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Enhancement of the luminous efficiency of a ceramic phosphor plate by post-annealing for high-power LED applications
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Enhancement of the luminous efficiency of a ceramic phosphor plate by post-annealing for high-power LED applications In-Jae Leea,b, Young-Gyun Kima,c, Vijayakumar Elayappana, Jimin Kima, Kihyun Kima, Hyun Sung Noha, Haigun Leea,∗ a
Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea LG Innotek Components R&D Center, Ansan-si, Gyeonggi-do, 15588, Republic of Korea c DEMO Technology Division, National Fusion Research Institute Daejeon, 31433, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: High-power LED Automotive headlamp Post-annealing PiG plates Luminous efficiency
Currently, phosphor-converted white light-emitting diodes offer low energy consumption, good environmental stability, and a long lifetime. Hence, they are widely utilized in high-power light-emitting diode (LED) applications such as those in the automotive headlamp industries. However, obtaining high luminous efficiency of such diodes is challenging because of their internal structural properties such as micropores. Herein, we developed phosphor-in-glass (PiG) plates by mixing a blue LED chip and yellow phosphor to create high-power white LEDs (w-LEDs). In addition, the influence of post-annealing on the prepared PiG plates at different temperatures (350°C-550 °C) was investigated. Post-annealing, a treatment that facilitates the mobility of the ceramic matrix encapsulating the phosphor powder, decreases an LED's porosity, thereby enhancing its overall luminous efficiency. Results show that PiG plates post-annealed at 450 °C exhibit superior optical performance and effective color properties than PiG plates that were non-annealed or post-annealed at 350 °C, 400 °C, 500 °C, and 550 °C. Therefore, post-annealed PiG plates are more suitable potential materials for application in the highpower LED industry.
1. Introduction In recent decades, light-emitting diodes (LEDs) have received considerable attention as light sources among researchers owing to their specific properties such as high efficiency, long-endurance, environmental friendliness, and low power consumption. Additionally, LED applications benefit both electronic devices in decorative lighting and the high-power automotive headlamp industry [1–5]. Typically, LEDs utilize group III–V nitride semiconductors, such as GaN, that are structurally stable at high temperatures and can be applied to highpower devices [6]. Further, white LEDs (w-LEDs) are currently perceived as the next generation of solid-state lighting sources and an alternative to conventional incandescent and fluorescent lamps with wide applications in display lighting, medical services, as indicators and automobile headlamps, and in conventional indoor and outdoor lighting [7–9]. These widely applicable w-LEDs are produced with uniform blending of yellow Y3Al5O12: Ce3+ (YAG) phosphor on a GaN-based blue LED chip including organic binders, namely, polymer materials such as organic or silicon resins [10,11]. This method is advantageous ∗
in terms of processing and cost; however, it has some disadvantages related to high-power lighting. When the LED chip and silicon resin are in contact, some light generated from the chip is lost as heat (for conventional high-power w-LED with power exceeding 3 W and junction temperature exceeding 150 °C), deteriorating the phosphor characteristics; this affects the luminous efficacy and long-term reliability of the resin based LED chips [12–15]. To overcome these issues of polymer binders, a packaging method that produces phosphor in the form of a plate and mounts this plate directly onto the chip can be applied. In this regard, recently, inorganic materials, such as transparent phosphor ceramics and glass ceramics, have demonstrated excellent moisture and thermal stability and have become practicable alternatives to organic polymer binders [16–20]. Existing literature presents two types of phosphor plates: 1) phosphor-in-glass (PiG) plates, produced by mixing phosphor powder and glass powder; and 2) polycrystalline plates, produced by mixing phosphor powder with a small amount of organic resin. Of these, PiG has been widely used in applications that require high-power lighting, such as industrial and automobile lamps, owing to its excellent market competitiveness, high thermal stability, and good luminous efficacy
Corresponding author. E-mail address: [email protected] (H. Lee).
