Structural and Scintillation Properties of Ce³⁺:Gd₃Al₃Ga₂O₁₂ Translucent Ceramics Prepared by One-Step Sintering

Abstract

Cerium-doped gadolinium aluminum gallium garnet (Ce³⁺:Gd₃Al₃Ga₂O₁₂, Ce³⁺:GAGG) ceramic is a promising scintillation material. In this study, Ce³⁺:Gd₃Al₃Ga₂O₁₂ scintillation ceramics were prepared by the one-step sintering of commercially available Gd₂O₃, Al₂O₃, Ga₂O₃, and CeO₂ powders in a flowing oxygen atmosphere at 1600 °C by solid-phase reaction sintering. For all the Ce³⁺:Gd₃Al₃Ga₂O₁₂ ceramic samples doped with different amounts of Ce³⁺ doping, dense ceramics were obtained. The structure, photoluminescence, and scintillation properties of the Ce³⁺:Gd₃Al₃Ga₂O₁₂ ceramics have been investigated. The average grain size of samples sintered at 1600 °C is about 2 μm. The X-ray excitation luminescence peak is around 560 nm, which is consistent with that of Ce³⁺:Gd₃Al₃Ga₂O₁₂ single crystals, matching well with the computed tomography X-ray detector’s response sensitivity. The light yield is higher compared to the standard reference sample—lutetium yttrium orthosilicate single crystal.

1. Introduction

Scintillators are light-emitting materials that absorb high-energy particles or rays such as X-ray, γ-ray, and neutrons, and emit near-UV or visible photons [1]. Scintillation detectors have important applications in many fields, such as nuclear medicine, security, and high-energy physics [2]. As the key component in scintillation detectors, scintillator materials have various forms, such as single crystals [3], glass [4], transparent ceramics [5], plastics [6], and quantum dots [7].

In the past 20 years, scintillator detectors have been widely used in X-ray computed tomography (CT) systems. Currently, Pr:Gd₂O₂S (GOS), Ce:(Tb,Lu)₃Al₅O₁₂ (Gemstone^TM), and Eu:(Y,Gd)₂O₃ (YGO, HiLight^TM) are the three key scintillator materials commonly used in CT systems [8]. GOS was developed by Hitachi and widely used by medical device manufacturers such as Philips and Siemens. Gemstone^TM and HiLight^TM were developed by GE and are used in their CT products [9]. Ce³⁺: GAGG is a high-quality scintillator material, which has the advantages of high density (6.7 g/cm³), high light yield, short luminescence decay time, and no spontaneous radiation [10]. The effective atomic number of the GAGG crystal is as high as 54.4, and its emission peak is also well matched with the photodetector, making it good for applications in CT [11]. In addition, because Ce:GAGG contains the element Gd, the isotope of Gd has the largest known thermal neutron reaction cross section and is suitable for applications such as high-energy physics quantifiers and neutron detectors [12]. In 2012, Yanagida et al. [3] successfully grew a Ce:Gd₃Ga₃Al₂O₁₂ single crystal with a light yield of 46,000 ph/MeV at 662 keV and an energy resolution of about 4.9%. In 2016, Ce³⁺:GAGG single crystalline film scintillators prepared by Jan Bok et al. [13] were used for electron detection in SEM. In 2018, Lim et al. [14] prepared Ce³⁺:GAGG scintillation crystals for synchrotron X-ray radiography (SXR). In 2020, Ce³⁺:GAGG scintillator powders were prepared by Gerasymov et al. [15] for the development of X-ray radiography composite detectors.

The cubic crystal structure of GAGG makes it possible to obtain transparent ceramics. The sintering temperature of the ceramics is much lower compared to the growth temperature of single crystals, which largely inhibits the volatilization of gallium oxide. Compared with single-crystal scintillators, transparent ceramic scintillators have the advantages of uniform doping of rare earth ions, high doping concentration, simple and low-cost preparation, and it is easy to prepare large-sized samples [16]. In 2015, Chen et al. [5,9] prepared (Ce,Gd)₃Al₃Ga₂O₁₂ ceramics. They investigated the effects of doping with different concentrations of zirconia and different holding times without additives on the optical properties of the samples; however, no further study of the scintillation properties of the samples was carried out. In 2016, Ye et al. [8] prepared Ce³⁺-doped Gd₃Ga₃Al₂O₁₂ scintillation ceramics via two steps of sintering: first pre-sintering and then hot isostatic pressing (HIP). HIP plays a key role in increasing the optical transmittance of ceramic samples.

