Fabrication of Highly Transparent Y₂O₃ Ceramics via Colloidal Processing Using ZrO₂-Coated Y₂O₃ Nanoparticles

Abstract

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.

1. Introduction

Transparent yttria (Y₂O₃) is one of the most promising materials for application in high-power lasers [1], heat-resistive transparent windows [2], high-temperature refractories, and semiconductor devices [3], owing to its broad wavelength range of transparency (0.2–8.0 μm), high corrosion resistance, high melting point (2430 °C), high thermal conductivity, low phonon energy, and easy doping of rare-earth activator ions with a band gap energy of 6.3 eV [4]. To achieve fully densified ceramics with desired optical properties, fabrication of polycrystalline Y₂O₃ ceramics requires careful control of each step of ceramic processing, including powder synthesis, consolidation, and sintering. Simultaneously, each step has a direct impact on the following step.

By utilization of Y₂O₃ powder produced via established synthesis routes or directly using commercial powder, sintering methods and mechanisms have been mostly studied by researchers recently [5,6,7,8,9]. Typically, transparent Y₂O₃ ceramics can be sintered by pressure-assisted sintering processes such as spark plasma sintering (SPS) [5], hot pressing (HP) [6], and hot isostatic pressing (HIP) [7]. Pressureless sintering methods, such as vacuum sintering [10] and hydrogen atmosphere sintering [11], are also effective. Moreover, the HIP sintering method utilizes three-dimensional gas pressure to presintered bodies that are in the final stage of sintering, which is much more effective to eliminate pores compared to other sintering methods. Presintering in air followed by HIP treatment possesses more advantages than traditional vacuum presintering plus HIP treatment, including energy-saving, low cost, and efficient process without annealing [12]. On the other hand, fully dense Y₂O₃ ceramics can be obtained using ZrO₂ as sintering additives, which can decrease the grain boundary mobility, leading to Y₂O₃ ceramics with a fully dense fine microstructure and high optical properties [13]. According to the published literature, fabrication of highly transparent ceramics has mainly been achieved by vacuum presintering plus HIP treatment, which requires high-cost equipment and a long annealing process. Compared with vacuum presintering, presintering in air is a low-cost and efficient process without annealing. In 2015, Wang et al. [7] fabricated transparent Y₂O₃ ceramics through air presintering at 1550 °C plus HIPing at 1600 °C for 3 h. The in-line transmittance of the ceramic was 81.7% at 1064 nm, and the thickness of the sample was 1.0 mm. In 2016, Liu et al. [8] also reported transparent Y₂O₃ ceramics by air presintering plus HIPing. However, the sample only showed an IR transmittance of 83.0% in the 3.0 to 5.0 µm wavelength region

It is well known that colloidal processing has exhibited advantages over traditional dry pressing by producing green compacts with a homogenous microstructure, high relative density, and more versatile geometrical options [14,15,16]. There are, however, to the best of our knowledge, relatively few studies devoted to the colloidal processing of Y₂O₃ green bodies. Y₂O₃ powders are highly reactive in aqueous medium. Dissolving ions, including Y³⁺, Y(OH)²⁺, and ${\mathrm{Y}}_2{\left(\mathrm{OH}\right)}_2^{4+}$, which can suppress the electrical double layer, is the most critical challenge in preparing long-term well-dispersed suspensions with high solids loading [17,18,19,20]. In order to inhibit the hydrolysis issue of Y₂O₃ during colloidal processing, in 2014, Sun et al. [20] coated Y₂O₃ powder by a nucleophilic addition reaction between polyurethane and tetraethylene pentamine. In 2017, Xu et al. [21] prepared Y₂O₃ suspensions using alcohol as the dispersion medium. However, the boiling point of ethanol was as low as 78 °C, and the consolidated green compacts were easy to crack during the drying process. Until now, the solid loading of Y₂O₃ suspensions reported in previous literature has generally been in the range of 20 vol%–30.0 vol%, which is much lower than other oxide materials [19,21,22]. It has been found that coating is an effective method to alter the surface properties of the particles and is feasible to improve the stability and dispersity of the core material during colloidal processing [23,24]. Notably, several studies have shown that ZrO₂ possesses much higher inertness in aqueous suspension compared to Y₂O₃ [25,26]. In our previous research [10], particles consisting of a Y₂O₃ core coated with ZrO₂ were developed via the precipitation method by taking dual advantages of ZrO₂ as particle surface passivator and sintering aid, and high-transparency ZrO₂ doped Y₂O₃ ceramic was fabricated. Nevertheless, the coating process is complicated, and sintering still involves vacuum sintering.

