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High hardnesses of Tm3+-doped La2O3-Al2O3 luminescent glasses fabricated by containerless solidification
Journal of Non-Crystalline Solids 525 (2019) 119599
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High hardnesses of Tm3+-doped La2O3-Al2O3 luminescent glasses fabricated by containerless solidification
T
Xiaoyan Zhanga,b,c, Weicheng Yanga,b, Jinrong Zhanga,b, Jinsheng Lia,b,c, Luhong Jianga,b, ⁎ Xiwei Qia,b,c, qxw@ neuq.edu.cn a
School of materials science and Engineering, Northeastern University, Shenyang 110819, PR China School of Resources and Materials, Northeastern University at QinHuangDao, QinHuangDao 066004, PR China c Key Laboratory of Dielectric and Electrolyte Functional Material, Hebei Province, QinHuangDao 066004, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescent glass High Vicker’s hardness La2O3-Al2O3: Tm3+ Containerless method
A series of colorless and transparent (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) luminescent glasses were successfully prepared by the containerless method. The effects of Tm3+ substitution on the physical, thermal, optical and photoluminescence properties of La2O3-Al2O3 glasses were investigated. X-ray diffraction confirmed the amorphous natures of the as-prepared glasses. Excitated at 354 nm, La2O3-Al2O3:Tm3+ glasses emitted blue luminescence at 457 nm, corresponding to the 1D2→3F4 transition of Tm3+. These glasses showed high Vicker's hardnesses (7.2–8.8 GPa), and excellent thermal stabilities (Tg: 840–851 °C), which were both augmented with increasing content of Tm3+. They also exhibited high transmittances in a wide window ranging from 220 nm to 8.9 μm. In conclusion, the substitution of Tm2O3 for La2O3 increased the Vicker's hardness, thermal stability and transmittance window of La2O3-Al2O3 glasses, allowing potential applications to optical devices in the visible to infrared region.
1. Introduction Over the past few decades, rare earth doped luminescent glasses have attracted widespread attention in different fields, owing to facile fabrication, remarkable electric insulation performance, high refractive index and high transmittance in a wide window, especially in lightemitting diodes, optical devices, infrared (IR) detection as well as visible and IR solid-state lasers [1–5]. Among rare earth activators, Tm3+ ion is known as an effective blue light emission center with a simple and well-separated 4f level structure, with both down-conversion and up-conversion luminescences [6,7]. Tm3+-doped glasses have been considered as attractive candidates of host materials due to strong absorption at around the emission wavelength of commercial highpower 808 nm laser diode, efficient cross relaxation among Tm3+ ions, and large inhomogeneous broadening [8,9]. Among different matrices, La2O3-Al2O3 glasses play an indispensable role because of good glassforming ability, wide transmission region, low cost, together with thermal and chemical durabilities [10,11]. Although the spectroscopic properties and Judd-Ofelt parameters of Tm3+-doped La2O3-Al2O3 monocrystal have been studied [6], the multifunctional properties of La2O3-Al2O3:Tm3+ glasses, such as Vicker's hardness as well as thermal and optical properties, are still large unknown. Therefore, La2O3⁎
Al2O3:Tm3+ glassy materials are worthy of further studies. In this study, we aimed to obtain Tm3+-doped La2O3-Al2O3 luminescent glasses by a novel containerless method using an aerodynamic levitation furnace and to clarify the multifunctional properties, such as fundamental physical, thermal, optical and down-conversion luminescence properties. In additon, the parameters, such as ΔT (ΔT = Tx-Tg) and optical energy gap (Eopt) were also calculated to evaluate the glassforming ability and the UV absorption edge of these glasses. 2. Experimental 2.1. Synthesis Transparent Tm3+-doped glasses with molar compositions of (28-x) La2O3–72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) were prepared by the containerless method using an aerodynamic levitation furnace. High-purity Al2O3 (99.99%, Sinopharm Chemical Reagent Co. Ltd., China), La2O3 (99.99%, Sinopharm Chemical Reagent Co. Ltd., China), and Tm2O3 powders (99.99%, Aladdin Chemical Reagent Company) were used as starting materials. Weighted raw materials were uniformly mixed, compacted into disks with the diameter of 10 mm, and sintered at 1000 °C for 10 h. Then the samples were placed on a nozzle of
Corresponding author at: School of materials science and Engineering, Northeastern University, Shenyang 110819, PR China. E-mail address: [email protected] (X. Qi).
https://doi.org/10.1016/j.jnoncrysol.2019.119599 Received 25 June 2019; Received in revised form 2 August 2019; Accepted 3 August 2019 0022-3093/ © 2019 Published by Elsevier B.V.
