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Fabrication and characterization of highly transparent Yb3+: Y2O3 ceramics
Optical Materials xxx (2015) xxx–xxx
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and characterization of highly transparent Yb3+: Y2O3 ceramics Kaijie Ning a,c,⇑, Jun Wang b,c, Dewei Luo b,c, Jie Ma b, Jian Zhang a,c, Zhi Li Dong a,c, Ling Bing Kong c, Ding Yuan Tang a,b,⇑ a b c
Temasek Laboratories of Nanyang Technological University, Singapore 639798, Singapore School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Highly transparent ceramics Yb3+: Y2O3 Solid-state reaction method Characterization
a b s t r a c t Highly transparent Yb3+ doped Y2O3 (Yb3+: Y2O3) ceramics was fabricated by a solid-state reaction method using ZrO2 and La2O3 as additives. The morphology of the prepared powder was investigated and the phase of the sintered Yb3+: Y2O3 ceramics sample was identified. The microstructure, transmittance spectrum, Vickers hardness and fracture toughness for the as-sintered Yb3+: Y2O3 ceramics were measured. The average grain size was about 9.11 lm and the transmittance at the wavelength of 2000 nm was about 82.0%, which was 99% of the theoretical value. ZrO2 and La2O3 were useful additives for highly transparent Yb3+: Y2O3 ceramics fabrication and mechanical properties improvement. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The polycrystalline transparent ceramics have more advantages than single crystals on laser application [1,2], such as low cost, composite structural design, easy manufacture, large-size fabrication, high dopant concentration and comparable thermal conductivity. Transparent ceramics of YAG and Y2O3 are the two kinds of mostly investigated laser materials and are the promising alternatives for supplanting the single crystals. Because of the cubic structure of these two materials, no bad effects of the inherent birefringence are imposed on light transmittance. Transparent ceramics of YAG and Y2O3 can be fabricated properly by raw material purity controlling [3] and sintering techniques optimization [4–7]. Although YAG transparent ceramics has been demonstrated as one good laser host material [8], the relatively lower thermal conductivity [9] is not benefit for high power lasers compared with Y2O3 transparent ceramics. Normally, the high melting point and phase transition [10] lead to much more difficulties for high-quality and large-scale Y2O3 single crystal growth. But transparent ceramics of Y2O3 [4–7] could be fabricated with large-scale at a relatively lower temperature by adding sintering aids. Yb3+ ion owns merits of the high quantum efficiency, the relatively long ⇑ Corresponding authors at: Temasek Laboratories of Nanyang Technological University, Singapore 639798, Singapore. E-mail addresses: [email protected] (K. Ning), [email protected] (D.Y. Tang).
fluorescence lifetime, the wide absorption and emission band, and is demonstrated as a suitable active ion for high efficient diode-pump solid-state lasers (DPSSLs) [11–13]. So Yb3+ doped Y2O3 (Yb3+: Y2O3) transparent ceramics for laser application have attracted many researchers’ focus. In this paper, highly transparent Yb3+: Y2O3 ceramics was fabricated by a solid-state reaction method using ZrO2 and La2O3 as additives. Characteristics of the prepared powders and the sintered ceramics samples were investigated. 2. Experimental 2.1. Powder preparation and ceramics sintering The transparent 5 at.% Yb3+: Y2O3 ceramics samples were sintered by using a solid-state reaction method in a high temperature vacuum furnace. The commercially available high-purity powders of Y2O3 (99.999% purity, Jiahua Advanced Material Resources Co., Ltd, China) and Yb2O3 (99.995% purity, Jiahua Advanced Material Resources Co., Ltd, China) were employed as starting materials. The sintering additives were ZrO2 and La2O3 with the corresponding ratios of 1.6 wt.% and 0.7 wt.%. The whole well weighted raw powders by adding 0.5 ml PEI as disperser in ethanol were fully mixed on a planetary ball milling machine for 15 h. The fully dispersed powders were then sent to a hot air dryer at 60 °C for 12 h to evaporate the ethanol. After sieving, the powders were calcined at 800 °C for 3 h. The green bodies were obtained in an
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Please cite this article in press as: K. Ning et al., Opt. Mater. (2015), http://dx.doi.org/10.1016/j.optmat.2015.03.032
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Structure Database (ICSD) without any additional peaks. Only single yttria phase with cubic structure (S.G. Ia-3, No. 206, Z = 16) was obtained. The density of the as-sintered Yb3+: Y2O3 ceramics was estimated based on the Archimedes’ principle and was about 99.6% of the theoretical density.
