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Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations Dongyue Yan a,b, Xiaodong Xu a,b, Hao Lu a,b, Yuwei Wang a,b, Peng Liu a,b,n, Jian Zhang a,b,n a Jiangsu Key Laboratories of Advanced Laser Materials and Devices, School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou, 221116 China b Jiangsu Collaborative Innovation Center of Advanced Laser Technology and Emerging Industry, Jiangsu Normal University, Xuzhou, 221116 China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 12 July 2016 Accepted 13 July 2016
Transparent Y2O3 ceramics were fabricated by the solid-state reaction and vacuum sintering method using La2O3, ZrO2 and Al2O3 as sintering aids. The microstructure of the Y2O3 ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y2O3 transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 mm. Furthermore, the transmittance of Y2O3 ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y2O3 transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Y2O3 Microstructure Grain size Optical properties
1. Introduction Due to its excellent chemical stability, low emission and small absorption coefficient in the IR region at high temperature, the Y2O3 transparent ceramics were used as excellent IR-windows materials [1,2]. In addition, compared with the conventional transmitting infrared window materials such as sapphire, AlON and MgAl2O4, Y2O3 ceramic also has longer cutoff wavelength and broad transparency range which is extremely important for IRwindow applications [1–4]. For the window materials, the flexural strength of the materials is also very important as well as the transmittance of the materials. However, the flexural strength of the Y2O3 transparent ceramics is not very high at present [3,4]. According to the HallPetch relations, the ceramic with the smaller average grain size owns the higher flexural strength [5]. The fine grain size of the ceramics can be achieved by many approaches, such as using the nanocrystalline powder as the starting materials, two-step sintering method to enhance the densification with the minimum grain growth, and applying the sintering aids to suppress grain growth and etc [6–11]. In recent years, many sintering aids have been developed to improve the optical and flexural properties of the Y2O3 transparent ceramics [11–13]. The ThO2 and HfO2 were used as sintering aids in Y2O3 system, but they were less applied at n
Corresponding author at: Jiangsu Key Laboratories of Advanced Laser Materials and Devices, School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou 221116, China. E-mail addresses: [email protected] (P. Liu), [email protected] (J. Zhang).
present because of the toxicity and high expense [10]. In addition, high sintering temperature (2050–2100 °C) is necessary to achieve the high optical quality. Afterwards, La2O3, ZrO2 as well as their combinations began to be used as sintering aids to fabricate high optical quality transparent ceramics [11–14]. Yang et al. has reported that transparent Y2O3 ceramics with the in-line transmittance over 80% at 1–6 mm can be fabricated by using La2O3 as the sintering aids [11]. In their studies, high doping concentration of La2O3 up to 10 mol% were used. Qing Yi et al. [12] has reported that the Y2O3 transparent ceramics were fabricated by co-doping with La2O3 and ZrO2 for the first time. The transmittance of the samples was 79.93% at the wavelength around 1100 nm, and the average grain size was about 10 mm. Although the highly transparent Y2O3 ceramics have been fabricated, the average grain size of the Y2O3 fabricated is still over 10 mm, which is not favor to obtain the higher flexural strength [15]. In this work, highly transparent Y2O3 ceramics were prepared by a solid-state reaction method with La2O3, ZrO2 and Al2O3 as composite additives. The microstructural evolution and optical transmittance of the Y2O3 samples which were vacuum sintered at different temperature were investigated.
