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Tb3+-activated SiO2–Al2O3–CaO–CaF2 oxyfluoride scintillating glass ceramics
Nuclear Instruments and Methods in Physics Research A 621 (2010) 322–325
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tb3 + -activated SiO2–Al2O3–CaO–CaF2 oxyfluoride scintillating glass ceramics Xin-yuan Sun a,n, Shi-ming Huang b a b
Department of Physics, Jinggangshan University, Ji’an 343009, PR China Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Department of Physics, Tongji University, 200092 Shanghai, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 February 2010 Received in revised form 6 April 2010 Accepted 7 April 2010 Available online 13 April 2010
In this paper, transparent glass ceramics containing CaF2 nanocrystals were synthesized by heattreating as-made Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glass. The precipitated crystalline phase in the glass ceramics is CaF2 nanocrystals identified by X-ray diffraction (XRD) patterns. Both photoluminescence (PL) and X-ray excited luminescence (XEL) results show that enhancement of Tb3 + 542 nm emission intensity by a factor of about 2–3 is achieved in glass ceramics with respect to the as-made glass. XEL intensity of the investigated glass ceramics is comparable to that of the commercial glass sciltillatior (LHK-6 type), which suggests its potential for scintillation application. & 2010 Elsevier B.V. All rights reserved.
Keywords: Transparent glass ceramic CaF2 nanocrystals Photoluminescence X-ray excited luminescence
1. Introduction Glass is an attractive scintillating material in view of its lowcost, large-volume production possibility and easy shaping of elements [1–3]. Glass, however, being the lack of long-range order and the presence of many point defects gives rise to trapping sites responsible for non-radiative recombinations [2,3], which result in its low light yields. One of the reasons is that activators generally require a crystalline environment to scintillate effectively, because the probability of radiative recombination of excitons in ordered (crystalline) environment is usually larger than in disordered (amorphous) ones [4]. Hence glass scintillators have been less frequently studied with respect to scintillating crystals in recent years. Oxyfluoride glass ceramics doped with rare earths were studied extensively in the past decades, since such materials provide a desirable low phonon energy fluoride environment for activator ions while maintaining the advantages of an oxide glass, such as high mechanical strength, chemical durability, and thermal stability. Glass ceramics of this kind usually contain small crystalline phases such as nanosize PbxCd1 xF2 crystals in the host glass, which can improve optical properties without loss of transparency [5,6]. However, both CdF2 and PbF2 raw materials are highly poisonous and cannot be used widely from the environment point of view. In 1998, a novel glass ceramic containing LaF3 nanocrystals was developed by Dejneka [7]. The efficiency of b-induced luminescence of this kind of glass ceramic
n
Corresponding author. Tel.: + 86 796 8100488. E-mail address: [email protected] (X.-y. Sun).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.032
is reported to be comparable or nearly equal to that of the Schott IQI-301 product, which is encouraging for its scintillation potential [8]. However, the cost of LaF3 material is an obstacle for industrial applications, and large efforts have been made to synthesize some novel glass ceramics containing nanocrystals such as NaYF4 and Sr2GdF7 to reduce the synthesis cost [9,10]. At the present time, some transparent glass ceramics containing MF2 (M ¼Ca, Sr, Ba) nanocrystals have been paid more attention due to both lower cost and lesser toxicity, and they have been found to be useful in various fields such as upconversion [11,12], quantum cutting [13], white light-emitting-diodes (LEDs) [14], and scintillation applications [15,16]. Moreover, crystalline CaF2 is highly transparent from 0.13 to 9.5 mm and gives good matching of refractive index with the aluminosilicate host glass [17], so this glass ceramic has been interesting for physics and materials scientists. The luminescent properties of Tb3 + in transparent glass and glass ceramics containing CaF2 nanocrystals were investigated in our previous work [15]. The objective of this paper is to investigate the scintillating properties of Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glasses by XRD patterns, PL, and XEL spectra. Gd3 + ions are codoped due to their remarkable sensitizing effect on Tb3 + ions [18–20], and results suggest that the investigated glass ceramics have potential for scintillation applications.
