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Abstract
Lu3Al5O12:Ce (LuAG:Ce) phosphor ceramics (PCs) with the excellent thermal stability and high saturation threshold are considered as the best green-fluorescent converters for high-power laser diodes (LDs) lighting. In this study, the effects of sintering additives and sintering processes on the transmittance and microstructure of LuAG:Ce PCs were systematically studied, and the luminescence performance of ceramics with different transmittance was compared. LuAG:Ce PCs with the transmittance of 80% (@800 nm, 1.5 mm) were obtained by using 0.1 wt.% MgO and 0.5 wt.% TEOS as sintering additives, combined with optimized vacuum pre-sintering and hot isostatic pressing. Compared to the non-HIP samples, the transmittance had increased by 11%. The microstructure of ceramics indicated that high transparency was closely related to the decrease in intergranular pores. Notably, the luminous efficiency of 253 lm/W and its saturation thresholds of > 46 W/mm2 were obtained simultaneously in green-emitting LDs devices. Moreover, under 3W laser irradiation, highly transparent ceramics had the low surface temperature of 66.4 °C, indicating the good heat dissipation performance. The observed high luminous efficiency and high saturation threshold of LuAG:Ce PCs were attributed to fewer pores and oxygen vacancies. Therefore, this work proves that highly transparent LuAG:Ce PCs are promising green-fluorescent converters for high-power LDs lighting.
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1. Introduction
Nowadays, solid state lighting has attracted much attention in both academy and industry due to its long life, high luminous efficiency (LE), energy conservation and environmental protection [1–4]. Light emitting diodes (LEDs) have been commercialized and used in white light lighting, liquid crystal display (LCD), and plant growing [5–7]. However, LEDs cannot meet the requirements of high-power lighting applications because of the well-known problem of an “efficiency drop” at high input power density (> 3 W/cm2) [8]. Compared to LEDs, LDs have a higher threshold current, which effectively avoids efficiency droop [9]. Additionally, LDs can provide higher peak efficiency at higher power densities, smaller emitting areas, and lower beam divergence. This advantage has led to the widespread application of high-power, high-brightness laser lighting in next-generation lighting products, such as laser projection, automotive headlights, airport lighting, laser TVs, and etc. [10–12].
High-power excitation density of LDs will be accompanied by plenty of heat accumulation during the fluorescent converter, which requires phosphor conversion materials with the excellent thermal conductivity. Traditional organic packaging materials for phosphors have a low thermal conductivity (0.1∼0.4 W·m-1·K-1), which makes it easy to age and carbonize under high-power laser irradiation [13,14]. To meet the heat dissipation requirements for high-power LDs lighting, lots of all-inorganic phosphor converters such as phosphor crystals [15,16], phosphor in glass (PIG) [17,18] and PCs [19,20] have been extensively explored. Compared to crystals and PIG, PCs with significant advantages of low cost, flexible ion doping and high thermal conductivity become a more suitable choice for LDs lighting.
Currently, white LDs lighting mostly uses a superposition of yellow-green light emitted by Ce3+ and the remaining blue light from LDs [21,22]. Y3Al5O12: Ce (YAG: Ce) ceramics are the most studied PCs material, which mainly emit yellow-light under the excitation of the blue LDs [23–25]. However, the lack of red-light and green-light components severely restricts its use in high-quality white lighting. Therefore, it is urgently needed to study the high-performance green-color and red-color emitting PCs under the excitation of high-power LDs [26–28]. As a green phosphor, LuAG:Ce has the same garnet structure as YAG: Ce, but shows better thermal stability and higher LE [20]. Consequently, LuAG:Ce PCs are regarded as one of optimal green-fluorescent converters for high-power LDs lighting [3,29]. Scientists have made lots of efforts to improve the luminescence performance of LuAG:Ce, mainly focusing on chemical doping. Co-doping Mg2+/Si4 + [27], Ba2+/Si4 + [3] and Y3+/Sc4 + [30] pairs in LuAG host can realize the spectral red shift and full width at half maximum (FWHM) increase by regulating crystal field of LuAG:Ce PCs to improve color rendering index (CRI) and decrease correlated color temperature (CCT). However, to further optimize the luminescence performance of ceramics excited by high-power density laser lighting, not only the peak position of the emission spectra and FWHM value need to be considered, but also the LE and saturation threshold (ST) should be paid more attention. In 2023, Zhou et al [31]. reported HA-LuAG:Ce composite PCs with the high transmittance (T) of 78% (@800 nm, 0.8 mm) realized an ultra-high LE of 300.48 lm/W and an enhanced ST of 7.9 W (here LE represents pure optical efficacy). Obviously, the T of PCs plays a significant role in the LE and ST of phosphor converters. Nonetheless, the effect of T on luminescence performance in PCs is not very clear. On one hand, it has been reported that the micro-pores of translucent PCs could enhance the scattering, increase the extraction rate of blue light and achieve the uniformity of luminescence, but the micro-pores also cause uneven heat dissipation and low ST [32,33]. On the other hand, the highly transparent PCs could have good heat dissipation performance and high ST, but they would cause the loss of incident light and non-uniform luminescence (“yellow-ring” effect) due to the lack of scattering centers [33,34]. Therefore, revealing the relationship between T and luminescence performance in the PCs is very necessary. In this work, we systematically studied the influence of sintering process and sintering additives on the T of LuAG:Ce PCs, and investigated the impact of T on the luminescence performance through microstructure and infrared thermal imaging analysis. Moreover, LDs lighting mostly adopts remote packaging technology, which also requires LuAG:Ce PCs have a relatively high T [35,36]. Therefore, it is necessary to prepare highly T LuAG:Ce PCs and investigate the its luminous performance excited by high-power density LDs.