Journal of Non-Crystalline Solids 525 (2019) 119599
X. Zhang, et al.
aerodynamic levitation furnace and melted by CO2 laser device. Through oxygen gas flow, the melt was levitated and kept in the molten state for a few seconds to ensure homogenization. Afterwards, the laser was turned off, and transparent colorless glassy-spheres with diameters of 2–4 mm were finally obtained by cooling down the melt rapidly to room temperature. A part of the glassy spheres were ground into powders for the luminescence test, and the remaining were carefully polished into disks for X-ray diffraction (XRD), optical and hardness analyses.
Table 1 Densities (ρ) and Vicker's hardnesses of (28-x)La2O3-72Al2O3:xTm3+ glasses. x
ρ(g/cm3)
Vicker's hardness (GPa)
0 1 1.5 2 3
4.16 4.18 4.20 4.21 4.23
7.2 7.5 7.7 8.2 8.8
substitution of the relatively lighter La2O3 (325.84 g/mol) by the heavier Tm2O3 (385.86 g/mol). Meanwhile, all glasses have high Vicker's hardnesses, which also rises as x increased. According to the literature [12], the hardness is inversely proportional to the square of the distance between ions. The ionic radius of the substituted ion Tm3+ (88 pm) is smaller than that of La3+ (103.2 pm), resulting the rising Vicker's hardness as Tm3+ content increased. The similar results can be found in the reference [13], in which the hardness of (30-x)CaO-05Y2O310B2O3-55P2O5:xNiO glasses increases with the content of Ni2+. The highest Vicker's hardness is 8.8 GPa for x = 3, exceeding those of lanthanum aluminate glass 0.8LaAlO3-0.2Nb2O5 (8.3 GPa) [14], rare earth Er3+/Yb3+-doped glass TiO2-La2O3-Ta2O5 (7.7 GPa) [15] and La2O3Lu2O3-TiO2 (8.26 GPa) [16], and even being comparable to the maximum values reported for oxide glasses 81.8Al2O3-18.2Y2O3 (~9 GPa) [17] and 29.3Al2O3-50.2SiO2 -20.5Sc2O3 (9.4 GPa) [18].
2.2. Characterizations of samples The sample structure was tested by XRD, using Ultima IV X-ray diffractometer (Rigaku, Japan). The density was measured by the Archimedes' method using ME204 electronic balance and accompanied density components (Mettler Toledo, China). MHV-52 digital Vicker's hardness tester (Sctmc, China) equipped with a diamond Vicker's indenter was used to obtain the Vicker's hardness, which was measured three times for each sample and then averaged. Differential thermal analysis (DTA) was conducted by Setsys Evolution system (Setaram, France) in an argon atmosphere at a heating rate of 10 °C/min. The optical transmittance spectra were determined on Cary 5000 UV–visNIR spectrometer (Varian, USA) and Excalibur 3100 Fourier transform IR spectrometer (Varian, USA). Refractive indices were collected by SE 850 DUV SPEC ellipsometer (Sentech, Germany). Emission and excitation spectra were detected by F-7000 fluorescence spectrofluorometer (Hitachi, Japan) employing a 150 W Xe lamp. All the experiments were performed at room temperature.
