1. Introduction

Scintillating glass shows a promising alternative to scintillating crystals or ceramics owing to the advantages of its low-cost, large-volume production, and easy shaping of elements [1–3]. In recent years, fast Ce³⁺-activated scintillating glasses above 5.0 g/cm³ have received intensive attentions in the fields of high energy nuclear physics and medical imaging [4–7]. However, the reducing atmosphere provided by either CO or H₂/N₂ mixed gases is necessary to reduce Ce⁴⁺ to Ce³⁺ as completely as possible during glass synthesis, which not only complicates the experimental process, but also increases the production cost. Interestingly, we have successfully synthesized Ce³⁺-activated borogermanate glasses in air atmosphere by adding the strong reducing Si₃N₄ agent [8]. Furthermore, the relatively low light-yield of scintillating glasses derived from the intermediate- and long-range disorder structure may be one of the most difficult obstacles on the practical way. To some extent, the shortcoming of the low light-yield mentioned can be compensated by the strategies of energy transfer or nanocrystallization of the precursor glass to form transparent glass-ceramic [7, 9].

To our knowledge, there are a few reports on the bismuth-doped luminescent glasses induced by the melting temperature [10–12], and the valent states of bismuth ion can be conveniently controlled by tailing the melting temperature, which results in different luminescence properties. However, there is no report on luminescence behaviors of Ce³⁺-doped glass induced by melting temperature. Therefore, in this work, we have easily enhanced the light-yield of Ce³⁺-activated borogermanate glasses only by tailoring the melting temperature. A deep insight into the luminescent properties of Ce³⁺-activated scintillating glasses is given by the optical transmittance, X-ray absorption near edge spectroscopy (XANES), photoluminescence (including fluorescence decay curves) and X-ray excited luminescence (XEL) spectra. The plausible reason for the enhanced mechanism is also discussed via both the glass density and energy-dispersive spectroscopy (EDS) analysis.

2. Experimental

Borogermanate scintillating glasses with the nominal composition of 25B₂O₃-40GeO₂-34Gd₂O₃-1CeO₂-0.31Si₃N₄ are synthesized by a melt-quenching method at different temperature of 1350, 1375, 1400, 1425 and 1450 °C, respectively. The role of the additive Si₃N₄ is to effectively reduce Ce⁴⁺ to Ce³⁺ during glass synthesis in air atmosphere [8]. The raw materials derives from H₃BO₃ (99.9%, Shanghai Chemical Reagents Co. Ltd., Shanghai, China), GeO₂ (99.999%, Nanjing Hope Tech. Development Inc. Nanjing, China), Gd₂O₃ (99.99%, Jiangxi Ketai Advanced Materials Co. Ltd., Nanchang, China), CeO₂ (99.99%, Shanghai Chemical Reagents Co. Ltd., Shanghai, China) and Si₃N₄ (99.99%, Shanghai Jingchun Reagent Co. Ltd., Shanghai, China). Batches of about 20 g raw materials were mixed homogeneously in an agate mortar and melted in an alumina crucible for about 1 h. 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.

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 L_III-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 glasses 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 home-made X-ray excited spectrometer, where a JF-2 X-ray tube (Mo anticathode target, Liaodong Radioative Instrument Co. Ltd) was used as the X-ray source operating at 50 kV and 40 mA, and a SBP-300 spectrometer (Beijing Zonix Instrument Co. Ltd) was coupled to the injected surface of the samples at 90° to the X-ray direction. Field emission scanning electron microscopy (FESEM, NOVA NANOSEM 230) equipped with an energy-dispersive spectroscopy (EDS, AZTec X-Max 80) was operated at 30 kV of accelerating voltage and applied for the analysis of glass composition. All the measurements were carried out at room temperature.

3. Results and discussion

Ttransmittance spectra of Ce³⁺-activated borogermanate glasses are shown in Fig. 1(a).It is clear that the transmittance spectra with the similar intensity and profiles seem to be independent on the melting temperature, and the linear transmittance coefficient exceeds 82% in 400-800 nm region. The same cut-off edges approximately at 368 nm result from their same concentration of Ce³⁺ ions. The noticeable reducing effect of Si₃N₄ may be illustrated by the digital photographs of the transparent and colourless borogermanate scintillating glasses, as shown in the inset of Fig. 1(a).

 

Fig. 1 Transmittance spectra (a), Ce L_III-edge XANES spectra of Ce³⁺-activated borogermanate scintillating glasses (b) and XANES spectra of Ce⁴⁺ and Ce³⁺ reference samples (c). Inset shows the digital photographs.

Download Full Size | PDF

Figure 1(c) displays the Ce L_III-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 [13, 14], the double peaks (peak A and B) arising from final-state configuration is associated with the 2p4f¹5d¹ and 2p4f⁰5d¹ of Ce⁴⁺, respectively. While peak C at about 5723 eV is assigned to Ce³⁺ peak, it is associated with the 2p4f¹5d dipole allowed transition. To qualitatively determine the valence state of cerium in borogermanate glasses, the normalized Ce L_III-edge XANES spectra are compared in Fig. 1(b). It is surprising that the normalized Ce L_III-edge XANES spectra is highly similar. The absorption edges on the low energy side of all the XANES spectra match well with the features of Ce³⁺ ions (Peak C). However, the very weak signal of the featured Ce⁴⁺ ions (peaks B and A) coexists in all the Ce³⁺-activated borogermanate glasses. From the results of LFC method, the concentration of Ce³⁺ ions in borogermanate glasses is fitted to be about 81.4, 86.1, 88.1, 83.7 and 85.8% with an increase in melting temperature, respectively. In a word, the Ce³⁺ concentration in the investigated borogermanate glass is in the vicinity of 85%, which is in well line with the transmittance results.

