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Transparent and colorless CeO2-activated borogermanate glasses with about 5.60 g/cm3 were successfully synthesized by a melt-quenching method in air atmosphere. Both the optical transmittance and X-ray absorption near edge spectroscopy (XANES) spectra confirm that Ce4+ can be effectively reduced to its trivalent state, i.e. Ce3+ ions, by minor quantity of Si3N4 addition as a strong reducing agent. The luminescence characteristics excited by both ultraviolet and X-ray light were studied and the optimal content of Si3N4 was determined. The developed dense Ce3+-activated borogermanate glasses is featured with a broad emission band centered at 430 nm and a decay time of about 30 ns, which will be a promising candidate to scintillating crystals and ceramics.
© 2015 Optical Society of America
1. Introduction
Scintillating glass is one of the most promising candidates to scintillating crystals in medical imaging, homeland security, high energy and nuclear physics, owing to the advantages of low-cost, large-volume production and easy shaping of elements [1–3]. From the viewpoint of practical application, scintillating glasses characterized with high density and fast decay time nature are particularly paid extensive attentions in recent years [4–9]. Because a dense scintillating material (usually exceeding 5.0 g/cm3) enlarges the X-ray absorption cross-section and improves the image signal-to-noise ratio [2]. While fast decay (usually several to tens of nanoseconds) of a scintillator is significant for the avoidance of ghost imaging [10]. Trivalent cerium, Ce3+, is one of the most powerful activator in the designing of fast scintillating materials. However, the reducing atmosphere provided by either CO or H2/N2 mixed gases is necessary during synthesis to reduce Ce4+ to Ce3+ as completely as possible. The provided reducing atmosphere not only complicates the synthesis process, but also increases the relatively economic cost.
It is reported that the lower valent Eu2+ ions can be well controlled in air atmosphere under strict conditions, which includes the special crystal structure and site requirements, as well as both the valence and radii between the incorporated and substituted ions of the host crystals [11]. The self-reduction of Eu3+ ions in glasses are also observed in recent years [12, 13]. Except for the successful synthesis of CeO2-activated borogermanate scintillating glasses in air by adding minor quantity of Si3N4 as a strong reducing agent [14], there is no report on Ce3+-activeted scintillating glasses synthesized in air atmosphere. Although added as CeO2, cerium is assumed to be easily converted to the trivalent state in the non-crystalline glasses as a result of reaction with the added Si3N4 [15]:
In this work, we have extended our investigation on the effect of minor quantity of Si3N4 addition on the structure and luminescence properties of CeO2-activated borogermanate glass by the FT-IR, optical transmittance, XANES, photoluminescence and X-ray excited luminescence (XEL) spectra in detail.
2. Experimental
Besides for the host glass (25B2O3-40GeO2-35Gd2O3), there are five pieces of CeO2-activated borogermanate glasses with the nominal compositions of 25B2O3-40GeO2-34Gd2O3-1CeO2-xSi3N4 (x = 0.0, 0.13, 0.31, 0.44 and 0.57) were synthesized from H3BO3 (99.9%, Shanghai Chemical Reagents Co. Ltd.), GeO2 (99.999%, Nanjing Hope Tech. Development Inc.), Gd2O3(99.99%, Jiangxi Ketai Advanced Materials Co. Ltd.), CeO2 (99.99%, Shanghai Chemical Reagents Co. Ltd.) and Si3N4(99.99%, Shanghai Alada Co. Ltd.). Batches of about 20 g raw materials were mixed well in an agate mortar and melted in an alumina crucible at 1350 °C for about 1 h in atmosphere. Then the homogeneous melts were quickly poured onto a preheated stainless steel mold. The quenched glasses were finally annealed at 600 °C for 4-8 h followed by cooling down naturally to room temperature.
FT-IR absorption spectra were always accumulated for 16 scans on a Nicolet IS5 FT-IR spectrophotometer operating with 4 cm−1 resolution. Transmittance spectra were collected on a Perkin Elmer Lambda 750S UV/VIS spectrometer in 200-800 nm region. X-ray absorption near edge structure (XANES) spectra at Ce LIII-edge were recorded with a Si (111) double-crystal monochromator at beam line 1W2B of Beijing Synchrotron Radiation Facility (BSRF). Linear combination fitting (LCF) method was used to estimate the content of Ce3+ and Ce4+ in our glass by Athena software. Excitation and emission spectra were carried out on a Hitachi F-7000 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Luminescence decay curves were recorded in an Edinburgh FLS980 spectrometer using TCSPC technique excited by the 375 nm EPLED light source featured with 75 ps pulse width and 2 MHz frequency, respectively. The XEL spectra were examined using an X-ray excited spectrometer, in which an Au anticathode target was used as the X-ray source operating at 40 kV and 50 µA. All the measurements were carried out at room temperature.