2. Experimental Oxyfluoride glasses with the nominal composition of 45SiO2– 20Al2O3–10CaO–25CaF2 were prepared using high-purity grade oxide or salts, including SiO2, Al2O3, CaCO3, CaF2, Tb2O3
X.-y. Sun, S.-m. Huang / Nuclear Instruments and Methods in Physics Research A 621 (2010) 322–325
(111)
(220)
GC725
GC700
GC675
GC650
G
20
30
40 50 2 Theta (deg.)
Dhkl ¼
Kl
o cos y
where Dhkl is the crystal size in the vertical direction of (h k l), l the ˚ y the angle of diffraction, w the wavelength of X-ray (Cu Ka: 1.541 A), full width at half maximum (FWHM) of the diffraction peak, and constant K¼0.89. The Scherrer formula is applied to the (2 2 0) reflection of cubic phase CaF2 at about 471. All line profiles are fitted by a Lorentzian function, as shown typically in Fig. 1(b). The calculated results indicate that the size of precipitated CaF2 nanocrystals increases with growing temperature, and the biggest size of CaF2 nanocrystals in GC725 sample is estimated to be about 24 nm. Based on Hopper’s theory [21], the critical size of participating CaF2 nanocrystals (less than 30 nm) is crucial for getting high transparent glass ceramics, which is of significance for scinitillation application.
Intensity (a.u.)
46.5
47.0 47.5 2 Theta (Deg.)
48.0
Fig. 1. (a) XRD patterns of the as-made glass and glass ceramics and (b) the XRD peak used for particle size analysis of GC725 sample and the corresponding Lorentzian fitting curve.
200
7F 5
150 5D
5 3
G GC650 GC675 GC700 GC725
D4
100 7
F6
7
F5
50
7F 4 7
F6
7F 2
7F 4
7F 3
3.2. PL spectra Fig. 2 shows PL spectra of the as-made glass and glass ceramics under excitation by 225 nm light. The Tb3 + emissions are composed of two groups of transitions [15]: the blue emission bands centering at 382, 415, 437, 456, and 471 nm attributed to 5 D3-7FJ (J¼6, 5, 4, 3, and 2), while the green ones are located at 488, 542, 586, and 620 nm assigned to 5D4-7FJ (J¼6, 5, 4, and 3). The PL intensity of Tb3 + 542 nm emission increases with growing heat-treating temperature until reaching a maximum at 675 1C, and then it decreases.
70
ω = 0.36
46.0
Intensity (a.u.)
Fig. 1(a) shows XRD patterns of the as-made glass and the corresponding glass ceramics. XRD pattern of the as-made glass shows no diffraction peaks, indicating its amorphous structure in nature. After heat-treating, the diffraction peaks of glass ceramics can be easily assigned to cubic CaF2 crystalline phase and indexed as (1 1 1), (2 2 0), and (3 1 1) in Fig. 1(a). Moreover, the width of the peaks becomes sharper with higher heat-treating temperature, which indicates the gradual formation of CaF2 nanocrystals in the glass ceramics. From the obtained peak width of XRD patterns, the crystal size of CaF2 in glass ceramics can be estimated by the Scherrer formula [11]:
60
GC725 glass-ceramic Model: Lorentz y = y0 + (2*A/PI)* (w/(4*(x-xc)^2 + w^2)) y0 99.68056 ± 5.856 xc 46.77639 ± 0.00774 w 0.36202 ± 0.03095
3. Results and discussion 3.1. XRD pattern analysis
(311)
Intensity (a.u.)