In summary, some of the past studies used two-step sintering, and the additional hot isostatic pressure made the preparation costly. Some of the sintering methods make the sintering temperature higher, which also makes the preparation cost increased. Therefore, the one-step high-temperature solid-phase reaction method used in this paper has good prospects for application.

In this paper, Ce³⁺:GAGG scintillation ceramics were prepared by one-step high-temperature solid-phase reactive sintering in oxygen atmosphere, and the phase composition, crystal structure, microstructure, luminescence spectra, and scintillation decay behavior of the ceramics were investigated.

2. Materials and Methods

Ce³⁺:GAGG scintillation ceramics were prepared by high-temperature solid-phase reactive sintering [3]. The purity of the raw material has a great influence on the performance of the ceramics, and the chemical composition of Ce³⁺:Gd₃Al₃Ga₂O₁₂ shows the highest light yield [17]. High-purity Gd₂O₃ (99.99%, aladdin, Shanghai, China), Al₂O₃ (99.99%, aladdin), Ga₂O₃ (99.99%, aladdin, Shanghai, China), CeO₂ (99.99%, aladdin, Shanghai, China), and MgO (99.99%, aladdin, Shanghai, China) commercial powders were used as raw materials and accurately weighed according to the (Ce_xGd_1−x)₃Al₃Ga₂O₁₂ (x = 0.0005, 0.002, 0.0035 and 0.005) composition. The mixed powders were prepared by wet ball milling, and the balls were high purity ZrO₂ balls. The ball-to-material weight ratio was about 3:1. 1 wt% PEG-400 was added as the dispersant, 0.02 wt% MgO and 0.5 wt% tetra-ethyl orthosilicate (TEOS) were added as sintering aids [18]. The mixed powder was ball-milled in anhydrous ethanol at 250 rpm in a planetary ball mill (QM-3SP04, Nanjing Nanda Instrument Co., Ltd., Nanjing, China) for 12 h. After ball milling, the mixed slurry was dried at 100 °C. After drying completely, the powders were granulated through 100 mesh sieves. The obtained powders were calcined at 850 °C in air in a muffle furnace to remove the organic ingredients, then finally the powders for ceramic preparation were obtained. The resulting powders were loaded into a stainless steel mold, uniaxially pressed into Φ20 mm tablets at 10 MPa pressure, and further processed by cold isostatic pressing at 250 MPa. No binder was added during the forming process. In order to suppress the volatilization of Ga₂O₃ during sintering, 0.6 L/min of flowing oxygen was introduced into the tube furnace and sintered at 1600 °C [19]. The sintering temperature schedule goes as follows: rising from room temperature to 1100 °C at a rate of 5 °C/min and holding for 15 min; from 1100 °C to 1600 °C at a temperature rise rate of 3 °C/min and then holding for 300 min; reducing the temperature from 1600 °C to 1500 °C at a cooling rate of 3 °C/min and then naturally cooling down to room temperature. The oxygen flow was stopped when the temperature was below 500 °C. Finally, the samples were polished on both sides.

The phase composition and crystal structure of ceramic samples were investigated using an X-ray diffractometer (XRD, Rigaku, MiniFlex 600 type, Tokyo, Japan) equipped with Cu K_α radiation in the 2θ range of 10–90° and a scanning step of 0.02°. The grain size and morphology of the ceramic samples were characterized by scanning electron microscopy (SEM, Quanta FEG 250, Washington, DC, USA). The optical absorption spectra were measured at room temperature using a UV-Vis-NIR spectrophotometer (SolidSpec-3700i/3700i, Shimadzu, Kyoto, Japan). Photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra, temperature-dependent PL spectra, and PL decay curves were measured using a fluorescence spectrometer (FLS-1000, Edinburgh, UK). The X-ray source for X-ray excitation luminescence (XEL) spectroscopy was an X-ray tube (F30III-2) equipped with a tungsten target and a maximum output power of 108 W (72 kV, 1.5 mA). The light yield of the samples was tested using a test system consisting of a ¹³⁷Cs gamma ray source, CR173 photomultiplier manufactured by Beijing Hamamatsu Photon Techniques INC, and a multi-channel analyzer with silicone gel coupling between the photomultiplier and the ceramic.

