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The effect of lithium fluoride on the thermal stability and thermoluminescence properties of borosilicate glass and glass-ceramics
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Original Article
The effect of lithium fluoride on the thermal stability and thermoluminescence properties of borosilicate glass and glass-ceramics ⁎
Marcin Środaa, Szymon Świonteka, , Wojciech Gieszczykb, Paweł Bilskib a b
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Poland The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass-ceramics LiF Oxyfluoride glass Borosilicate glass Thermoluminescence TL Alpha and beta radiation
The oxyfluoride glass and glass-ceramics from the LiF-B2O3-SiO2 system are developed. The stable glass can be produced in the range of 20–40 mol% LiF. The effect of LiF admixture on the thermal stability of the glass as well as the thermoluminescence (TL) properties such as glow curves shape is studied. The results show that the increase of lithium fluoride content in the borosilicate glass causes efficiency enhancement of the thermoluminescence signal. We have clearly stated that the process of controlled crystallization of the oxyfluoride glasses can lead again to increased intensity of the TL process. The glass-ceramics with 40 mol% LiF reveals similar level of TL signal to commercially used doped LiF material and can be considered as active material for alpha and beta radiation detectors.
1. Introduction The lithium fluoride has structure analogous to NaCl but it is much less soluble in water. Because of the wide and direct band gap for LiF (13.6 eV), its crystals are transparent to short wavelength ultraviolet radiation, far more than any other material [1]. Therefore, lithium fluoride crystals can be used as a means to record ionizing radiation exposure for gamma rays, beta particles and neutrons in thermoluminescent dosimeters (TLDs) [2–4]. However, LiF is toxic and it is impossible to use this material directly in biological environments. In some cases, i.e. brachytherapy, special detectors measuring ionizing radiation inside biologic tissues is required [5]. The first crucial feature of ionizing radiation detector is similar to human tissue equivalent absorption coefficient. It is necessary to use a detector which has similar effective atomic number (Zeff) [6]. It turns out that modified borosilicate glasses can be characterized by similar shielding parameters as for human organs [7,8]. However, elemental composition of the batch has the principal influence on the final thermoluminescence properties [9]. The presence of silica in borosilicate glass increases its mechanical strength and made the glass more chemical resistant especially for dissolution. The chemical resistance for environmental factors is the key advantage of glass dosimeters over crystalline ones [10,11]. The crystalline substances which exhibit thermoluminescence phenomenon could be extremely toxic for human health [12]. We believe that advantages of the glass and glass-ceramic
⁎
detectors could be its simply process of manufacturing, forming to particular shape and less toxicity. G. Anjaiah et al. studied the luminescence properties of rare earth doped lead fluoroborate glasses [13]. They showed that lead fluoroborate glasses have potential applicability for thermoluminescence purposes due to the wide optical energy band gap. The rare earth doped fluoroborate glasses as well as CaF2 with higher content of the luminescence centers was also studied [14–17]. Moreover, wider energy gap together with negligibly small saturation effect allows detector to measure in wide range of high-energy radiation without underestimation error. Additionally, deeper location of luminescence centers improves thermal stability of radiation history of the detector [18,19]. Due to the well known luminescence properties of crystalline LiF [20] we developed a new LiF-B2O3-SiO2 glass and glass-ceramics based on this system which could be less expensive and more utilitarian than currently used. The objective of this study was to characterize thermoluminescence properties of the oxyfluoride glasses in conjunction with their thermal stability and tendency to crystallization of the fluoride phases. 2. Experimental 2.1. Glass preparation All of the examined glasses were prepared by conventional method
Corresponding author. E-mail address: [email protected] (S. Świontek).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.037 Received 6 June 2019; Received in revised form 20 September 2019; Accepted 21 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Marcin Środa, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.037
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Table 1 The chemical composition of oxyfluoride borate glasses with their effective atomic number values. Glass no.
10LiF 20LiF 30LiF 40LiF 50LiF Zeff
Chemical composition of glass, mol% LiF
B2O3
SiO2
Zeff
10 20 30 40 50 LiF 8.2 [21]
45 40 35 30 25 B2O3 6.9 [22]
45 40 35 30 25 SiO2 11.1 [23]
9.3 9.2 9.1 9.0 8.9
of melting and supercooling of the melt. The nominal chemical composition of the glasses is presented in Table 1. Chemically pure compounds (LiF, SiO2, H3BO3) were used for the batch preparation. The composition of each batch was calculated for melting 30 g of glass. After the process of homogenization in the ceramic mortar, the batches were melted in the platinum crucible, in the electric furnace under the cover in order to restrain the evaporation of compounds at the temperature of 1350 °C for 30 min. The melts were supercooled by pouring them on pre-heated brass plate to about 350 °C and annealed at the temperature of 400 °C. Due to volatility of LiF and H3BO3 the correction of glass composition was estimated on level 20–15%. The values of volatility are based on mass lost analysis,
2.2. Methods of study DSC method using NETZSCH STA 449 F3 equipment and a platinum crucible in air atmosphere was applied to study the thermal stability of glass. Approximately 60 mg of grinded glass below 0.05 mm were used for DSC measurement from 250 to 1000 °C at the constant heating rate of 10 °C/s. The transformation temperature (Tg) established on inflection point and peak temperature of crystallization (Tc) were determined. Δcp at Tg temperature and ΔH of crystallization were calculated using NETZSCH software. Scanning electron microscope (SEM) images of the 10LiF and 50LiF glasses were taken with the use of Nova NanoSEM microscope. The X-ray diffraction analysis (Philips X’Pert Diffractometer with CuKα radiation) was used to confirm the amorphous nature of the glass and crystallization products after the thermal treatment. The thermoluminescence measurements were performed for as-made glass and glass-ceramics to analyze the influence of crystallization. Each sample was irradiated with the same dose of alpha and beta radiation, respectively. Before the second irradiations the samples were heated to the temperature of 400 °C at the constant heating rate of 5 °C/s to erase the residual signal coming from the previous exposures. FT-IR spectra were recorded in the middle infrared region (MIR) with Bruker Company Vertex 70 v spectrometer. Spectra were collected at room temperature in the range of 4000–500 cm–1 using typically 128 scans at 4 cm–1 resolution. The samples were prepared by the standard KBr pellet method. Glass and glass-ceramic samples were grinded to fine powders less than 100 μm for thermoluminescence measurements. The samples of glass were irradiated with beta radiation (90Sr/90Y source) dose of c.a. 9.4 Gy and with alpha particles (241Am) fluence of 1.21·108 1/scm2 (c.a. 19.85Gy in terms of an average dose delivered to the irradiated volume of the sample). The response of samples to alpha and beta radiation was studied in consecutive measurements with one additional erasing readout between them. The automatic Risø TL/OSL-DA-20 reader was used for measurements. Detailed specification of the reader and its performance is described by Bilski et al. [24]. The measurements were carried out with the use of BG39 optical filter which transmits blue and green light. All the measured curves were analyzed using GlowVIEW
Fig. 1. Scanning electron microscope (SEM) images with EDX analysis of (a) 10LiF sample (Magnification 10000×), (b) 50LiF sample (Magnification 20000×), (c) 30LiF sample heat treated at 720 °C (Magnification 5000×).