(1.5 mol%), and Gd2O3 (0.75 mol%) as the starting materials. About 100 g batches were mixed well and melted at 1400–1450 1C for 2 h in a platinum crucible in air. The melts were quickly poured to a preheated stainless steel mold. The resultant glass was annealed in a muffle furnace at 600 1C for 3 h to release inner stress. Glass samples (denoted as G) with a regular size of 20 mm 20 mm 2 mm were finally obtained after being cut and polished carefully. The glass samples were then annealed precisely for another 4 h at 650, 675, 700, and 725 1C to induce crystallization and prepare transparent glass ceramics (denoted as GC650, GC675, GC700, and GC725, respectively). Crystalline phase were identified by X-ray diffraction (Brucker D8) using Cu Ka radiation with 2y from 201 to 601. PL spectra were recorded on a Perkin-Elmer luminescence spectrometer LS55, equipped with a Xe lamp as an excitation source. XEL spectra were performed by an X-ray excited spectrometer, FluorMain, where an F-30 X-ray tube (W anticathode target) was the X-ray source, and operated under 80 kV and 6 mA. Luminescence spectra were recorded with a 44 W plate grating monochromator and Hamamatsu R928-28 photomultiplier and data acquired by a computer. All measurements were carried out at room temperature.
323
0 350
400
450
500
550
600
7F 3
650
Wavelength (nm) Fig. 2. PL spectra of the as-made glass and glass ceramics (lex ¼ 225 nm).
It is well known that the probability of radiative recombination of excitons in ordered crystalline environment is usually larger than in disordered (amorphous) environment [4]. As indicated by the
324
X.-y. Sun, S.-m. Huang / Nuclear Instruments and Methods in Physics Research A 621 (2010) 322–325
G GC650 GC675 GC700 GC725
1500
Intensity (a.u.)
Intensity (a.u.)
1200
GC675 LKH-6 glass
1500
900 600
1000
500
300 0 450
0 500
550
600
650
450
500
550
600
650
Wavelength (nm)
Wavelength (nm) Fig. 3. XEL spectra of the as-made glass and glass ceramics (80 kV, 6 mA).
Fig. 4. XEL spectra of GC675 glass ceramic and LKH-6 glass.
XRD results, the crystal size of CaF2 nanocrystals in glass ceramics increases with growing temperature, which suggests that more Tb3 + ions can enter the fluorine environment, and form CaF2:Tb3 + nanocrystals [15,22]. Compared to a silicate-based host (the energy of Si–O vibration is 1100 cm 1), CaF2 crystals have a lower phonon energy of 320 cm 1 [23]. Excited Tb3 + therefore have a reduced multiphonon decay rate, resulting in an enhanced radiative emission rate. The higher the heat-treating temperature, the larger the number of CaF2:Tb3 + nanocrystals. As a result, stronger emissions of Tb3 + are observed in these glass ceramics. On the other hand, the reduction of Tb3 + PL intensity in GC700 and GC725 samples may be associated with both concentration quenching and reduction of their transparency [15]. Since more Tb3 + ions enter the precipitated CaF2 nanocrystals with growing temperature, their blue and green emissions can be quenched with a cross-relaxation process by means of Tb3 + (5D3)+Tb3 + (7F6)-Tb3 + (5D4)+Tb3 + (7F0), as shown by the blue emissions in Fig. 2.
energies, because the interaction mechanism differs remarkably in the case of ultraviolet light and X-ray excitation [19,20]. Under ultraviolet excitation, energy is deposited by direct excitation into the terbium ions. X-rays first interact with the host materials and produce electrons and holes, which then interact with the terbium ions. Fig. 4 shows the XEL spectra of the GC675 sample and the commercial LKH-6 type glass scintillator under the same experimental conditions. It is encouraging that the XEL intensity of GC675 sample is comparable to that of commercial LKH-6 type glass scintillator. This implies that it will be a potential scintillator by emphasis on optimizing the composition of oxyfluoride glass and the concentrations of Tb3 + and Gd3 + ions.
