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Mechanical properties and machinability of GYGAG:Ce ceramic scintillators
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Mechanical properties and machinability of GYGAG:Ce ceramic scintillators Xuting Qiua,b, Zhaohua Luoa,∗, Jiyun Zhanga, Haochuan Jianga,∗∗, Jun Jianga,∗∗∗ a b
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: GYGAG:Ce ceramic scintillators Scintillator array Machinability
Cerium doped gadolinium yttrium gallium aluminum garnet ceramics, GYGAG:Ce for short, have drawn a lot of attention and showed a very promising application in medical CT detectors. This mainly attributes to their excellent scintillation properties, such as high light output, low afterglow, fast decay and high x-ray stopping power. In order to provide more accurate lesion location in real CT diagnosis, ceramic scintillators need to be machined into regular pixelated arrays, and the pixels should well match with the photodiode elements. Therefore, in addition to the scintillation characteristics, the mechanical properties and machinability of these ceramic scintillators should have been studied further, but such data and references are limited. In this research, we focused on the mechanical properties and the machinability behaviour of GYGAG:Ce ceramics. It was observed that GYGAG:Ce ceramics with grain size around 8 μm have the highest mechanical strength, and are highly suitable for machining process to achieve fine array pixels, for which a strict dimensional tolerance needs to be held.
1. Introduction Scintillation is the process that high energy rays or particles, such as X-ray and γ-rays, are converted to visible light. Materials that realize this conversion are called scintillators. For decades, scintillators have been widely used in high energy rays detection or medical imaging. In a medical computerized tomography (CT), the scintillator is a critical component in the detector which receives the X rays and provides scintillation light that can be converted to digital electronic signals further for the image reconstruction. There are mainly three commercial ceramic scintillators in the current medical CT systems around the world, namely HighlightTM, GemstoneTM and GOS [1–5]. Among them, HighlightTM and GemstoneTM are produced by GE but not sold separately. GOS is the only ceramic scintillator available for the rest of medical CT detectors on market. In recent years, cerium doped gadolinium gallium aluminum garnet scintillators, referred to as GGAG:Ce, have drawn much attention and showed potential of commercial applications, because of their outstanding characteristics, such as high density (6.63 g/cm3) [6], high light output (4,8000 photons/MeV) [7], low afterglow (< 0.005 @ 100 m s) [8] and fast decay (50 ns) [9]. However, the crystal structure of GGAG is thermodynamically metastable and is likely to decompose into second phases, perovskite, for example. In order to enhance the stability of its crystal structure, partial
substitution of Gd with smaller ions has been successfully used to achieve this goal [10,11]. For instance, the group from Lawrence Livermore National Laboratory have conducted a series of research on Y3+ doped GGAG:Ce ceramics and made their energy resolution as low as 3.5% at 662 keV [12–14]. In our previous work, we fabricated a series of GYGAG:Ce transparent ceramics with different Y doping level, and the highest light output of ceramic scintillators achieved 61,000 ± 1200 photons/MeV [15]. Additionally, the afterglow was 0.004% @ 100 m s and the scintillation decay was about 58 ns. With the improvement of powder synthesizing technology, the in-line transmittance of GYGAG:Ce ceramics was further improved to 81% [16]. All these unique characteristics make GYGAG:Ce ceramics suitable for nuclear medical imaging, such as CT and PET (Positron Emission Computed Tomography). It should be noted that, in real CT detector applications, scintillators have to be machined into arrays with fine pixelated structure. More importantly, to achieve an optical coupling with the rear photodiode with high accuracy, the dimensional tolerance for this pixelated structure is really tight. So, the mechanical properties and machinability behaviour are also important for CT scintillators. However, they have not been a frequent subject of studies. In this work, GYGAG:Ce powders were first synthesized by chemical co-precipitation method, and then sintered into dense ceramics in
∗
Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail address: [email protected] (Z. Luo). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.183 Received 2 September 2019; Received in revised form 8 October 2019; Accepted 20 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Xuting Qiu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.183
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oxygen atmosphere, followed by hot isostatic pressing. The 1 and 2 dimensional ceramic scintillator arrays were then produced by scribing. We will focus on the mechanical properties and their machinability of GYGAG:Ce ceramics, in order to acquire critical data to support the practical applications. 2. Experimental section 2.1. Synthesis The starting high-purity raw materials (Gd2O3, 5 N; Ga2O3, 5 N; Y2O3, 5 N) and reagents (NH4Al(SO4)2·12H2O, AR; Ce2(CO3)3·6H2O, 5 N) were first dissolved in hot nitric acid according to the formula of Ce0.03Y0.9Gd2.07Ga3Al2O12 (GYGAG:Ce) to obtain a salt solution with concentration of 0.3 mol/L. Ammonium hydrogen carbonate and ammonium hydroxide were dissolved in deionized water to make the precipitant solution with 2 mol/L. Then, the salt solutions were slowly dropped into the mixed precipitants under vigorous mechanical stirring. The process of co-precipitation was under ultrasonic condition, which was described in our previous works [11,15]. After precipitation, the precipitate was filtered, washed, dried and ground to pass through the 200-mesh screen. The precursor powder was calcined at 850 °C for 2 h. The powder was pressed in a steel die of 50 mm × 50 mm size, followed by cold isostatic pressing at 250 MPa. To prevent the evaporation of Ga at high temperature, the dense green bodies were sintered at 1550–1700 °C for 2 h in oxygen atmosphere followed by hot isostatic pressing and annealing treatment. To verify the machinability of GYGAG:Ce ceramics, linear and 2- dimensional ceramic arrays have been processed by dicing saw, respectively. The 2- dimensional array was composed of 16 × 32 rectangular pixels, and the size for each single pixel was 1 mm × 1 mm × 2 mm BaSO4 powder was mixed with epoxy resin and the mixture was filled into the gaps among pixels as the reflector.
