Granulation of Y₂O₃ powders by a vibration method for the preparation of transparent ceramics

Abstract

A modified vibrating screen method was employed to prepare granulated Y₂O₃ powders, which were used to fabricate transparent ceramics. The granulation process was carried out with yttrium-stabilized zirconia balls (5 mm in diameter) in a 200-mesh screen, which was vibrated by a Heidolph shaker. After passage through the vibrating screen, the sub-micron Y₂O₃ particles were converted into large granules and collected in the bottom chamber of the sieve. Compared with the original powders, the granulated powders had a smaller repose angle of 34.09° and larger apparent and tap densities with sharp rises of 50.54 and 32.07%, respectively, indicating a good flowability and filling properties. A high transmittance of 82% at 2500 nm was achieved for ceramics prepared using the granulated powders by vacuum sintering and hot isostatic pressing (HIP).

---

1. Introduction

Transparent ceramics are polycrystalline transparent materials prepared through raw material processing, molding and sintering process.¹ Compared to single crystal, polycrystalline ceramics offer advantages in terms of ease to fabrication, control of dopant contents and complicated shapes.² In the past decades, transparent ceramics have attracted considerable interest and great progress was realized with regard to their applications in many fields.^3–5 However, the preparation of transparent ceramics usually encounters technical challenges, for example the increase in scattering defects and degradation in optical properties resulting from inhomogeneity of green bodies.^6,7 An important reason for such a non-uniformity, especially when cold isostatic pressing (CIP) is performed, is poor flowability of the raw powders used for the preparation of transparent ceramics, which are generally sub-micron or nano-sized.^8–10

In order to improve the flowability of the powders, a granulation process is usually employed to convert fine particles into micron-sized or even larger spherical granules.¹¹ Spray drying is a widely used granulation technique, which transforms the original particles into spherical granules with a higher packing density by the use of binders.^12–15 Zhang et al. optimized yttrium aluminium garnet (YAG) granules by adjusting the parameters of spray drying, and fabricated high quality transparent YAG ceramics. Kim et al. established the correlation between the morphology of Y₂O₃ granules and spray drying conditions.⁶ Despite the excellent granulation properties of the resultant granules, it is notable that the diameter of the drying chamber is normally over meter sized for obtaining spherical granules in spray drying processes.^16,17 This means that the minimum throughput is in the range of kilograms. However, as most types of transparent ceramics presently are in the experimental stage rather than in the industrial application stage, it is difficult to produce adequate powders for spray drying. Furthermore, although the binders used in spray drying could be eliminated by the calcination of green bodies, the residual pores by the removal of the binders might be an unsolved problem for transparency.¹⁶ Therefore, the development of a convenient granulation technique is quite necessary for the fabrication of transparent ceramics, in laboratory scale. Ku et al. pointed out, fine particles tend to form agglomerates upon vibration.¹⁸ In the present study, a modified binderless vibrating screen method was employed to prepare granulated Y₂O₃ powders, which were then used to manufacture transparent ceramics. The granule powders were characterized and their sintering behaviours were investigated, with regard to repose angles, apparent and tap densities, sintering curves, and transmittance spectra.

2. Experimental details

2.1 Sample preparation

Transparent Y₂O₃ ceramics were prepared using 99.99% pure Y₂O₃ powder purchased from Aladdin. A 5 at% ZrO₂ nanopowder (99.99%, Aladdin) was introduced as the sintering additive to prepare the ceramics at low temperature. Y₂O₃, ZrO₂, and anhydrous alcohol (99.95%, Jiangsu Qiangsheng Chemical Corporation) were milled in a YSZ (yttrium-stabilized zirconia) jar with YSZ balls (5 mm in diameter) as milling media on a planetary mill for 24 h. After milling, the mixtures along with the YSZ balls were dried at 70 °C for 20 h.

The granulation process combined with a sieving process was carried out using the equipment illustrated in Fig. 1. A 200-mesh sieve was temporarily fixed on a shaker (Heidolph, Vibramax 100, Germany). The amplitude of shaker was 3 mm, and the frequency was adjustable between 0 and 1350 rpm (revolutions per minute). The dried mixtures were directly fed onto the screen and sieved with the YSZ balls. Subsequently, they were subjected to the granulation process in the bottom chamber. The bouncing of the balls and the collision between the balls and the mixtures broke the big particles, and promoted the sieving. The sub-micron powders were granulated because of the horizontal mechanical vibration of the shaker.¹⁸ We call the powders prepared by sieving, ‘original powders’, whether they were manually or automatically vibrated/sieved.

Fig. 1 Schematic illustration of the sieving and granulation equipment.