In this work, a simple method to obtain ZrO₂-coated Y₂O₃ nanoparticles by thermal decomposition of zirconium nitrate was described to improve the dispersion of Y₂O₃ in aqueous suspensions. Highly transparent Y₂O₃ ceramics were prepared by a simple shaping approach. followed by air presintering plus HIPing without annealing. The effect of coating on the dispersity and stability of the Y₂O₃ suspensions during colloidal processing was investigated in comparison with uncoated powder.

2. Materials and Methods

2.1. Starting Materials

High-purity commercial Y₂O₃ nanopowder (99.99% pure; Huizhou Ruier Rare Chemical Hi-Tech Co., Ltd., Huizhou, China) was adopted as the starting material. Zirconium nitrate (Zr(NO₃)₄·5H₂O, analytical reagent, Sinopharm chemical reagent, Shanghai, China) was adopted to prepare a 0.2 mol/L solution. Triammonium citrate (TAC) (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as dispersant. The pH value of the suspensions was adjusted by HCl or KOH solution.

2.2. Coating Procedure and Suspension Preparation

Y₂O₃ powder was dispersed in ethyl alcohol with 2.0 wt.% dispersant added. After milling with ZrO₂ balls for 12 h, the suspension was then dried at 90 °C in an oven. The dried powder was sieved through a 200-mesh nylon screen. The powders were then calcinated at 1100 °C for 4 h, which was named starting powder (S.P.).

The S.P. was dispersed into ethyl alcohol, followed by the addition of a corresponding ratio of 5.0 at% Zr⁴⁺ zirconium nitrate solution, and then the suspension was milled for 24 h. Subsequently, the suspension was dried at 80 °C for 12 h. The dried hygroscopic powder (D.P.) consisting of a complex hydrate of dried metal nitrate salt was consecutively calcined at 600, 700, and 800 °C for 4 h in a furnace to obtain ZrO₂-coated Y₂O₃ nanopowder. The mass ratio of TAC for different solid loadings of suspensions was 1.5 wt.%.

2.3. Consolidation and Sintering

The centrifugal slip-casting method was adopted using a laboratory centrifuge at 3000 rpm for 40 min. The green compacts were left at room temperature for 24 h, followed by drying at 80 °C for 12 h in an oven. The compacted samples were then presintered at 1500 °C for 16 h in air to reach the final stage of sintering. Finally, the presintered samples were post-HIPed at 1550 °C for 2 h under 200 MPa in argon atmosphere. Figure 1 shows a flowchart of the experimental procedure.

2.4. Characterization

The crystal phase was identified by XRD (X’pert PROMPD, PANalytical, Almelo, the Netherlands) using nickel-filtered CuKα radiation. The morphologies of the powders were observed by transmission electron microscopy (TEM, Model JEM-2100F, JEOL, Tokyo, Japan). Zeta potential measurements were adopted to analyze the charged state of the particles’ surface (DT-1202, Dispersion Technology Inc., New York, NY, USA). The rheological properties of the suspensions were tested by a cone-plate viscometer (Brookfield DV-II+Pro, Brookfield Engineering Laboratories, Middleboro, MA, USA). The relative densities of the green compact and sintered bodies were measured by the Archimedes method with regard to the theoretical density of Y₂O₃ (5.03 g/cm³). The morphologies of the sintered bodies were observed by FE-SEM using a Hitachi scanning electron microscope (S-4800, Tokyo, Japan). The samples after polishing on both surfaces were used to measure the in-line transmittance by using an ultraviolet/visible/near-infrared spectrophotometer (Model Lambda 750S, PerkinElmer, Waltham, MA, USA).

