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Molten-salt synthesis of large micron-sized YAG:Ce3+ phosphors for laser diode applications
Ceramics International 45 (2019) 21657–21660
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Molten-salt synthesis of large micron-sized YAG:Ce3+ phosphors for laser diode applications
T
Haohao Wanga,*, Meisheng Wub, Baoliang Maa, Liangshu Weia, Yibin Chenc, Langkai Lic,** a
Department of Physics, College of Science, Nanjing Agricultural University, Nanjing, 210095, PR China Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing, 210095, PR China c Department of Materials Science and Engineering, Xiamen University, Xiamen, 361005, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphors Thermal-quenching Phosphor wheel Laser Radiant efficiency
Large micron-sized Y3Al5O12:Ce3+ phosphors were successfully synthesized by a cost-effective method using eutectic mixtures of YF3–AlF3 as the molten-salt flux for high-power laser diodes applications. The crystallinity, morphology, particle size, thermal quenching properties of the phosphors were measured, respectively. The phosphor particle size increased with increasing amount of flux, and the medium size (D50) could reach 40 μm when adding 25 wt% YF3–AlF3. The phosphors with larger particle size did not only exhibit superior optical performances but also excellent thermal-quenching characteristics, leading to a lower surface temperature and a higher radiant efficiency when excited by a high-power laser.
1. Introduction Y3Al5O12:Ce3+ (YAG:Ce3+) is the most commonly used commercial yellow phosphor for white light-emitting diodes (WLEDs) due to its suitable photoluminescence (PL) properties, excited by an InGaN blue chip, as well as its high efficiency and physicochemical stability [1–4]. In general, the phosphor is prepared in powder form, which can scatter incident light and affect blue-light absorption and yellow-light emission. Hence, the particle size is one of the key factors affecting the phosphor's optical performance, including its luminous efficacy, color quality, and thermal-quenching properties. According to the literatures, the phosphor particle size for lighting applications is usually in the nano-to-several micrometer range [5–9]. Although phosphors with a smaller particle size can provide equivalent coverage densities at lower weights, the brightness decreases with decreasing particle size owing to the increased number of surface defects [10]. The best particle-size range for industrial applications is between 1 and 8 μm without aggregation [9]. The development of high-power WLEDs poses higher heat-dissipation requirements for phosphors. This issue becomes more critical when blue laser diodes (LDs) are desired as primary light sources, for example, in applications such as laser projectors. Hence, transparent ceramics (TCs) [11–13] and phosphor single crystals (SCs) [14–17] were proposed to overcome the abovementioned problems. However, compared to conventional phosphor powders, it is complex and *
expensive to produce TCs and SCs. The raw materials have to be pressed under high pressure and the products have to be polished after calcination to produce TCs. Specific equipment and particular technique are required to grow the SCs and the process is quite time-consuming. Moreover, the luminescence of TCs and SCs are not adjustable after sintering. For the phosphor particles, this adjustment is easy to implement by changing the phosphor ratio using red- or green-emitting phosphors. Molten salts form ionic liquids at relatively low temperatures and they can act as a chemical-reaction medium to create a liquid environment with high temperature for crystal growth [18–20]. Inorganic molten salts usually have several specific properties, such as high oxidizing potential, high thermal conductivity, and low viscosities and densities. Thus, the molten-salt method is a cost-effective approach to control the growth of crystal targets and obtain pure single-phase powders. Up to date, the criteria for choosing the appropriate molten salts for different kinds of inorganic phosphor materials are not fully understood. In the literatures, there are reports on the preparation of YAG:Ce3+ phosphor using BaF2, NaCl, YF3, H3BO3, NaCl–KCl, and NaNO2–KNO2 as molten salts [21–26]. Significant effects regarding the various particle sizes and morphology refinements obtained by the molten-salt method have been reported in the literature. To the best of our knowledge, the application performance of YAG:Ce3+ phosphors with super large particle size in high-power WLEDs and LDs have not been reported so far.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (L. Li).
**
https://doi.org/10.1016/j.ceramint.2019.07.163 Received 9 July 2019; Received in revised form 13 July 2019; Accepted 14 July 2019 Available online 15 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 21657–21660