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Fig. 3. DSC curves for oxyfluoride glass samples.
Table 2 Characteristic temperatures for oxyfluoride glass samples where Tg – transformation temperature calculated as the deflection point, TX – the temperature of onset crystallization, Tc – the temperature of peak crystallization, kgl – Hruby coefficient, ΔH – enthalpy of crystallization, Δcp – change of specific heat capacity at thetransformation range.
Fig. 2. XRD patterns of 10LiF sample (a) and 50LiF sample (b).
software [25].
Glass no.
Tg ( ± 3 °C)
Tx ( ± 3 °C)
Tc ( ± 3 °C)
kgl
ΔH (J/g)
Δcp (J/ (g K))
10LiF 20LiF 30LiF 40LiF
419 473 423 466
— 658 648 618
— 694 672 644
— 1.52 2.45 1.01
— 156.0 79.6 104.5
0.044 0.608 0.312 0.456
3. Results 3.1. Thermal stability of glass – SEM/XRD/DSC analysis All prepared glasses, except for 10LiF and 50LiF, were good quality, homogeneous and transparent. 10LiF sample showed loss of translucency and 50LiF sample became uniformly white. Fig. 1a presents SEM image of 10LiF sample with heterogeneity, which appear as lighter or darker regions. Due to limitation of the technique, the light elements with low electron energy, i.e. Li, B are not observed in EDX analysis. But a difference in the Si content between the two regions can be distinguished. No crystalline phase was confirmed by XRD analysis in 10LiF sample (Fig. 2a). On the other hand, the 50LiF sample underwent spontaneous crystallization of LiF (JCPDS card no: 01-089-3610) during melt cooling (Fig. 2b) which is also observed in SEM image (Fig. 1b). After process of heat-treatment at 720 °C, homogeneous and transparent LiF samples crystallized to LiBF4 phase (Fig. 1c). EDX analysis confirmed that forming crystallites contain fluoride. Fig. 3 and Table 2 show DSC curves and thermal parameters of the glasses, respectively. The glasses are characterized by transformation temperature and crystallization in the range 410–460 °C and 620–720 °C, respectively. The 10LiF glass is different from the other and shows no effect of the crystallization. With the increase of LiF the crystallization peak shifts towards lower temperature. This broad exothermal peak comes from crystallization of LiBF4 (JCPDS card no: 00040-1431) and α-SiO2 (JCPDS card no: 01-089-3433) according to XRD analysis (Fig. 4). Following the XRD analysis we confirmed the phase separation without any crystallization in 10LiF sample, crystallization of LiBF4 and α-SiO2 after thermal treatment of the 20LiF, 30LiF, 40LiF glasses and spontaneous crystallization of 50LiF during melting process.
Fig. 4. XRD pattern of 40LiF glass sample heat treated at 720 °C for 60 min. Stars and circles indicate LiBF4 and α-SiO2 phases, respectively.
significant information about bond vibrations, which involves the bending, stretching and rotating vibrational motions in molecule structure [26]. The FT-IR spectra of the analyzed glasses are characterized by the presence of three main active infrared spectral regions in the range: 700–800 cm−1, 800–1200 cm−1 and 1200–1650 cm−1. Due to the constant ratio of (B2O3/SiO2) = 1 of the all studied glasses, no significant changes in the intensity of the bands are observed. However, we confirmed that LiF affects the position of absorption maxima. The absorption bands shift towards higher wavenumbers as the LiF content increases in the glass. This results from the fact that Li atoms act as a network modifier. Increase of their content depolymerize network of the glass. It is worth to notice that LiF itself shows no bands in the range due to the lack of a change in dipole moment as a result of the vibration.
3.2. FT-IR study Fig. 5 illustrates the FT-IR spectra of all tested glasses and provides 3
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Fig. 5. FT-IR spectra and their deconvolution for oxyfluoride borosilicate glasses.
3.3.2. Glass-ceramic samples Fig. 7 shows thermoluminescence glow curves for the heat treated glass samples at 600 °C and 720 °C, respectively. Thermoluminescence of samples after alpha radiation is significantly lower then after beta radiation due to lower penetrability of the first one and this phenomena remains unchanged after ceramization. Thermal treatment of the glasses after beta radiation significant changes the TL signal as to numbers of maxima, intensity and temperature of the peaks. Thus, we observe two peaks at 80 °C and above 270 °C for 20LiF sample heat treated at 600 °C. Three peaks at 80 °C, 125 °C and 270 °C are visible on the glow curve of the sample after ceramization at 720 °C. For 30LiF sample heat treated at 600 °C glow curve consists of broad peak which maximum corresponds to the temperature of 95 °C and with shoulder at 140 °C. The intensity of TL signal for the heat treated sample is greater than seventy times in comparison with the as-made glass. Moreover, the sample treated at temperature of 720 °C shows enhancement of TL signal more than three times compared to the sample annealed at 600 °C without significant shifting the peak position. 40LiF sample after heat treatment reveals much more narrower peaks than other samples. The main one is visible at 175 °C and the second lower peak at 100 °C for sample heat treated at 600 °C. The increase in temperature of the thermal treatment to 720 °C shifts the peaks to lower temperature (maximum at 160 °C and 90 °C,
3.3. Thermoluminescence study 3.3.1. As-made glasses The thermoluminescence (TL) glow curves for lithium fluoroborate glass samples are shown in Fig. 6. The glow curve of 10LiF sample consist of two low peaks at 80 °C and 235 °C, respectively. On the other hand the glow curves of 20LiF and 30LiF samples consist of one dominant peak which corresponds to the temperature of 235 °C and 220 °C, respectively. The intensity of TL signals after alpha and beta exposition is very low for 20LiF sample and it is comparable to the background noise in case of the sample after alfa radiation exposition. The increase of LiF content induces higher TL signal which is greater five and ten times in 30LiF sample compared to 20LiF for alpha and beta radiation, respectively. The glow curves of 40LiF sample showed the highest TL response with two peaks at the temperatures of 130 °C and 205 °C and two shoulders at 225 °C and 270 °C. The intensity of the main peak is greater than two hundred times in comparison with 30LiF sample after beta radiation exposition. For 50LiF sample which spontaneously crystallized the intensity of main peak is lower than for 40LiF but slightly higher than for 30LiF. Moreover, the peak position of maximum for 50LiF sample is similar to 10LiF for which phase separation was confirmed (Fig. 1a).
