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Fabrication and luminescent properties of fine-grained cerium-doped lutetium-yttrium oxyorthosilicate ceramics
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Fabrication and luminescent properties of fine-grained cerium-doped lutetium-yttrium oxyorthosilicate ceramics Lingcong Fan a, *, Yiquan Wu b, **, Zhijun Zhang a, Ying Shi a, ***, Jianjun Xie a a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, 14802, New York, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: LYSO SPS Fine grain Luminescence Oxygen vacancy
When compared to Lu2SiO5:Ce (LSO:Ce), cerium-doped lutetium-yttrium oxyorthosilicate solid solutions not only possess a similar density, but they also exhibit good scintillation properties (high light yield, short decay time and good energy resolution). Fine-grained Lu1.8Y0.2SiO5:Ce (LYSO:Ce) ceramics were fabricated via Spark Plasma Sintering (SPS) at a temperature of 1050 � C and an applied pressure of 100 MPa from a nano-sized LYSO:Ce powder. The average grain size of the sintered LYSO:Ce ceramics is 210 nm. To eliminate the oxygen vacancies that result from the sintering process, the sintered LYSO:Ce ceramics were subsequently annealed at 950 � C for 40 h in air. The in-line transmittance of the LYSO:Ce ceramics is 1.7% at 420 nm. The light yield of the annealed LYSO:Ce ceramics is 23,600 photons/MeV. It is suggested that SPS combined with annealing in air is a promising route to fabricate fine-grained LYSO:Ce ceramics with good scintillation performance.
1. Introduction Inorganic scintillators are a class of luminescent materials with high density which can convert high energy radiation (X-rays orγ-rays) or the energy of high energy particles into near ultraviolet or/and visible light. These materials have a wide range of applications are commonly used in oil well logging, high-energy physics, safety inspection, industrial diagnosis and nuclear medicine imaging [1–3]. Cerium-doped lutetium oxyorthosilicate (Lu2SiO5:Ce, LSO:Ce) is an excellent scintillator. Cerium-doped Lu2SiO5–Y2SiO5 solid solution has been demonstrated to be a very successful alternative material that retains favorable scintil lation properties of LSO:Ce along costs associated with Y2SiO5 [4]. Compared with LSO:Ce, cerium-doped lutetium-yttrium oxy orthosilicate (LYSO:Ce) not only possess good scintillation properties, such as high light yield (27,000–31,000 photons/MeV), short decay time (~40 ns) and good energy resolution (8–13%), but also have a compa rable density (7.11 g/cm3) to LSO (7.40 g/cm3) [5–7]. In addition, LYSO possesses some additional advantages including lower melting point (2100 � C for LYSO, 2150 � C for LSO) [8] (which is favorable for the growth of oxyorthosilicate single crystals), cheaper raw materials, and higher doping concentration of cerium. Furthermore, the lower melting
point of LYSO:Ce is also favorable for sintering of fine-grained oxy orthosilicate ceramics. LSO:Ce ceramics have been fabricated successfully by pressureless sintering [9], Hot Pressing (HP) [10], Hot Isostatic Pressing (HIP) [11–14] and Spark Plasma Sintering (SPS) [15,16]. The results of ex periments suggest that the scintillation properties of the LSO:Ce ce ramics are comparable to that of their single crystal counterpart. However, achieving transparency in LSO:Ce ceramics remains a chal lenge due to their optical anisotropy deriving from their monoclinic crystal structure, especially for coarse-grained ceramics which further lead to low in-line transmittance [9,17]. According to Rayleigh-Gans-Debye (RGD) theory [18], the in-line transmittance of non-cubic ceramics can be elevated by suppressing their grain size. The grain size of optical ceramics can be suppressed to sub micrometer through pressure-assisted sintering technology. The resulting fully dense and fine-grained ceramics possesses good in-line transmission. This is the reason why many studies have recently been conducted on non-cubic ceramics fabricated via pressure-assisted sintering technology [19–22]. This work is devoted to the fabrication of fine-grained LYSO:Ce ceramics through the combination of nanosized powder processing and spark plasma sintering.
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Fan), [email protected] (Y. Wu), [email protected] (Y. Shi). https://doi.org/10.1016/j.optmat.2019.109563 Received 29 September 2019; Received in revised form 9 November 2019; Accepted 19 November 2019 0925-3467/© 2019 Published by Elsevier B.V.
Please cite this article as: Lingcong Fan, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109563
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2. Experimental
3. Results and discussion
Lu1.8Y0.2SiO5:Ce (LYSO:Ce) powder was synthesized via a sol-gel process. The Ce ion concentration relative to the sum of the Lu, Ce and Y ions is 0.5 mol%. LuCl3⋅6H2O (99.99%), YCl3⋅6H2O (99.99%), CeCl3⋅7H2O (99.9%) and tetraethyl orthosilicate (TEOS, AR) with the desired stoichiometric ratios were dispersed in isopropanol (AR). Pro pylene oxide (PPO, AR) with the mole ratio of rare earth elements to PPO of 1:20 was added in the mixture and mechanically stirred for 24 h resulting in a gel to be obtained. The gel was then dried at 80 � C for 24 h in air. The LYSO:Ce precursor was calcined at 1000 � C for 2 h in air. 4 g of LYSO:Ce powder was loaded in a graphite mold with a diameter of 18.75 mm. The LYSO:Ce powder was separated by graphite foil with BN spraying coatings from the mold. The LYSO:Ce sample was sintered in an FCT HP D 25 SPS furnace (FCT Systeme GmbH, Rauenstein, Germany), with a heating rate of 50 � C/min under a loading pressure of 100 MPa and dwelled at 1050–1200 � C for 15 min. The morphology of the LYSO:Ce powder was observed on a Scanning Electron Microscope (SEM, XL-30, Philips, Eindhoven, the Netherlands) and Transmission Electron Microscope (TEM, JEM-2010F, JEOL, Japan). The sintered LYSO:Ce ceramics was lapped and mirror-polished down to 1 mm. The X-ray diffraction (XRD) of the powder and ceramics was performed on a Bruker D2 Phaser diffractometer (Madison, WI, USA) using Cu Kα radiation. For SEM observation, small parts were cut from the polished LYSO:Ce ceramic. All of the cut LYSO:Ce ceramics were thermally etched in air for 10 h. The thermal etching temperatures of the LYSO:Ce ceramics sintered at 1050, 1150 and 1100 � C were 50 � C lower than their corresponding sintering temperature, respectively. The LYSO:Ce ceramics sintered at 1200 � C were thermally etched at 1100 � C. The microstructure of the etched LYSO:Ce ceramics was observed via SEM (FEI Quanta 200, Hillsboro, OR, USA). To eliminate oxygen va cancies, the LYSO:Ce ceramics sintered at 1050 � C were annealed at 950 � C for 40 h in air. The in-line transmittance of the LYSO:Ce ceramic was measured using a Hitachi U2910 UV–Vis–NIR spectrophotometer (Tokyo, Japan). The photoluminescence (PL) and Photoluminescence Excitation (PLE) spectra of the LYSO:Ce ceramics were obtained at room temperature via a Shimadzu RF-5301PC fluorescence spectrofluorom eter (Kyoto, Japan). The luminescence decay time of LYSO:Ce ceramics was measured on a spectrofluorometer (FLSP920, Edinburgh, Living ston, UK), and a deuterium lamp was used as the excitation source. The Cathodoluminescence (CL) studies were carried out using a GATAN Mono CL system in a field emission SEM (FEI Quanta 400 F, Hillsboro, OR, USA) with an accelerating voltage of 20 kV. The light yield of the LYSO:Ce ceramics was carried out using a Hamamatsu photomultiplier tube (PMT, 1306, Hamamatsu City, Japan) with a bi-alkali photo-cath ode and quartz window, under 662 keV γ-ray (137Cs source) excitation. All of the aforementioned measurements were performed at room temperature.
