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Investigation of Ce3 + luminescence in borate-rich borosilicate glasses
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
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Investigation of Ce3 + luminescence in borate-rich borosilicate glasses M.W. Kieltya, M. Dettmannb, V. Herrigb, M.G. Chapmana, M.R. Marchewkaa, A.A. Trofimova, U. Akgunb, L.G. Jacobsohna,c,⁎ a b c
Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA Physics Department, Coe College, Cedar Rapids, IA 52402, USA COMSET – Center for Optical Materials Science and Engineering Technologies, Clemson University, Anderson, SC 29625, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Borosilicate glass Cerium Raman Luminescence Defects
An investigation of Ce-activated borate-rich borosilicate glasses was performed in a composition range previously unexplored, with high B2O3:SiO2 ratios from 4.83:1 to 8.75:1. Glass samples were prepared by the melt quenching method in air. Samples were individually heated for 15 min at 1000 °C, evaluated for weight loss due to the release of volatile species, followed by a final melt for 15 min at 1000 °C, and then annealed at 350 °C for one hour to enhance stability. Particular emphasis was placed on the luminescence of the glasses investigated under X-ray and ultraviolet excitation. Luminescence was interpreted in terms of defects and Ce3 + in different environments. The results indicated it was possible to achieve a composition of the host matrix that exhibited ultraviolet transparency cutoff at 326 nm and a well-defined emission from Ce3 +, especially under X-ray excitation.
1. Introduction Despite the long availability of glass scintillators [1–4], there is growing interest in the development of new rare-earth (RE) activated scintillating glasses as alternatives to single crystals for the detection and measurement of ionizing radiation [5]. The primary advantages glasses offer over single crystals are that glasses can be manufactured faster and at lower-cost, which can be combined with the ability to easily manufacture large volumes in virtually any shape. Glasses must satisfy numerous conditions to be an effective scintillator, including optical transparency to the emission of the activator, high luminosity, and effective ionizing radiation stopping power. Recently, the focus has been given to glasses with relatively large amounts of REs to achieve high densities aiming at more efficient X- and gamma-ray scintillators [5–10]. REs are also used as the main luminescence activators of scintillating glasses [11–13], with Ce3 + being the best activator due to its fast decay time of tens of nanoseconds, and matching of its emission wavelength to the peak operating efficiency of photomultiplier tubes. Further, the luminescence of Ce3 + involves the 5d1 → 4f parity allowed transition that has a large transition probability. Notably, the 4f ground state is spin-orbit split into two levels, 2F5/2 and 2F7/2, separated by about 2000 cm− 1 (~ 0.25 eV) thus generating a double emission band. Since the 5d1 level lacks shielding from outer electrons, the 5d1 → 4f transition is greatly affected by the chemical and crystallographic nature of the host [14]. ⁎
Luminescent materials containing boron, and particularly boratecontaining glasses [3,15–17], have the potential for being thermal neutron scintillators due to the high absorption cross-section of 10B combined with its naturally large isotopic abundance of 19.9% [18]. Further, the composition of the glasses can be made exclusively with chemical elements with low atomic numbers to suppress gamma-ray absorption. While extensive investigation of Ce3 + luminescence in glasses has been performed, Ce-activated borosilicate glasses received limited attention with research focused on glass compositions with low B2O3:SiO2 ratios, from about 0.09:1 to 1.8:1 [10,19–22]. This work aims at bridging this gap through the fabrication and investigation of Ce-activated borosilicate glasses with high B2O3:SiO2 ratios, from 4.83:1 to 8.75:1. 2. Experimental procedure Glass samples were prepared by the melt quenching method in platinum crucibles in air. The amount of precursors used reflected the correct molar ratios for each sample according to the batch procedure described in [23], and then the precursors were thoroughly mixed for 5 min to ensure homogeneity of the melt. Ten gram batches were produced with the following starting materials: boric acid (H3BO3, Sigma-Aldrich, 99.5%), silicon dioxide (SiO2, Sigma-Aldrich, 99.6%), aluminum oxide, (Al2O3, Brockmann I, Sigma-Aldrich), cerium chloride (CeCl3, 99.999%), lithium carbonate (Li2CO3, Sigma-Aldrich, 99.0%),
Corresponding author at: Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA. E-mail address: [email protected] (L.G. Jacobsohn).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.022 Received 24 March 2017; Received in revised form 15 June 2017; Accepted 17 June 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Kielty, M.W., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.022
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Table 1 Glass label (#), B2O3:SiO2 ratio, nominal glass composition, density, transmission cutoff wavelength (λcutoff), and index of refraction for increasing Li2O content. Glass #
2 6 14 9 10 1 11
B2O3:SiO2
8.75 5.83 5 4.83 5 8.75 8.75
Nominal composition (mol%) B2O3
SiO2
Li2O
Na2O
Al2O3
Ce2O3
70 70 70 70 70 70 70
8 12 14 14.5 14 8 8
– 12 13 13.75 14 20 20.8
20 – – – – – –
1 1 1 1 1 1 1
1 5 2 0.75 1 1 0.2
