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Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics
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Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics Yi-Chen Chen, Yung-Tang Nien ∗ Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 63201, Taiwan
a r t i c l e
i n f o
Article history: Received 12 June 2016 Received in revised form 22 July 2016 Accepted 25 July 2016 Available online xxx Keyword: Laser Sintering Phosphor Yttrium aluminum garnet Light emitting diode
a b s t r a c t Cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor ceramics were fabricated by CO2 laser sintering under various powers. X-ray diffraction results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. The reaction profile caused by laser ablation increased from 0.28 mm (15 W) to 2.68 mm (40 W), and it appeared deeper in the scanning sequence because of the low rigidity of YAG melts compared with solid precursors. Photoluminescence (PL) measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation presented more intense yellow emission (enhancement of 54%) as compared with YAG:Ce by solid-state reaction (SSR, 1500 ◦ C). The laser beams were believed to distribute activators (Ce) more homogeneously compared with SSR. Additionally, a rougher surface of YAG:Ce phosphor ceramics with periodic holes caused by laser ablation resulted in more excitation and emissivity of PL. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Phosphor materials have been studied over 100 years and applied in various areas and products, such as fluorescent lamps, x-ray images, cathode ray tubes, electroluminescence and plasma display panels, by converting incident electrons, photons, or energetic particles to visible photons. Most importantly, phosphors have gained much attention because of the rapid development of white light emitting diodes (WLEDs) [1]. The first WLED product was introduced by Nichia Chemical Industries Ltd. in 1996 with an efficiency of only 5 lumens/W, using InGaN blue LED chip to excite cerium-doped yttrium aluminum garnet (Y3 Al5 O12 :Ce or YAG:Ce) phosphor powders with a yellow emission peak at 550–580 nm [2]. By tuning the output ratios of yellow/blue lights or adding red phosphors, a color temperature range of 2800–6500 K white light can be obtained. Therefore, WLED solid state lighting technology can speed up the replacement of traditional fluorescence products and is a potential solution to global warming [3]. The preparation methods of YAG phosphor powders for WLED applications can be roughly classified to solid state reaction (SSR) [4], sol-gel [5], solvothermal one-pot synthesis [6], hydrothermal [7], and microwave sintering methods [8]. Among these methods, SSR has the advantages of simple process, mass production, and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y.-T. Nien).
low cost, but it leads to intermediate phase formations, e.g., yttrium aluminum perovskite (YAlO3 or YAP) and yttrium aluminum monoclinic (YAl4 O9 or YAM). Song et al. pointed out that heat treatments with fluxes at a high temperature of 1500 ◦ C can eliminate impurity phases and obtain better luminescence performance of YAG:Ce [9]. However, three stages in SSR, namely, slow heating, high temperature holding, and furnace cooling, generally require much time to obtain a homogeneous distribution of dopants, reduce internal pores, and avoid large volume contractions of ceramic compacts. Therefore, a reliable furnace to maintain temperature uniformity and avoid an uneven temperature distribution is required for SSR to prepare phosphors. Moreover, the phosphor powders interspersed in the epoxy resin may distribute randomly to cause serious scattering and non-uniform emission. Phosphor ceramics have been proposed in resin-free WLED packages to overcome the problems of working temperature, current, and package density [10,11]. Laser sintering was recently proposed as a new type of heat treatment and forming technology. Laser sintering does not only create complex and accurate 3D parts, but also achieves compact density by high temperature reactions in a rapid time, i.e., one order less compared with traditional SSR processes [12]. Moreover, the extremely short contact time of the laser beam and materials significantly defines the reaction areas to avoid side effects to surrounding microstructures [13,14]. In the production of ceramic parts by laser sintering, an organic polymer material is mostly used as a binder to link ceramic powers. It is then burned out by ignition to densify ceramic pellets or parts [15]. To the best of our
http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Y.-C. Chen, Y.-T. Nien, Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032