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Fig. 6. TL glow curves of oxyfluoride borosilicate glasses with the use of BG39 optical filter (solid line refers to beta radiation exposition while dash line refers to alpha radiation exposition): a) 10LiF, b) 20LiF, c) 30LiF, d) 40LiF, e) 50LiF.
With the low content of LiF (10LiF sample) no crystallization (Fig. 2a) but phase separation (Fig. 1a) appears. When the content of both glass formers decreases to 50 mol % (50LiF sample) the spontaneous devitrification is observed as a formation of LiF (Fig. 2b). In this case the glass network is low polymerized what promotes process of the crystallization. The fluorides are often used as opacificiers to produce milky silicate glass in glass technology, e.g. for the production of lampshades [28] and they play a role of nucleation agents. In our glass, LiF facilitates mixing silica and borate units in the glass network. We observe the phase separation only in the 10LiF glass with the lowest content of LiF. As shown in Fig. 1, the spinoidal decomposition is revealed for the glass. Moreover, no crystallization is confirmed by XRD analysis (Fig. 2). The additional confirmation of the process is shown on DSC curve (Fig. 3) where a broad transformation range is visible with the lowest transformation temperature (Tg = 419 °C). Our study reveals
respectively). In addition, the third low peak appears at 220 °C after heat treatment at 720 °C. 4. Discussion Glasses involving simultaneously both SiO2 and B2O3 shows strong tendency to phase separation and this effect was widely investigated, especially in the ternary Na2O-Ba2O3-SiO2 [27]. A. K. Varshneya indicates that the large cations as K+, Rb+, Cs+ show decreased tendency to phase separation. This is mainly for three reasons: 1) higher number of components increases the entropy of the system, 2) an incorporated modifier facilitates the formation of common borosilicate network due to compensation of charge of B3+ and formation of [BO4] tetrahedral, 3) the flexibility of the network increases and different structural units can coexist together forming mix-network glass. Account taken of the above, the stable glass can be obtained in the range 20–40 mol% LiF. 5
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Fig. 7. TL glow curves of heat treated glass samples with the use of BG39 optical filter (solid lines refer to beta radiation exposition while dash lines refer to alpha radiation exposition). a) 20LiF, b) 30LiF, c) 40LiF.
that not only the large cations, but also LiF admixture increases homogeneity and thermal stability of the borosilicate glass. However, this effect is observed only to a limited extend. The highest Hruby parameter kgl = 2.45 which is an indicator of thermal stability was obtained for 30LiF glass. It is assumed that glasses with kgl > 1 are believed to have high thermal stability. Our glasses with LiF content in the range from 20 to 40 mol% meet this condition. On the other hand, the glass with the highest content of LiF (50% mol) demonstrates spontaneous crystallization of griceite (LiF). It is accepted that the network of binary alkali borate glasses contains trigonal planar [BO3] units and tetrahedral [BO4] units. It is known that the variation of properties in borate glasses follows the variation of [BO3]/[BO3 + BO4] ratio, which is depended on the alkali metal oxide content [29]. FT-IR curves of our samples show the same character with the three main regions [30–33]: 1) above 1200 cm−1 due to the asymmetric stretching vibration of the BeO bond of trigonal [BO3] structural units, 2) in the range 1200 cm−1 and 800 cm−1 including the BeO and SieO bonds asymmetric stretching vibration of tetrahedral [BO4] and [SiO4] structural units, respectively, 3) between 800–600 cm−1 because of the bending vibration of BeOeB and SieOeSi linkages in the borosilicate network.
Fig. 8. The area ratio of the envelops assigned to [BX3] and [BX4] with [SiX4] units. The line is drawn as guide to the eye.
series of glass samples. This parameter was calculated as surface area under envelope from 1600 to 1250 cm−1 units divided by the area of envelope in the range 1250–750 cm−1 as representative of [BX3] and tetrahedral [BX4] and [SiX4], respectively. The contribution of [BX3] diminishes in the glass structure with increasing LiF content, as seen in Fig. 8. Our study shows that two factors determine the thermoluminescence of the borosilicate glasses: a content of LiF in the glass and temperature of the heat treatment. With the greater amount of LiF in the glass, the higher efficiency of TL is observed, but only the stable glass can be obtained up to 40 mol% of LiF. The thermal treatment of glass leads to crystallization of LiBF4 and α-SiO2 and formation of glassceramic, thereby enhancing the TL signal. After heat treatment at the
Deconvolution of the spectra reveals some changes in a position of the absorption bends (Fig. 5). We observe a monotonic shift of the maxima towards higher wavenumbers with the increase of LiF. This phenomena also appears in the Li2O-B2O3 glasses with an increasing content of Li2O [34]. It indicates a continuous polymerization of the network. To confirm the effect of the LiF on the glass structure, we calculated the area of the bands derived from [BX3], [BX4] and [SiX4] vibrational regions of FT-IR spectra, where X refers to O and F atoms, simultaneously. Fig. 8 shows the [BX3]/([BX4] + [SiX4]) ratio for our 6
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borosilicate glass were described by Clark et al. [37]. The glow curve for pure borosilicate glass consists of one broad peak at c.a. 150 °C. What is more, the reproducibility of TL response for borosilicate glass was proved by E. Salama et al. [38]. However, P. Szajerski et al. [39] described simultaneous glass network disruption in borosilicate glasses which caused formation of low energy traps and shifting TL peak positions to lower temperatures. 5. Conclusion In this research oxyfluoride borate glasses and glass-ceramics from LiF-B2O3-SiO2 system were developed. Our studies revealed that up to 40 wt. % of LiF can be introduced to the borosilicate glass network. LiF significantly decreases the phase separation phenomena in the glass and increases the thermal stability, even though glass-ceramics with optical active LiBF4 was obtained. The glass exhibits thermoluminescence effect, which efficiency increases with the higher content of lithium fluoride in the glass. Moreover, the glass-ceramics with lithium tetrafluoroborate (LiBF4) shows higher TL signal and sensitivity than the glass. The level of TL signal of our glass-ceramics is comparable to the commercial material based on LiF:Mg,Ti but with the lower temperature (about 50 °C) and narrow signal of the luminescence response. The results show that glass-ceramics from LiF-B2O3-SiO2 system with 40 mol % of LiF could be considered as an active material in the TL detectors.