and sodium carbonate (Na2CO3, Sigma-Aldrich, 99.5%). Each sample was individually heated for 15 min at 1000 °C, and after the initial melt, samples were weighed to compare the experimental weight loss to the predicted weight loss due to the release of volatile species during the melt. All samples had less than 1% difference in weight before and after the initial melt. Samples underwent a final melt for 15 min at 1000 °C, were poured into a graphite mold, and annealed at 350 °C for one hour to enhance stability. Table 1 summarizes the glass compositions investigated in this work organized as a function of the Li2O content. Density measurements were made using a Quantachrome MicroUltrapycnometer 1000. Density values reported in this work corresponded to the average of multiple measurements for each sample. The standard deviation of the measurements was less than 0.01 g/cm3 for all samples. The index of refraction was estimated for each composition, including the activator content, by means of the Gladstone-Dale relation [24,25] using the experimentally-determined density as input. Raman scattering measurements were executed using a JASCO NRS Imaging Raman spectrometer using the 488 nm line of an argon ion laser with 50 mW of power. Optical transmission spectra were collected using a Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer. Radioluminescence (RL) measurements were executed using a custom-designed Freiberg Instruments Lexsyg Research spectrofluorometer equipped with a Varian Medical Systems VF-50 J X-ray tube with a tungsten target and operated at 40 kV and 1 mA coupled with an ionization chamber for dose measurement, an Andor Technology Shamrock 163 spectrograph, and an Andor Technology DU920P-BU Newton CCD camera. For these measurements, samples were powderized to fill 8 mm diameter 1 mm deep cups thus allowing for relative RL intensity comparison between different samples. Results were not corrected for the spectral sensitivity of the system. Photoluminescence (PL) spectra were obtained using a Horiba Jobin Yvon Fluorolog 3 spectrophotometer equipped with double monochromators for both excitation and detection, and a 450 W xenon arc lamp as the excitation source. All measurements were performed with excitation and detection spectral resolution of 1 nm.
Density (g/cm3)
λcutoff (nm)
n
2.25 2.31 2.18 2.12 2.14 2.22 2.25
367 435 404 379 326 378 314
1.47 1.48 1.47 1.47 1.47 1.50 1.49
Fig. 1. Density as a function of the alkali oxide:SiO2 ratio. The nominal Ce content is shown for each glass. The dotted line is a guide to the eye only.
higher alkali oxide:SiO2 ratios. For low alkali oxide:SiO2 ratios, around 1, density increased for higher Ce contents while for higher alkali oxide:SiO2 ratios, around 2.5, density seemed insensitive to the Ce content. According to previous works, the structure of boron oxide glass consists of 3-coordinated B2O3 forming boroxol rings interconnected by BeOeB bridges. Accordingly, Raman spectra of borate glass are dominated by a narrow peak at 808 cm− 1 assigned to the symmetric breathing of oxygen ions within these rings [28,29]. In addition to that, Raman spectra of borate glass contain a variety of other less intense bands originated from different BeO units, including a band at about 1260 cm− 1 assigned to a transverse optical mode from delocalized BeO stretching involving both the boroxol ring and contributions from the continuous random network [30,31]. It is noted that the major constituent of the glasses investigated in this work was B2O3 (70 mol%) combined with Li2O (from 12 to 20.8 mol%, with the exception of glass #2) and SiO2 (from 8 to 14.5 mol%). The composition of the glasses was such that Li2O + SiO2 compounds to 24–28.8 mol% total. The incorporation of alkali oxides in borate glasses is known to alter the network, disrupting the boroxol rings [32]. The ratio of 4-coordinated boron to 3-coordinated boron increases for higher alkali oxide:B2O3 ratios through the formation of BO4− units with an adjacent alkali ion for charge balance up to a certain amount of Li [33,34]. In terms of Raman measurements, these changes are manifested by the appearance of a Raman peak at 780 cm− 1 at the expense of the 808 cm− 1 peak [32]. The 780 cm− 1 peak has been attributed to sixmembered rings containing both planar BO3 and BO4 tetrahedra units [28,35]. In xLi2O·(1 − x)B2O3 glasses, the 808 cm− 1 peak has been reported to disappear for x ~ 0.2–0.25 [33,36]. In the Raman spectra of alkali borate glasses, the presence of diborate groups is manifested by a band around 1100 cm− 1 [35]. On the other hand, binary borosilicate glasses still present boroxol rings (808 cm− 1 Raman band) even with
3. Results and discussion As discussed earlier, this work focused on the investigation of borate-rich borosilicate glasses, with all glasses being composed of 70 mol % B2O3, 1 mol% Al2O3, SiO2 varying from 8 to 14.5 mol%, Li2O from 12 to 20 mol%, and Ce2O3 from 0.2 to 5 mol% such that the summation of all components is 100 mol%. In glass #2, Na2O fully substituted for 20 mol% Li2O. The high borate concentration and the relative small variation of the composition was reflected in the relatively low and narrow range of densities obtained, from 2.12 to 2.31 g/cm3 (cf. Table 1). These values were in overall agreement with the density of other borosilicate glasses [3,16,26] and similar to commercially available Ce-activated glass scintillators, e.g., [27]. Fig. 1 shows the dependence of the density on the alkali oxide:SiO2 ratio and on the Ce content. For a fixed Ce content (1 mol%), density tended to increase for 2
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agreement with the shoulder observed at about 1470 cm− 1 and reinforced the conclusion that vibrational modes above ~1200 cm− 1 were related to the boron network. Additional bands were observed at about 781 and 808 cm− 1, though in glasses #2, 11, and 14 the later band was manifested through the asymmetry of the 781 cm− 1 band. Overall, Gaussian deconvolution showed the 781/808 band area ratio increased for higher alkali oxide contents, revealing the relative increase of triborate rings at the expense of boroxol rings. The assignment of these bands was as described above and the results were in agreement with similar Li2O borosilicate glasses [38]. The band at about 490 cm− 1 was originated in bridging bonds BeOeB, BeOeSi, and SieOeSi that compose the three-dimensional network. In some glasses, a weak broad band centered at about 300 cm− 1 was also observed, though the unknown nature of this band hindered further analysis. In summary, despite variations in the chemical composition, all glasses presented the same structural features with the Raman spectra being dominated by bands related to the boron network. Glasses were also characterized in their optical properties, with particular attention to the transmission in the ultraviolet (UV)-blue region of the spectrum characterized by the transmission cutoff wavelength due to its strong effect on the Ce3 + luminescence of the glasses. These results are presented in Table 1 and Fig. 3. The cutoff wavelength depended strongly on the composition, ranging from 314 to 435 nm (Fig. 3a) and was controlled by the Ce content as shown in Fig. 3b.
