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Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics Xianqiang Chen a , Haiming Qin b , Ye Zhang b , Jun Jiang b,∗ , Yiquan Wu a,∗ , Haochuan Jiang b,∗ a b
Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
a r t i c l e
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Article history: Received 6 February 2017 Received in revised form 4 May 2017 Accepted 13 May 2017 Available online xxx Keywords: Ce-doped GAGG Optical properties Oxygen sintering Transparent ceramics
a b s t r a c t Ce3+ -doped Gd3 (Al1-x Gax )5 O12 (Ce:GAGG) transparent ceramics were successfully prepared via a solid state reaction/oxygen sintering method. The effects of Ga substitution on the structure and optical properties of the ceramics were investigated. The highest quantum yield and relatively high scintillation light yield were achieved in the Gd3 (Al0.6 Ga0.4 )5 O12 . The investigated processing technique demonstrated advantages such as increased flexibility and short processing time, thus being very cost effective. The investigated approach provides a much more economical alternative to the conventional melt growth processes used to fabricate single crystals. Published by Elsevier Ltd.
1. Introduction In recent years, Ce3+ -doped gadolinium-aluminum-based garnet (Ce:GAG-based) materials have attracted significant attention in various fields, such as white light emitting diodes (LEDs) [1–4], scintillators [5–11] and yellow persistent phosphors [12]. However, studies on Ce:GAG have mainly focused on the luminescence and scintillation characteristics of single crystal and powder materials, [6,9–11,13,14] with only a few reports in the literature concerning the crystal structure and luminescence properties of ceramic Ce:GAG. The ceramic form of Ce:GAG is more favorable for certain applications, such as lasers and scintillators. For example, ceramics have the advantages of much lower cost and much higher activator dopant concentration compared to single crystals. In 2011, Shotaro Nishiura et al. reported the fabrication of transparent Ce:GdYAG ceramic phosphors for white LEDs [3,4]. In 2012, Tsuneyuki Kanai et al. reported the preparation of Ce3+ -doped Gd3 (Al,Ga)5 O12 (Ce:GAGG) ceramics via a hot pressing method, with a specimen of 1.8 mm thickness exhibiting a transmittance of ∼33% at 550 nm [7]. In 2014, Jumpei Ueda et al. reported the preparation of a yellow persistent phosphor of Ce3+ -Cr3+ -codoped GAGG transparent ceramic by a solid-state reaction method in
∗ Corresponding authors. E-mail addresses: [email protected] (J. Jiang), [email protected] (Y. Wu), [email protected] (H. Jiang).
vacuum. Ueda reported that a 0.542 mm thick specimen exhibited a transmittance of approximately 50% at 800 nm [12]. In addition, other authors have employed a combined pre-sintering and hot isostatic pressing (HIPing) treatment to prepare transparent Ce:GAGG-based ceramics, which has led to samples with excellent scintillation light yield and optical properties [15–17]. However, samples prepared by vacuum sintering, hot pressing, or the sintering-HIPing approach, are dark due to the formation of an oxygen deficient phase. As a result, the ceramics must be postannealed in air for a long time; a process that is quite costly and time consuming. Moreover, the significant potential for reduction of gallium oxide to gallium metal, which evaporates at high temperature, must be considered when processing GAGG-based materials in air or reducing atmospheres [10,18]. Recently, researchers at Ningbo Institute of Materials Technology and Engineering (NIMTE) have developed a pressureless sintering process to prepare transparent GAGG ceramics in a pure oxygen atmosphere, in order to reduce preparation costs and cycle time [19]. To our best knowledge, no comprehensive study exists in the literature concerning the effect of Ga3+ co-doping on the crystal structure and luminescence properties of GAG-based transparent ceramics. As previously mentioned, work on Ce:GAG has mainly focused on materials in single crystal or powder form. In the case of co-doped Ce:Y2 Gd1 Al5-x Gax O12 single crystals, the substitution of octahedrally-coordinated Al3+ with larger ions (such as Ga3+ ) results in a blueshift of the Ce3+ 5d → 4f luminescence band [20].
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024 0955-2219/Published by Elsevier Ltd.
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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In this study, GAGG-based transparent ceramics were prepared in a pure oxygen atmosphere, and the effects of Ga3+ substitution on the fabrication, luminescence and scintillation characteristics of Ce:GAGG transparent ceramics were intensively studied.
