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Fabrication, microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders
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Fabrication, microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders Qiang Liua, Jinbang Lia,b, Jiawei Daib,c, Zewang Hub,c, Cong Chena,b, Xiaopu Chenb,c, ⁎ Yagang Fengb,c, Jiang Lib, a
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China c University of Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb:Lu2O3 Transparent ceramics Fabrication Spectroscopic properties
Ytterbium doped lutetium oxide (Yb:Lu2O3) transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) of the powders synthesized by the co-precipitation method. The effects of calcination temperature on the composition and morphology of the powders were investigated. Fine and well dispersed 5 at% Yb:Lu2O3 powders with the mean particle size of 67 nm were obtained when calcined at 1100 °C for 4 h. Using the synthesized powders as starting material, we fabricated 5 at% Yb:Lu2O3 ceramics by presintering at different temperatures combined with HIP post-treatment. The influence of pre-sintering temperature on the densities, microstructures and optical quality of the 5 at% Yb:Lu2O3 ceramics was studied. The ceramic sample pre-sintered at 1500 °C for 2 h with HIP post-treating at 1700 °C for 8 h has the highest in-line transmittance of 78.2% at 1100 nm and the average grain size of 2.6 µm. In addition, the absorption and emission cross sections of the 5 at% Yb:Lu2O3 ceramics were also calculated.
1. Introduction Solid-state laser has been successfully used in industrial, military, medical and other fields due to its small size, high efficiency, and excellent performance [1]. In the past decades, rare-earth-doped yttrium aluminum garnet (YAG) as the host material for high-power solid-state lasers has been rapidly developed [2,3]. The sesquioxides such as Sc2O3, Y2O3 and Lu2O3 possess higher thermal conductivity than YAG and have been considered to have significant potential as host materials for high-power and high efficiency solid-state lasers [4–6]. Among them, Yb-doped Lu2O3 stands out as the best host material especially when the dopant concentration of Yb3+ ion is high [7]. Because of the similar ionic radii and bonding forces of Lu3+ and Yb3+ ion, the Yb3+ ion can easily substitute for Lu3+ ion. In addition, the overall thermal conductivity is almost not affected by the doping concentration, which is due to the negligible scattering of the propagating phonons. Furthermore, Yb3+ ion has high quantum efficiency and simple energy manifolds (the 2F7/2 ground state and the 2F5/2 excited state), which prevent the excited state absorption and upper level conversion [8–10]. The melting point of Lu2O3 is about 2450 °C, which is higher than the melting temperature of iridium crucible [11–13]. Therefore, it is quite difficult to fabricate Lu2O3 single crystals with large size and high
⁎
optical quality by the conventional melt growth process [14]. The ceramic processing technology provides an alternative way to fabricate transparent Lu2O3 materials since ceramic processing requires 25–30% lower temperature than the melting temperature. Compared with single crystals, transparent ceramics are advantageous in many ways including higher doping concentration, possibility of larger size, and more function design freedom [15]. Powders with high purity, good dispersity and uniform size are the key to obtain high optical quality ceramics [16]. However, commercial Yb3+ doped Lu2O3 powders are not common available. Lots of methods have been used to synthesize rare-earth-doped Lu2O3 powders, such as, sol-gel combustion [17], hydrothermal synthesis [18] and co-precipitation methods [7,12,19]. Among these methods, co-precipitation route has been proven to be an effective route to synthesize rare-earthdoped Lu2O3 powders with homogeneous composition and good dispersion. Calcination temperature has a big influence on the purity and agglomeration of the powders [20,21]. Optimization of calcination temperature to obtain highly pure and less-agglomerated Yb:Lu2O3 powders is necessary. In addition, vacuum pre-sintering followed by HIP post-treatment has been employed to achieve Nd: YAG [22] and Y2O3 [23] ceramics with high optical quality. The pre-sintering temperature has a big influence on the optical quality and microstructure of
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.ceramint.2018.03.238 Received 7 February 2018; Received in revised form 16 March 2018; Accepted 26 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Q., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.238
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the final ceramics [24]. As a result, optimization of pre-sintering temperature to obtain highly transparent Yb:Lu2O3 ceramics also needs to be systematically investigated. In this work, 5 at% Yb:Lu2O3 transparent ceramics were successfully fabricated by vacuum sintering combined with hot isostatic pressing from carbonate co-precipitated nano-powders. The influence of calcination temperature on the phase composition, particle size and morphology of the powders was systematically investigated. In addition, the influence of pre-sintering temperature on the densities, microstructures and optical quality of the ceramics was also studied. Finally, the absorption and emission cross-sections of the 5 at% Yb:Lu2O3 ceramics were also calculated.
2. Experimental procedure The rare earth raw materials used in the experiment were Lu2O3 (99.999%, Zhongkai New Materials Co., Ltd., Jining, China) and Yb2O3 (99.99%, Alfa Aesar, USA). Lu(NO3)3 solution and Yb(NO3)3 solution were prepared by dissolving Lu2O3 and Yb2O3 in nitric acid at 80 °C, respectively. Then the solutions were filtered to remove any undissolved particles and impurities with 0.3 µm filter paper. Lu3+ and Yb3+ concentrations of the nitrate solutions were assayed by chemical analysis. Lu(NO3)3 solution and Yb(NO3)3 solution were mixed together in stoichiometric proportions of Yb.05Lu.95O3 (5 at% Yb:Lu2O3). (NH4)2SO4 (99.0%, Sinopharm Chemical Reagent Co., Ltd, china) as the dispersant was added to the mixed metal ion solution. The molar ratio of (NH4)2SO4 to metal ion was 1:1. Finally the solution was diluted with deionized water, and the concentration of Lu3+ was set to 0.2 mol/L. Precipitant solution was obtained by dissolving Ammonium hydrogen carbonate (AHC) (Aladdin Industrial Corporation, China) in deionized water with a concentration of 1 M. The Yb:Lu2O3 precursor was prepared by adding 360 mL AHC solution at a speed of 3 mL/min into 500 mL mixed metal solution under stirring. After aging for 3 h, the resulting precipitate was washed for three times with deionized water and alcohol, respectively, and then dried at 70 °C for 48 h in an oven. Then, the precipitate was sieved through a 200-mesh screen and calcined at 600–1200 °C for 4 h in air. The nano-powders calcined at 1100 °C were uniaxially pressed into pellets of 18 mm in diameter at 25 MPa and then cold isostatically pressed under 250 MPa. For ceramic fabrication, the green pellets were pre-sintered at 1450–1750 °C for 2 h in a vacuum furnace (10−5 Pa) and subsequently hot isostatic pressed at 1700 °C in an Ar pressure of 100 MPa for 8 h. The sintered ceramics were annealed in air at 1350 °C for 10 h to remove oxygen vacancies. At last, the samples were double-side polished with 1 mm thick for the transmittance test. The phase compositions of the powders were determined by X-ray diffraction (XRD, Model D/max2200 PC, Rigaku, Japan) in the range of 2 θ = 10–80° using nickel-filtered Cu-Kα radiation. Fourier transform infrared spectrum (FTIR) was performed on an infrared spectrometer (FTIR, Bruker VERTEX 70 spectrophotometer, Ettlingen, Germany) using the standard KBr method in the range of 4000 cm−1–400 cm−1. The specific surface area (SBET) of the precursor and calcined powders was performed by Norcross ASAP 2010 micromeritics with N2 as the absorption gas at 77 K. The density of the ceramics was measured by the Archimedes method using deionized water as the immersion medium. The microstructure of the polished samples (thermally etched at 1350 °C for 3 h) was observed by a field emission scanning electron microscopy (FESEM, SU8220, Hitachi, Japan). The average grain sizes of sintered samples were measured by the linear intercept method from SEM images and the average intercept length was multiplied by 1.56 [25]. The in-line transmittance of the samples was measured by a UV–VIS–NIR spectrophotometer (Model Carry-5000, Varian, USA). The pore size of the HIPed ceramics was measured by the optical microscope (BX51, Olympus, Japan).