Fig. 9. The effect of LiF content and thermal treatment on peak temperature of the TL signal. The line is drawn as guide to the eye.
Acknowledgements This research has been supported by the EU Project POWR.03.02.00-00-I004/16 and the statutory funds of AGH University of Science and Technology Department of Materials Science and Ceramics AGH number WIMiC No 11.11.160.365 in 2019. References [1] R.C. Chaney, E.E. Lafon, C.C. Lin, Energy band structure of lithium fluoride crystals by the method of tight binding, Phys. Rev. B 4 (1971) 2734–2741. [2] B. Obryk, P. Bilski, M. Budzanowski, M. Fuerstner, C. Ilgner, F. Jaquenod, P. Olko, M. Puchalska, H. Vincke, The response of different types of TL lithium fluoride detectors to high-energy mixed radiation fields, Radiat. Meas. 43 (2008) 1144–1148. [3] B. Obryk, P. Bilski, M. Glaser, M. Fuerstner, M. Budzanowski, P. Olko, A. Pajor, The response of TL lithium fluoride detectors to 24 GeV/c protons for doses ranging up to 1 MGy, Radiat. Meas. 45 (2010) 643–1645. [4] H.J. Khoury, B. Obryk, V.S. Barros, P.L. Guzzo, C.G. Ferreira, P. Bilski, P. Olko, Response of TL lithium fluoride detectors (MTS) to high gamma radiation doses, Radiat. Meas. 46 (2011) 1878–1881. [5] J. Skowronek, M. Wierzbicka, M. Leszczyńska, W. Szyfter, Brachyterapia paliatywna PDR i HDR w leczeniu nawrotów miejscowych nowotworów głowy i szyi, Rep. Pract. Oncol. Radiother. 8 (2003) 362–368. [6] M.I. Sayyed, S.A.M. Issa, H.O. Tekin, Y.B. Saddeek, Comparative study of gammaray shielding and elastic properties of BaO–Bi2O3–B2O3 and ZnO–Bi2O3–B2O3 glass systems, Mater. Chem. Phys. 217 (2018) 11–22. [7] N. Chanthima, J. Kaewkhao, Investigation on radiation shielding parameters of bismuth borosilicate glass from 1 keV to 100 GeV, Ann. Nucl. Energy 55 (2013) 23–28. [8] Y. Isokawa, S. Hirano, G. Okada, N. Kawaguchi, T. Yanagida, Characterization of cedoped lithium borosilicate glasses as tissue equivalent phosphors for radiation measurements, Radiat. Meas. 111 (2018) 13–18. [9] R.A. Clark, J.D. Robertson, J.M. Schwantes, Intrinsic dosimetry: elemental composition effects on the thermoluminescence of commercial borosilicate glass, Radiat. Meas. 59 (2013) 270–276. [10] R. Laopaiboon, C. Bootjomchai, Physical properties and thermoluminescence of glasses designed for radiation dosimetry measurements, Mater. Des. 80 (2015) 20–27. [11] R. Laopaiboon, C. Bootjomchai, Thermoluminescence studies on alkali-silicate glass doped with dysprosium oxide for use in radiation dosimetry measurement, J. Lumin. 158 (2015) 275–280. [12] M. Isik, E. Bulur, N.M. Gasanly, Photo-transferred thermoluminescence of shallow traps in β-irradiated BeO ceramics, J. Lumin. 187 (2017) 290–294. [13] G. Anjaiah, S.N. Rasool, P. Kistaiah, Spectroscopic and visible luminescence properties of rare earth ions in lead fluoroborate glasses, J. Lumin. 159 (2015) 110–118. [14] S. Ibrahim, F.H. ElBatal, A.M. Abdelghany, Optical character enrichment of NdF3 – doped lithium fluoroborate glasses, J. Non-Cryst. Solids 453 (2016) 16–22. [15] Y.C. Ratnakaram, A. Balakrishna, D. Rajesh, M. Seshadri, Influence of modifier oxides on spectroscopic properties of Sm3+ doped lithium fluoroborate glass, J.
Fig. 10. The comparison of TL sensitivity results for 40LiF samples with data obtained for commercially used doped LiF material [40,41].
peak temperature obtained from DSC analysis, the thermoluminescence efficiency grows with the increase of lithium fluoride in the glass. In addition, the main peak on the glow curve shifts to lower temperature after the heat treatment as shown in Fig. 9. Moreover, the change is linear only for the as-made glasses and shows minimum for glassceramics at 30 mol% LiF. The sample 40LiF after the process of ceramization at 720 °C reveals similar level of TL signal to the commercial material LiF:Mg,Ti (Fig. 10) but with the lower temperature of the response. The differences observed in TL signal are caused by changes of the glass structure and the nearest surrounding of Li+ ions which are responsible for the interaction of alpha and beta particles by producing free electrons and holes. Pure lithium fluoride crystals exhibit the thermoluminescence phenomena [35] and the TL comes from two types of vacancies which are present in fcc structure of LiF (tetrahedral and octahedral sites). During exposition, the radiation creates in the LiF structure F centers (anion vacancies trapping electrons), which tend to aggregate forming more complex defects. Among them F2 and F3+ color centers are of special interest. F2 center is composed of two anion vacancies with two bounded electrons, while F3+ of three vacancies with two electrons. Typically, thermoluminescence peaks occur in the range of 140–310 °C for lithium fluoride [36]. Unfortunately there are not publications according to thermoluminescence of lithium fluoride glass. Elemental composition effects on the thermoluminescence of 7
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
The effect of lithium fluoride on the thermal stability and thermoluminescence properties of borosilicate glass and glass-ceramics ⁎
Marcin Środaa, Szymon Świonteka, , Wojciech Gieszczykb, Paweł Bilskib a b
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Poland The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass-ceramics LiF Oxyfluoride glass Borosilicate glass Thermoluminescence TL Alpha and beta radiation
The oxyfluoride glass and glass-ceramics from the LiF-B2O3-SiO2 system are developed. The stable glass can be produced in the range of 20–40 mol% LiF. The effect of LiF admixture on the thermal stability of the glass as well as the thermoluminescence (TL) properties such as glow curves shape is studied. The results show that the increase of lithium fluoride content in the borosilicate glass causes efficiency enhancement of the thermoluminescence signal. We have clearly stated that the process of controlled crystallization of the oxyfluoride glasses can lead again to increased intensity of the TL process. The glass-ceramics with 40 mol% LiF reveals similar level of TL signal to commercially used doped LiF material and can be considered as active material for alpha and beta radiation detectors.