Fig. 2. Raman scattering spectra vertically shifted to enhance visual clarity.
the incorporation of up to 60 mol% SiO2 [37], while a band at 475 cm− 1 increases relative to the 808 cm− 1 for increasing SiO2 contents. This band has been assigned to bending or rocking of BeOeB, BeOeSi, and SieOeSi bridging bonds [37]. Moreover, the addition of SiO2 alters the bands related to BeO stretching in the 1250–1500 cm− 1 region, leading to the appearance of two bands due to SieO stretching within 1000–1200 cm− 1 [37]. In the case of borosilicate glasses with 15 mol% SiO2 and 10 or 20 mol% Li2O, the presence of boroxol rings, tetraborate, diborate, and SiO4 tetrahedra groups with four bridging oxygen ions have been identified, with the band at 475 cm− 1 shifting to higher wavenumbers for higher Li2O contents and the band at 810 cm− 1 disappearing for the glass with 20 mol% Li2O [38]. The structure of RLi2O·B2O3·KSiO2 glasses has also been investigated by NMR, focusing on 29Si and revealing a simple or nearly so proportional sharing of the alkali between the borate and silicate structural groups [34]. Raman scattering measurements were used to gain insight into the atomic arrangement of the glasses. These results are presented in Fig. 2 where the spectra were shifted vertically to enhance visual clarity. The spectra were organized top-down for decreasing Li2O content, from 20.8 to 12 mol%, and overall for increasing Si2O content, with the first two top spectra with 8 mol% SiO2, and the following spectra with 12–14.5 mol% SiO2. The spectrum at the bottom of Fig. 2 has 20 mol% of Na2O instead of Li2O. All the spectra were similar and dominated by a broad band between about 950 and 1800 cm− 1 and peaked around 1280 cm− 1. This band resulted from the convolution of several bands and, in most cases, presented shoulders centered at about 1130, 1470, and 1630 cm− 1. Broad intense Raman bands are observed around ~ 1000–1300 cm− 1 in binary alkali silicate glasses [39], and within ~ 1200–1600 cm− 1 and possibly higher wavenumbers for binary alkali borate glasses [32,33,36]. It is thus suggested that the broad band within ~ 1100–1800 cm− 1 observed in this work corresponds to the superposition of all these contributions. The dominating feature peaking at about 1280 cm− 1 was assigned to delocalized BeO stretching within the boroxol ring with possible contribution from three-coordinated metaborate units, while the band centered at about 1130 cm− 1 was assigned to diborate groups [35] together with the SieO stretching contributions that occur within 1000–1200 cm− 1 [37]. Further, a band at around 1465–1495 cm− 1 was assigned to BeO vibrations in Na2O·SiO2·B2O3 glasses with its position depending on the specific composition [40]. The position of this band was in general
Fig. 3. a) Partial optical transmission spectra highlighting the cutoff wavelength of each glass. b) Cutoff wavelength as a function of the nominal Ce content. Dotted lines are guides to the eye only.
3
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Fig. 5. Integral RL as a function of the cutoff wavelength. Dotted lines are linear best fits.