3.3. XEL spectra Fig. 3 shows the XEL spectra of the as-made glass and glass ceramics. It is obviously seen that the green emissions of Tb3 + ions always increase with growing temperature. Compared with the as-made glass, the XEL intensity of 542 nm line is enhanced by a factor of 3 in the GC675, GC700, or GC725 glass ceramics. There are usually three stages involved in a scintillating process [1,8]: (a) absorption of incident radiation by the host and conversion of energy into thermalized electrons and holes, (b) transfer of some fraction of the excited electrons and holes to luminescence centers, and (c) luminescence process. The efficiency of a scintillator is determined by the product of those for each stage. Compared with fully crystalline materials, glasses are expected to be less efficient in stage (b), since more point defects are present for its disordered structure [2,3]. The efficiency in stage (a) is degraded in materials with high phonon frequencies, but is not dependent on crystallinity of the scintillator. However, the efficiency of stage (c) is principally determined by the quantum yield of Tb3 + ions, which is high and not very hostdependent. The enhancement of Tb3 + XEL intensity is attributed to the CaF2:Tb3 + nanocrystals with lower phonon energy, as illustrated in the case of PL spectra, but the XEL intensity of Tb3 + emissions does not decrease in the investigated glass ceramics on X-ray excitation, which may be associated with different incident
4. Conclusions A novel transparent scintillating glass ceramic containing CaF2 nanocrystals has been successfully synthesized by heat-treating the as-made Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glass. It is found that the inclusion of CaF2:Tb3 + nanocrystals in the host glass is beneficial for more Tb3 + emission under both UV and X-ray excitations. The main reason for this is that Tb3 + prefers entering the precipitated CaF2 phase and forms CaF2:Tb3 + nanocrystals with low phonon energy. The XEL intensity of the investigated glass ceramics is comparable to that of the commercial glass scintillator (LKH-6 type), which suggests its potential for scintillation applications.
Acknowledgements This work was supported by the National Natural Science Fund of China (Grant no. 10904114), Natural Science Foundation of Jiangxi Province (Grant no. 2009GQ W0010), Technology Project of Jiangxi Provincial Department of Education (Grant No. GJJ10203), and the Program for Young Excellent Doctors in Jinggangshan University. References [1] M.J. Webber, J. Lumin. 100 (2002) 35. [2] M. Nikl, J.A. Mares, E. Mihokova, K. Nitsch, N. Solovieva, V. Babin, A. Krasnikov, S. Zazubovich, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, S. Baccaro, Radiat. Meas. 33 (2001) 593.
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[3] G.P. Pazzi, P. Fabeni, C. Susini, M. Nikl, E. Mihokova, N. Solovieva, K. Nitsch M. Martini, A. Vedda, S. Baccaro, A. Cevilia, V. Babin, Nucl. Instr. and Meth. B 191 (2002) 366. [4] G. Chen, J. Hohnson, S. Schweizer, Proc. SPIE 6142 (2006) 6142X-1. [5] Y. Wang, J. Ohwaki, Appl. Phys. Lett. 63 (1993) 3268. [6] G. Dantelle, M. Mortier, G. Patriarche, D. Vivien, J. Solid State Chem. 179 (2006) 1995. [7] M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. [8] Z. Pan, K. James, Y. Cui, A. Burger, N. Cherepy, S.A. Payne, R. Mu, S.H. Morgan, Nucl. Instr. and Meth. A 594 (2008) 215. [9] F. Liu, E. Ma, D. Chen, Y. Yu, Y. Wang, J. Phys. Chem. B 110 (2006) 20843. [10] X. Qiao, X. Fan, M. Wang, Appl. Phys. Lett. 89 (2006) 111919-1. [11] L. Huang, G. Qin, Y. Arail, R. Jose, T. Suzuki, Y. Ohishi, T. Yamashita Y. Akimoto, J. Appl. Phys. 102 (2007) 093506. [12] X. Qiao, X. Fan, M. Wang, Scr. Mater. 55 (2006) 211. [13] S. Ye, B. Zhu, J. Chen, J. Luo, J. Qiu, Appl. Phys. Lett. 99 (2008) 141112.