H. Wang, et al.
In the present study, YAG:Ce3+ phosphor with large particle size (D50 ≈ 40 μm) was successfully synthesized using an eutectic mixture of YF3–AlF3 as the molten salt based on plenty experimental results. The morphology, particle size and thermal-quenching properties of YAG:Ce3+ phosphors with different particle sizes applied in LDs were investigated and compared. 2. Experimental Commercially available Y2O3, Al2O3 and CeO2 were used as the starting raw materials. YF3–AlF3 (1:1 wt ratio) powder flux was added to the starting materials at weight percentages of 0%, 10% and 25%. The nominal composition of the phosphor was determined to be Y2.94Al5O12:0.06Ce3+ (YAG:Ce3+) after experimental optimization. The abovementioned regent-grade materials were purchased from Sinopharm Chemical Reagent Co., Ltd. The reactants were weighted stoichiometrically and thoroughly ground in an agate mortar. Then, the powders were moved into a corundum crucible and calcined at 1500 °C for 10 h under a reducing atmosphere (95%N2 + 5%H2). The products were cooled down to room temperature. The as-synthesized phosphor powders were crushed and washed several times with a 10% hot nitric acid solution and deionized water to remove the molten salts. Finally, the powders were dried at 120 °C for 4 h. The phase purities were analyzed by powder X-ray diffraction (XRD, D/MAX-Ultima, Rigaku) using Cu Kα radiation for 2θ = 20–80°. A fluorescence spectrophotometer (EVERFINE, EX-1000, Yuanfang) was used to measure the PL properties, external quantum efficiency (EQE), and temperature-dependent emission spectra. Scanning electron microscopy (SEM) images were taken using a Hitachi TM3000 microscope. The particle-size distribution was determined using a dynamic light-scattering system (DLS, LS-POP, Oumeike) to obtain the particle diameters D10, D50, and D90. Fig. 1 present the laser projector setup, which is comprised of a 455 nm blue laser light and YAG:Ce3+ phosphor wheel as the excitation source and light-converter, respectively. After the generation of white light, the green and red light can be filtered out using specific optical filters. Hence, the standard white light can be generated by the combination of green, red, and blue light. The detailed preparation process of laser light source and phosphor wheel are described as follows. Silica gel, which acts as the binder, was added to the phosphor powders with a weight percentage of 70%. Then, the mixture was put into a vacuum de-aeration mixer to remove the bubbles. After that, the as-obtained mixture was coated onto the surface of an aluminum mirror, supported by a disc, and dried at 120 °C for 1 h. Finally, a phosphor wheel (H: 2 mm, inner diam: 52 mm, outer diam: 60 mm) was formed. The disc, which was driven by a motor, was rotated at a high speed. Six semiconductor laser groupware (455 nm) was used as the laser source, laser power then can be achieved higher by the combination of six laser output than single output of laser. The combined laser beam was focused and irradiated onto the surface of the rotated phosphor wheel. Then, the combined fluorescence and 455 nm laser light was focused by
Fig. 1. Illustration of the setup of the laser-source device converted by the phosphor wheel.
Fig. 2. XRD patterns of YAG:Ce3+ phosphors synthesized (a) without adding fluxes, (b) adding 10 and (c) 25 wt% YF3–AlF3 fluxes.
a set of collection lens and reflected to a power meter (FL250A, Ophir) by a dichroic filter. The radiant efficiency (RE) was obtained by calculating the power ratio of the combined light and the 455 nm laser. The temperature was detected at the focusing point on the wheel surface. Three points were selected to measure the RE and temperature. 3. Results and discussion Fig. 2 shows the XRD patterns of YAG:Ce3+ phosphors synthesized with different YF3–AlF3 amounts as well as the standard reference. It can be concluded that the as-synthesized phosphor products were well crystallized, and the formed phase was readily indexed as YAG (JCPDS Card 33-0040), indicating that the additives of various amount of fluxes did not have an effect on the crystallization of the YAG phase. The SEM images of phosphor samples synthesized using different amount of fluxes are illustrated in Fig. 3. It is obvious that the morphology of the phosphor particles obtained with flux (Fig. 3a and b) is far better than that of the particles without flux (Fig. 3c). The particles formed without fluxes were coarser and non-uniform, whereas those formed by the molten-salt method were well shaped and evenly distributed. The SEM image of the YAG:Ce3+ phosphor is also provided in Fig. S1 for comparison. The average particle size clearly increased with
Fig. 3. SEM images of YAG:Ce3+ phosphors synthesized (a) without adding fluxes, (b) adding 10 and (c) 25 wt% YF3–AlF3 fluxes. (d) The size distribution corresponds to samples (a)–(c) and commercial LED phosphors. Scale bar in the SEM images: 50 μm.
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Ceramics International 45 (2019) 21657–21660
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Fig. 5. Radiant efficiency of phosphor samples with increasing laser power in comparison with commercial LED phosphors.
Fig. 4. (a) PL spectra and (b) temperature-dependent normalized emission spectra of YAG:Ce3+ phosphors synthesized without adding any fluxes, and adding 10 and 25 wt% YF3–AlF3 fluxes at 450 nm excitation in comparison with commercial LED phosphors upon heating to 465 K. Inset (a): EQE of the corresponding phosphor samples.