2. Experimental procedure Commercial Gd2 O3 (GanZhou QianDong Rare Earths Group Co., Ltd., China, 5 m), ␣-Al2 O3 (Aladdin Industrial Inc., United States, 200 nm), Ga2 O3 (Chalco Henan Aluminum Fabrication CO., LTD, China, 2 m), and CeO2 (Aladdin Industrial Inc., United States, 100 nm) powders with 99.99% purity were employed as starting materials. The powders were batched to form the compositions of Ce0.015 :Gd2.985 (Al1-x Gax )5 O12 (Ce:GAGG, x = 0.1, 0.3, 0.4, 0.5, 0.7, 0.9), and then mixed via ball milling for 12 h in alcohol, at a rotation speed of 300 r/min. The mixtures were then dried, calcined, dry-pressed into 25 mm diameter pellets, and cold isostatically pressed under a pressure of 300 MPa. No binder was added to aid in the forming process. The compacts were then sintered at 1650 ◦ C for 20 h in flowing dry oxygen at a flow rate of 0.6 L/min, in order to achieve high transparency. No further annealing treatment was applied. Finally, the specimen disks of ∼20 mm diameter were mirror-polished on both sides to a thickness of 1.7 mm. Select samples were thermally etched at 1300 ◦ C for 30 min to reveal grain boundaries for grain size measurements. The phase compositions of the samples were identified by Xray Diffraction (XRD, Model D8 Advance, Bruker AXS Co., Germany) using CuK␣ radiation, over the 2 range of 10◦ −90◦ . The bulk densities of the ceramics were determined by the Archimedes’ Principle method. The grain size and morphology of the ceramics were characterized by Scanning Electron Microscopy (SEM, Quanta FEG 250, FEI Co., USA). The grain size of the sintered samples was obtained by the linear intercept method (200 grains counted), where [21] the average grain size was calculated by multiplying the average linear intercept distance by 1.56. The optical transmittance of the samples was measured on a spectrometer (Lambda 950, Perkin Elmer Co., USA) over the wavelength range of 200–800 nm. Excitation and emission spectra were measured at room temperature on a fluorescence spectrometer (F-4600, Hitachi, Tokyo, Japan). Samples’ relative light yields were measured at room temperature using a custom-built X-ray excited luminescence apparatus (120Kv, 5.33 mA). Quantum yield was determined by measurements on a quantum efficiency measurement system (QE-2100, Otsuka electronics Co., LTD, Japan) at room temperature.
Fig. 1. (a) X-ray diffraction patterns for the Ce:GAGG-based ceramics oxygensintered at 1650 ◦ C for 20 h. (b) Expanded view of the 2 diffraction peak between 30◦ and 35◦ .
3. Results and discussion Fig. 1(a) shows the XRD patterns of the Ce:GAGG ceramics sintered at 1650 ◦ C for 20 h. All samples are single-phased, with the phase indexed to the garnet crystal structure. It can be observed that with increasing gallium content, the XRD peaks shift to lower 2 values (Fig. 1(b)), corresponding to increasing unit cell parameter. The calculated lattice parameters are listed in Fig. 2. The unit cell parameter increases linearly with increasing Ga3+ concentration, following Vegard’s law, indicating the formation of homogeneous solid solutions. Such behavior can be attributed to the ionic radii difference between the Ga and Al ions. The ionic radius of Ga3+ (0.62 nm) is larger than that of Al3+ (0.535 nm) [22], thus the incorporation of Ga3+ increases the lattice parameter of the material. The substitution of corresponding molar amounts of Ga2 O3 for Al2 O3 in Ce:GAGG causes Ga3+ to substitute on Al3+ sites, to form a solid solution of the form Ce0.015 Gd2.985 (Al1-x Gax )5 O12 . Through
Fig. 2. The calculated lattice constants.
XRD, the density of the solid solution, dcal , can be calculated according to the following equation: dcal =
8 [0.015MCe + 2.985MGd + 5 (1 − x) MAl + 5xMGa ] a3 NA
where a is the lattice constant of the solid solution at room temperature, NA is Avogadro’s constant, and M denotes the atomic weight of the compound. The calculated and measured (by the Archimedes’ method) sample densities are summarized in Fig. 3. The measured values are in good agreement with the calculated values, and as expected, the density of the material increases with increasing Ga
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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Fig. 3. The results of the calculated and measured density.
content. For x = 0.3, the measured density is 6.30 g/cm3 , which is very close to the calculated density of 6.32 g/cm3 . Fig. 4(a) shows a picture of the transparent ceramics fabricated for this study, and the microstructures of the ceramics with varying Ga content. The sintered pellets are ∼20 mm in diameter and ∼1.7 mm in thickness. The color of the samples changes from orange to greenish yellow with increasing gallium substitution. The microstructures of the ceramics also show different characteristics with varying Ga content. For the ceramic with a Ga content of x = 0.1, some micropores and second phase can be observed, as shown in Fig. 4(b). From the SEM images presented in Fig. 4(b), (c) and (d), it can be seen that the ceramics have uniform grain sizes. In addition, all ceramic samples showed similar grain sizes independent of Ga content, of around 20 m on average. The transmission spectra from 200 to 800 nm of select Ce:GAGG transparent ceramics samples are shown in Fig. 5. An absorption band located from around 400–500 nm can be ascribed to the 4f → 5d1 transition of Ce3+ . The band increases in intensity and shifts to shorter wavelengths as Ga3+ concentration increases, which is in good agreement with observations of the normalized excitation spectra from 390 to 540 nm, shown in Fig. 6. However, when the Ga content of the sample is increased to x=0.9, strong quenching occurs, and no excitation and emission are observed. Such behavior can be attributed to approximation and admixture
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Fig. 5. Transmission spectra of Ce:GAGG transparent ceramics with different substitution levels of Ga3+ for Al3+ .