3.3. Thermal properties
Table 1 summarizes the densities (ρ) and Vicker's hardnesses of (28x)La2O3-72Al2O3: xTm3+ glasses. Adding Tm3+ to 28La2O3–72Al2O3 system increases the density of the as-prepared glass, from 4.16 to 4.23 g/cm3 for x = 0 to 3, respectively. It can be ascribed to the
To investigate the thermal stability and glass-forming ability of (28x)La2O3-72Al2O3: xTm3+ glasses, the DTA curves are recorded and shown in Fig. 2. The curve of each glass presents a clear glass transition and a strong exothermic peak due to crystallization. The corresponding glass-transition temperature Tg and first crystallization temperature Tx are displayed in Fig. 3. Since the melting point of Tm2O3 (2425 °C) is higher than that of Al2O3 (2054 °C), Tm2O3 substitution improves the thermal stability of these glasses and Tg slightly increases with rising x from 840 to 851 °C. The values significantly surpass those of silicate, borate, aluminate and germanosilicate glasses [19–22], confirming a higher thermal stability. Generally, the glass-forming ability is measured using the temperature gap ΔT (ΔT = Tx-Tg) (inset of Fig. 3) [16,23]. A larger ΔT indicates better anti-crystallization property, i.e. harder devitrification of a glass in the preparation process. In this study, the reduction in ΔT suggests that the glass-forming ability of the present system is attenuated after Tm2O3 substitution. In other words, it is due to the fact that Tm2O3 is network modifier while Al2O3 is an intermediate that in this case can play glass-former role [24].
Fig. 1. XRD patterns of (28-x)La2O3-72Al2O3:xTm3+ glasses.
Fig. 2. DTA curves of (28-x)La2O3-72Al2O3: xTm3+ glasses.
3. Results and discussion 3.1. Structural analyses The XRD patterns of (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) samples are presented in Fig. 1. It is well-known that typical hump located at low angle is attributed to glassy phase. Obviously, each sample exhibits only a broad diffusive diffraction peak centered at 2θ of 29°, verifying the amorphous nature. 3.2. Physical properties
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Journal of Non-Crystalline Solids 525 (2019) 119599
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Fig. 3. Tg and Tx for (28-x)La2O3-72Al2O3: xTm3+ glasses. Inset shows ΔT (ΔT = Tx-Tg) of these glasses.
Fig. 6. Transmittances of (28-x)La2O3-72Al2O3: xTm3+ glasses in IR region. The inset gives the photograph of well-polished sample of x = 1.5.
Fig. 7. Refractive index dispersion of 27 La2O3-72 Al2O3:1 Tm3+ glass. The dotted curve in the inset is obtained using the Drude–Voigt relationship, and the red curve is the fitted linear curve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Transmittances of (28-x)La2O3-72Al2O3: xTm3+ glasses in UV-VIS-NIR region. Inset is the transmittances from 200 nm to 260 nm.
Fig. 5. Representation of (αhv)1/2 versus hv of (28-x) La2O3-72 Al2O3: xTm3+ glasses. Inset gives the optical energy gap Eopt of these glasses. Fig. 8. The composition dependence of the refractive index nd of (28-x)La2O372Al2O3:xTm3+ glasses.
3.4. Optical properties Fig. 4 shows the transmittance spectra of all samples in the wavelength range from 200 to 1500 nm. The transmittance of 28La2O372Al2O3 glass is about 80%. As the Tm2O3 content increases, six 3
Journal of Non-Crystalline Solids 525 (2019) 119599
X. Zhang, et al.
Fig. 9. PLE (a) and PL (b) spectra of (28-x)La2O3-72Al2 O3: xTm3+ (x = 1, 1.5, 2, 3)glasses.
3.5. Photoluminescence properties
absorption bands centered at around 1210, 790, 682, 467, 354 and 260 nm appear, representing transitions from the 3H6 ground state of Tm3+ to excited states 3H5, 3H4, 3F2,3, 1G4, 1D2 and 3P2, respectively [9,25,26]. UV absorption edge shifts irregularly, as shown in the inset in Fig. 4. Practically, the UV edge is determined by the optical band gap energy Eopt, which is affected by the strong internal forces of electron transition from the valence band to the conduction band [27,28]. Thus, Eopt is calculated using the following equation:
αhv = A (hv − Eopt )2
Fig. 9(a) exhibits the photoluminescence excitation spectra of (28-x) La2O3-72Al2O3: xTm3+ (x = 0, 1, 1.5, 2, 3) glasses recorded by monitoring the emission at 457 nm assigned to the 1D2 → 3F4 transition of Tm3+ ions. The spectra of these glasses consist of four excitation peaks centered at 260 nm, 275 nm, 287 nm and 354 nm, corresponding to the transitions of Tm3+ from the lowest ground state 3H6 to the excited upper states 3P2, 3P1, 3P0 and 1D2, respectively [32,33]. The emission spectra of (28-x)La2O3-72Al2O3:xTm3+ glasses excited at 354 nm (3H6–1D2) are shown in Fig. 9 (b). There are a strong blue emission peak centered at 457 nm and a very weak emission peak at 476 nm, which can be assigned to the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+ [34–36]. Clearly, the emission intensity increases with rising Tm3+ content up to 1.5% and then gradually decreases as the content exceeds 1.5% because of concentration quenching [37].