https://doi.org/10.1016/j.ceramint.2019.10.013 Received 19 July 2019; Received in revised form 2 October 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: In-Jae Lee, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.013
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[21]. However, the phosphor in a PiG plate emits light only at angles lower than the critical angle. At angles greater than the critical angle, light is trapped in the plate because of total reflection, resulting in lower luminous efficacy. Moreover, the light emitted from the phosphor on the blue chip is trapped in the micropores of the PiG plate, thus lowering luminous efficacy [22,23]. To resolve these issues, eliminating the microspores present inside the phosphor glass matrices is desirable. To the best of our knowledge, removal of micropores present inside PiG plates via post-annealing has not been investigated thus far. Herein, PiG plates for automotive LED applications were prepared by mixing yellow phosphors (Lu3Al5O12: Ce3+ (LuAG)) and a small amount of red phosphors (CaAlSiN3: Eu2+ (CaSN); to adjust color temperature) with Pb-free glass powder (B2O3–ZnO–SiO2). The prepared PiG plates were subjected to post-annealing treatments at various temperatures ranging from 350 °C to 550 °C. Subsequently, the properties of post-annealed PiG plates were systematically investigated using physical characterization techniques and their luminescence performance was compared to that of a non-annealed PiG plate.
Fig. 1. Photographs of the phosphor plate fabricated for this study.
2. Experimental 2.1. Preparation of PiG plate To fabricate the PiG plate, Lu3Al5O12: Ce3+ (LuAG; Intematix) phosphor was used with a wavelength interval of 540–550 nm and particle size of 15–20 μm. A small amount of red phosphor (SiAlON, Denka) powder was added to adjust the color temperature. Then, glass powder based on B2O3–ZnO–SiO2 (Central Glass Co.) was used as a transparent ceramic powder to cover and protect the phosphor powder. In addition, the glass powder had a particle size of 5–7 μm and a glass transition temperature (Tg) of 410°C-490 °C. In the preparation stage, phosphor powders and glass powders were weighed at a ratio of 20:80 wt% and then uniformly mixed for 24 h using a ball mill (ZrO2 balls with a diameter of 3 mm) [24,25]. Next, the mixed powder was placed in a metal mold (Ø 25 mm; Hantech, Korea), uniaxially pressed at a pressure of 60–80 MPa using a hydraulic press, and maintained at room temperature for 5 min until a compact green body was formed. The green compact was placed in an electric furnace, heat treated at 650 °C in an air atmosphere at a heating rate of 10 °C/min, and
Fig. 2. XRD patterns and corresponding scanning electron microscopy (SEM) images of (a) only glass; (b) red phosphor; (c) yellow phosphor; and (d) a PiG plate. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. SEM images of PiG plates: (a) non-post-annealed; (b) post-annealed at 350 °C; (c) 400 °C; (d) 450 °C; (e) 500 °C; and (f) 550 °C. 2
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pure LuAG phase and no impurity phases. In addition, the amorphosity of glass materials influenced the lowered intensity peaks of the phosphor materials, indicating that the phosphor was well blended in a uniform manner onto the glass matrices. The impact of various post-annealing temperatures (350 °C, 400 °C, 450 °C, 500 °C, and 550 °C) on the structures of the prepared PiG plates was systematically investigated via SEM, as shown in Fig. 3. In the case of the non-annealed sintered ceramic plate, the shape and size of the trapped pores formed during heat treatment depended on the sintering temperature, which eventually determined the viscosity of the glass powder. The measured diameter of the largest pore in this phosphor plate was approximately > 10 μm. Observation of the phosphor particles’ morphology after post-annealing showed that their glass matrices also exhibited the characteristics of an amorphous structure. The internal pore size tends to decrease while the annealing temperature increases; however, this tendency of increasing pore size is observed at temperatures above 500 °C. Larger pores were formed by the coalescence of small pores, which are mobile owing to the lowered viscosity of the glass matrix in the post-annealed phosphor plate. Additionally, the overall porosity on the surface of the PiG plate with and without post-annealing was analyzed via optical image analysis to macroscopically and statistically investigate the effects of postannealing temperatures on the porosity of the PiG plate. Table 1 summarizes the porosity analysis results of the phosphor plate samples. Fig. 4 shows the optical images of phosphor plate samples at various post-annealing temperatures and a reference sample. As expected, porosity decreases while the post-annealing temperature increases; however, an increasing tendency of porosity is observed above 500 °C. This tendency corresponds with the graphical analysis of the SEM images (Fig. 3), revealing that the increase in temperature lowers the viscosity of ceramic materials. Fig. 5 depicts a graph showing pore size and porosity as a function of the post-annealing temperature. The transmittance was determined using the Lambert-Beer equation. The difference in optical properties caused by the presence of pores inside the plate was also investigated [27–29].