between the conduction band and 5d excitation band of Ce3+ . As shown in Fig. 6, two Ce3+ 5d excitation bands can be observed; one between 390 and 550 nm, and another between 320 and 380 nm, corresponding to the 5d1 and 5d2 transitions, respectively. These two bands correspond to transitions to the two lowest 5d levels of the Ce3+ ion. The band positions are similar to those of other Ce3+ doped garnets [14,20,23]. From the normalized excitation spectra of the 5d1 and 5d2 bands, the shift of the 5d band maxima was investigated. It was determined that with increasing Ga3+ concentration, the 5d1 band shifts towards a higher energy position, whereas the opposite shift direction is observed for the 5d2 band. Fig. 7 depicts the normalized emission spectra for Ce:GAGG transparent ceramics under 470 nm excitation at room temperature. A distinguished Ce3+ 5d → 4f emission band between 480 and 800 nm is apparent. It can be observed that the emission peaks shift to shorter wavelengths with increasing Ga3+ content. For a Ga content of x=0.1, the emission peak is located at 574 nm, whereas for x = 0.7, the peak shifts to 549 nm, which is consistent with the analysis of the Ce:GAGG band structure scheme. Fig. 8 presents the transition energies of the 5d band excitation and emission maxima extracted from Figs. 6 and 7. The crystal field splitting between the 5d1 and 5d2 absorption bands was also investigated. The transition and crystal field splitting energy values
Fig. 4. (a) The appearance of the transparent ceramics (sample thickness ∼1.7 mm) and SEM images of microstructure of sintered transparent ceramics of (b) 10at.%Ga, (c)50at.%Ga and (d) 90at.%Ga.
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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Fig. 6. Normalized excitation spectra of the Ce:GAGG transparent ceramics with different substitution levels of Ga3+ for Al3+, recorded by monitoring the wavelength of 550 nm. The D district shows the normalized excitation bands of the 5d2 state.
Fig. 7. Normalized emission spectra for the Ce:GAGG transparent ceramics measured for excitation at 470 nm at room temperature.
Fig. 8. Ga3+ content dependence of transition energies of the Ce3+ in the Ce:GAGG transparent ceramic samples.
of Ce3+ in Ce:GAGG transparent ceramics are shown as a function of Ga3+ concentration. With increasing Ga3+ concentration, a decrease in the splitting of these bands occurs, which can be attributed to the substitution of Al3+ with the larger Ga3+ ion. Such results are consistent with similar studies on Ce:YAG [24–26]. It has been theorized that with a reduction of the crystal field strength around Ce3+ via Ga3+ substitution, the environment of the Ce3+ ion becomes more cubic [26,27].
Fig. 9. The 5d1 , 5d2 energy levels of Ce3+ and conduction band vary with Ga3+ doping. CB and VB are abbreviations of conduction and valence bands, respectively.
Based on the excitation and emission spectra of the analyzed ceramic samples and the results of others studies [13,20,24,28–30], an energy level diagram has been proposed, which is shown in Fig. 9. The influence of Ga3+ substitution on the optical properties of Ce:GAGG transparent ceramics can be classified using this diagram. In the diagram, the position of the Ce3+ ground state is designated as 0 eV. It should be noted that the relative distance between the Ce3+ ground state and the valence band is considered to be a fixed value, since the valence band of the GAGG matrix is composed of oxygen orbital states, which are not expected to be strongly influenced by changing the surrounding cations [13]. With increasing Ga3+ content, the 5d emission band shifts to a higher energy level, resulting in a blue shift of the Ce3+ 5d → 4f luminescence band. In recent theoretical studies [28], it was explained that this effect is primarily due to geometrical distortions of the host lattice, while the direct electronic effects of Ga3+ co-doping were shown to be negligible. It is well-known that the garnet structure has three cation sites, i.e., the dodecahedral, octahedral, and tetrahedral sites, as shown in Fig. 10. The eightfold-coordinated dodecahedral sites are occupied by Gd3+ and Ce3+ ions, whereas the sixfold-coordinated octahedral and fourfold-coordinated tetrahedral sites are occupied by Al3+ and Ga3+ ions. In fact, the octahedral and tetrahedral sites are unique. Experimental studies on Ce:YAG have shown that when Ga3+ is introduced into the lattice, it first occupies octahedral sites, and only when they become unavailable does it begin to occupy tetrahedral sites [25,31], a notion that is supported by other recent theoretical work [28]. It is assumed that such preferential substitution is also valid for Ce:GAGG ceramics. It is interesting to observe that Ce0.015 :Gd2.985 Al3 Ga2 O12 , with
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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Fig. 10. Schematic representation of the three cation sites in Ce:GAGG garnet structure.