an increase of in the amount of YF3–AlF3 as described in the size-distribution table in Fig. 3. A particle size of D50 ≈ 40 μm was obtained upon adding 25 wt% YF3–AlF3; this value is much larger than the particle sizes obtained for the samples prepared without fluxes (D50 ≈ 6 μm), and with 10 wt% fluxes (D50 ≈ 20 μm). This particle size is also larger than that obtained for the commercial phosphor product. The trend observed for the particle-size parameters D10 and D90 was also consistent with that observed for D50 in different phosphor samples. The abovementioned results indicate that YF3–AlF3 fluxes play a critical role in the synthesis of high quality YAG:Ce3+ crystals. As the increase of additive of YF3–AlF3 fluxes in the sintering process, fluid environment formed by low-melting point fluxes improved the mass diffusion, which would contribute the faster growth of crystals leading to the larger particle size. The PL curves of all the phosphor samples excited at 450 nm showed similar shapes and depicted a well-known broad fluorescence band ranging from 480 to 700 nm, with a peak at around 553 nm. The maximum remained at the same wavelength value for every sample. The PL results (Fig. 4a) clearly revealed that the phosphors without fluxes presented lower emission intensities, while the emission intensities of the phosphors remained constant with increasing particle size. The PL intensity decrease in small particles could be ascribed to material defects in the microstructure, attributed to a stop in the crystal formation in an early step [27]. The phosphor sample D50 ≈ 40 μm
(25 wt% YF3–AlF3) could scarcely contribute to improve the PL intensity compared with the sample D50 ≈ 20 μm (adding 10 wt% YF3–AlF3), which could be ascribed to their similar EQE values, as shown in the inset of Fig. 4a. Fig. S2 shows the normalized emission spectrum of the YAG:Ce3+ phosphor sample obtained with adding 25 wt% YF3–AlF3. The peak position gradually shifted toward the red side, which was related to the crystal-field effect on the Ce energy levels, indicating that the crystal-field effect is weaker, leading to the emission of lower energy photons [28]. The temperature-dependent emission intensities (λex = 450 nm) of phosphor samples show a dependence trend for increasing particle sizes (Fig. 4b). The phosphor samples having the largest particle sizes (D50 ≈ 40 μm) showed the best thermal-quenching properties with 13% emission loss at temperatures up to 465 K. The samples having the smallest particle size (D50 ≈ 6 μm) presented 21% loss at the same temperature. The thermal stability of the phosphor samples with larger particle size could be attributed to the presence of less surface defects. Fig. 5 shows the RE variations of the phosphors upon adding YF3–AlF3 compared to commercial LED phosphors at different laser powers. The RE values of all the phosphor samples decreased as the laser power increased from 25 to 120 W. The phosphor sample having the largest particle size (D50 ≈ 40 μm) shows higher RE values compared to the sample with particle size D50 ≈ 20 μm (10 wt% YF3–AlF3) and to the commercial LED phosphor at all the laser powers. Table 1 shows that the RE value of phosphor sample with the largest particle size was 31.33%, while the RE values for the phosphor samples with particle size D50 ≈ 20 μm and for the commercial LED phosphor were 30.40 and 30.83, respectively. Accordingly, the temperature detected at the laser focused point was 381.7, 394.3, and 393.3 K for phosphor samples with 25 and 10 wt% YF3–AlF3 and for commercial phosphor, respectively. The optical properties were not influenced by the particle size excited by 450 nm Xenon lamp at room temperature (Table 1). Two main reasons could explain the abovementioned results, as illustrated in Fig. 6. First, as can be known, larger particles can assist the thermal conductivity due to the presence of less boundary defects. Second, the larger particles have less contact area, which can shorten the thermal transfer distance in comparison with that of smaller particles at the same wheel thickness. In contrast, smaller particles can accumulate thermal energy owing to their greater boundary area and longer thermal transfer distance. 4. Conclusion In summary, we have prepared well-crystallized YAG:Ce3+ phosphors at 1500 °C for 10 h by the molten-salt method. The particle size
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Ceramics International 45 (2019) 21657–21660
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Table 1 Comparison of the emission peak value, CIE chromaticity coordinates and EQE properties of phosphor samples excited by a 450 nm Xenon lamp, and comparison of the RE and surface temperature of phosphor samples excited by a 445 nm laser source (120 W). Samples
10 wt% fluxes 25 wt% fluxes Commercial
Excited by 450 nm Xenon lamp
Excited by 445 nm Laser source (120 W)
Peak (λ, nm)
CIE (x, y)
RE (η, %)
Surface temperature (T, K)
552.8 553.2 553.6
(0.439, 0.542) (0.440, 0.542) (0.440, 0.542)
30.40 ± 0.08 31.33 ± 0.11 30.83 ± 0.07
394.3 ± 1.2 381.7 ± 2.1 393.3 ± 1.5
Fig. 6. Thermal conduction in the phosphor wheels formed by adding (a) 10 and (b) 25 wt% YF3–AlF3 when excited by 445 nm laser source.