3.1. Microstructure and phase composition of LYSO:Ce powder In Fig. 1(a), the morphology of the LYSO:Ce powder calcined at 1000 C in air is displayed. The figure shows that the particles exhibit very little agglomeration and are nearly spherical and uniform. TEM image in Fig. 1(b) show that the average grain size of the LYSO:Ce powder is 70 nm. The Selected Area Electron Diffraction (SAED) pattern indicates a monoclinic crystal structure. Lutetiunm/yttrium oxyorthosilicate exists in two monoclinic phases; the metaphase crystallizes in space group P21/c, whereas the stable phase crystallizes in space group C2/c [23]. Phase evolution from powder to ceramics is presented in Fig. 2. The XRD patterns of the LYSO:Ce powder and the sintered ceramics are consistent with the standard data of the JCPDS card (PDF#97-008-9624 for pow der (metaphase) [24], PDF#97-015-9308 for ceramics (stable phase) [25,26]). No second phases can be observed in either pattern. This suggests that the LYSO:Ce powder transforms from metaphase, with a space group of P21/c, into stable phase, with a space group of C2/c, through sintering. This phase transformation from powder to ceramics also was observed in polycrystalline LSO:Ce [13]. The metaphase LYSO/LSO do not appear in phase diagrams since they are not formed at equilibrium. They are found in the wet chemical processes [27–29] rather than solid phase synthesis [30]. The phase-transformation tem perature from the space group of P21/c to C2/c for LYSO/LSO is in the
�
Fig. 2. XRD patterns of LYSO:Ce powder calcined at 1000 � C and ceramics sintered calcined at 1050 � C.
Fig. 1. SEM image (a) and TEM image (b) of LYSO:Ce powder. Inset in (b) is SAED pattern. 2
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range of 1000–1100 � C [27–29].
increasing average grain size (2r) from 210 to 2630 nm (n ¼ 1.808, Δn ¼ 0.028, t ¼ 1 mm). The in-line transmittance of the LYSO:Ce ceramics sintered at 1050 � C is 1.7% at 420 nm. This value is slightly greater (1.2–1.4%) than that of LSO:Ce ceramics [12,13,35], which is attributed to the smaller average grain size of LYSO:Ce ceramics resulting from a lower sintering temperature compared with that of LSO:Ce ceramics. The difference between the measured data and calculation result from the RGD model can attributed to very few micro pores in the LYSO:Ce ceramics. The RGD model is based on fully dense optical materials, whereas sintered ceramics always exists residual pores. The pores (even for the ceramics with porosity of 0.02%) can result in noteworthy scattering of light. At short wavelengths (<420 nm), since the pore size is too large compared with the wavelength, the contribution to the back scattering of light decreases and resultantly the transmission increases with decreasing wavelength [36].
3.2. Microstructure and optical property of LYSO:Ce ceramics Fig. 3 shows the thermally-etched surface micrograph of LYSO:Ce ceramics sintered at different temperatures under 100 MPa for 15 min by SPS. No residual pores and second phase can be observed in the ce ramics sintered at 1050 and 1100 � C. The sintering temperature to achieve pore-free LYSO:Ce ceramics is 300–650 � C lower than that of LSO:Ce ceramics sintered by SPS [16], HP [10,31] and HIP [11–13]. This can be attributed to the lower melting point of LYSO:Ce. A small amount of pores appear within the grains and in the triangular grain boundaries of the LYSO:Ce ceramics sintered at 1150 and 1200 � C. The pores entrapped within the grains indicate that the grain growth is faster than the pore diffusion at 1150/1200 � C under an applied pressure of 100 MPa. The grain size of the LYSO:Ce ceramics increases remarkably with the increasing SPS sintering temperature. The average grain sizes of the ceramics sintered at 1050 � C, 1100 � C, 1150 � C and 1200 � C are 0.21, 1.36, 2.01 and 2.63 μm, respectively. Some intergranular cracks were observed in the LYSO:Ce ceramics sintered at 1150 � C and 1200 � C. These cracks propagate from the grain boundary, or triple point, to the grain. Intergranular cracks were also observed in the hot-pressed LSO:Ce ceramics [12]. These cracks can result from the expansion of gas trapped in an isolated pore and/or residual stress due to anisotropy in coefficient of thermal expansion of LYSO [32,33]. The theoretical in-line transmittance of dense ceramics (no pores nor second phases) can be predicted through the Rayleighe-Ganse-Debye (RGD) light scattering theory [18]; Tth ¼ [16n2/(nþ1)4]exp [-3π2 (Δn)2rt/(n2λ2)], where Tth is the theoretical in-line transmittance of the ceramics, n is the average refractive index of uniaxial or biaxial crystal, Δn is the maximum difference between the refractive indices of the crystal, r is half of the average grain size of the ceramics, t is the thickness of the polycrystalline ceramics. LYSO possesses the same structure as biaxial crystal LSO. The maximum difference between the refractive indexes of LYSO is 0.028 at its maximum emission wavelength (420 nm) [34]. Optical anisotropy can result in light scattering when light passes through the grain boundary of polycrystalline ceramics and therefore gives rise to low in-line transmittance in polycrystalline LYSO: Ce ceramics. As shown in Fig. 4, the calculated in-line transmittance values (at 420 nm) of LYSO:Ce ceramics sharply decrease with
3.3. Luminescence properties and light yield of LYSO:Ce ceramics Stable-phase LYSO exists in two crystallographic sites; the RE1 site is coordinated by seven oxygen atoms (five silicon-bonded oxygens and two non-silicon-bonded oxygens), and the RE2 site is coordinated by six oxygen atoms (four silicon-bonded oxygens and two non-silicon-bonded oxygens) [26,27]. Cerium occupies both the RE1 and RE2 sites, so there are two emission centers in LYSO:Ce that are referred to as Ce1 and Ce2, respectively, which present different luminescence characteristics [37–39]. Fig. 5 shows the PL and PLE spectra of the as-sintered and annealed LYSO:Ce ceramics. The wavelengths were chosen to separate excitation and emission bands from Ce1 and Ce2 centers. The PLE spectrum recorded for the 400 nm emission display four excitation peaks centered at 228 nm (5.45 eV), 263 nm (4.73 eV), 302 nm (4.12 eV) and 365 nm (3.41 eV), which are attributed to the 4f→5d4, 5d3, 5d2, and 5d1 tran sitions of Ce1 [40,41], respectively. The 4f→5d2, and 5d1 transitions of Ce2 are responsible for the excitation peaks located at 325 nm (3.82eV) and 375 nm (3.31 eV) in the PLE spectrum recorded for the 500 nm emission [42]. The excitation band centered at 325 nm (3.82eV) in the PLE spectrum of the annealed LYSO:Ce ceramics became more over lapped with the 365 nm (3.41 eV) excitation band of Ce1 when compared with those of the as-sintered ceramics. The excitation band of Ce2 is overlapped with the emission band of Ce1 in the wavelength range from 365 to 410 nm, which indicates that the Ce1 emission in the high energy range can be reabsorbed by Ce2 [43], i.e. there is an energy transfer from Ce1 to Ce2 which occurred in the LYSO:Ce ceramics. This suggests that the energy transfer from Ce1 to Ce2 in LYSO:Ce ceramics is
Fig. 3. SEM micrographs of thermally etched surface of the LYSO:Ce ceramics sintered by SPS at (a) 1050 � C, (b) 1100 � C, (c) 1150 � C and (d) 1200 � C under 100 MPa applied pressure.