Fig. 11. (a) Ga3+ content dependence of quantum yield, (b) Ga3+ content dependence relative light yield, (c) the Ce:GAGG ceramic samples under blue light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
all octahedral sites occupied by Ga3+ , shows the highest quantum yield and relative scintillation light yield, as shown in Fig. 11(a) and (b). Fig. 11(c) shows Ce:GAGG ceramics under blue light, and it is clearly apparent that the brightest sample is that with a Ga content of x=0.4, i.e., Ce0.015 :Gd2.985 Al3 Ga2 O12 . This result is consistent with studies of Ce:Y2 Gd1 Al5-x Gax O12 single crystals [20], Ce:(Lu,Gd)3 (Ga,Al)5 O12 single crystals [6] and Ce:Gd3 (Ga,Al)5 O12 powders [13].
4. Conclusions Transparent Ce3+ -doped Gd3 (Al1-x Gax )5 O12 ceramics, where x ranges from 0.1 to 0.9, can be prepared via a facile solid state reaction/oxygen sintering method. The effects of Ga content on the structure, morphology and optical properties of the Ce:GAGG ceramics were investigated. The shift of the Ce3+ energy levels with varying Ga content in the GAGG host has been analyzed. With increasing Ga content, a blue shift is observed in the 5d1 excita-
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tion band and the 5d emission band of Ce3+ , whereas a red shift is observed for the 5d2 excitation band. The highest quantum yield and relative scintillation light yield were obtained in ceramics with the composition of Ce0.015 :Gd2.985 Al3 Ga2 O12 . Acknowledgements This research was partially supported by NSFC (51502308), and National Key Research and Development Plan (2016YFC0101800). Drs. X. Q. Chen and Y. Q. Wu also gratefully acknowledge the AFOSR (contract FA9550-14-1-0155) for partially funding their work. Yiquan Wu also gratefully acknowledges the NSF CAREER grant (1554094) for partially funding the work. References [1] J.G. Kang, M.-K. Kim, K.B. Kim, Preparation and luminescence characterization of GGAG: Ce3+ , B3+ for a white light-emitting diode, Mater. Res. Bull. 43 (8) (2008) 1982–1988. [2] J.Y. Park, H.C. Jung, G.S.R. Raju, B.K. Moon, J.H. Jeong, S.-M. Son, et al., Sintering temperature effect on structural and luminescence properties of 10 mol% Y substituted Gd3 Al5 O12 : Ce phosphors, Opt. Mater. 32 (2) (2009) 293–296. [3] S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, Ce3+ : GdYAG ceramic phosphors for white LED, Proc. SPIE (2011), 793404-04-6. [4] S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, Preparation of transparent Ce3+ : GdYAG ceramics phosphors for white LED, IOP Conf. Ser.: Mater. Sci. Eng. 18 (2011) 102005. [5] T. Kanai, M. Satoh, I. Miura, Characteristics of a nonstoichiometric Gd3+␦ (Al, Ga)5- ␦O12 : Ce garnet scintillator, J. Am. Ceram. Soc. 91 (2) (2008) 456–462. [6] K. Kamada, T. Endo, K. Tsutumi, T. Yanagida, Y. Fujimoto, A. Fukabori, et al., Composition engineering in cerium-doped (Lu, Gd)3 (Ga, Al)5 O12 single-crystal scintillators, Cryst. Growth Des. 11 (10) (2011) 4484–4490. [7] T. Kanai, M. Satoh, I. Miura, Hot-pressing method to consolidate Gd3 (Al, Ga)5 O12 : Ce garnet scintillator powder for use in an X-ray CT detector, Int. J. Appl. Ceram. Technol. 10 (s1) (2013) E1–E10. [8] A. Suzuki, S. Kurosawa, J. Pejchal, V. Babin, Y. Fujimoto, A. Yamaji, et al., The effect of different oxidative growth conditions on the scintillation properties of Ce: Gd3 Al3 Ga2 O12 crystal, Phys. Status Solidi C 9 (12) (2012) 2251–2254. [9] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl, et al., 2 inch diameter single crystal growth and scintillation properties of Ce: Gd3 Al2 Ga3 O12 , J. Cryst. Growth 352 (1) (2012) 88–90. [10] A. Yoshikawa, Y. Fujimoto, A. Yamaji, S. Kurosawa, J. Pejchal, M. Sugiyama, et al., Crystal growth and characterization of Ce: Gd3 (Ga, Al)5 O12 single crystal using floating zone method in different O2 partial pressure, Opt. Mater. 35 (11) (2013) 1882–1886. [11] S. Kurosawa, Y. Shoji, Y. Yokota, K. Kamada, V.I. Chani, A. Yoshikawa, Czochralski growth of Gd3 (Al5-x Gax )O12 (GAGG) single crystals and their scintillation properties, J. Cryst. Growth 39 (2014) 134–137. [12] J. Ueda, K. Kuroishi, S. Tanabe, Yellow persistent luminescence in Ce3+ -Cr3+ -codoped gadolinium aluminum gallium garnet transparent ceramics after blue-light excitation, Appl. Phys. Express. 7 (6) (2014) 062201.