2. Experimental procedure Commercial high-purity Y2O3 (5 N, Jiahua Corp. Ltd., China) powders were used as the starting materials. High-purity La2O3 (4 N, Jiahua Corp. Ltd., China), ZrO2 (99.5%, metal base, Alfa-Aesar, UK) and Al2O3 (4 N, Sumitomo Chemicals, Japan) powders were
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Please cite this article as: D. Yan, et al., Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.089i
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used as sintering aids, and the doping concentration were fixed at 0.5 wt%, 3.0 wt% and 0.004 wt%, respectively. The weighted powders were mixed with ethanol by planetary ball-milled for 15 h with zirconia balls (3 mm, Tosoh, Japan). After dried at 55 °C for 24 h, the homogeneous powders were obtained. Then the powders were sieved through a 60 mesh screen. These homogeneous powders were calcined at 800 °C for 6 h and dry pressed into green bodies with a 16 mm stainless steel mold at 10 MPa. After that, the green bodies were cold isostatic pressed at 200 MPa. Eventually, green bodies were sintered at different temperature from 1550 to 1800 °C for 8 h under high vacuum degree (below 10 3 Pa). Then, the sintered samples were annealed at 1350 °C for 10 h in air. At last, the transparent Y2O3 ceramics were cut and polished, and samples with thickness of 1 mm were obtained. In order to obtain the flexural strength by three point bending tests, the samples were processed into the rectangular bars with the size of 3 mm*4 mm*36 mm. In addition, the surfaces of the rectangular bars were mirror polished and the edges of the bars were chamfered into 45°. The morphology of the raw powders, the mixture powders after ball milling, the thermal etching surface and the fracture surface of the Y2O3 ceramics were recorded on a scanning electron microscope (SEM, JSM- 6510, JEOL, Kariya, Japan). The optical transmittance of Y2O3 ceramics was obtained by a UV–VIS–NIR spectrophotometer (Lambda 950, Perkin-Elmer, Waltham, MA) and Fourier transform infrared spectroscopy (Tensor 27, BRUKER OPTIK GmbH, Ettlingen, Germany). The flexural strength was obtained by Instron-5566 universal material testing machine (Norwood, MA, American). Furthermore, their refractive index under different temperature were tested by spectroscopic ellipsometry method (IR-VASE, J.A. Woollam, Lincoln, NE). The average grain size was measured by averaging over 200 grains and using a mean linear method. The density of the bulk ceramics was tested by the Archimedes method.
3. Result and conclusion Fig. 1(a) and (b) presents the SEM image of the starting Y2O3, Al2O3 powders respectively. Fig. 1(c) shows the morphology of powder mixtures with sintering aids after ball milling. The average particle sizes of original Y2O3 powders was 4–5 mm with heavily agglomeration. The Al2O3 powders were homogeneous distributed and the mean particle sizes was about 200 nm. However, it can be obviously observed that the agglomeration of the particles was easily crashed by ball milling process, which is shown in Fig. 1(c). After the ball milling, the fine particle size powder mixtures appeared. Fig. 2 shows the sintering map of Y2O3 ceramics in the temperature range of 1550–1800 °C. In addition to the different sintering temperature, all the holding time at the different sintering temperature was kept at 8 h, constantly. The grain size and the
Fig. 2. Densification and grain growth behavior of the samples sintered at 1550– 1800 °C for 8 h.
relative density increased with temperature increasing in general. It can be clearly found that below the 1650 °C, the density of sintered Y2O3 body increased rapidly, with the very slow grain growth rate. When the sintering temperature further raised to 1700 °C, the density was closed to the theoretical density, and the grain size was only 1.8 mm. With the sintering temperature further increase, the rapid grain growth happened. As the sintering temperature reached 1800 °C, the average grain size was around 3.5 mm, which was much smaller than the results reported by Ning et al. so that flexural strength value can reach to 203.2 MPa. In their work, La2O3 and ZrO2 were used as the sintering aids and the average grain size of the Y2O3 ceramic sintered at the similar conditions was 9.11 mm [16]. This phenomenon indicated that the inhibition of grain growth have been carried out during sintering process through doping with three additives. It has been reported that La3 þ ions enhanced the mobility of grain boundary while the Zr4 þ ions strongly suppressed the migration of grain boundary [12–14]. Compared with the reference 16, the content of the Zr4 þ in the study is higher than that of in the reference, which can cause the reduction of the average grain sizes. According to the literature [17], the small amount of Al2O3 can react with Y2O3, which were forming the eutectic Y4Al2O9 (YAM) phase during the sintering process. The presence of the eutectic phase resulted in a significant increase in ionic diffusion rate. Furthermore, the increasing of the ionic diffusion rate at the grain boundaries can contribute to the higher grain-boundary mobility. All of these will improve the densification rate of the Y2O3 during sintering process after Al2O3 added. Therefore, the Y2O3 ceramics can be reach the full dense under the lower sintering temperature. And the transmittance of the Y2O3 can reach 82.1% around 1100 nm when it was vacuum sintered just at 1800 °C. The thermal etching surface of the as-prepared Y2O3 ceramics
Fig. 1. SEM micrograph of the (a) raw powder of Y2O3; (b) raw powder of Al2O3; (c) powder mixture after ball-milling.