Herein, (Lu1-xCex)3Al5O12 (LuAG:Ce) (x = 0.3%) PCs with high T were fabricated and applied to LDs lighting. Sintering additives and sintering processes were systematically studied to improve the T of LuAG:Ce PCs. Using 0.1% MgO and 0.5% TEOS as sintering additives, combined with optimized vacuum pre-sintering (VPS) and hot isostatic pressing (HIP), the T of LuAG:Ce PCs could reach 80% (@800 nm, 1.5 mm). Particularly, green LDs devices constructed by combining LuAG:Ce PCs with a 460 nm laser source were found to have the high LE of 253lm/W and the high ST of >46W/mm2. Combined with microstructure and infrared thermal images analysis, it was believed that the high LE and high ST of phosphor converters were closely related to fewer pores and oxygen vacancies in LuAG:Ce PCs. These results indicate that highly transparent LuAG:Ce PCs are promising green-fluorescent converters for high-power LDs lighting.
2. Experimental method
The study used commercial Al2O3 (99.99%, Alfa Aesar, Ward Hill, MA, USA), Lu2O3 (99.99%, Alfa Aesar, Ward Hill, USA), CeO2 (99.99%, Alfa Aesar, Ward Hill, MA, USA) as starting materials, and weighed the powders for (Lu1-xCex)3Al5O12 (x = 0.3%) according to the chemical formula. Different concentrations of MgO (99.99%, Alfa Aesar, Ward Hill, USA) and TEOS (99.99%, Alfa Aesar, Ward Hill, MA, USA) were selected as composite sintering additives. Based on our previous research [27,30], we determined the selection and dosage range of additives, in which the additive amount of MgO ranged from 0.05 to 0.15 wt.%, and the additive amount of TEOS ranged from 0.3 to 0.7 wt.%. The detailed combinations of composite sintering additives are listed in Table 1. For the sake of simplicity, the samples with different contents of MgO and TEOS composite additives were labeled as M1T3, M1T5, M1T7, M0.5T5, M1.5T5. The powders were ball-milled with high-purity alumina balls in ethanol for 15 hours, followed by drying in an oven at 55°C for 12 hours. After grinding and sieving through a 100-mesh sieve, the dried powders were calcined at 800°C for 6 hours to remove impurities. Afterwards, the powders were uniaxially pressed into slices at 5 MPa using a stainless steel mold (Ф = 22 mm). The resulting green bodies were cold isostatically pressed (CIPed) at 220 MPa for 200s. The CIPed green bodies were annealed at 700°C in air for 6 h to remove organic residues. Then the samples were sintered at 1720, 1740, 1760, 1780, and 1800°C for 8 h in a vacuum furnace with 10−5 Torr. After VPS, the samples were put into a HIP for sintering at a temperature of 1750°C for 2 h. Finally, the HIP phosphor ceramic pieces were placed in a tube furnace and kept at 1450°C for 10 h to eliminate oxygen vacancies. After two-side mirror polishing, the final prepared LAG:Ce PCs with a thickness of 1.5 mm, a diameters of 16 mm and a roughness of Ra = 3∼4 nm were obtained (Supplement 1, Fig S1). The flowchart of whole preparation process was shown in Fig. 1.
Table 1. Composite sintering additives of the LuAG:Ce PCs.