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knowledge, the fabrication of YAG phosphor ceramics using energetic laser beams has not been reported. This study focused on the fabrication of YAG:Ce by blending precursor powders with an organic polymer to form pellets through uniaxial pressing, followed by sintering by a CO2 laser under various powers. The phase purity, surface morphology, microstructure, and photoluminescence (PL) of laser sintered YAG:Ce will be discussed and compared with that by a traditional SSR at 1500 ◦ C. 2. Experimental procedure The precursor materials of alumina (Al2 O3 , 99.9%, 3–5 m, Showa Chemicals), yttria (Y2 O3 , 99.9%, 10 m, Alfa Aesar), and Ce(NO3 )3 ·6H2 O (99.9%, Aencore Chemical) were used without further purification. According to the PL intensity results of various contents of Ce in YAG, an optimal content of Ce (0.055) was chosen for all samples, either in laser sintering or SSR. The above starting materials were ball-milled in 40 ml of ethanol (99.5%, Echo Chemical) for 20 h in a stoichiometric ratio of 2.89:5:0.055 (Y:Al:Ce). The ball mills used in this study were pure alumina with a diameter of 2 mm and maintained at a weight ratio of 1.2 (powder/ball). After ball milling, the above raw materials were dried in an oven at 90 ◦ C for 6 h and transferred to an alumina mortar for grinding (10 min first) with 5 wt% silica powders (99.9%, 80 nm, Alfa Aesar) and 1 wt% polyvinyl alcohol (PVA, 98%–99%, Alfa Aesar) in ethanol for 1 and 0.5 h, respectively. Silica functions as a sintering aid to enhance the diffusion of precursors [16]. After mixing, the above raw materials were dried again at 90 ◦ C for 6 h and ground for 10 min. The above mixture was shaped into pellets in a diameter of 10 mm by uniaxial pressing of 35 kg/cm2 for 10 min. After preparing the above pellets, a CO2 laser (3AXLE DC-6040N) was utilized to sinter areas of 6 mm × 6 mm or 14 mm × 14 mm in a scanning speed of 0.2 mm/s, a spot interval of 0.5 mm, and an irradiation time of 0.1 s for each spot under various powers of 15, 20, 30, and 40 W. The diameter of the laser spot was approximately 0.5 mm. For comparison, the same pellet without PVA but with 1 wt% NaF addition (99%, Alfa Aesar) was sintered twice in a traditional furnace at 1500 ◦ C for 12 and 24 h in air. The crystal structure, microstructure, and surface morphology of laser sintered samples were investigated using an x-ray diffractometer (XRD, Bruker D8A25), a scanning electron microscope (SEM, Hitachi SU3500), and an optical microscope (OM, Olympus DSX500). For PL measurements, YAG:Ce pellets were excited by a peak wavelength of 453 nm and detected via a fiber probe with six surrounding fibers (excitation) and a central fiber (emission collection) to an optical spectrometer (Ocean Optics USB2000). 3. Results and discussion The crystal structure and surface morphology of yttria-alumina precursor powders under various laser powers were studied using the XRD patterns and OM images shown in Figs. 1 and 2, respectively. As shown in Fig. 1a, all samples in the power range of 15–40 W crystallized well as a single phase of cubic YAG (JCPDS 791891) without any obvious residual or impurity phase, e.g., cubic yttria (JCPDS 01-0831), hexagonal alumina (JCPDS 01-1296), monoclinic YAM (JCPDS 34-0368), or orthorhombic YAP (JCPDS 70-1677). Both yttrium silicate and aluminosilicate were also not found in all samples even 5% silica was added in the precursors [16]. The details can be seen by the magnified patterns in Fig. 1b–d. By observing the diffraction intensity of (420) in Fig. 1a, the peak intensity revealed no remarkable difference under a laser power range of 15–40 W. However, the crystallinity decreased with increasing laser power as revealed by the peak of (400) in Fig. 1b. This finding indicated that better crystallinity was obtained at low laser powers of 15
Fig. 1. (a) XRD patterns of precursors by laser sintering under various powers as indicated by I (15 W) to IV (40 W). Magnified patterns in the ranges of (b) 28–31◦ , (c) 33–36◦ , and (d) 57–60◦ .
and 20 W. The number of holes induced by laser beam ablation decreased but the size increased with the laser powers, as revealed by the OM images in Fig. 2. The laser produced a heat-affected zone (HAZ) to melt around the target area. The insets in the OM images of Fig. 2a (15 W) and 2d (40 W) show the front and back sides of pellets after laser sintering, respectively. These images revealed some black rings around the laser ablation holes caused by burn out or carbonization of the PVA binder. In addition, the surfaces of all samples exhibited a shiny appearance. Therefore, a high laser power produced more liquids in HAZ during laser sintering and filled the holes as observed by fewer open holes on their surfaces. Notably, the melts or liquids in HAZ may solidify quickly with less crystallinity or even non-crystallinity because of the occurrence of quick temperature drops after the removal of laser beams. Hence, reduced crystallinity was found in the samples of 30 and 40 W laser powers as discussed in Fig. 1b. More detailed microstructures of laser sintered YAG:Ce surface and cross sections by SEM are shown in Fig. 3 and their corresponding insets. The surface around the laser-drilled holes in Fig. 3a of 15 W was smooth, which evidently indicated the formation of a liquid phase from precursors and subsequent solidification to form YAG. As the laser powers increased, a rough surface with some large particles was easily found because of large melt pools. In particular, many temperature quenching-induced small voids at 40 W are shown in Fig. 3d. This quick temperature drop led to a decrease
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Fig. 2. OM images of laser sintered YAG:Ce pellets under various powers: (a) 15, (b) 20, (c) 30, and (d) 40 W Insets in (a) and (d) reveal the corresponding front and back sides of the pellets with a diameter of 10 mm and a scale bar of 400 m, respectively.