Fig. 4. In-line transmittance curves of LYSO:Ce ceramics sintered at 1050 � C and RGD model calculation (n ¼ 1.808, Δn ¼ 0.028, t ¼ 1 mm). 3
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Fig. 5. Normalized excitation (a) and emission (b) spectra of the as-sintered (top) and annealed (bottom) LYSO:Ce ceramics.
more efficient after undergoing annealing in air. This effect can be attributed to a smaller number of defects (oxygen vacancies) in the LYSO lattice, which therefore decreases the possibility that carriers were trapped. The PL spectra of the as-sintered LYSO:Ce ceramics presents a broad emission band, which can be decomposed into three peaks centered at 391 nm (3.18 eV), 425 nm (2.92 eV), and 472 nm (2.63 eV) through deconvolution in energy scale, which is in accord with the Gaussian decomposition results of PL spectra of LYSO:Ce single crystal [44]. The first two peaks originate from Ce1 emission (5d1→4F5/2 and 4 F7/2 transitions), and the third peak is mainly attributed to Ce2 emis sion. After annealing, the center of the Ce2 emission band shifted to 2.67 eV from 2.63 eV under 325 nm excitation, to 2.71 eV from 2.63 eV under 340 nm excitation, respectively. Recently, ab initio study in dicates that the low-energy emission band of LSO:Ce originates from the contribution of ten Ce3þ luminescent centers, one being CeLu2 and nine being different combinations of one substitutional Ce3þ ion and one neutral oxygen vacancy from a [SiO4] tetrahedral site (Ce-VO, five close to Ce1 and four close to Ce2) [45]. The spread related to multi-center luminescence give a good explanation for the absence of doublet structure of Ce2 emission from experiments in LSO:Ce [17,39], LYSO:Ce [40,44] and YSO [46]. Indeed, annealing in air can significantly lower the concentration of oxygen vacancies in LYSO:Ce ceramics fabricated from a strongly reduced atmosphere (carbon from graphite die), which can lower the emission from the combinations of one substitutional Ce3þ ion and one neutral oxygen vacancy in the low-energy range and lead to a blue shift of the low-energy emission band. The decay time of cerium-doped rare earth oxyorthosilicate is on the order of several tens of nanoseconds due to the spin-and-parity-allowed 5 d→4f transitions of Ce3þ. The decay time of Ce3þ is directly related to how long the electron stays in the excited state level of Ce3þ. An electron which stays in the excited state level of Ce3þ is readily captured by other centers (impurity ions, oxygen vacancies etc.) near the Ce3þ, which is a nonradiative process and is faster than radiative 5 d→4f transitions. Fig. 6 presents the decay curves of the as-sintered and annealed LYSO:Ce
ceramics under the excitation and emission wavelength to separate Ce1 and Ce2. The decay values of all the curves were achieved via single or two exponential fitting. The decay times of the as-sintered LYSO:Ce ceramics are 7 ns (77%) and 20 ns (23%) for Ce1 and 8 ns (80%), 23 ns (20%) for Ce2, respectively, while the values of the annealed LYSO:Ce ceramics are 5 ns (83%) and 24 ns (17%) for Ce1 and 36.5 ns for Ce2, respectively. The decay times of Ce1 and Ce2 of the annealed LYSO:Ce ceramics are slower than those of the as-sintered ceramics, especially for Ce2. This indicates that oxygen vacancies exist in both the as-sintered and annealed ceramics and concentrations of oxygen vacancies in LYSO:Ce ceramics can be dramatically lowered through annealing in air. These decay times of LYSO:Ce ceramics is much faster than those of LYSO:Ce single crystal [44], which can be attributed to thermally induced ionization of some excited Ce1/Ce2 center. The thermally induced ionization through the relaxation of 5d1 state can shorten the nanosecond decay time of LYSO:Ce. The band characteristics of CL spectra of the as-sintered and annealed LYSO:Ce ceramics in Fig. 7 is in accordance with that of the corresponding PL spectra in Fig. 5. The in tegrated CL intensity of the annealed ceramics reaches values 2.05 times that of the as-sintered ceramics. The 137Cs energy spectra in Fig. 8 in dicates that the light yield of the annealed LYSO:Ce ceramics is 23,600 photons/MeV, which is 2.46 times that of the as-sintered ceramics and is comparable with that of typical LYSO:Ce single crystals [47,48]; this is in accordance with the CL results. This high light yield and CL intensity of the annealed ceramics results from less oxygen vacancies in the ce ramics, which is favorable for radiative 5 d→4f transitions of Ce3þ. 4. Conclusion Phase-pure LYSO:Ce powder with an average grain size of 70 nm was synthesized via a sol-gel route. Starting from a synthesized nano powder, fine-grained LYSO:Ce ceramics were fabricated at 1050 � C under an applied pressure of 100 MPa via SPS. The average grain size of the LYSO: Ce ceramics is 210 nm. The in-line transmittance of the ceramics is 1.7% 4
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Fig. 6. Decay profiles of the as-sintered (a, b) and annealed (c, d) LYSO:Ce ceramics.