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Fig. 3. Microstructures of Y2O3 sintered at (a) 1550 °C, (b) 1600 °C, (c) 1650 °C, (d) 1700 °C, (e) 1750 °C, (f) 1800 °C.
are shown in Fig. 3. Porosity and the pore diameter decreased with the increasing of sintering temperature, and pores completely disappeared with the temperature rising to 1700 °C. In addition, as the sintering temperature increased from 1750 °C to 1800 °C, the grain grew rapidly. The microstructure evolution as well as porosity changes was consistent with the results as shown in Fig. 2. Fig. 4 illustrates the in-line transmittance spectra of Y2O3 doped with La2O3, ZrO2 and Al2O3 sintered at 1750 and 1800 °C for 8 h. The inset image shows the photograph of the Y2O3 transparent ceramics sintered at different temperature. When the applied sintering temperature was lower than 1700 °C, all the as-prepared samples were opaque. According to the SEM images shown in Fig. 3, most of the pores are located near the grain boundary areas in the microstructures. Meanwhile, small amount of intracrystalline pores were found to appear in the microstructure, which is shown in Fig. 5(a). These residue pores acted as the main scattering centers and greatly decreased the optical transparency of the ceramics. When the samples were sintered at 1750 °C or above, few
residue pore can be found in the microstructure of the sintered ceramic. However from the Fig. 4, it can be found that Y2O3 sintered at 1800 °C shows the slightly higher transmittance than that of 1750 °C sintered Y2O3 ceramic, especially at the short wavelength range. From the SEM pictures shown in Fig. 5(b) and (c), both ceramics showed the really dense microstructures. In transparent MgAl2O4 ceramic related work, A. Krell et al. reported the existence of nano-porosity in the sintered ceramic which is extremely difficult to be observed by using the normal SEM can contribute to the lower transparency of transparent ceramics [18]. Moreover, the as-prepared Y2O3 ceramics sintered at 1800 °C had high transmittance from 0.5 to 6 mm and the long wavelength cutoff was over 8 mm, which was in favor of the application for infrared window. For optical window applications, the accurate refractive index measurements of the window materials is very important. In this studies, the refractive index of Y2O3 ceramics sintered at 1800 °C were measured in the wavelength range of 2.0–10.0 mm. And the dependent of Y2O3 refractive index on the temperature from 30 °C to 400 °C were also measured, which were shown in Fig. 6. The inset table shows the refractive index of as-prepared Y2O3 ceramic at 2.0 mm, 3.0 mm 4.0 mm and 4.5 mm. In general, with the temperature increasing from 30 °C to 400 °C, the refractive index of Y2O3 ceramic slightly decreased. In the wavelength range from 2.0 to 2.5 mm, there were some refractive index changes can be found with the temperature increasing. However, at the longer wavelength, the difference of the Y2O3 refractive index at different temperatures were negligible.
4. Conclusion
Fig. 4. Transmittance spectra of Y2O3 sintered at 1750 °C and 1800 °C for 8 h. The inset image show a photograph of corresponding Y2O3 ceramics.
The highly transparent Y2O3 ceramics were fabricated by solidstate reaction using La2O3, ZrO2 and Al2O3 combinations as the sintering aids with short dwelling time at 1800 °C. The average grain size of as-prepared Y2O3 ceramic was only 3.5 mm. Moreover, the fabricated Y2O3 ceramics owns very broad transmission range and the overall transmittance reached 80% in the wavelength range from 0.5 to 6.0 mm. With the temperature increasing from 30 °C to 400 °C, the Y2O3 ceramics shows the relatively stable refractive index, which is extremely important for its potential
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Fig. 5. SEM images of the fracture surfaces of samples sintered at (a) 1700 °C, (b) 1750 °C (c) 1800 °C for 8 h.
Fig. 6. The refractive index of 1800 °C sintered Y2O3 ceramic at 30 °C, 100 °C, 200 °C, 300 °C and 400 °C, The inset table shows the refractive index of as-prepared Y2O3 ceramic at 2.0 mm, 3.0 mm 4.0 mm and 4.5 mm.
applications such as missile domes, windows, and other optical elements.
Acknowledgements The authors gratefully acknowledge Dr. Dan Han for the contributions to the measurement of flexural strength.