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Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics Xianqiang Chen a , Haiming Qin b , Ye Zhang b , Jun Jiang b,∗ , Yiquan Wu a,∗ , Haochuan Jiang b,∗ a b
Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
a r t i c l e
i n f o
Article history: Received 6 February 2017 Received in revised form 4 May 2017 Accepted 13 May 2017 Available online xxx Keywords: Ce-doped GAGG Optical properties Oxygen sintering Transparent ceramics
a b s t r a c t Ce3+ -doped Gd3 (Al1-x Gax )5 O12 (Ce:GAGG) transparent ceramics were successfully prepared via a solid state reaction/oxygen sintering method. The effects of Ga substitution on the structure and optical properties of the ceramics were investigated. The highest quantum yield and relatively high scintillation light yield were achieved in the Gd3 (Al0.6 Ga0.4 )5 O12 . The investigated processing technique demonstrated advantages such as increased flexibility and short processing time, thus being very cost effective. The investigated approach provides a much more economical alternative to the conventional melt growth processes used to fabricate single crystals. Published by Elsevier Ltd.
1. Introduction In recent years, Ce3+ -doped gadolinium-aluminum-based garnet (Ce:GAG-based) materials have attracted significant attention in various fields, such as white light emitting diodes (LEDs) [1–4], scintillators [5–11] and yellow persistent phosphors [12]. However, studies on Ce:GAG have mainly focused on the luminescence and scintillation characteristics of single crystal and powder materials, [6,9–11,13,14] with only a few reports in the literature concerning the crystal structure and luminescence properties of ceramic Ce:GAG. The ceramic form of Ce:GAG is more favorable for certain applications, such as lasers and scintillators. For example, ceramics have the advantages of much lower cost and much higher activator dopant concentration compared to single crystals. In 2011, Shotaro Nishiura et al. reported the fabrication of transparent Ce:GdYAG ceramic phosphors for white LEDs [3,4]. In 2012, Tsuneyuki Kanai et al. reported the preparation of Ce3+ -doped Gd3 (Al,Ga)5 O12 (Ce:GAGG) ceramics via a hot pressing method, with a specimen of 1.8 mm thickness exhibiting a transmittance of ∼33% at 550 nm [7]. In 2014, Jumpei Ueda et al. reported the preparation of a yellow persistent phosphor of Ce3+ -Cr3+ -codoped GAGG transparent ceramic by a solid-state reaction method in
∗ Corresponding authors. E-mail addresses: [email protected] (J. Jiang), [email protected] (Y. Wu), [email protected] (H. Jiang).
vacuum. Ueda reported that a 0.542 mm thick specimen exhibited a transmittance of approximately 50% at 800 nm [12]. In addition, other authors have employed a combined pre-sintering and hot isostatic pressing (HIPing) treatment to prepare transparent Ce:GAGG-based ceramics, which has led to samples with excellent scintillation light yield and optical properties [15–17]. However, samples prepared by vacuum sintering, hot pressing, or the sintering-HIPing approach, are dark due to the formation of an oxygen deficient phase. As a result, the ceramics must be postannealed in air for a long time; a process that is quite costly and time consuming. Moreover, the significant potential for reduction of gallium oxide to gallium metal, which evaporates at high temperature, must be considered when processing GAGG-based materials in air or reducing atmospheres [10,18]. Recently, researchers at Ningbo Institute of Materials Technology and Engineering (NIMTE) have developed a pressureless sintering process to prepare transparent GAGG ceramics in a pure oxygen atmosphere, in order to reduce preparation costs and cycle time [19]. To our best knowledge, no comprehensive study exists in the literature concerning the effect of Ga3+ co-doping on the crystal structure and luminescence properties of GAG-based transparent ceramics. As previously mentioned, work on Ce:GAG has mainly focused on materials in single crystal or powder form. In the case of co-doped Ce:Y2 Gd1 Al5-x Gax O12 single crystals, the substitution of octahedrally-coordinated Al3+ with larger ions (such as Ga3+ ) results in a blueshift of the Ce3+ 5d → 4f luminescence band [20].
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024 0955-2219/Published by Elsevier Ltd.
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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In this study, GAGG-based transparent ceramics were prepared in a pure oxygen atmosphere, and the effects of Ga3+ substitution on the fabrication, luminescence and scintillation characteristics of Ce:GAGG transparent ceramics were intensively studied.