Table 1 Porosity of the post-annealed ceramic phosphor plate. Temperature (°C)
Porosity (%)
Non-post-annealed 350 400 450 500 550
5.630 3.062 1.674 0.355 3.974 4.850
subsequently maintained at 650 °C for 30 min to achieve the desired viscosity of the glass powder. The fabricated sintered compact was surface-polished to a thickness of 150–200 μm. It was sliced into squares of 1 × 1 mm2 (see Fig. 1) and directly attached to the LED chip. Next, the sintered compact was subjected to post-annealing at various temperatures ranging from 350 °C, 400 °C, 450 °C, 500 °C–550 °C. For the post-annealing treatment of the PiG plates, the sintered compact was placed in an electric furnace, followed by heat treatment at various temperatures between 350 °C and 550 °C in air atmosphere at a heating rate of 10 °C/min and maintained therein for 30 min. 2.2. Measurements and characterization The crystalline phase of each material used herein was identified via powder X-ray diffraction (XRD; Rigaku, MAX-2500V) with the CuKα target aligned to 10° ≤ 2θ ≤ 90°. The microstructure of PiG plates with and without post-annealing was examined via scanning electron microscopy (SEM; FEI, INSPECT S50). To improve the luminous efficacy, the PiG plate was post-annealed between 350°C and 550 °C, and the optical properties of the fabricated PiG plate were measured. An integrating sphere (LMS-400) was used for measuring the plate's optical properties including luminous efficacy, color temperature, and color coordinates. The photoluminescence (PL) spectrum of the reference sample and post-annealed PiG plates was measured using a spectrophotometer (J&C Tech. Co., LMS-400) equipped with a 200 W Xe lamp as the excitation source.
T (λ ) = (1 − Rs ) exp (−Csca•t )
3. Results and discussion
Csca = Np × Gp × Qsca = Fig. 2 shows XRD analysis results of the glass powder, phosphor powder, and the fabricated PiG plate. The obtained XRD results of the PiG plate are indexed as a cubic garnet structure of LuAG (JCPDS card No. 1.53–0272) [26]. The fabricated PiG was highly crystalline with a
3Vp 2d
Qsca
(1)
(2)
Here, Rs is the reflectivity, t is the sample thickness (160 μm in this study), Csca is the scattering coefficient measured via Mie scattering theory using porosity (Vp), pore size (d), and scattering efficiency (Qsca)
Fig. 4. Optical image analysis results of the post-annealed ceramic phosphor plate: (a) non-post-annealed; (b) 350 °C; (c) 400 °C; (d) 450 °C; (e) 500 °C; and (f) 550 °C. 3
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annealing proceeds, the viscosity increases after densification, causing pore growth. Moreover, the increasing pore size may trigger refraction or scattering of light, thus affecting transmittance and optical properties. Furthermore, transmittance changes affect the luminous efficiency of the LED package [31]. Fig. 6 shows the International Commission on Illumination (CIE) chromaticity diagram of post-annealed phosphor plates obtained at various post-annealing temperatures. Table 2 shows the range of colorcorrelated temperature (CCT) changes, with the post-annealing temperature ranging from 350 °C to 550 °C. Compared to the reference sample, the post-annealed plates exhibit a similar chromaticity diagram. However, chromaticity coordinate shifts were observed with different post-annealing temperatures applied on phosphor plate samples. This result implies that luminous properties of blue light, such as transmittance and reflection, were changed owing to the rearrangement of the internal