Fig. 8. Energy spectra of the as-sintered and annealed LYSO:Ce ceramics, the insets are photographs of the LYSO:Ce ceramics.
Fig. 7. CL spectra of the as-sintered and annealed LYSO:Ce ceramics.
at 420 nm. The PL, PLE and CL spectra indicate that oxygen vacancies in LYSO:Ce ceramics can be reduced effectively through annealing in air. The light yield of the annealed LYSO:Ce ceramics reaches 23,600 pho tons/MeV, which is comparable to that of a typical LYSO:Ce single crystal.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 5
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the work reported in this paper.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers: 51802186, U1732128 and 51172139) and Shanghai Young University Teachers’ Training Subsidy Scheme (ZZSD18008). The authors are grateful to Dr. X. D. Gao and Prof. X. H. Zeng at Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences for their help in CL measurement and fruitful discussion. Y. Q. Wu gratefully acknowledges NSF CAREER grant (1554094) for funding his research work. References [1] D.S. McGregor, Materials for gamma-ray spectrometers: inorganic scintillators, Annu. Rev. Mater. Res. 48 (2018) 245–277. [2] M. Nikl, A. Yoshikawa, Recent R&D trends in inorganic single-crystal scintillator materials for radiation detection, Adv. Opt. Mater. 3 (2015) 463–481. [3] C. Dujardin, E. Auffray, E. Bourret-Courchesne, P. Dorenbos, P. Lecoq, M. Nikl, A. N. Vasil’ev, A. Yoshikawa, R.Y. Zhu, Needs, trends, and advances in inorganic scintillators, IEEE Trans. Nucl. Sci. 65 (2018) 1977–1997. [4] D.W. Cooke, K.J. McClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R. E. Muenchausen, R.C. Sze, Crystal growth and optical characterization of ceriumdoped Lu1.8Y0.2SiO5, J. Appl. Phys. 88 (2000) 7360–7362. [5] J. Chen, R. Mao, L. Zhang, R.-Y. Zhu, Large size LSO and LYSO crystals for future high energy physics experiments, IEEE Trans. Nucl. Sci. 54 (2007) 718–724. [6] R. Mao, L. Zhang, R.-Y. Zhu, Emission spectra of LSO and LYSO crystals excited by UV light, X-ray and γ-ray, IEEE Trans. Nucl. Sci. 55 (2008) 1759–1766. [7] C.M. Pepin, P. Berard, A.L. Perrot, C. Pepin, D. Houde, R. Lecomte, C.L. Melcher, H. Dautet, Properties of LYSO and recent LSO scintillators for phoswich PET detectors, IEEE Trans. Nucl. Sci. 51 (2004) 789–795. [8] B. Hautefeuille, K. Lebbou, C. Dujardin, J. Fourmigue, L. Grosvalet, O. Tillement, C. Pedrini, Shaped crystal growth of Ce3þ-doped Lu2(1 x)Y2xSiO5 oxyorthosilicate for scintillator applications by pulling-down technique, J. Cryst. Growth 289 (2006) 172–177. [9] L. Fan, Y. Shi, J. Xu, J. Xie, F. Lei, Consolidation of translucent Ce3þ-doped Lu2SiO5 scintillation ceramics by pressureless sintering, J. Mater. Res. 29 (2014) 2252–2259. [10] A. Lempicki, C. Brecher, H. Lingertat, S.R. Miller, J. Glodo, V.K. Sarin, A ceramic version of the LSO scintillator, IEEE Trans. Nucl. Sci. 55 (2008) 1148–1151. [11] Y. Wang, E.v. Loef, W.H. Rhodes, J. Glodo, C. Brecher, L. Nguyen, A. Lempicki, G. Baldoni, W.M. Higgins, K.S. Shah, Lu2SiO5:Ce optical ceramic scintillator for PET, IEEE Trans. Nucl. Sci. 56 (2009) 887–891. [12] S. Roy, H. Lingertat, C. Brecher, V. Sarin, Optical properties of anisotropic polycrystalline Ce3þ activated LSO, Opt. Mater. 35 (2012) 827–832. [13] L. Fan, M. Jiang, D. Lin, D. Zhou, Y. Shi, Y. Wu, H. Yao, F. Xu, J. Xie, F. Lei, L. Zhang, J. Zhang, Densification of cerium-doped lutetium oxyorthosilicate scintillation ceramics by hot isostatic pressing, J. Alloy. Comp. 720 (2017) 161–168. [14] L.-C. FAN, Y. SHI, J.-J. XIE, Fabrication and luminescent property of polycrystalline cerium-doped lutetium oxyorthsilicate scintillation ceramics, J. Inorg. Mater. 33 (2018) 237–244. [15] T. LIN, Z.-B. XU, L.-Y. DENG, Y.-Y. REN, Y. SHI, J.-J. XIE, Spark plasma sintering of Ce3þ:Lu2SiO5 scintillation ceramics and its luminescent characteristics, J. Inorg. Mater. 26 (2011) 1210–1214. [16] J. Xie, Y. Shi, L. Fan, Z. Xu, Microstructure and luminescent properties of Ce: Lu2SiO5 ceramic scintillator by spark plasma sintering, Opt. Mater. 35 (2013) 744–747. [17] L. Fan, X. Zhang, D. Lin, Y. Shi, J. Zhang, J. Xie, F. Lei, L. Zhang, L. Chen, H. Yuan, Luminescence characteristics of Lu2SiO5:Ce3þ (LSO:Ce) ceramic scintillators under VUV–UV excitation, Nucl. Instrum. Methods Phys. Res., Sect. A 806 (2016) 325–329. [18] R. Apetz, M.P.B.v. Bruggen, Transparent alumina:A light scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486. [19] A. Krell, J. Klimke, T. Hutzler, Advanced spinel and sub-μm Al2O3 for transparent armour applications, J. Eur. Ceram. Soc. 29 (2009) 275–281. [20] N. Roussel, L. Lallemant, J.Y. Chane-Ching, S. Guillemet-Fristch, B. Durand, V. Garnier, G. Bonnefont, G. Fantozzi, L. Bonneau, S. Trombert, D. GarciaGutierrez, A. Krell, Highly dense, transparent α-Al2O3 ceramics from ultrafine nanoparticles via a standard SPS sintering, J. Am. Ceram. Soc. 96 (2013) 1039–1042. [21] B.-N. Kim, K. Hiraga, K. Morita, H. Yoshida, Spark plasma sintering of transparent alumina, Scr. Mater. 57 (2007) 607–610.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Fabrication and luminescent properties of fine-grained cerium-doped lutetium-yttrium oxyorthosilicate ceramics Lingcong Fan a, *, Yiquan Wu b, **, Zhijun Zhang a, Ying Shi a, ***, Jianjun Xie a a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, 14802, New York, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: LYSO SPS Fine grain Luminescence Oxygen vacancy
When compared to Lu2SiO5:Ce (LSO:Ce), cerium-doped lutetium-yttrium oxyorthosilicate solid solutions not only possess a similar density, but they also exhibit good scintillation properties (high light yield, short decay time and good energy resolution). Fine-grained Lu1.8Y0.2SiO5:Ce (LYSO:Ce) ceramics were fabricated via Spark Plasma Sintering (SPS) at a temperature of 1050 � C and an applied pressure of 100 MPa from a nano-sized LYSO:Ce powder. The average grain size of the sintered LYSO:Ce ceramics is 210 nm. To eliminate the oxygen vacancies that result from the sintering process, the sintered LYSO:Ce ceramics were subsequently annealed at 950 � C for 40 h in air. The in-line transmittance of the LYSO:Ce ceramics is 1.7% at 420 nm. The light yield of the annealed LYSO:Ce ceramics is 23,600 photons/MeV. It is suggested that SPS combined with annealing in air is a promising route to fabricate fine-grained LYSO:Ce ceramics with good scintillation performance.