Fig. 1. Picture of the as-sintered Yb3+: Y2O3 transparent ceramics after double-face polishing.
ironed die set and then cold isostatic pressure (CIP) of 200 MPa was imposed for 4 min. The green bodies were sintered at the temperature of 1820 °C for 30 h with a high vacuum below 103 Pa. The sintered tablet samples were annealed at the temperature of 1400 °C for 15 h in air atmosphere to remove the color centers. After polishing two faces of the annealed tablets, transparent Yb3+: Y2O3 ceramics wafers with thickness of 1 mm were obtained and one of the wafers is shown in Fig. 1.
Fig. 2. Morphology of the raw powders after ball milling.
2.2. Characterization The morphology of the well mixed raw powders after ball milling was detected on a FESEM machine (JSM-6340F, JEOL Co. Ltd, Japan) after ultrasonic bath process. The phase of the sintered ceramics was identified from X-ray diffraction (XRD) patterns (Cu Ka radiation, XRD-6000, Shimadzu Co. Ltd, Japan). The thermal etching face, indentation and indentation crack were observed using a SEM machine (JSM-5410, JEOL Co. Ltd, Japan). The transmittance spectrum was recorded on a UV–VIS–NIR spectrophotometer (Cary 5000, Varian Co. Ltd, US) in the wavelength range of 200–2000 nm. The Vickers hardness and fracture toughness were concluded using a digital hardness tester (FV-700e, FUTURE-TECH Co. Ltd, Japan) and these tests were performed from 15 indentations. All the above measurements were carried out at room temperature. 3. Results and discussion 3.1. Powder characterization and phase identification The morphology of the well mixed raw powders after ball milling was investigated and is illustrated in Fig. 2. The original Y2O3 raw powders without ball milling from the same company have ever been measured using the SEM machine and was reported in Ref. [14]. The original Y2O3 powders present intense agglomeration with average particle size of about 5 lm [14]. But after ball milling process, the well mixed raw powders are easily broken down to individual spheres with very soft agglomeration. The diameters of individual spheres were estimated to be 50–150 nm from the FESEM photomicrograph in Fig. 2. This soft agglomerated raw powers after ball milling are benefit for the flowability during the die-filling and the die-pressing to produce compact green bodies. The relative density of the compact green bodies after the effect of 200 MPa CIP was increased to about 55% of the theoretical density calculated from the geometric dimension. XRD patterns shown in Fig. 3 were used to identify the phase of as-sintered Yb3+: Y2O3 ceramics. The measured XRD patterns confirm with the standard PDF Card No. 86-1326 in Inorganic Crystal
Fig. 3. XRD patterns of the as-sintered Yb3+: Y2O3 transparent ceramics.
Fig. 4. SEM image of the polished and etched surface for the as-sintered Yb3+: Y2O3 transparent ceramics.