2. Experimental procedure Commercial Gd2 O3 (GanZhou QianDong Rare Earths Group Co., Ltd., China, 5 m), ␣-Al2 O3 (Aladdin Industrial Inc., United States, 200 nm), Ga2 O3 (Chalco Henan Aluminum Fabrication CO., LTD, China, 2 m), and CeO2 (Aladdin Industrial Inc., United States, 100 nm) powders with 99.99% purity were employed as starting materials. The powders were batched to form the compositions of Ce0.015 :Gd2.985 (Al1-x Gax )5 O12 (Ce:GAGG, x = 0.1, 0.3, 0.4, 0.5, 0.7, 0.9), and then mixed via ball milling for 12 h in alcohol, at a rotation speed of 300 r/min. The mixtures were then dried, calcined, dry-pressed into 25 mm diameter pellets, and cold isostatically pressed under a pressure of 300 MPa. No binder was added to aid in the forming process. The compacts were then sintered at 1650 ◦ C for 20 h in flowing dry oxygen at a flow rate of 0.6 L/min, in order to achieve high transparency. No further annealing treatment was applied. Finally, the specimen disks of ∼20 mm diameter were mirror-polished on both sides to a thickness of 1.7 mm. Select samples were thermally etched at 1300 ◦ C for 30 min to reveal grain boundaries for grain size measurements. The phase compositions of the samples were identified by Xray Diffraction (XRD, Model D8 Advance, Bruker AXS Co., Germany) using CuK␣ radiation, over the 2 range of 10◦ −90◦ . The bulk densities of the ceramics were determined by the Archimedes’ Principle method. The grain size and morphology of the ceramics were characterized by Scanning Electron Microscopy (SEM, Quanta FEG 250, FEI Co., USA). The grain size of the sintered samples was obtained by the linear intercept method (200 grains counted), where [21] the average grain size was calculated by multiplying the average linear intercept distance by 1.56. The optical transmittance of the samples was measured on a spectrometer (Lambda 950, Perkin Elmer Co., USA) over the wavelength range of 200–800 nm. Excitation and emission spectra were measured at room temperature on a fluorescence spectrometer (F-4600, Hitachi, Tokyo, Japan). Samples’ relative light yields were measured at room temperature using a custom-built X-ray excited luminescence apparatus (120Kv, 5.33 mA). Quantum yield was determined by measurements on a quantum efficiency measurement system (QE-2100, Otsuka electronics Co., LTD, Japan) at room temperature.
Fig. 1. (a) X-ray diffraction patterns for the Ce:GAGG-based ceramics oxygensintered at 1650 ◦ C for 20 h. (b) Expanded view of the 2 diffraction peak between 30◦ and 35◦ .
3. Results and discussion Fig. 1(a) shows the XRD patterns of the Ce:GAGG ceramics sintered at 1650 ◦ C for 20 h. All samples are single-phased, with the phase indexed to the garnet crystal structure. It can be observed that with increasing gallium content, the XRD peaks shift to lower 2 values (Fig. 1(b)), corresponding to increasing unit cell parameter. The calculated lattice parameters are listed in Fig. 2. The unit cell parameter increases linearly with increasing Ga3+ concentration, following Vegard’s law, indicating the formation of homogeneous solid solutions. Such behavior can be attributed to the ionic radii difference between the Ga and Al ions. The ionic radius of Ga3+ (0.62 nm) is larger than that of Al3+ (0.535 nm) [22], thus the incorporation of Ga3+ increases the lattice parameter of the material. The substitution of corresponding molar amounts of Ga2 O3 for Al2 O3 in Ce:GAGG causes Ga3+ to substitute on Al3+ sites, to form a solid solution of the form Ce0.015 Gd2.985 (Al1-x Gax )5 O12 . Through
Fig. 2. The calculated lattice constants.
XRD, the density of the solid solution, dcal , can be calculated according to the following equation: dcal =
8 [0.015MCe + 2.985MGd + 5 (1 − x) MAl + 5xMGa ] a3 NA
where a is the lattice constant of the solid solution at room temperature, NA is Avogadro’s constant, and M denotes the atomic weight of the compound. The calculated and measured (by the Archimedes’ method) sample densities are summarized in Fig. 3. The measured values are in good agreement with the calculated values, and as expected, the density of the material increases with increasing Ga
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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Fig. 3. The results of the calculated and measured density.
content. For x = 0.3, the measured density is 6.30 g/cm3 , which is very close to the calculated density of 6.32 g/cm3 . Fig. 4(a) shows a picture of the transparent ceramics fabricated for this study, and the microstructures of the ceramics with varying Ga content. The sintered pellets are ∼20 mm in diameter and ∼1.7 mm in thickness. The color of the samples changes from orange to greenish yellow with increasing gallium substitution. The microstructures of the ceramics also show different characteristics with varying Ga content. For the ceramic with a Ga content of x = 0.1, some micropores and second phase can be observed, as shown in Fig. 4(b). From the SEM images presented in Fig. 4(b), (c) and (d), it can be seen that the ceramics have uniform grain sizes. In addition, all ceramic samples showed similar grain sizes independent of Ga content, of around 20 m on average. The transmission spectra from 200 to 800 nm of select Ce:GAGG transparent ceramics samples are shown in Fig. 5. An absorption band located from around 400–500 nm can be ascribed to the 4f → 5d1 transition of Ce3+ . The band increases in intensity and shifts to shorter wavelengths as Ga3+ concentration increases, which is in good agreement with observations of the normalized excitation spectra from 390 to 540 nm, shown in Fig. 6. However, when the Ga content of the sample is increased to x=0.9, strong quenching occurs, and no excitation and emission are observed. Such behavior can be attributed to approximation and admixture
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Fig. 5. Transmission spectra of Ce:GAGG transparent ceramics with different substitution levels of Ga3+ for Al3+ .