1. Introduction Inorganic scintillators are a class of luminescent materials with high density which can convert high energy radiation (X-rays orγ-rays) or the energy of high energy particles into near ultraviolet or/and visible light. These materials have a wide range of applications are commonly used in oil well logging, high-energy physics, safety inspection, industrial diagnosis and nuclear medicine imaging [1–3]. Cerium-doped lutetium oxyorthosilicate (Lu2SiO5:Ce, LSO:Ce) is an excellent scintillator. Cerium-doped Lu2SiO5–Y2SiO5 solid solution has been demonstrated to be a very successful alternative material that retains favorable scintil lation properties of LSO:Ce along costs associated with Y2SiO5 [4]. Compared with LSO:Ce, cerium-doped lutetium-yttrium oxy orthosilicate (LYSO:Ce) not only possess good scintillation properties, such as high light yield (27,000–31,000 photons/MeV), short decay time (~40 ns) and good energy resolution (8–13%), but also have a compa rable density (7.11 g/cm3) to LSO (7.40 g/cm3) [5–7]. In addition, LYSO possesses some additional advantages including lower melting point (2100 � C for LYSO, 2150 � C for LSO) [8] (which is favorable for the growth of oxyorthosilicate single crystals), cheaper raw materials, and higher doping concentration of cerium. Furthermore, the lower melting
point of LYSO:Ce is also favorable for sintering of fine-grained oxy orthosilicate ceramics. LSO:Ce ceramics have been fabricated successfully by pressureless sintering [9], Hot Pressing (HP) [10], Hot Isostatic Pressing (HIP) [11–14] and Spark Plasma Sintering (SPS) [15,16]. The results of ex periments suggest that the scintillation properties of the LSO:Ce ce ramics are comparable to that of their single crystal counterpart. However, achieving transparency in LSO:Ce ceramics remains a chal lenge due to their optical anisotropy deriving from their monoclinic crystal structure, especially for coarse-grained ceramics which further lead to low in-line transmittance [9,17]. According to Rayleigh-Gans-Debye (RGD) theory [18], the in-line transmittance of non-cubic ceramics can be elevated by suppressing their grain size. The grain size of optical ceramics can be suppressed to sub micrometer through pressure-assisted sintering technology. The resulting fully dense and fine-grained ceramics possesses good in-line transmission. This is the reason why many studies have recently been conducted on non-cubic ceramics fabricated via pressure-assisted sintering technology [19–22]. This work is devoted to the fabrication of fine-grained LYSO:Ce ceramics through the combination of nanosized powder processing and spark plasma sintering.
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Fan), [email protected] (Y. Wu), [email protected] (Y. Shi). https://doi.org/10.1016/j.optmat.2019.109563 Received 29 September 2019; Received in revised form 9 November 2019; Accepted 19 November 2019 0925-3467/© 2019 Published by Elsevier B.V.
Please cite this article as: Lingcong Fan, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109563
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2. Experimental
3. Results and discussion
Lu1.8Y0.2SiO5:Ce (LYSO:Ce) powder was synthesized via a sol-gel process. The Ce ion concentration relative to the sum of the Lu, Ce and Y ions is 0.5 mol%. LuCl3⋅6H2O (99.99%), YCl3⋅6H2O (99.99%), CeCl3⋅7H2O (99.9%) and tetraethyl orthosilicate (TEOS, AR) with the desired stoichiometric ratios were dispersed in isopropanol (AR). Pro pylene oxide (PPO, AR) with the mole ratio of rare earth elements to PPO of 1:20 was added in the mixture and mechanically stirred for 24 h resulting in a gel to be obtained. The gel was then dried at 80 � C for 24 h in air. The LYSO:Ce precursor was calcined at 1000 � C for 2 h in air. 4 g of LYSO:Ce powder was loaded in a graphite mold with a diameter of 18.75 mm. The LYSO:Ce powder was separated by graphite foil with BN spraying coatings from the mold. The LYSO:Ce sample was sintered in an FCT HP D 25 SPS furnace (FCT Systeme GmbH, Rauenstein, Germany), with a heating rate of 50 � C/min under a loading pressure of 100 MPa and dwelled at 1050–1200 � C for 15 min. The morphology of the LYSO:Ce powder was observed on a Scanning Electron Microscope (SEM, XL-30, Philips, Eindhoven, the Netherlands) and Transmission Electron Microscope (TEM, JEM-2010F, JEOL, Japan). The sintered LYSO:Ce ceramics was lapped and mirror-polished down to 1 mm. The X-ray diffraction (XRD) of the powder and ceramics was performed on a Bruker D2 Phaser diffractometer (Madison, WI, USA) using Cu Kα radiation. For SEM observation, small parts were cut from the polished LYSO:Ce ceramic. All of the cut LYSO:Ce ceramics were thermally etched in air for 10 h. The thermal etching temperatures of the LYSO:Ce ceramics sintered at 1050, 1150 and 1100 � C were 50 � C lower than their corresponding sintering temperature, respectively. The LYSO:Ce ceramics sintered at 1200 � C were thermally etched at 1100 � C. The microstructure of the etched LYSO:Ce ceramics was observed via SEM (FEI Quanta 200, Hillsboro, OR, USA). To eliminate oxygen va cancies, the LYSO:Ce ceramics sintered at 1050 � C were annealed at 950 � C for 40 h in air. The in-line transmittance of the LYSO:Ce ceramic was measured using a Hitachi U2910 UV–Vis–NIR spectrophotometer (Tokyo, Japan). The photoluminescence (PL) and Photoluminescence Excitation (PLE) spectra of the LYSO:Ce ceramics were obtained at room temperature via a Shimadzu RF-5301PC fluorescence spectrofluorom eter (Kyoto, Japan). The luminescence decay time of LYSO:Ce ceramics was measured on a spectrofluorometer (FLSP920, Edinburgh, Living ston, UK), and a deuterium lamp was used as the excitation source. The Cathodoluminescence (CL) studies were carried out using a GATAN Mono CL system in a field emission SEM (FEI Quanta 400 F, Hillsboro, OR, USA) with an accelerating voltage of 20 kV. The light yield of the LYSO:Ce ceramics was carried out using a Hamamatsu photomultiplier tube (PMT, 1306, Hamamatsu City, Japan) with a bi-alkali photo-cath ode and quartz window, under 662 keV γ-ray (137Cs source) excitation. All of the aforementioned measurements were performed at room temperature.