(1)
where α is the absorption coefficient, A is an energy-independent constant, and hν is the incident radiation photon energy. Eopt is evaluated by extrapolating the linear part of (αhv)1/2 versus hv curve to (αhv)1/2 = 0(Fig. 5), with the values plotted in the inset. With increasing amount of Tm2O3, Eopt decreases to 5.125 eV for x = 1, and then increases with rising x to 5.209 eV. Hence, the transition between valence and conduction bands requires more energy and the cutoff wavelength hypochromatically shifted, being consistent with the variations of absorption edge in Fig. 4. The photograph of well-polished x = 1.5 sample and the IR transmittance spectra of (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) glasses are shown in Fig. 6. The absorption band at around 3.4 μm can be attributed to the free hydroxyl group which generally exists in oxide glasses [29]. The transmittance of each sample is above 80%, and the IR absorption edge related to multiphonon processes appears at the wavelength over 5.2 μm. The absorption cut-off wavelength at 3% transmittance shifts from 6.7 μm to 8.9 μm, indicating that doped Tm3+ increases the cut-off wavelength and Tm2O3 substitution enlarges the wide transmittance window of La2O3-Al2O3 glasses. Therefore, La2O3Al2O3 glasses are typified by such a wide transmittance range from 220 nm to 8.9 μm, allowing applications to glass materials with wide optical windows. The dependence of refractive index of 27La2O3-72Al2O3:1Tm3+ glass on wavelength is shown in Fig. 7. According to the single oscillator model from the Drude-Voigt relationship, the plot of 1/(n2−1) vs. 1/λ2 is a straight line [30]. As displayed in the inset of Fig. 7, the measured values (the dotted curve) are well linearly related, indicating that the single oscillator model adequately describes the refractive index dispersion in 27La2O3-72Al2O3:1Tm3+ glass. Fig. 8 presents the composition dependences of refractive indices nd of (28-x)La2O3-72 Al2O3:xTm3+ glasses at 587.6 nm. With rising x from 0 to 3, nd changes from 1.93 to 2.08, reaching the maximum at x = 1. In general, an oxide glass with nd of >2.0 is eligible for preparing highvalue lenses used in small optical devices such as smart phones with digital cameras and endoscopes [31].
4. Conclusions In conclusion, transparent (28-x)La2O3-72Al2O3: xTm3+ (x = 0, 1, 1.5, 2, 3) glasses were fabricated successfully by containerless processing. The as-prepared glasses emitted blue emission at 457 nm attributed to the 1D2 → 3F4 transition of Tm3+. These glasses had high Vicker's hardnesses (up to 8.8 GPa), excellent thermal stabilities (Tg up to 851 °C) and long cut-off wavelengths (up to 8.9 μm) in the IR region, which were effectively augmented through Tm2O3 substitution. They also exhibited high transmittances in a wide window. The highest refractive index was 2.08 for x = 1. Hence, Tm3+-doped La2O3-Al2O3 glasses may have optical applicability in the visible to IR region, such as optical devices of smart phones with digital cameras and endoscopes. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Nature Science Foundation of China (No.51602042) and the Natural Science Foundation of Heibei Province (No. E2018501042, B2018407058). References [1] Y. Yu, Z. Liu, N. Dai, Y. Sheng, H. Luan, J. Peng, Z. Jiang, H. Li, J. Li, L. Yang, Ce-TbMn co-doped white light emitting glasses suitable for long-wavelength UV excitation, Opt. Express 19 (2011) 19473.