Fig. 3. Surface SEM images of laser sintered YAG:Ce under various powers: 15 (a), 20 (b), 30 (c), and 40 W (d). Inset: Corresponding cross section SEM images of laser sintered YAG:Ce under various powers.
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Fig. 5. Schematics of blue light excitation (Iex ), reflection (Iex -Iab ), and yellow emission (Iem ) in YAG:Ce phosphor ceramics prepared by laser sintering (left) and solid state reaction (SSR, right). Note that the length of arrows roughly indicates the light intensity. Fig. 4. Full spectra of residual blue light excitation and yellow emission of YAG:Ce ceramics by various laser sintering powers and solid state reaction (SSR). Upper right inset: Integrated intensity comparison of residual blue light excitation and yellow emission by normalizing to laser sintered sample of 15 W. Lower right inset reveals the measurement setup.
in crystallinity as discussed in Fig. 1. The corresponding insets in Fig. 3 reveal the cross section profiles under various laser powers, showing an increasing reaction depth from 0.28 mm (15 W) to 2.68 mm (40 W). Moreover, the drilled holes appeared deeper from right to left as the scanning direction in the same sample (insets of Fig. 3), which were explained as less rigid YAG melts by previous laser scanning compared with initial solid precursors. Meanwhile, the isolated voids noted in the deeper locations may be caused by incomplete liquid flows to drilled holes during laser sintering. Given that a high laser power can cause more vaporization or heavy ablation of powders, the density, surface smoothness, and crystallinity of laser sintered YAG all decreased as revealed by the severe depressions in the insets of Fig. 3. Fig. 4 shows the full spectra of residual blue light excitation and yellow emission of YAG:Ce ceramics, which were prepared by various laser sintering powers (15–40 W) and SSR. The residual blue light excitation is defined in the lower right inset of Fig. 4 as the reflection (Iex -Iab ) of excitation (Iex ) after absorption by the YAG:Ce ceramics (Iab ). The yellow emission bands were broad and asymmetric from 480 nm to 700 nm with a peak centered at around 532 nm, which resulted from the electronic transitions (2 Dj → 7 F7/2 ,5/2 ) inducing radiation of Ce3+ ions in the YAG host, as revealed by the energy levels in the figure [4–9,14]. The samples of 15 and 20 W exhibited a low intensity of reflection (Iex -Iab ) or residual blue light excitation but a high emission intensity (Iem ), indicating a high absorption (Iab ) of blue light excitation and a good conversion efficiency of blue light into yellow (Iem /Iex ). By contrast, the higher intensity of residual blue light excitation and the lower intensity of yellow emission in the sample of 40 W, or the lower intensity of both residual blue light and yellow emission in the sample of 30 W indicated a relatively lower conversion efficiency compared with the other samples (15 and 20 W). As shown by the integrated intensity of both residual blue light excitation (430–470 nm) and yellow emission (480–700 nm) in the upper right inset of Fig. 4, the yellow emission intensity (open circles) of samples of 15 and 20 W was higher than those of samples of 30 and 40 W by approximately 25–30%. In terms of internal conversion efficiency (Iem /Iab ), the emission intensity of samples of 15 (100%) and 20 W (106%) was quite similar (±2.5% error), but the sample of 15 W was optimal because it exhibited a higher intensity of residual blue light excitation (100%,
lower Iab ) than that of 20 W (85%, higher Iab ). A rough surface with more periodic holes in the sample of 15 W, caused by laser ablation and drilling as shown by Fig. 2a, resulted in a thorough penetration of blue light into ceramic materials to excite YAG:Ce sufficiently. Therefore, the locally smooth surface of 40 W sample with no obvious open holes may cause the strong backscattering of blue light excitation (139%) and low yellow emission output (75%). To compare the luminous intensity of YAG:Ce ceramics prepared by traditional SSR at 1500 ◦ C with this study, the sample by laser sintering (15 W) exhibited a much higher emission intensity and a lower residual blue light excitation (Fig. 4 and its inset), similarly indicating a higher conversion efficiency as discussed above. The enhancement of yellow emission intensity was around 54% (46% → 100%) upon comparing laser sintering (15 W) with SSR. Both SSR and laser sintering could facilitate precursor interdiffusion and reaction by heating to high temperatures. However, the laser beams could easily approach the melting points of the