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Abstract
We presented the fabrication of both U-doped and (Ce, U) co-doped YAG transparent ceramics by the vacuum sintering method for neutron detection, which has not been seen in literature to date. The microstructure and optical property of the samples were investigated. Tetravalent was found to be the only valence state of U in YAG under high vacuum and with Ca(NO3)2·4H2O as the sintering aid. No pores and a second phase were found in Ce0.2,U0.2:YAG ceramics, and the grain size is uniform and of micron scale. The in-line transmittance of U0.2:YAG and Ce0.2,U0.2:YAG ceramics were measured to be as high as 79.04% and 80.24% in the visible light region, respectively. The results indicate that this material would be a promising candidate for the potential applications of neutron detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Neutron detection plays a vital role in the research of high energy physics, especially in the fields of nuclear energy, production of radioisotopes and applied nuclear physics [1, 2]. Unlike charged particles, neutron can’t be detected directly by ionization due to its electrical neutrality. Instead, neutron is characterized by reaction products, which is obtained from the interaction between neutron and nucleus. Signal-to noise ratio (SNR) of the detection system would be an important and challenging parameter because γ-ray also generated in the interaction can also be detected, which would cause serious interference [3, 4]. Among the several interactions, nuclear fission is superior with high SNR, benefiting from its high-energy fission fragments as the detected object, the energy of which is far higher than that of γ-ray. As the heaviest element in nature, Uranium (U) is the main participant in nuclear fission, which is reasonable to be one of the most suitable candidates for neutron detection [5].
As the carrier of U, the selection of matrix material is essential [6]. Yttrium aluminum garnet (Y3Al5O12, hereinafter referred to as YAG) has been well-known as one of the best host materials because of its high density (4.56 g/cm3), excellent thermal and chemical stability, mechanical properties and optical transparency. Rare earth (RE) ions have been doped into YAG and applied in laser device, scintillator, light-emitting diodes, etc [7–10]. Moreover, the ionic radius of U4+ (1.00 Å) is close to that of yttrium(Y3+, 1.019Å) in YAG, which suggests a strong possibility of replacement [11]. However, for neutron detection, U doped YAG is not sufficient because the high ionization energy cannot be converted directly into electric codes by photomultiplier tube. Ce,U co-doped YAG (Ce,U:YAG) may be a considerable solution. Because Ce3+ ions can absorb the ionization energy and then emit photon, and the emission spectrum is about 530 nm, which fits the absorption of photomultiplier tube [12]. Meanwhile, Ce3+ has been widely used in scintillator due to its high light yield and nanosecond fluorescence lifetime [13]. Single crystal is not capable of multi-doping or heavily doping due to the effective segregation coefficient [14], while transparent ceramic material is quite competitive for its excellent capacity of ions doping and large-size fabrication. Meanwhile, in recent years, many researchers reported that the optical property of transparent ceramic is comparable to that of single crystal [15–17]. To date, studies on Ce,U:YAG transparent ceramics for neutron detection have been scarcely reported.
In this paper, U doped YAG (U:YAG) and Ce,U:YAG transparent ceramics were prepared by vacuum sintering method. Doping different concentrations of uranium from 0.1 at% to 1.0 at%, the crystalline structure and optical property of U:YAG were analyzed. The valence state of uranium in YAG was analyzed by XPS and transmittance spectrum. The optical property and microstructure of Ce,U co-doped YAG transparent ceramics were also investigated. Ce0.2%U0.2%:YAG transparent ceramic was confirmed to be the optimal U, Ce co-doped YAG sample.
2. Experimental procedure
In order to obtain the ceramics with high transparency, High-purity Al2O3 (99.99%), Y2O3 with 99.99%, Ce2O3 with 99.99%, along with 99% uranyl nitrate (UO2(NO3)2·6H2O) and 99.99% calcium nitrate (Ca(NO3)2·4H2O) sintering aids were weighted according to stoichiometric formula. In this experience, we took two steps. Firstly, sintering was carried out through the concentrations of different uranium ions, and the best single doping concentrations of U:YAG transparent ceramics was obtained by comparing the results of the transmittance test results. Then, the mixture of Ce and U ions with a ratio of 1: 1 was co-doped to achieve Ce,U:YAG transparent ceramics. Among them, the divalent Ca2+ was introduced as sintering aids to form charge-coupled to enhanced the stability of U substitution for Y in YAG phase [11]. Vacuum sintering method synthesized in a vacuum sintering furnace [10, 18]. The chemicals were mixed in required a ratio for ball milling in ethyl alcohol as solvent medium for 24 h. After stirring, the milled slurry was then dried in a circulation drying oven along with 55 °C for 20 h. Then the sample powders were sieved through mesh screen of 150, 200 and 325 and pressed into 30 mm diameter pellets followed by a 45 MPa. (In terms of optical properties and material performances, there is almost no different among the YAG powders with different particle sizes. So it was no longer discussed in the following section.) Then the specimens were calcined at 800 °C for 10 h in oxygen atmosphere [19, 20]. After cooling, the organisms was removed from the the ceramic green body to obtain more purity phase. Moreover, the green body then were applied cold isostatic press at 200 MPa after simple vacuum pump. Eventually, the green body was sintered in a tungsten heating element vacuum furnace at 1900 °C for 10 h.
