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Abstract
High power and high brightness laser lighting puts forward new requirements for phosphor converters such as high luminous efficiency, high thermal conductivity and high saturation threshold due to the severe thermal effect. The structure design of phosphor converters is proposed as what we believe to be a novel strategy for less heat production and more heat conduction. In this work, the rod-shaped YAG:Ce phosphor ceramics (PCs) and disc-shaped YAG:Ce PCs as control group were fabricated by the gel casting and vacuum sintering, to comparatively study the luminescence performance for LD lighting, on the premise that the total number of transverse Ce3+ ions and the volume of samples from two comparison groups were same. All rod YAG:Ce PCs with low Ce3+ concentration exhibited the high luminous efficiency and better thermal stability than YAG:Ce discs with high Ce3+ concentration. Under the laser power density of 47.8 W/mm2, the luminous saturation was never observed in all rod-shaped YAG:Ce PCs. The high luminous efficacy of 245∼274 lm/W, CRI of 56.3∼59.5 and CCT of 4509∼4478 K were achieved. More importantly, due to the extremely low Ce3+ doping concentration (0.01 at%), rod-shaped ceramics based LDs devices showed the excellent thermal performance and their surface temperatures were even below 30.5 °C surprisingly under the laser power density of 20.3 W·mm-2 (2 W). These results indicate that the rod shape of phosphor converter is a promising structure engineering for high power laser lighting.
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1. Introduction
Laser-driven lighting has many advantages of high-power, high-brightness and high-collimation, making it very promising in emerging applications such as car headlights, high-definition projection displays and drone searchlights [1–6]. At present, white laser diodes (LDs) lighting mostly uses a combination of blue LD chips and yellow-green light emitted by phosphor conversion materials [7–9]. However, high-power excitation density of the LDs would be accompanied by plenty of heat accumulation in the body of phosphor converters [10–12]. Therefore, the thermal properties of converters are the key technical parameters for laser lighting [13,14].
As a phosphor converter, Y3Al5O12:Ce (YAG:Ce) possesses the superior properties including high absorption of blue light, broadband emission and good thermal stability [15–20], and has been commercialized in the white light-emitting diode (LED) lighting by packaging it with organic silicone. But a low thermal conductivity (0.1∼0.4 W·m-1·K-1) of traditional organic packaging materials severely limits its application in high-power and high-brightness LDs lighting [21]. To overcome the above problem, many inorganic phosphor converters such as phosphor in glass (PIG) [22–24] and phosphor ceramics (PCs) [25,26] have been specially studied. PIG has a very simple preparation process of embedding phosphor powders in glass, and is easy to regulate its correlated color temperature (CCT) and color rendering index (CRI) just by choosing different phosphors [27]. However, PIG is also unable to withstand the high power density (≥10W·mm-2) excitation due to the limited thermal conductivity of glass (1.0∼3.0 W·m-1·K-1) [28]. PCs possess many advantages of low cost, flexible ion doping and high thermal conductivity (9.0∼14.0 W·m-1·K-1), making it a more suitable choice for LDs lighting.
In addition to high thermal conductivity of the phosphor converter materials, an excellent optical structure design is essential in the LDs lighting system. Introducing concave and convex microstructures on the surface of ceramics and constructing the secondary phase could realize the high luminous efficiency (LE) by improving light extraction efficiency [29–33]. Zheng et al. tunned the surface roughness (322.86 nm) of YAG:Ce PCs to get high LE of 178.5 lm·W-1 under LDs excitation [34]. Cozzan’s group designed “porous Al2O3-YAG:Ce” multiphase ceramics by spark plasma sintering [29]. The ceramics based laser illumination device achieved an ultra-high LE of 305 lm·W-1 by the combined effect of the stomatal and secondary phases. Meanwhile, the geometric design was also effective method to improve lighting performance. Our group designed a clad-core structure ceramic including Al2O3-YAG:Ce core and YAG clad for LDs lighting [35]. The high LE of 210 lm·W-1 and the high saturation threshold (ST) of 60 W·mm-2 were obtained simultaneously in white LDs devices. However, due to the convenience of the dry pressing process, most studies on the shape of phosphor ceramics have been designed to be disc-shaped. Dubey et al. reported a uniform laser lighting source using a cylindrical acrylic rod coated with YAG:Ce phosphor [36]. The rod structure was designed to allow most laser beams to pass through the entire length of the rod through total internal reflection (TIR) and achieved uniform irradiation in 360° full-angle. Unfortunately, the maximal incident power of the blue LD was only 1.0 W due to the poor thermal conductivity of acrylic. Considering rod-shaped ceramics have the high thermal conductivity of 9∼14 W·m-1·K-1, if applying ceramic materials instead of acrylic, it is expected that both LE and heat dissipation properties would be improved significantly. Nevertheless, it could be found that there are very few relevant reports about rod-shaped YAG:Ce PCs in recent years.