Fig. 1. FTIR spectra of the precursor and the 5at%Yb:Lu2O3 powders calcined at different temperatures for 4 h.
3. Results and discussion Fig. 1 shows the FT-IR spectra of the precursor and the 5 at% Yb:Lu2O3 powders calcined at different temperatures for 4 h. The wide absorption bands centered at 3450 cm−1 are related to O-H stretching of molecular water. The bands at 844 cm−1 are assigned to the deformation vibration of C-O in CO32-, while the bands at 1527 and 1405 cm−1 are corresponding to the asymmetric stretch of the C-O bond in CO32-. The bands at around 1100 cm−1 are attributed to SO42-. The bands at 2360 cm−1 are caused by the asymmetric stretch of CO2 absorbed in air. The FT-IR spectrum of the precursor indicates that the precursor contains functional groups such as OH−1, CO32- and H2O. With the increase of calcination temperature, intensities of the bands of OH−1, CO32-, SO42- and H2O become weaker. Peaks of CO32- and SO42disappear when the calcination temperature is higher than 1000 °C. When the calcination temperature is 600 °C, new band at 575 cm−1 related to the stretching of Lu–O bond appears, which results from the crystallization of Yb:Lu2O3 from the precursor [12]. This result is consistent with the results of XRD analysis. Fig. 2 shows the XRD patterns of the 5 at% Yb:Lu2O3 precursor and the powders calcined at different temperatures for 4 h. It can be seen
Fig. 2. XRD patterns of the precursor and the 5at%Yb:Lu2O3 powders calcined at different temperatures for 4 h. 2
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ytterbium ion can substitute for lutetium ion and they form a solid solution. With the increase of calcination temperature, the diffraction peaks of Lu2O3 phase become much sharper, indicating the improvement of crystallization. The mean crystallite sizes (DXRD) of the 5 at% Yb:Lu2O3 powders can be calculated by the X-ray line broadening technique performed on the (2 2 2) diffraction of the Yb:Lu2O3 from Scherrer equation:
Table 1 The specific surface areas, average particle sizes (DBET), mean crystallite sizes (DXRD) and mean primary particle sizes (DSEM) of the precursor and the powders calcined at different temperatures for 4 h. Calcination Temperature (°C)
Specific surface area (m2/g)
DBET (nm)
DXRD (nm)
Precursor 600 °C 800 °C 1000 °C 1100 °C 1200 °C
38.9 33.4 23.6 12.2 7.6 7.3
20 27 52 84 87
12 14 22 43 52
DSEM (nm)
D XRD = 0.89 λ/(β⋅ cos θ)
(1)
where β is the full width at half-maximum (FWHM) of a diffraction peak at Bragg angel θ and λ is the wavelength of Cu Kα radiation. The mean crystallite sizes of 5 at% Yb:Lu2O3 powders calculated from XRD peaks are shown in Table 1. It can be seen that the average crystallite size of the powders increases from 12 to 52 nm when the calcination temperature increases from 600 to 1200 °C. Fig. 3 displays the SEM micrographs of the precursor and the calcined powders. The precursor is a flake structure. The powders calcined at 600 and 800 °C show analogous morphology compared with that of the precursor. For the powders calcined at 1000 °C, the flake structure
41 67 83
that the precursor exhibits low crystallinity and is almost amorphous. After being calcined at 600 °C and above, all the characteristic diffraction peaks of the powders are corresponding to the cubic crystal structure of Lu2O3 (JCPDS 43–1021). No secondary phases are observed in the powders calcined at different temperatures, indicating that
Fig. 3. SEM micrographs of the (a) precursor and the 5at%Yb:Lu2O3 powders calcined for 4 h at (b) 600 °C, (c) 800 °C, (d) 1000 °C, (e) 1100 °C and (f) 1200 °C. 3
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began to decompose into small particles with clear edge. The average particle size of the powders increases when calcined at 1100 °C and the flake structure almost disappears. However, as the calcination temperature increases to 1200 °C, the grain grows up apparently and the agglomeration is aggravated. The mean primary particle size measured by the linear intercept method from SEM images is denoted as DSEM. The DSEM of the powders calcined at 1000, 1100 and 1200 °C are 41, 67 and 83 nm, respectively. The specific surface areas of the precursor and the powders are also listed in Table 1. It can be seen that the specific surface area of the 5 at% Yb:Lu2O3 powders reduces distinctly from 33.4 to 7.3 m2/g as the calcination temperature increases from 600° to 1200°C. The average particle size (DBET) of the 5 at% Yb:Lu2O3 powders is calculated from the following formula:
Fig. 5. Photograph of the 5at%Yb:Lu2O3 ceramics (1.0 mm thickness) before and after HIP post-treatment with different vacuum pre-sintering temperatures.