3. Results and Discussion

The images of the polished Ce³⁺:GAGG ceramics with different sintering temperatures and different concentrations are shown in Figure 1a. All samples are of a diameter of 1.5 cm and a thickness of 0.1 cm. The text underneath the sample is visible, and the color of the sample gradually changes from light yellow to yellow with increasing Ce³⁺ doping concentration. Under the excitation of UV lamp (365 nm) irradiation, the samples emitted strong yellowish green luminescence, as shown in Figure 1b. The SEM, optical absorption spectra, and PLE and PL spectra are discussed later with a 0.05%Ce³⁺:GAGG ceramic sample as an example. The sintering temperatures of each sample in Figure 1a are shown in

Table 1. The sintering temperature of each sample in Figure 1a.

Sample Location	Sintering Holding Temperature
First in the first row	1550 °C
Second in the first row	1575 °C
Third in the first row	1625 °C
Fourth in the first row	1650 °C
Second row	1600 °C

.

The XRD patterns of (Ce_xGd_1−x)₃Al₃Ga₂O₁₂ (x = 0.0005, 0.002, 0.0035, and 0.005) ceramics sintered at 1600 °C are shown in Figure 2a. All the Ce³⁺:GAGG ceramic samples from 10° to 90° diffraction peaks can match the standard card of Gd₃Al₃Ga₂O₁₂ (No. #46-0448) well, and no other impurity peaks were detected, which indicates that the pure-phase Ce³⁺:GAGG ceramics have been successfully prepared. As the cerium content increases, the position of the main XRD peaks shifts toward the lower angles, as shown in Figure 2b. Since the ionic radius of Ce³⁺ (102 pm) is larger than that of Gd³⁺ (94 pm), the doping of Ce³⁺ increases the lattice parameters [20]. According to the Bragg formula, the change in the angle of the X-ray diffraction is caused by the change in the crystal plane spacing. The lattice parameters of the samples with different concentrations can be calculated from the XRD patterns. The calculated lattice parameters are shown in Figure 3. The lattice parameters increase with the increase in Ce³⁺ concentration.

The structure of garnet has three cationic sites, that is, the dodecahedral, the octahedral, and the tetrahedral lattices. The crystal structure of GAGG drawn using Diamond^TM is shown in Figure 4, where Gd³⁺ occupies the eight-O²⁻-coordinated dodecahedral site, and Al³⁺ and Ga³⁺ occupy the six-O²⁻-coordinated octahedral and four-O²⁻-coordinated tetrahedral sites, respectively. When Ce³⁺ is doped, it generally enters the dodecahedral lattices. GAGG crystals are derived from GGG crystals, which are an excellent host material for laser gain media. Because Al and Ga are the same main group elements and the radius of Al³⁺ is smaller, Al³⁺ makes random disorderly substitutions for Ga³⁺ in the tetrahedral and the octahedral sites; Ga₂O₃ is volatile at high temperatures, and the price of Al₂O₃ as a raw material is lower, so it is suitable to replace part of Ga³⁺ with Al³⁺. More importantly, the optimization of ceramic properties can be achieved through the regulation of the Ga³⁺ and Al³⁺ ratio [2].

The removal of residual pores between grains during grain growth is an important step in the process of ceramic sintering to achieve a dense microstructure [21]. The SEM cross-sectional images of the Ce³⁺:GAGG ceramic sample sintered at 1600 °C are shown in Figure 5. The ceramics have a uniform grain size with an average particle size of 2 μm. The fracture surface morphology shows a dense microstructure with no obvious pores or second phases, indicating the absence of impurities. No abnormal grain growth was also observed. Compared with other studies in the literature, where the average grain sizes are around 5 μm or even larger [5,8,9,20], the grain size of ceramics is smaller.

The results of the EDS element mapping of the 0.05%Ce³⁺:GAGG ceramic are shown in Figure 5. The element mapping shows that Gd, Al, Ga, O, and Ce are uniformly distributed throughout the sample, further demonstrating the homogeneous microstructure of the ceramic samples. Mg elements were not detected in the plots due to the small amount of addition.

The optical transmittance spectra of the 0.05%Ce:GAGG scintillation ceramic measured at room temperature are shown in Figure 6a. The transmittance of the sample at 563 nm is 45.5%, and for scintillation ceramics, if the sample has a high transmittance, the scintillation photons are more easily detected by the diode through the ceramic, resulting in better scintillation performance [22]. The transmission spectrum shows that there are two absorption bands located at 338 nm and 445 nm, which are characteristic absorption bands of Ce³⁺, mainly due to the external electron leap of 4f→5d₂ and 4f→5d₁ of Ce³⁺ [23]. In addition to this absorption band, another absorption band is seen between 220 nm and 320 nm. The absorption band here may be formed due to the presence of charge transfer between Ce⁴⁺ and O²⁻ [24].