structures of post-annealed ceramic phosphor plates. Table 2 summarizes the results from the luminous flux measurement and chromaticity diagram analysis of phosphor plate samples. The luminous flux of phosphor plates highly depends on their porosity values, and the phosphor plate post-annealed at 450 °C was the most efficient sample, achieving the highest white light intensity. In addition, the CIE color coordinates of emission in each sample provide a precise comparison of the luminous properties of phosphor samples. The results of this comparison clearly demonstrate the similarity of the CIE color coordinates of each sample with CCT values ranging between 5,000 K and 5,700 K. Fig. 7 shows the separated spectra of ceramic phosphor plates obtained via PL measurement. Fig. 7(a) and (b) present blue light excitation and yellow light emission, respectively. In Fig. 7(a), the PL intensity of the blue excitation decreased owing to post-annealing as it reduces the phosphor plates’ porosity and enhances their glass transmittance, followed by increased blue light absorption. Furthermore, the increased blue light absorption of the plates enables the emission of yellow light with higher PL intensity, as shown in Fig. 7(b). A key reason for this phenomenon is that the pores trapped inside the phosphor plates act as scattering centers, causing loss of light by restricting the transmittance of yellow light. Fig. 7(b) presents a graph showing the PL characteristics of the yellow spectrum as a function of post-annealing temperature. The spectrum of 570-nm excitation, which is the central wavelength of yellow light, exhibits a decreasing trend after an initial increase because of the increase in post-annealing temperature (Fig. 7(a)). These results indicate that the post-annealed phosphor plate can readily absorb blue light and subsequently emit yellow light with higher intensity compared to its non-post-annealed counterpart. 4. Conclusion In summary, we investigated the luminous properties of phosphor plates fabricated via post-annealing at various temperatures ranging from 350 °C to 550 °C. The highest luminous efficacy was observed at 450 °C, and low luminous efficacy was observed at 500 °C or at higher temperatures. The difference in luminous efficacy caused by different post-annealing temperatures primarily depended on internal pore size, which impacted the minimization of the loss of light. Overall, PiG plates post-annealed at 450 °C exhibited superior optical performance and effective color properties compared to other samples, providing great potential for applications in the high-power LED industry. Further research is necessary for investigating the effect of post-annealing on phosphor powder's thermal aging.
Fig. 5. Properties of the post-annealed ceramic phosphor plate: (a) porosity vs. temperature; (b) pore size vs. temperature; and (c) transmittance vs. mean of porosity.
Declaration of competing interest
[30]. The porosity and pore size initially decreased and subsequently increased. However, the number and size of micropores reduced at 450 °C ranged between 4 and 5 μm, as shown in Fig. 4(d). Potentially, as
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. 4
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Fig. 6. CIE 1931 chromaticity diagram of the post-annealed ceramic phosphor plate.
Acknowledgments
Table 2 Luminous flux and chromaticity diagram of the post-annealed ceramic phosphor plate. Parameters Luminous flux [lm] Chromaticity diagram (CIE 1971) CCT (K)
Cx Cy
Ref.