reactants (Y2 O3 : 2425 ◦ C and Al2 O3 : 2072 ◦ C), which induced liquid phases for better transportation and distribution of doping activators (Ce) compared with SSR. [12] Notably, the SSR sample possessed a much smoother surface and dense microstructure with very few open pores. Therefore, YAG:Ce by SSR and laser sintering may lead to significantly different amounts of reflection of blue light excitation (SSR: 147%) and yellow emission as discussed previously and demonstrated by Fig. 5. A rough surface of YAG:Ce phosphor ceramics with periodic holes by laser sintering and drilling (left of Fig. 5) resulted in less reflection (dashed lines, Iex -Iab ) or more absorption of excitation (Iab ), leading to higher PL (dotted lines, Iem ). 4. Conclusions YAG:Ce phosphor ceramics were prepared by CO2 laser sintering. XRD results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. Moreover, the samples by 15–20 W laser power exhibited better crystallinity by comparing their (400) peak intensities, indicating that a quick solidification from YAG melts by a high laser power (40 W) could induce defects or incomplete crystallization. The reaction depth caused by laser beams largely increased from 0.28 mm (15 W) to 2.68 mm (40 W), and appeared deeper in the scanning sequence, which resulted from the low rigidity of YAG melts compared with initial solid precursors. PL measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation of 453 nm presented a more intense emission peak centered at 532 nm than those at other laser powers. Moreover, this sample was enhanced by 54% as compared with YAG:Ce by SSR. The
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laser beams could approach the melting point of the reactants and induce liquid phases for better transportation and distribution of doping activators (Ce) compared with SSR. Additionally, a rough surface of YAG:Ce phosphor ceramics with periodic holes by laser sintering and drilling could avoid light backscattering and result in enhanced absorption of excitation and emissivity of PL. Acknowledgments The author (YTN) would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under the grants 104-2221-E-150-006 and 105-2218-E-006-017. References [1] L. Chen, C.C. Lin, C.W. Yeh, R.S. Liu, Light converting inorganic phosphors for white light-emitting diodes, Materials 3 (2010) 2172–2195. [2] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [3] E.F. Schubert, J.K. Kim, Solid-state light sources getting smart, Science 308 (2005) 1274–1278. [4] H.M. Lee, C.C. Cheng, C.Y. Huang, The synthesis and optical property of solid-state-prepared YAG:Ce phosphor by a spray-drying method, Mater. Res. Bull. 44 (2009) 1081–1085. [5] C.H. Lu, H.C. Hong, R. Jagannathan, Sol–gel synthesis and photoluminescent properties of cerium-ion doped yttrium aluminum garnet powders, J. Mater. Chem. 12 (2002) 2525–2530.
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[6] W.T. Lin, Y.C. Wu, One-pot synthesis of submicrometer-sized Ce:YAG spherical particles by solvothermal process using alcohol solvents, J. Am. Ceram. Soc. 98 (2015) 2754–2759. [7] H. Yang, L. Yuan, G. Zhu, A. Yu, H. Xu, Luminescent properties of YAG:Ce3+ phosphor powders prepared by hydrothermal-homogeneous precipitation method, Mater. Lett. 63 (2009) 2271–2273. [8] A. Bhaskar, H.Y. Chang, T.H. Chang, S.Y. Cheng, Microwave annealing of YAG:Ce nanophosphors, Mater. Lett. 78 (2012) 124–126. [9] 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. [10] X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen, Q. Su, Highly thermally stable single-component white-emitting silicate glass for organic-resin-free white-light-emitting diodes, ACS Appl. Mater. Interfaces 6 (2014) 2709–2717. [11] K. Haladejová, A. Prnová, R. Klement, W.-H. Tuan, S.-J. Shih, D. Galusek, Aluminate glass based phosphors for LED applications, J. Eur. Ceram. Soc. 36 (2016) 2969–2973. [12] T. Kasai, Y. Ozaki, H. Noda, K. Kawasaki, K. Tanemoto, Laser-sintered barium titanate, J. Am. Ceram. Soc. 72 (1989) 1716–1718. [13] B. Qian, Z. Shen, Laser sintering of ceramics, J. Asian Ceram. Soc. 1 (2013) 315–321. [14] Y.T. Nien, C.W. Ma, I.G. Chen, Effect of laser drilling on the microstructure and luminescence of YAG:Ce,Si phosphor ceramics, Int. J. Appl. Ceram. Technol. 12 (2015) 745–749. [15] K. Subramanian, N. Vail, J. Barlow, H. Marcus, Selective laser sintering of alumina with polymer binders, Rapid Prototyping J. 1 (1995) 24–35. [16] Y.T. Nien, T.H. Lu, V.R. Bandi, I.G. Chen, Microstructure and photoluminescence characterizations of Y3 Al5 O12 :Ce phosphor ceramics sintered with silica, J. Am. Ceram. Soc. 95 (2012) 1378–1382.