In this study, the comparative study of rod-shaped YAG:Ce PCs and disc-shaped YAG:Ce PCs was carried out on the premise that the number of transverse Ce ions was same. All samples of YAG:Ce were fabricated through gelcasting and vacuum sintering. Meanwhile, in the process of comparison, to keep heat dissipation performance of rods and discs as consistent as possible, the volume of the comparison group samples was consistent. The luminous flux (LF), LE, CCT, CRI and ST were also studied in detail by combining PCs with high-power blue LDs. All rod-shaped YAG:Ce PCs did not show luminous saturation when the pumped power density even reached 47.8 W·mm-2. Moreover, all YAG:Ce rods with low Ce3+ concentration exhibited the better LE and the high thermal stability than YAG:Ce discs with high Ce3+ concentration. Notably, the LE of 245∼274 lm·W-1 in YAG:Ce rods was even superior to most of the highly doped YAG:Ce samples reported. Under 20.3 W·mm-2 (2 W) laser irradiation, YAG:Ce rods had the low surface temperature of 30.5 °C, indicating the excellent heat dissipation performance. These results confirm that the YAG:Ce phosphor ceramic with a low Ce3+ doping concentration is a promising candidate for laser lighting.
2. Experimental method
The α-Al2O3 (4N, Alfa Aesar, Ward Hill, America), Y2O3 (4N, Alfa Aesar, Ward Hill, USA), and CeO2 (4N, Shandong Xiya, Shandong, China) were selected as the raw materials. 0.1wt.% MgO (4N, Alfa Aesar, Ward Hill, America) and 0.5wt.% tetraethyl orthosilicate (TEOS, 4N, Alfa Aesar, Ward Hill, USA) were added as sintering additives. Ammonium citrate tribasic (TAC was used as a dispersant for stabilizing the dispersion of pulp particles. The ideal pH pulp could be adjusted by adding tetramethylammonium hydroxide (TMAH, C4H13NO, Aladdin, Shanghai, China). To form three-dimensional gel, N, N'-methyl enebisacrylamide (MBAM, C7H10N2O2, Aladdin, China) was chosen as the gelling system. The ammonium persulfate (APS, H8N2O8S2, Aladdin, China) was selected as the initiator.
Detailed gelcasting preparation procedures of ceramic rods could be found in the previous studies [37,38]. The raw powders were mixed together based on the stoichiometric ratio of (Y1-xCex)3Al5O12 (where x = 0.0001, 0.000125, 0.0002, 0.002, 0.005, and 0.01) and were ball-milled for 15 hours at a speed of 180 r/min. Then, dispersant PEI and ethanol were added, and the dried mixture powder was sieved through a 200-mesh sieve. Figure 1 shows the gelcasting and process of Ce: YAG transparent ceramic flowchart. Firstly, ceramic raw materials with a solid content of 52 vol.% were weighed and added to the premixed solution in three increments. At a rotation speed of 220 r/min, a uniform slurry was obtained after ball milling for 24 h. The ball-milled slurry with ammonium persulfate was vacuum degassed for 3 min and then immediately injected into a mold. A suction force was applied through the vacuum pump to prepare rod-shaped ceramics. After gelation at room temperature, the mold was removed, and the sample was placed in a drying oven at 50 °C for degassing. After drying, the green bodies were desorbed at 500 °C for 10 h to completely remove the organic matter. Ceramics were carried out by vacuum sintering at 1780 °C for 8 hours, followed by annealing in air atmosphere at 1450 °C for 10 hours. For the sake of simplicity, we denoted these samples as R0.01, R0.0125, R0.02, D0.20, D0.50 and D1.00, respectively. The detailed composition design in this study is listed in Table 1.