Figures 2(a) and 2(b) show the excitation and emission spectra of Ce³⁺-activated borogermanate glasses, respectively. In the excitation spectrum, two groups of broad bands ranging 200-300 and 325-400 nm are ascribed to the 4f-5d_3,4,5 and 4f-5d_1,2 transition, respectively [8]. Under direct excitation at the 4f-5d_1,2, a typical emission of Ce³⁺ ions assigned to 5d¹-²F_5/2,7/2 transition in 375-650 region is clearly observed. Both the excitation and emission intensity of Ce³⁺-activated borogermanate glasses increases remarkably with an increase in the melting temperature from 1350 to 1450 °C. To our interesting, a very limited blue-shift within 2-4 nm is noticed in both the excitation spectra and emission spectra, as clearly illustrated in the inset of emission spectra of Fig. 2(b). The very limited blue shift implies that weak modification of the glass mirostructure around Ce³⁺ ions. The crystal field strength (D_q) around the Ce³⁺ is estimated approximately by ${\text{D}}_{\text{q}}\propto \text{Z}/{\text{R}}^{\text{5}}$ [15].Where Z is the charge on the anion and R represents the distance between the central ion and the surrounding ligands. If Z is fixed, the strength of the crystal field is inversely proportional to the fifth power of the bond length. Since the molar concentration of Ce³⁺ ions is fixed in borogermanate glasses, and the glass densities were measured to a decrease from 5.62 to 5.37 g/cm³ with an increase in melting temperature, the number of Ce³⁺ per unit volume gets lower. It suggests the longer distance between Ce³⁺ and its surrounding ligands, which results in the lower crystal field strength (D_q) around the Ce³⁺ ions and lead to the slight blue shift of Ce³⁺ ions in the excitation and emission bands, as shown in Figs. 2(a) and 2(b).

 

Fig. 2 Excitation (a), emission (b) spectra, luminescence decay curves (c) and XEL spectra (d). Inset shows the normalized emission spectra in 412.5-462.5 nm region.

Download Full Size | PDF

The luminescence decay curves (λ_ex = 375 nm, λ_em = 433 nm) of Ce³⁺-activated borogermanate glasses are shown in Fig. 2(c). All the luminescence decay curves of Ce³⁺ ions significantly deviate from the single exponential rule, but they can be well fitted by two exponentials. Thus, the mean lifetimes of all the non-exponential curves are evaluated by a sum of two exponential functions [8]. The mean lifetimes of Ce³⁺-activated borogermanate glasses are evaluated to be about 32.73, 32.12, 32.83, 33.58 and 33.92 ns with an increase in melting temperature, respectively.

As a promising candidate to scintillating crystals to detect high-energy X-, alpha- and beta-rays, the XEL spectra of Ce³⁺-activated borogermanate glasses are displayed in Fig. 2(d). The XEL spectra is very similar to the emission spectra (see the Fig. 2(b)), and the XEL intensity of Ce³⁺-activated borogermanate glasses are also observed to be enhanced dramatically with an increase in the melting temperature. However, some detectable Eu³⁺ emission at about 625 and 654 nm also appear in the XEL spectra, which originates from the impurity of Gd₂O₃ raw materials because the impurity Eu³⁺ ions always exists in Gd₂O₃ raw material [8].

To find out the plausible reason for the enhanced Ce³⁺ emission induced by an increase in melting temperature, we have mentioned that the glass density decrease from 5.62 to 5.37 g/cm³ with an increase in melting temperature. This law may be related to the modification of not only the glass structure, but also the glass composition. The glass structure is hardly changed according to the unpolarized Raman scattering spectra (not shown here), because there is no new Raman shift detectable besides our previously assigned band centered at about 320, 580, 810, 940 and 1410 cm⁻¹ [16, 17]. It is worthy to point out that all Ce³⁺-activated borogermanate glass with the same composition is synthesized in an Al₂O₃ crucible, the difference is only the melting temperature. Therefore, we speculate the dissolved Al³⁺ from crucible should not be neglected and be responsible for the enhanced emission intensity of Ce³⁺ ions, because the deeper corrosion trace of aluminum cubicle at higher temperature are observed. To further confirm this speculation, we have carried out and shown the EDS spectra of the T = 1350, 1400 and 1450 °C glasses in Fig. 3, respectively.The analysis results persuasively suggest that the concentration of Al³⁺ ions increase, from 5.27 (T = 1350 °C) to 7.09 (T = 1400 °C) and 8.21 at% (T = 1450 °C), with an increase in melting temperature. As a fact of matter, the optical properties of luminescent glasses induced by either the intended incorporation of Al³⁺ or the evitable dissolution of Al³⁺ from crucible has been received attentions in recent years [18–20]. The role of Al³⁺ ions is well known to isolate the RE-O-RE bonds and form Al-O-RE bonds, such declustering effect, leading to larger RE separation, which results in the enhanced emission intensity [18–20].

 

Fig. 3 EDS spectra of the T = 1350 (a), T = 1400 (b) and T = 1450 °C (c) glasses

Download Full Size | PDF

4. Conclusions

More than 80% Ce⁴⁺ is effectively reduced to Ce³⁺ ions in borogermanate glasses synthesized in air atmosphere at different melting temperature in the 1350-1450 °C region. The luminescence intensity of Ce³⁺ ions increases noticeably with an increasing melting temperature, which attributes to the declustering effect of the dissolved Al³⁺ from Al₂O₃ crucible. The developed Ce³⁺-activated borogermanate shows a broad emission centered at 430 nm and a decay time of about 33 ns, which will be promising in the scintillating fields of high energy physics engineering and medical imaging.