Fig. 1. Influence of sintering temperature on the density and grain size of GYGAG:Ce ceramics.
by scanning electron microscopy (SEM, Hitachi, TM-1000). The photoluminescence emission and fluorescence decay curves were measured with a Horiba FL3-111 spectrometer. 3. Results and discussion 3.1. Mechanical properties The density and average grain size of GYGAG:Ce ceramics sintered at different temperatures were presented in Fig. 1. It shows that the density of the ceramics increased as the sintering temperature increased from 1550 °C to 1600 °C, and reached 99.2% of the theoretical value (6.30 g/cm3). The density change becomes less significant as the sintering temperature increased further. The average grain sizes of the ceramics increased gradually from 2.8 ± 0.6 μm for the sintering temperature of 1550 °C to 15.1 ± 3.7 μm for 1650 °C. When the sintering temperature increased to 1700 °C, the grain size grew significantly to 38.9 ± 7.1 μm. It should be noted that the grain size of GOS ceramic scintillators always vary from 30-40 μm to 80–100 μm [18,19] because of its tabular crystal structure. However, in order to possess good mechanical properties for equiaxed crystal, such as garnet scintillators, the grain sizes need to be kept much smaller and are usually around 6–8 μm. The fracture surface of GYGAG:Ce ceramics sintered at different temperatures are presented in Fig. 2. It can be observed that there are a few residual pores in samples after sintering at 1550 °C (Fig. 2a), which is in correspondence with its relatively low density shown in Fig. 1. Residual pores are known to be strong light-scattering centers for transparent ceramics and should be eliminated as much as possible. Evidently, the residual pores disappeared when the sintering temperature was above 1600 °C. In addition, it shows trans-granular fracture behaviour for samples sintered below 1600 °C with an average grain size of less than 10 μm (Fig. 2a and b). As the sintering temperature increased further, the grains became coarsening and the fracture behaviour turned into mixed mode (trans and inter-granular) at T = 1650 °C (Fig. 2c) and intergranular mode at T = 1700 °C (Fig. 2d). The change of fracture behaviour was probably due to the fact that the binding force among intergranular boundaries became weak because of high sintering temperature, and subsequently, cracks preferred to propagate intergranularly [20]. It is well known that the grain size G is an important factor that determines the strength and reliability of polycrystalline materials [21]. For cubic materials, the average grain size is usually taken into consideration. The 3-point flexural strength as a function of average grain
2.2. Characterization Samples for mechanical measurements were cut and machined to the size of 3 mm × 4 mm × 36 mm, while those for spectrum testing were cut and machined to 10 mm × 10 mm × 2 mm and double-side polished. The densities of the sintered samples were measured by Archimedean method. The 3-point flexure strength of the sintered specimens was tested on an Instron 5566 testing system with the span width and the crosshead speed of 20 mm and 0.5 mm/min, respectively. There were at least five specimens for each batch of flexure strength test. The Young's Modulus E for all these samples could also be calculated according to the following equation:
E=
L3 D 4BH3 ( P
D0 ) P0
−
(1)
where L is the span width, B is the width thickness of sample, H is the thickness of sample, P is the load, D is the displacement, P0 and D0 stand for the adjusted values for load and displacement, respectively. The Vickers hardness and fracture toughness were calculated based on indentation test with a load of 1 kg for 10 s on a Wilson-wolpert Tukon-2100B Vickers hardness tester. The apparent fracture toughness KIC was calculated from the following equation [17]: −3
KIC =
P (πc ) tgβ
2
(2)
where c is half length of the radial cracks, β is slant angle of the indenter and equals to 68° in this measurement. The specimens for machining property testing were first incised on a scriber with the blade width of 100 μm, and the condition of kerfs were examined on a confocal laser scanning microscope (CLSM). The microstructures of cross sections of the sintered ceramics were characterized 2
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Fig. 2. Microstructure of cross sections of GYGAG:Ce ceramics after sintering at different temperatures.
correlation between the flexure strength σ and G−1/2 follows well with the Hall-Petch relation with a slope of 0.46 MPa m−1/2. The slope is close to that of MgAl2O4 and the finer G branch of Al2O3 [22]. Based on a lot of references, Rice tried to work out the relationship between the mechanical strength and the grain size, and concluded that the dσ/d (G−1/2) slope would be less than the fracture toughness [21]. In this work, the dσ/d(G−1/2) slope is actually less than the fracture toughness of GYGAG ceramics which is about 1.0 MPa m1/2 (Table 1). It should be noted that, because of its incomplete densification, the sample with the finest grain size in Fig. 3 is an outlier. In addition, the flexure strength of the GYGAG:Ce in this work was slightly higher than those of YAG:Nd listed in Ref. [23]. The Young's modulus for all these samples were in the range of 194–229 GPa. The Vickers indentation and the crack propagation under a load of 1 kg on the GYGAG:Ce ceramics with different grain sizes are displayed in Fig. 4. The corresponding Vickers hardness as a function of average grain sizes is also presented in Fig. 4. The indentation marks are clear and the micro-cracks emanated from each corners of the indentations. It is worth mentioning that, the Vicker's hardness and the fractural toughness for ceramics with grain size larger than 38.9 μm were not available because of the presence of spalling and crack deflection. The Vickers hardness of GYGAG ceramics decreased when the average grain size increased from 2.8 μm to 15.1 μm (Fig. 4e). This might be due to the increase of grain size and resulting decrease of microplastic deformation. It is worth to note that, the hardness values of these samples also satisfy the Hall-Patch relation with a slope of 0.62 (Fig. 4e). The Vickers hardness for GYGAG:Ce ceramics is comparable to those of YAG
Fig. 3. 3-Points flexure strength as a function of grain size.
size is presented in Fig. 3. It shows that the flexure strength of ceramics is strongly affected by the grain size. The highest average strength was determined to be 263.0 ± 45.5 MPa when G = 8.4 ± 3.1 μm. As the grain size increased from 8.4 ± 3.0 μm to 38.9 ± 7.1 μm, the flexure strength deceased by 32% and reduced to 177.6 ± 30.1 MPa. The Table 1 Mechanical properties of GYGAG:Ce ceramics sintered from 1550 °C to 1700 °C. Sintering Temperature (oC)
Average grain size (μm)
Bending Strength (MPa)
Young's Modulus (GPa)
Vickers Hardness (GPa)
Fracture Toughness (MPa·m1/2)
1550 1600 1650 1700
2.8 ± 0.6 8.4 ± 3.1 15.1 ± 3.7 38.9 ± 7.1
233.4 263.0 223.2 177.6
200.6 218.0 228.6 193.9
12.1 ± 0.2 12.0 ± 0.1 11.9 ± 0.1 –
1.01 ± 0.00 1.08 ± 0.04 1.04 ± 0.02 –
± ± ± ±
26.3 45.5 32.6 30.1
3
± ± ± ±
9.4 6.0 8.0 11.6
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Fig. 4. The Vickers indentation marks under a 1 kg load, for GYGAG:Ce ceramics with different average grain sizes 2.8 μm (a), 8.4 μm (b), 15.1 μm (c), 38.9 μm (d), and the Vickers hardness as a function of average grain size G (e).