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[14] P. Babu, K.H. Jang, E.S. Kim, L. Shi, H.J. Seo, F.E. Lopez, U.R. RodriguezMendoza, V. Lavin, R. Vijaya, C.K. Jayasankarm, L.R. Moorthy, J. Appl. Phys. 105 (2009) 013516. [15] X. Sun, M. Gu, S. Huang, X. Jie, X. Liu, B. Liu, C. Ni, J. Lumin. 129 (2009) 773. [16] J. Fu, M. Kobayashi, S. Sugimoto, J.M. Parker, J. Am. Ceram. Soc. 92 (2009) 2119. [17] X. Qiao, X. Fan, M. Wang, J. Adam, X. Zhang, J. Phys.: Condens. Matter 18 (2006) 6397. [18] X. Sun, S. Huang, M. Gu, Q. Gao, X. Gong, Z. Ye, Physica B 405 (2010) 569. [19] R.A. Buchanan, C. Bueno, US Patent, Pat. No. 5120970, 1992. [20] R.A. Buchanan, C. Bueno, US Patent, Pat. No. 5391320, 1995. [21] G.H. Beall, L.R. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [22] D. Chen, Y. Wang, Y. Yu, E. Ma, F. Bao, Z. Hu, Y. Cheng, Mater. Chem. Phys. 95 (2006) 264. [23] W.B. Lacina, P.S. Pershan, Phys. Rev. B 1 (1970) 1756.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tb3 + -activated SiO2–Al2O3–CaO–CaF2 oxyfluoride scintillating glass ceramics Xin-yuan Sun a,n, Shi-ming Huang b a b
Department of Physics, Jinggangshan University, Ji’an 343009, PR China Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Department of Physics, Tongji University, 200092 Shanghai, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 February 2010 Received in revised form 6 April 2010 Accepted 7 April 2010 Available online 13 April 2010
In this paper, transparent glass ceramics containing CaF2 nanocrystals were synthesized by heattreating as-made Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glass. The precipitated crystalline phase in the glass ceramics is CaF2 nanocrystals identified by X-ray diffraction (XRD) patterns. Both photoluminescence (PL) and X-ray excited luminescence (XEL) results show that enhancement of Tb3 + 542 nm emission intensity by a factor of about 2–3 is achieved in glass ceramics with respect to the as-made glass. XEL intensity of the investigated glass ceramics is comparable to that of the commercial glass sciltillatior (LHK-6 type), which suggests its potential for scintillation application. & 2010 Elsevier B.V. All rights reserved.
Keywords: Transparent glass ceramic CaF2 nanocrystals Photoluminescence X-ray excited luminescence
1. Introduction Glass is an attractive scintillating material in view of its lowcost, large-volume production possibility and easy shaping of elements [1–3]. Glass, however, being the lack of long-range order and the presence of many point defects gives rise to trapping sites responsible for non-radiative recombinations [2,3], which result in its low light yields. One of the reasons is that activators generally require a crystalline environment to scintillate effectively, because the probability of radiative recombination of excitons in ordered (crystalline) environment is usually larger than in disordered (amorphous) ones [4]. Hence glass scintillators have been less frequently studied with respect to scintillating crystals in recent years. Oxyfluoride glass ceramics doped with rare earths were studied extensively in the past decades, since such materials provide a desirable low phonon energy fluoride environment for activator ions while maintaining the advantages of an oxide glass, such as high mechanical strength, chemical durability, and thermal stability. Glass ceramics of this kind usually contain small crystalline phases such as nanosize PbxCd1 xF2 crystals in the host glass, which can improve optical properties without loss of transparency [5,6]. However, both CdF2 and PbF2 raw materials are highly poisonous and cannot be used widely from the environment point of view. In 1998, a novel glass ceramic containing LaF3 nanocrystals was developed by Dejneka [7]. The efficiency of b-induced luminescence of this kind of glass ceramic