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3.2. Microstructure
3.3. Transmittance spectrum
The SEM image of the polished and etched surface for the assintered Yb3+: Y2O3 ceramics is displayed in Fig. 4. The grain boundary is apparent and narrow, and no observed pores are trapped inside of the grains. There are no extra large-sized grains in the as-sintered Yb3+: Y2O3 ceramics. The maximum and average grain sizes were estimated to be 19.14 and 9.11 lm from the statistical evaluation which were calculated from a SEM image with the magnification of 1000. The average grain size for our sintered Yb3+: Y2O3 ceramics is much smaller than that (48.5 lm) in the reported Tm3+: Y2O3 ceramics using the same additives at the temperature of 1800 °C [15] which achieved near our sintering temperature. The concentrations of ZrO2 additive in our sintered Yb3+: Y2O3 ceramics and the reported Tm3+: Y2O3 ceramics were almost the same. The main difference between them was the concentration of La2O3 additive. It is found that the concentration of La2O3 additive in our sintered Yb3+: Y2O3 ceramics is much lower, only about 1/6 of that by weight in the reported Tm3+: Y2O3 ceramics [15]. Thus, we include that the grain size in yttria ceramics could be suppressed by decreasing the amount of the La2O3 additive when the amount of ZrO2 additive is kept constant. These relatively smaller grain size controlling would enhance the densification and avoid micro-pores trapped into the sintered yttria ceramics. As a laser material, sintered yttria transparent ceramics with smaller grain size would improve the thermomechanical property and would be beneficial to laser oscillation.
Fig. 5 illustrates the transmittance spectrum of the as-sintered Yb3+: Y2O3 transparent ceramics. The absorption band between 820 and 1060 nm in the transmittance spectrum comes from the transition of Yb3+ ground state 2F7/2 to Yb3+ excited state 2F5/2. At the wavelength of 2000 nm, transmittance for the as-sintered Yb3+: Y2O3 ceramics is about 82.0%, which approaches 99% of the theoretical transmittance (theoretical transmittance was 82.8% at the wavelength of 2000 nm, which was calculated from formula of T = 100[1 ((n 1)/(n + 1))2]2, where n is the refractive index of Y2O3 single crystal [16]). So, the as-sintered Yb3+: Y2O3 ceramics is almost totally transparent in the near infrared (NIR) wavelength range. In the visible light range, transmittance at the wavelength of 600 nm is also as high as 75.7%, although which is relatively lower compared with that in the NIR light range. This is probably affected by the unobserved residual nanometer range micro-pores which activate relatively high scattering when light passes through the sintered ceramics. When the scatter center is smaller than the transmittance wavelength, the scatter intensity increases proportionally to k4 (k is the wavelength) from Rayleigh’s equation [17]. 3.4. Hardness and fracture toughness The Vickers hardness can be calculated from the following formula [18,19],
P HV ¼ k 2 d
ð1Þ
where P is the load applying to the indenter (here is 29.4 N), d is the diagonal length of the indention, k is the shape factor of the indenter (1.854 for a pyramid-shaped). The fracture toughness can be calculated with the following equation [18,19],
K IC ¼ 0:16ðE=HV Þ1=2 ðP=C 3=2 Þ
Fig. 5. Transmittance spectrum of the polished Yb3+: Y2O3 transparent ceramics.
ð2Þ
where E is the Young’s modulus (here is 179.8 GPa from Ref. [20]), C is indentation crack length. The SEM images of indentation and indentation crack for as-sintered Yb3+: Y2O3 transparent ceramics are illustrated in Fig. 6. The Vickers hardness and fracture toughness for the as-sintered Yb3+: Y2O3 ceramics were calculated to be 8.65 GPa and 0.95 MPa m1/2 respectively, and are listed in Table 1. In order to have a comparison, the other Y2O3 ceramics were also added in Table 1. Both hardness and fracture toughness of our sintered Yb3+: Y2O3 ceramics with ZrO2 and La2O3 as additives show the medium values which lie between the HIPed two step sintered
Fig. 6. SEM images of indentation (a) and indentation crack (b) for the as-sintered Yb3+: Y2O3 transparent ceramics.