The green compaction with dimension of 20 mm in diameter formed at 3 MPa were further densified through CIP at 210 MPa before pre-sintering in a tungsten coil furnace at different temperatures (1100–1850 °C) for 5 h and HIP at 1600 °C for 3 h with high purity argon (99.999%) as medium at a pressure of 200 MPa. The ceramics were polished on both surfaces with a thickness of 3 mm before any optical measurements were made.

2.2 Characterizations

The yield of the products was calculated by weighing the throughput of granulated powder and comparing it with the theoretical value. The morphology of the powders and the fracture surfaces of the sintered samples were observed by scanning electron microscopy (SEM, ZEISS, Germany). The size distribution of the original powder and granulated powders were separately measured by a BI-XDC particle-size analyzer (BIC, USA) and the software of Nano Measurer 1.2 with the using of the existing SEM results. The repose angle of the powders was measured according to an experimental method described in the literature.¹⁹ Repose angle refers to the maximum angle of the horizontal with the slope of a powder heap formed by powder freely flow and accumulation from a funnel to a horizontal plane, which is determined by the following formula: θ = arctan(2h/d), where θ, h and d are the repose angle, the height and the diameter of the powder heap. The apparent and tap densities were measured according to reports in the literature by the mass divided by the occupied volume without and with tapping a powder-filled cylinder against a flat surface until no volume change of the powder was observed.^20,21 The optical transmittance spectra were recorded using a Lambda 750 ultraviolet/visible/near-infrared (UV/VIS/NIR) spectrophotometer (Perkin Elmer, America) in the wavelength range from 200 to 3000 nm. The relative densities of the pre-sintered specimens were calculated using the gravimetric method. A theoretical density of 5.031 g cm⁻³ was used for the measurement of the relative density.²²

3. Results and discussion

The granulation process was carried out at frequencies from 500 rpm to 1350 rpm. The relationship of the yield of granulated powders with increasing the time and vibrational frequencies is shown in Table 1. At a low frequency of 500 rpm, the vibrational energy was insufficient for the movement of the mixtures and YSZ balls, leading to a yield as low as 26.69% in 20 min. As the frequency was increased to 800 rpm, the mixtures and YSZ balls started to move and rotate around the screen centre, resulting in an increase in the yield to about 51% in 20 min. The yield increased gradually as time increasing from 10 to 20 min and frequency elevating from 800 to 1100 rpm. At a higher frequency of 1200 rpm, the dried particles were more easily destroyed by collision between the particles and YSZ balls, and thus, combined with the bouncing of the balls owing to the strong vibration, led to a yield of 97.82% in 20 min. In comparison, a 30 min continuous manual operation produced a yield of ∼90% with the same amount of raw materials. Therefore, it can be concluded that the granulation method allows us to save time and labour, as well as improve the powder yield.

Table 1 Relationship of yield versus vibrational frequency and time for the granulated powders

Frequency (rpm)	500	800	900	1000	1100	1200	1350
10 min	—	45.67	58.12	63.21	75.94	86.16	91.76
15 min	19.02	48.92	62.10	72.04	81.93	93.52	96.02
20 min	26.69	51.12	65.16	78.95	85.66	97.82	98.46
30 min	—	—	—	86.14	91.31	98.23	99.02

---

As we all know, a higher yield depends on both the longer running time and the higher vibrational frequency of the shaker. However, a further prolonging the time and elevating the frequency is meaningless for the minor improvement of yield and the possibility of the damage of instrument at the maximum frequency. And in practice, the longer running time and the higher vibrational frequency mean higher energy consumption and longer process time. Therefore, by a comprehensive assessment of the synthetic efficiency, shorter running time of 20 min and smaller frequency of 1200 rpm as well as 500 rpm and 800 rpm have been chosen in this study according to the results in Table 1.

The relationship between the morphology of the powders and the vibrational frequencies has also been studied. Fig. 2 and 3 show the morphology and the size distribution of the original and the granulated Y₂O₃ powders prepared at the frequencies of 500, 800, and 1200 rpm. As shown in Fig. 2a and 3a, the size of the original powders is in submicron scale. However, for the granulated powders obtained at the low vibrational frequencies of 500 and 800 rpm, the grains agglomerated to form larger and granules due to the vibration. Furthermore, the weak vibrational energy was insufficient to generate abrasion between the big granules, resulting in particles with an irregular shape and loose edges with sizes reaching up to about 100 μm, as shown in Fig. 2b and c. Upon increasing the frequency to 1200 rpm, on account of the severe abrasion between large particles by strong vibration and relative movement, the granulated particles exhibited nearly spherical shaped particles, and the size of the particles was reduced to 10–50 μm, much smaller than that obtained at 500 and 800 rpm. As shown in Fig. 3b–d, the size of the granulated powders indeed reduced as the frequency increasing from 500 rpm to 1200 rpm. Furthermore, Fig. 3 also confirmed that the size distribution of the particles shaken at 1200 rpm is narrower than at low frequency. It is expected that more uniform and smaller granules could be formed by further increasing the vibrational frequency. In the present study, a frequency of 1200 rpm was employed and the obtained results are discussed as follows.