3.1. Microstructure and phase composition of LYSO:Ce powder In Fig. 1(a), the morphology of the LYSO:Ce powder calcined at 1000 C in air is displayed. The figure shows that the particles exhibit very little agglomeration and are nearly spherical and uniform. TEM image in Fig. 1(b) show that the average grain size of the LYSO:Ce powder is 70 nm. The Selected Area Electron Diffraction (SAED) pattern indicates a monoclinic crystal structure. Lutetiunm/yttrium oxyorthosilicate exists in two monoclinic phases; the metaphase crystallizes in space group P21/c, whereas the stable phase crystallizes in space group C2/c [23]. Phase evolution from powder to ceramics is presented in Fig. 2. The XRD patterns of the LYSO:Ce powder and the sintered ceramics are consistent with the standard data of the JCPDS card (PDF#97-008-9624 for pow der (metaphase) [24], PDF#97-015-9308 for ceramics (stable phase) [25,26]). No second phases can be observed in either pattern. This suggests that the LYSO:Ce powder transforms from metaphase, with a space group of P21/c, into stable phase, with a space group of C2/c, through sintering. This phase transformation from powder to ceramics also was observed in polycrystalline LSO:Ce [13]. The metaphase LYSO/LSO do not appear in phase diagrams since they are not formed at equilibrium. They are found in the wet chemical processes [27–29] rather than solid phase synthesis [30]. The phase-transformation tem perature from the space group of P21/c to C2/c for LYSO/LSO is in the
�
Fig. 2. XRD patterns of LYSO:Ce powder calcined at 1000 � C and ceramics sintered calcined at 1050 � C.
Fig. 1. SEM image (a) and TEM image (b) of LYSO:Ce powder. Inset in (b) is SAED pattern. 2
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range of 1000–1100 � C [27–29].
increasing average grain size (2r) from 210 to 2630 nm (n ¼ 1.808, Δn ¼ 0.028, t ¼ 1 mm). The in-line transmittance of the LYSO:Ce ceramics sintered at 1050 � C is 1.7% at 420 nm. This value is slightly greater (1.2–1.4%) than that of LSO:Ce ceramics [12,13,35], which is attributed to the smaller average grain size of LYSO:Ce ceramics resulting from a lower sintering temperature compared with that of LSO:Ce ceramics. The difference between the measured data and calculation result from the RGD model can attributed to very few micro pores in the LYSO:Ce ceramics. The RGD model is based on fully dense optical materials, whereas sintered ceramics always exists residual pores. The pores (even for the ceramics with porosity of 0.02%) can result in noteworthy scattering of light. At short wavelengths (<420 nm), since the pore size is too large compared with the wavelength, the contribution to the back scattering of light decreases and resultantly the transmission increases with decreasing wavelength [36].
3.2. Microstructure and optical property of LYSO:Ce ceramics Fig. 3 shows the thermally-etched surface micrograph of LYSO:Ce ceramics sintered at different temperatures under 100 MPa for 15 min by SPS. No residual pores and second phase can be observed in the ce ramics sintered at 1050 and 1100 � C. The sintering temperature to achieve pore-free LYSO:Ce ceramics is 300–650 � C lower than that of LSO:Ce ceramics sintered by SPS [16], HP [10,31] and HIP [11–13]. This can be attributed to the lower melting point of LYSO:Ce. A small amount of pores appear within the grains and in the triangular grain boundaries of the LYSO:Ce ceramics sintered at 1150 and 1200 � C. The pores entrapped within the grains indicate that the grain growth is faster than the pore diffusion at 1150/1200 � C under an applied pressure of 100 MPa. The grain size of the LYSO:Ce ceramics increases remarkably with the increasing SPS sintering temperature. The average grain sizes of the ceramics sintered at 1050 � C, 1100 � C, 1150 � C and 1200 � C are 0.21, 1.36, 2.01 and 2.63 μm, respectively. Some intergranular cracks were observed in the LYSO:Ce ceramics sintered at 1150 � C and 1200 � C. These cracks propagate from the grain boundary, or triple point, to the grain. Intergranular cracks were also observed in the hot-pressed LSO:Ce ceramics [12]. These cracks can result from the expansion of gas trapped in an isolated pore and/or residual stress due to anisotropy in coefficient of thermal expansion of LYSO [32,33]. The theoretical in-line transmittance of dense ceramics (no pores nor second phases) can be predicted through the Rayleighe-Ganse-Debye (RGD) light scattering theory [18]; Tth ¼ [16n2/(nþ1)4]exp [-3π2 (Δn)2rt/(n2λ2)], where Tth is the theoretical in-line transmittance of the ceramics, n is the average refractive index of uniaxial or biaxial crystal, Δn is the maximum difference between the refractive indices of the crystal, r is half of the average grain size of the ceramics, t is the thickness of the polycrystalline ceramics. LYSO possesses the same structure as biaxial crystal LSO. The maximum difference between the refractive indexes of LYSO is 0.028 at its maximum emission wavelength (420 nm) [34]. Optical anisotropy can result in light scattering when light passes through the grain boundary of polycrystalline ceramics and therefore gives rise to low in-line transmittance in polycrystalline LYSO: Ce ceramics. As shown in Fig. 4, the calculated in-line transmittance values (at 420 nm) of LYSO:Ce ceramics sharply decrease with
3.3. Luminescence properties and light yield of LYSO:Ce ceramics Stable-phase LYSO exists in two crystallographic sites; the RE1 site is coordinated by seven oxygen atoms (five silicon-bonded oxygens and two non-silicon-bonded oxygens), and the RE2 site is coordinated by six oxygen atoms (four silicon-bonded oxygens and two non-silicon-bonded oxygens) [26,27]. Cerium occupies both the RE1 and RE2 sites, so there are two emission centers in LYSO:Ce that are referred to as Ce1 and Ce2, respectively, which present different luminescence characteristics [37–39]. Fig. 5 shows the PL and PLE spectra of the as-sintered and annealed LYSO:Ce ceramics. The wavelengths were chosen to separate excitation and emission bands from Ce1 and Ce2 centers. The PLE spectrum recorded for the 400 nm emission display four excitation peaks centered at 228 nm (5.45 eV), 263 nm (4.73 eV), 302 nm (4.12 eV) and 365 nm (3.41 eV), which are attributed to the 4f→5d4, 5d3, 5d2, and 5d1 tran sitions of Ce1 [40,41], respectively. The 4f→5d2, and 5d1 transitions of Ce2 are responsible for the excitation peaks located at 325 nm (3.82eV) and 375 nm (3.31 eV) in the PLE spectrum recorded for the 500 nm emission [42]. The excitation band centered at 325 nm (3.82eV) in the PLE spectrum of the annealed LYSO:Ce ceramics became more over lapped with the 365 nm (3.41 eV) excitation band of Ce1 when compared with those of the as-sintered ceramics. The excitation band of Ce2 is overlapped with the emission band of Ce1 in the wavelength range from 365 to 410 nm, which indicates that the Ce1 emission in the high energy range can be reabsorbed by Ce2 [43], i.e. there is an energy transfer from Ce1 to Ce2 which occurred in the LYSO:Ce ceramics. This suggests that the energy transfer from Ce1 to Ce2 in LYSO:Ce ceramics is
Fig. 3. SEM micrographs of thermally etched surface of the LYSO:Ce ceramics sintered by SPS at (a) 1050 � C, (b) 1100 � C, (c) 1150 � C and (d) 1200 � C under 100 MPa applied pressure.