could be tuned by adjusting the amount of eutectic mixtures of YF3–AlF3, and the particle size D50 could reach 40 μm when adding 25 wt% YF3–AlF3. The photoluminescence properties revealed that the emission intensities and external quantum efficiencies obtained after adding fluxes were higher than those of the phosphors without added fluxes. A YAG:Ce3+ phosphor wheel made with the largest particle sizes (D50 ≈ 40 μm) presented the highest radiant efficiency (31.33%) and the lowest surface temperature (381.7 K), which was attributed to the excellent thermal-quenching characteristics. In addition, LuAG:Ce3+, GdxY1-xYAG:Ce3+ phosphors were successfully synthesized by the molten-salt method using eutectic mixtures of YF3–AlF3. Hence, the emission wavelength can be adjusted from around 518 to 576 nm. The SEM images and emission spectra of selected samples are shown in Figs. S3–S4. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.07.163. References [1] D. Bucevac, V. Krstic, The effect of SiC addition on photoluminescence of YAG:Ce phosphor for white LED, J. Eur. Ceram. Soc. 38 (2018) 5519–5524. [2] L. Zhang, B. Sun, L. Gu, W. Bu, X. Fu, R. Sun, T. Zhou, F.A. Selim, C. Wong, H. Chen, Enhanced light extraction of single-surface textured YAG:Ce transparent ceramics for high power white LEDs, Appl. Surf. Sci. 455 (2018) 425–432. [3] C.C. Chiang, M.S. Tsai, C.S. Hsiao, M.H. Hon, Synthesis of YAG:Ce phosphor via different aluminum sources and precipitation processes, J. Alloy. Comp. 416 (2006) 265–269. [4] D. Haranath, H. Chander, P. Sharma, S. Singh, Enhanced luminescence of Y3Al5O12:Ce3+ nanophosphor for white light-emitting diodes, Appl. Phys. Lett. 89 (2006). [5] H. Yang, D.-K. Lee, Y.-S. Kim, Spectral variations of nano-sized Y3Al5O12: Ce phosphors via codoping/substitution and their white LED characteristics, Mater. Chem. Phys. 114 (2009) 665–669. [6] H. Yang, G. Zhu, L. Yuan, C. Zhang, F. Li, H. Xu, A. Yu, Characterization and luminescence properties of YAG: Ce3+ phosphors by molten salt synthesis, J. Am. Ceram. Soc. 95 (2012) 49–51.
[7] Z.-Y. Liu, C. Li, B.-H. Yu, Y.-H. Wang, H.-B. Niu, Effects of YAG: Ce phosphor particle size on luminous flux and angular color uniformity of phosphor-converted white LEDs, J. Disp. Technol. 8 (2012) 329–335. [8] C. Wu, A. Luo, G. Du, X. Qin, W. Shi, Synthesis and luminescent properties of nonaggregated YAG: Ce3+ phosphors via the molten salt synthesis method, Mater. Sci. Semicond. Process. 16 (2013) 679–685. [9] Y. Shi, W. Li, Y. Wen, Micro-sized K2SiF6: Mn4+ red phosphors for light emitting diodes synthesized by a simple method, Funct. Mater. Lett. 10 (2017) 1750016. [10] Y. Shi, Y. Wang, D. Wang, B. Liu, Y. Li, L. Wei, Synthesis of hexagonal prism (La, Ce, Tb) PO4 phosphors by precipitation method, Cryst. Growth Des. 12 (2012) 1785–1791. [11] L.-f. He, G.-h. Fan, M.-y. Lei, Z.-l. Lou, Z.-w. Chen, Y. Xiao, S.-w. Zheng, T. Zhang, Preparation and optical properties of MgAl2O4/Ce:YAG transparent ceramics, Spectrosc. Spectr. Anal. 33 (2013) 1175–1179. [12] R. Zheng, D. Luo, Y. Yuan, Z. Wang, Y. Zhang, W. Wei, L.B. Kong, D. Tang, Dy3+/ Ce3+ codoped YAG transparent ceramics for single‐composition tunable white‐light phosphor, J. Am. Ceram. Soc. 98 (2015) 3231–3235. [13] G. Liu, Z. Zhou, Y. Shi, Q. Liu, J. Wan, Y. Pan, Ce: YAG transparent ceramics for applications of high power LEDs: thickness effects and high temperature performance, Mater. Lett. 139 (2015) 480–482. [14] A. Latynina, M. Watanabe, D. Inomata, K. Aoki, Y. Sugahara, E.G. Villora, K. Shimamura, Properties of Czochralski grown Ce, Gd:Y3Al5O12 single crystal for white light-emitting diode, J. Alloy. Comp. 553 (2013) 89–92. [15] C. Yang, G. Gu, X. Zhao, X. Liang, W. Xiang, The growth and luminescence properties of Y3Al5O12:Ce3+ single crystal by doping Gd3+ for W-LEDs, Mater. Lett. 