between the conduction band and 5d excitation band of Ce3+ . As shown in Fig. 6, two Ce3+ 5d excitation bands can be observed; one between 390 and 550 nm, and another between 320 and 380 nm, corresponding to the 5d1 and 5d2 transitions, respectively. These two bands correspond to transitions to the two lowest 5d levels of the Ce3+ ion. The band positions are similar to those of other Ce3+ doped garnets [14,20,23]. From the normalized excitation spectra of the 5d1 and 5d2 bands, the shift of the 5d band maxima was investigated. It was determined that with increasing Ga3+ concentration, the 5d1 band shifts towards a higher energy position, whereas the opposite shift direction is observed for the 5d2 band. Fig. 7 depicts the normalized emission spectra for Ce:GAGG transparent ceramics under 470 nm excitation at room temperature. A distinguished Ce3+ 5d → 4f emission band between 480 and 800 nm is apparent. It can be observed that the emission peaks shift to shorter wavelengths with increasing Ga3+ content. For a Ga content of x=0.1, the emission peak is located at 574 nm, whereas for x = 0.7, the peak shifts to 549 nm, which is consistent with the analysis of the Ce:GAGG band structure scheme. Fig. 8 presents the transition energies of the 5d band excitation and emission maxima extracted from Figs. 6 and 7. The crystal field splitting between the 5d1 and 5d2 absorption bands was also investigated. The transition and crystal field splitting energy values
Fig. 4. (a) The appearance of the transparent ceramics (sample thickness ∼1.7 mm) and SEM images of microstructure of sintered transparent ceramics of (b) 10at.%Ga, (c)50at.%Ga and (d) 90at.%Ga.
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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Fig. 6. Normalized excitation spectra of the Ce:GAGG transparent ceramics with different substitution levels of Ga3+ for Al3+, recorded by monitoring the wavelength of 550 nm. The D district shows the normalized excitation bands of the 5d2 state.
Fig. 7. Normalized emission spectra for the Ce:GAGG transparent ceramics measured for excitation at 470 nm at room temperature.
Fig. 8. Ga3+ content dependence of transition energies of the Ce3+ in the Ce:GAGG transparent ceramic samples.
of Ce3+ in Ce:GAGG transparent ceramics are shown as a function of Ga3+ concentration. With increasing Ga3+ concentration, a decrease in the splitting of these bands occurs, which can be attributed to the substitution of Al3+ with the larger Ga3+ ion. Such results are consistent with similar studies on Ce:YAG [24–26]. It has been theorized that with a reduction of the crystal field strength around Ce3+ via Ga3+ substitution, the environment of the Ce3+ ion becomes more cubic [26,27].
Fig. 9. The 5d1 , 5d2 energy levels of Ce3+ and conduction band vary with Ga3+ doping. CB and VB are abbreviations of conduction and valence bands, respectively.
Based on the excitation and emission spectra of the analyzed ceramic samples and the results of others studies [13,20,24,28–30], an energy level diagram has been proposed, which is shown in Fig. 9. The influence of Ga3+ substitution on the optical properties of Ce:GAGG transparent ceramics can be classified using this diagram. In the diagram, the position of the Ce3+ ground state is designated as 0 eV. It should be noted that the relative distance between the Ce3+ ground state and the valence band is considered to be a fixed value, since the valence band of the GAGG matrix is composed of oxygen orbital states, which are not expected to be strongly influenced by changing the surrounding cations [13]. With increasing Ga3+ content, the 5d emission band shifts to a higher energy level, resulting in a blue shift of the Ce3+ 5d → 4f luminescence band. In recent theoretical studies [28], it was explained that this effect is primarily due to geometrical distortions of the host lattice, while the direct electronic effects of Ga3+ co-doping were shown to be negligible. It is well-known that the garnet structure has three cation sites, i.e., the dodecahedral, octahedral, and tetrahedral sites, as shown in Fig. 10. The eightfold-coordinated dodecahedral sites are occupied by Gd3+ and Ce3+ ions, whereas the sixfold-coordinated octahedral and fourfold-coordinated tetrahedral sites are occupied by Al3+ and Ga3+ ions. In fact, the octahedral and tetrahedral sites are unique. Experimental studies on Ce:YAG have shown that when Ga3+ is introduced into the lattice, it first occupies octahedral sites, and only when they become unavailable does it begin to occupy tetrahedral sites [25,31], a notion that is supported by other recent theoretical work [28]. It is assumed that such preferential substitution is also valid for Ce:GAGG ceramics. It is interesting to observe that Ce0.015 :Gd2.985 Al3 Ga2 O12 , with
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Fig. 10. Schematic representation of the three cation sites in Ce:GAGG garnet structure.