1. Introduction The lithium fluoride has structure analogous to NaCl but it is much less soluble in water. Because of the wide and direct band gap for LiF (13.6 eV), its crystals are transparent to short wavelength ultraviolet radiation, far more than any other material [1]. Therefore, lithium fluoride crystals can be used as a means to record ionizing radiation exposure for gamma rays, beta particles and neutrons in thermoluminescent dosimeters (TLDs) [2–4]. However, LiF is toxic and it is impossible to use this material directly in biological environments. In some cases, i.e. brachytherapy, special detectors measuring ionizing radiation inside biologic tissues is required [5]. The first crucial feature of ionizing radiation detector is similar to human tissue equivalent absorption coefficient. It is necessary to use a detector which has similar effective atomic number (Zeff) [6]. It turns out that modified borosilicate glasses can be characterized by similar shielding parameters as for human organs [7,8]. However, elemental composition of the batch has the principal influence on the final thermoluminescence properties [9]. The presence of silica in borosilicate glass increases its mechanical strength and made the glass more chemical resistant especially for dissolution. The chemical resistance for environmental factors is the key advantage of glass dosimeters over crystalline ones [10,11]. The crystalline substances which exhibit thermoluminescence phenomenon could be extremely toxic for human health [12]. We believe that advantages of the glass and glass-ceramic
⁎
detectors could be its simply process of manufacturing, forming to particular shape and less toxicity. G. Anjaiah et al. studied the luminescence properties of rare earth doped lead fluoroborate glasses [13]. They showed that lead fluoroborate glasses have potential applicability for thermoluminescence purposes due to the wide optical energy band gap. The rare earth doped fluoroborate glasses as well as CaF2 with higher content of the luminescence centers was also studied [14–17]. Moreover, wider energy gap together with negligibly small saturation effect allows detector to measure in wide range of high-energy radiation without underestimation error. Additionally, deeper location of luminescence centers improves thermal stability of radiation history of the detector [18,19]. Due to the well known luminescence properties of crystalline LiF [20] we developed a new LiF-B2O3-SiO2 glass and glass-ceramics based on this system which could be less expensive and more utilitarian than currently used. The objective of this study was to characterize thermoluminescence properties of the oxyfluoride glasses in conjunction with their thermal stability and tendency to crystallization of the fluoride phases. 2. Experimental 2.1. Glass preparation All of the examined glasses were prepared by conventional method
Corresponding author. E-mail address: [email protected] (S. Świontek).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.037 Received 6 June 2019; Received in revised form 20 September 2019; Accepted 21 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Marcin Środa, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.037
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Table 1 The chemical composition of oxyfluoride borate glasses with their effective atomic number values. Glass no.
10LiF 20LiF 30LiF 40LiF 50LiF Zeff
Chemical composition of glass, mol% LiF
B2O3
SiO2
Zeff
10 20 30 40 50 LiF 8.2 [21]
45 40 35 30 25 B2O3 6.9 [22]
45 40 35 30 25 SiO2 11.1 [23]
9.3 9.2 9.1 9.0 8.9
of melting and supercooling of the melt. The nominal chemical composition of the glasses is presented in Table 1. Chemically pure compounds (LiF, SiO2, H3BO3) were used for the batch preparation. The composition of each batch was calculated for melting 30 g of glass. After the process of homogenization in the ceramic mortar, the batches were melted in the platinum crucible, in the electric furnace under the cover in order to restrain the evaporation of compounds at the temperature of 1350 °C for 30 min. The melts were supercooled by pouring them on pre-heated brass plate to about 350 °C and annealed at the temperature of 400 °C. Due to volatility of LiF and H3BO3 the correction of glass composition was estimated on level 20–15%. The values of volatility are based on mass lost analysis,
2.2. Methods of study DSC method using NETZSCH STA 449 F3 equipment and a platinum crucible in air atmosphere was applied to study the thermal stability of glass. Approximately 60 mg of grinded glass below 0.05 mm were used for DSC measurement from 250 to 1000 °C at the constant heating rate of 10 °C/s. The transformation temperature (Tg) established on inflection point and peak temperature of crystallization (Tc) were determined. Δcp at Tg temperature and ΔH of crystallization were calculated using NETZSCH software. Scanning electron microscope (SEM) images of the 10LiF and 50LiF glasses were taken with the use of Nova NanoSEM microscope. The X-ray diffraction analysis (Philips X’Pert Diffractometer with CuKα radiation) was used to confirm the amorphous nature of the glass and crystallization products after the thermal treatment. The thermoluminescence measurements were performed for as-made glass and glass-ceramics to analyze the influence of crystallization. Each sample was irradiated with the same dose of alpha and beta radiation, respectively. Before the second irradiations the samples were heated to the temperature of 400 °C at the constant heating rate of 5 °C/s to erase the residual signal coming from the previous exposures. FT-IR spectra were recorded in the middle infrared region (MIR) with Bruker Company Vertex 70 v spectrometer. Spectra were collected at room temperature in the range of 4000–500 cm–1 using typically 128 scans at 4 cm–1 resolution. The samples were prepared by the standard KBr pellet method. Glass and glass-ceramic samples were grinded to fine powders less than 100 μm for thermoluminescence measurements. The samples of glass were irradiated with beta radiation (90Sr/90Y source) dose of c.a. 9.4 Gy and with alpha particles (241Am) fluence of 1.21·108 1/scm2 (c.a. 19.85Gy in terms of an average dose delivered to the irradiated volume of the sample). The response of samples to alpha and beta radiation was studied in consecutive measurements with one additional erasing readout between them. The automatic Risø TL/OSL-DA-20 reader was used for measurements. Detailed specification of the reader and its performance is described by Bilski et al. [24]. The measurements were carried out with the use of BG39 optical filter which transmits blue and green light. All the measured curves were analyzed using GlowVIEW
Fig. 1. Scanning electron microscope (SEM) images with EDX analysis of (a) 10LiF sample (Magnification 10000×), (b) 50LiF sample (Magnification 20000×), (c) 30LiF sample heat treated at 720 °C (Magnification 5000×).