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High hardnesses of Tm3+-doped La2O3-Al2O3 luminescent glasses fabricated by containerless solidification
T
Xiaoyan Zhanga,b,c, Weicheng Yanga,b, Jinrong Zhanga,b, Jinsheng Lia,b,c, Luhong Jianga,b, ⁎ Xiwei Qia,b,c, qxw@ neuq.edu.cn a
School of materials science and Engineering, Northeastern University, Shenyang 110819, PR China School of Resources and Materials, Northeastern University at QinHuangDao, QinHuangDao 066004, PR China c Key Laboratory of Dielectric and Electrolyte Functional Material, Hebei Province, QinHuangDao 066004, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescent glass High Vicker’s hardness La2O3-Al2O3: Tm3+ Containerless method
A series of colorless and transparent (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) luminescent glasses were successfully prepared by the containerless method. The effects of Tm3+ substitution on the physical, thermal, optical and photoluminescence properties of La2O3-Al2O3 glasses were investigated. X-ray diffraction confirmed the amorphous natures of the as-prepared glasses. Excitated at 354 nm, La2O3-Al2O3:Tm3+ glasses emitted blue luminescence at 457 nm, corresponding to the 1D2→3F4 transition of Tm3+. These glasses showed high Vicker's hardnesses (7.2–8.8 GPa), and excellent thermal stabilities (Tg: 840–851 °C), which were both augmented with increasing content of Tm3+. They also exhibited high transmittances in a wide window ranging from 220 nm to 8.9 μm. In conclusion, the substitution of Tm2O3 for La2O3 increased the Vicker's hardness, thermal stability and transmittance window of La2O3-Al2O3 glasses, allowing potential applications to optical devices in the visible to infrared region.
1. Introduction Over the past few decades, rare earth doped luminescent glasses have attracted widespread attention in different fields, owing to facile fabrication, remarkable electric insulation performance, high refractive index and high transmittance in a wide window, especially in lightemitting diodes, optical devices, infrared (IR) detection as well as visible and IR solid-state lasers [1–5]. Among rare earth activators, Tm3+ ion is known as an effective blue light emission center with a simple and well-separated 4f level structure, with both down-conversion and up-conversion luminescences [6,7]. Tm3+-doped glasses have been considered as attractive candidates of host materials due to strong absorption at around the emission wavelength of commercial highpower 808 nm laser diode, efficient cross relaxation among Tm3+ ions, and large inhomogeneous broadening [8,9]. Among different matrices, La2O3-Al2O3 glasses play an indispensable role because of good glassforming ability, wide transmission region, low cost, together with thermal and chemical durabilities [10,11]. Although the spectroscopic properties and Judd-Ofelt parameters of Tm3+-doped La2O3-Al2O3 monocrystal have been studied [6], the multifunctional properties of La2O3-Al2O3:Tm3+ glasses, such as Vicker's hardness as well as thermal and optical properties, are still large unknown. Therefore, La2O3⁎
Al2O3:Tm3+ glassy materials are worthy of further studies. In this study, we aimed to obtain Tm3+-doped La2O3-Al2O3 luminescent glasses by a novel containerless method using an aerodynamic levitation furnace and to clarify the multifunctional properties, such as fundamental physical, thermal, optical and down-conversion luminescence properties. In additon, the parameters, such as ΔT (ΔT = Tx-Tg) and optical energy gap (Eopt) were also calculated to evaluate the glassforming ability and the UV absorption edge of these glasses. 2. Experimental 2.1. Synthesis Transparent Tm3+-doped glasses with molar compositions of (28-x) La2O3–72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) were prepared by the containerless method using an aerodynamic levitation furnace. High-purity Al2O3 (99.99%, Sinopharm Chemical Reagent Co. Ltd., China), La2O3 (99.99%, Sinopharm Chemical Reagent Co. Ltd., China), and Tm2O3 powders (99.99%, Aladdin Chemical Reagent Company) were used as starting materials. Weighted raw materials were uniformly mixed, compacted into disks with the diameter of 10 mm, and sintered at 1000 °C for 10 h. Then the samples were placed on a nozzle of
Corresponding author at: School of materials science and Engineering, Northeastern University, Shenyang 110819, PR China. E-mail address: [email protected] (X. Qi).
https://doi.org/10.1016/j.jnoncrysol.2019.119599 Received 25 June 2019; Received in revised form 2 August 2019; Accepted 3 August 2019 0022-3093/ © 2019 Published by Elsevier B.V.