ceramics measured by other researchers [23,24]. However, the fractural toughness is insensitive to grain size and is much lower than that of YAG ceramics in the current studies [23,25,26]. All the mechanical properties are summarized in Table I. 3.2. Machinability and arrays To identify the machinability of GYGAG:Ce ceramic scintillators with different grain sizes, the ceramics were scribed into arrays with the blade thickness of about 100 μm. Fig. 5 presents the picture of the linear ceramic arrays after scribing. Macroscopically the arrays remained intact and the kerfs were regular. If we take a closer look at the condition of the kerf under a confocal laser scanning microscope, it can be found that some differences still exist (Fig. 6). In general, the kerf widths were slightly wider than 100 μm and exhibited uniformity for the ceramics tested in this work. The quality of machining is similar with the condition of cross sections presented in Fig. 2, and it is also highly dependent on the grain size of ceramics. Ceramics with an average grain size of around 8 μm show the optimal cutting quality, which is flat and smooth. This unique quality would enable the GYGAG:Ce ceramics to be made into smaller pixel size, which would be beneficial for the spatial resolution of CT imaging. When the grain size overgrew to above 38 μm, damage starts to appear at both edges of the kerf (Fig. 6d), which should be avoided in real application. In order to investigate the machinability of GYGAG:Ce ceramics
Fig. 6. The CLSM micrograph of the kerf for ceramic with different grain sizes 2.8 μm (a), 8.4 μm (b), 15.1 μm (c) and 38.9 μm (d), respectively.
Fig. 7. Photo of 16 × 32 pixelate 2-dimensional GYGAG:Ce ceramic scintillator array made of 1 mm × 1 mm pixels.
further, we processed these ceramics into 2-dimensional arrays and filled the kerfs with previously described reflector mixture (Fig. 7). The original ceramic wafer was sintered at 1600 °C for 2 h and the average grain size is about 8.4 μm. The array was made up of 16 × 32 pixels,
Fig. 5. Picture of the linear ceramic arrays after cutting. 4
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emission intensity might benefit from the increase of density and the improvement of transmittance of ceramics, while the decrease would be related to increase of grain boundary scattering. The normalized fluorescence decay profiles of GYGAG:Ce ceramics by monitoring the 5d1-4f emission (λem = 547 nm) under the 435 nm excitation are presented in Fig. 9. It appears that all samples tested have similar decay behaviour, which indicates that the ceramic scintillators have uniform activator concentration and microstructure. These decay curves can be well fitted by a double-exponential function. The fast and slow components of the decay time are 13 ns and 50 ns, respectively. The slow decay component is in reasonable agreement with the intrinsic 5d-4f transition of Ce3+ ions in garnet hosts. The fast decay component at the beginning of the curves might be caused by the nonradiative transfer from Ce3+ ions to the impurity or lattice defects. The primary speed of a scintillator is affected by both decay and rise time, but it is mainly determined by the decay time of the emission. This is because of that the rise time for most scintillators is very short and usually in range of a few nanoseconds. It is well known that the kinetic process is different between scintillation and luminescence, and the rise time of scintillation is longer than that of photoluminescence. However, the scintillation decay of Ce3+ in the same host would be the same as that of the photoluminescence decay. It is reasonable to say that the primary decay speed of GYGAG:Ce ceramic scintillators is fast enough to satisfy the demand of the modern medical CT detectors.
Fig. 8. Photoluminescence spectra of GYGAG:Ce ceramic scintillators.
4. Conclusions In order to provide insight into the scintillator array manufacturing and facilitate practical application for GYGAG:Ce ceramics, we studied the mechanical properties and machinability behaviour of these materials. It was found that the ceramics sintered at 1600 °C with an average grain size of around 8.4 μm have optimum overall performances for mechanical processing. The flexure strength, Young's modulus, Vickers hardness and fracture toughness were 263.0 ± 45.5 MPa, 218.0 ± 6.0 GPa, 12.0 ± 0.1 GPa and 1.08 ± 0.04 MPa m1/2, respectively. After dicing, the kerfs of the linear ceramic array exhibited good flatness and surface finishing. In addition, for the 2-dimensional array filled up with reflector, all pixels are near-perfectly square and uniform in dimensions, while the surface of the array remains very flat. Acknowledgement
Fig. 9. Fluorescence decay curves of Ce3+ in GYGAG ceramic scintillators.
This work is financially supported by National Key Research and Development Program of China (2016YFC0104502, 2017YFC0111602), National Natural Science Foundation of China (51672286), Zhejiang Provincial Natural Science Foundation of China (LY17E020011), Youth Innovation Promotion Association of Chinese of Chinese Academy of Science (2016271), Fujian Institute of Innovation, Chinese Academy of Sciences (FJCXY18040203).
and the size for each pixel was 1 mm × 1 mm, while the width of each kerf was 100 μm. The pixels of the arrays appear to be square with uniform dimensions. The reflector lines are straight and uniform. The main function of the reflectors is to absorb and reflect the scintillation light that generated from very single element, and reduce the crosstalk (due to light transmission between pixels) as much as possible. Otherwise, the crosstalk between pixels will dramatically reduce the spatial resolution of the system. Additionally, the surface of the 2-dimensional scintillator array remains very flat, indicating that the ceramic was sufficiently strong to withstand the shrinkage stress caused by the curing of the epoxy resin.
References [1] W. Rossner, B.C. Grabmaier, Phosphors for X-ray detectors in computed tomography, J. Lumin. 48–9 (1991) 29–36. [2] C. Greskovich, S. Duclos, Ceramic scintillators, Annu. Rev. Mater. Sci. 27 (1997) 69–88. [3] N.J. Cherepy, J.D. Kuntz, J.J. Roberts, T.A. Hurst, O.B. Drury, R.D. Sanner, T.M. Tillotson, S.A. Payne, Transparent ceramic scintillator fabrication, properties and applications, in: A. Burger, L.A. Franks, R.B. James (Eds.) Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X2008. [4] L. Wang, B. Liu, X.-w. Wu, J. Wang, Y. Zhou, W.-q. Wang, X.-h. Zhu, Y.-q. Yu, X.h. Li, S. Zhang, Y. Shen, Correlation between CT attenuation value and iodine concentration in vitro: discrepancy between gemstone spectral imaging on singlesource dual-energy CT and traditional polychromatic X-ray imaging, J. Med. Imaging Radiat. Oncol. 56 (2012) 379–383. [5] R. Nakamura, Improvements in the X-ray characteristics of Gd2O2S:Pr ceramic scintillators, J. Am. Ceram. Soc. 82 (1999) 2407–2410. [6] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl, Y. Fujimoto, A. Fukabori, A. Yoshikawa, 2 inch diameter single crystal growth and scintillation properties of Ce:Gd3Al2Ga3O12, J. Cryst. Growth 352 (2012) 88–90.