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Domes - Past and Present, 2004. [3] H. Eilers, Fabrication, optical transmittance, and hardness of IR-transparent ceramics made from nanophase yttria, J. Eur. Ceram. Soc. 27 (2007) 4711–4717. [4] L. Gan, Y.J. Park, H. Kim, J.M. Kim, J.W. KO, J.W. Lee, Fabrication of submicrongrained IR-transparent Y2O3, ceramics from commercial nano-raw powders, Ceram. Int. 41 (9) (2015) 11992–11998. [5] S. Takeuchi, The mechanism of the inverse Hall-Petch relation of nanocrystals, Scr. Mater. 44 (8-9) (2001) 1483–1487. [6] Z.G. Huang, X.D. Sun, Z.M. Xiu, S.W. Chen, C.T. Tsai, Precipitation synthesis and sintering of yttria nanopowders, Mater. Lett. 58 (15) (2004) 2137–2142. [7] P.-L. Chen, I.-W. Chen, Sintering of fine oxide powders: II, sintering mechanism, J. Am. Ceram. Soc. 80 (3) (1997) 637–645. [8] J. Zhang, L. An, M. Liu, S. Shimai, S.-W. Wang, Sintering of Yb3 þ :Y2O3 transparent ceramics in hydrogen atmosphere, J. Eur. Ceram. Soc. 29 (2) (2008) 305–309. [9] L. Gan, Y.J. Park, H. Kim, J.M. Kim, J.W. KO, J.W. Lee, Effects of pre-sintering and annealing on the optical transmittance of Zr-doped Y2O3, transparent ceramics fabricated by vacuum sintering conjugated with post-hot-isostatic pressing, Ceram. Int. 41 (2015) 9622–9627. [10] A.-S. Kaygorodov, V.-V. Ivanov, V.-R. Khrustov, Y.-A. Kotov, A.-I. Medvedev, V.V. Osipov, M.-G. Ivanov, M.-G. Ivanov, A.-N. Orlov, A.-M. Murzakave, Fabrication of Nd:Y2O3 transparent ceramics by pulsed compaction and sintering of weakly agglomerated nanopowders, J. Eur. Ceram. Soc. 27 (2) (2007) 1165–1169. [11] A. Ikesue, K. Kamata, K. Yoshida, Synthesis of transparent Nd-doped HfO2 -Y2O3 ceramics using HIP, J. AM. Ceram. Soc. 79 (2) (1996) 359–364. [12] H.J. Zhang, Q.H. Yang, H.X. Zhou, J. Xu, Effect of La2O3 on properties of Yb3 þ doped yttrium oxide transparent laser ceramics, Chin. J. Inorg. Chem. 25 (4) (2009) 578–583. [13] X.R. Hou, S.M. Zhou, Y.K. Li, W.J. Li, Effect of ZrO2, on the sinterability and spectral properties of (Yb0.05 Y0.95 )2O3, transparent ceramic, Opt. Mater. 32 (9) (2010) 920–923. [14] Q. Yi, S. Zhou, H. Teng, H. Lin, T.T. Jia, Structural and optical properties of Tm: Y2O3, transparent ceramic with La 2O3, ZrO2, as composite sintering aid, J. Eur. Ceram. Soc. 32 (2) (2012) 381–388. [15] K. Serivalsatit, Fabrication of transparent ceramic laser media for high energy laser applications, CLEMSON University, 2010. [16] K.J. Ning, J. Wan, D.W. Luo, J. Ma, J. Zhang, Z.L. Dong, L.B. Kong, D.Y. Tang, et al., Fabrication and characterization of highly transparent Yb3 þ : Y2O3 ceramics, Opt. Mater. 2 (2015) 21–24. [17] J. Wang, J. Zhang, K.J Ning, D.W. Luo, H. Yang, D.L. Yin, D.Y. Tang, L.B. Kong, Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering, J. Am. Ceram. Soc., 2016. [18] A. Krell, K. Waetzig, J. Klimke, Effects and elimination of nanoporosity in transparent sintered spinel, in: R.W. Tustison (Ed.), Proceedings of the SPIE: defense, security þ sensing-window and dome technologies and materials XII, vol. 8016, SPIE, Bellingham, Orlando, FL, USA, 25–29 April, 2011, 801602, pp. 1–10.