170 (2016) 58–61. [16] L. Feng, Y. Li, H. Xiong, S. Wang, J. Wang, W. Ding, Y. Zhang, F. Yun, Freestanding GaN-based light-emitting diode membranes on Y3Al5O12:Ce3+ crystal phosphor plate for efficient white light emission, Appl. Phys. Express 9 (2016). [17] S. Arjoca, E.G. Villora, D. Inomata, K. Aoki, Y. Sugahara, K. Shimamura, Temperature dependence of Ce:YAG single-crystal phosphors for high-brightness white LEDs/LDs, Mater. Res. Express 2 (2015). [18] X. Wen, C. He, B. Wu, X. Huang, Z. Huang, Z. Yin, Y. Liu, M. Fang, X. Wu, X. Min, Molten salt synthesis, growth mechanism, and photoluminescence of rod chlorapatite microcrystallites, CrystEngComm 21 (2019) 1809–1817. [19] B.-r. Li, Y. Yang, Molten salt solvent synthesis of La2Mo2O9 nano-wires by controlling the subsequent calcinations process, Mater. Chem. Phys. 147 (2014) 735–743. [20] M.K. Ekmekci, M. Erdem, A. Mergen, G. Ozen, B. Di Bartolo, Molten salt synthesis and spectral properties of Nd3+ doped CdNb2O6 columbite phosphors, J. Alloy. Comp. 591 (2014) 230–233. [21] Z. Song, J. Liao, X. Ding, X. Liu, Q. Liu, Synthesis of YAG phosphor particles with excellent morphology by solid state reaction, J. Cryst. Growth 365 (2013) 24–28. [22] L. Song, Y. Dong, Q. Shao, J. Jiang, Preparation of dispersed submicron YAG:Ce3+ phosphors via the molten salt method, J. Mater. Sci. Mater. Electron. 29 (2018) 5761–5767. [23] K. Kanai, Y. Fukui, T. Kozawa, A. Kondo, M. Naito, Effect of flux powder addition on the synthesis of YAG phosphor by mechanical method, Adv. Powder Technol. 29 (2018) 457–461. [24] C.-H. Chiang, T.-H. Liu, H.-Y. Lin, H.-Y. Kuo, S.-Y. Chu, Effects of flux additives on the characteristics of Y2.95Al5O12:0.05Ce3+ phosphor: particle growth mechanism and luminescence, J. Appl. Phys. 114 (2013). [25] L. Gan, Z.-Y. Mao, F.-F. Xu, Y.-C. Zhu, X.-J. Liu, Molten salt synthesis of YAG:Ce3+ phosphors from oxide raw materials, Ceram. Int. 40 (2014) 5067–5071. [26] C. Wu, A. Luo, G. Du, X. Qin, W. Shi, Synthesis and luminescent properties of nonaggregated YAG:Ce3+ phosphors via the molten salt synthesis method, Mater. Sci. Semicond. Process. 16 (2013) 679–685. [27] L.T. Su, A.I.Y. Tok, F.Y.C. Boey, X.H. Zhang, J.L. Woodhead, C.J. Summers, Photoluminescence phenomena of Ce3+-doped Y3Al5O12 nanophosphors, J. Appl. Phys. 102 (2007). [28] M. Acta PolymericaBorlaf, M. Frankowska, W.W. Kubiak, T. Graule, Strong photoluminescence emission at low dopant amount in YAG:Ce and YAG:Eu phosphors, Mater. Res. Bull. 100 (2018) 413–419.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Molten-salt synthesis of large micron-sized YAG:Ce3+ phosphors for laser diode applications
T
Haohao Wanga,*, Meisheng Wub, Baoliang Maa, Liangshu Weia, Yibin Chenc, Langkai Lic,** a
Department of Physics, College of Science, Nanjing Agricultural University, Nanjing, 210095, PR China Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing, 210095, PR China c Department of Materials Science and Engineering, Xiamen University, Xiamen, 361005, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphors Thermal-quenching Phosphor wheel Laser Radiant efficiency
Large micron-sized Y3Al5O12:Ce3+ phosphors were successfully synthesized by a cost-effective method using eutectic mixtures of YF3–AlF3 as the molten-salt flux for high-power laser diodes applications. The crystallinity, morphology, particle size, thermal quenching properties of the phosphors were measured, respectively. The phosphor particle size increased with increasing amount of flux, and the medium size (D50) could reach 40 μm when adding 25 wt% YF3–AlF3. The phosphors with larger particle size did not only exhibit superior optical performances but also excellent thermal-quenching characteristics, leading to a lower surface temperature and a higher radiant efficiency when excited by a high-power laser.