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Table 1 Vickers hardness and fracture toughness for different Y2O3 ceramics. Ceramics 3+
Er : Y2O3 (Ref.[19]) Yb3+: Y2O3 (This work) Y2O3 (Ref.[22]) Er3+: Y2O3 (Ref.[19])
Sintering method
Additives
Hardness (GPa)
Fracture toughness (MPa m1/2)
Grain size (lm)
HIPed two step sintering Conventional sintering HIPed two step sintering Conventional sintering
None ZrO2 and La2O3 None None
9.09(0.41) 8.65(0.35) 7.67 7.23(0.35)
1.39(0.07) 0.95(0.05) – 0.81(0.07)
0.34 9.11 21.5 328
Er3+: Y2O3 and the conventional sintered Er3+: Y2O3 ceramics. Commonly, the ceramics hardness will increase by decreasing the grain size. The grain boundary plays a function of dislocation blocking [21] when indenter interaction is imposed on ceramics. In Fig. 6(b), the indentation crack of the as-sintered Yb3+: Y2O3 ceramics is not a straight line and the grain boundary blocking is clearly observed across the indentation crack (identified by the arrows). As a result, mechanical properties of the as-sintered Yb3+: Y2O3 ceramics are improved by additives of ZrO2 and La2O3 compared with Er3+: Y2O3 ceramics without additives from the same traditional sintering method. 4. Conclusions Highly transparent Yb3+: Y2O3 ceramics was fabricated by a solid-state reaction method at the temperature of 1820 °C using ZrO2 and La2O3 as additives. The raw powders after ball milling showed the soft agglomeration which enhanced the flowability for producing compact green bodies. The phase identification revealed that the as-sintered Yb3+: Y2O3 ceramics had a cubic structural yttria. The relative density of the sintered Yb3+: Y2O3 ceramics was achieved to 99.6% of the theoretical value. The maximum and the average grain size were 19.14 and 9.11 lm, respectively. The absorption band between 820 and 1060 nm was assigned as the transition from Yb3+ ground state 2F7/2 to Yb3+ excited state 2F5/2. The transmittance at the wavelength of 2000 nm was about 82.0%, which was 99% of the theoretical transmittance. In the visible wavelength range, the transmittance at the wavelength of 600 nm was also as high as 75.7%. The Vickers hardness and fracture toughness were calculated and compared with other fabricated Y2O3 ceramics. ZrO2 and La2O3 were useful
additives for highly transparent Yb3+: Y2O3 ceramics fabrication and mechanical properties improvement. References [1] A. Ikesue, Y.L. Aung, Nat. Photon. 2 (2008) 721. [2] Q. Lou, J. Zhou, Y. Qi, H. Cai, Laser Applications of Transparent Polycrystalline Ceramic: Advances in Ceramics, InTech, 2011. [3] R.L. Gentilman, M. Greenberg, T. Hartnett, US Patent 4,761,390, 1988. [4] K. Schmidt, US Patent 3,248,590, 1966. [5] F.J. Reid, W.H. Rhodes, US Patent 4,166,831, 1979. [6] R.C. Anderson, US Patent 3,545,987, 1970. [7] W.H. Rhodes, US Patent 4,115,134, 1978. [8] J. Sanghera, W. Kim, G. Villalobos, B. Shaw, C. Baker, J. Frantz, B. Sadowski, I. Aggarwal, Opt. Mater. 35 (2013) 693. [9] J. Lu, K. Takaichi, T. Uematsu, A. Shirakawa, M. Musha, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Jpn. J. Appl. Phys. 41 (2002) L1373. [10] J. Sanghera, S. Bayya, G. Villalobos, W. Kim, J. Frantz, B. Shaw, B. Sadowski, R. Miklos, C. Baker, M. Hunt, I. Aggarwal, F. Kung, D. Reicher, S. Peplinski, A. Ogloza, P. Langston, C. Lamar, P. Varmette, M. Dubinskiy, L. DeSandre, Opt. Mater. 33 (2011) 511. [11] W.F. Krupke, IEEE J. Sel. Top. Quant. Electron. 6 (2000) 1287. [12] S. Chénais, F. Druon, S. Forget, F. Balembois, P. Georges, Prog. Quant. Electron. 