3.3. Luminescence performances The photoluminescence (PL) spectra excited at 435 nm for samples sintered at different temperatures are presented in Fig. 8. The peaks for all PL spectra were located at around 547 nm, which corresponds to the typical de-excitation of Ce3+ from 5d1 to 4f [27]. The consistency of the shape and peak of all the emission spectra indicates that the coordination environment around Ce3+ ions did not change as the sintering temperature changed. However, the emission intensity increases with the increase of temperature from 1550 °C to 1650 °C and decreases when the sintering temperature increased to 1700 °C. The increase of 5
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precipitation, Ultrason. Sonochem. 39 (2017) 792–797. [17] A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. [18] E.I. Gorokhova, V.A. Demidenko, S.B. Eron'ko, E.A. Oreshchenko, P.A. Rodnyi, S.B. Mikhrin, Luminescence and scintillation properties of Gd2O2S:Eu optical ceramic, J. Opt. Technol. 77 (2010) 50–58. [19] Y. Ito, H. Yamada, M. Yoshida, H. Fujii, G. Toda, H. Takeuchi, Y. Tsukuda, Hot isostatic pressed Gd2O2S:Pr,Ce,F translucent scintillator ceramics for X-Ray computed tomography detectors, Jpn. J. Appl. Phys 2-Lett. Exp. Lett. 27 (1988) L1371–L1373. [20] M. Boniecki, Z. Librant, A. Wajler, W. Wesolowski, H. Weglarz, Fracture toughness, strength and creep of transparent ceramics at high temperature, Ceram. Int. 38 (2012) 4517–4524. [21] R.W. Rice, Ceramic tensile strength grain size relations: grain sizes, slopes, and branch intersections, J. Mater. Sci. 32 (1997) 1673–1692. [22] M. Sokol, S. Kalabukhov, R. Shneck, E. Zaretsky, N. Frage, Effect of grain size on the static and dynamic mechanical properties of magnesium aluminate spinel (MgAl2O4), J. Eur. Ceram. Soc. 37 (2017) 3417–3424. [23] J. Li, Y.S. Wu, Y.B. Pan, W.B. Liu, L.P. Huang, J.K. Guo, Fabrication, Microstructure and properties of highly transparent Nd:YAG laser ceramics, Opt. Mater. 31 (2008) 6–17. [24] S. Nakayama, A. Ikesue, M. Sakamoto, Preparation of transparent YAG ceramic and its application to window material of infrared spectrophotometer, Nippon Kagaku Kaishi (2000) 437–440. [25] L. Mezeix, D.J. Green, Comparison of the mechanical properties of single crystal and polycrystalline yttrium aluminum garnet, Int. J. Appl. Ceram. Technol. 3 (2006) 166–176. [26] T.I. Mah, T.A. Parthasarathy, H.D. Lee, Polycrystalline YAG; structural or functional? J. Ceram. Process. Res. 5 (2004) 369–379. [27] Z. Luo, H. Jiang, J. Jiang, R. Mao, Microstructure and optical characteristics of Ce:Gd3(Ga,Al)5O12 ceramic for scintillator application, Ceram. Int. 41 (2015) 873–876.
[7] Y.T. Wu, Z.H. Luo, H.C. Jiang, F. Meng, M. Koschan, C.L. Melcher, Single crystal and optical ceramic multicomponent garnet scintillators: a comparative study, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 780 (2015) 45–50. [8] T. Kanai, M. Satoh, I. Miura, Characteristics of a nonstoichiometric Gd3+δ(Al,Ga)5δO12:Ce garnet scintillator, J. Am. Ceram. Soc. 91 (2008) 456–462. [9] M. Mori, J. Xu, G. Okada, T. Yanagida, J. Ueda, S. Tanabe, Comparative study of optical and scintillation properties of Ce:YAGG, Ce:GAGG and Ce:LuAGG transparent ceramics, J. Ceram. Soc. Jpn. 124 (2016) 569–573. [10] W. Chewpraditkul, N. Pattanaboonmee, W. Chewpraditkul, K. Kamada, A. Yoshikawa, M. Nikl, Luminescence and scintillation response of YGd2Al2Ga3O12:Ce and LuGd2Al2Ga3O12:Ce scintillators, Radiat. Meas. 90 (2016) 153–156. [11] Z.-H. Luo, Y.-F. Liu, C.-H. Zhang, J.-X. Zhang, H.-M. Qin, H.-C. Jiang, J. Jiang, Effect of Yb3+ on the crystal structural modification and photoluminescence properties of GGAG:Ce3+, Inorg. Chem. 55 (2016) 3040–3046. [12] N.J. Cherepy, Z.M. Seeley, S.A. Payne, P.R. Beck, E.L. Swanberg, S. Hunter, L. Ahle, S.E. Fisher, C. Melcher, H. Wei, T. Stefanik, Y.S. Chung, J. Kindem, High Energy Resolution Transparent Ceramic Garnet Scintillators, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics Xvi, (2014), p. 9213. [13] Z.M. Seeley, N.J. Cherepy, S.A. Payne, Homogeneity of Gd-based garnet transparent ceramic scintillators for gamma spectroscopy, J. Cryst. Growth 379 (2013) 79–83. [14] N.J. Cherepy, Z.M. Seeley, S.A. Payne, P.R. Beck, O.B. Drury, S.P. O'Neal, K.M. Figueroa, S. Hunter, L. Ahle, P.A. Thelin, T. Stefanik, J. Kindem, Development of transparent ceramic Ce-doped gadolinium garnet gamma spectrometers, IEEE Trans. Nucl. Sci. 60 (2013) 2330–2335. [15] J.Y. Zhang, Z.H. Luo, Y.F. Liu, H.C. Jiang, J. Jiang, G.Q. Liu, J.X. Zhang, H.M. Qin, Cation-substitution induced stable GGAG:Ce3+ ceramics with improved optical and scintillation properties, J. Eur. Ceram. Soc. 37 (2017) 4925–4930. [16] J.Y. Zhang, Z.H. Luo, H.C. Jiang, J. Jiang, C.H. Chen, J.X. Zhang, Z.Z. Gui, N. Xiao, Highly transparent cerium doped gadolinium gallium aluminum garnet ceramic prepared with precursors fabricated by ultrasonic enhanced chemical co-
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Mechanical properties and machinability of GYGAG:Ce ceramic scintillators Xuting Qiua,b, Zhaohua Luoa,∗, Jiyun Zhanga, Haochuan Jianga,∗∗, Jun Jianga,∗∗∗ a b
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: GYGAG:Ce ceramic scintillators Scintillator array Machinability
Cerium doped gadolinium yttrium gallium aluminum garnet ceramics, GYGAG:Ce for short, have drawn a lot of attention and showed a very promising application in medical CT detectors. This mainly attributes to their excellent scintillation properties, such as high light output, low afterglow, fast decay and high x-ray stopping power. In order to provide more accurate lesion location in real CT diagnosis, ceramic scintillators need to be machined into regular pixelated arrays, and the pixels should well match with the photodiode elements. Therefore, in addition to the scintillation characteristics, the mechanical properties and machinability of these ceramic scintillators should have been studied further, but such data and references are limited. In this research, we focused on the mechanical properties and the machinability behaviour of GYGAG:Ce ceramics. It was observed that GYGAG:Ce ceramics with grain size around 8 μm have the highest mechanical strength, and are highly suitable for machining process to achieve fine array pixels, for which a strict dimensional tolerance needs to be held.