Fig. 1. Schematic diagram of fabricating rod-shaped YAG:Ce PCs and disc-shaped YAG:Ce PCs by gelcasting.
Table 1. Comparison group samples details of ceramic rods (R) and ceramic discs (D)
Phase compositions of all samples were characterized using an X-ray diffraction (XRD) device with a copper target X-ray tube (D 8 Advance, Bruker, Karlsruhe, Germany) with the scanning range of 10-80° and a dwelling time of 0.02 s per step. Microstructures of PCs were obtained using a scanning electron microscope (SEM, JSM-6510, JEOL, Tokyo, Japan. Photoluminescence (PL) and photoluminescence excitation (PLE) of PCs were measured using a fluorescence spectrophotometer (OmniFluo 900, Beijing, China) equipped with a scintillating xenon lamp. White LD devices based on the reflected mode were fabricated by combining the as-prepared ceramics with 460 nm laser source. The electroluminescence (EL) spectra, CRI, CCT, and CIE coordinates of the as-fabricated white LD devices were recorded using an integrating sphere spectroradiometer system (HASS-2000, Hangzhou, China).
3. Results and discussion
Figure 2 shows the XRD patterns of the YAG:Ce PCs with different Ce3+ doping concentrations and schematic crystal structure sketch of YAG:Ce PCs (Fig. 2(b)). All rod-shaped ceramics were crushed into ceramic powders by a mortar, and then tested by the X-ray diffraction device. All diffraction peaks were well corresponded to the standard peaks of YAG garnet structure (JCPDS#88-2048), and there was no impurity phase observed, demonstrating the YAG phase had been well formed by the combination of gelcasting and vacuum sintering. As shown in Fig. 2(a), all diffraction peaks did not exhibit the obvious shift with increasing Ce3+ concentration. The result indicated that the adopted Ce3+ doping concentration (less than 1 at.%) did not cause a significant lattice variation. Generally, the radius of Ce3+ ions (0.114 nm, CN = 8) was very similar to that of Y3+ ions (r = 0.1019 nm, CN = 8) [39]. Therefore, Ce3+ ions would enter the Y3+ lattice sites based on the similar ionic radius [26,40].
Fig. 2. (a) XRD patterns of the YAG:Ce PCs with different Ce3+ doping concentrations; (b) schematic crystal structure sketch of YAG:Ce PCs.
At present, the transmittance of ceramic rods was usually evaluated by observing the rods immersed in refractive index matching liquid with an optical microscope. Obviously, this method could not show the transmittance of ceramic rods. To more accurately characterize the transmittance of ceramic rods, the YAG:Ce ceramic discs with the same doping concentration were fabricated by the same preparation conditions of YAG:Ce ceramic rods (D0.01, D0.0125, D0.02). And their in-line transmittance was used to evaluate the optical quality of YAG:Ce ceramic rods. Figure 3 gave the appearances and in-line transmittance spectra of the fabricated YAG:Ce ceramic rods and YAG:Ce ceramic discs. All disc-shape samples were very transparent, and the words behind them could be clearly resolved. However, all rod-shaped ceramic samples showed the translucent appearance, and the words behind them were not very clear. The main reason for this was that the surfaces of the ceramic rods were not mirror-polished. Due to the “diffuse reflection effect”, the rough surface would cause strong scattering of incident light, thereby reducing the transmittance of ceramic rods. As shown in Fig. 3(b), two broad absorption bands appeared at 340 and 460 nm due to the 4f→5d1 and 4f→5d2 transitions of Ce3+ ion [41,42]. In general, Ce3+ doping had little effect on the transmittance of PCs, and their transmittances at 800 nm fluctuated around 80%. The sample with 0.01 at.% Ce3+ concentration had the highest transmittance (T = 82.1% @800 nm). With increasing the Ce3+ concentration, the T at 800 nm of samples decreased first and then increased. The reason for T difference between samples was that CeO2 raw material could serve as sintering additives during sintering, which affected the densification behavior of PCs [43]. Meanwhile, Ce3+ incorporation would also change the properties of melting point and refractive index, affecting its theoretical transmittance and optimal sintering temperature [44]. Thus, the combined effect of “sintering additives” and “incorporation” influenced the final T of YAG:Ce PCs.