The ceramic phase identification of uranium-doped YAG and uranium co-doped YAG:Ce with the presence of Ca2+ was examined by X-ray diffraction analysis using Cu Kα radiation (XRD, Miniflex600, Rigaku,Japan). The microstructure of surface was investigated by scanning electron microscopy (SEM, JSM-6700F, JEOL, Japan). Mirror-polished samples on both surfaces (1 mm thickness) were grinded by superfine powder. The oxidation state of uranium was examined by XPS measurement (ESCALAB250, Thermo Scientific, USA). And the optical transmittance was measured by UV/visible/near infrared reflection (NIR) spectrophotometer (wavelength from 200 to 900 nm, Lambda-950, perkinElmer, USA).
3. Results and discussion
Figure 1 shows the crystalline structure of (Y1-xUx)3Al5O12 (x = 0.001, 0.002, 0.003, 0.005, 0.008) transparent ceramics. For all the samples, all the diffraction peaks can be attributed to cubic Y3Al5O12 phase (JCPDS NO.33-0040) [21], and no impurity phases were detected. The diffraction peaks were sharp and intense, demonstrating their highly crystalline nature.
Fig. 1 XRD patterns of (Y1-xUx)3Al5O12 ceramics (x = 0.001, x = 0.002, x = 0.003, x = 0.005, x = 0.008).
Photographs of transparent U:YAG ceramics plates with different U doping concentrations were shown in Fig. 2. The (Y1-xUx)3Al5O12 samples (x ≤ 0.002) were fully transparent, and the letters behind these samples could be observed quite clearly. However, with further increase of U doping concentrations (x > 0.02), the corresponding internal defect gradually increased, and the color of samples gradually turned from light green to dark green, and even fully opaque for (Y0.992U0.008)3Al5O12. The corresponding transmittance as shown in Fig. 3, were 75.9%, 79.04%, 57.60%, 41.53% and 6.8% (@ 714 nm) for the (Y1-xUx)3Al5O12 samples with x = 0.001, 0.002, 0.003, 0.005, and 0.008, respectively. It is noteworthy that 0.2 at% U would be the optimal concentration for the highest transmittance and considerable doping concentration.
Fig. 2 Photograph of (Y1-xUx)3Al5O12 ceramics (x = 0.001, x = 0.002, x = 0.003, x = 0.005, x = 0.008).
Tetravalent uranium is the target valence state for the stability in YAG structure and its luminescence properties [11, 22, 23]. Reduction atmosphere is helpful to form U4+ and it will turn to higher hexavalent uranium in oxidation atmosphere, which is mentioned in the previous report [24]. In this work, the ceramics were sintered under high vacuum atmosphere to maintain the U+4 oxidation state. Figure 4 shows the U 4f XPS spectra of U:YAG transparent ceramic. The peak located around 393 eV could be fitted with two peaks. These two peaks located at 393.8 eV and 391.08 eV can be attributed to Y 3s and U 4f5/2, respectively. The peak located at 380.18 eV is attributed to the U 4f7/2. There is about 10.9 eV apart between U 4f5/2 and U 4f7/2 [25], that proves the valence state of uranium in YAG is tetravalent, which is consistent with the conclusion reported by Q. H. Zhang’s team [11]. In this study, calcium nitrate was used as sintering aids. The added Ca2+ ions also played an important role in the charge balance.
More evidence can be found from the transmittance spectrum. As shown in Fig. 3, from 300 nm to 1000 nm there are many absorbance peaks which resulted from the electron transitions of the 5f2 configuration to the 5f16d1 energy levels transition of uranium [26]. The absorption peaks correspond to the transition from 3P2→3H4 (473 nm), 1I6→3H4 (537 nm), 3P1→3H4 (586 nm), 3P2→3H5 (633 nm), 1D2→3H4 (674 nm), 3P2→3F3 (740 nm), 1I6→1G4 (797 nm), respectively [27]. These several bands have been attributed to the transition from the electronic configurational 5f16d1 to the states of the ground electronic configurational 5f2 in U4+:YAG. The corresponding schematic diagram of the energy levels in U4+:YAG transparent ceramic are shown in Fig. 5. The zero-phonon line at 377 nm can be seen by U4+ ions embedded in YAG and energy levels for the 5f2 electronic configuration of U4+ ions transition radiation are display. All the dates agree well with the previous research [28], which further confirmed that U4+ ions were successfully doped into YAG transparent ceramics.