Please cite this article as: D. Yan, et al., Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.089i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations Dongyue Yan a,b, Xiaodong Xu a,b, Hao Lu a,b, Yuwei Wang a,b, Peng Liu a,b,n, Jian Zhang a,b,n a Jiangsu Key Laboratories of Advanced Laser Materials and Devices, School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou, 221116 China b Jiangsu Collaborative Innovation Center of Advanced Laser Technology and Emerging Industry, Jiangsu Normal University, Xuzhou, 221116 China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 12 July 2016 Accepted 13 July 2016
Transparent Y2O3 ceramics were fabricated by the solid-state reaction and vacuum sintering method using La2O3, ZrO2 and Al2O3 as sintering aids. The microstructure of the Y2O3 ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y2O3 transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 mm. Furthermore, the transmittance of Y2O3 ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y2O3 transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Y2O3 Microstructure Grain size Optical properties
1. Introduction Due to its excellent chemical stability, low emission and small absorption coefficient in the IR region at high temperature, the Y2O3 transparent ceramics were used as excellent IR-windows materials [1,2]. In addition, compared with the conventional transmitting infrared window materials such as sapphire, AlON and MgAl2O4, Y2O3 ceramic also has longer cutoff wavelength and broad transparency range which is extremely important for IRwindow applications [1–4]. For the window materials, the flexural strength of the materials is also very important as well as the transmittance of the materials. However, the flexural strength of the Y2O3 transparent ceramics is not very high at present [3,4]. According to the HallPetch relations, the ceramic with the smaller average grain size owns the higher flexural strength [5]. The fine grain size of the ceramics can be achieved by many approaches, such as using the nanocrystalline powder as the starting materials, two-step sintering method to enhance the densification with the minimum grain growth, and applying the sintering aids to suppress grain growth and etc [6–11]. In recent years, many sintering aids have been developed to improve the optical and flexural properties of the Y2O3 transparent ceramics [11–13]. The ThO2 and HfO2 were used as sintering aids in Y2O3 system, but they were less applied at n
Corresponding author at: Jiangsu Key Laboratories of Advanced Laser Materials and Devices, School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou 221116, China. E-mail addresses: [email protected] (P. Liu), [email protected] (J. Zhang).
present because of the toxicity and high expense [10]. In addition, high sintering temperature (2050–2100 °C) is necessary to achieve the high optical quality. Afterwards, La2O3, ZrO2 as well as their combinations began to be used as sintering aids to fabricate high optical quality transparent ceramics [11–14]. Yang et al. has reported that transparent Y2O3 ceramics with the in-line transmittance over 80% at 1–6 mm can be fabricated by using La2O3 as the sintering aids [11]. In their studies, high doping concentration of La2O3 up to 10 mol% were used. Qing Yi et al. [12] has reported that the Y2O3 transparent ceramics were fabricated by co-doping with La2O3 and ZrO2 for the first time. The transmittance of the samples was 79.93% at the wavelength around 1100 nm, and the average grain size was about 10 mm. Although the highly transparent Y2O3 ceramics have been fabricated, the average grain size of the Y2O3 fabricated is still over 10 mm, which is not favor to obtain the higher flexural strength [15]. In this work, highly transparent Y2O3 ceramics were prepared by a solid-state reaction method with La2O3, ZrO2 and Al2O3 as composite additives. The microstructural evolution and optical transmittance of the Y2O3 samples which were vacuum sintered at different temperature were investigated.