therein]. The bands peaked within 2.69–2.77 eV from glasses #2, 6, 10, and 11 were in agreement with the emission of Ce3 + incorporated in silica [43,44], but they were also in agreement with the emission of oxygen vacancy (dicoordinated silicon) in silica reported to luminesce at 2.7–2.8 eV [42]. Glasses #1, 2, 6, 9, and 14 presented an emission band within 2.93–3.33 eV, in agreement with previous results from a 8%B2O3·92%SiO2 glass where Ce3 + emission was reported at 3.02 eV [45], and from a 40%Li2O·40%B2O3·20%SiO2 glass where it varied from 3.15 to 3.40 eV as Ce content increased from 0.1 to 2 mol% [46]. The emission of Ce3 + ions that are not fully incorporated into the silica network is commonly found in glasses treated at temperatures below about 450 °C, as it is the case in this work. This has been associated to an emission band within about 3.47–3.54 eV [43,47–49], and was tentatively assigned as the origin of the bands within 3.51–3.64 eV from glasses #9, 10, 11, and 14. This is in agreement with the reported Ce3 + emission from a 25%Na2O ⋅ 75% B2O3 glass at 3.44 eV, noting that the glass in that work was treated at around 450 °C [50]. The integrated RL emission is presented as a function of the cutoff wavelength in Fig. 5. The dotted lines correspond to the linear best fits to the cutoff wavelength values of glasses #2, 1, 9, 14, and 6, and of glasses #2 and 10, respectively. Overall, these results showed that RL output increased for lower cutoff wavelengths with the exception of glass #11 caused by its much lower Ce content, nearly 4× lower than the next lowest Ce concentration (0.75 mol% of glass #9). More importantly, it was found that glass #10 was at least 3× brighter than the other glasses. Glass #10 was selected for further investigation due to its superior UV transparency combined with its considerably higher relative RL brightness. The RL measurement revealed the presence of three bands centered at 1.85, 2.75, and 3.63 eV. Accordingly, PL emission and excitation measurements of these bands were executed, as shown in Fig. 6. In Fig. 6a, excitation spectrum monitored at 3.63 eV showed one band at 4.08 eV, while excitation at 4.08 eV yielded a main band at 3.55 and a weaker band at 2.79 eV. Best fit Gaussian deconvolution of the excitation spectrum revealed it to be composed of two superimposed bands centered at 4.01 and 4.40 eV. Fig. 6b shows the excitation spectrum monitored at 2.75 eV that peaked at 4.28 eV, and the emission spectrum excited at 4.28 eV that, again, yielded a main band at 3.58 and a weaker band at 2.76 eV. The results of the best fit Gaussian deconvolution of the excitation spectrum were similar to those presented in Fig. 6a, yielding two bands centered at 3.96 and 4.46 eV. These results suggested the emission band around 2.75 eV to be from Ce3 +, since the reported absorption/excitation band of oxygen vacancies is at 3.15 and 5.0 eV [42,51], and no such bands were observed. Fig. 6c presents the excitation spectrum monitored at 1.85 eV yielding one band centered at 4.20 eV. Best fit Gaussian deconvolution of the excitation spectrum revealed it to be composed of two superimposed bands centered at 4.08 and 4.49 eV. Excitation at 4.20 eV
Fig. 4. RL spectra (black line) together with the best fit of each spectrum (red line) and the individual Gaussian bands (green lines) used in the fitting. The peak position of each Gaussian band is indicated for each band in eV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Similar results were reported from Ce-activated binary Li2O ⋅B2O3 glasses prepared within a large range of compositions [20]. The substitution of Na2O for Li2O slightly improved UV transparency (glasses #1 and 2). The index of refraction was estimated using the Gladstone-Dale equation. In addition to the simplicity, previous work on heavy metal oxide glasses showed this approach to be reasonably accurate [40]. These results are presented in Table 1, showing the index of refraction values were relatively low and within a narrow range, from 1.47 to 1.50, in agreement with the relatively small compositional variation among the samples that are dominated by 70 mol% B2O3. These values were in overall agreement with previously obtained index of refraction values for Li2O ⋅ borate glasses [16]. Luminescence was investigated under X-ray (RL) and UV (PL) excitation. Fig. 4 summarizes the RL results (black line), together with the best fit of each spectrum (red line) and the individual Gaussian bands (green line) used in the fitting. The spectra presented more than one emission band, and all spectra could be fit with 3 Gaussian bands with the exception of glass #1 where only 2 bands were needed to obtain a high quality fitting. The peak position of each Gaussian band was also included in the figure in eV. Luminescence from substitutional Fe3 + impurity in quartz was reported at 1.75 eV [41] and is expected to be in a similar position in silica. The bands within 1.75–1.82 eV from glasses #1, 2 and 14 were assigned to this impurity. It is noted that the band peaked at 1.64 eV from glass #9 was mounted on a relatively strong background and its peak was not clearly defined. Consequently, there was higher uncertainty in the determination of the peak position. This band was tentatively assigned to Fe3 + impurities in silica. Non-bridging oxygen hole center (NBOHC) in silica luminesces at 1.85–1.95 eV [42], matching the bands peaking within 1.85–1.91 eV from glasses #6, 10, and 11 eV. Multiple emission bands in undoped and Ce-activated silica have been reported within about 2.70 to 3.61 eV [43 and references 4
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[21] [22] [23] Fig. 6. PL emission and excitation spectra (black lines) together with the best fit of each spectrum (red lines) and the individual Gaussian bands (green lines) used in the fitting. The peak position of each Gaussian band is indicated for each band in eV: a) excitation spectrum monitored at 3.63 eV, and emission spectrum excited at 4.08 eV; b) excitation spectrum monitored at 2.75 eV, and emission spectrum excited at 4.28 eV; and c) excitation spectrum monitored at 1.85 eV, and emission spectrum excited at 4.20 eV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Acknowledgements
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This material is based upon work supported by the National Science Foundation under Grants No. 1207080 and 1407404.
