmittances in the mid-infrared range were measured using a Fourier-transform infrared spectrometer (FTIR, TENSOR 27, Bruker, Germany). All the measurements were carried out at room temperature.

3. Results and Discussion

Figure 1 compares the evolution of microstructures in Y₂O₃ ceramics as a function of sintering temperature during the final sintering stage under flowing oxygen and vacuum conditions. Samples sintered in an oxygen atmosphere exhibit smaller average grain sizes and lower porosity compared to those sintered in vacuum at identical temperatures (Figure 1a–d,f–i). Notably, the oxygen-sintered specimens show significantly narrower grain size distributions (Figure 1e,j), which can be attributed to two primary factors. First, the oxygen-rich environment promotes the formation of oxygen interstitials within the crystal lattice, effectively reducing the grain boundary mobility of Y₂O₃ and thereby suppressing grain growth [12]. Second, the forced convection induced by the flowing gas atmosphere enhances heat transfer efficiency within the furnace chamber. In contrast, vacuum sintering primarily relies on thermal radiation for heat transfer, leading to less efficient temperature distribution. The improved thermal uniformity in oxygen sintering ultimately contributes to a more homogeneous microstructure in the sintered specimens.

Figure 2a compares the evolution of the relative density of Y₂O₃ ceramics as a function of sintering temperature under different sintering atmospheres. When the sintering temperature is below 1430 °C, samples sintered in vacuum exhibit slightly higher relative density than those sintered in an oxygen atmosphere. This can be attributed to the increased concentration of oxygen vacancies in vacuum, which enhances grain boundary diffusion in Y₂O₃ ceramics, thereby promoting densification during the intermediate stages of sintering. However, during the final stage of sintering, the number of residual pores decreases significantly, leading to a substantial reduction in the pinning effect on grain boundaries. At this stage, the high grain boundary diffusion rate facilitates rapid grain growth, as shown in Figure 2b. As grain growth progresses, residual pores tend to coalesce and migrate along with the grain boundaries, resulting in a gradual decrease in pore number and an increase in pore size, as illustrated in Figure 3. With the continuous increase in pore size, their mobility declines, and they finally become trapped within the rapidly migrating grain boundaries, forming intragranular pores. As shown in Figure 2 and Figure 3, Y₂O₃ ceramics sintered in an oxygen atmosphere exhibit smaller average grain sizes, lower porosity and smaller average pore sizes at the same sintering temperature compared to those sintered in vacuum. This can be attributed to enhanced microstructural uniformity and the suppression of excessive grain growth.

To further eliminate residual pores in Y₂O₃ ceramics, samples pre-sintered under O₂ and vacuum atmospheres were both subjected to hot isostatic pressing. Figure 4 compares the XRD of Y₂O₃ ceramics that underwent vacuum or oxygen pre-sintering at 1460 °C, followed by HIP at 1440 °C. As shown in Figure 4a, the XRD patterns of the two ceramics correspond to the diffraction peaks of the pure cubic Y₂O₃ phase (PDF no.74-1828), and no extra impurity peak is found, indicating that the sample does not produce a second phase. As shown in Figure 4b, compared with the vacuum pre-sintering sample, the X-ray diffraction peak of oxygen pre-sintering Y₂O₃ ceramics moves slightly to the small angle direction. This is because the introduction of oxygen interstitial point defects leads to an increase in the lattice constant of Y₂O₃, which shows that the X-ray diffraction peak of vacuum pre-sintering Y₂O₃ ceramics deviates to the small angle direction.

Figure 5 compares the transmittance of Y₂O₃ ceramics that underwent vacuum or oxygen pre-sintering at 1460 °C, followed by HIP at 1440 °C. Representative sample properties of the transparent Y₂O₃ ceramics in recent work are also listed in

Table 1. Optical properties of Y₂O₃ transparent ceramics from the literature and this work.

Sample	Sintering Method	Sintering Additive	Transmittance	Ref.
Y2O3	VS + HIP	None	83.0%@3–5 μm	[15]
Y2O3	AS + HIP	None	81.8%@800 nm	[16]
Y2O3	VS	ZrO2	80%@3–6 μm	[17]
Y2O3	OS	ZrO2	80%@800 nm	[9]
Y2O3	VS + HIP	None	81.6%@800 nm 84.0%@3.3 μm	This work
Y2O3	OS + HIP	None	81.9%@800 nm 80.8%@3.3 μm	This work

for comparison. As shown in Figure 5a, Y₂O₃ ceramics pre-sintered under both atmospheres exhibit high in-line transmittance in the visible and near-infrared regions (the sample thickness is 4.3 mm). The transmittance exceeds 80% at wavelengths above 700 nm, indicating that the samples pre-sintered in either vacuum or oxygen exhibit good optical quality. However, as seen in Figure 5b, the sample pre-sintered in oxygen exhibits a broad absorption peak in the 2.8–4.1 μm range, which corresponds to the characteristic absorption of hydroxyl groups [13]. This phenomenon indicates that hydroxyl groups in Y₂O₃ ceramics can only be effectively removed through sintering in a vacuum or a reducing atmosphere. Their resistance to removal at high temperatures under oxygen suggests that hydroxyl groups are not simply present in the form of absorbed water molecules within the Y₂O₃ green bodies but are chemically bonded to yttrium ions. During high-temperature sintering, the desorption reaction of hydroxyl groups from the material is shown in Reaction (1) [14].