30 (2006) 89. [13] I. Matsushima, A. Tanabashi, Opt. Rev. 17 (2010) 294. [14] H. Yang, X. Qin, J. Zhang, S. Wang, J. Ma, L. Wang, Q. Zhang, J. Alloys Compd. 509 (2011) 5274. [15] Q. Yi, S. Zhou, H. Teng, H. Lin, X. Hou, T. Jia, J. Eur. Ceram. Soc. 32 (2012) 381. [16] Y. Nigara, Jpn. J. Appl. Phys. 7 (1968) 404. [17] A. Ikesue, K. Yoshida, T. Yamamoto, J. Am. Ceram. Soc. 80 (1997) 1517. [18] K. Serivalsatit, B. Kokuoz, B. Yazgan-Kokuoz, M. Kennedy, J. Ballatow, J. Am. Ceram. Soc. 93 (2010) 1320. [19] A.A. Kaminskii, M.S. Akchurin, R.V. Gainutdinov, K. Takaichi, A. Shirakava, H. Yagi, T. Yanagitani, K. Ueda, Crystallogr. Rep. 50 (2005) 869. [20] O. Yeheskel, O. Tevet, J. Am. Ceram. Soc. 82 (1999) 136. [21] I.J. McColm, Ceramic Hardness, Plenum Press, New York, 1990. [22] H. Eilers, J. Eur. Ceram. Soc. 27 (2007) 4711.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and characterization of highly transparent Yb3+: Y2O3 ceramics Kaijie Ning a,c,⇑, Jun Wang b,c, Dewei Luo b,c, Jie Ma b, Jian Zhang a,c, Zhi Li Dong a,c, Ling Bing Kong c, Ding Yuan Tang a,b,⇑ a b c
Temasek Laboratories of Nanyang Technological University, Singapore 639798, Singapore School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Highly transparent ceramics Yb3+: Y2O3 Solid-state reaction method Characterization
a b s t r a c t Highly transparent Yb3+ doped Y2O3 (Yb3+: Y2O3) ceramics was fabricated by a solid-state reaction method using ZrO2 and La2O3 as additives. The morphology of the prepared powder was investigated and the phase of the sintered Yb3+: Y2O3 ceramics sample was identified. The microstructure, transmittance spectrum, Vickers hardness and fracture toughness for the as-sintered Yb3+: Y2O3 ceramics were measured. The average grain size was about 9.11 lm and the transmittance at the wavelength of 2000 nm was about 82.0%, which was 99% of the theoretical value. ZrO2 and La2O3 were useful additives for highly transparent Yb3+: Y2O3 ceramics fabrication and mechanical properties improvement. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The polycrystalline transparent ceramics have more advantages than single crystals on laser application [1,2], such as low cost, composite structural design, easy manufacture, large-size fabrication, high dopant concentration and comparable thermal conductivity. Transparent ceramics of YAG and Y2O3 are the two kinds of mostly investigated laser materials and are the promising alternatives for supplanting the single crystals. Because of the cubic structure of these two materials, no bad effects of the inherent birefringence are imposed on light transmittance. Transparent ceramics of YAG and Y2O3 can be fabricated properly by raw material purity controlling [3] and sintering techniques optimization [4–7]. Although YAG transparent ceramics has been demonstrated as one good laser host material [8], the relatively lower thermal conductivity [9] is not benefit for high power lasers compared with Y2O3 transparent ceramics. Normally, the high melting point and phase transition [10] lead to much more difficulties for high-quality and large-scale Y2O3 single crystal growth. But transparent ceramics of Y2O3 [4–7] could be fabricated with large-scale at a relatively lower temperature by adding sintering aids. Yb3+ ion owns merits of the high quantum efficiency, the relatively long ⇑ Corresponding authors at: Temasek Laboratories of Nanyang Technological University, Singapore 639798, Singapore. E-mail addresses: [email protected] (K. Ning), [email protected] (D.Y. Tang).