Journal of Non-Crystalline Solids 525 (2019) 119599
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aerodynamic levitation furnace and melted by CO2 laser device. Through oxygen gas flow, the melt was levitated and kept in the molten state for a few seconds to ensure homogenization. Afterwards, the laser was turned off, and transparent colorless glassy-spheres with diameters of 2–4 mm were finally obtained by cooling down the melt rapidly to room temperature. A part of the glassy spheres were ground into powders for the luminescence test, and the remaining were carefully polished into disks for X-ray diffraction (XRD), optical and hardness analyses.
Table 1 Densities (ρ) and Vicker's hardnesses of (28-x)La2O3-72Al2O3:xTm3+ glasses. x
ρ(g/cm3)
Vicker's hardness (GPa)
0 1 1.5 2 3
4.16 4.18 4.20 4.21 4.23
7.2 7.5 7.7 8.2 8.8
substitution of the relatively lighter La2O3 (325.84 g/mol) by the heavier Tm2O3 (385.86 g/mol). Meanwhile, all glasses have high Vicker's hardnesses, which also rises as x increased. According to the literature [12], the hardness is inversely proportional to the square of the distance between ions. The ionic radius of the substituted ion Tm3+ (88 pm) is smaller than that of La3+ (103.2 pm), resulting the rising Vicker's hardness as Tm3+ content increased. The similar results can be found in the reference [13], in which the hardness of (30-x)CaO-05Y2O310B2O3-55P2O5:xNiO glasses increases with the content of Ni2+. The highest Vicker's hardness is 8.8 GPa for x = 3, exceeding those of lanthanum aluminate glass 0.8LaAlO3-0.2Nb2O5 (8.3 GPa) [14], rare earth Er3+/Yb3+-doped glass TiO2-La2O3-Ta2O5 (7.7 GPa) [15] and La2O3Lu2O3-TiO2 (8.26 GPa) [16], and even being comparable to the maximum values reported for oxide glasses 81.8Al2O3-18.2Y2O3 (~9 GPa) [17] and 29.3Al2O3-50.2SiO2 -20.5Sc2O3 (9.4 GPa) [18].
2.2. Characterizations of samples The sample structure was tested by XRD, using Ultima IV X-ray diffractometer (Rigaku, Japan). The density was measured by the Archimedes' method using ME204 electronic balance and accompanied density components (Mettler Toledo, China). MHV-52 digital Vicker's hardness tester (Sctmc, China) equipped with a diamond Vicker's indenter was used to obtain the Vicker's hardness, which was measured three times for each sample and then averaged. Differential thermal analysis (DTA) was conducted by Setsys Evolution system (Setaram, France) in an argon atmosphere at a heating rate of 10 °C/min. The optical transmittance spectra were determined on Cary 5000 UV–visNIR spectrometer (Varian, USA) and Excalibur 3100 Fourier transform IR spectrometer (Varian, USA). Refractive indices were collected by SE 850 DUV SPEC ellipsometer (Sentech, Germany). Emission and excitation spectra were detected by F-7000 fluorescence spectrofluorometer (Hitachi, Japan) employing a 150 W Xe lamp. All the experiments were performed at room temperature.