1. Introduction Y3Al5O12:Ce3+ (YAG:Ce3+) is the most commonly used commercial yellow phosphor for white light-emitting diodes (WLEDs) due to its suitable photoluminescence (PL) properties, excited by an InGaN blue chip, as well as its high efficiency and physicochemical stability [1–4]. In general, the phosphor is prepared in powder form, which can scatter incident light and affect blue-light absorption and yellow-light emission. Hence, the particle size is one of the key factors affecting the phosphor's optical performance, including its luminous efficacy, color quality, and thermal-quenching properties. According to the literatures, the phosphor particle size for lighting applications is usually in the nano-to-several micrometer range [5–9]. Although phosphors with a smaller particle size can provide equivalent coverage densities at lower weights, the brightness decreases with decreasing particle size owing to the increased number of surface defects [10]. The best particle-size range for industrial applications is between 1 and 8 μm without aggregation [9]. The development of high-power WLEDs poses higher heat-dissipation requirements for phosphors. This issue becomes more critical when blue laser diodes (LDs) are desired as primary light sources, for example, in applications such as laser projectors. Hence, transparent ceramics (TCs) [11–13] and phosphor single crystals (SCs) [14–17] were proposed to overcome the abovementioned problems. However, compared to conventional phosphor powders, it is complex and *
expensive to produce TCs and SCs. The raw materials have to be pressed under high pressure and the products have to be polished after calcination to produce TCs. Specific equipment and particular technique are required to grow the SCs and the process is quite time-consuming. Moreover, the luminescence of TCs and SCs are not adjustable after sintering. For the phosphor particles, this adjustment is easy to implement by changing the phosphor ratio using red- or green-emitting phosphors. Molten salts form ionic liquids at relatively low temperatures and they can act as a chemical-reaction medium to create a liquid environment with high temperature for crystal growth [18–20]. Inorganic molten salts usually have several specific properties, such as high oxidizing potential, high thermal conductivity, and low viscosities and densities. Thus, the molten-salt method is a cost-effective approach to control the growth of crystal targets and obtain pure single-phase powders. Up to date, the criteria for choosing the appropriate molten salts for different kinds of inorganic phosphor materials are not fully understood. In the literatures, there are reports on the preparation of YAG:Ce3+ phosphor using BaF2, NaCl, YF3, H3BO3, NaCl–KCl, and NaNO2–KNO2 as molten salts [21–26]. Significant effects regarding the various particle sizes and morphology refinements obtained by the molten-salt method have been reported in the literature. To the best of our knowledge, the application performance of YAG:Ce3+ phosphors with super large particle size in high-power WLEDs and LDs have not been reported so far.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (L. Li).
**
https://doi.org/10.1016/j.ceramint.2019.07.163 Received 9 July 2019; Received in revised form 13 July 2019; Accepted 14 July 2019 Available online 15 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 21657–21660
H. Wang, et al.
In the present study, YAG:Ce3+ phosphor with large particle size (D50 ≈ 40 μm) was successfully synthesized using an eutectic mixture of YF3–AlF3 as the molten salt based on plenty experimental results. The morphology, particle size and thermal-quenching properties of YAG:Ce3+ phosphors with different particle sizes applied in LDs were investigated and compared. 2. Experimental Commercially available Y2O3, Al2O3 and CeO2 were used as the starting raw materials. YF3–AlF3 (1:1 wt ratio) powder flux was added to the starting materials at weight percentages of 0%, 10% and 25%. The nominal composition of the phosphor was determined to be Y2.94Al5O12:0.06Ce3+ (YAG:Ce3+) after experimental optimization. The abovementioned regent-grade materials were purchased from Sinopharm Chemical Reagent Co., Ltd. The reactants were weighted stoichiometrically and thoroughly ground in an agate mortar. Then, the powders were moved into a corundum crucible and calcined at 1500 °C for 10 h under a reducing atmosphere (95%N2 + 5%H2). The products were cooled down to room temperature. The as-synthesized phosphor powders were crushed and washed several times with a 10% hot nitric acid solution and deionized water to remove the molten salts. Finally, the powders were dried at 120 °C for 4 h. The phase purities were analyzed by powder X-ray diffraction (XRD, D/MAX-Ultima, Rigaku) using Cu Kα radiation for 2θ = 20–80°. A fluorescence spectrophotometer (EVERFINE, EX-1000, Yuanfang) was used to measure the PL properties, external quantum efficiency (EQE), and temperature-dependent emission spectra. Scanning electron microscopy (SEM) images were taken using a Hitachi TM3000 microscope. The particle-size distribution was determined using a dynamic light-scattering system (DLS, LS-POP, Oumeike) to obtain the particle diameters D10, D50, and D90. Fig. 1 present the laser projector setup, which is comprised of a 455 nm blue laser light and YAG:Ce3+ phosphor wheel as the excitation source and light-converter, respectively. After the generation of white light, the green and red light can be filtered out using specific optical filters. Hence, the standard white light can be generated by the combination of green, red, and blue light. The detailed preparation process of laser light source and phosphor wheel are described as follows. Silica gel, which acts as the binder, was added to the phosphor powders with a weight percentage of 70%. Then, the mixture was put into a vacuum de-aeration mixer to remove the bubbles. After that, the as-obtained mixture was coated onto the surface of an aluminum mirror, supported by a disc, and dried at 120 °C for 1 h. Finally, a phosphor wheel (H: 2 mm, inner diam: 52 mm, outer diam: 60 mm) was formed. The disc, which was driven by a motor, was rotated at a high speed. Six semiconductor laser groupware (455 nm) was used as the laser source, laser power then can be achieved higher by the combination of six laser output than single output of laser. The combined laser beam was focused and irradiated onto the surface of the rotated phosphor wheel. Then, the combined fluorescence and 455 nm laser light was focused by
Fig. 1. Illustration of the setup of the laser-source device converted by the phosphor wheel.