Fig. 4. In-line transmittance curves of LYSO:Ce ceramics sintered at 1050 � C and RGD model calculation (n ¼ 1.808, Δn ¼ 0.028, t ¼ 1 mm). 3
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Fig. 5. Normalized excitation (a) and emission (b) spectra of the as-sintered (top) and annealed (bottom) LYSO:Ce ceramics.
more efficient after undergoing annealing in air. This effect can be attributed to a smaller number of defects (oxygen vacancies) in the LYSO lattice, which therefore decreases the possibility that carriers were trapped. The PL spectra of the as-sintered LYSO:Ce ceramics presents a broad emission band, which can be decomposed into three peaks centered at 391 nm (3.18 eV), 425 nm (2.92 eV), and 472 nm (2.63 eV) through deconvolution in energy scale, which is in accord with the Gaussian decomposition results of PL spectra of LYSO:Ce single crystal [44]. The first two peaks originate from Ce1 emission (5d1→4F5/2 and 4 F7/2 transitions), and the third peak is mainly attributed to Ce2 emis sion. After annealing, the center of the Ce2 emission band shifted to 2.67 eV from 2.63 eV under 325 nm excitation, to 2.71 eV from 2.63 eV under 340 nm excitation, respectively. Recently, ab initio study in dicates that the low-energy emission band of LSO:Ce originates from the contribution of ten Ce3þ luminescent centers, one being CeLu2 and nine being different combinations of one substitutional Ce3þ ion and one neutral oxygen vacancy from a [SiO4] tetrahedral site (Ce-VO, five close to Ce1 and four close to Ce2) [45]. The spread related to multi-center luminescence give a good explanation for the absence of doublet structure of Ce2 emission from experiments in LSO:Ce [17,39], LYSO:Ce [40,44] and YSO [46]. Indeed, annealing in air can significantly lower the concentration of oxygen vacancies in LYSO:Ce ceramics fabricated from a strongly reduced atmosphere (carbon from graphite die), which can lower the emission from the combinations of one substitutional Ce3þ ion and one neutral oxygen vacancy in the low-energy range and lead to a blue shift of the low-energy emission band. The decay time of cerium-doped rare earth oxyorthosilicate is on the order of several tens of nanoseconds due to the spin-and-parity-allowed 5 d→4f transitions of Ce3þ. The decay time of Ce3þ is directly related to how long the electron stays in the excited state level of Ce3þ. An electron which stays in the excited state level of Ce3þ is readily captured by other centers (impurity ions, oxygen vacancies etc.) near the Ce3þ, which is a nonradiative process and is faster than radiative 5 d→4f transitions. Fig. 6 presents the decay curves of the as-sintered and annealed LYSO:Ce
ceramics under the excitation and emission wavelength to separate Ce1 and Ce2. The decay values of all the curves were achieved via single or two exponential fitting. The decay times of the as-sintered LYSO:Ce ceramics are 7 ns (77%) and 20 ns (23%) for Ce1 and 8 ns (80%), 23 ns (20%) for Ce2, respectively, while the values of the annealed LYSO:Ce ceramics are 5 ns (83%) and 24 ns (17%) for Ce1 and 36.5 ns for Ce2, respectively. The decay times of Ce1 and Ce2 of the annealed LYSO:Ce ceramics are slower than those of the as-sintered ceramics, especially for Ce2. This indicates that oxygen vacancies exist in both the as-sintered and annealed ceramics and concentrations of oxygen vacancies in LYSO:Ce ceramics can be dramatically lowered through annealing in air. These decay times of LYSO:Ce ceramics is much faster than those of LYSO:Ce single crystal [44], which can be attributed to thermally induced ionization of some excited Ce1/Ce2 center. The thermally induced ionization through the relaxation of 5d1 state can shorten the nanosecond decay time of LYSO:Ce. The band characteristics of CL spectra of the as-sintered and annealed LYSO:Ce ceramics in Fig. 7 is in accordance with that of the corresponding PL spectra in Fig. 5. The in tegrated CL intensity of the annealed ceramics reaches values 2.05 times that of the as-sintered ceramics. The 137Cs energy spectra in Fig. 8 in dicates that the light yield of the annealed LYSO:Ce ceramics is 23,600 photons/MeV, which is 2.46 times that of the as-sintered ceramics and is comparable with that of typical LYSO:Ce single crystals [47,48]; this is in accordance with the CL results. This high light yield and CL intensity of the annealed ceramics results from less oxygen vacancies in the ce ramics, which is favorable for radiative 5 d→4f transitions of Ce3þ. 4. Conclusion Phase-pure LYSO:Ce powder with an average grain size of 70 nm was synthesized via a sol-gel route. Starting from a synthesized nano powder, fine-grained LYSO:Ce ceramics were fabricated at 1050 � C under an applied pressure of 100 MPa via SPS. The average grain size of the LYSO: Ce ceramics is 210 nm. The in-line transmittance of the ceramics is 1.7% 4
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Fig. 6. Decay profiles of the as-sintered (a, b) and annealed (c, d) LYSO:Ce ceramics.