where ρ (9.40 g/cm3) is the theoretical density of 5 at% Yb:Lu2O3 nanopowders calculated from the lattice parameters of the 5 at% Yb:Lu2O3 nano-powders. SBET is the specific surface area determined by BET measurement. The DBET increases from 20 to 87 nm when the calcination temperature increases from 600 to 1200 °C, and the DBET is larger than DXRD, indicating that the particles are polycrystalline. In our previous work [26], the calcination temperature has to be at least above 1000 °C in order to completely remove the carbonate and sulfate from the Yb:Sc2O3 precursor. If the carbonate and sulfate cannot be removed, the residual carbonate and sulfate will form contamination in the sintering process of ceramics, which will degrade the optical quality of the ceramics. However, when the calcination temperature is as high as1200°C, the grain grows obviously accompanied with hard agglomeration and will reduce the sintering activity of the Yb:Sc2O3 powder. So in this work, we chose the powders calcined at 1100 °C to fabricate Yb:Lu2O3 transparent ceramics. Fig. 4 shows the relative densities of the 5 at% Yb:Lu2O3 ceramics before and after HIP post-treatment. The relative density of the presintered ceramic increases from 92.7% to 98.6% with the sintering temperature increasing from 1450 to 1750 °C. After the HIP posttreatment, the relative densities of all the samples are increased. For the sample pre-sintered at 1450 °C, the relative density only increases to 96.0% after HIP post-treatment. It is because that the sample pre-sintered at 1450 °C still has many open-pores (as can be seen in Fig. 6a), which leads to the infiltration of argon and eliminates the driving force for densification during HIP treatment. When the pre-sintering temperature is between 1500 and 1650 °C, the relative densities of the ceramic samples after HIP post-treatment exceed 99.8%, indicating that the residual pores in the pre-sintered Yb:Lu2O3 ceramics can be effectively removed. However, further increase of pre-sintering temperature
leads to the decrease of the relative density of the HIPed ceramic sample. It may be caused by the intragranular pores remained in the ceramics, which is difficult to be eliminated even by the HIP posttreatment. Fig. 5 shows the photograph of the 5 at% Yb:Lu2O3 ceramics vacuum sintered at different temperatures for 2 h and then HIP posttreated at 1700 °C for 8 h. The samples pre-sintered at different temperatures are opaque. The reason for the poor sintering activity of the Yb:Lu2O3 powders under vacuum sintering is due to the relatively hard agglomeration of the synthesized powders. After the HIP post-treatment, the samples pre-sintered at the temperature range from 1500 to 1750 °C become transparent and the letters under the ceramics can be seen, but the sample pre-sintered at 1450 °C is still opaque. It indicates that the pre-sintered temperature has an important effect on the transparency of the ceramic. Fig. 6 shows the in-line transmittance of the 5 at% Yb:Lu2O3 samples after HIP post-treatment. The sample pre-sintered at 1450 °C is opaque after HIP post-treatment due to the insufficient densification. The sample pre-sintered at 1500 °C has the highest transmittance of 78.2% at the wavelength of 1100 nm. When the pre-sintering temperature is higher than 1500 °C, the transmittance of the samples gradually decreases. It is because that the increased grain size is not conducive to the removal of pores during the HIP process. The results are also consistent with those reported in the literature [27]. Furthermore, when the pre-sintering temperature is high, the grain growth rate is fast and it will form intragranular pores, which are difficult to be eliminated by the HIP post-treatment, as can be confirmed by Fig. 7, Fig. 8 and Fig. 9. The decrease of optical transmittance at shorter wavelength is mainly caused by the scattering of remnant pores. The transparency of the Yb:Lu2O3 ceramics could be further enhanced by optimizing the fabrication process, for instance, to improve the dispersity and morphology of the raw powders, to select a better shaping method, or to
Fig. 4. Relative density of the pre-sintered and post-HIPed 5at%Yb:Lu2O3 ceramics with different vacuum pre-sintering temperatures.
Fig. 6. In-line transmittance of the post-HIPed 5at%Yb:Lu2O3 ceramics (1.0 mm thickness) pre-sintered at different temperatures.
DBET = 6/(ρ⋅SBET)
(2)
4
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Fig. 7. SEM images of the thermally etched surfaces of the 5at%Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h (a) 1450 °C; (b) 1500 °C; (c) 1550 °C; (d) 1600 °C;(e) 1650 °C; (f) 1700 °C; (g)1750 °C.
5
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Fig. 8. SEM images of the thermally etched surfaces of the 5at%Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h and HIP post-treated at 1700 °C for 8 h (a) 1450 °C; (b) 1500 °C; (c) 1550 °C; (d) 1600 °C;(e) 1650 °C; (f) 1700 °C; (g)1750 °C.
6
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Fig. 10. Absorption cross section of the 5at%Yb:Lu2O3 ceramics pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C. Fig. 9. The average grain sizes of the pre-sintered and post-HIPed 5at% Yb:Lu2O3 ceramics with different vacuum pre-sintering temperatures.
post-treated at 1700 °C for 8 h. After the HIP post-treatment, the sample pre-sintered at 1450 °C still has many open pores. Open pores formed during vacuum pre-sintering will lead to the infiltration of argon and eliminate the driving force for densification during HIP treatment. When the pre-sintering temperature is between 1500 and 1750 °C, the residual pores at the grain boundaries are almost completely removed and the ceramics become transparent (as shown in Fig. 5). In the HIPed ceramics pre-sintered at 1700 and 1750 °C, intragranular pores formed during vacuum pre-sintering still exist because intragranular pores are hard to eliminate by HIP post-treatment. Only bulk diffusion can be served for the elimination of intragranular pores. This diffusion style is much slower than grain boundary diffusion. Therefore, intragranular pores are virtually frozen in place and hardly to be removed in limited time, even under the high temperature and high pressure during HIP process. It can be seen that after the HIP post-treatment, the average grain size of all the ceramics increases. As shown in Fig. 9, the grain sizes of all ceramics increase from 2.1 to 8.2 µm with the pre-sintering temperatures increasing from 1450 to 1750°C. The average grain size of HIPed Yb:Lu2O3 ceramics pre-sintered at 1500 °C is only 2.6 µm, which endows the ceramic with the possibility to have good mechanical properties. Fig. 10 shows the absorption cross section of the 5 at% Yb:Lu2O3 ceramic pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C for 8 h. There are four main strong absorption peaks at 903, 947, 975 and 1040 nm corresponding to 2F7/2-2F5/2 transitions of Yb3+. The absorption cross-section (σabs) is calculated by the following equations [29].