350 °C
400 °C
450 °C
500 °C
550 °C
1019.1 0.3350 0.3382
1029.2 0.3347 0.3371
1060.9 0.3358 0.3392
1085.2 0.3412 0.3486
1067.5 0.3376 0.3421
1066.6 0.3381 0.3433
5,383
5,396
5,349
5,142
5,276
5,257
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Fig. 7. Photoluminescence (PL) properties of the post-annealed ceramic phosphor plate: (a) depicts PL properties during blue light excitation; and (b) depicts PL properties during yellow light emission. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhancement of the luminous efficiency of a ceramic phosphor plate by post-annealing for high-power LED applications In-Jae Leea,b, Young-Gyun Kima,c, Vijayakumar Elayappana, Jimin Kima, Kihyun Kima, Hyun Sung Noha, Haigun Leea,∗ a
Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea LG Innotek Components R&D Center, Ansan-si, Gyeonggi-do, 15588, Republic of Korea c DEMO Technology Division, National Fusion Research Institute Daejeon, 31433, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: High-power LED Automotive headlamp Post-annealing PiG plates Luminous efficiency
Currently, phosphor-converted white light-emitting diodes offer low energy consumption, good environmental stability, and a long lifetime. Hence, they are widely utilized in high-power light-emitting diode (LED) applications such as those in the automotive headlamp industries. However, obtaining high luminous efficiency of such diodes is challenging because of their internal structural properties such as micropores. Herein, we developed phosphor-in-glass (PiG) plates by mixing a blue LED chip and yellow phosphor to create high-power white LEDs (w-LEDs). In addition, the influence of post-annealing on the prepared PiG plates at different temperatures (350°C-550 °C) was investigated. Post-annealing, a treatment that facilitates the mobility of the ceramic matrix encapsulating the phosphor powder, decreases an LED's porosity, thereby enhancing its overall luminous efficiency. Results show that PiG plates post-annealed at 450 °C exhibit superior optical performance and effective color properties than PiG plates that were non-annealed or post-annealed at 350 °C, 400 °C, 500 °C, and 550 °C. Therefore, post-annealed PiG plates are more suitable potential materials for application in the highpower LED industry.
1. Introduction In recent decades, light-emitting diodes (LEDs) have received considerable attention as light sources among researchers owing to their specific properties such as high efficiency, long-endurance, environmental friendliness, and low power consumption. Additionally, LED applications benefit both electronic devices in decorative lighting and the high-power automotive headlamp industry [1–5]. Typically, LEDs utilize group III–V nitride semiconductors, such as GaN, that are structurally stable at high temperatures and can be applied to highpower devices [6]. Further, white LEDs (w-LEDs) are currently perceived as the next generation of solid-state lighting sources and an alternative to conventional incandescent and fluorescent lamps with wide applications in display lighting, medical services, as indicators and automobile headlamps, and in conventional indoor and outdoor lighting [7–9]. These widely applicable w-LEDs are produced with uniform blending of yellow Y3Al5O12: Ce3+ (YAG) phosphor on a GaN-based blue LED chip including organic binders, namely, polymer materials such as organic or silicon resins [10,11]. This method is advantageous ∗
in terms of processing and cost; however, it has some disadvantages related to high-power lighting. When the LED chip and silicon resin are in contact, some light generated from the chip is lost as heat (for conventional high-power w-LED with power exceeding 3 W and junction temperature exceeding 150 °C), deteriorating the phosphor characteristics; this affects the luminous efficacy and long-term reliability of the resin based LED chips [12–15]. To overcome these issues of polymer binders, a packaging method that produces phosphor in the form of a plate and mounts this plate directly onto the chip can be applied. In this regard, recently, inorganic materials, such as transparent phosphor ceramics and glass ceramics, have demonstrated excellent moisture and thermal stability and have become practicable alternatives to organic polymer binders [16–20]. Existing literature presents two types of phosphor plates: 1) phosphor-in-glass (PiG) plates, produced by mixing phosphor powder and glass powder; and 2) polycrystalline plates, produced by mixing phosphor powder with a small amount of organic resin. Of these, PiG has been widely used in applications that require high-power lighting, such as industrial and automobile lamps, owing to its excellent market competitiveness, high thermal stability, and good luminous efficacy
Corresponding author. E-mail address: [email protected] (H. Lee).