n
Corresponding author. Tel.: + 86 796 8100488. E-mail address: [email protected] (X.-y. Sun).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.032
is reported to be comparable or nearly equal to that of the Schott IQI-301 product, which is encouraging for its scintillation potential [8]. However, the cost of LaF3 material is an obstacle for industrial applications, and large efforts have been made to synthesize some novel glass ceramics containing nanocrystals such as NaYF4 and Sr2GdF7 to reduce the synthesis cost [9,10]. At the present time, some transparent glass ceramics containing MF2 (M ¼Ca, Sr, Ba) nanocrystals have been paid more attention due to both lower cost and lesser toxicity, and they have been found to be useful in various fields such as upconversion [11,12], quantum cutting [13], white light-emitting-diodes (LEDs) [14], and scintillation applications [15,16]. Moreover, crystalline CaF2 is highly transparent from 0.13 to 9.5 mm and gives good matching of refractive index with the aluminosilicate host glass [17], so this glass ceramic has been interesting for physics and materials scientists. The luminescent properties of Tb3 + in transparent glass and glass ceramics containing CaF2 nanocrystals were investigated in our previous work [15]. The objective of this paper is to investigate the scintillating properties of Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glasses by XRD patterns, PL, and XEL spectra. Gd3 + ions are codoped due to their remarkable sensitizing effect on Tb3 + ions [18–20], and results suggest that the investigated glass ceramics have potential for scintillation applications.
2. Experimental Oxyfluoride glasses with the nominal composition of 45SiO2– 20Al2O3–10CaO–25CaF2 were prepared using high-purity grade oxide or salts, including SiO2, Al2O3, CaCO3, CaF2, Tb2O3
X.-y. Sun, S.-m. Huang / Nuclear Instruments and Methods in Physics Research A 621 (2010) 322–325
(111)
(220)
GC725
GC700
GC675
GC650
G
20
30
40 50 2 Theta (deg.)
Dhkl ¼
Kl
o cos y
where Dhkl is the crystal size in the vertical direction of (h k l), l the ˚ y the angle of diffraction, w the wavelength of X-ray (Cu Ka: 1.541 A), full width at half maximum (FWHM) of the diffraction peak, and constant K¼0.89. The Scherrer formula is applied to the (2 2 0) reflection of cubic phase CaF2 at about 471. All line profiles are fitted by a Lorentzian function, as shown typically in Fig. 1(b). The calculated results indicate that the size of precipitated CaF2 nanocrystals increases with growing temperature, and the biggest size of CaF2 nanocrystals in GC725 sample is estimated to be about 24 nm. Based on Hopper’s theory [21], the critical size of participating CaF2 nanocrystals (less than 30 nm) is crucial for getting high transparent glass ceramics, which is of significance for scinitillation application.
Intensity (a.u.)
46.5
47.0 47.5 2 Theta (Deg.)
48.0
Fig. 1. (a) XRD patterns of the as-made glass and glass ceramics and (b) the XRD peak used for particle size analysis of GC725 sample and the corresponding Lorentzian fitting curve.
200
7F 5
150 5D
5 3
G GC650 GC675 GC700 GC725
D4
100 7
F6
7
F5
50
7F 4 7
F6
7F 2
7F 4
7F 3
3.2. PL spectra Fig. 2 shows PL spectra of the as-made glass and glass ceramics under excitation by 225 nm light. The Tb3 + emissions are composed of two groups of transitions [15]: the blue emission bands centering at 382, 415, 437, 456, and 471 nm attributed to 5 D3-7FJ (J¼6, 5, 4, 3, and 2), while the green ones are located at 488, 542, 586, and 620 nm assigned to 5D4-7FJ (J¼6, 5, 4, and 3). The PL intensity of Tb3 + 542 nm emission increases with growing heat-treating temperature until reaching a maximum at 675 1C, and then it decreases.