Fig. 3. (a) Appearances of the fabricated YAG:Ce ceramic rods and YAG:Ce ceramic discs; (b) In-line transmittance of ceramic discs with different Ce3+ doping concentrations.
Figure 4 gives the SEM images of YAG:Ce ceramic rods and YAG:Ce ceramic discs. From the polished surfaces of YAG:Ce, it could be seen that all the samples showed a dense microstructure with homogeneous grains and clean grain. Besides, no residual visible pores or secondary phases were detected, which corroborated the high T of YAG:Ce in Fig. 3. As the Ce3+ concentration increased from 0.2 at.% to 1 at.%, the average grain size of YAG:Ce ceramic discs decreased monotonically from 11.15 µm to 6.03 µm, which indicated that Ce addition could affect the sintering behavior of ceramics such as grain growth kinetics. For the high Ce3+ concentration samples, a portion of Ce element could locate at grain boundaries that inhibit the grain boundary migration (solute drag effect), resulting in the appearance of very small grains in Fig. 4(f). Notably, compared with ceramic discs, the average grain size of ceramic rods with different Ce3+ concentration (0.01∼0.02 at.%) was very similar and fluctuated around 3.5 µm, which was much smaller than that of ceramic discs. We believed that the reason of this phenomenon might be related to the geometric curvature of ceramic rods, and the relevant study would be reported in the future work.
Fig. 4. SEM micrographs of the surfaces of (a) R0.01, (b) R0.0125, (c) R0.02, (d) D0.20, (e) D0.50 and (f) D1.00.
To investigate the spectral characteristics of YAG:Ce PCs, the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of D0.20 are displayed in Fig. 5. Monitored at 530 nm, a strong excitation band situated at 460 nm and a weak excitation band situated at 340 nm could be observed, which originated from the 4f7/2-5d1 transition and the 4f5/2-5d2 transition of Ce3+ ion, respectively. The result indicated that YAG:Ce PCs could be excited by ultraviolet light and blue light. Moreover, from the PL spectra it could be seen that a broad yellow light band situated at 530 nm could be measured by exciting D0.20 under blue light at 460 nm. This emission was corresponded to the spin-allowed 5d1-4f transition of Ce3+ ion. The similar PLE and PL spectra could be found in the YAG:Ce PCs with different Ce3+ doping concentrations and previous research [45–47].
Figure 6(a) exhibits our laser lighting test system mainly included an integrating sphere, a fiber coupled blue LD and YAG:Ce rods. A transmission-type white light LDs device was constructed by placing YAG:Ce rods above the terminal of optical fiber. The selected pump power and excitation wavelength of the laser source were 1.3 W and 460 nm, respectively. For analyzing the characteristics of rod-shaped YAG:Ce PCs based white LDs, the comparative study of rod-shaped YAG:Ce PCs and disc-shaped YAG:Ce PCs were carried out on the premise that the number of transverse Ce ions were the same. At the same time, the volume of the comparison group samples was consistent to keep heat dissipation performance of rods and discs as consistent as possible. From the chromaticity parameters shown in Fig. 6(b), it could be clearly seen that the color hue was changed from yellow-green light (0.3826, 0.4981) to yellow-orange (0.4597, 0.4804) with increasing Ce3+ concentration. The reason why the color coordinates of rod-shaped ceramics shifted toward yellow-green with increasing Ce3+ concentration was thought to be the result of a joint effect of an increase in red light and a significant reduction in blue light. Meanwhile, compared with samples of ceramic discs, the chromaticity parameters of R0.01 (0.3826, 0.4981) was closer to that of the ideal white light (0.333,0.333). The electroluminescent spectra (EL) of YAG:Ce based white LDs were shown in Fig. 6(c). It was obvious that the sharp blue emission (460 nm) from disc-shaped ceramics based LDs was weaker than that from rod-shaped ceramics based LDs, which might be due to the high Ce3+ concentration disc-shaped ceramics. Notably, the emission intensity of rod-shaped YAG:Ce PCs with low Ce3+ concentration was superior to that of disc-shaped YAG:Ce PCs with high Ce3+ concentration in the comparison groups. Moreover, as shown in the inset of Fig. 6(c), the real sense emitting color of the white LDs was mainly yellow.