https://doi.org/10.1016/j.ceramint.2019.10.013 Received 19 July 2019; Received in revised form 2 October 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: In-Jae Lee, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.013
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[21]. However, the phosphor in a PiG plate emits light only at angles lower than the critical angle. At angles greater than the critical angle, light is trapped in the plate because of total reflection, resulting in lower luminous efficacy. Moreover, the light emitted from the phosphor on the blue chip is trapped in the micropores of the PiG plate, thus lowering luminous efficacy [22,23]. To resolve these issues, eliminating the microspores present inside the phosphor glass matrices is desirable. To the best of our knowledge, removal of micropores present inside PiG plates via post-annealing has not been investigated thus far. Herein, PiG plates for automotive LED applications were prepared by mixing yellow phosphors (Lu3Al5O12: Ce3+ (LuAG)) and a small amount of red phosphors (CaAlSiN3: Eu2+ (CaSN); to adjust color temperature) with Pb-free glass powder (B2O3–ZnO–SiO2). The prepared PiG plates were subjected to post-annealing treatments at various temperatures ranging from 350 °C to 550 °C. Subsequently, the properties of post-annealed PiG plates were systematically investigated using physical characterization techniques and their luminescence performance was compared to that of a non-annealed PiG plate.
Fig. 1. Photographs of the phosphor plate fabricated for this study.
2. Experimental 2.1. Preparation of PiG plate To fabricate the PiG plate, Lu3Al5O12: Ce3+ (LuAG; Intematix) phosphor was used with a wavelength interval of 540–550 nm and particle size of 15–20 μm. A small amount of red phosphor (SiAlON, Denka) powder was added to adjust the color temperature. Then, glass powder based on B2O3–ZnO–SiO2 (Central Glass Co.) was used as a transparent ceramic powder to cover and protect the phosphor powder. In addition, the glass powder had a particle size of 5–7 μm and a glass transition temperature (Tg) of 410°C-490 °C. In the preparation stage, phosphor powders and glass powders were weighed at a ratio of 20:80 wt% and then uniformly mixed for 24 h using a ball mill (ZrO2 balls with a diameter of 3 mm) [24,25]. Next, the mixed powder was placed in a metal mold (Ø 25 mm; Hantech, Korea), uniaxially pressed at a pressure of 60–80 MPa using a hydraulic press, and maintained at room temperature for 5 min until a compact green body was formed. The green compact was placed in an electric furnace, heat treated at 650 °C in an air atmosphere at a heating rate of 10 °C/min, and
Fig. 2. XRD patterns and corresponding scanning electron microscopy (SEM) images of (a) only glass; (b) red phosphor; (c) yellow phosphor; and (d) a PiG plate. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. SEM images of PiG plates: (a) non-post-annealed; (b) post-annealed at 350 °C; (c) 400 °C; (d) 450 °C; (e) 500 °C; and (f) 550 °C. 2
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pure LuAG phase and no impurity phases. In addition, the amorphosity of glass materials influenced the lowered intensity peaks of the phosphor materials, indicating that the phosphor was well blended in a uniform manner onto the glass matrices. The impact of various post-annealing temperatures (350 °C, 400 °C, 450 °C, 500 °C, and 550 °C) on the structures of the prepared PiG plates was systematically investigated via SEM, as shown in Fig. 3. In the case of the non-annealed sintered ceramic plate, the shape and size of the trapped pores formed during heat treatment depended on the sintering temperature, which eventually determined the viscosity of the glass powder. The measured diameter of the largest pore in this phosphor plate was approximately > 10 μm. Observation of the phosphor particles’ morphology after post-annealing showed that their glass matrices also exhibited the characteristics of an amorphous structure. The internal pore size tends to decrease while the annealing temperature increases; however, this tendency of increasing pore size is observed at temperatures above 500 °C. Larger pores were formed by the coalescence of small pores, which are mobile owing to the lowered viscosity of the glass matrix in the post-annealed phosphor plate. Additionally, the overall porosity on the surface of the PiG plate with and without post-annealing was analyzed via optical image analysis to macroscopically and statistically investigate the effects of postannealing temperatures on the porosity of the PiG plate. Table 1 summarizes the porosity analysis results of the phosphor plate samples. Fig. 4 shows the optical images of phosphor plate samples at various post-annealing temperatures and a reference sample. As expected, porosity decreases while the post-annealing temperature increases; however, an increasing tendency of porosity is observed above 500 °C. This tendency corresponds with the graphical analysis of the SEM images (Fig. 3), revealing that the increase in temperature lowers the viscosity of ceramic materials. Fig. 5 depicts a graph showing pore size and porosity as a function of the post-annealing temperature. The transmittance was determined using the Lambert-Beer equation. The difference in optical properties caused by the presence of pores inside the plate was also investigated [27–29].