3. Results and Discussion

3.1. Coating Y₂O₃ Particle with Zirconium Nitrate via Thermal Decomposition Method

XRD patterns of the S.P., D.P., and powder after calcination at different temperatures are shown in Figure 2. All the peaks match the standard cubic Y₂O₃ phase (JCPDS card, No. 43-1036), and no ZrO₂ phase was detected. Compared with the S.P., the diffraction peak intensity of the D.P. was weakened, indicating the presence of zirconium nitrate crystal on the surface of the Y₂O₃ particle. The radius of Zr⁴⁺ ions (r = 0.72 Å) is smaller than Y³⁺ ions (r = 0.89 Å), which will shift the diffraction peaks to larger angles [27]. The diffraction peaks of the powders shift to larger angles after 600 °C calcination, indicating the dissolution of Zr⁴⁺ within the Y₂O₃ lattice gradually with the decomposition of Zr(NO₃)₄. With a further increase in the calcination temperatures from 600 to 800 °C, the diffraction peaks of Y₂O₃ continuously shift towards larger angles (Figure 2b), which leads us to conclude that the increased dissolution of ZrO₂ within the Y₂O₃ lattice coincides with the increase in calcination temperature.

In order to deeply investigate the evolution during the coating procedure, the morphological characteristics of the powders were analyzed by transmission electron microscopy, as shown in Figure 3. The particles for the S.P. possess high crystallinity with flat and clear edges. The average size of the particles is ~100.0 nm (Figure 3a). In contrast, the D.P. exhibits rough surfaces with loosely packed tiny particles of zirconium nitrate crystal, as shown in Figure 3b. On account of that, the S.P. was already calcined at 1100 °C before the coating process, and there was no significant change in the morphology of the core particle after different temperature calcination. However, it is worth noting that with the increasing calcination temperature, the tiny particles consisting of the coating layer gradually decreased and disappeared (Figure 3c,d,f). As we can see from Figure 3d,e, after calcination at 700 °C, a coating layer with a thickness of 5–10 nm could be observed. On the one hand, the d-spacing of the coating layer of 2.94 Å could be indexed to the ZrO₂ (tetragonal) structure of the (011) crystal plane. On the other hand, the d-spacing of 5.29 Å is the (200) plane for the Y₂O₃ (cubic) structure. With the further increase in calcination temperature to 800 °C (Figure 3f), the coating layer nearly disappeared, indicating that ZrO₂ gradually dissolved into the Y₂O₃ lattice with the increase in calcination temperature.

During calcination, Zr(NO₃)₄ decomposed and adhered onto the surfaces of Y₂O₃ particles. ZrO₂ dissolved into the Y₂O₃ lattice with the increase in calcination temperature. The diffusion depth of Zr⁴⁺ into Y₂O₃ lattice for the sample calcinated at 700 °C for 4 h was evaluated using the following equations [28]:

$$D_{\mathrm{lattice}}=\exp (-423 (\mathrm{kJ}/\mathrm{mol})/{\mathrm{R}T) \mathrm{m}}^2{\mathrm{s}}^{-1}$$

(1)

where $D_{\mathrm{lattice}}$ is the diffusion coefficient, and T is temperature. According to Boniecki’s research, the diffusion coefficient of Zr⁴⁺ in the Y₂O₃ lattice at 1600 °C was 7.04 × 10⁻²² m²s⁻¹; hence, we can calculate the diffusion coefficient of Zr⁴⁺ in the Y₂O₃ lattice at 700 °C as follows [29]:

$$D_{\mathrm{lattice}}^{700}=\frac{\exp (-423 (\mathrm{kJ}/\mathrm{mol})/{\mathrm{Rg}927) \mathrm{m}}^2{\mathrm{s}}^{-1}}{\exp (-423 (\mathrm{kJ}/\mathrm{mol})/{\mathrm{Rg}1827) \mathrm{m}}^2{\mathrm{s}}^{-1}} \times 7{.04 \times 10}^{-22}$$