2. Experimental procedure Commercial high-purity Y2O3 (5 N, Jiahua Corp. Ltd., China) powders were used as the starting materials. High-purity La2O3 (4 N, Jiahua Corp. Ltd., China), ZrO2 (99.5%, metal base, Alfa-Aesar, UK) and Al2O3 (4 N, Sumitomo Chemicals, Japan) powders were
http://dx.doi.org/10.1016/j.ceramint.2016.07.089 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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used as sintering aids, and the doping concentration were fixed at 0.5 wt%, 3.0 wt% and 0.004 wt%, respectively. The weighted powders were mixed with ethanol by planetary ball-milled for 15 h with zirconia balls (3 mm, Tosoh, Japan). After dried at 55 °C for 24 h, the homogeneous powders were obtained. Then the powders were sieved through a 60 mesh screen. These homogeneous powders were calcined at 800 °C for 6 h and dry pressed into green bodies with a 16 mm stainless steel mold at 10 MPa. After that, the green bodies were cold isostatic pressed at 200 MPa. Eventually, green bodies were sintered at different temperature from 1550 to 1800 °C for 8 h under high vacuum degree (below 10 3 Pa). Then, the sintered samples were annealed at 1350 °C for 10 h in air. At last, the transparent Y2O3 ceramics were cut and polished, and samples with thickness of 1 mm were obtained. In order to obtain the flexural strength by three point bending tests, the samples were processed into the rectangular bars with the size of 3 mm*4 mm*36 mm. In addition, the surfaces of the rectangular bars were mirror polished and the edges of the bars were chamfered into 45°. The morphology of the raw powders, the mixture powders after ball milling, the thermal etching surface and the fracture surface of the Y2O3 ceramics were recorded on a scanning electron microscope (SEM, JSM- 6510, JEOL, Kariya, Japan). The optical transmittance of Y2O3 ceramics was obtained by a UV–VIS–NIR spectrophotometer (Lambda 950, Perkin-Elmer, Waltham, MA) and Fourier transform infrared spectroscopy (Tensor 27, BRUKER OPTIK GmbH, Ettlingen, Germany). The flexural strength was obtained by Instron-5566 universal material testing machine (Norwood, MA, American). Furthermore, their refractive index under different temperature were tested by spectroscopic ellipsometry method (IR-VASE, J.A. Woollam, Lincoln, NE). The average grain size was measured by averaging over 200 grains and using a mean linear method. The density of the bulk ceramics was tested by the Archimedes method.
3. Result and conclusion Fig. 1(a) and (b) presents the SEM image of the starting Y2O3, Al2O3 powders respectively. Fig. 1(c) shows the morphology of powder mixtures with sintering aids after ball milling. The average particle sizes of original Y2O3 powders was 4–5 mm with heavily agglomeration. The Al2O3 powders were homogeneous distributed and the mean particle sizes was about 200 nm. However, it can be obviously observed that the agglomeration of the particles was easily crashed by ball milling process, which is shown in Fig. 1(c). After the ball milling, the fine particle size powder mixtures appeared. Fig. 2 shows the sintering map of Y2O3 ceramics in the temperature range of 1550–1800 °C. In addition to the different sintering temperature, all the holding time at the different sintering temperature was kept at 8 h, constantly. The grain size and the
Fig. 2. Densification and grain growth behavior of the samples sintered at 1550– 1800 °C for 8 h.
relative density increased with temperature increasing in general. It can be clearly found that below the 1650 °C, the density of sintered Y2O3 body increased rapidly, with the very slow grain growth rate. When the sintering temperature further raised to 1700 °C, the density was closed to the theoretical density, and the grain size was only 1.8 mm. With the sintering temperature further increase, the rapid grain growth happened. As the sintering temperature reached 1800 °C, the average grain size was around 3.5 mm, which was much smaller than the results reported by Ning et al. so that flexural strength value can reach to 203.2 MPa. In their work, La2O3 and ZrO2 were used as the sintering aids and the average grain size of the Y2O3 ceramic sintered at the similar conditions was 9.11 mm [16]. This phenomenon indicated that the inhibition of grain growth have been carried out during sintering process through doping with three additives. It has been reported that La3 þ ions enhanced the mobility of grain boundary while the Zr4 þ ions strongly suppressed the migration of grain boundary [12–14]. Compared with the reference 16, the content of the Zr4 þ in the study is higher than that of in the reference, which can cause the reduction of the average grain sizes. According to the literature [17], the small amount of Al2O3 can react with Y2O3, which were forming the eutectic Y4Al2O9 (YAM) phase during the sintering process. The presence of the eutectic phase resulted in a significant increase in ionic diffusion rate. Furthermore, the increasing of the ionic diffusion rate at the grain boundaries can contribute to the higher grain-boundary mobility. All of these will improve the densification rate of the Y2O3 during sintering process after Al2O3 added. Therefore, the Y2O3 ceramics can be reach the full dense under the lower sintering temperature. And the transmittance of the Y2O3 can reach 82.1% around 1100 nm when it was vacuum sintered just at 1800 °C. The thermal etching surface of the as-prepared Y2O3 ceramics
Fig. 1. SEM micrograph of the (a) raw powder of Y2O3; (b) raw powder of Al2O3; (c) powder mixture after ball-milling.