3. Results and discussion
To investigate the effect of minor quantity of Si3N4 addition on the glass structure, the FT-IR absorption spectra are illustrated in Fig. 1. The strong absorption band located at ca 705 cm−1 is ascribed to the bending and stretching vibrations of T-O-T (T = B, Ge) linkages [16]. The high-frequency region of IR spectra centered at 1180 cm−1 corresponds to the vibrations of [BO3] triangle group [16], and the band at ca 1330 cm−1 should be assigned to the B-O- bonds vibration occurring in a large borate network and not to a specific group [17]. As shown in the inset of Fig. 1(b), the absorption peak at about 950 cm−1 begins to form with the addition of Si3N4 agent, which may be associated with the stretching vibration of B-O-Si linkage [18]. Additionally, the relative absorption intensity gets stronger with the increasing concentration of Si3N4.
Fig. 1 FT-IR absorption spectra (a) and the corresponding normalized FT-IR absorption spectra at about 705 cm−1 (b). Inset shows the enlargement wavenumber region of 850-1050 cm−1.
Optical transmittance spectra of Ce3+-activated borogermanate glasses are shown in Fig. 2(a). The cut-off edge of the host glass is the shortest one at approximately 290 nm, therefore the featured absorption peaks at 253, 275, 307 and 312 nm assigned to Gd3+ ions are clearly observed. With the incorporation of cerium ions, the cut-off edge of the x = 0.0 glass shift remarkably towards 405 nm. It is regarded as the role of cerium dopants, both the electronic transition of Ce3+ ion from its 4f ground state to 5d excited and the charge transfer band of O2--Ce4+ will be responsible for the total absorption of 290-405 nm region [19, 20]. The powerful reducing nature of Si3N4 decreases remarkably the quantity of Ce4+ to Ce3+ ions, which is speculated by the obvious blue-shifting of the cut-off edge from 405 nm (x = 0.0) to 363 nm (x = 0.31). Furthermore, the borogermanate glasses show linear transmittance coefficient of above 82% in the 400-800 nm region, which is of significance for the stronger light yield output of Ce3+-activated borogermanate glasses. The noticeable reducing effect of Si3N4 is also illustrated by the digital photographs of borogermanate glasses, as shown in Fig. 2(b). The colour of the x = 0.0 glass is heavy brown, while those of the other Ce3+-activated borogermanate glasses are transparent and colourless. The colour change from brown to colourless suggests that more quantity of Ce3+ ions exist in borogermanate glass with the addition of Si3N4. Interestingly, the nearly similar UV cut-off edges with adding Si3N4 agent imply the similar concentration of Ce3+ ions in borogermanate glass.
Figure 3(b) displays the Ce LIII-edge XANES spectra of CeF3 and CeO2 measured as reference samples of trivalent and tetravalent Ce compounds, respectively. According to many-body calculations with Anderson impurity model [21, 22], the double peaks (peak A and B) arising from final-state configuration is associated with the 2p4f15d1 and 2p4f05d1 of Ce4+, respectively. Peak C at about 5723 eV is Ce3+ peak, which is associated with the 2p4f15d dipole allowed transition. To quantitatively determine the valence state of cerium in borogermanate scintillating glasses, the normalized Ce LIII-edge XANES spectra are compared in Fig. 3(a). The absorption edges on the low energy side of all XANES spectra match well the characteristic of Ce3+ ion, namely Peak C. All XANES spectra of borogermanate glass show the evident characteristic of Ce4+ ion, namely peaks A and B. From the results of LFC method, the concentration of Ce3+ ions increase rapidly from 62.2% (x = 0.0) to about 81% (x = 0.13-0.57). The resolved concentration of Ce3+ ions is in good accordance with the transmittance results. However, the concentration of Ce3+ ions in x = 0.44 glass is not resolved for the bad XANES data.
Fig. 3 Ce LIII-edge XANES spectra of borogermanate scintillating glasses, and XANES spectra of CeO2 and CeF3 reference samples (b).