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Fig. 3. DSC curves for oxyfluoride glass samples.
Table 2 Characteristic temperatures for oxyfluoride glass samples where Tg – transformation temperature calculated as the deflection point, TX – the temperature of onset crystallization, Tc – the temperature of peak crystallization, kgl – Hruby coefficient, ΔH – enthalpy of crystallization, Δcp – change of specific heat capacity at thetransformation range.
Fig. 2. XRD patterns of 10LiF sample (a) and 50LiF sample (b).
software [25].
Glass no.
Tg ( ± 3 °C)
Tx ( ± 3 °C)
Tc ( ± 3 °C)
kgl
ΔH (J/g)
Δcp (J/ (g K))
10LiF 20LiF 30LiF 40LiF
419 473 423 466
— 658 648 618
— 694 672 644
— 1.52 2.45 1.01
— 156.0 79.6 104.5
0.044 0.608 0.312 0.456
3. Results 3.1. Thermal stability of glass – SEM/XRD/DSC analysis All prepared glasses, except for 10LiF and 50LiF, were good quality, homogeneous and transparent. 10LiF sample showed loss of translucency and 50LiF sample became uniformly white. Fig. 1a presents SEM image of 10LiF sample with heterogeneity, which appear as lighter or darker regions. Due to limitation of the technique, the light elements with low electron energy, i.e. Li, B are not observed in EDX analysis. But a difference in the Si content between the two regions can be distinguished. No crystalline phase was confirmed by XRD analysis in 10LiF sample (Fig. 2a). On the other hand, the 50LiF sample underwent spontaneous crystallization of LiF (JCPDS card no: 01-089-3610) during melt cooling (Fig. 2b) which is also observed in SEM image (Fig. 1b). After process of heat-treatment at 720 °C, homogeneous and transparent LiF samples crystallized to LiBF4 phase (Fig. 1c). EDX analysis confirmed that forming crystallites contain fluoride. Fig. 3 and Table 2 show DSC curves and thermal parameters of the glasses, respectively. The glasses are characterized by transformation temperature and crystallization in the range 410–460 °C and 620–720 °C, respectively. The 10LiF glass is different from the other and shows no effect of the crystallization. With the increase of LiF the crystallization peak shifts towards lower temperature. This broad exothermal peak comes from crystallization of LiBF4 (JCPDS card no: 00040-1431) and α-SiO2 (JCPDS card no: 01-089-3433) according to XRD analysis (Fig. 4). Following the XRD analysis we confirmed the phase separation without any crystallization in 10LiF sample, crystallization of LiBF4 and α-SiO2 after thermal treatment of the 20LiF, 30LiF, 40LiF glasses and spontaneous crystallization of 50LiF during melting process.
Fig. 4. XRD pattern of 40LiF glass sample heat treated at 720 °C for 60 min. Stars and circles indicate LiBF4 and α-SiO2 phases, respectively.
significant information about bond vibrations, which involves the bending, stretching and rotating vibrational motions in molecule structure [26]. The FT-IR spectra of the analyzed glasses are characterized by the presence of three main active infrared spectral regions in the range: 700–800 cm−1, 800–1200 cm−1 and 1200–1650 cm−1. Due to the constant ratio of (B2O3/SiO2) = 1 of the all studied glasses, no significant changes in the intensity of the bands are observed. However, we confirmed that LiF affects the position of absorption maxima. The absorption bands shift towards higher wavenumbers as the LiF content increases in the glass. This results from the fact that Li atoms act as a network modifier. Increase of their content depolymerize network of the glass. It is worth to notice that LiF itself shows no bands in the range due to the lack of a change in dipole moment as a result of the vibration.
3.2. FT-IR study Fig. 5 illustrates the FT-IR spectra of all tested glasses and provides 3
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Fig. 5. FT-IR spectra and their deconvolution for oxyfluoride borosilicate glasses.
3.3.2. Glass-ceramic samples Fig. 7 shows thermoluminescence glow curves for the heat treated glass samples at 600 °C and 720 °C, respectively. Thermoluminescence of samples after alpha radiation is significantly lower then after beta radiation due to lower penetrability of the first one and this phenomena remains unchanged after ceramization. Thermal treatment of the glasses after beta radiation significant changes the TL signal as to numbers of maxima, intensity and temperature of the peaks. Thus, we observe two peaks at 80 °C and above 270 °C for 20LiF sample heat treated at 600 °C. Three peaks at 80 °C, 125 °C and 270 °C are visible on the glow curve of the sample after ceramization at 720 °C. For 30LiF sample heat treated at 600 °C glow curve consists of broad peak which maximum corresponds to the temperature of 95 °C and with shoulder at 140 °C. The intensity of TL signal for the heat treated sample is greater than seventy times in comparison with the as-made glass. Moreover, the sample treated at temperature of 720 °C shows enhancement of TL signal more than three times compared to the sample annealed at 600 °C without significant shifting the peak position. 40LiF sample after heat treatment reveals much more narrower peaks than other samples. The main one is visible at 175 °C and the second lower peak at 100 °C for sample heat treated at 600 °C. The increase in temperature of the thermal treatment to 720 °C shifts the peaks to lower temperature (maximum at 160 °C and 90 °C,
3.3. Thermoluminescence study 3.3.1. As-made glasses The thermoluminescence (TL) glow curves for lithium fluoroborate glass samples are shown in Fig. 6. The glow curve of 10LiF sample consist of two low peaks at 80 °C and 235 °C, respectively. On the other hand the glow curves of 20LiF and 30LiF samples consist of one dominant peak which corresponds to the temperature of 235 °C and 220 °C, respectively. The intensity of TL signals after alpha and beta exposition is very low for 20LiF sample and it is comparable to the background noise in case of the sample after alfa radiation exposition. The increase of LiF content induces higher TL signal which is greater five and ten times in 30LiF sample compared to 20LiF for alpha and beta radiation, respectively. The glow curves of 40LiF sample showed the highest TL response with two peaks at the temperatures of 130 °C and 205 °C and two shoulders at 225 °C and 270 °C. The intensity of the main peak is greater than two hundred times in comparison with 30LiF sample after beta radiation exposition. For 50LiF sample which spontaneously crystallized the intensity of main peak is lower than for 40LiF but slightly higher than for 30LiF. Moreover, the peak position of maximum for 50LiF sample is similar to 10LiF for which phase separation was confirmed (Fig. 1a).