70
ω = 0.36
46.0
Intensity (a.u.)
Fig. 1(a) shows XRD patterns of the as-made glass and the corresponding glass ceramics. XRD pattern of the as-made glass shows no diffraction peaks, indicating its amorphous structure in nature. After heat-treating, the diffraction peaks of glass ceramics can be easily assigned to cubic CaF2 crystalline phase and indexed as (1 1 1), (2 2 0), and (3 1 1) in Fig. 1(a). Moreover, the width of the peaks becomes sharper with higher heat-treating temperature, which indicates the gradual formation of CaF2 nanocrystals in the glass ceramics. From the obtained peak width of XRD patterns, the crystal size of CaF2 in glass ceramics can be estimated by the Scherrer formula [11]:
60
GC725 glass-ceramic Model: Lorentz y = y0 + (2*A/PI)* (w/(4*(x-xc)^2 + w^2)) y0 99.68056 ± 5.856 xc 46.77639 ± 0.00774 w 0.36202 ± 0.03095
3. Results and discussion 3.1. XRD pattern analysis
(311)
Intensity (a.u.)
(1.5 mol%), and Gd2O3 (0.75 mol%) as the starting materials. About 100 g batches were mixed well and melted at 1400–1450 1C for 2 h in a platinum crucible in air. The melts were quickly poured to a preheated stainless steel mold. The resultant glass was annealed in a muffle furnace at 600 1C for 3 h to release inner stress. Glass samples (denoted as G) with a regular size of 20 mm 20 mm 2 mm were finally obtained after being cut and polished carefully. The glass samples were then annealed precisely for another 4 h at 650, 675, 700, and 725 1C to induce crystallization and prepare transparent glass ceramics (denoted as GC650, GC675, GC700, and GC725, respectively). Crystalline phase were identified by X-ray diffraction (Brucker D8) using Cu Ka radiation with 2y from 201 to 601. PL spectra were recorded on a Perkin-Elmer luminescence spectrometer LS55, equipped with a Xe lamp as an excitation source. XEL spectra were performed by an X-ray excited spectrometer, FluorMain, where an F-30 X-ray tube (W anticathode target) was the X-ray source, and operated under 80 kV and 6 mA. Luminescence spectra were recorded with a 44 W plate grating monochromator and Hamamatsu R928-28 photomultiplier and data acquired by a computer. All measurements were carried out at room temperature.
323
0 350
400
450
500
550
600
7F 3
650
Wavelength (nm) Fig. 2. PL spectra of the as-made glass and glass ceramics (lex ¼ 225 nm).
It is well known that the probability of radiative recombination of excitons in ordered crystalline environment is usually larger than in disordered (amorphous) environment [4]. As indicated by the
324
X.-y. Sun, S.-m. Huang / Nuclear Instruments and Methods in Physics Research A 621 (2010) 322–325
G GC650 GC675 GC700 GC725
1500
Intensity (a.u.)
Intensity (a.u.)
1200
GC675 LKH-6 glass
1500
900 600
1000
500
300 0 450
0 500
550
600
650
450
500
550
600
650
Wavelength (nm)
Wavelength (nm) Fig. 3. XEL spectra of the as-made glass and glass ceramics (80 kV, 6 mA).
Fig. 4. XEL spectra of GC675 glass ceramic and LKH-6 glass.