Figure 4 show the excitation and emission spectra of Ce3+-activated borogermanate glasses, respectively. Both the excitation and emission spectra are very similar except for the x = 0.0 glass. As shown in the excitation spectrum of Fig. 4(a), two groups of broad bands in 250-300 and 325-400 nm regions are ascribed to the 4f-5d3,4,5 and 4f-5d1,2 transition, respectively [14]. Under direct excitation at the 4f-5d1,2, a typical emission of Ce3+ ions extending from 375 to 600 nm is observed, as shown in the emission spectra of Fig. 4(b). It is clear that the emission intensity is enhanced by at least 20 times with the addition of Si3N4. The emission intensity increases with an increasing amount of Si3N4 agent, and reaches maximum in the x = 0.31 glass, then it decreases with further increasing amount of Si3N4 agent. This attributes to the higher concentration of reduced Ce3+ ions (see Fig. 3), which also decrease the self-absorption of Ce4+ ions, as shown in the transmittance spectra of Fig. 2(a).
The luminescence decay curves (λex = 375 nm, λem = 433 nm) of CeO2-activated borogermanate glasses were measured and shown in Fig. 5. All the luminescence decay curves of Ce3+ ions significantly deviate from the single exponential rule. However, they can be well fitted by two exponentials according to the following equation:
where I(t) is the emission intensity of Ce3+ ions at a given time, t, A1 and A2 are constants, and τ1 and τ2 are the short- and long-decay components, respectively. Thus, the evaluated mean lifetime τ is obtained by [14]:The mean lifetimes were evaluated to be about 22.44, 29.95, 29.88, 31.04 and 31.09 ns, respectively. In a word, the mean lifetimes of Ce3+-activated borogermanate glasses slightly increases from 22.44 to about 30 ns with the higher amount of Si3N4 agent.
The mean lifetime of Ce3+ ions is well known to be dependent on the host glass, it varies within 20-80 ns, as summarized in Table 1. The lifetimes of Ce3+ ions in borosilicate and oxyfluoride borosilicate glasses is about 33-50 ns [7–9], while it prolongs to about 50 and 68 ns in the CaO-Al2O3-BaO and oxyfluoride aluminasilicate glasses, respectively [4, 23]. The longest lifetime of Ce3+ ions is about 80 ns in the SiO2:Ce3+ sol-gel glass [24]. The lifetime with about 30 ns in the investigated Ce3+-activated borogermante glass is very close to those in phosphate and fluorohafnate scintillating glasses [1,6]. Bearing the high density, fast decay time and the economic cost of a scintillator in mind, the developed Ce3+-activated borogermanate glass is considered to be one of the most competitive scintillating materials in high-energy physics engineering and medical imaging.
Table 1. The mean lifetimes of Ce3+ ions in various scintillating glasses
As a promising candidate to scintillating crystals for high-energy rays detection, the XEL spectra of Ce3+-activated borogermanate scintillating glasses are presented in Fig. 6. The XEL intensity of the featured Gd3+ (314 nm) emission equals approximately to that of the impurity Eu3+ (592, 614, 652 and 702 nm) in the host glass, the impurity Eu3+ originate from the Gd2O3 agent because the Eu3+ trace is always presented in Gd2O3 raw material [25]. When a amount of Ce3+ ions appear in the x = 0.0 glass, the varnishing of Gd3+ emission peaks indicates the existence of efficient energy transfer from Gd3+ to Ce3+ ions, part of energy absorbed by Gd3+ is also transferred to the Eu3+ ions. With higher content of Ce3+ ions derived from the strong reduction effect of Si3N4 in borogermanate glass, the XEL spectra begin to be dominated by the Ce3+ emission band without any Gd3+ and Eu3+ peaks. The XEL intensity increases with an increase in the amount of Si3N4 agent, and reaches maxmium in the x = 0.31 glass, then it decreases with further increasing amount of Si3N4 agent. They are in well line with the emission spectra of Fig. 4(b).
4. Conclusion
More than 80% tetravalent cerium is effectively reduced to its trivalent state in borogermanate scintillating glasses synthesized in air atmosphere by adding minor quantity of Si3N4 addition. The reduced Ce3+ ions remarkably lowers the self-absorption of Ce4+ ions and results in the enhancement of Ce3+ emission intensity by at least 20 times. The strongest emission intensity under both ultraviolet and X-ray light is reached when the optimal content of Si3N4 agent is x = 0.31. The developed dense Ce3+-activated borogermanate scintillating glasses shows a broad emission centered at 433 nm and a decay time of about 30 ns, which will be promising in the scintillating fields of high energy physics engineering and medical imaging.
Acknowledgments
This work is supported by the Natural Science Fund of China (11165010, 51202098, 11465010 and 11465010), the Natural Science Fund of Jiangxi Province (20142BAB202006, 20152ACB21017), the Training Program of Young Scientists (JingGang Star) in Jiangxi Province (20133BCB23023), the Key Subject of Atomic & Molecular Physics in Jiangxi Province (2011-2015), and the Open Fund of Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics of Chinese Academy of Sciences.
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