Please cite this article in press as: Y.-C. Chen, Y.-T. Nien, Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032
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Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Feature article
Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics Yi-Chen Chen, Yung-Tang Nien ∗ Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 63201, Taiwan
a r t i c l e
i n f o
Article history: Received 12 June 2016 Received in revised form 22 July 2016 Accepted 25 July 2016 Available online xxx Keyword: Laser Sintering Phosphor Yttrium aluminum garnet Light emitting diode
a b s t r a c t Cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor ceramics were fabricated by CO2 laser sintering under various powers. X-ray diffraction results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. The reaction profile caused by laser ablation increased from 0.28 mm (15 W) to 2.68 mm (40 W), and it appeared deeper in the scanning sequence because of the low rigidity of YAG melts compared with solid precursors. Photoluminescence (PL) measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation presented more intense yellow emission (enhancement of 54%) as compared with YAG:Ce by solid-state reaction (SSR, 1500 ◦ C). The laser beams were believed to distribute activators (Ce) more homogeneously compared with SSR. Additionally, a rougher surface of YAG:Ce phosphor ceramics with periodic holes caused by laser ablation resulted in more excitation and emissivity of PL. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Phosphor materials have been studied over 100 years and applied in various areas and products, such as fluorescent lamps, x-ray images, cathode ray tubes, electroluminescence and plasma display panels, by converting incident electrons, photons, or energetic particles to visible photons. Most importantly, phosphors have gained much attention because of the rapid development of white light emitting diodes (WLEDs) [1]. The first WLED product was introduced by Nichia Chemical Industries Ltd. in 1996 with an efficiency of only 5 lumens/W, using InGaN blue LED chip to excite cerium-doped yttrium aluminum garnet (Y3 Al5 O12 :Ce or YAG:Ce) phosphor powders with a yellow emission peak at 550–580 nm [2]. By tuning the output ratios of yellow/blue lights or adding red phosphors, a color temperature range of 2800–6500 K white light can be obtained. Therefore, WLED solid state lighting technology can speed up the replacement of traditional fluorescence products and is a potential solution to global warming [3]. The preparation methods of YAG phosphor powders for WLED applications can be roughly classified to solid state reaction (SSR) [4], sol-gel [5], solvothermal one-pot synthesis [6], hydrothermal [7], and microwave sintering methods [8]. Among these methods, SSR has the advantages of simple process, mass production, and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y.-T. Nien).