optimize the HIP post-treatment schedules, etc. Fig. 7 shows the thermally etched surfaces of the 5 at% Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h. It can be seen that the pre-sintering temperature greatly influences the type of the pores and the grain sizes. For the sample pre-sintered at 1450 °C, many open-pores as well as some closed pores can be seen, which lead to a relatively lower density. When the pre-sintering temperature increases to 1500 °C and above, almost no open-pores can be observed. However, when the sintering temperature increases to 1700 and 1750 °C, some intragranular pores appear. It is because that the grain growth rate exceeds the removal rate of the pores at high sintering temperatures [28]. With the increase of pre-sintering temperature, the porosity decreases continuously, and this is consistent with the relative density shown in Fig. 4. The average grain sizes of the ceramics measured by the linear intercept method from SEM micrographs are given in Fig. 9. It can be seen that, at the temperature range of 1450–1600 °C, the grain size increases slowly from 0.6 to 1.3 µm. However, when the sintering temperature is higher than 1650 °C, the grain growth rate of the sample is accelerated. The grain size of the samples increases from 1.9 to 3.6 µm when the pre-sintering temperature increases from 1650 to 1700°C, and reaches 6.2 µm at 1750 °C. Hot isostatic pressing can provide high driving force for densification and be used to eliminate intergranular pores. During the HIP treatment, the applied compressive stress diffuses onto the pores in the grain boundaries, where the vacancy concentration can be decreased and the pores will shrink and move due to plastic deformation of surrounding grains. Meanwhile, the plastic deformation is driven by the grain boundary sliding and the smaller grain means more active for grain boundary sliding [24]. In order to effectively eliminate the pores during HIP treatment, the samples with closed pores while the pores exist at the grain boundaries are in favor. Furthermore, the grain size should be as small as possible to maintain high grain boundary sliding. Based on these principles, the ceramics pre-sintered between 1500 and 1550 °C have the more preferred microstructures to be used for HIP treatment. The porosity of the HIPed ceramics can be calculated from the relative density of the 5 at% Yb:Lu2O3 ceramics pre-vacuum sintered at 1500, 1550, 1600, 1650, 1700 and 1750 °C, whose values are about 0.1%, 0.1%, 0.1%, 0.2%, 0.4% and 0.7%. The average pore sizes of the HIPed samples pre-vacuum sintered at 1500, 1550, 1600 °C are all about 0.50 µm. And they increase from 0.54 µm to 0.55 and 0.62 µm with increasing the pre-vacuum sintering temperatures from 1650 °C to 1700 and 1750 °C. Fig. 8 shows the micrograph of the 5 at% Yb:Lu2O3 ceramics HIP
1 − n ⎞2 R=⎛ ⎝1 + n ⎠ T=
(1 − R)2 exp(−αb) 1 − R2 exp(−2αb)
σabs =
α N
(3)
(4) (5)
where T is the transmittance of the 5 at% Yb:Lu2O3 ceramic, α and b are the absorption coefficient and the thickness of the sample, N is the Yb3+ concentration per unit volume. The refractive index at different wavelengths of Lu2O3 can be estimated from the following formula reported by Kaminskii et al. [30].
n2 = D1 +
D2 − D4 λ2 λ2 − D3
(6)
where D1 is 3.6196(9), D2 is 0.04131(4), D3 is 0.0238(1) and D4 is 0.00856(2). The absorption cross-sections at 947 and 976 nm are 7
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Wang, Synthesis of Er3+:Lu2O3 nanopowders by carbonate co-precipitation process and fabrication of transparent ceramics, J. Alloy. Compd. 652 (2015) 281–286. [20] X. Lu, B.X. Jiang, J. Li, W.B. Liu, L. Wang, X.W. Ba, C. Hu, B.L. Liu, Y. Pan, Synthesis of highly sinterable Yb:Sc2O3 nanopowders for transparent ceramic, Ceram. Int. 39 (2013) 4695–4700. [21] J.W. Dai, Y.B. Pan, W. Wang, W. Luo, T.F. Xie, H.M. Kou, J. Li, Fabrication of Tb3Al5O12 transparent ceramics using co-precipitated nanopowders, Opt. Mater. 73 (2017) 38–44. [22] S.H. Lee, E.R. Kupp, A.J. Stevenson, J.M. Anderson, G.L. Messing, X. Li, E.C. Dickey, J.Q. Dumm, V.K. Simonaitis-Castillo, G.J. Quarles, Hot isostatic pressing of transparent Nd:YAG ceramics, J. Am. Ceram. Soc. 92 (2009) 1456–1463. [23] J. Mouzon, A. Maitre, L. Frisk, N. Lehto, M. Odén, Fabrication of transparent yttria by HIP and the glass-encapsulation method, J. Eur. Ceram. Soc. 29 (2009) 311–316. [24] L. Zhang, Y. Ben, H. Chen, D.Y. Tang, X.Z. Fu, R. Sun, B. Song, C.P. Wong, Low temperature-sintering and microstructure of highly transparent yttria ceramics, J. Alloy. Compd. 695 (2017) 2580–2586. [25] M.I. Mendelson, Average grain size in polycrystalline ceramics, J. Am. Ceram. Soc. 52 (1969) 443–446. [26] Q. Liu, Z.F. Dai, D. Hreniak, S.S. Li, W.B. Liu, W. Wang, W. Luo, C.Y. Li, J.W. Dai, H.H. Chen, H.M. Kou, Y. Shi, Y.P. Pan, J. Li, Fabrication of Yb:Sc2O3 laser ceramics by vacuum sintering co-precipitated nano-powders, Opt. Mater. 72 (2017). [27] W. Luo, P. Ma, T.F. Xie, J.W. Dai, Y.B. Pan, H.M. Kou, J. Li, Fabrication and spectroscopic properties of Co: MgAl2O4 transparent ceramics by the HIP posttreatment, Opt. Mater. 69 (2017) 152–157. [28] Z.M. Seeley, J.D. Kuntz, N.J. Cherepy, S.A. Payne, Transparent Lu2O3:Eu ceramics by sinter and HIP optimization, Opt. Mater. 33 (2011) 1721–1726. [29] P. Ma, T.F. Xie, L.X. Wu, H.H. Chen, H.M. Kou, Y. Shi, Y.P. Pan, J. Li, Fabrication of
Fig. 11. Emission cross section of 5at%Yb:Lu2O3 ceramics pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C.
calculated to be 0.62 × 10−20 and 1.03 × 10−20 cm2, which is close to the value of the Yb:Lu2O3 single crystal [31]. The large absorption cross section and broad absorption band make it suitable for pumping by high-power InGaAs laser diodes, the pump efficiency will be high. Fig. 11 shows the room-temperature emission cross section of the 5 at% Yb:Lu2O3 ceramic pre-sintered at 1500 °C for 2 h and HIP posttreated at 1700 °C for 8 h. The emission cross section σems(λ) is obtained by reciprocity method, as derived from the absorption spectrum:
σem (λ ) = σabs (λ )
Zl c c exp ⎜⎛ ⎛h − h ⎞/ kT ⎞⎟ Zu λ λ ZL ⎠ ⎠ ⎝⎝ ⎜
⎟
(7)
where σabs (λ) is the absorption cross section at the wavelength λ, Zl and Zu are the lower and upper manifold partition functions, which are calculated from the energy level structure of Yb:Lu2O3 shown in literature [32]. Planck constant is h and c is the speed of light, k is the Boltzmann constant, T is the absolute temperature. The emission cross sections of at 1033 and 1080 nm are 0.84 × 10−20 and 0.21 × 10−20 cm2, respectively. 4. Conclusions In this work, 5 at% Yb:Lu2O3 transparent ceramics were successfully fabricated by vacuum sintering combined with hot isostatic pressing (HIP) sintering using carbonate co-precipitated nano-powders. 1100 °C was found to be the optimal calcination temperature of the precursor to obtain fine and well dispersed 5 at% Yb:Lu2O3 powders. The ceramic pre-sintered at 1500 °C for 2 h with HIP post-treating at 1700 °C for 8 h shows the highest transmittance of 78.2% at the wavelength of 1100 nm. The average grain size of the sample is as small as 2.6 µm. The absorption cross-sections of the 5 at% Yb:Lu2O3 ceramic at 903, 947, 976 nm were calculated to be 0.36 × 10−20, 0.62 × 10−20 and 1.02 × 10−20 cm2, respectively. The emission cross sections of the 5 at % Yb:Lu2O3 ceramic at 1033 and 1080 nm were calculated to be 0.84 × 10−20 and 0.21 × 10−20 cm2, respectively. The prepared 5 at% Yb:Lu2O3 transparent ceramic is a promising candidate for the highpower solid-state lasers. Acknowledgements This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the Key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). 8
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[31] L. laversenne, Y. Guyot, C. Goutaudier, M.T. Cohen-adad, G. Boulon, Optimization of spectroscopic propeties of Yb3+-doped refractory sesquioxides: cubic Y2O3, Lu2O3 and monoclinic Gd2O3, Opt. Mater. 16 (2001) 475–483. [32] J.H. Mun, A. Jouini, A. Yoshikawa, J.H. Kim, T. Fukuda, J.S. Lee, Thermal and optical properties of Yb-doped Lu2O3 single crystal grown by the micro-pullingdown method, J. Ceram. Proc. Res. 14 (2013) 636–640.