Fig. 2 Morphology of (a) the original powder and the granulated powders prepared at different vibrational frequencies (b) 500 rpm, (c) 800 rpm and (d) 1200 rpm.

Fig. 3 Particles size distribution of (a) the original powder and the granulated powders prepared at different vibrational frequencies (b) 500 rpm, (c) 800 rpm and (d) 1200 rpm.

Table 2 reports the repose angle, apparent density, and tap density of the original and granulated Y₂O₃ powders. As shown in Table 2, the repose angle of the original powders was 44.98°, while that of the granulated powders was reduced to 34.09°. The repose angle is considered as the most effective evidence to evaluate the flowability of powders.¹⁹ As the definition of repose angle indicates, a lower repose angle reveals a lower powder heap height and a weaker internal friction between particles of powder, which is harder to suppress the flow of powder. Therefore, a lower repose angle powder was reasonable indicator for its higher flowability. Generally, a small repose angle of the granules contributes to a better flowability in the preparation of fully densified ceramics.

Table 2 Repose angle, apparent density, and tap density of the original and granulated powders

Sample	Original powders	Granulated powders
Repose angle (°)	44.98	34.09
Apparent density (g cm^−3)	0.60	0.91
Tap density (g cm^−3)	0.92	1.21

---

The apparent and tap density values are also given in Table 2. The apparent density of the granulated powders raised to 0.91 g cm⁻³ compared with that of the original powders (0.60 g cm⁻³), and the tap density also increased from 0.92 to 1.21 g cm⁻³ due to the particle rearrangement during the tapping process.²³ Overall, the apparent and tap densities of the granulated powders strongly increased up to 50.54 and 32.07%, respectively, revealing the better flowability of the obtained Y₂O₃ granules.

In order to produce transparent Y₂O₃ ceramics, the original and granulated powders were pre-pressed at 3 MPa followed by a CIP treatment at 210 MPa. The fracture surfaces of the obtained green bodies are shown in Fig. 4. The rearrangement of the particles and the breakage of the granules occurred under pressure.⁶ A small fluctuation could be observed on the fracture surface of pre-pressed sample fabricated from the original powders (Fig. 4a). After the CIP treatment, the fluctuation nearly disappeared, leading to a plat fracture surface (Fig. 4b). With regard to the granulated samples, the micron-sized granules were diminished by applying a small pressure of 3 MPa, leading to a relatively rough morphology of the fracture surfaces in Fig. 4c. This indicates that the granules are easily crushed at low pressure. Finally, a much more homogeneous fracture structure without any voids was formed by CIP treatment, as shown in Fig. 4d. The fracture surfaces are introduced to account for the low particle intensity that result in comparable morphologies of CIPed green bodies that separately fabricated with original powder and granulated powder.

Fig. 4 Fracture surfaces of the green bodies fabricated with (a) original powders, pre-pressing; (b) original powders, CIP treatment; (c) granulated powders, pre-pressing; and (d) granulated powders, CIP treatment.

In addition, the green bodies of the original and granulated powders have close relative density values of 56.38 ± 0.86 and 55.79 ± 1.03%, respectively. The close relative density brings about similar sintering properties of the two specimens, which is beneficial to fabricate transparent ceramics by using the granulated powders. Otherwise, the improved granulation process might carry out at the expense of transmittance for the change of sintering properties. The sintering process focused on the green bodies fabricated with granulated powders.

During the sintering process of the ceramic products, the green bodies fabricated with granulated Y₂O₃ powders were pre-sintered at 1100–1850 °C. The relative densities of the obtained ceramic samples are presented in Fig. 5. It can be seen that, when the pre-sintering temperature was below 1300 °C, the relative density of the samples remained unchanged compared with that of the green bodies, while showing a growing tendency with increasing temperatures (1300–1700 °C). It is worth noting that, a higher temperature can lead to a more rapid increase in relative density, which can reach about 100% at 1750 °C. Previous research established that a relative density above 92% is required for the formation of ceramics with high optical transparency by HIP.²⁴ Thus, a minimum pre-sintering temperature of about 1700 °C was necessary in the present work, as shown in Fig. 5.