fluorescence lifetime, the wide absorption and emission band, and is demonstrated as a suitable active ion for high efficient diode-pump solid-state lasers (DPSSLs) [11–13]. So Yb3+ doped Y2O3 (Yb3+: Y2O3) transparent ceramics for laser application have attracted many researchers’ focus. In this paper, highly transparent Yb3+: Y2O3 ceramics was fabricated by a solid-state reaction method using ZrO2 and La2O3 as additives. Characteristics of the prepared powders and the sintered ceramics samples were investigated. 2. Experimental 2.1. Powder preparation and ceramics sintering The transparent 5 at.% Yb3+: Y2O3 ceramics samples were sintered by using a solid-state reaction method in a high temperature vacuum furnace. The commercially available high-purity powders of Y2O3 (99.999% purity, Jiahua Advanced Material Resources Co., Ltd, China) and Yb2O3 (99.995% purity, Jiahua Advanced Material Resources Co., Ltd, China) were employed as starting materials. The sintering additives were ZrO2 and La2O3 with the corresponding ratios of 1.6 wt.% and 0.7 wt.%. The whole well weighted raw powders by adding 0.5 ml PEI as disperser in ethanol were fully mixed on a planetary ball milling machine for 15 h. The fully dispersed powders were then sent to a hot air dryer at 60 °C for 12 h to evaporate the ethanol. After sieving, the powders were calcined at 800 °C for 3 h. The green bodies were obtained in an
http://dx.doi.org/10.1016/j.optmat.2015.03.032 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Ning et al., Opt. Mater. (2015), http://dx.doi.org/10.1016/j.optmat.2015.03.032
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Structure Database (ICSD) without any additional peaks. Only single yttria phase with cubic structure (S.G. Ia-3, No. 206, Z = 16) was obtained. The density of the as-sintered Yb3+: Y2O3 ceramics was estimated based on the Archimedes’ principle and was about 99.6% of the theoretical density.
Fig. 1. Picture of the as-sintered Yb3+: Y2O3 transparent ceramics after double-face polishing.
ironed die set and then cold isostatic pressure (CIP) of 200 MPa was imposed for 4 min. The green bodies were sintered at the temperature of 1820 °C for 30 h with a high vacuum below 103 Pa. The sintered tablet samples were annealed at the temperature of 1400 °C for 15 h in air atmosphere to remove the color centers. After polishing two faces of the annealed tablets, transparent Yb3+: Y2O3 ceramics wafers with thickness of 1 mm were obtained and one of the wafers is shown in Fig. 1.
Fig. 2. Morphology of the raw powders after ball milling.
2.2. Characterization The morphology of the well mixed raw powders after ball milling was detected on a FESEM machine (JSM-6340F, JEOL Co. Ltd, Japan) after ultrasonic bath process. The phase of the sintered ceramics was identified from X-ray diffraction (XRD) patterns (Cu Ka radiation, XRD-6000, Shimadzu Co. Ltd, Japan). The thermal etching face, indentation and indentation crack were observed using a SEM machine (JSM-5410, JEOL Co. Ltd, Japan). The transmittance spectrum was recorded on a UV–VIS–NIR spectrophotometer (Cary 5000, Varian Co. Ltd, US) in the wavelength range of 200–2000 nm. The Vickers hardness and fracture toughness were concluded using a digital hardness tester (FV-700e, FUTURE-TECH Co. Ltd, Japan) and these tests were performed from 15 indentations. All the above measurements were carried out at room temperature. 3. Results and discussion 3.1. Powder characterization and phase identification The morphology of the well mixed raw powders after ball milling was investigated and is illustrated in Fig. 2. The original Y2O3 raw powders without ball milling from the same company have ever been measured using the SEM machine and was reported in Ref. [14]. The original Y2O3 powders present intense agglomeration with average particle size of about 5 lm [14]. But after ball milling process, the well mixed raw powders are easily broken down to individual spheres with very soft agglomeration. The diameters of individual spheres were estimated to be 50–150 nm from the FESEM photomicrograph in Fig. 2. This soft agglomerated raw powers after ball milling are benefit for the flowability during the die-filling and the die-pressing to produce compact green bodies. The relative density of the compact green bodies after the effect of 200 MPa CIP was increased to about 55% of the theoretical density calculated from the geometric dimension. XRD patterns shown in Fig. 3 were used to identify the phase of as-sintered Yb3+: Y2O3 ceramics. The measured XRD patterns confirm with the standard PDF Card No. 86-1326 in Inorganic Crystal
Fig. 3. XRD patterns of the as-sintered Yb3+: Y2O3 transparent ceramics.