Nd:Lu2.7Gd0.3Al5O12 transparent ceramics by solid-state reactive sintering, Opt. Mater. 66 (2017) 422–427. [30] A.A. Kaminskii, M.S. Akchurin, P. Becker, K. Ueda, L. Bohatý, A. Shirakawa, M. Tokurakawa, K. Takaichi, H. Yagi, J. Dong, T. Yanagitani, Mechanical and optical properties of Lu2O3 host-ceramics for Ln3+ lasants, Laser Phys. Lett. 5 (2008) 300–303.
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Fabrication, microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders Qiang Liua, Jinbang Lia,b, Jiawei Daib,c, Zewang Hub,c, Cong Chena,b, Xiaopu Chenb,c, ⁎ Yagang Fengb,c, Jiang Lib, a
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China c University of Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb:Lu2O3 Transparent ceramics Fabrication Spectroscopic properties
Ytterbium doped lutetium oxide (Yb:Lu2O3) transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) of the powders synthesized by the co-precipitation method. The effects of calcination temperature on the composition and morphology of the powders were investigated. Fine and well dispersed 5 at% Yb:Lu2O3 powders with the mean particle size of 67 nm were obtained when calcined at 1100 °C for 4 h. Using the synthesized powders as starting material, we fabricated 5 at% Yb:Lu2O3 ceramics by presintering at different temperatures combined with HIP post-treatment. The influence of pre-sintering temperature on the densities, microstructures and optical quality of the 5 at% Yb:Lu2O3 ceramics was studied. The ceramic sample pre-sintered at 1500 °C for 2 h with HIP post-treating at 1700 °C for 8 h has the highest in-line transmittance of 78.2% at 1100 nm and the average grain size of 2.6 µm. In addition, the absorption and emission cross sections of the 5 at% Yb:Lu2O3 ceramics were also calculated.
1. Introduction Solid-state laser has been successfully used in industrial, military, medical and other fields due to its small size, high efficiency, and excellent performance [1]. In the past decades, rare-earth-doped yttrium aluminum garnet (YAG) as the host material for high-power solid-state lasers has been rapidly developed [2,3]. The sesquioxides such as Sc2O3, Y2O3 and Lu2O3 possess higher thermal conductivity than YAG and have been considered to have significant potential as host materials for high-power and high efficiency solid-state lasers [4–6]. Among them, Yb-doped Lu2O3 stands out as the best host material especially when the dopant concentration of Yb3+ ion is high [7]. Because of the similar ionic radii and bonding forces of Lu3+ and Yb3+ ion, the Yb3+ ion can easily substitute for Lu3+ ion. In addition, the overall thermal conductivity is almost not affected by the doping concentration, which is due to the negligible scattering of the propagating phonons. Furthermore, Yb3+ ion has high quantum efficiency and simple energy manifolds (the 2F7/2 ground state and the 2F5/2 excited state), which prevent the excited state absorption and upper level conversion [8–10]. The melting point of Lu2O3 is about 2450 °C, which is higher than the melting temperature of iridium crucible [11–13]. Therefore, it is quite difficult to fabricate Lu2O3 single crystals with large size and high
⁎
optical quality by the conventional melt growth process [14]. The ceramic processing technology provides an alternative way to fabricate transparent Lu2O3 materials since ceramic processing requires 25–30% lower temperature than the melting temperature. Compared with single crystals, transparent ceramics are advantageous in many ways including higher doping concentration, possibility of larger size, and more function design freedom [15]. Powders with high purity, good dispersity and uniform size are the key to obtain high optical quality ceramics [16]. However, commercial Yb3+ doped Lu2O3 powders are not common available. Lots of methods have been used to synthesize rare-earth-doped Lu2O3 powders, such as, sol-gel combustion [17], hydrothermal synthesis [18] and co-precipitation methods [7,12,19]. Among these methods, co-precipitation route has been proven to be an effective route to synthesize rare-earthdoped Lu2O3 powders with homogeneous composition and good dispersion. Calcination temperature has a big influence on the purity and agglomeration of the powders [20,21]. Optimization of calcination temperature to obtain highly pure and less-agglomerated Yb:Lu2O3 powders is necessary. In addition, vacuum pre-sintering followed by HIP post-treatment has been employed to achieve Nd: YAG [22] and Y2O3 [23] ceramics with high optical quality. The pre-sintering temperature has a big influence on the optical quality and microstructure of
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.ceramint.2018.03.238 Received 7 February 2018; Received in revised form 16 March 2018; Accepted 26 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Q., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.238
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the final ceramics [24]. As a result, optimization of pre-sintering temperature to obtain highly transparent Yb:Lu2O3 ceramics also needs to be systematically investigated. In this work, 5 at% Yb:Lu2O3 transparent ceramics were successfully fabricated by vacuum sintering combined with hot isostatic pressing from carbonate co-precipitated nano-powders. The influence of calcination temperature on the phase composition, particle size and morphology of the powders was systematically investigated. In addition, the influence of pre-sintering temperature on the densities, microstructures and optical quality of the ceramics was also studied. Finally, the absorption and emission cross-sections of the 5 at% Yb:Lu2O3 ceramics were also calculated.