Fig. 5 Relative densities of the green bodies fabricated with granulated powders pre-sintered at different temperatures.

The ultimate goal of this study concerning the granulation of Y₂O₃ powders is the achievement of transparent ceramics. Thus, the HIP method was employed to process the pre-sintered ceramic samples mentioned above. The samples were white opaque after pre-sintering and turned into transparent ceramics after the HIP treatment. The transmittance spectra of the HIP fabricated Y₂O₃ ceramics pre-sintered in the temperature range from 1700 to 1850 °C are shown in Fig. 6. An excellent optical transmittance of ∼82% at 2500 nm was achieved for all Y₂O₃ ceramics with different pre-sintering temperatures. However, in the short wavelength region from 200 to 1500 nm, higher pre-sintering temperatures led to a better transmittance. This could be attributed to the higher number of micropores in Y₂O₃ ceramics pre-sintered at lower temperatures, which could not be completely removed by the HIP treatment. The number of scattering pores could be reduced by increasing the pre-sintering temperature, in agreement with the scattering theory, which matched well with the results of this study.²⁵ In addition, the absorption peak at around 2850 nm originated from the OH asymmetric stretching vibration of the ceramics.²⁶

Fig. 6 Optical transmittance spectra of the HIPed Y₂O₃ ceramics that were pre-sintered at (a) 1700 °C, (b) 1750 °C, (c) 1800 °C and (d) 1850 °C.

The fracture surfaces of the Y₂O₃ ceramic pre-sintered at 1800 °C and the final HIP treated sample are shown in Fig. 7. It can be seen that, after pre-sintering, the ceramic experienced mixed inter- and intra-granular fracture modes with some inter-granular pores in triple junction, as shown in Fig. 7a. These pores are the main source of the scattering that leads to the low transmittance of the pre-sintered Y₂O₃ ceramics.²⁷ The inter-granular pores could be removed by exerting pressure during the HIP process,²⁴ resulting in clean grain boundary and high transmittance of the ceramic, as shown in Fig. 7b and 6.

Fig. 7 Fracture surfaces of the Y₂O₃ ceramics (a) pre-sintered at 1800 °C and (b) HIP treated after the 1800 °C pre-sintering.

Next, we further investigated the effect of the granulated Y₂O₃ powders on the transparency of the final ceramics. The transmittances of the HIP treated ceramics fabricated with the granulated and original powders are provided in Fig. 8b and a, respectively. These results show that the ceramic fabricated with the granulated powders possess a transmittance comparable to that of the sample fabricated with the original powders, indicating that the granulation approach does not affect the sintering properties of the original powders.

Fig. 8 Optical transmittance spectra of HIPed ceramics pre-sintered at 1800 °C and fabricated with (a) original powders and (b) granulated powders.

Based on the results analyzed above, it can be concluded that the granulation of Y₂O₃ powders can lead to a better flowability of raw materials, as well as a high transmittance of the resulting transparent ceramics. Most importantly, the granulation approach proposed in this study represents an efficient way to produce small amounts of powders and shows potential to be applied to the fabrication of complex shaped transparent ceramics.

4. Conclusions

In the present study, micron-sized Y₂O₃ powders were granulated by a modified vibrating screen method. A vibrational frequency of 1200 rpm led to a 97.82% yield of smaller and rounder granulated powders. Compared with the original powders, the granulated powders have a smaller repose angle of 34.09° and higher apparent and tap densities, with rises by 50.54 and 32.07%, respectively. In addition, the transmittance of the ceramics fabricated with the granulated powders was comparable to that of the sample fabricated with the original powders, which possessed a high transmittance of ∼82% at 2500 nm.

Acknowledgements

This work was supported by the National Youth Natural Science Foundation of China (No. 61405221 and 51302284) and the China Scholarship Council.