(2)

where $\mathrm{Rg}$ is the ideal gas content, and 927 and 1827 are the absolute temperatures of 700 and 1600 °C. As the holding time of calcination was 4 h, the diffusion area was achieved as follows:

$${\mathrm{S}=D}_{\mathrm{lattice}}^{700} \times 4 \times 3600 \approx 9{.30 \mathrm{nm}}^2$$

(3)

where S is the diffusion area. The diffusion depth could then be evaluated as 3.44 nm, which was far below the radius of the raw particles (100 nm). On the basis of our findings, one can conclude that powder after 700 °C calcination consisted of surface-coated ZrO₂ and Zr⁴⁺ solid solution outside of the Y₂O₃ lattice, which was beneficial to the inhibition of hydrolysis issue.

3.2. Dispersion Properties of ZrO₂-Coated Y₂O₃ Powders

Figure 4 shows the rheological behaviors of suspensions for the S.P. and coated powders with a solid loading of 10.0 vol% at pH = 10.1 with TAC added. The viscosity of the suspension with S.P. (S-suspension) decreases as the shear rate increases, showing a shear-thinning behavior. For the suspension with coated powder calcinated at different temperatures, there was a significant decrease in the viscosity, which exhibits a relatively constant viscosity corresponding to Newtonian. The lowest viscosity occurred when the calcination temperature was 700 °C. According to the above analysis, suspensions formulated with powders calcined at 700 °C (C-suspension) were adopted to be a better choice for the following steps to fabricate transparent ceramics.

The zeta potential of the ZrO₂-coated Y₂O₃ powders calcined at 700 °C for 4 h calcination, the S.P., and the synthesized ZrO₂ powder treated under identical conditions are shown in Figure 5. The isoelectric point (IEP) of the S.P. and synthesized ZrO₂ powder are 10.1 and 8.0, respectively. The IEP for ZrO₂-coated Y₂O₃ powders showed between the S.P. and synthesized ZrO₂ powder. The coating of ZrO₂ reduced the IEP of Y₂O₃ particles from 10.1 to 9.3. The negatively charged groups of TAC were absorbed on the surfaces of ZrO₂-coated yttria nanoparticles. As observed from this figure, the addition of TAC shifted the IEP value from 10.1 to 5.6. Moreover, the absolute zeta potential of particles in suspensions increases with the increase in pH, and it reaches 49 mV at the pH of 11. The good dispersion of coated Y₂O₃ powder in aqueous suspension is favorable for the preparation of well-dispersed and high solids loading solution in an easy method.

3.3. Consolidation and Sintering of Y₂O₃ Transparent Ceramics

Figure 6 shows the SEM images of the green bodies consolidated from two suspensions with a solid loading of 35.0 vol%. Larger micron-scale pores can be observed for the sample consolidated using S-suspensions (Figure 6a), which are difficult to eliminate during the sintering process. In sharp contrast, the green bodies of the C-suspension yielded a more homogeneous and compact microstructure (Figure 6b). Only nanosized pores can be seen, and the particles are in close contact with each other. The relative densities of the green bodies are 50.2% and 29.2% for the C-suspension and S-suspension, respectively. As we all know, green compacts with smaller pore size distribution and a homogeneous microstructure show high sinterability and were easy to densify at lower temperatures.

Figure 7 shows SEM images of the Y₂O₃ ceramics sintered at 1500 °C for 16 h in air. For the sample prepared from the S-suspension, a porous structure with the presence of open pore channels is shown in Figure 7a. The relative density of the sample is 86.4%, which is the typical feature of intermediate-stage sintering [30]. In comparison, for the sample prepared from the C-suspension (Figure 7b), the number of pores is much less than that in the S-suspension one. Moreover, it can also be noticed that the sample has a much smaller average pore size. The relative density of the sample reaches 96.4%, and there are no open pores found in the sample, suggesting that it is already at the final stage of sintering [27]. From the results we obtained, one can conclude that the higher relative density and more homogeneous microstructure improved the sinterability.