2. Experimental procedure The rare earth raw materials used in the experiment were Lu2O3 (99.999%, Zhongkai New Materials Co., Ltd., Jining, China) and Yb2O3 (99.99%, Alfa Aesar, USA). Lu(NO3)3 solution and Yb(NO3)3 solution were prepared by dissolving Lu2O3 and Yb2O3 in nitric acid at 80 °C, respectively. Then the solutions were filtered to remove any undissolved particles and impurities with 0.3 µm filter paper. Lu3+ and Yb3+ concentrations of the nitrate solutions were assayed by chemical analysis. Lu(NO3)3 solution and Yb(NO3)3 solution were mixed together in stoichiometric proportions of Yb.05Lu.95O3 (5 at% Yb:Lu2O3). (NH4)2SO4 (99.0%, Sinopharm Chemical Reagent Co., Ltd, china) as the dispersant was added to the mixed metal ion solution. The molar ratio of (NH4)2SO4 to metal ion was 1:1. Finally the solution was diluted with deionized water, and the concentration of Lu3+ was set to 0.2 mol/L. Precipitant solution was obtained by dissolving Ammonium hydrogen carbonate (AHC) (Aladdin Industrial Corporation, China) in deionized water with a concentration of 1 M. The Yb:Lu2O3 precursor was prepared by adding 360 mL AHC solution at a speed of 3 mL/min into 500 mL mixed metal solution under stirring. After aging for 3 h, the resulting precipitate was washed for three times with deionized water and alcohol, respectively, and then dried at 70 °C for 48 h in an oven. Then, the precipitate was sieved through a 200-mesh screen and calcined at 600–1200 °C for 4 h in air. The nano-powders calcined at 1100 °C were uniaxially pressed into pellets of 18 mm in diameter at 25 MPa and then cold isostatically pressed under 250 MPa. For ceramic fabrication, the green pellets were pre-sintered at 1450–1750 °C for 2 h in a vacuum furnace (10−5 Pa) and subsequently hot isostatic pressed at 1700 °C in an Ar pressure of 100 MPa for 8 h. The sintered ceramics were annealed in air at 1350 °C for 10 h to remove oxygen vacancies. At last, the samples were double-side polished with 1 mm thick for the transmittance test. The phase compositions of the powders were determined by X-ray diffraction (XRD, Model D/max2200 PC, Rigaku, Japan) in the range of 2 θ = 10–80° using nickel-filtered Cu-Kα radiation. Fourier transform infrared spectrum (FTIR) was performed on an infrared spectrometer (FTIR, Bruker VERTEX 70 spectrophotometer, Ettlingen, Germany) using the standard KBr method in the range of 4000 cm−1–400 cm−1. The specific surface area (SBET) of the precursor and calcined powders was performed by Norcross ASAP 2010 micromeritics with N2 as the absorption gas at 77 K. The density of the ceramics was measured by the Archimedes method using deionized water as the immersion medium. The microstructure of the polished samples (thermally etched at 1350 °C for 3 h) was observed by a field emission scanning electron microscopy (FESEM, SU8220, Hitachi, Japan). The average grain sizes of sintered samples were measured by the linear intercept method from SEM images and the average intercept length was multiplied by 1.56 [25]. The in-line transmittance of the samples was measured by a UV–VIS–NIR spectrophotometer (Model Carry-5000, Varian, USA). The pore size of the HIPed ceramics was measured by the optical microscope (BX51, Olympus, Japan).
Fig. 1. FTIR spectra of the precursor and the 5at%Yb:Lu2O3 powders calcined at different temperatures for 4 h.
3. Results and discussion Fig. 1 shows the FT-IR spectra of the precursor and the 5 at% Yb:Lu2O3 powders calcined at different temperatures for 4 h. The wide absorption bands centered at 3450 cm−1 are related to O-H stretching of molecular water. The bands at 844 cm−1 are assigned to the deformation vibration of C-O in CO32-, while the bands at 1527 and 1405 cm−1 are corresponding to the asymmetric stretch of the C-O bond in CO32-. The bands at around 1100 cm−1 are attributed to SO42-. The bands at 2360 cm−1 are caused by the asymmetric stretch of CO2 absorbed in air. The FT-IR spectrum of the precursor indicates that the precursor contains functional groups such as OH−1, CO32- and H2O. With the increase of calcination temperature, intensities of the bands of OH−1, CO32-, SO42- and H2O become weaker. Peaks of CO32- and SO42disappear when the calcination temperature is higher than 1000 °C. When the calcination temperature is 600 °C, new band at 575 cm−1 related to the stretching of Lu–O bond appears, which results from the crystallization of Yb:Lu2O3 from the precursor [12]. This result is consistent with the results of XRD analysis. Fig. 2 shows the XRD patterns of the 5 at% Yb:Lu2O3 precursor and the powders calcined at different temperatures for 4 h. It can be seen
Fig. 2. XRD patterns of the precursor and the 5at%Yb:Lu2O3 powders calcined at different temperatures for 4 h. 2
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ytterbium ion can substitute for lutetium ion and they form a solid solution. With the increase of calcination temperature, the diffraction peaks of Lu2O3 phase become much sharper, indicating the improvement of crystallization. The mean crystallite sizes (DXRD) of the 5 at% Yb:Lu2O3 powders can be calculated by the X-ray line broadening technique performed on the (2 2 2) diffraction of the Yb:Lu2O3 from Scherrer equation:
Table 1 The specific surface areas, average particle sizes (DBET), mean crystallite sizes (DXRD) and mean primary particle sizes (DSEM) of the precursor and the powders calcined at different temperatures for 4 h. Calcination Temperature (°C)
Specific surface area (m2/g)
DBET (nm)
DXRD (nm)
Precursor 600 °C 800 °C 1000 °C 1100 °C 1200 °C
38.9 33.4 23.6 12.2 7.6 7.3
20 27 52 84 87
12 14 22 43 52
DSEM (nm)
D XRD = 0.89 λ/(β⋅ cos θ)
(1)
where β is the full width at half-maximum (FWHM) of a diffraction peak at Bragg angel θ and λ is the wavelength of Cu Kα radiation. The mean crystallite sizes of 5 at% Yb:Lu2O3 powders calculated from XRD peaks are shown in Table 1. It can be seen that the average crystallite size of the powders increases from 12 to 52 nm when the calcination temperature increases from 600 to 1200 °C. Fig. 3 displays the SEM micrographs of the precursor and the calcined powders. The precursor is a flake structure. The powders calcined at 600 and 800 °C show analogous morphology compared with that of the precursor. For the powders calcined at 1000 °C, the flake structure
41 67 83
that the precursor exhibits low crystallinity and is almost amorphous. After being calcined at 600 °C and above, all the characteristic diffraction peaks of the powders are corresponding to the cubic crystal structure of Lu2O3 (JCPDS 43–1021). No secondary phases are observed in the powders calcined at different temperatures, indicating that
Fig. 3. SEM micrographs of the (a) precursor and the 5at%Yb:Lu2O3 powders calcined for 4 h at (b) 600 °C, (c) 800 °C, (d) 1000 °C, (e) 1100 °C and (f) 1200 °C. 3
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began to decompose into small particles with clear edge. The average particle size of the powders increases when calcined at 1100 °C and the flake structure almost disappears. However, as the calcination temperature increases to 1200 °C, the grain grows up apparently and the agglomeration is aggravated. The mean primary particle size measured by the linear intercept method from SEM images is denoted as DSEM. The DSEM of the powders calcined at 1000, 1100 and 1200 °C are 41, 67 and 83 nm, respectively. The specific surface areas of the precursor and the powders are also listed in Table 1. It can be seen that the specific surface area of the 5 at% Yb:Lu2O3 powders reduces distinctly from 33.4 to 7.3 m2/g as the calcination temperature increases from 600° to 1200°C. The average particle size (DBET) of the 5 at% Yb:Lu2O3 powders is calculated from the following formula:
Fig. 5. Photograph of the 5at%Yb:Lu2O3 ceramics (1.0 mm thickness) before and after HIP post-treatment with different vacuum pre-sintering temperatures.