XRD results, the crystal size of CaF2 nanocrystals in glass ceramics increases with growing temperature, which suggests that more Tb3 + ions can enter the fluorine environment, and form CaF2:Tb3 + nanocrystals [15,22]. Compared to a silicate-based host (the energy of Si–O vibration is 1100 cm 1), CaF2 crystals have a lower phonon energy of 320 cm 1 [23]. Excited Tb3 + therefore have a reduced multiphonon decay rate, resulting in an enhanced radiative emission rate. The higher the heat-treating temperature, the larger the number of CaF2:Tb3 + nanocrystals. As a result, stronger emissions of Tb3 + are observed in these glass ceramics. On the other hand, the reduction of Tb3 + PL intensity in GC700 and GC725 samples may be associated with both concentration quenching and reduction of their transparency [15]. Since more Tb3 + ions enter the precipitated CaF2 nanocrystals with growing temperature, their blue and green emissions can be quenched with a cross-relaxation process by means of Tb3 + (5D3)+Tb3 + (7F6)-Tb3 + (5D4)+Tb3 + (7F0), as shown by the blue emissions in Fig. 2.
energies, because the interaction mechanism differs remarkably in the case of ultraviolet light and X-ray excitation [19,20]. Under ultraviolet excitation, energy is deposited by direct excitation into the terbium ions. X-rays first interact with the host materials and produce electrons and holes, which then interact with the terbium ions. Fig. 4 shows the XEL spectra of the GC675 sample and the commercial LKH-6 type glass scintillator under the same experimental conditions. It is encouraging that the XEL intensity of GC675 sample is comparable to that of commercial LKH-6 type glass scintillator. This implies that it will be a potential scintillator by emphasis on optimizing the composition of oxyfluoride glass and the concentrations of Tb3 + and Gd3 + ions.
3.3. XEL spectra Fig. 3 shows the XEL spectra of the as-made glass and glass ceramics. It is obviously seen that the green emissions of Tb3 + ions always increase with growing temperature. Compared with the as-made glass, the XEL intensity of 542 nm line is enhanced by a factor of 3 in the GC675, GC700, or GC725 glass ceramics. There are usually three stages involved in a scintillating process [1,8]: (a) absorption of incident radiation by the host and conversion of energy into thermalized electrons and holes, (b) transfer of some fraction of the excited electrons and holes to luminescence centers, and (c) luminescence process. The efficiency of a scintillator is determined by the product of those for each stage. Compared with fully crystalline materials, glasses are expected to be less efficient in stage (b), since more point defects are present for its disordered structure [2,3]. The efficiency in stage (a) is degraded in materials with high phonon frequencies, but is not dependent on crystallinity of the scintillator. However, the efficiency of stage (c) is principally determined by the quantum yield of Tb3 + ions, which is high and not very hostdependent. The enhancement of Tb3 + XEL intensity is attributed to the CaF2:Tb3 + nanocrystals with lower phonon energy, as illustrated in the case of PL spectra, but the XEL intensity of Tb3 + emissions does not decrease in the investigated glass ceramics on X-ray excitation, which may be associated with different incident
4. Conclusions A novel transparent scintillating glass ceramic containing CaF2 nanocrystals has been successfully synthesized by heat-treating the as-made Tb3 + /Gd3 + -codoped 45SiO2–20Al2O3–10CaO–25CaF2 oxyfluoride glass. It is found that the inclusion of CaF2:Tb3 + nanocrystals in the host glass is beneficial for more Tb3 + emission under both UV and X-ray excitations. The main reason for this is that Tb3 + prefers entering the precipitated CaF2 phase and forms CaF2:Tb3 + nanocrystals with low phonon energy. The XEL intensity of the investigated glass ceramics is comparable to that of the commercial glass scintillator (LKH-6 type), which suggests its potential for scintillation applications.
Acknowledgements This work was supported by the National Natural Science Fund of China (Grant no. 10904114), Natural Science Foundation of Jiangxi Province (Grant no. 2009GQ W0010), Technology Project of Jiangxi Provincial Department of Education (Grant No. GJJ10203), and the Program for Young Excellent Doctors in Jinggangshan University. References [1] M.J. Webber, J. Lumin. 100 (2002) 35. [2] M. Nikl, J.A. Mares, E. Mihokova, K. Nitsch, N. Solovieva, V. Babin, A. Krasnikov, S. Zazubovich, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, S. Baccaro, Radiat. Meas. 33 (2001) 593.
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