Table 1 Porosity of the post-annealed ceramic phosphor plate. Temperature (°C)
Porosity (%)
Non-post-annealed 350 400 450 500 550
5.630 3.062 1.674 0.355 3.974 4.850
subsequently maintained at 650 °C for 30 min to achieve the desired viscosity of the glass powder. The fabricated sintered compact was surface-polished to a thickness of 150–200 μm. It was sliced into squares of 1 × 1 mm2 (see Fig. 1) and directly attached to the LED chip. Next, the sintered compact was subjected to post-annealing at various temperatures ranging from 350 °C, 400 °C, 450 °C, 500 °C–550 °C. For the post-annealing treatment of the PiG plates, the sintered compact was placed in an electric furnace, followed by heat treatment at various temperatures between 350 °C and 550 °C in air atmosphere at a heating rate of 10 °C/min and maintained therein for 30 min. 2.2. Measurements and characterization The crystalline phase of each material used herein was identified via powder X-ray diffraction (XRD; Rigaku, MAX-2500V) with the CuKα target aligned to 10° ≤ 2θ ≤ 90°. The microstructure of PiG plates with and without post-annealing was examined via scanning electron microscopy (SEM; FEI, INSPECT S50). To improve the luminous efficacy, the PiG plate was post-annealed between 350°C and 550 °C, and the optical properties of the fabricated PiG plate were measured. An integrating sphere (LMS-400) was used for measuring the plate's optical properties including luminous efficacy, color temperature, and color coordinates. The photoluminescence (PL) spectrum of the reference sample and post-annealed PiG plates was measured using a spectrophotometer (J&C Tech. Co., LMS-400) equipped with a 200 W Xe lamp as the excitation source.
T (λ ) = (1 − Rs ) exp (−Csca•t )
3. Results and discussion
Csca = Np × Gp × Qsca = Fig. 2 shows XRD analysis results of the glass powder, phosphor powder, and the fabricated PiG plate. The obtained XRD results of the PiG plate are indexed as a cubic garnet structure of LuAG (JCPDS card No. 1.53–0272) [26]. The fabricated PiG was highly crystalline with a
3Vp 2d
Qsca
(1)
(2)
Here, Rs is the reflectivity, t is the sample thickness (160 μm in this study), Csca is the scattering coefficient measured via Mie scattering theory using porosity (Vp), pore size (d), and scattering efficiency (Qsca)
Fig. 4. Optical image analysis results of the post-annealed ceramic phosphor plate: (a) non-post-annealed; (b) 350 °C; (c) 400 °C; (d) 450 °C; (e) 500 °C; and (f) 550 °C. 3
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annealing proceeds, the viscosity increases after densification, causing pore growth. Moreover, the increasing pore size may trigger refraction or scattering of light, thus affecting transmittance and optical properties. Furthermore, transmittance changes affect the luminous efficiency of the LED package [31]. Fig. 6 shows the International Commission on Illumination (CIE) chromaticity diagram of post-annealed phosphor plates obtained at various post-annealing temperatures. Table 2 shows the range of colorcorrelated temperature (CCT) changes, with the post-annealing temperature ranging from 350 °C to 550 °C. Compared to the reference sample, the post-annealed plates exhibit a similar chromaticity diagram. However, chromaticity coordinate shifts were observed with different post-annealing temperatures applied on phosphor plate samples. This result implies that luminous properties of blue light, such as transmittance and reflection, were changed owing to the rearrangement of the internal structures of post-annealed ceramic phosphor plates. Table 2 summarizes the results from the luminous flux measurement and chromaticity diagram analysis of phosphor plate samples. The luminous flux of phosphor plates highly depends on their porosity values, and the phosphor plate