References

1. S. F. Wang, J. Zhang and D. W. Luo, et al., Transparent ceramics: processing, materials and applications, Prog. Solid State Chem., 2013, 41(1–2), 20–54.
2. V. Lupei, A. Lupei and A. Ikesue, Single crystal and transparent ceramic Nd-doped oxide laser materials: a comparative spectroscopic investigation, J. Alloys Compd., 2004, 380(1–2), 61–70.
3. J. A. Cox, P. Greenlaw and G. Terry, et al., Comparative study of advanced IR transmissive materials, Proc. SPIE, 1986, 683, 49–62.
4. S. Q. Qiao, Y. Zhang and X. C. Shi, et al., Spectral properties and laser performance of Nd:Lu₃Al₅O₁₂ ceramic, Chin. Opt. Lett., 2015, 13(5), 051602.
5. D. H. Titterton, J. Sanghera and W. Kim, et al., Development of ceramic laser host materials, Proc. SPIE, 2011, 8187, 81870G.
6. C. S. Kim, M. J. Kim and H. Cho, et al., Fabrication and plasma resistance of Y₂O₃ ceramics, Ceram. Int., 2015, 41(10), 12757–12762.
7. W. C. Wang, F. Tang and X. Y. Yuan, et al., Fabrication and properties of tape-casting transparent Ho:Y₃Al₅O₁₂ ceramic, Chin. Opt. Lett., 2015, 13(5), 051404.
8. C. B. Willingham, J. M. Wahi and P. K. Hogan, et al., Densification of nano-yttria powders for IR window applications, Proc. SPIE, 2003, 5078, 179–188.
9. M. Suarez, A. Fernandez and J. L. Menendez, et al., Hot isostatic pressing of optically active Nd:YAG powders doped by a colloidal processing route, J. Eur. Ceram. Soc., 2010, 30(6), 1489–1494.
10. Q. Wen, S. G. Xiao and X. J. Gao, et al., Photochromic effect of HoPO₄:Li⁺ powder, Chin. Opt. Lett., 2015, 13(3), 031601.
11. K. T. Walter, A process for granulation of a particulate material, European patent, EP1064990B1, 2000.
12. L. Zhang, Y. Z. Li and X. Y. Li, et al., Characterization of spray granulated Nd:YAG particles for transparent ceramics, J. Alloys Compd., 2015, 639, 244–251.
13. I. G. Song and J. S. Kim, Enhancement of the mechanical properties of alumina ceramics by a granulation process and Y₂O₃ additive, Korean J. Met. Mater., 2015, 53(4), 262–269.
14. J. S. Cho, K. Y. Jung and Y. C. Kang, Two-step spray-drying synthesis of dense and highly luminescent YAG:Ce³⁺ phosphor powders with spherical shape, RSC Adv., 2015, 5, 8345–8350.
15. L. Zhang, H. Yang and X. B. Qiao, et al., Systematic optimization of spray drying for YAG transparent ceramics, J. Eur. Ceram. Soc., 2015, 35(8), 2391–2401.
16. W. B. Liu, W. X. Zhang and J. Li, et al., Preparation of spray-dried powders leading to Nd:YAG ceramics: the effect of PVB adhesive, Ceram. Int., 2012, 38(1), 259–264.
17. M. Serantoni, A. Piancastelli and A. L. Costa, et al., Improvements in the production of Yb:YAG transparent ceramic materials: spray drying optimisation, Opt. Mater., 2012, 34(6), 995–1001.
18. N. Ku, C. Hare and M. Ghadiri, et al., Auto-granulation of fine cohesive powder by mechanical vibration, Procedia Eng., 2015, 102, 72–80.
19. A. C. Y. Wong, Use of angle of repose and bulk densities for powder characterization and the prediction of minimum fluidization and minimum bubbling velocities, Chem. Eng. Sci., 2002, 57(14), 2635–2640.
20. H. H. Zhu, J. Y. H. Fuh and L. Lu, The influence of powder apparent density on the density in direct laser-sintered metallic parts, Int. J. Mach. Tool. Manufact., 2007, 47(2), 294–298.
21. I. G. Shah, K. J. Ely and W. C. Stagner, Effect of tapped density, compacted density, and drug concentration on light-induced fluorescence response as a process analytical tool, J. Pharm. Sci., 2015, 104(5), 1732–1740.
22. Z. Y. Wang, L. Zhang and H. Yang, et al., High optical quality Y₂O₃ transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment, Ceram. Int., 2016, 42(3), 4238–4245.
23. J. Mouzon, T. Lindback and M. Oden, Influence of agglomeration on the transparency of yttria ceramics, J. Am. Ceram. Soc., 2008, 91(10), 3380–3387.
24. J. Mouzon, A. Maitre and L. Frisk, et al., Fabrication of transparent yttria by HIP and the glass-encapsulation method, J. Eur. Ceram. Soc., 2009, 29(2), 311–316.
25. J. G. J. Peelen and R. Metselaa, Light-scattering by pores in polycrystalline materials – transmission properties of alumina, J. Appl. Phys., 1974, 45(1), 216–220.
26. Y. Cui, H. Li and K. Yi, et al., Moisture absorption characteristics of a SiO₂ film from 2 to 3 μm, Chin. Opt. Lett., 2015, 13(2), 023101.
27. Y. Feng, H. Lin and C. Chen, et al., Fabrication of transparent Tb₃Ga₅O₁₂ ceramic, Chin. Opt. Lett., 2015, 13(3), 031602.

---