where ρ (9.40 g/cm3) is the theoretical density of 5 at% Yb:Lu2O3 nanopowders calculated from the lattice parameters of the 5 at% Yb:Lu2O3 nano-powders. SBET is the specific surface area determined by BET measurement. The DBET increases from 20 to 87 nm when the calcination temperature increases from 600 to 1200 °C, and the DBET is larger than DXRD, indicating that the particles are polycrystalline. In our previous work [26], the calcination temperature has to be at least above 1000 °C in order to completely remove the carbonate and sulfate from the Yb:Sc2O3 precursor. If the carbonate and sulfate cannot be removed, the residual carbonate and sulfate will form contamination in the sintering process of ceramics, which will degrade the optical quality of the ceramics. However, when the calcination temperature is as high as1200°C, the grain grows obviously accompanied with hard agglomeration and will reduce the sintering activity of the Yb:Sc2O3 powder. So in this work, we chose the powders calcined at 1100 °C to fabricate Yb:Lu2O3 transparent ceramics. Fig. 4 shows the relative densities of the 5 at% Yb:Lu2O3 ceramics before and after HIP post-treatment. The relative density of the presintered ceramic increases from 92.7% to 98.6% with the sintering temperature increasing from 1450 to 1750 °C. After the HIP posttreatment, the relative densities of all the samples are increased. For the sample pre-sintered at 1450 °C, the relative density only increases to 96.0% after HIP post-treatment. It is because that the sample pre-sintered at 1450 °C still has many open-pores (as can be seen in Fig. 6a), which leads to the infiltration of argon and eliminates the driving force for densification during HIP treatment. When the pre-sintering temperature is between 1500 and 1650 °C, the relative densities of the ceramic samples after HIP post-treatment exceed 99.8%, indicating that the residual pores in the pre-sintered Yb:Lu2O3 ceramics can be effectively removed. However, further increase of pre-sintering temperature
leads to the decrease of the relative density of the HIPed ceramic sample. It may be caused by the intragranular pores remained in the ceramics, which is difficult to be eliminated even by the HIP posttreatment. Fig. 5 shows the photograph of the 5 at% Yb:Lu2O3 ceramics vacuum sintered at different temperatures for 2 h and then HIP posttreated at 1700 °C for 8 h. The samples pre-sintered at different temperatures are opaque. The reason for the poor sintering activity of the Yb:Lu2O3 powders under vacuum sintering is due to the relatively hard agglomeration of the synthesized powders. After the HIP post-treatment, the samples pre-sintered at the temperature range from 1500 to 1750 °C become transparent and the letters under the ceramics can be seen, but the sample pre-sintered at 1450 °C is still opaque. It indicates that the pre-sintered temperature has an important effect on the transparency of the ceramic. Fig. 6 shows the in-line transmittance of the 5 at% Yb:Lu2O3 samples after HIP post-treatment. The sample pre-sintered at 1450 °C is opaque after HIP post-treatment due to the insufficient densification. The sample pre-sintered at 1500 °C has the highest transmittance of 78.2% at the wavelength of 1100 nm. When the pre-sintering temperature is higher than 1500 °C, the transmittance of the samples gradually decreases. It is because that the increased grain size is not conducive to the removal of pores during the HIP process. The results are also consistent with those reported in the literature [27]. Furthermore, when the pre-sintering temperature is high, the grain growth rate is fast and it will form intragranular pores, which are difficult to be eliminated by the HIP post-treatment, as can be confirmed by Fig. 7, Fig. 8 and Fig. 9. The decrease of optical transmittance at shorter wavelength is mainly caused by the scattering of remnant pores. The transparency of the Yb:Lu2O3 ceramics could be further enhanced by optimizing the fabrication process, for instance, to improve the dispersity and morphology of the raw powders, to select a better shaping method, or to
Fig. 4. Relative density of the pre-sintered and post-HIPed 5at%Yb:Lu2O3 ceramics with different vacuum pre-sintering temperatures.
Fig. 6. In-line transmittance of the post-HIPed 5at%Yb:Lu2O3 ceramics (1.0 mm thickness) pre-sintered at different temperatures.
DBET = 6/(ρ⋅SBET)
(2)
4
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Fig. 7. SEM images of the thermally etched surfaces of the 5at%Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h (a) 1450 °C; (b) 1500 °C; (c) 1550 °C; (d) 1600 °C;(e) 1650 °C; (f) 1700 °C; (g)1750 °C.
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Fig. 8. SEM images of the thermally etched surfaces of the 5at%Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h and HIP post-treated at 1700 °C for 8 h (a) 1450 °C; (b) 1500 °C; (c) 1550 °C; (d) 1600 °C;(e) 1650 °C; (f) 1700 °C; (g)1750 °C.
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Fig. 10. Absorption cross section of the 5at%Yb:Lu2O3 ceramics pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C. Fig. 9. The average grain sizes of the pre-sintered and post-HIPed 5at% Yb:Lu2O3 ceramics with different vacuum pre-sintering temperatures.
post-treated at 1700 °C for 8 h. After the HIP post-treatment, the sample pre-sintered at 1450 °C still has many open pores. Open pores formed during vacuum pre-sintering will lead to the infiltration of argon and eliminate the driving force for densification during HIP treatment. When the pre-sintering temperature is between 1500 and 1750 °C, the residual pores at the grain boundaries are almost completely removed and the ceramics become transparent (as shown in Fig. 5). In the HIPed ceramics pre-sintered at 1700 and 1750 °C, intragranular pores formed during vacuum pre-sintering still exist because intragranular pores are hard to eliminate by HIP post-treatment. Only bulk diffusion can be served for the elimination of intragranular pores. This diffusion style is much slower than grain boundary diffusion. Therefore, intragranular pores are virtually frozen in place and hardly to be removed in limited time, even under the high temperature and high pressure during HIP process. It can be seen that after the HIP post-treatment, the average grain size of all the ceramics increases. As shown in Fig. 9, the grain sizes of all ceramics increase from 2.1 to 8.2 µm with the pre-sintering temperatures increasing from 1450 to 1750°C. The average grain size of HIPed Yb:Lu2O3 ceramics pre-sintered at 1500 °C is only 2.6 µm, which endows the ceramic with the possibility to have good mechanical properties. Fig. 10 shows the absorption cross section of the 5 at% Yb:Lu2O3 ceramic pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C for 8 h. There are four main strong absorption peaks at 903, 947, 975 and 1040 nm corresponding to 2F7/2-2F5/2 transitions of Yb3+. The absorption cross-section (σabs) is calculated by the following equations [29].