Fig. 4. SEM image of the polished and etched surface for the as-sintered Yb3+: Y2O3 transparent ceramics.
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3.2. Microstructure
3.3. Transmittance spectrum
The SEM image of the polished and etched surface for the assintered Yb3+: Y2O3 ceramics is displayed in Fig. 4. The grain boundary is apparent and narrow, and no observed pores are trapped inside of the grains. There are no extra large-sized grains in the as-sintered Yb3+: Y2O3 ceramics. The maximum and average grain sizes were estimated to be 19.14 and 9.11 lm from the statistical evaluation which were calculated from a SEM image with the magnification of 1000. The average grain size for our sintered Yb3+: Y2O3 ceramics is much smaller than that (48.5 lm) in the reported Tm3+: Y2O3 ceramics using the same additives at the temperature of 1800 °C [15] which achieved near our sintering temperature. The concentrations of ZrO2 additive in our sintered Yb3+: Y2O3 ceramics and the reported Tm3+: Y2O3 ceramics were almost the same. The main difference between them was the concentration of La2O3 additive. It is found that the concentration of La2O3 additive in our sintered Yb3+: Y2O3 ceramics is much lower, only about 1/6 of that by weight in the reported Tm3+: Y2O3 ceramics [15]. Thus, we include that the grain size in yttria ceramics could be suppressed by decreasing the amount of the La2O3 additive when the amount of ZrO2 additive is kept constant. These relatively smaller grain size controlling would enhance the densification and avoid micro-pores trapped into the sintered yttria ceramics. As a laser material, sintered yttria transparent ceramics with smaller grain size would improve the thermomechanical property and would be beneficial to laser oscillation.
Fig. 5 illustrates the transmittance spectrum of the as-sintered Yb3+: Y2O3 transparent ceramics. The absorption band between 820 and 1060 nm in the transmittance spectrum comes from the transition of Yb3+ ground state 2F7/2 to Yb3+ excited state 2F5/2. At the wavelength of 2000 nm, transmittance for the as-sintered Yb3+: Y2O3 ceramics is about 82.0%, which approaches 99% of the theoretical transmittance (theoretical transmittance was 82.8% at the wavelength of 2000 nm, which was calculated from formula of T = 100[1 ((n 1)/(n + 1))2]2, where n is the refractive index of Y2O3 single crystal [16]). So, the as-sintered Yb3+: Y2O3 ceramics is almost totally transparent in the near infrared (NIR) wavelength range. In the visible light range, transmittance at the wavelength of 600 nm is also as high as 75.7%, although which is relatively lower compared with that in the NIR light range. This is probably affected by the unobserved residual nanometer range micro-pores which activate relatively high scattering when light passes through the sintered ceramics. When the scatter center is smaller than the transmittance wavelength, the scatter intensity increases proportionally to k4 (k is the wavelength) from Rayleigh’s equation [17]. 3.4. Hardness and fracture toughness The Vickers hardness can be calculated from the following formula [18,19],
P HV ¼ k 2 d
ð1Þ
where P is the load applying to the indenter (here is 29.4 N), d is the diagonal length of the indention, k is the shape factor of the indenter (1.854 for a pyramid-shaped). The fracture toughness can be calculated with the following equation [18,19],
K IC ¼ 0:16ðE=HV Þ1=2 ðP=C 3=2 Þ
Fig. 5. Transmittance spectrum of the polished Yb3+: Y2O3 transparent ceramics.