low cost, but it leads to intermediate phase formations, e.g., yttrium aluminum perovskite (YAlO3 or YAP) and yttrium aluminum monoclinic (YAl4 O9 or YAM). Song et al. pointed out that heat treatments with fluxes at a high temperature of 1500 ◦ C can eliminate impurity phases and obtain better luminescence performance of YAG:Ce [9]. However, three stages in SSR, namely, slow heating, high temperature holding, and furnace cooling, generally require much time to obtain a homogeneous distribution of dopants, reduce internal pores, and avoid large volume contractions of ceramic compacts. Therefore, a reliable furnace to maintain temperature uniformity and avoid an uneven temperature distribution is required for SSR to prepare phosphors. Moreover, the phosphor powders interspersed in the epoxy resin may distribute randomly to cause serious scattering and non-uniform emission. Phosphor ceramics have been proposed in resin-free WLED packages to overcome the problems of working temperature, current, and package density [10,11]. Laser sintering was recently proposed as a new type of heat treatment and forming technology. Laser sintering does not only create complex and accurate 3D parts, but also achieves compact density by high temperature reactions in a rapid time, i.e., one order less compared with traditional SSR processes [12]. Moreover, the extremely short contact time of the laser beam and materials significantly defines the reaction areas to avoid side effects to surrounding microstructures [13,14]. In the production of ceramic parts by laser sintering, an organic polymer material is mostly used as a binder to link ceramic powers. It is then burned out by ignition to densify ceramic pellets or parts [15]. To the best of our
http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Y.-C. Chen, Y.-T. Nien, Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032
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knowledge, the fabrication of YAG phosphor ceramics using energetic laser beams has not been reported. This study focused on the fabrication of YAG:Ce by blending precursor powders with an organic polymer to form pellets through uniaxial pressing, followed by sintering by a CO2 laser under various powers. The phase purity, surface morphology, microstructure, and photoluminescence (PL) of laser sintered YAG:Ce will be discussed and compared with that by a traditional SSR at 1500 ◦ C. 2. Experimental procedure The precursor materials of alumina (Al2 O3 , 99.9%, 3–5 m, Showa Chemicals), yttria (Y2 O3 , 99.9%, 10 m, Alfa Aesar), and Ce(NO3 )3 ·6H2 O (99.9%, Aencore Chemical) were used without further purification. According to the PL intensity results of various contents of Ce in YAG, an optimal content of Ce (0.055) was chosen for all samples, either in laser sintering or SSR. The above starting materials were ball-milled in 40 ml of ethanol (99.5%, Echo Chemical) for 20 h in a stoichiometric ratio of 2.89:5:0.055 (Y:Al:Ce). The ball mills used in this study were pure alumina with a diameter of 2 mm and maintained at a weight ratio of 1.2 (powder/ball). After ball milling, the above raw materials were dried in an oven at 90 ◦ C for 6 h and transferred to an alumina mortar for grinding (10 min first) with 5 wt% silica powders (99.9%, 80 nm, Alfa Aesar) and 1 wt% polyvinyl alcohol (PVA, 98%–99%, Alfa Aesar) in ethanol for 1 and 0.5 h, respectively. Silica functions as a sintering aid to enhance the diffusion of precursors [16]. After mixing, the above raw materials were dried again at 90 ◦ C for 6 h and ground for 10 min. The above mixture was shaped into pellets in a diameter of 10 mm by uniaxial pressing of 35 kg/cm2 for 10 min. After preparing the above pellets, a CO2 laser (3AXLE DC-6040N) was utilized to sinter areas of 6 mm × 6 mm or 14 mm × 14 mm in a scanning speed of 0.2 mm/s, a spot interval of 0.5 mm, and an irradiation time of 0.1 s for each spot under various powers of 15, 20, 30, and 40 W. The diameter of the laser spot was approximately 0.5 mm. For comparison, the same pellet without PVA but with 1 wt% NaF addition (99%, Alfa Aesar) was sintered twice in a traditional furnace at 1500 ◦ C for 12 and 24 h in air. The crystal structure, microstructure, and surface morphology of laser sintered samples were investigated using an x-ray diffractometer (XRD, Bruker D8A25), a scanning electron microscope (SEM, Hitachi SU3500), and an optical microscope (OM, Olympus DSX500). For PL measurements, YAG:Ce pellets were excited by a peak wavelength of 453 nm and detected via a fiber probe with six surrounding fibers (excitation) and a central fiber (emission collection) to an optical spectrometer (Ocean Optics USB2000). 3. Results and discussion The crystal structure and surface morphology of yttria-alumina precursor powders under various laser powers were studied using the XRD patterns and OM images shown in Figs. 1 and 2, respectively. As shown in Fig. 1a, all samples in the power range of 15–40 W crystallized well as a single phase of cubic YAG (JCPDS 791891) without any obvious residual or impurity phase, e.g., cubic yttria (JCPDS 01-0831), hexagonal alumina (JCPDS 01-1296), monoclinic YAM (JCPDS 34-0368), or orthorhombic YAP (JCPDS 70-1677). Both yttrium silicate and aluminosilicate were also not found in all samples even 5% silica was added in the precursors [16]. The details can be seen by the magnified patterns in Fig. 1b–d. By observing the diffraction intensity of (420) in Fig. 1a, the peak intensity revealed no remarkable difference under a laser power range of 15–40 W. However, the crystallinity decreased with increasing laser power as revealed by the peak of (400) in Fig. 1b. This finding indicated that better crystallinity was obtained at low laser powers of 15
Fig. 1. (a) XRD patterns of precursors by laser sintering under various powers as indicated by I (15 W) to IV (40 W). Magnified patterns in the ranges of (b) 28–31◦ , (c) 33–36◦ , and (d) 57–60◦ .