optimize the HIP post-treatment schedules, etc. Fig. 7 shows the thermally etched surfaces of the 5 at% Yb:Lu2O3 ceramics pre-sintered at different temperatures for 2 h. It can be seen that the pre-sintering temperature greatly influences the type of the pores and the grain sizes. For the sample pre-sintered at 1450 °C, many open-pores as well as some closed pores can be seen, which lead to a relatively lower density. When the pre-sintering temperature increases to 1500 °C and above, almost no open-pores can be observed. However, when the sintering temperature increases to 1700 and 1750 °C, some intragranular pores appear. It is because that the grain growth rate exceeds the removal rate of the pores at high sintering temperatures [28]. With the increase of pre-sintering temperature, the porosity decreases continuously, and this is consistent with the relative density shown in Fig. 4. The average grain sizes of the ceramics measured by the linear intercept method from SEM micrographs are given in Fig. 9. It can be seen that, at the temperature range of 1450–1600 °C, the grain size increases slowly from 0.6 to 1.3 µm. However, when the sintering temperature is higher than 1650 °C, the grain growth rate of the sample is accelerated. The grain size of the samples increases from 1.9 to 3.6 µm when the pre-sintering temperature increases from 1650 to 1700°C, and reaches 6.2 µm at 1750 °C. Hot isostatic pressing can provide high driving force for densification and be used to eliminate intergranular pores. During the HIP treatment, the applied compressive stress diffuses onto the pores in the grain boundaries, where the vacancy concentration can be decreased and the pores will shrink and move due to plastic deformation of surrounding grains. Meanwhile, the plastic deformation is driven by the grain boundary sliding and the smaller grain means more active for grain boundary sliding [24]. In order to effectively eliminate the pores during HIP treatment, the samples with closed pores while the pores exist at the grain boundaries are in favor. Furthermore, the grain size should be as small as possible to maintain high grain boundary sliding. Based on these principles, the ceramics pre-sintered between 1500 and 1550 °C have the more preferred microstructures to be used for HIP treatment. The porosity of the HIPed ceramics can be calculated from the relative density of the 5 at% Yb:Lu2O3 ceramics pre-vacuum sintered at 1500, 1550, 1600, 1650, 1700 and 1750 °C, whose values are about 0.1%, 0.1%, 0.1%, 0.2%, 0.4% and 0.7%. The average pore sizes of the HIPed samples pre-vacuum sintered at 1500, 1550, 1600 °C are all about 0.50 µm. And they increase from 0.54 µm to 0.55 and 0.62 µm with increasing the pre-vacuum sintering temperatures from 1650 °C to 1700 and 1750 °C. Fig. 8 shows the micrograph of the 5 at% Yb:Lu2O3 ceramics HIP
1 − n ⎞2 R=⎛ ⎝1 + n ⎠ T=
(1 − R)2 exp(−αb) 1 − R2 exp(−2αb)
σabs =
α N
(3)
(4) (5)
where T is the transmittance of the 5 at% Yb:Lu2O3 ceramic, α and b are the absorption coefficient and the thickness of the sample, N is the Yb3+ concentration per unit volume. The refractive index at different wavelengths of Lu2O3 can be estimated from the following formula reported by Kaminskii et al. [30].
n2 = D1 +
D2 − D4 λ2 λ2 − D3
(6)
where D1 is 3.6196(9), D2 is 0.04131(4), D3 is 0.0238(1) and D4 is 0.00856(2). The absorption cross-sections at 947 and 976 nm are 7
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Fig. 11. Emission cross section of 5at%Yb:Lu2O3 ceramics pre-sintered at 1500 °C for 2 h and HIP post-treated at 1700 °C.
calculated to be 0.62 × 10−20 and 1.03 × 10−20 cm2, which is close to the value of the Yb:Lu2O3 single crystal [31]. The large absorption cross section and broad absorption band make it suitable for pumping by high-power InGaAs laser diodes, the pump efficiency will be high. Fig. 11 shows the room-temperature emission cross section of the 5 at% Yb:Lu2O3 ceramic pre-sintered at 1500 °C for 2 h and HIP posttreated at 1700 °C for 8 h. The emission cross section σems(λ) is obtained by reciprocity method, as derived from the absorption spectrum:
σem (λ ) = σabs (λ )
Zl c c exp ⎜⎛ ⎛h − h ⎞/ kT ⎞⎟ Zu λ λ ZL ⎠ ⎠ ⎝⎝ ⎜
⎟
(7)
where σabs (λ) is the absorption cross section at the wavelength λ, Zl and Zu are the lower and upper manifold partition functions, which are calculated from the energy level structure of Yb:Lu2O3 shown in literature [32]. Planck constant is h and c is the speed of light, k is the Boltzmann constant, T is the absolute temperature. The emission cross sections of at 1033 and 1080 nm are 0.84 × 10−20 and 0.21 × 10−20 cm2, respectively. 4. Conclusions In this work, 5 at% Yb:Lu2O3 transparent ceramics were successfully fabricated by vacuum sintering combined with hot isostatic pressing (HIP) sintering using carbonate co-precipitated nano-powders. 1100 °C was found to be the optimal calcination temperature of the precursor to obtain fine and well dispersed 5 at% Yb:Lu2O3 powders. The ceramic pre-sintered at 1500 °C for 2 h with HIP post-treating at 1700 °C for 8 h shows the highest transmittance of 78.2% at the wavelength of 1100 nm. The average grain size of the sample is as small as 2.6 µm. The absorption cross-sections of the 5 at% Yb:Lu2O3 ceramic at 903, 947, 976 nm were calculated to be 0.36 × 10−20, 0.62 × 10−20 and 1.02 × 10−20 cm2, respectively. The emission cross sections of the 5 at % Yb:Lu2O3 ceramic at 1033 and 1080 nm were calculated to be 0.84 × 10−20 and 0.21 × 10−20 cm2, respectively. The prepared 5 at% Yb:Lu2O3 transparent ceramic is a promising candidate for the highpower solid-state lasers. Acknowledgements This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the Key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). 8
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