The in-line transmittance curves of the samples are shown in Figure 8. The transmittance of the sample prepared by the S.P. is lower than its theoretical value. The relative density of the sample is 95.5%. In contrast, the sample prepared with coated powder after 700 °C calcination has a remarkable improvement in transmittance observed. The relative density of the sample is 99.9%. The in-line transparency (ILT) at 1100 nm is 81.4%. At the wavelength of 2000 nm, the ILT approaches 82.6%, which is very close to the theoretical value of Y₂O₃ crystal. The inset photograph shows the optical images of the as-obtained Y₂O₃ ceramic prepared with coated powder after polishing, and the characters below the ceramic sample can be seen very clearly.

It is worth mentioning that the absorption edge shifts to the slong wavelength of the sample prepared by the S.P. at the wavelength of 250–350 nm range could be observed. The addition of Zr⁴⁺ to Y₂O₃ will produce oxygen interstitial of ${\mathrm{O}}_{\mathrm{i}}^{″}$ during the sintering process and inhibit the grain boundary mobility to promote the densification of Y₂O₃ ceramics. Herein, the shift of the undoped sample could be due to the change of band gap energy and the oxygen vacancies [31] formed during the sintering process. Moreover, the very tiny little residual pores in the sintered body could also be an important issue for the decrease in the transmittance at short wavelengths.

Figure 9 shows the microstructure of the fracture surfaces for the HIPed samples. Compared with the sample prepared by the C-suspension, the sample prepared by the S-suspension (shown in Figure 9a) exhibited an exaggerated grain size and lots of intergranular pores and intragranular pores act as light scattering centers in transparent ceramics, resulting in a decrease in transmittance. The average grain size of the sample obtained from the S.P. is 3.3 μm. However, fully densified and no pores are observed in the sample prepared by ZrO₂-coated powder (Figure 9b,c), which helps to explain their higher optical quality. The typical grain size distribution is shown in Figure 9d. The mean grain size is 1.7 μm, indicating that ZrO₂ effectively promoted the densification and inhibited the grain growth of the sintered body.

Compared with Wang and Liu’s results [7,8], we achieved highly transparent Y₂O₃ ceramic with fine grain size, and the ILT was ~80.0% in the visible wavelength range by air presintering plus HIPing at a lower temperature. Herein, we proposed a new method to synthesize ZrO₂-coated Y₂O₃ powder for colloidal processing. By adopting ZrO₂-coated Y₂O₃ powder, a well-dispersed suspension with high solid loading (35.0 vol%) was achieved. After consolidation, the green compacts were sintered in an energy-saving and effective method, which is low-temperature air presintering plus HIPing without annealing. Moreover, the sintered body was fully densified, and the average grain size is less than 2.0 μm, which may make it a promising fabrication method in the preparation of sesquioxide transparent ceramics. We point out that this is the most effective and simple method to fabricate highly transparent ceramics with fine grain size.

4. Conclusions

In summary, the coating of Y₂O₃ particles with a thin ZrO₂ layer via an easy method was reported, and it was found to be an effective method to inhibit hydrolysis issues during suspension preparation. Consequently, a homogeneous green body with a relative density of 50.2% was obtained, which is beneficial for fabrication of highly transparent Y₂O₃ ceramics with fine grain size via low-temperature air presintering plus HIPing treatment. The Y₂O₃ ceramic presintered at 1500 °C × 16 h and post-HIPed at 1550 °C × 2 h showed a grain size of 1.7 μm and high in-line transmittance (81.4% at 1100 nm, thickness: ~1.0 mm), very close to the theoretical value of Y₂O₃. This is the most effective and simple method to fabricate highly transparent ceramics with fine grain size.