The XEL spectra of Ce³⁺:GAGG ceramics with different concentrations are shown in Figure 6b, and they have a broad emission with a peak wavelength at about 560 nm, which is associated with the 5d-4f electron transition of Ce³⁺. The profile of the XEL spectra is consistent with the counterpart of the PL spectrum. This luminescence profile matches the receiving wavelength of the photodiode well, which is more suitable for the CT X-ray detector’s response sensitivity.

The PL and PLE spectra of the 0.05%Ce³⁺:GAGG ceramics are shown in Figure 7a. The spectra show a yellow-green emission band with the peak emission located around 563 nm under light excitation at 450 nm, which is related to the 5d-4f electron transition. The two main excitation peaks are located around 340 nm and 450 nm, which are related to the 4f→5d₂ and 4f→5d₁ electron transitions of Ce³⁺, respectively. In addition, the excitation peaks located at 275 nm, 308 nm, and 313 nm are due to the Gd³⁺ 4f-4f electron transitions, which also confirms the existence of energy transfer between Gd³⁺ and Ce³⁺. When the 4f-5d₁ electron transition is monitored at 450 nm, the emission band is located between 500 nm and 650 nm, which is due to the electron transition of Ce³⁺ from 5d level to 4f level [25].

The PL decay curves of (Ce_xGd_1−x)₃Al₃Ga₂O₁₂ (x = 0.0005, 0.002, 0.0035, and 0.005) measured at 450 nm excitation at room temperature are shown in Figure 7b. All the decay curves can be well fitted using a single exponential function as follows:

$$I\left(t\right)=A_{1}exp\left(-t/t_1\right)+I_0$$

where I(t) is the luminescence intensity at time t, I₀ is the initial luminescence intensity, A₁ is a constant, t is the time, and t₁ is the luminescence decay time. The fluorescence lifetime values of Ce³⁺ in all ceramic samples were 53.01, 55.36, 54.05, and 52.41 ns, respectively.

The ¹³⁷Cs pulse height spectrum of Ce³⁺:GAGG ceramics is shown in Figure 8 [26,27]. The light yield of the ceramics was calculated by comparing it with the LYSO of known light yield. First, measure the channel number of a standard sample LYSO with known light yield. Then, measure the channel number of the sample to be tested under the same conditions. Since samples with different luminescence wavelengths are matched to the photomultiplier differently, the light yield of the sample to be tested is calculated by the following equation:

$${LY}_m=\frac{{LO}_m}{{LO}_s}·{LY}_s·\eta$$

where LY_m is the light yield of Ce³⁺:GAGG to be measured and LY_s is the light yield of the standard reference sample LYSO. LO_m is the number of channels of Ce³⁺:GAGG to be measured and LO_s is the number of channels of the standard reference sample LYSO. η is calculated from the ratio of the wavelength of luminescence emission of different materials to the optimum wavelength of the photomultiplier. The light yield of 0.35%Ce³⁺:GAGG was calculated to be 31,500 ph/MeV, which is slightly higher than the light yield of the standard reference sample. It is expected to further increase the light yield of the sample by appropriately increasing the grain size of the sample.

4. Conclusions

Pure garnet-phase Ce³⁺:Gd₃Al₃Ga₂O₁₂ scintillation ceramics were prepared by a one-step solid-phase reactive sintering in flowing pure oxygen. The luminescence properties of the Ce³⁺:Gd₃Al₃Ga₂O₁₂ ceramics, including photoluminescence spectra, photoluminescence decay curves, and X-ray excitation luminescence spectra, were investigated in detail. In the photoluminescence spectra and X-ray excitation luminescence spectra, the luminescence peaks match well with the receiving wavelength of the photodiode. The decay time of Ce³⁺:Gd₃Al₃Ga₂O₁₂ scintillation ceramics under 450 nm excitation can be well fitted with a single exponential function. The 50 ms decay time meets the requirements of rapid scintillator decay. The light yield of Ce³⁺:Gd₃Al₃Ga₂O₁₂ scintillation ceramics is up to 31,500 ph/MeV, which is better than the standard reference sample lutetium yttrium orthosilicate single crystal, and further improvement by changing the grain size is expected subsequently.