5
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Comment
Investigation of Ce3 + luminescence in borate-rich borosilicate glasses M.W. Kieltya, M. Dettmannb, V. Herrigb, M.G. Chapmana, M.R. Marchewkaa, A.A. Trofimova, U. Akgunb, L.G. Jacobsohna,c,⁎ a b c
Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA Physics Department, Coe College, Cedar Rapids, IA 52402, USA COMSET – Center for Optical Materials Science and Engineering Technologies, Clemson University, Anderson, SC 29625, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Borosilicate glass Cerium Raman Luminescence Defects
An investigation of Ce-activated borate-rich borosilicate glasses was performed in a composition range previously unexplored, with high B2O3:SiO2 ratios from 4.83:1 to 8.75:1. Glass samples were prepared by the melt quenching method in air. Samples were individually heated for 15 min at 1000 °C, evaluated for weight loss due to the release of volatile species, followed by a final melt for 15 min at 1000 °C, and then annealed at 350 °C for one hour to enhance stability. Particular emphasis was placed on the luminescence of the glasses investigated under X-ray and ultraviolet excitation. Luminescence was interpreted in terms of defects and Ce3 + in different environments. The results indicated it was possible to achieve a composition of the host matrix that exhibited ultraviolet transparency cutoff at 326 nm and a well-defined emission from Ce3 +, especially under X-ray excitation.
1. Introduction Despite the long availability of glass scintillators [1–4], there is growing interest in the development of new rare-earth (RE) activated scintillating glasses as alternatives to single crystals for the detection and measurement of ionizing radiation [5]. The primary advantages glasses offer over single crystals are that glasses can be manufactured faster and at lower-cost, which can be combined with the ability to easily manufacture large volumes in virtually any shape. Glasses must satisfy numerous conditions to be an effective scintillator, including optical transparency to the emission of the activator, high luminosity, and effective ionizing radiation stopping power. Recently, the focus has been given to glasses with relatively large amounts of REs to achieve high densities aiming at more efficient X- and gamma-ray scintillators [5–10]. REs are also used as the main luminescence activators of scintillating glasses [11–13], with Ce3 + being the best activator due to its fast decay time of tens of nanoseconds, and matching of its emission wavelength to the peak operating efficiency of photomultiplier tubes. Further, the luminescence of Ce3 + involves the 5d1 → 4f parity allowed transition that has a large transition probability. Notably, the 4f ground state is spin-orbit split into two levels, 2F5/2 and 2F7/2, separated by about 2000 cm− 1 (~ 0.25 eV) thus generating a double emission band. Since the 5d1 level lacks shielding from outer electrons, the 5d1 → 4f transition is greatly affected by the chemical and crystallographic nature of the host [14]. ⁎
Luminescent materials containing boron, and particularly boratecontaining glasses [3,15–17], have the potential for being thermal neutron scintillators due to the high absorption cross-section of 10B combined with its naturally large isotopic abundance of 19.9% [18]. Further, the composition of the glasses can be made exclusively with chemical elements with low atomic numbers to suppress gamma-ray absorption. While extensive investigation of Ce3 + luminescence in glasses has been performed, Ce-activated borosilicate glasses received limited attention with research focused on glass compositions with low B2O3:SiO2 ratios, from about 0.09:1 to 1.8:1 [10,19–22]. This work aims at bridging this gap through the fabrication and investigation of Ce-activated borosilicate glasses with high B2O3:SiO2 ratios, from 4.83:1 to 8.75:1. 2. Experimental procedure Glass samples were prepared by the melt quenching method in platinum crucibles in air. The amount of precursors used reflected the correct molar ratios for each sample according to the batch procedure described in [23], and then the precursors were thoroughly mixed for 5 min to ensure homogeneity of the melt. Ten gram batches were produced with the following starting materials: boric acid (H3BO3, Sigma-Aldrich, 99.5%), silicon dioxide (SiO2, Sigma-Aldrich, 99.6%), aluminum oxide, (Al2O3, Brockmann I, Sigma-Aldrich), cerium chloride (CeCl3, 99.999%), lithium carbonate (Li2CO3, Sigma-Aldrich, 99.0%),
Corresponding author at: Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA. E-mail address: [email protected] (L.G. Jacobsohn).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.022 Received 24 March 2017; Received in revised form 15 June 2017; Accepted 17 June 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Kielty, M.W., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.022
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Table 1 Glass label (#), B2O3:SiO2 ratio, nominal glass composition, density, transmission cutoff wavelength (λcutoff), and index of refraction for increasing Li2O content. Glass #
2 6 14 9 10 1 11
B2O3:SiO2
8.75 5.83 5 4.83 5 8.75 8.75
Nominal composition (mol%) B2O3
SiO2
Li2O
Na2O
Al2O3
Ce2O3
70 70 70 70 70 70 70
8 12 14 14.5 14 8 8
– 12 13 13.75 14 20 20.8
20 – – – – – –
1 1 1 1 1 1 1
1 5 2 0.75 1 1 0.2