and 20 W. The number of holes induced by laser beam ablation decreased but the size increased with the laser powers, as revealed by the OM images in Fig. 2. The laser produced a heat-affected zone (HAZ) to melt around the target area. The insets in the OM images of Fig. 2a (15 W) and 2d (40 W) show the front and back sides of pellets after laser sintering, respectively. These images revealed some black rings around the laser ablation holes caused by burn out or carbonization of the PVA binder. In addition, the surfaces of all samples exhibited a shiny appearance. Therefore, a high laser power produced more liquids in HAZ during laser sintering and filled the holes as observed by fewer open holes on their surfaces. Notably, the melts or liquids in HAZ may solidify quickly with less crystallinity or even non-crystallinity because of the occurrence of quick temperature drops after the removal of laser beams. Hence, reduced crystallinity was found in the samples of 30 and 40 W laser powers as discussed in Fig. 1b. More detailed microstructures of laser sintered YAG:Ce surface and cross sections by SEM are shown in Fig. 3 and their corresponding insets. The surface around the laser-drilled holes in Fig. 3a of 15 W was smooth, which evidently indicated the formation of a liquid phase from precursors and subsequent solidification to form YAG. As the laser powers increased, a rough surface with some large particles was easily found because of large melt pools. In particular, many temperature quenching-induced small voids at 40 W are shown in Fig. 3d. This quick temperature drop led to a decrease
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Fig. 2. OM images of laser sintered YAG:Ce pellets under various powers: (a) 15, (b) 20, (c) 30, and (d) 40 W Insets in (a) and (d) reveal the corresponding front and back sides of the pellets with a diameter of 10 mm and a scale bar of 400 m, respectively.
Fig. 3. Surface SEM images of laser sintered YAG:Ce under various powers: 15 (a), 20 (b), 30 (c), and 40 W (d). Inset: Corresponding cross section SEM images of laser sintered YAG:Ce under various powers.
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Fig. 5. Schematics of blue light excitation (Iex ), reflection (Iex -Iab ), and yellow emission (Iem ) in YAG:Ce phosphor ceramics prepared by laser sintering (left) and solid state reaction (SSR, right). Note that the length of arrows roughly indicates the light intensity. Fig. 4. Full spectra of residual blue light excitation and yellow emission of YAG:Ce ceramics by various laser sintering powers and solid state reaction (SSR). Upper right inset: Integrated intensity comparison of residual blue light excitation and yellow emission by normalizing to laser sintered sample of 15 W. Lower right inset reveals the measurement setup.
in crystallinity as discussed in Fig. 1. The corresponding insets in Fig. 3 reveal the cross section profiles under various laser powers, showing an increasing reaction depth from 0.28 mm (15 W) to 2.68 mm (40 W). Moreover, the drilled holes appeared deeper from right to left as the scanning direction in the same sample (insets of Fig. 3), which were explained as less rigid YAG melts by previous laser scanning compared with initial solid precursors. Meanwhile, the isolated voids noted in the deeper locations may be caused by incomplete liquid flows to drilled holes during laser sintering. Given that a high laser power can cause more vaporization or heavy ablation of powders, the density, surface smoothness, and crystallinity of laser sintered YAG all decreased as revealed by the severe depressions in the insets of Fig. 3. Fig. 4 shows the full spectra of residual blue light excitation and yellow emission of YAG:Ce ceramics, which were prepared by various laser sintering powers (15–40 W) and SSR. The residual blue light excitation is defined in the lower right inset of Fig. 4 as the reflection (Iex -Iab ) of excitation (Iex ) after absorption by the YAG:Ce ceramics (Iab ). The yellow emission bands were broad and asymmetric from 480 nm to 700 nm with a peak centered at around 532 nm, which resulted from the electronic transitions (2 Dj → 7 F7/2 ,5/2 ) inducing radiation of Ce3+ ions in the YAG host, as revealed by the energy levels in the figure [4–9,14]. The samples of 15 and 20 W exhibited a low intensity of reflection (Iex -Iab ) or residual blue light excitation but a high emission intensity (Iem ), indicating a high absorption (Iab ) of blue light excitation and a good conversion efficiency of blue light into yellow (Iem /Iex ). By contrast, the higher intensity of residual blue light excitation and the lower intensity of yellow emission in the sample of 40 W, or the lower intensity of both residual blue light and yellow emission in the sample of 30 W indicated a relatively lower conversion efficiency compared with the other samples (15 and 20 W). As shown by the integrated intensity of both residual blue light excitation (430–470 nm) and yellow emission (480–700 nm) in the upper right inset of Fig. 4, the yellow emission intensity (open circles) of samples of 15 and 20 W was higher than those of samples of 30 and 40 W by approximately 25–30%. In terms of internal conversion efficiency (Iem /Iab ), the emission intensity of samples of 15 (100%) and 20 W (106%) was quite similar (±2.5% error), but the sample of 15 W was optimal because it exhibited a higher intensity of residual blue light excitation (100%,