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Fig. 6. TL glow curves of oxyfluoride borosilicate glasses with the use of BG39 optical filter (solid line refers to beta radiation exposition while dash line refers to alpha radiation exposition): a) 10LiF, b) 20LiF, c) 30LiF, d) 40LiF, e) 50LiF.
With the low content of LiF (10LiF sample) no crystallization (Fig. 2a) but phase separation (Fig. 1a) appears. When the content of both glass formers decreases to 50 mol % (50LiF sample) the spontaneous devitrification is observed as a formation of LiF (Fig. 2b). In this case the glass network is low polymerized what promotes process of the crystallization. The fluorides are often used as opacificiers to produce milky silicate glass in glass technology, e.g. for the production of lampshades [28] and they play a role of nucleation agents. In our glass, LiF facilitates mixing silica and borate units in the glass network. We observe the phase separation only in the 10LiF glass with the lowest content of LiF. As shown in Fig. 1, the spinoidal decomposition is revealed for the glass. Moreover, no crystallization is confirmed by XRD analysis (Fig. 2). The additional confirmation of the process is shown on DSC curve (Fig. 3) where a broad transformation range is visible with the lowest transformation temperature (Tg = 419 °C). Our study reveals
respectively). In addition, the third low peak appears at 220 °C after heat treatment at 720 °C. 4. Discussion Glasses involving simultaneously both SiO2 and B2O3 shows strong tendency to phase separation and this effect was widely investigated, especially in the ternary Na2O-Ba2O3-SiO2 [27]. A. K. Varshneya indicates that the large cations as K+, Rb+, Cs+ show decreased tendency to phase separation. This is mainly for three reasons: 1) higher number of components increases the entropy of the system, 2) an incorporated modifier facilitates the formation of common borosilicate network due to compensation of charge of B3+ and formation of [BO4] tetrahedral, 3) the flexibility of the network increases and different structural units can coexist together forming mix-network glass. Account taken of the above, the stable glass can be obtained in the range 20–40 mol% LiF. 5
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Fig. 7. TL glow curves of heat treated glass samples with the use of BG39 optical filter (solid lines refer to beta radiation exposition while dash lines refer to alpha radiation exposition). a) 20LiF, b) 30LiF, c) 40LiF.
that not only the large cations, but also LiF admixture increases homogeneity and thermal stability of the borosilicate glass. However, this effect is observed only to a limited extend. The highest Hruby parameter kgl = 2.45 which is an indicator of thermal stability was obtained for 30LiF glass. It is assumed that glasses with kgl > 1 are believed to have high thermal stability. Our glasses with LiF content in the range from 20 to 40 mol% meet this condition. On the other hand, the glass with the highest content of LiF (50% mol) demonstrates spontaneous crystallization of griceite (LiF). It is accepted that the network of binary alkali borate glasses contains trigonal planar [BO3] units and tetrahedral [BO4] units. It is known that the variation of properties in borate glasses follows the variation of [BO3]/[BO3 + BO4] ratio, which is depended on the alkali metal oxide content [29]. FT-IR curves of our samples show the same character with the three main regions [30–33]: 1) above 1200 cm−1 due to the asymmetric stretching vibration of the BeO bond of trigonal [BO3] structural units, 2) in the range 1200 cm−1 and 800 cm−1 including the BeO and SieO bonds asymmetric stretching vibration of tetrahedral [BO4] and [SiO4] structural units, respectively, 3) between 800–600 cm−1 because of the bending vibration of BeOeB and SieOeSi linkages in the borosilicate network.
Fig. 8. The area ratio of the envelops assigned to [BX3] and [BX4] with [SiX4] units. The line is drawn as guide to the eye.
series of glass samples. This parameter was calculated as surface area under envelope from 1600 to 1250 cm−1 units divided by the area of envelope in the range 1250–750 cm−1 as representative of [BX3] and tetrahedral [BX4] and [SiX4], respectively. The contribution of [BX3] diminishes in the glass structure with increasing LiF content, as seen in Fig. 8. Our study shows that two factors determine the thermoluminescence of the borosilicate glasses: a content of LiF in the glass and temperature of the heat treatment. With the greater amount of LiF in the glass, the higher efficiency of TL is observed, but only the stable glass can be obtained up to 40 mol% of LiF. The thermal treatment of glass leads to crystallization of LiBF4 and α-SiO2 and formation of glassceramic, thereby enhancing the TL signal. After heat treatment at the
Deconvolution of the spectra reveals some changes in a position of the absorption bends (Fig. 5). We observe a monotonic shift of the maxima towards higher wavenumbers with the increase of LiF. This phenomena also appears in the Li2O-B2O3 glasses with an increasing content of Li2O [34]. It indicates a continuous polymerization of the network. To confirm the effect of the LiF on the glass structure, we calculated the area of the bands derived from [BX3], [BX4] and [SiX4] vibrational regions of FT-IR spectra, where X refers to O and F atoms, simultaneously. Fig. 8 shows the [BX3]/([BX4] + [SiX4]) ratio for our 6
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borosilicate glass were described by Clark et al. [37]. The glow curve for pure borosilicate glass consists of one broad peak at c.a. 150 °C. What is more, the reproducibility of TL response for borosilicate glass was proved by E. Salama et al. [38]. However, P. Szajerski et al. [39] described simultaneous glass network disruption in borosilicate glasses which caused formation of low energy traps and shifting TL peak positions to lower temperatures. 5. Conclusion In this research oxyfluoride borate glasses and glass-ceramics from LiF-B2O3-SiO2 system were developed. Our studies revealed that up to 40 wt. % of LiF can be introduced to the borosilicate glass network. LiF significantly decreases the phase separation phenomena in the glass and increases the thermal stability, even though glass-ceramics with optical active LiBF4 was obtained. The glass exhibits thermoluminescence effect, which efficiency increases with the higher content of lithium fluoride in the glass. Moreover, the glass-ceramics with lithium tetrafluoroborate (LiBF4) shows higher TL signal and sensitivity than the glass. The level of TL signal of our glass-ceramics is comparable to the commercial material based on LiF:Mg,Ti but with the lower temperature (about 50 °C) and narrow signal of the luminescence response. The results show that glass-ceramics from LiF-B2O3-SiO2 system with 40 mol % of LiF could be considered as an active material in the TL detectors.
