1. Introduction

More and more inorganic transparent ceramics have been developed recently for scintillation applications such as radiation detection, medical imaging, and security inspection [1–5]. Among some primary characteristics such as density, light yield, decay time and radiation discrimination, decay time of the scintillator has always been considered an important parameter that needs to be minimized. In most circumstances where high count-rate is required, slow scintillation will cause light pulse pile-up and background build-up in the photomultiplier tube (PMT) signal, deteriorating the energy and spatial resolution of the camera [6]. However, scintillators with long decay times of a few milliseconds can now be used with new devices other than conventional PMT-based gamma cameras, such as electron-multiplying charge-coupled devices (EMCCDs) [6].

Transparent oxide ceramics doped with Ce³⁺ or Pr³⁺ exhibit fast scintillation decay and high light yield because of their dominating parity and spin allowed 5d-4f transition [7–9]. Similarly, Eu²⁺ doped non-oxide materials also show intense scintillation and acceptable decay time faster than 1 μs [10]. However, Eu³⁺ doped transparent oxide ceramics have been demonstrated to possess long decay time slightly above 1 ms due to the parity forbidden 4f-4f transitions [11], which may be applied with EMCCDs. Research reports about Eu²⁺ doped oxide materials, such as Y₂O₃, Lu₂O₃, Y₃Al₅O₁₂ (YAG) [12–14], are not common, mainly because Eu²⁺ easily becomes Eu³⁺ even for samples synthesized in a reducing atmosphere. Therefore, it is difficult to prepare high Eu²⁺ concentration or pure Eu²⁺ doped oxides, especially transparent ceramics, although some efforts have been presented by reduction of Eu³⁺ ions with aliovalent additives such as Zr⁴⁺ [14], or by the spark plasma sintering (SPS) technique [12].

In this work, Eu²⁺ doped YAG and Eu³⁺ doped YAG ceramics with high transmittance in the visible region were reported for the first time. These transparent ceramics were obtained from solid state reactive sintering with vacuum pressure as high as 10⁻⁷ torr. Vacuum sintered samples were annealed at different temperatures, and their photoluminescence spectra were studied to identify the characteristic peaks of Eu²⁺ and Eu³⁺ emissions. A YAG transparent ceramic codoped with Eu and Zr was also prepared and compared with other samples for photoluminescence.

2. Experimental

The starting material, Y₂O₃ powder with 99.99% purity, was purchased from Jiahua Advanced Materials Resources Co. Ltd.; the Al₂O₃ powder, with 99.99% purity, was ordered from Baikowski Malakoff Inc.; the Eu₂O₃ powder, also with 99.99% purity, along with Y₂O₃ stabilized ZrO₂ nanopowder and tetraethyl orthosilicate (1 wt% used in the mixture) were purchased from Sigma-Aldrich. The chemicals were mixed in a required ratio for ball milling, which was completed using ethanol as a solvent and alumina ball as a media, for 20 h. After drying and 800 °C calcination, the powder was sieved through a 200 mesh screen, and dry pressed into 20 mm diameter pellets, followed by a 200 MPa cold isostatic pressing. The green body was then sintered at 1780 °C in a graphite heating element vacuum furnace with a vacuum pressure of 10⁻⁷ torr during 12 h dwelling, while YAG phase was formed through a solid state reaction. The transparent samples after sintering were annealed with different temperature and summarized in Table 1

Table 1. Conditions and major emission property of all the samples

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X-ray diffraction (XRD) data of the ceramic samples were recorded using a Cu Kα radiation source with a Bruker D8 Phaser diffractometer. Absorption coefficients of the polished ceramic samples were measured in a PerkinElmer Lambda 950 UV-VIS-NIR spectrophotometer. The photoluminescence spectra were acquired at room temperature by a Jobin Yvon Flurolog-3 spectrofluorometer with excitation at wavelength of 400 nm for all samples.

3. Results and Discussion

YAG has a cubic crystal structure with a lattice constant of 12.000 Å [15]. The XRD patterns of the 1 mol% Eu: YAG, 6 mol% Eu: YAG, 1 mol% Eu, 1 mol% Zr: YAG and 1400 °C annealed 1 mol% Eu: YAG transparent ceramic samples in Fig. 1 showed that every peak from 15° to 140° 2θ could be indexed to the available standard YAG diffractions (JCPDS 88-2048, la-3d), with refined lattice parameters of 12.0053 Å, 12.01324 Å, 12.0098 Å and 12.0049 Å respectively. No second phases could be observed in the XRD patterns. In the YAG lattice, Y³⁺ (115.9 pm) is coordinated by 8 O^2- ions in the form of a triangular dodecahedron, while Al³⁺ ions occupy two types of sites: one with coordination number (CN) 4 (53 pm), and another with CN 6 (67.5 pm). Given the ionic radii and CN of the doping ions and sintering aids, Eu²⁺ (139 pm, CN 8), Eu³⁺ (120.6 pm, CN 8), Zr⁴⁺ (98 pm, CN 8) and Si⁴⁺ would most likely substitute for the Y³⁺, Y³⁺, Y³⁺ and Al³⁺ sites, respectively. Therefore, it was reasonable that 6 mol% Eu: YAG sample presented the highest lattice constant with Zr, Eu co-doped sample being the smallest. However, it was not logical that the lattice constant of 1400 °C annealed sample (Eu³⁺, a = 12.0098) was higher than that of the as sintered sample (Eu²⁺, a = 12.0053) by only considering the Eu ion size. The plausible explanation was that after 12 h vacuum sintering, there were still some Si⁴⁺ ions in the YAG lattice that acted as negative effect as compared to Eu for lattice expansion. After 72 h annealing, with the decrease of Si⁴⁺ ion amount in the lattice, the lattice constant increased although Eu²⁺ turned into the smaller size Eu³⁺.