Besides U4+ ions, Ce3+ ions were doped into YAG for realization of scintillation characteristics. High optical property, especially low scattering centers, ensures high light yield for neutron detection. As mentioned above, 0.2 at% would be the optimal doping concentration of uranium in YAG. The doping concentration of Ce3+ ions we chose was 0.2 at% because the effect of concentration quenching enhanced rapidly when the doping content is over 0.2%, which was studied in our previous research [29, 30]. Given the ionic radius and coordination number (CN) of the doping ions and sintering aids, Ce3+(1.143Å, CN 8) and inset with U4+(1Å, CN 8) would most likely substitute the Y3+(1.019Å, CN 8). Therefore, it is more likely that Ce3+ and U4+ replace Y3+ sites in YAG. Figure 6 shows the XRD patterns of standard cubic structure of YAG (JCPDS card NO.33-0040, la-3d) and Ce0.2%U0.2%:YAG transparent ceramic after sintering at 1900 °C for 10 h. The result suggested that the prepared materials were completely agreed with the date of PDF#33-0040, with a refined lattice parameter of 12.009 Å while YAG has a cubic crystal structure with a lattice constant of 12.000 Å [31]. The slight change of lattice parameter resulted from the different ionic radius between the doping ions (U, Ce) and Y.
Figure 7 shows the in-line transmittance of the polished Ce0.2%U0.2%:YAG transparent ceramic (ф = 25 mm, 1.0 mm thickness), of which the color is light yellow-green. It could be seen that the sample was transparent and the letters could be observed clearly. The transmittance reached as high as 80.24% in visible band, close to the theoretical value. Absorption peak located at 340 nm and 450 nm were attributed to the transition between 4f and higher 5d levels of Ce3+. After the Excitation by ionization energy, the electrons excited to 5d levels would return to 4f energy level and emit photons (the wavelength is around 530 nm), which can be detected by photomultiplier tube.
Figure 8 shows the FE-SEM image of (Y0.996U0.002Ce0.002)3Al5O12 ceramic. Clear and thin grain boundary can be observed, and no precipitation or second phase is found. The well-grown grains can be found and there are no obvious pores observed inside or outside grains. The grain size of ceramic varied typically between 0.5 and 8 μm from over 260 grains in the SEM images and the average grain size is ~3.19 μm, as shown in Fig. 9. Ikesue has proved that the optical property would be excellent when the average grain size was uniform and over a few microns [32]. The effect of Rayleigh scattering would get serious when the grain size is close to the wavelength of photon. Peelen and Metselaar [33, 34] have concluded that light scattering at grain boundaries can be neglected. The realization of control on grain size can be achieved by the concentration of Ca2+ ions and sintering temperature. These conclusions are reported in recent reports [16, 30, 33, 34], which are in conformity with our experimental results. The uniform grain size of Ce0.2%U0.2%:YAG ceramic was the main reason of its high transmittance. In future, the scintillation properties of the transparent Ce,U:YAG ceramics and its potential application in neutron detection will be tested and analyzed.
4. Conclusion
In summary, both of U-doped and (Ce, U) co-doped YAG transparent ceramics were fabricated successfully at 1900 °C for 10 h. The optimal doping concentration in YAG is 0.2 at%. U0.2%:YAG has high in-line transmittance (79.04%). The oxidation state of uranium was tetravalent, which was confirmed by U 4f XPS spectrum and in-line transmittance spectrum. 0.2 at% Ce3+ ion was co-doped into U:YAG ceramics for the scintillation characteristics. The in-line transmittance of Ce0.2%U0.2%:YAG ceramic sample reached as high as 80.24% in visible band. Microstructure of highly densification was observed, no pores and second phase was found. To sum up, Ce0.2%U0.2%:YAG transparent ceramic would be a competent material for neutron detection and other potential applications. The further test for neutron detection would be done in future work.
Funding
National Key R & D Program of China (NO. 2016YFB0701003); Key Lab of Optoelectronic Materials Chemistry and Physics.
Acknowledgments
This work was supported by the National Key R & D Program of China (NO. 2016YFB0701003). Authors gratefully acknowledge the Key Lab of Optoelectronic Materials Chemistry and Physics for financial support this research.
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