Fig. 9. The effect of LiF content and thermal treatment on peak temperature of the TL signal. The line is drawn as guide to the eye.
Acknowledgements This research has been supported by the EU Project POWR.03.02.00-00-I004/16 and the statutory funds of AGH University of Science and Technology Department of Materials Science and Ceramics AGH number WIMiC No 11.11.160.365 in 2019. References [1] R.C. Chaney, E.E. Lafon, C.C. Lin, Energy band structure of lithium fluoride crystals by the method of tight binding, Phys. Rev. B 4 (1971) 2734–2741. [2] B. Obryk, P. Bilski, M. Budzanowski, M. Fuerstner, C. Ilgner, F. Jaquenod, P. Olko, M. Puchalska, H. Vincke, The response of different types of TL lithium fluoride detectors to high-energy mixed radiation fields, Radiat. Meas. 43 (2008) 1144–1148. [3] B. Obryk, P. Bilski, M. Glaser, M. Fuerstner, M. Budzanowski, P. Olko, A. Pajor, The response of TL lithium fluoride detectors to 24 GeV/c protons for doses ranging up to 1 MGy, Radiat. Meas. 45 (2010) 643–1645. [4] H.J. Khoury, B. Obryk, V.S. Barros, P.L. Guzzo, C.G. Ferreira, P. Bilski, P. Olko, Response of TL lithium fluoride detectors (MTS) to high gamma radiation doses, Radiat. Meas. 46 (2011) 1878–1881. [5] J. Skowronek, M. Wierzbicka, M. Leszczyńska, W. Szyfter, Brachyterapia paliatywna PDR i HDR w leczeniu nawrotów miejscowych nowotworów głowy i szyi, Rep. Pract. Oncol. Radiother. 8 (2003) 362–368. [6] M.I. Sayyed, S.A.M. Issa, H.O. Tekin, Y.B. Saddeek, Comparative study of gammaray shielding and elastic properties of BaO–Bi2O3–B2O3 and ZnO–Bi2O3–B2O3 glass systems, Mater. Chem. Phys. 217 (2018) 11–22. [7] N. Chanthima, J. Kaewkhao, Investigation on radiation shielding parameters of bismuth borosilicate glass from 1 keV to 100 GeV, Ann. Nucl. Energy 55 (2013) 23–28. [8] Y. Isokawa, S. Hirano, G. Okada, N. Kawaguchi, T. Yanagida, Characterization of cedoped lithium borosilicate glasses as tissue equivalent phosphors for radiation measurements, Radiat. Meas. 111 (2018) 13–18. [9] R.A. Clark, J.D. Robertson, J.M. Schwantes, Intrinsic dosimetry: elemental composition effects on the thermoluminescence of commercial borosilicate glass, Radiat. Meas. 59 (2013) 270–276. [10] R. Laopaiboon, C. Bootjomchai, Physical properties and thermoluminescence of glasses designed for radiation dosimetry measurements, Mater. Des. 80 (2015) 20–27. [11] R. Laopaiboon, C. Bootjomchai, Thermoluminescence studies on alkali-silicate glass doped with dysprosium oxide for use in radiation dosimetry measurement, J. Lumin. 158 (2015) 275–280. [12] M. Isik, E. Bulur, N.M. Gasanly, Photo-transferred thermoluminescence of shallow traps in β-irradiated BeO ceramics, J. Lumin. 187 (2017) 290–294. [13] G. Anjaiah, S.N. Rasool, P. Kistaiah, Spectroscopic and visible luminescence properties of rare earth ions in lead fluoroborate glasses, J. Lumin. 159 (2015) 110–118. [14] S. Ibrahim, F.H. ElBatal, A.M. Abdelghany, Optical character enrichment of NdF3 – doped lithium fluoroborate glasses, J. Non-Cryst. Solids 453 (2016) 16–22. [15] Y.C. Ratnakaram, A. Balakrishna, D. Rajesh, M. Seshadri, Influence of modifier oxides on spectroscopic properties of Sm3+ doped lithium fluoroborate glass, J.
Fig. 10. The comparison of TL sensitivity results for 40LiF samples with data obtained for commercially used doped LiF material [40,41].
peak temperature obtained from DSC analysis, the thermoluminescence efficiency grows with the increase of lithium fluoride in the glass. In addition, the main peak on the glow curve shifts to lower temperature after the heat treatment as shown in Fig. 9. Moreover, the change is linear only for the as-made glasses and shows minimum for glassceramics at 30 mol% LiF. The sample 40LiF after the process of ceramization at 720 °C reveals similar level of TL signal to the commercial material LiF:Mg,Ti (Fig. 10) but with the lower temperature of the response. The differences observed in TL signal are caused by changes of the glass structure and the nearest surrounding of Li+ ions which are responsible for the interaction of alpha and beta particles by producing free electrons and holes. Pure lithium fluoride crystals exhibit the thermoluminescence phenomena [35] and the TL comes from two types of vacancies which are present in fcc structure of LiF (tetrahedral and octahedral sites). During exposition, the radiation creates in the LiF structure F centers (anion vacancies trapping electrons), which tend to aggregate forming more complex defects. Among them F2 and F3+ color centers are of special interest. F2 center is composed of two anion vacancies with two bounded electrons, while F3+ of three vacancies with two electrons. Typically, thermoluminescence peaks occur in the range of 140–310 °C for lithium fluoride [36]. Unfortunately there are not publications according to thermoluminescence of lithium fluoride glass. Elemental composition effects on the thermoluminescence of 7
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