and sodium carbonate (Na2CO3, Sigma-Aldrich, 99.5%). Each sample was individually heated for 15 min at 1000 °C, and after the initial melt, samples were weighed to compare the experimental weight loss to the predicted weight loss due to the release of volatile species during the melt. All samples had less than 1% difference in weight before and after the initial melt. Samples underwent a final melt for 15 min at 1000 °C, were poured into a graphite mold, and annealed at 350 °C for one hour to enhance stability. Table 1 summarizes the glass compositions investigated in this work organized as a function of the Li2O content. Density measurements were made using a Quantachrome MicroUltrapycnometer 1000. Density values reported in this work corresponded to the average of multiple measurements for each sample. The standard deviation of the measurements was less than 0.01 g/cm3 for all samples. The index of refraction was estimated for each composition, including the activator content, by means of the Gladstone-Dale relation [24,25] using the experimentally-determined density as input. Raman scattering measurements were executed using a JASCO NRS Imaging Raman spectrometer using the 488 nm line of an argon ion laser with 50 mW of power. Optical transmission spectra were collected using a Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer. Radioluminescence (RL) measurements were executed using a custom-designed Freiberg Instruments Lexsyg Research spectrofluorometer equipped with a Varian Medical Systems VF-50 J X-ray tube with a tungsten target and operated at 40 kV and 1 mA coupled with an ionization chamber for dose measurement, an Andor Technology Shamrock 163 spectrograph, and an Andor Technology DU920P-BU Newton CCD camera. For these measurements, samples were powderized to fill 8 mm diameter 1 mm deep cups thus allowing for relative RL intensity comparison between different samples. Results were not corrected for the spectral sensitivity of the system. Photoluminescence (PL) spectra were obtained using a Horiba Jobin Yvon Fluorolog 3 spectrophotometer equipped with double monochromators for both excitation and detection, and a 450 W xenon arc lamp as the excitation source. All measurements were performed with excitation and detection spectral resolution of 1 nm.
Density (g/cm3)
λcutoff (nm)
n
2.25 2.31 2.18 2.12 2.14 2.22 2.25
367 435 404 379 326 378 314
1.47 1.48 1.47 1.47 1.47 1.50 1.49
Fig. 1. Density as a function of the alkali oxide:SiO2 ratio. The nominal Ce content is shown for each glass. The dotted line is a guide to the eye only.
higher alkali oxide:SiO2 ratios. For low alkali oxide:SiO2 ratios, around 1, density increased for higher Ce contents while for higher alkali oxide:SiO2 ratios, around 2.5, density seemed insensitive to the Ce content. According to previous works, the structure of boron oxide glass consists of 3-coordinated B2O3 forming boroxol rings interconnected by BeOeB bridges. Accordingly, Raman spectra of borate glass are dominated by a narrow peak at 808 cm− 1 assigned to the symmetric breathing of oxygen ions within these rings [28,29]. In addition to that, Raman spectra of borate glass contain a variety of other less intense bands originated from different BeO units, including a band at about 1260 cm− 1 assigned to a transverse optical mode from delocalized BeO stretching involving both the boroxol ring and contributions from the continuous random network [30,31]. It is noted that the major constituent of the glasses investigated in this work was B2O3 (70 mol%) combined with Li2O (from 12 to 20.8 mol%, with the exception of glass #2) and SiO2 (from 8 to 14.5 mol%). The composition of the glasses was such that Li2O + SiO2 compounds to 24–28.8 mol% total. The incorporation of alkali oxides in borate glasses is known to alter the network, disrupting the boroxol rings [32]. The ratio of 4-coordinated boron to 3-coordinated boron increases for higher alkali oxide:B2O3 ratios through the formation of BO4− units with an adjacent alkali ion for charge balance up to a certain amount of Li [33,34]. In terms of Raman measurements, these changes are manifested by the appearance of a Raman peak at 780 cm− 1 at the expense of the 808 cm− 1 peak [32]. The 780 cm− 1 peak has been attributed to sixmembered rings containing both planar BO3 and BO4 tetrahedra units [28,35]. In xLi2O·(1 − x)B2O3 glasses, the 808 cm− 1 peak has been reported to disappear for x ~ 0.2–0.25 [33,36]. In the Raman spectra of alkali borate glasses, the presence of diborate groups is manifested by a band around 1100 cm− 1 [35]. On the other hand, binary borosilicate glasses still present boroxol rings (808 cm− 1 Raman band) even with
3. Results and discussion As discussed earlier, this work focused on the investigation of borate-rich borosilicate glasses, with all glasses being composed of 70 mol % B2O3, 1 mol% Al2O3, SiO2 varying from 8 to 14.5 mol%, Li2O from 12 to 20 mol%, and Ce2O3 from 0.2 to 5 mol% such that the summation of all components is 100 mol%. In glass #2, Na2O fully substituted for 20 mol% Li2O. The high borate concentration and the relative small variation of the composition was reflected in the relatively low and narrow range of densities obtained, from 2.12 to 2.31 g/cm3 (cf. Table 1). These values were in overall agreement with the density of other borosilicate glasses [3,16,26] and similar to commercially available Ce-activated glass scintillators, e.g., [27]. Fig. 1 shows the dependence of the density on the alkali oxide:SiO2 ratio and on the Ce content. For a fixed Ce content (1 mol%), density tended to increase for 2
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agreement with the shoulder observed at about 1470 cm− 1 and reinforced the conclusion that vibrational modes above ~1200 cm− 1 were related to the boron network. Additional bands were observed at about 781 and 808 cm− 1, though in glasses #2, 11, and 14 the later band was manifested through the asymmetry of the 781 cm− 1 band. Overall, Gaussian deconvolution showed the 781/808 band area ratio increased for higher alkali oxide contents, revealing the relative increase of triborate rings at the expense of boroxol rings. The assignment of these bands was as described above and the results were in agreement with similar Li2O borosilicate glasses [38]. The band at about 490 cm− 1 was originated in bridging bonds BeOeB, BeOeSi, and SieOeSi that compose the three-dimensional network. In some glasses, a weak broad band centered at about 300 cm− 1 was also observed, though the unknown nature of this band hindered further analysis. In summary, despite variations in the chemical composition, all glasses presented the same structural features with the Raman spectra being dominated by bands related to the boron network. Glasses were also characterized in their optical properties, with particular attention to the transmission in the ultraviolet (UV)-blue region of the spectrum characterized by the transmission cutoff wavelength due to its strong effect on the Ce3 + luminescence of the glasses. These results are presented in Table 1 and Fig. 3. The cutoff wavelength depended strongly on the composition, ranging from 314 to 435 nm (Fig. 3a) and was controlled by the Ce content as shown in Fig. 3b.