Fig. 8. Energy spectra of the as-sintered and annealed LYSO:Ce ceramics, the insets are photographs of the LYSO:Ce ceramics.
Fig. 7. CL spectra of the as-sintered and annealed LYSO:Ce ceramics.
at 420 nm. The PL, PLE and CL spectra indicate that oxygen vacancies in LYSO:Ce ceramics can be reduced effectively through annealing in air. The light yield of the annealed LYSO:Ce ceramics reaches 23,600 pho tons/MeV, which is comparable to that of a typical LYSO:Ce single crystal.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 5
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the work reported in this paper.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers: 51802186, U1732128 and 51172139) and Shanghai Young University Teachers’ Training Subsidy Scheme (ZZSD18008). The authors are grateful to Dr. X. D. Gao and Prof. X. H. Zeng at Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences for their help in CL measurement and fruitful discussion. Y. Q. Wu gratefully acknowledges NSF CAREER grant (1554094) for funding his research work. References [1] D.S. McGregor, Materials for gamma-ray spectrometers: inorganic scintillators, Annu. Rev. Mater. Res. 48 (2018) 245–277. [2] M. Nikl, A. Yoshikawa, Recent R&D trends in inorganic single-crystal scintillator materials for radiation detection, Adv. Opt. Mater. 3 (2015) 463–481. [3] C. Dujardin, E. Auffray, E. Bourret-Courchesne, P. Dorenbos, P. Lecoq, M. Nikl, A. N. Vasil’ev, A. Yoshikawa, R.Y. Zhu, Needs, trends, and advances in inorganic scintillators, IEEE Trans. Nucl. Sci. 65 (2018) 1977–1997. [4] D.W. Cooke, K.J. McClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R. E. Muenchausen, R.C. Sze, Crystal growth and optical characterization of ceriumdoped Lu1.8Y0.2SiO5, J. Appl. Phys. 88 (2000) 7360–7362. [5] J. Chen, R. Mao, L. Zhang, R.-Y. Zhu, Large size LSO and LYSO crystals for future high energy physics experiments, IEEE Trans. Nucl. Sci. 54 (2007) 718–724. [6] R. Mao, L. Zhang, R.-Y. Zhu, Emission spectra of LSO and LYSO crystals excited by UV light, X-ray and γ-ray, IEEE Trans. Nucl. Sci. 55 (2008) 1759–1766. [7] C.M. Pepin, P. Berard, A.L. Perrot, C. Pepin, D. Houde, R. Lecomte, C.L. Melcher, H. Dautet, Properties of LYSO and recent LSO scintillators for phoswich PET detectors, IEEE Trans. Nucl. Sci. 51 (2004) 789–795. [8] B. Hautefeuille, K. Lebbou, C. Dujardin, J. Fourmigue, L. Grosvalet, O. Tillement, C. Pedrini, Shaped crystal growth of Ce3þ-doped Lu2(1 x)Y2xSiO5 oxyorthosilicate for scintillator applications by pulling-down technique, J. Cryst. Growth 289 (2006) 172–177. [9] L. Fan, Y. Shi, J. Xu, J. Xie, F. Lei, Consolidation of translucent Ce3þ-doped Lu2SiO5 scintillation ceramics by pressureless sintering, J. Mater. Res. 29 (2014) 2252–2259. [10] A. Lempicki, C. Brecher, H. Lingertat, S.R. Miller, J. Glodo, V.K. Sarin, A ceramic version of the LSO scintillator, IEEE Trans. Nucl. Sci. 55 (2008) 1148–1151. [11] Y. Wang, E.v. Loef, W.H. Rhodes, J. Glodo, C. Brecher, L. Nguyen, A. Lempicki, G. Baldoni, W.M. Higgins, K.S. Shah, Lu2SiO5:Ce optical ceramic scintillator for PET, IEEE Trans. Nucl. Sci. 56 (2009) 887–891. [12] S. Roy, H. Lingertat, C. Brecher, V. Sarin, Optical properties of anisotropic polycrystalline Ce3þ activated LSO, Opt. Mater. 35 (2012) 827–832. [13] L. Fan, M. Jiang, D. Lin, D. Zhou, Y. Shi, Y. Wu, H. Yao, F. Xu, J. Xie, F. Lei, L. Zhang, J. Zhang, Densification of cerium-doped lutetium oxyorthosilicate scintillation ceramics by hot isostatic pressing, J. Alloy. Comp. 720 (2017) 161–168. [14] L.-C. FAN, Y. SHI, J.-J. XIE, Fabrication and luminescent property of polycrystalline cerium-doped lutetium oxyorthsilicate scintillation ceramics, J. Inorg. Mater. 33 (2018) 237–244. [15] T. LIN, Z.-B. XU, L.-Y. DENG, Y.-Y. REN, Y. SHI, J.-J. XIE, Spark plasma sintering of Ce3þ:Lu2SiO5 scintillation ceramics and its luminescent characteristics, J. Inorg. Mater. 26 (2011) 1210–1214. [16] J. Xie, Y. Shi, L. Fan, Z. Xu, Microstructure and luminescent properties of Ce: Lu2SiO5 ceramic scintillator by spark plasma sintering, Opt. Mater. 35 (2013) 744–747. [17] L. Fan, X. Zhang, D. Lin, Y. Shi, J. Zhang, J. Xie, F. Lei, L. Zhang, L. Chen, H. Yuan, Luminescence characteristics of Lu2SiO5:Ce3þ (LSO:Ce) ceramic scintillators under VUV–UV excitation, Nucl. Instrum. Methods Phys. Res., Sect. A 806 (2016) 325–329. [18] R. Apetz, M.P.B.v. Bruggen, Transparent alumina:A light scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486. [19] A. Krell, J. Klimke, T. Hutzler, Advanced spinel and sub-μm Al2O3 for transparent armour applications, J. Eur. Ceram. Soc. 29 (2009) 275–281. [20] N. Roussel, L. Lallemant, J.Y. Chane-Ching, S. Guillemet-Fristch, B. Durand, V. Garnier, G. Bonnefont, G. Fantozzi, L. Bonneau, S. Trombert, D. GarciaGutierrez, A. Krell, Highly dense, transparent α-Al2O3 ceramics from ultrafine nanoparticles via a standard SPS sintering, J. Am. Ceram. Soc. 96 (2013) 1039–1042. [21] B.-N. Kim, K. Hiraga, K. Morita, H. Yoshida, Spark plasma sintering of transparent alumina, Scr. Mater. 57 (2007) 607–610.
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