 

Fig. 1 XRD patterns of as sintered and annealed transparent YAG ceramics with different element and concentration doping.

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The incorporation of Eu²⁺ into the host lattice brought a proportional concentration of oxygen vacancies, which could capture electrons forming F⁺, F and even F-aggregate centers. As shown clearly in the absorption spectra in Fig. 2 , several strong, wide and bell-shaped bands lie in the ultraviolet (UV) and visible region of the spectrum for the as sintered 1 mol% Eu: YAG and 1 mol% Eu, 1 mol% Zr: YAG ceramic samples. These absorption peaks disappeared after annealing the samples at 1400 °C for 72 h and 1200 °C for 90 h in air. The absorption with wavelength smaller than 300 nm of the as sintered ceramics could be attributed to the 4f⁷ to 4f⁶5d transition in Eu²⁺, which was much lower after annealing. It is difficult to identify the type and structural surrounding of the color centers with broad band peaks at 375 nm and 540 nm. Only a very weak color center emission band centered at 1325 nm could be observed, with the similar band shape to the two absorption bands where the excitation wavelength was chosen.. The photographs of the 1 mol% Eu: YAG ceramic (1.1 mm thick, 14 mm in diameter) and annealed 1 mol% Eu, 1 mol% Zr: YAG ceramic (0.8 mm thick, 14 mm in diameter) were presented in the inset of Fig. 2. The two ceramics were held around 2 cm above the printed paper, where the letters could be clearly seen through, although the as sintered ceramic was strongly colored. The transmittance of the 1.1 mm thick blue color sample was measured to be 61% at wavelength of 900 nm, while the recently reported transmittance value for 0.8 mm thick Eu²⁺ doped YAG ceramic was only 6% at the same wavelength [12].

 

Fig. 2 Absorption spectra of as sintered and annealed Eu: YAG and Eu, Zr: YAG transparent ceramics. The inset shows the photographs of as sintered 1 mol% Eu: YAG (a) and 1200°C annealed 1 mol% Eu, 1 mol% Zr:YAG (b) transparent ceramics around 0.8 mm thickness on top of printed paper.

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Photoluminescence spectra in Fig. 3 demonstrated the optical characteristics of Eu²⁺ ions in the as sintered ceramic samples. The emission ratio of Eu²⁺ to Eu³⁺ luminescence was much higher than the ratio in other reported Eu²⁺ doped YAG ceramics [12–14]. The very broad emission band from wavelength lower than 420 nm to higher than 700 nm could be ascribed to 4f⁶5d¹ to 4f⁷ transitions of Eu²⁺ ions, while several sharp emission peaks around wavelength of 600 nm originated from the ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3) transitions of Eu³⁺ ions. It was revealed that the energy absorbed by Eu²⁺ could be transferred to Eu³⁺ ions resulting in the residual Eu³⁺ ion luminescence [16]. Furthermore, it was clear that higher doping concentration brought stronger luminescent intensity for the emission features of both Eu²⁺ and Eu³⁺ ions. However, Zr and Eu co-doped YAG showed lower emission intensity than that of Eu doped YAG sample. The O^2- to Zr⁴⁺ charge transfer transition in the CN 8 dodecahedron unit only resulted in absorption around 180 nm, which could not be the reason for intensity decrease. Similarly, it was reported that the light yield slightly decreased after Zr codoping of Ce: YAP samples, and Ce-Zr pairing was proposed as the possible reason [17, 18].

 

Fig. 3 Photoluminescence spectra of vacuum as sintered YAG transparent ceramics with different element and concentration doping, with excitation at wavelength of 400 nm.

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As sintered 1 mol% Eu: YAG ceramic samples were annealed at different temperatures to study the photoluminescence properties. As presented in Fig. 4, the intensity of the broad emission decreased, while the intensity of the sharp Eu³⁺ emission increased, when the annealing temperature increased from 500 °C to 700 °C. This indicated that the defects accounting for whole emission weak intensity did not change while a small amount of Eu²⁺ had been transferred to Eu³⁺, and thus the oxidation of Eu²⁺ ions into Eu³⁺ ions was more energetically favorable. As annealing temperature increased to 900 °C, both Eu²⁺ emission and Eu³⁺ emission strongly increased with much higher Eu³⁺ emission ratio, meaning both defects and Eu²⁺ ion amount were decreasing.

 

Fig. 4 Photoluminescence spectra of 1 mol% Eu doped YAG transparent ceramics annealed at different low temperatures, with excitation at wavelength of 400 nm.

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The ceramic sample annealed at much higher temperature exhibited spectra with only Eu³⁺ emission features as shown in Fig. 5.The emission intensity for these annealed samples increased dramatically as compared with the as sintered 6 mol% Eu: YAG transparent ceramic. There were no emission peaks related to Eu²⁺ emission observed in the photoluminescence spectra. With the successful fabrication of both Eu²⁺ and Eu³⁺ doped transparent YAG ceramics, further research will be focused on the defect structure and applications with PMT or CCD detectors.

 

Fig. 5 Photoluminescence spectra of YAG transparent ceramics annealed at high temperature comparing with the spectrum of as sintered ceramic, with excitation at wavelength of 400 nm.

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4. Conclusion

Eu²⁺ or Eu³⁺ doped YAG transparent ceramics were fabricated by a reactive sintering method under high vacuum. Absorption coefficients of the as sintered ceramics exhibited strong and broad absorption peaks centered at 375 nm and 540 nm. Photoluminescence spectra of different temperature annealed and different doping concentration ceramics were compared, and Eu²⁺ or Eu³⁺ emission peaks and emission intensity ratio change were studied.