1. Introduction Scintillation is the process that high energy rays or particles, such as X-ray and γ-rays, are converted to visible light. Materials that realize this conversion are called scintillators. For decades, scintillators have been widely used in high energy rays detection or medical imaging. In a medical computerized tomography (CT), the scintillator is a critical component in the detector which receives the X rays and provides scintillation light that can be converted to digital electronic signals further for the image reconstruction. There are mainly three commercial ceramic scintillators in the current medical CT systems around the world, namely HighlightTM, GemstoneTM and GOS [1–5]. Among them, HighlightTM and GemstoneTM are produced by GE but not sold separately. GOS is the only ceramic scintillator available for the rest of medical CT detectors on market. In recent years, cerium doped gadolinium gallium aluminum garnet scintillators, referred to as GGAG:Ce, have drawn much attention and showed potential of commercial applications, because of their outstanding characteristics, such as high density (6.63 g/cm3) [6], high light output (4,8000 photons/MeV) [7], low afterglow (< 0.005 @ 100 m s) [8] and fast decay (50 ns) [9]. However, the crystal structure of GGAG is thermodynamically metastable and is likely to decompose into second phases, perovskite, for example. In order to enhance the stability of its crystal structure, partial
substitution of Gd with smaller ions has been successfully used to achieve this goal [10,11]. For instance, the group from Lawrence Livermore National Laboratory have conducted a series of research on Y3+ doped GGAG:Ce ceramics and made their energy resolution as low as 3.5% at 662 keV [12–14]. In our previous work, we fabricated a series of GYGAG:Ce transparent ceramics with different Y doping level, and the highest light output of ceramic scintillators achieved 61,000 ± 1200 photons/MeV [15]. Additionally, the afterglow was 0.004% @ 100 m s and the scintillation decay was about 58 ns. With the improvement of powder synthesizing technology, the in-line transmittance of GYGAG:Ce ceramics was further improved to 81% [16]. All these unique characteristics make GYGAG:Ce ceramics suitable for nuclear medical imaging, such as CT and PET (Positron Emission Computed Tomography). It should be noted that, in real CT detector applications, scintillators have to be machined into arrays with fine pixelated structure. More importantly, to achieve an optical coupling with the rear photodiode with high accuracy, the dimensional tolerance for this pixelated structure is really tight. So, the mechanical properties and machinability behaviour are also important for CT scintillators. However, they have not been a frequent subject of studies. In this work, GYGAG:Ce powders were first synthesized by chemical co-precipitation method, and then sintered into dense ceramics in
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Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail address: [email protected] (Z. Luo). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.183 Received 2 September 2019; Received in revised form 8 October 2019; Accepted 20 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Xuting Qiu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.183
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oxygen atmosphere, followed by hot isostatic pressing. The 1 and 2 dimensional ceramic scintillator arrays were then produced by scribing. We will focus on the mechanical properties and their machinability of GYGAG:Ce ceramics, in order to acquire critical data to support the practical applications. 2. Experimental section 2.1. Synthesis The starting high-purity raw materials (Gd2O3, 5 N; Ga2O3, 5 N; Y2O3, 5 N) and reagents (NH4Al(SO4)2·12H2O, AR; Ce2(CO3)3·6H2O, 5 N) were first dissolved in hot nitric acid according to the formula of Ce0.03Y0.9Gd2.07Ga3Al2O12 (GYGAG:Ce) to obtain a salt solution with concentration of 0.3 mol/L. Ammonium hydrogen carbonate and ammonium hydroxide were dissolved in deionized water to make the precipitant solution with 2 mol/L. Then, the salt solutions were slowly dropped into the mixed precipitants under vigorous mechanical stirring. The process of co-precipitation was under ultrasonic condition, which was described in our previous works [11,15]. After precipitation, the precipitate was filtered, washed, dried and ground to pass through the 200-mesh screen. The precursor powder was calcined at 850 °C for 2 h. The powder was pressed in a steel die of 50 mm × 50 mm size, followed by cold isostatic pressing at 250 MPa. To prevent the evaporation of Ga at high temperature, the dense green bodies were sintered at 1550–1700 °C for 2 h in oxygen atmosphere followed by hot isostatic pressing and annealing treatment. To verify the machinability of GYGAG:Ce ceramics, linear and 2- dimensional ceramic arrays have been processed by dicing saw, respectively. The 2- dimensional array was composed of 16 × 32 rectangular pixels, and the size for each single pixel was 1 mm × 1 mm × 2 mm BaSO4 powder was mixed with epoxy resin and the mixture was filled into the gaps among pixels as the reflector.