post-annealed at 450 °C was the most efficient sample, achieving the highest white light intensity. In addition, the CIE color coordinates of emission in each sample provide a precise comparison of the luminous properties of phosphor samples. The results of this comparison clearly demonstrate the similarity of the CIE color coordinates of each sample with CCT values ranging between 5,000 K and 5,700 K. Fig. 7 shows the separated spectra of ceramic phosphor plates obtained via PL measurement. Fig. 7(a) and (b) present blue light excitation and yellow light emission, respectively. In Fig. 7(a), the PL intensity of the blue excitation decreased owing to post-annealing as it reduces the phosphor plates’ porosity and enhances their glass transmittance, followed by increased blue light absorption. Furthermore, the increased blue light absorption of the plates enables the emission of yellow light with higher PL intensity, as shown in Fig. 7(b). A key reason for this phenomenon is that the pores trapped inside the phosphor plates act as scattering centers, causing loss of light by restricting the transmittance of yellow light. Fig. 7(b) presents a graph showing the PL characteristics of the yellow spectrum as a function of post-annealing temperature. The spectrum of 570-nm excitation, which is the central wavelength of yellow light, exhibits a decreasing trend after an initial increase because of the increase in post-annealing temperature (Fig. 7(a)). These results indicate that the post-annealed phosphor plate can readily absorb blue light and subsequently emit yellow light with higher intensity compared to its non-post-annealed counterpart. 4. Conclusion In summary, we investigated the luminous properties of phosphor plates fabricated via post-annealing at various temperatures ranging from 350 °C to 550 °C. The highest luminous efficacy was observed at 450 °C, and low luminous efficacy was observed at 500 °C or at higher temperatures. The difference in luminous efficacy caused by different post-annealing temperatures primarily depended on internal pore size, which impacted the minimization of the loss of light. Overall, PiG plates post-annealed at 450 °C exhibited superior optical performance and effective color properties compared to other samples, providing great potential for applications in the high-power LED industry. Further research is necessary for investigating the effect of post-annealing on phosphor powder's thermal aging.
Fig. 5. Properties of the post-annealed ceramic phosphor plate: (a) porosity vs. temperature; (b) pore size vs. temperature; and (c) transmittance vs. mean of porosity.
Declaration of competing interest
[30]. The porosity and pore size initially decreased and subsequently increased. However, the number and size of micropores reduced at 450 °C ranged between 4 and 5 μm, as shown in Fig. 4(d). Potentially, as
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. 4
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Fig. 6. CIE 1931 chromaticity diagram of the post-annealed ceramic phosphor plate.
Acknowledgments
Table 2 Luminous flux and chromaticity diagram of the post-annealed ceramic phosphor plate. Parameters Luminous flux [lm] Chromaticity diagram (CIE 1971) CCT (K)
Cx Cy
Ref.
350 °C
400 °C
450 °C
500 °C
550 °C
1019.1 0.3350 0.3382
1029.2 0.3347 0.3371
1060.9 0.3358 0.3392
1085.2 0.3412 0.3486
1067.5 0.3376 0.3421
1066.6 0.3381 0.3433
5,383
5,396
5,349
5,142
5,276
5,257
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Fig. 7. Photoluminescence (PL) properties of the post-annealed ceramic phosphor plate: (a) depicts PL properties during blue light excitation; and (b) depicts PL properties during yellow light emission. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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