lower Iab ) than that of 20 W (85%, higher Iab ). A rough surface with more periodic holes in the sample of 15 W, caused by laser ablation and drilling as shown by Fig. 2a, resulted in a thorough penetration of blue light into ceramic materials to excite YAG:Ce sufficiently. Therefore, the locally smooth surface of 40 W sample with no obvious open holes may cause the strong backscattering of blue light excitation (139%) and low yellow emission output (75%). To compare the luminous intensity of YAG:Ce ceramics prepared by traditional SSR at 1500 ◦ C with this study, the sample by laser sintering (15 W) exhibited a much higher emission intensity and a lower residual blue light excitation (Fig. 4 and its inset), similarly indicating a higher conversion efficiency as discussed above. The enhancement of yellow emission intensity was around 54% (46% → 100%) upon comparing laser sintering (15 W) with SSR. Both SSR and laser sintering could facilitate precursor interdiffusion and reaction by heating to high temperatures. However, the laser beams could easily approach the melting points of the reactants (Y2 O3 : 2425 ◦ C and Al2 O3 : 2072 ◦ C), which induced liquid phases for better transportation and distribution of doping activators (Ce) compared with SSR. [12] Notably, the SSR sample possessed a much smoother surface and dense microstructure with very few open pores. Therefore, YAG:Ce by SSR and laser sintering may lead to significantly different amounts of reflection of blue light excitation (SSR: 147%) and yellow emission as discussed previously and demonstrated by Fig. 5. A rough surface of YAG:Ce phosphor ceramics with periodic holes by laser sintering and drilling (left of Fig. 5) resulted in less reflection (dashed lines, Iex -Iab ) or more absorption of excitation (Iab ), leading to higher PL (dotted lines, Iem ). 4. Conclusions YAG:Ce phosphor ceramics were prepared by CO2 laser sintering. XRD results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. Moreover, the samples by 15–20 W laser power exhibited better crystallinity by comparing their (400) peak intensities, indicating that a quick solidification from YAG melts by a high laser power (40 W) could induce defects or incomplete crystallization. The reaction depth caused by laser beams largely increased from 0.28 mm (15 W) to 2.68 mm (40 W), and appeared deeper in the scanning sequence, which resulted from the low rigidity of YAG melts compared with initial solid precursors. PL measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation of 453 nm presented a more intense emission peak centered at 532 nm than those at other laser powers. Moreover, this sample was enhanced by 54% as compared with YAG:Ce by SSR. The
Please cite this article in press as: Y.-C. Chen, Y.-T. Nien, Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032
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laser beams could approach the melting point of the reactants and induce liquid phases for better transportation and distribution of doping activators (Ce) compared with SSR. Additionally, a rough surface of YAG:Ce phosphor ceramics with periodic holes by laser sintering and drilling could avoid light backscattering and result in enhanced absorption of excitation and emissivity of PL. Acknowledgments The author (YTN) would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under the grants 104-2221-E-150-006 and 105-2218-E-006-017. References [1] L. Chen, C.C. Lin, C.W. Yeh, R.S. Liu, Light converting inorganic phosphors for white light-emitting diodes, Materials 3 (2010) 2172–2195. [2] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [3] E.F. Schubert, J.K. Kim, Solid-state light sources getting smart, Science 308 (2005) 1274–1278. [4] H.M. Lee, C.C. Cheng, C.Y. Huang, The synthesis and optical property of solid-state-prepared YAG:Ce phosphor by a spray-drying method, Mater. Res. Bull. 44 (2009) 1081–1085. [5] C.H. Lu, H.C. Hong, R. Jagannathan, Sol–gel synthesis and photoluminescent properties of cerium-ion doped yttrium aluminum garnet powders, J. Mater. Chem. 12 (2002) 2525–2530.
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Please cite this article in press as: Y.-C. Chen, Y.-T. Nien, Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.032