Fig. 2. Raman scattering spectra vertically shifted to enhance visual clarity.
the incorporation of up to 60 mol% SiO2 [37], while a band at 475 cm− 1 increases relative to the 808 cm− 1 for increasing SiO2 contents. This band has been assigned to bending or rocking of BeOeB, BeOeSi, and SieOeSi bridging bonds [37]. Moreover, the addition of SiO2 alters the bands related to BeO stretching in the 1250–1500 cm− 1 region, leading to the appearance of two bands due to SieO stretching within 1000–1200 cm− 1 [37]. In the case of borosilicate glasses with 15 mol% SiO2 and 10 or 20 mol% Li2O, the presence of boroxol rings, tetraborate, diborate, and SiO4 tetrahedra groups with four bridging oxygen ions have been identified, with the band at 475 cm− 1 shifting to higher wavenumbers for higher Li2O contents and the band at 810 cm− 1 disappearing for the glass with 20 mol% Li2O [38]. The structure of RLi2O·B2O3·KSiO2 glasses has also been investigated by NMR, focusing on 29Si and revealing a simple or nearly so proportional sharing of the alkali between the borate and silicate structural groups [34]. Raman scattering measurements were used to gain insight into the atomic arrangement of the glasses. These results are presented in Fig. 2 where the spectra were shifted vertically to enhance visual clarity. The spectra were organized top-down for decreasing Li2O content, from 20.8 to 12 mol%, and overall for increasing Si2O content, with the first two top spectra with 8 mol% SiO2, and the following spectra with 12–14.5 mol% SiO2. The spectrum at the bottom of Fig. 2 has 20 mol% of Na2O instead of Li2O. All the spectra were similar and dominated by a broad band between about 950 and 1800 cm− 1 and peaked around 1280 cm− 1. This band resulted from the convolution of several bands and, in most cases, presented shoulders centered at about 1130, 1470, and 1630 cm− 1. Broad intense Raman bands are observed around ~ 1000–1300 cm− 1 in binary alkali silicate glasses [39], and within ~ 1200–1600 cm− 1 and possibly higher wavenumbers for binary alkali borate glasses [32,33,36]. It is thus suggested that the broad band within ~ 1100–1800 cm− 1 observed in this work corresponds to the superposition of all these contributions. The dominating feature peaking at about 1280 cm− 1 was assigned to delocalized BeO stretching within the boroxol ring with possible contribution from three-coordinated metaborate units, while the band centered at about 1130 cm− 1 was assigned to diborate groups [35] together with the SieO stretching contributions that occur within 1000–1200 cm− 1 [37]. Further, a band at around 1465–1495 cm− 1 was assigned to BeO vibrations in Na2O·SiO2·B2O3 glasses with its position depending on the specific composition [40]. The position of this band was in general
Fig. 3. a) Partial optical transmission spectra highlighting the cutoff wavelength of each glass. b) Cutoff wavelength as a function of the nominal Ce content. Dotted lines are guides to the eye only.
3
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Fig. 5. Integral RL as a function of the cutoff wavelength. Dotted lines are linear best fits.