Fig. 6. (a) Schematic diagram of laser lighting test system, (b) Chromaticity parameters, (c) Appearances of the YAG:Ce TCs based white LDs and the EL spectra.
For exploring the luminescence performance of rod-shaped YAG:Ce PCs under high power density excitation, the variations of the LF, LE and ST with increasing power density is shown in Fig. 7. Under the premise of the same transverse Ce3+ ion concentration, LF and LE of rod-shaped ceramics in each comparison group were superior to that of disc-shaped ceramics. The highest LF and LE of rod-shaped ceramics with the low Ce3+ doping concentration were 1084∼1250 lm and 245∼274 lm·W-1, respectively. And the ST of all rod-shaped ceramics could reach 47.80 W·mm-2. However, the luminous saturation phenomenon could be observed in D0.50 and D1.00 samples due to the high Ce3+ doping concentration, which was consistent with the previous results that the behavior of luminous decline began to occur when the Ce content ≥ 0.2 at.%. Low Ce3+ doping concentration had positive effects on both intensity quenching and thermal quenching, which could explain the high ST of rod-shaped ceramics [48,49]. Moreover, in this transmissive configuration, the rod-shaped ceramics in each comparison group could improve the extraction rate of incident light through the total internal reflection (TIR) effect, explaining the improved LF and LE [36,50]. Therefore, a high brightness light source with the LF of 1084∼1250 lm and LE of 245∼274 lm·W-1 was got here in the transmissive configuration by geometric designing with R0.01, R0.0125 and R0.02.
Fig. 7. (a) LF, (b) LE for disc-shaped ceramics and rod-shaped ceramics as a function of the incident laser power density.
Figure 8 shows schematic diagram of geometric design effects on luminescence performance in YAG:Ce PCs. It could be understood from Fig. 8(a, a’) that when PCs based LDs adopted the transmission mode, rod structure would make most blue laser beams to pass through the entire length of the rod through TIR, which might allow Ce3+ ions absorb more blue light and improve the extraction rate of incident light. This could explain the enhanced LF and LE in rod-shaped ceramics. Our group found that the critical angle $\Phi$ of TIR in YAG:Ce PCs were about 33° [51]. Meanwhile, under the premise of the same transverse Ce3+ ion concentration and the same volume in comparison group samples, low Ce3+ doping concentration of rod-shaped ceramics could have positive effects on both intensity quenching and thermal quenching [35,48,49], which would make Ce3+ ion emit more yellow light and produce less heat than high Ce3+ doping concentration of disc-shaped ceramics, thereby resulting in high LE and high ST of PCs. Accordingly, high-quality phosphor converters for high-power LDs could be obtained by the rod structure design.
Fig. 8. Schematic diagram of geometric design effects on luminescence performance in YAG:Ce PCs: the TIR effects for disc (a) and rod (a’), the LE and ST of disc-shaped ceramics (b) and rod-shaped ceramics (b’).The critical angle $\Phi$ = 33°, sin$\Phi$c = nYAG/nair.
Table 2 compares the fabricated rod-shaped YAG:Ce PCs with several other representative YAG:Ce ceramics reported in literature from the aspect of Ce3+ doping concentration, LF, LE and ST. As could be seen, The rod-shaped YAG:Ce PCs with low Ce3+ doping concentration had a high LF of 1084∼1250 lm, LE of 245∼274 lm·W-1 and ST of > 47 W·mm-2 among the various ceramics. Therefore, rod-shaped YAG:Ce PCs with low Ce3+ doping concentration possessed excellent luminescent performance, greatly satisfying the technical requirements for laser illumination.