$${\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{O}}^{2-}+{\mathrm{H}}_2\mathrm{O}\uparrow$$

(1)

The above reaction is reversible, which effectively explains why hydroxyl groups cannot be removed even by high-temperature calcination under flowing oxygen conditions. The presence of oxygen molecules increases the concentration of ${\mathrm{O}}^{2-}$, which, as a product, drives the reaction in the reverse direction. In contrast, under vacuum conditions, the removal of water vapor products is accelerated, which promotes the reaction to proceed in the forward direction.

To investigate the temperature range at which hydroxyl groups begin to decompose during vacuum sintering, Y₂O₃ green bodies were subjected to vacuum heat treatment within the temperature range of 600–1460 °C. To prevent moisture absorption, the samples were immediately densified in a flowing oxygen atmosphere at 1460 °C after the respective vacuum heat treatment, ensuring complete closure of the pores. The samples were then subjected to hot isostatic pressing at 1440 °C to achieve full densification. Figure 6 shows the FTIR spectra of the densified Y₂O₃ samples with different vacuum heat treatment temperatures. It can be observed that after low-temperature (600 °C) vacuum heat treatment, the hydroxyl content in the samples significantly decreased. The above phenomenon indicates that the primary driving force for hydroxyl desorption in Y₂O₃ green bodies is the rapid removal of water vapor in a vacuum environment, rather than the high-temperature decomposition of Y–OH bonds. This finding provides new insights into the desorption behavior of hydroxyl groups in oxide-based infrared optical materials, offering a potential strategy for controlling hydroxyl-related absorption in similar systems.

Figure 7a illustrates the relationship between the output power and the absorbed pump power for the Yb:Y₂O₃ ceramic sample pre-sintered in an oxygen atmosphere and further densified by hot isostatic pressing (HIP) in a simple two-mirror laser cavity. Using a fiber coupled semiconductor laser with a central wavelength of 976 nm, the numerical aperture is 0.22, and the fiber diameter is 105 μm. The pump light output from the laser is collimated through lens F1 (focal length f = 50 mm) and then focused into the ceramic gain medium by lens F2 (focal length f = 125 mm). In order to reduce the thermal effect caused by high-power laser pumping, indium foil was used to tightly wrap the laser ceramic and fix it in the copper heat sink, and the cooling water temperature was set at 20 °C. The resonant cavity structure used in this study is a double mirror flat cavity structure. Among them, the planar input mirror M1 is coated with a two-color dielectric film, which has high transmittance (T > 98%) in the pump light band and high reflectivity (R > 99.9%) in the laser band. The output coupling mirror OC is a planar structure [18]. The Yb:Y₂O₃ ceramic sample had dimensions of 3 mm × 3 mm × 3 mm and was used without an anti-reflection coating. The threshold absorbed pump powers were 1.5 W, 3.0 W, and 3.0 W with the output couplers (OCs) of 5%, 10%, and 15%, respectively. With a 10% OC, a maximum output power of 12.85 W was achieved at an absorbed pump power of 38.7 W, corresponding to a slope efficiency of 84.4%. The laser efficiency and output power were comparable to those obtained from vacuum-sintered Yb:Y₂O₃ ceramics [18]. This result indicates that the presence of hydroxyl in Yb:Y₂O₃ ceramics does not adversely affect laser performance in the 1 μm wavelength range. Therefore, it is reasonable to believe that oxygen-atmosphere sintering might be a promising approach for fabricating rare-earth-doped Y₂O₃ ceramics for laser applications below 2.9 μm wavelengths.

4. Conclusions

This paper investigated the effects of two sintering atmospheres—vacuum and flowing oxygen—on the sintering behavior of Y₂O₃ ceramics, focusing on densification characteristics and microstructural evolutions. The results indicate that, compared with vacuum sintering, oxygen-atmosphere sintering effectively suppresses grain growth during the final sintering stage and enhances microstructural uniformity. At equivalent sintering densities, oxygen-sintered samples exhibit significantly smaller average grain sizes than those sintered under vacuum. In the 2.8–4.1 μm mid-infrared range, oxygen-sintered samples show distinct hydroxyl absorption peaks. In contrast, vacuum sintering effectively eliminated hydroxyl groups, thereby reducing mid-infrared absorption. Despite the presence of hydroxyl groups, the laser performance of Yb:Y₂O₃ ceramics densified in an oxygen atmosphere demonstrated comparable laser efficiency and output power in the 1 μm wavelength range, underscoring their potential for laser applications outside the 2.8–4.1 μm absorption regions. In summary, oxygen sintering offers a more cost-effective and scalable route for fabricating laser ceramics, while vacuum sintering is preferable for mid-infrared window applications due to its superior hydroxyl removal and reduced mid-infrared absorption.