ð2Þ
where E is the Young’s modulus (here is 179.8 GPa from Ref. [20]), C is indentation crack length. The SEM images of indentation and indentation crack for as-sintered Yb3+: Y2O3 transparent ceramics are illustrated in Fig. 6. The Vickers hardness and fracture toughness for the as-sintered Yb3+: Y2O3 ceramics were calculated to be 8.65 GPa and 0.95 MPa m1/2 respectively, and are listed in Table 1. In order to have a comparison, the other Y2O3 ceramics were also added in Table 1. Both hardness and fracture toughness of our sintered Yb3+: Y2O3 ceramics with ZrO2 and La2O3 as additives show the medium values which lie between the HIPed two step sintered
Fig. 6. SEM images of indentation (a) and indentation crack (b) for the as-sintered Yb3+: Y2O3 transparent ceramics.
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Table 1 Vickers hardness and fracture toughness for different Y2O3 ceramics. Ceramics 3+
Er : Y2O3 (Ref.[19]) Yb3+: Y2O3 (This work) Y2O3 (Ref.[22]) Er3+: Y2O3 (Ref.[19])
Sintering method
Additives
Hardness (GPa)
Fracture toughness (MPa m1/2)
Grain size (lm)
HIPed two step sintering Conventional sintering HIPed two step sintering Conventional sintering
None ZrO2 and La2O3 None None
9.09(0.41) 8.65(0.35) 7.67 7.23(0.35)
1.39(0.07) 0.95(0.05) – 0.81(0.07)
0.34 9.11 21.5 328
Er3+: Y2O3 and the conventional sintered Er3+: Y2O3 ceramics. Commonly, the ceramics hardness will increase by decreasing the grain size. The grain boundary plays a function of dislocation blocking [21] when indenter interaction is imposed on ceramics. In Fig. 6(b), the indentation crack of the as-sintered Yb3+: Y2O3 ceramics is not a straight line and the grain boundary blocking is clearly observed across the indentation crack (identified by the arrows). As a result, mechanical properties of the as-sintered Yb3+: Y2O3 ceramics are improved by additives of ZrO2 and La2O3 compared with Er3+: Y2O3 ceramics without additives from the same traditional sintering method. 4. Conclusions Highly transparent Yb3+: Y2O3 ceramics was fabricated by a solid-state reaction method at the temperature of 1820 °C using ZrO2 and La2O3 as additives. The raw powders after ball milling showed the soft agglomeration which enhanced the flowability for producing compact green bodies. The phase identification revealed that the as-sintered Yb3+: Y2O3 ceramics had a cubic structural yttria. The relative density of the sintered Yb3+: Y2O3 ceramics was achieved to 99.6% of the theoretical value. The maximum and the average grain size were 19.14 and 9.11 lm, respectively. The absorption band between 820 and 1060 nm was assigned as the transition from Yb3+ ground state 2F7/2 to Yb3+ excited state 2F5/2. The transmittance at the wavelength of 2000 nm was about 82.0%, which was 99% of the theoretical transmittance. In the visible wavelength range, the transmittance at the wavelength of 600 nm was also as high as 75.7%. The Vickers hardness and fracture toughness were calculated and compared with other fabricated Y2O3 ceramics. ZrO2 and La2O3 were useful
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Please cite this article in press as: K. Ning et al., Opt. Mater. (2015), http://dx.doi.org/10.1016/j.optmat.2015.03.032