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3
Fig. 3. Microstructures of Y2O3 sintered at (a) 1550 °C, (b) 1600 °C, (c) 1650 °C, (d) 1700 °C, (e) 1750 °C, (f) 1800 °C.
are shown in Fig. 3. Porosity and the pore diameter decreased with the increasing of sintering temperature, and pores completely disappeared with the temperature rising to 1700 °C. In addition, as the sintering temperature increased from 1750 °C to 1800 °C, the grain grew rapidly. The microstructure evolution as well as porosity changes was consistent with the results as shown in Fig. 2. Fig. 4 illustrates the in-line transmittance spectra of Y2O3 doped with La2O3, ZrO2 and Al2O3 sintered at 1750 and 1800 °C for 8 h. The inset image shows the photograph of the Y2O3 transparent ceramics sintered at different temperature. When the applied sintering temperature was lower than 1700 °C, all the as-prepared samples were opaque. According to the SEM images shown in Fig. 3, most of the pores are located near the grain boundary areas in the microstructures. Meanwhile, small amount of intracrystalline pores were found to appear in the microstructure, which is shown in Fig. 5(a). These residue pores acted as the main scattering centers and greatly decreased the optical transparency of the ceramics. When the samples were sintered at 1750 °C or above, few
residue pore can be found in the microstructure of the sintered ceramic. However from the Fig. 4, it can be found that Y2O3 sintered at 1800 °C shows the slightly higher transmittance than that of 1750 °C sintered Y2O3 ceramic, especially at the short wavelength range. From the SEM pictures shown in Fig. 5(b) and (c), both ceramics showed the really dense microstructures. In transparent MgAl2O4 ceramic related work, A. Krell et al. reported the existence of nano-porosity in the sintered ceramic which is extremely difficult to be observed by using the normal SEM can contribute to the lower transparency of transparent ceramics [18]. Moreover, the as-prepared Y2O3 ceramics sintered at 1800 °C had high transmittance from 0.5 to 6 mm and the long wavelength cutoff was over 8 mm, which was in favor of the application for infrared window. For optical window applications, the accurate refractive index measurements of the window materials is very important. In this studies, the refractive index of Y2O3 ceramics sintered at 1800 °C were measured in the wavelength range of 2.0–10.0 mm. And the dependent of Y2O3 refractive index on the temperature from 30 °C to 400 °C were also measured, which were shown in Fig. 6. The inset table shows the refractive index of as-prepared Y2O3 ceramic at 2.0 mm, 3.0 mm 4.0 mm and 4.5 mm. In general, with the temperature increasing from 30 °C to 400 °C, the refractive index of Y2O3 ceramic slightly decreased. In the wavelength range from 2.0 to 2.5 mm, there were some refractive index changes can be found with the temperature increasing. However, at the longer wavelength, the difference of the Y2O3 refractive index at different temperatures were negligible.
4. Conclusion
Fig. 4. Transmittance spectra of Y2O3 sintered at 1750 °C and 1800 °C for 8 h. The inset image show a photograph of corresponding Y2O3 ceramics.
The highly transparent Y2O3 ceramics were fabricated by solidstate reaction using La2O3, ZrO2 and Al2O3 combinations as the sintering aids with short dwelling time at 1800 °C. The average grain size of as-prepared Y2O3 ceramic was only 3.5 mm. Moreover, the fabricated Y2O3 ceramics owns very broad transmission range and the overall transmittance reached 80% in the wavelength range from 0.5 to 6.0 mm. With the temperature increasing from 30 °C to 400 °C, the Y2O3 ceramics shows the relatively stable refractive index, which is extremely important for its potential
Please cite this article as: D. Yan, et al., Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.089i
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Fig. 5. SEM images of the fracture surfaces of samples sintered at (a) 1700 °C, (b) 1750 °C (c) 1800 °C for 8 h.
Fig. 6. The refractive index of 1800 °C sintered Y2O3 ceramic at 30 °C, 100 °C, 200 °C, 300 °C and 400 °C, The inset table shows the refractive index of as-prepared Y2O3 ceramic at 2.0 mm, 3.0 mm 4.0 mm and 4.5 mm.
applications such as missile domes, windows, and other optical elements.
Acknowledgements The authors gratefully acknowledge Dr. Dan Han for the contributions to the measurement of flexural strength.
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Please cite this article as: D. Yan, et al., Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.089i