Fig. 2. XRD patterns of YAG:Ce3+ phosphors synthesized (a) without adding fluxes, (b) adding 10 and (c) 25 wt% YF3–AlF3 fluxes.
a set of collection lens and reflected to a power meter (FL250A, Ophir) by a dichroic filter. The radiant efficiency (RE) was obtained by calculating the power ratio of the combined light and the 455 nm laser. The temperature was detected at the focusing point on the wheel surface. Three points were selected to measure the RE and temperature. 3. Results and discussion Fig. 2 shows the XRD patterns of YAG:Ce3+ phosphors synthesized with different YF3–AlF3 amounts as well as the standard reference. It can be concluded that the as-synthesized phosphor products were well crystallized, and the formed phase was readily indexed as YAG (JCPDS Card 33-0040), indicating that the additives of various amount of fluxes did not have an effect on the crystallization of the YAG phase. The SEM images of phosphor samples synthesized using different amount of fluxes are illustrated in Fig. 3. It is obvious that the morphology of the phosphor particles obtained with flux (Fig. 3a and b) is far better than that of the particles without flux (Fig. 3c). The particles formed without fluxes were coarser and non-uniform, whereas those formed by the molten-salt method were well shaped and evenly distributed. The SEM image of the YAG:Ce3+ phosphor is also provided in Fig. S1 for comparison. The average particle size clearly increased with
Fig. 3. SEM images of YAG:Ce3+ phosphors synthesized (a) without adding fluxes, (b) adding 10 and (c) 25 wt% YF3–AlF3 fluxes. (d) The size distribution corresponds to samples (a)–(c) and commercial LED phosphors. Scale bar in the SEM images: 50 μm.
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Fig. 5. Radiant efficiency of phosphor samples with increasing laser power in comparison with commercial LED phosphors.
Fig. 4. (a) PL spectra and (b) temperature-dependent normalized emission spectra of YAG:Ce3+ phosphors synthesized without adding any fluxes, and adding 10 and 25 wt% YF3–AlF3 fluxes at 450 nm excitation in comparison with commercial LED phosphors upon heating to 465 K. Inset (a): EQE of the corresponding phosphor samples.
an increase of in the amount of YF3–AlF3 as described in the size-distribution table in Fig. 3. A particle size of D50 ≈ 40 μm was obtained upon adding 25 wt% YF3–AlF3; this value is much larger than the particle sizes obtained for the samples prepared without fluxes (D50 ≈ 6 μm), and with 10 wt% fluxes (D50 ≈ 20 μm). This particle size is also larger than that obtained for the commercial phosphor product. The trend observed for the particle-size parameters D10 and D90 was also consistent with that observed for D50 in different phosphor samples. The abovementioned results indicate that YF3–AlF3 fluxes play a critical role in the synthesis of high quality YAG:Ce3+ crystals. As the increase of additive of YF3–AlF3 fluxes in the sintering process, fluid environment formed by low-melting point fluxes improved the mass diffusion, which would contribute the faster growth of crystals leading to the larger particle size. The PL curves of all the phosphor samples excited at 450 nm showed similar shapes and depicted a well-known broad fluorescence band ranging from 480 to 700 nm, with a peak at around 553 nm. The maximum remained at the same wavelength value for every sample. The PL results (Fig. 4a) clearly revealed that the phosphors without fluxes presented lower emission intensities, while the emission intensities of the phosphors remained constant with increasing particle size. The PL intensity decrease in small particles could be ascribed to material defects in the microstructure, attributed to a stop in the crystal formation in an early step [27]. The phosphor sample D50 ≈ 40 μm
(25 wt% YF3–AlF3) could scarcely contribute to improve the PL intensity compared with the sample D50 ≈ 20 μm (adding 10 wt% YF3–AlF3), which could be ascribed to their similar EQE values, as shown in the inset of Fig. 4a. Fig. S2 shows the normalized emission spectrum of the YAG:Ce3+ phosphor sample obtained with adding 25 wt% YF3–AlF3. The peak position gradually shifted toward the red side, which was related to the crystal-field effect on the Ce energy levels, indicating that the crystal-field effect is weaker, leading to the emission of lower energy photons [28]. The temperature-dependent emission intensities (λex = 450 nm) of phosphor samples show a dependence trend for increasing particle sizes (Fig. 4b). The phosphor samples having the largest particle sizes (D50 ≈ 40 μm) showed the best thermal-quenching properties with 13% emission loss at temperatures up to 465 K. The samples having the smallest particle size (D50 ≈ 6 μm) presented 21% loss at the same temperature. The thermal stability of the phosphor samples with larger particle size could be attributed to the presence of less surface defects. Fig. 5 shows the RE variations of the phosphors upon adding YF3–AlF3 compared to commercial LED phosphors at different laser powers. The RE values of all the phosphor samples decreased as the laser power increased from 25 to 120 W. The phosphor sample having the largest particle size (D50 ≈ 40 μm) shows higher RE values compared to the sample with particle size D50 ≈ 20 μm (10 wt% YF3–AlF3) and to the commercial LED phosphor at all the laser powers. Table 1 shows that the RE value of phosphor sample with the largest particle size was 31.33%, while the RE values for the phosphor samples with particle size D50 ≈ 20 μm and for the commercial LED phosphor were 30.40 and 30.83, respectively. Accordingly, the temperature detected at the laser focused point was 381.7, 394.3, and 393.3 K for phosphor samples with 25 and 10 wt% YF3–AlF3 and for commercial phosphor, respectively. The optical properties were not influenced by the particle size excited by 450 nm Xenon lamp at room temperature (Table 1). Two main reasons could explain the abovementioned results, as illustrated in Fig. 6. First, as can be known, larger particles can assist the thermal conductivity due to the presence of less boundary defects. Second, the larger particles have less contact area, which can shorten the thermal transfer distance in comparison with that of smaller particles at the same wheel thickness. In contrast, smaller particles can accumulate thermal energy owing to their greater boundary area and longer thermal transfer distance. 4. Conclusion In summary, we have prepared well-crystallized YAG:Ce3+ phosphors at 1500 °C for 10 h by the molten-salt method. The particle size
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Table 1 Comparison of the emission peak value, CIE chromaticity coordinates and EQE properties of phosphor samples excited by a 450 nm Xenon lamp, and comparison of the RE and surface temperature of phosphor samples excited by a 445 nm laser source (120 W). Samples
10 wt% fluxes 25 wt% fluxes Commercial
Excited by 450 nm Xenon lamp
Excited by 445 nm Laser source (120 W)
Peak (λ, nm)
CIE (x, y)
RE (η, %)
Surface temperature (T, K)
552.8 553.2 553.6
(0.439, 0.542) (0.440, 0.542) (0.440, 0.542)
30.40 ± 0.08 31.33 ± 0.11 30.83 ± 0.07
394.3 ± 1.2 381.7 ± 2.1 393.3 ± 1.5
Fig. 6. Thermal conduction in the phosphor wheels formed by adding (a) 10 and (b) 25 wt% YF3–AlF3 when excited by 445 nm laser source.
could be tuned by adjusting the amount of eutectic mixtures of YF3–AlF3, and the particle size D50 could reach 40 μm when adding 25 wt% YF3–AlF3. The photoluminescence properties revealed that the emission intensities and external quantum efficiencies obtained after adding fluxes were higher than those of the phosphors without added fluxes. A YAG:Ce3+ phosphor wheel made with the largest particle sizes (D50 ≈ 40 μm) presented the highest radiant efficiency (31.33%) and the lowest surface temperature (381.7 K), which was attributed to the excellent thermal-quenching characteristics. In addition, LuAG:Ce3+, GdxY1-xYAG:Ce3+ phosphors were successfully synthesized by the molten-salt method using eutectic mixtures of YF3–AlF3. Hence, the emission wavelength can be adjusted from around 518 to 576 nm. The SEM images and emission spectra of selected samples are shown in Figs. S3–S4. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.07.163. References [1] D. Bucevac, V. Krstic, The effect of SiC addition on photoluminescence of YAG:Ce phosphor for white LED, J. Eur. Ceram. Soc. 38 (2018) 5519–5524. [2] L. Zhang, B. Sun, L. Gu, W. Bu, X. Fu, R. Sun, T. Zhou, F.A. Selim, C. Wong, H. Chen, Enhanced light extraction of single-surface textured YAG:Ce transparent ceramics for high power white LEDs, Appl. Surf. Sci. 455 (2018) 425–432. [3] C.C. Chiang, M.S. Tsai, C.S. Hsiao, M.H. Hon, Synthesis of YAG:Ce phosphor via different aluminum sources and precipitation processes, J. Alloy. Comp. 416 (2006) 265–269. [4] D. Haranath, H. Chander, P. Sharma, S. Singh, Enhanced luminescence of Y3Al5O12:Ce3+ nanophosphor for white light-emitting diodes, Appl. Phys. Lett. 89 (2006). [5] H. Yang, D.-K. Lee, Y.-S. Kim, Spectral variations of nano-sized Y3Al5O12: Ce phosphors via codoping/substitution and their white LED characteristics, Mater. Chem. Phys. 114 (2009) 665–669. [6] H. Yang, G. Zhu, L. Yuan, C. Zhang, F. Li, H. Xu, A. Yu, Characterization and luminescence properties of YAG: Ce3+ phosphors by molten salt synthesis, J. Am. Ceram. Soc. 95 (2012) 49–51.
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