3.3. Thermal properties
Table 1 summarizes the densities (ρ) and Vicker's hardnesses of (28x)La2O3-72Al2O3: xTm3+ glasses. Adding Tm3+ to 28La2O3–72Al2O3 system increases the density of the as-prepared glass, from 4.16 to 4.23 g/cm3 for x = 0 to 3, respectively. It can be ascribed to the
To investigate the thermal stability and glass-forming ability of (28x)La2O3-72Al2O3: xTm3+ glasses, the DTA curves are recorded and shown in Fig. 2. The curve of each glass presents a clear glass transition and a strong exothermic peak due to crystallization. The corresponding glass-transition temperature Tg and first crystallization temperature Tx are displayed in Fig. 3. Since the melting point of Tm2O3 (2425 °C) is higher than that of Al2O3 (2054 °C), Tm2O3 substitution improves the thermal stability of these glasses and Tg slightly increases with rising x from 840 to 851 °C. The values significantly surpass those of silicate, borate, aluminate and germanosilicate glasses [19–22], confirming a higher thermal stability. Generally, the glass-forming ability is measured using the temperature gap ΔT (ΔT = Tx-Tg) (inset of Fig. 3) [16,23]. A larger ΔT indicates better anti-crystallization property, i.e. harder devitrification of a glass in the preparation process. In this study, the reduction in ΔT suggests that the glass-forming ability of the present system is attenuated after Tm2O3 substitution. In other words, it is due to the fact that Tm2O3 is network modifier while Al2O3 is an intermediate that in this case can play glass-former role [24].
Fig. 1. XRD patterns of (28-x)La2O3-72Al2O3:xTm3+ glasses.
Fig. 2. DTA curves of (28-x)La2O3-72Al2O3: xTm3+ glasses.
3. Results and discussion 3.1. Structural analyses The XRD patterns of (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) samples are presented in Fig. 1. It is well-known that typical hump located at low angle is attributed to glassy phase. Obviously, each sample exhibits only a broad diffusive diffraction peak centered at 2θ of 29°, verifying the amorphous nature. 3.2. Physical properties
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Fig. 3. Tg and Tx for (28-x)La2O3-72Al2O3: xTm3+ glasses. Inset shows ΔT (ΔT = Tx-Tg) of these glasses.
Fig. 6. Transmittances of (28-x)La2O3-72Al2O3: xTm3+ glasses in IR region. The inset gives the photograph of well-polished sample of x = 1.5.
Fig. 7. Refractive index dispersion of 27 La2O3-72 Al2O3:1 Tm3+ glass. The dotted curve in the inset is obtained using the Drude–Voigt relationship, and the red curve is the fitted linear curve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Transmittances of (28-x)La2O3-72Al2O3: xTm3+ glasses in UV-VIS-NIR region. Inset is the transmittances from 200 nm to 260 nm.
Fig. 5. Representation of (αhv)1/2 versus hv of (28-x) La2O3-72 Al2O3: xTm3+ glasses. Inset gives the optical energy gap Eopt of these glasses. Fig. 8. The composition dependence of the refractive index nd of (28-x)La2O372Al2O3:xTm3+ glasses.
3.4. Optical properties Fig. 4 shows the transmittance spectra of all samples in the wavelength range from 200 to 1500 nm. The transmittance of 28La2O372Al2O3 glass is about 80%. As the Tm2O3 content increases, six 3
Journal of Non-Crystalline Solids 525 (2019) 119599
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Fig. 9. PLE (a) and PL (b) spectra of (28-x)La2O3-72Al2 O3: xTm3+ (x = 1, 1.5, 2, 3)glasses.
3.5. Photoluminescence properties
absorption bands centered at around 1210, 790, 682, 467, 354 and 260 nm appear, representing transitions from the 3H6 ground state of Tm3+ to excited states 3H5, 3H4, 3F2,3, 1G4, 1D2 and 3P2, respectively [9,25,26]. UV absorption edge shifts irregularly, as shown in the inset in Fig. 4. Practically, the UV edge is determined by the optical band gap energy Eopt, which is affected by the strong internal forces of electron transition from the valence band to the conduction band [27,28]. Thus, Eopt is calculated using the following equation:
αhv = A (hv − Eopt )2
Fig. 9(a) exhibits the photoluminescence excitation spectra of (28-x) La2O3-72Al2O3: xTm3+ (x = 0, 1, 1.5, 2, 3) glasses recorded by monitoring the emission at 457 nm assigned to the 1D2 → 3F4 transition of Tm3+ ions. The spectra of these glasses consist of four excitation peaks centered at 260 nm, 275 nm, 287 nm and 354 nm, corresponding to the transitions of Tm3+ from the lowest ground state 3H6 to the excited upper states 3P2, 3P1, 3P0 and 1D2, respectively [32,33]. The emission spectra of (28-x)La2O3-72Al2O3:xTm3+ glasses excited at 354 nm (3H6–1D2) are shown in Fig. 9 (b). There are a strong blue emission peak centered at 457 nm and a very weak emission peak at 476 nm, which can be assigned to the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+ [34–36]. Clearly, the emission intensity increases with rising Tm3+ content up to 1.5% and then gradually decreases as the content exceeds 1.5% because of concentration quenching [37].