Fig. 11. (a) Ga3+ content dependence of quantum yield, (b) Ga3+ content dependence relative light yield, (c) the Ce:GAGG ceramic samples under blue light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
all octahedral sites occupied by Ga3+ , shows the highest quantum yield and relative scintillation light yield, as shown in Fig. 11(a) and (b). Fig. 11(c) shows Ce:GAGG ceramics under blue light, and it is clearly apparent that the brightest sample is that with a Ga content of x=0.4, i.e., Ce0.015 :Gd2.985 Al3 Ga2 O12 . This result is consistent with studies of Ce:Y2 Gd1 Al5-x Gax O12 single crystals [20], Ce:(Lu,Gd)3 (Ga,Al)5 O12 single crystals [6] and Ce:Gd3 (Ga,Al)5 O12 powders [13].
4. Conclusions Transparent Ce3+ -doped Gd3 (Al1-x Gax )5 O12 ceramics, where x ranges from 0.1 to 0.9, can be prepared via a facile solid state reaction/oxygen sintering method. The effects of Ga content on the structure, morphology and optical properties of the Ce:GAGG ceramics were investigated. The shift of the Ce3+ energy levels with varying Ga content in the GAGG host has been analyzed. With increasing Ga content, a blue shift is observed in the 5d1 excita-
Please cite this article in press as: X. Chen, et al., Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.024
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tion band and the 5d emission band of Ce3+ , whereas a red shift is observed for the 5d2 excitation band. The highest quantum yield and relative scintillation light yield were obtained in ceramics with the composition of Ce0.015 :Gd2.985 Al3 Ga2 O12 . Acknowledgements This research was partially supported by NSFC (51502308), and National Key Research and Development Plan (2016YFC0101800). Drs. X. Q. Chen and Y. Q. Wu also gratefully acknowledge the AFOSR (contract FA9550-14-1-0155) for partially funding their work. Yiquan Wu also gratefully acknowledges the NSF CAREER grant (1554094) for partially funding the work. References [1] J.G. Kang, M.-K. Kim, K.B. Kim, Preparation and luminescence characterization of GGAG: Ce3+ , B3+ for a white light-emitting diode, Mater. Res. Bull. 43 (8) (2008) 1982–1988. [2] J.Y. Park, H.C. Jung, G.S.R. Raju, B.K. Moon, J.H. Jeong, S.-M. Son, et al., Sintering temperature effect on structural and luminescence properties of 10 mol% Y substituted Gd3 Al5 O12 : Ce phosphors, Opt. Mater. 32 (2) (2009) 293–296. [3] S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, Ce3+ : GdYAG ceramic phosphors for white LED, Proc. SPIE (2011), 793404-04-6. [4] S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, Preparation of transparent Ce3+ : GdYAG ceramics phosphors for white LED, IOP Conf. Ser.: Mater. Sci. Eng. 18 (2011) 102005. [5] T. Kanai, M. Satoh, I. Miura, Characteristics of a nonstoichiometric Gd3+␦ (Al, Ga)5- ␦O12 : Ce garnet scintillator, J. Am. Ceram. Soc. 91 (2) (2008) 456–462. [6] K. Kamada, T. Endo, K. Tsutumi, T. Yanagida, Y. Fujimoto, A. Fukabori, et al., Composition engineering in cerium-doped (Lu, Gd)3 (Ga, Al)5 O12 single-crystal scintillators, Cryst. Growth Des. 11 (10) (2011) 4484–4490. [7] T. Kanai, M. Satoh, I. Miura, Hot-pressing method to consolidate Gd3 (Al, Ga)5 O12 : Ce garnet scintillator powder for use in an X-ray CT detector, Int. J. Appl. Ceram. Technol. 10 (s1) (2013) E1–E10. [8] A. Suzuki, S. Kurosawa, J. Pejchal, V. Babin, Y. Fujimoto, A. Yamaji, et al., The effect of different oxidative growth conditions on the scintillation properties of Ce: Gd3 Al3 Ga2 O12 crystal, Phys. Status Solidi C 9 (12) (2012) 2251–2254. [9] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl, et al., 2 inch diameter single crystal growth and scintillation properties of Ce: Gd3 Al2 Ga3 O12 , J. Cryst. Growth 352 (1) (2012) 88–90. [10] A. Yoshikawa, Y. Fujimoto, A. Yamaji, S. Kurosawa, J. Pejchal, M. Sugiyama, et al., Crystal growth and characterization of Ce: Gd3 (Ga, Al)5 O12 single crystal using floating zone method in different O2 partial pressure, Opt. Mater. 35 (11) (2013) 1882–1886. [11] S. Kurosawa, Y. Shoji, Y. Yokota, K. Kamada, V.I. Chani, A. Yoshikawa, Czochralski growth of Gd3 (Al5-x Gax )O12 (GAGG) single crystals and their scintillation properties, J. Cryst. Growth 39 (2014) 134–137. [12] J. Ueda, K. Kuroishi, S. Tanabe, Yellow persistent luminescence in Ce3+ -Cr3+ -codoped gadolinium aluminum gallium garnet transparent ceramics after blue-light excitation, Appl. Phys. Express. 7 (6) (2014) 062201.
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