Table 2. Performance comparison of YAG:Ce materials in previous work
Figure 9 shows the variations of the CRI and CCT with the increasing power density. Compared to high Ce3+ concentration doping disc-shaped ceramics, rod-shaped ceramics with low Ce3+ concentration showed higher CRI of 56.3∼59.5 and CCT of 4509∼4478 K. Moreover, it could be seen that the CRI and CCT of rod-shaped ceramics varied in quite small range, indicating excellent color stability when the power density increased to 47.8 W·mm-2. However, the disc-shaped ceramics appeared a noticeable shift of CRI and CCT for D0.50 and D1.00, which was attributed to the high Ce3+ doping concentration. Generally, high Ce3+ doping concentration is helpful in getting high CRI, but tends to cause worse thermal and intensity quenching. In this work, the high CRI of 59.5 and moderate CCT of 4550 K were obtained in rod-shaped ceramics with 0.01 at.% Ce3+ concentration, indicating that the ceramic rod with a low doping concentration was a promising candidate for laser lighting.
Fig. 9. (a) CCT, (b) CRI for disc-shaped ceramics and rod-shaped ceramics as a function of the incident laser power density.
As known, the thermal quenching effect of phosphor converters as a disadvantage of high-power lighting could greatly deteriorate the luminescence [58–60]. Figure 10 shows the surface temperature of disc-shaped ceramics and rod-shaped ceramics under a laser power of 20.3 W·mm-2 (2 W) by thermal imaging analysis. The reason for choosing the low power density of 20.3 W·mm-2 (2 W) was to select the samples without thermal quenching phenomenon to compare the influence of different shapes and concentrations on the thermal stability of the samples. It could be seen that the surface temperatures of the rod-shaped ceramics gradually increased from 30.5 °C to 40.0 °C with the increase of Ce3+ concentration, and were all below 40.0 °C. However, under the same power density excitation, the surface temperatures of disc-shaped ceramics gradually increased from 47.4 °C to 49.6 °C with the increase of Ce3+ concentration, and were all close to 50 °C. Above results indicated that the rod-shaped ceramics had good thermal management performance and the heat induced luminescence quenching would not occur even under the power density of 47.8 W·mm-2. We believed that the low Ce3+ doping concentration and the unique rod shape made rod-shaped ceramics exhibit better thermal properties than disc-shaped ceramics.
Fig. 10. Infrared thermal images and surface temperatures of (a) R0.01, (b) R0.0125, (c) R0.02, (d) D0.20, (e) D0.50, (f) D1.00 based based LDs.
4. Conclusion
In summary, the comparative study of rod-shaped YAG:Ce PCs and disc-shaped YAG:Ce PCs were carried out on the premise that the number of transverse Ce3+ ions and the volume of the comparison group samples were same. The study showed all rod-shaped YAG:Ce PCs had no luminous saturation even under the laser power density of 47.8 W·mm-2. Moreover, the rod-shaped YAG:Ce PCs obtained the high LE of 245∼274 lm·W-1 due to the improved extraction rate of incident light. More importantly, due to the extremely low Ce3+ doping concentration, the rod-shaped ceramics based LDs devices showed the excellent thermal performance. The surface temperatures of rod-shaped ceramics were all below 40.0 °C under a laser power density of 20.3 W·mm-2 (2 W). By placing R0.01 ceramics above the terminal of blue LD optical fiber, a high brightness transmission-type white LDs device with the LF over 1000 lm, LE of 247 lm·W-1, CCT of 4550 K, chromaticity parameters of (0.3826, 0.4981) and the operating temperature of 30.5 °C was obtained. These results indicate that the rod phosphor materials with low active ions concentration is a promising structure engineering strategy for high power laser lighting.
Funding
National Key Research and Development Program of China (2021YFB3501700); National Natural Science Foundation of China (52202135, 52302141, 61975070); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); International S&T Cooperation Program of Jiangsu Province (BZ2023007); Key Research and Development Project of Jiangsu Province (BE2021040, BE2023050); Natural Science Foundation of Jiangsu Province (BK20221226); Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX22_1257); Natural Science Foundation of the Jiangsu Higher Education Institutes of China (22KJB140011); Special Project for Technology Innovation of Xuzhou City (KC23380, KC22497, KC21379); Open Project of State Key Laboratory of Crystal Materials (KF2205).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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