(1)
where α is the absorption coefficient, A is an energy-independent constant, and hν is the incident radiation photon energy. Eopt is evaluated by extrapolating the linear part of (αhv)1/2 versus hv curve to (αhv)1/2 = 0(Fig. 5), with the values plotted in the inset. With increasing amount of Tm2O3, Eopt decreases to 5.125 eV for x = 1, and then increases with rising x to 5.209 eV. Hence, the transition between valence and conduction bands requires more energy and the cutoff wavelength hypochromatically shifted, being consistent with the variations of absorption edge in Fig. 4. The photograph of well-polished x = 1.5 sample and the IR transmittance spectra of (28-x)La2O3-72Al2O3:xTm3+ (x = 0, 1, 1.5, 2, 3) glasses are shown in Fig. 6. The absorption band at around 3.4 μm can be attributed to the free hydroxyl group which generally exists in oxide glasses [29]. The transmittance of each sample is above 80%, and the IR absorption edge related to multiphonon processes appears at the wavelength over 5.2 μm. The absorption cut-off wavelength at 3% transmittance shifts from 6.7 μm to 8.9 μm, indicating that doped Tm3+ increases the cut-off wavelength and Tm2O3 substitution enlarges the wide transmittance window of La2O3-Al2O3 glasses. Therefore, La2O3Al2O3 glasses are typified by such a wide transmittance range from 220 nm to 8.9 μm, allowing applications to glass materials with wide optical windows. The dependence of refractive index of 27La2O3-72Al2O3:1Tm3+ glass on wavelength is shown in Fig. 7. According to the single oscillator model from the Drude-Voigt relationship, the plot of 1/(n2−1) vs. 1/λ2 is a straight line [30]. As displayed in the inset of Fig. 7, the measured values (the dotted curve) are well linearly related, indicating that the single oscillator model adequately describes the refractive index dispersion in 27La2O3-72Al2O3:1Tm3+ glass. Fig. 8 presents the composition dependences of refractive indices nd of (28-x)La2O3-72 Al2O3:xTm3+ glasses at 587.6 nm. With rising x from 0 to 3, nd changes from 1.93 to 2.08, reaching the maximum at x = 1. In general, an oxide glass with nd of >2.0 is eligible for preparing highvalue lenses used in small optical devices such as smart phones with digital cameras and endoscopes [31].
4. Conclusions In conclusion, transparent (28-x)La2O3-72Al2O3: xTm3+ (x = 0, 1, 1.5, 2, 3) glasses were fabricated successfully by containerless processing. The as-prepared glasses emitted blue emission at 457 nm attributed to the 1D2 → 3F4 transition of Tm3+. These glasses had high Vicker's hardnesses (up to 8.8 GPa), excellent thermal stabilities (Tg up to 851 °C) and long cut-off wavelengths (up to 8.9 μm) in the IR region, which were effectively augmented through Tm2O3 substitution. They also exhibited high transmittances in a wide window. The highest refractive index was 2.08 for x = 1. Hence, Tm3+-doped La2O3-Al2O3 glasses may have optical applicability in the visible to IR region, such as optical devices of smart phones with digital cameras and endoscopes. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Nature Science Foundation of China (No.51602042) and the Natural Science Foundation of Heibei Province (No. E2018501042, B2018407058). References [1] Y. Yu, Z. Liu, N. Dai, Y. Sheng, H. Luan, J. Peng, Z. Jiang, H. Li, J. Li, L. Yang, Ce-TbMn co-doped white light emitting glasses suitable for long-wavelength UV excitation, Opt. Express 19 (2011) 19473.
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