therein]. The bands peaked within 2.69–2.77 eV from glasses #2, 6, 10, and 11 were in agreement with the emission of Ce3 + incorporated in silica [43,44], but they were also in agreement with the emission of oxygen vacancy (dicoordinated silicon) in silica reported to luminesce at 2.7–2.8 eV [42]. Glasses #1, 2, 6, 9, and 14 presented an emission band within 2.93–3.33 eV, in agreement with previous results from a 8%B2O3·92%SiO2 glass where Ce3 + emission was reported at 3.02 eV [45], and from a 40%Li2O·40%B2O3·20%SiO2 glass where it varied from 3.15 to 3.40 eV as Ce content increased from 0.1 to 2 mol% [46]. The emission of Ce3 + ions that are not fully incorporated into the silica network is commonly found in glasses treated at temperatures below about 450 °C, as it is the case in this work. This has been associated to an emission band within about 3.47–3.54 eV [43,47–49], and was tentatively assigned as the origin of the bands within 3.51–3.64 eV from glasses #9, 10, 11, and 14. This is in agreement with the reported Ce3 + emission from a 25%Na2O ⋅ 75% B2O3 glass at 3.44 eV, noting that the glass in that work was treated at around 450 °C [50]. The integrated RL emission is presented as a function of the cutoff wavelength in Fig. 5. The dotted lines correspond to the linear best fits to the cutoff wavelength values of glasses #2, 1, 9, 14, and 6, and of glasses #2 and 10, respectively. Overall, these results showed that RL output increased for lower cutoff wavelengths with the exception of glass #11 caused by its much lower Ce content, nearly 4× lower than the next lowest Ce concentration (0.75 mol% of glass #9). More importantly, it was found that glass #10 was at least 3× brighter than the other glasses. Glass #10 was selected for further investigation due to its superior UV transparency combined with its considerably higher relative RL brightness. The RL measurement revealed the presence of three bands centered at 1.85, 2.75, and 3.63 eV. Accordingly, PL emission and excitation measurements of these bands were executed, as shown in Fig. 6. In Fig. 6a, excitation spectrum monitored at 3.63 eV showed one band at 4.08 eV, while excitation at 4.08 eV yielded a main band at 3.55 and a weaker band at 2.79 eV. Best fit Gaussian deconvolution of the excitation spectrum revealed it to be composed of two superimposed bands centered at 4.01 and 4.40 eV. Fig. 6b shows the excitation spectrum monitored at 2.75 eV that peaked at 4.28 eV, and the emission spectrum excited at 4.28 eV that, again, yielded a main band at 3.58 and a weaker band at 2.76 eV. The results of the best fit Gaussian deconvolution of the excitation spectrum were similar to those presented in Fig. 6a, yielding two bands centered at 3.96 and 4.46 eV. These results suggested the emission band around 2.75 eV to be from Ce3 +, since the reported absorption/excitation band of oxygen vacancies is at 3.15 and 5.0 eV [42,51], and no such bands were observed. Fig. 6c presents the excitation spectrum monitored at 1.85 eV yielding one band centered at 4.20 eV. Best fit Gaussian deconvolution of the excitation spectrum revealed it to be composed of two superimposed bands centered at 4.08 and 4.49 eV. Excitation at 4.20 eV
Fig. 4. RL spectra (black line) together with the best fit of each spectrum (red line) and the individual Gaussian bands (green lines) used in the fitting. The peak position of each Gaussian band is indicated for each band in eV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Similar results were reported from Ce-activated binary Li2O ⋅B2O3 glasses prepared within a large range of compositions [20]. The substitution of Na2O for Li2O slightly improved UV transparency (glasses #1 and 2). The index of refraction was estimated using the Gladstone-Dale equation. In addition to the simplicity, previous work on heavy metal oxide glasses showed this approach to be reasonably accurate [40]. These results are presented in Table 1, showing the index of refraction values were relatively low and within a narrow range, from 1.47 to 1.50, in agreement with the relatively small compositional variation among the samples that are dominated by 70 mol% B2O3. These values were in overall agreement with previously obtained index of refraction values for Li2O ⋅ borate glasses [16]. Luminescence was investigated under X-ray (RL) and UV (PL) excitation. Fig. 4 summarizes the RL results (black line), together with the best fit of each spectrum (red line) and the individual Gaussian bands (green line) used in the fitting. The spectra presented more than one emission band, and all spectra could be fit with 3 Gaussian bands with the exception of glass #1 where only 2 bands were needed to obtain a high quality fitting. The peak position of each Gaussian band was also included in the figure in eV. Luminescence from substitutional Fe3 + impurity in quartz was reported at 1.75 eV [41] and is expected to be in a similar position in silica. The bands within 1.75–1.82 eV from glasses #1, 2 and 14 were assigned to this impurity. It is noted that the band peaked at 1.64 eV from glass #9 was mounted on a relatively strong background and its peak was not clearly defined. Consequently, there was higher uncertainty in the determination of the peak position. This band was tentatively assigned to Fe3 + impurities in silica. Non-bridging oxygen hole center (NBOHC) in silica luminesces at 1.85–1.95 eV [42], matching the bands peaking within 1.85–1.91 eV from glasses #6, 10, and 11 eV. Multiple emission bands in undoped and Ce-activated silica have been reported within about 2.70 to 3.61 eV [43 and references 4
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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[21] [22] [23] Fig. 6. PL emission and excitation spectra (black lines) together with the best fit of each spectrum (red lines) and the individual Gaussian bands (green lines) used in the fitting. The peak position of each Gaussian band is indicated for each band in eV: a) excitation spectrum monitored at 3.63 eV, and emission spectrum excited at 4.08 eV; b) excitation spectrum monitored at 2.75 eV, and emission spectrum excited at 4.28 eV; and c) excitation spectrum monitored at 1.85 eV, and emission spectrum excited at 4.20 eV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Acknowledgements
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This material is based upon work supported by the National Science Foundation under Grants No. 1207080 and 1407404.
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