Fig. 1. Influence of sintering temperature on the density and grain size of GYGAG:Ce ceramics.
by scanning electron microscopy (SEM, Hitachi, TM-1000). The photoluminescence emission and fluorescence decay curves were measured with a Horiba FL3-111 spectrometer. 3. Results and discussion 3.1. Mechanical properties The density and average grain size of GYGAG:Ce ceramics sintered at different temperatures were presented in Fig. 1. It shows that the density of the ceramics increased as the sintering temperature increased from 1550 °C to 1600 °C, and reached 99.2% of the theoretical value (6.30 g/cm3). The density change becomes less significant as the sintering temperature increased further. The average grain sizes of the ceramics increased gradually from 2.8 ± 0.6 μm for the sintering temperature of 1550 °C to 15.1 ± 3.7 μm for 1650 °C. When the sintering temperature increased to 1700 °C, the grain size grew significantly to 38.9 ± 7.1 μm. It should be noted that the grain size of GOS ceramic scintillators always vary from 30-40 μm to 80–100 μm [18,19] because of its tabular crystal structure. However, in order to possess good mechanical properties for equiaxed crystal, such as garnet scintillators, the grain sizes need to be kept much smaller and are usually around 6–8 μm. The fracture surface of GYGAG:Ce ceramics sintered at different temperatures are presented in Fig. 2. It can be observed that there are a few residual pores in samples after sintering at 1550 °C (Fig. 2a), which is in correspondence with its relatively low density shown in Fig. 1. Residual pores are known to be strong light-scattering centers for transparent ceramics and should be eliminated as much as possible. Evidently, the residual pores disappeared when the sintering temperature was above 1600 °C. In addition, it shows trans-granular fracture behaviour for samples sintered below 1600 °C with an average grain size of less than 10 μm (Fig. 2a and b). As the sintering temperature increased further, the grains became coarsening and the fracture behaviour turned into mixed mode (trans and inter-granular) at T = 1650 °C (Fig. 2c) and intergranular mode at T = 1700 °C (Fig. 2d). The change of fracture behaviour was probably due to the fact that the binding force among intergranular boundaries became weak because of high sintering temperature, and subsequently, cracks preferred to propagate intergranularly [20]. It is well known that the grain size G is an important factor that determines the strength and reliability of polycrystalline materials [21]. For cubic materials, the average grain size is usually taken into consideration. The 3-point flexural strength as a function of average grain
2.2. Characterization Samples for mechanical measurements were cut and machined to the size of 3 mm × 4 mm × 36 mm, while those for spectrum testing were cut and machined to 10 mm × 10 mm × 2 mm and double-side polished. The densities of the sintered samples were measured by Archimedean method. The 3-point flexure strength of the sintered specimens was tested on an Instron 5566 testing system with the span width and the crosshead speed of 20 mm and 0.5 mm/min, respectively. There were at least five specimens for each batch of flexure strength test. The Young's Modulus E for all these samples could also be calculated according to the following equation:
E=
L3 D 4BH3 ( P
D0 ) P0
−
(1)
where L is the span width, B is the width thickness of sample, H is the thickness of sample, P is the load, D is the displacement, P0 and D0 stand for the adjusted values for load and displacement, respectively. The Vickers hardness and fracture toughness were calculated based on indentation test with a load of 1 kg for 10 s on a Wilson-wolpert Tukon-2100B Vickers hardness tester. The apparent fracture toughness KIC was calculated from the following equation [17]: −3
KIC =
P (πc ) tgβ
2
(2)
where c is half length of the radial cracks, β is slant angle of the indenter and equals to 68° in this measurement. The specimens for machining property testing were first incised on a scriber with the blade width of 100 μm, and the condition of kerfs were examined on a confocal laser scanning microscope (CLSM). The microstructures of cross sections of the sintered ceramics were characterized 2
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Fig. 2. Microstructure of cross sections of GYGAG:Ce ceramics after sintering at different temperatures.
correlation between the flexure strength σ and G−1/2 follows well with the Hall-Petch relation with a slope of 0.46 MPa m−1/2. The slope is close to that of MgAl2O4 and the finer G branch of Al2O3 [22]. Based on a lot of references, Rice tried to work out the relationship between the mechanical strength and the grain size, and concluded that the dσ/d (G−1/2) slope would be less than the fracture toughness [21]. In this work, the dσ/d(G−1/2) slope is actually less than the fracture toughness of GYGAG ceramics which is about 1.0 MPa m1/2 (Table 1). It should be noted that, because of its incomplete densification, the sample with the finest grain size in Fig. 3 is an outlier. In addition, the flexure strength of the GYGAG:Ce in this work was slightly higher than those of YAG:Nd listed in Ref. [23]. The Young's modulus for all these samples were in the range of 194–229 GPa. The Vickers indentation and the crack propagation under a load of 1 kg on the GYGAG:Ce ceramics with different grain sizes are displayed in Fig. 4. The corresponding Vickers hardness as a function of average grain sizes is also presented in Fig. 4. The indentation marks are clear and the micro-cracks emanated from each corners of the indentations. It is worth mentioning that, the Vicker's hardness and the fractural toughness for ceramics with grain size larger than 38.9 μm were not available because of the presence of spalling and crack deflection. The Vickers hardness of GYGAG ceramics decreased when the average grain size increased from 2.8 μm to 15.1 μm (Fig. 4e). This might be due to the increase of grain size and resulting decrease of microplastic deformation. It is worth to note that, the hardness values of these samples also satisfy the Hall-Patch relation with a slope of 0.62 (Fig. 4e). The Vickers hardness for GYGAG:Ce ceramics is comparable to those of YAG
Fig. 3. 3-Points flexure strength as a function of grain size.
size is presented in Fig. 3. It shows that the flexure strength of ceramics is strongly affected by the grain size. The highest average strength was determined to be 263.0 ± 45.5 MPa when G = 8.4 ± 3.1 μm. As the grain size increased from 8.4 ± 3.0 μm to 38.9 ± 7.1 μm, the flexure strength deceased by 32% and reduced to 177.6 ± 30.1 MPa. The Table 1 Mechanical properties of GYGAG:Ce ceramics sintered from 1550 °C to 1700 °C. Sintering Temperature (oC)
Average grain size (μm)
Bending Strength (MPa)
Young's Modulus (GPa)
Vickers Hardness (GPa)
Fracture Toughness (MPa·m1/2)
1550 1600 1650 1700
2.8 ± 0.6 8.4 ± 3.1 15.1 ± 3.7 38.9 ± 7.1
233.4 263.0 223.2 177.6
200.6 218.0 228.6 193.9
12.1 ± 0.2 12.0 ± 0.1 11.9 ± 0.1 –
1.01 ± 0.00 1.08 ± 0.04 1.04 ± 0.02 –
± ± ± ±
26.3 45.5 32.6 30.1
3
± ± ± ±
9.4 6.0 8.0 11.6
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Fig. 4. The Vickers indentation marks under a 1 kg load, for GYGAG:Ce ceramics with different average grain sizes 2.8 μm (a), 8.4 μm (b), 15.1 μm (c), 38.9 μm (d), and the Vickers hardness as a function of average grain size G (e).
ceramics measured by other researchers [23,24]. However, the fractural toughness is insensitive to grain size and is much lower than that of YAG ceramics in the current studies [23,25,26]. All the mechanical properties are summarized in Table I. 3.2. Machinability and arrays To identify the machinability of GYGAG:Ce ceramic scintillators with different grain sizes, the ceramics were scribed into arrays with the blade thickness of about 100 μm. Fig. 5 presents the picture of the linear ceramic arrays after scribing. Macroscopically the arrays remained intact and the kerfs were regular. If we take a closer look at the condition of the kerf under a confocal laser scanning microscope, it can be found that some differences still exist (Fig. 6). In general, the kerf widths were slightly wider than 100 μm and exhibited uniformity for the ceramics tested in this work. The quality of machining is similar with the condition of cross sections presented in Fig. 2, and it is also highly dependent on the grain size of ceramics. Ceramics with an average grain size of around 8 μm show the optimal cutting quality, which is flat and smooth. This unique quality would enable the GYGAG:Ce ceramics to be made into smaller pixel size, which would be beneficial for the spatial resolution of CT imaging. When the grain size overgrew to above 38 μm, damage starts to appear at both edges of the kerf (Fig. 6d), which should be avoided in real application. In order to investigate the machinability of GYGAG:Ce ceramics
Fig. 6. The CLSM micrograph of the kerf for ceramic with different grain sizes 2.8 μm (a), 8.4 μm (b), 15.1 μm (c) and 38.9 μm (d), respectively.
Fig. 7. Photo of 16 × 32 pixelate 2-dimensional GYGAG:Ce ceramic scintillator array made of 1 mm × 1 mm pixels.
further, we processed these ceramics into 2-dimensional arrays and filled the kerfs with previously described reflector mixture (Fig. 7). The original ceramic wafer was sintered at 1600 °C for 2 h and the average grain size is about 8.4 μm. The array was made up of 16 × 32 pixels,
Fig. 5. Picture of the linear ceramic arrays after cutting. 4
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emission intensity might benefit from the increase of density and the improvement of transmittance of ceramics, while the decrease would be related to increase of grain boundary scattering. The normalized fluorescence decay profiles of GYGAG:Ce ceramics by monitoring the 5d1-4f emission (λem = 547 nm) under the 435 nm excitation are presented in Fig. 9. It appears that all samples tested have similar decay behaviour, which indicates that the ceramic scintillators have uniform activator concentration and microstructure. These decay curves can be well fitted by a double-exponential function. The fast and slow components of the decay time are 13 ns and 50 ns, respectively. The slow decay component is in reasonable agreement with the intrinsic 5d-4f transition of Ce3+ ions in garnet hosts. The fast decay component at the beginning of the curves might be caused by the nonradiative transfer from Ce3+ ions to the impurity or lattice defects. The primary speed of a scintillator is affected by both decay and rise time, but it is mainly determined by the decay time of the emission. This is because of that the rise time for most scintillators is very short and usually in range of a few nanoseconds. It is well known that the kinetic process is different between scintillation and luminescence, and the rise time of scintillation is longer than that of photoluminescence. However, the scintillation decay of Ce3+ in the same host would be the same as that of the photoluminescence decay. It is reasonable to say that the primary decay speed of GYGAG:Ce ceramic scintillators is fast enough to satisfy the demand of the modern medical CT detectors.
Fig. 8. Photoluminescence spectra of GYGAG:Ce ceramic scintillators.
4. Conclusions In order to provide insight into the scintillator array manufacturing and facilitate practical application for GYGAG:Ce ceramics, we studied the mechanical properties and machinability behaviour of these materials. It was found that the ceramics sintered at 1600 °C with an average grain size of around 8.4 μm have optimum overall performances for mechanical processing. The flexure strength, Young's modulus, Vickers hardness and fracture toughness were 263.0 ± 45.5 MPa, 218.0 ± 6.0 GPa, 12.0 ± 0.1 GPa and 1.08 ± 0.04 MPa m1/2, respectively. After dicing, the kerfs of the linear ceramic array exhibited good flatness and surface finishing. In addition, for the 2-dimensional array filled up with reflector, all pixels are near-perfectly square and uniform in dimensions, while the surface of the array remains very flat. Acknowledgement
Fig. 9. Fluorescence decay curves of Ce3+ in GYGAG ceramic scintillators.
This work is financially supported by National Key Research and Development Program of China (2016YFC0104502, 2017YFC0111602), National Natural Science Foundation of China (51672286), Zhejiang Provincial Natural Science Foundation of China (LY17E020011), Youth Innovation Promotion Association of Chinese of Chinese Academy of Science (2016271), Fujian Institute of Innovation, Chinese Academy of Sciences (FJCXY18040203).
and the size for each pixel was 1 mm × 1 mm, while the width of each kerf was 100 μm. The pixels of the arrays appear to be square with uniform dimensions. The reflector lines are straight and uniform. The main function of the reflectors is to absorb and reflect the scintillation light that generated from very single element, and reduce the crosstalk (due to light transmission between pixels) as much as possible. Otherwise, the crosstalk between pixels will dramatically reduce the spatial resolution of the system. Additionally, the surface of the 2-dimensional scintillator array remains very flat, indicating that the ceramic was sufficiently strong to withstand the shrinkage stress caused by the curing of the epoxy resin.
References [1] W. Rossner, B.C. Grabmaier, Phosphors for X-ray detectors in computed tomography, J. Lumin. 48–9 (1991) 29–36. [2] C. Greskovich, S. Duclos, Ceramic scintillators, Annu. Rev. Mater. Sci. 27 (1997) 69–88. [3] N.J. Cherepy, J.D. Kuntz, J.J. Roberts, T.A. Hurst, O.B. Drury, R.D. Sanner, T.M. Tillotson, S.A. Payne, Transparent ceramic scintillator fabrication, properties and applications, in: A. Burger, L.A. Franks, R.B. James (Eds.) Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X2008. [4] L. Wang, B. Liu, X.-w. Wu, J. Wang, Y. Zhou, W.-q. Wang, X.-h. Zhu, Y.-q. Yu, X.h. Li, S. Zhang, Y. Shen, Correlation between CT attenuation value and iodine concentration in vitro: discrepancy between gemstone spectral imaging on singlesource dual-energy CT and traditional polychromatic X-ray imaging, J. Med. Imaging Radiat. Oncol. 56 (2012) 379–383. [5] R. Nakamura, Improvements in the X-ray characteristics of Gd2O2S:Pr ceramic scintillators, J. Am. Ceram. Soc. 82 (1999) 2407–2410. [6] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl, Y. Fujimoto, A. Fukabori, A. Yoshikawa, 2 inch diameter single crystal growth and scintillation properties of Ce:Gd3Al2Ga3O12, J. Cryst. Growth 352 (2012) 88–90.
3.3. Luminescence performances The photoluminescence (PL) spectra excited at 435 nm for samples sintered at different temperatures are presented in Fig. 8. The peaks for all PL spectra were located at around 547 nm, which corresponds to the typical de-excitation of Ce3+ from 5d1 to 4f [27]. The consistency of the shape and peak of all the emission spectra indicates that the coordination environment around Ce3+ ions did not change as the sintering temperature changed. However, the emission intensity increases with the increase of temperature from 1550 °C to 1650 °C and decreases when the sintering temperature increased to 1700 °C. The increase of 5
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precipitation, Ultrason. Sonochem. 39 (2017) 792–797. [17] A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. [18] E.I. Gorokhova, V.A. Demidenko, S.B. Eron'ko, E.A. Oreshchenko, P.A. Rodnyi, S.B. Mikhrin, Luminescence and scintillation properties of Gd2O2S:Eu optical ceramic, J. Opt. Technol. 77 (2010) 50–58. [19] Y. Ito, H. Yamada, M. Yoshida, H. Fujii, G. Toda, H. Takeuchi, Y. Tsukuda, Hot isostatic pressed Gd2O2S:Pr,Ce,F translucent scintillator ceramics for X-Ray computed tomography detectors, Jpn. J. Appl. Phys 2-Lett. Exp. Lett. 27 (1988) L1371–L1373. [20] M. Boniecki, Z. Librant, A. Wajler, W. Wesolowski, H. Weglarz, Fracture toughness, strength and creep of transparent ceramics at high temperature, Ceram. Int. 38 (2012) 4517–4524. [21] R.W. Rice, Ceramic tensile strength grain size relations: grain sizes, slopes, and branch intersections, J. Mater. Sci. 32 (1997) 1673–1692. [22] M. Sokol, S. Kalabukhov, R. Shneck, E. Zaretsky, N. Frage, Effect of grain size on the static and dynamic mechanical properties of magnesium aluminate spinel (MgAl2O4), J. Eur. Ceram. Soc. 37 (2017) 3417–3424. [23] J. Li, Y.S. Wu, Y.B. Pan, W.B. Liu, L.P. Huang, J.K. Guo, Fabrication, Microstructure and properties of highly transparent Nd:YAG laser ceramics, Opt. Mater. 31 (2008) 6–17. [24] S. Nakayama, A. Ikesue, M. Sakamoto, Preparation of transparent YAG ceramic and its application to window material of infrared spectrophotometer, Nippon Kagaku Kaishi (2000) 437–440. [25] L. Mezeix, D.J. Green, Comparison of the mechanical properties of single crystal and polycrystalline yttrium aluminum garnet, Int. J. Appl. Ceram. Technol. 3 (2006) 166–176. [26] T.I. Mah, T.A. Parthasarathy, H.D. Lee, Polycrystalline YAG; structural or functional? J. Ceram. Process. Res. 5 (2004) 369–379. [27] Z. Luo, H. Jiang, J. Jiang, R. Mao, Microstructure and optical characteristics of Ce:Gd3(Ga,Al)5O12 ceramic for scintillator application, Ceram. Int. 41 (2015) 873–876.
[7] Y.T. Wu, Z.H. Luo, H.C. Jiang, F. Meng, M. Koschan, C.L. Melcher, Single crystal and optical ceramic multicomponent garnet scintillators: a comparative study, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 780 (2015) 45–50. [8] T. Kanai, M. Satoh, I. Miura, Characteristics of a nonstoichiometric Gd3+δ(Al,Ga)5δO12:Ce garnet scintillator, J. Am. Ceram. Soc. 91 (2008) 456–462. [9] M. Mori, J. Xu, G. Okada, T. Yanagida, J. Ueda, S. Tanabe, Comparative study of optical and scintillation properties of Ce:YAGG, Ce:GAGG and Ce:LuAGG transparent ceramics, J. Ceram. Soc. Jpn. 124 (2016) 569–573. [10] W. Chewpraditkul, N. Pattanaboonmee, W. Chewpraditkul, K. Kamada, A. Yoshikawa, M. Nikl, Luminescence and scintillation response of YGd2Al2Ga3O12:Ce and LuGd2Al2Ga3O12:Ce scintillators, Radiat. Meas. 90 (2016) 153–156. [11] Z.-H. Luo, Y.-F. Liu, C.-H. Zhang, J.-X. Zhang, H.-M. Qin, H.-C. Jiang, J. Jiang, Effect of Yb3+ on the crystal structural modification and photoluminescence properties of GGAG:Ce3+, Inorg. Chem. 55 (2016) 3040–3046. [12] N.J. Cherepy, Z.M. Seeley, S.A. Payne, P.R. Beck, E.L. Swanberg, S. Hunter, L. Ahle, S.E. Fisher, C. Melcher, H. Wei, T. Stefanik, Y.S. Chung, J. Kindem, High Energy Resolution Transparent Ceramic Garnet Scintillators, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics Xvi, (2014), p. 9213. [13] Z.M. Seeley, N.J. Cherepy, S.A. Payne, Homogeneity of Gd-based garnet transparent ceramic scintillators for gamma spectroscopy, J. Cryst. Growth 379 (2013) 79–83. [14] N.J. Cherepy, Z.M. Seeley, S.A. Payne, P.R. Beck, O.B. Drury, S.P. O'Neal, K.M. Figueroa, S. Hunter, L. Ahle, P.A. Thelin, T. Stefanik, J. Kindem, Development of transparent ceramic Ce-doped gadolinium garnet gamma spectrometers, IEEE Trans. Nucl. Sci. 60 (2013) 2330–2335. [15] J.Y. Zhang, Z.H. Luo, Y.F. Liu, H.C. Jiang, J. Jiang, G.Q. Liu, J.X. Zhang, H.M. Qin, Cation-substitution induced stable GGAG:Ce3+ ceramics with improved optical and scintillation properties, J. Eur. Ceram. Soc. 37 (2017) 4925–4930. [16] J.Y. Zhang, Z.H. Luo, H.C. Jiang, J. Jiang, C.H. Chen, J.X. Zhang, Z.Z. Gui, N. Xiao, Highly transparent cerium doped gadolinium gallium aluminum garnet ceramic prepared with precursors fabricated by ultrasonic enhanced chemical co-
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