Phase compositions of all the samples were characterized using an X-ray diffraction (XRD) machine equipped with a copper target X-ray tube (XRD; D2 Phaser Advance, Bruker, Karlsruhe, Germany) with the scanning range of 10-80°and a dwelling time of 0.01s per step. Morphologies of the all samples were observed using a field emission scanning electron microscopy (FESEM, SU-8010, Hitachi, Japan), while energy dispersive X-ray spectroscopy (EDS, Inca X-Max, Oxford Instruments, Oxford, UK) was utilized for elemental mapping of the samples. In-line T of the mirror polished ceramics were measured using a UV-VIS-NIR spectrophotometer (Lambda 950, Perkin Elmer, Waltham, MA, USA), with a scanning range from 800 nm to 200 nm and a scanning speed of 200 nm/min. Furthermore, photoluminescence (PL) and photoluminescence excitation (PLE) were conducted using a fluorescence spectrophotometer (FLS 920, Edinburgh, UK) with a 450 W Xenon lamp as the excitation source. The chromaticity parameters of the samples were measured using an integrating sphere (R98, Everfine, Hangzhou, China) excited by a 460 nm blue LD chip, and the laser spot area was 0.0984 mm2. Surface temperature distribution of LuAG:Ce PCs were recorded using an infrared thermal imaging instrument (Fotric 226 s, Dallas, TX, USA). All the measurements were carried out at room temperature.
3. Results and discussion
The appropriate sintering additives are necessary for preparing high-quality transparent ceramics. The effects of additives on phase structures, microstructures and T were systematically investigated in order to obtain highly transparent LuAG:Ce PCs. It is well-known that phase structures will determine the T of ceramics [37]. Figure 2(a) shows the XRD patterns of LuAG:Ce PCs with different contents of MgO and TEOS composite additives. Among these combinations, all diffraction peaks were corresponded to the standard peaks of LuAG garnet structure (JCPDS#73-1368). The enlarged view of local angle around 34° showed that FWHM of samples changed with different contents of composite additives. It could be seen that M1T5 and M1.5T5 had the lowest FWHM. To obtain more accurate results, the crystal structure refinement was implemented using the GSAS software [38–40]. As shown in Fig. 2(b), there were no intermediate or impurity phases in the refinement results of M1T5 samples. The above results indicate the designed composite sintering additives doesn’t affect the main crystalline phase of the LuAG due to the relatively low additive amount.
Fig. 2. (a) XRD patterns of the LuAG:Ce PCs added with different composite additives and vacuum sintered at 1720 °C, and enlarged view of local angle around 34°; (b) Rietveld refinement of M1T5 samples.
Figure 3 gives the FESEM images of the polished surfaces of LuAG:Ce PCs with different composite additives and vacuum sintered at 1800 °C. As shown in Fig. 3(a∼c), when the TEOS content was fixed as 0.5 wt.%, the average grain size (GS) of LuAG:Ce decreased with the increase of MgO content. Generally speaking, MgO additives could replace Al3+ to form oxygen vacancies, which helped to improve the lattice diffusion coefficient of ceramics and eliminate gas pores. Meanwhile, MgO also precipitated at grain boundaries, hindering grain growth by “pinning effects” [41]. Thus, the GS value of ceramics was about 7.1 µm at MgO contents of 0.05 wt.%, and would become smaller when MgO contents were increased to 0.15 wt.%. However, when the MgO content was fixed as 0.1 wt.%, the GS of LuAG:Ce increased first and then decreased with the increase of TEOS content shown in Fig. 3 (b, d, e). As for TEOS, a small amount of SiO2 and LuAG could react to form a liquid phase at a relatively low temperature, promoting ceramic sintering. But excess liquid phase would cause the grain boundary segregation and second-phase formation, which restrained the sintering processes, so the GS would become smaller when TEOS contents were increased to 0.7 wt.%. Among these combinations, LuAG:Ce PCs with 0.1 wt.% MgO and 0.5 wt.% TEOS (M1T5) could be observed a dense microstructure with clean grain boundaries, and its average GS value was about 6.2 µm. The synergistic effect of TEOS and MgO additives could be verified with the microstructure (Fig. 3) and in-line transmission spectra of samples with different combinations of additives. Therefore, it is believed that MIT5 is considered to be the best combination of additives.
Fig. 3. FESEM images of the polished surfaces of (a)M0.5T5, (b)M1T5, (c)M1.5T5, (d)M1T3 and (e)M1T7.
To further analysis the microstructure of LuAG:Ce PCs with M1T5, EDS elemental scanning was conducted on the surface of ceramics for Ce, Lu, Al, and O elements, as shown in Fig. 4(a∼f). It was evident that all the relevant elements (Ce, Lu, Al and O) were uniformly distributed within the selected region, indicating that there were no intermediate or impurity phases. Additionally, it could be observed from Fig. 4(g) that the molar ratio of Lu3+/Al3+ ions was close to 3:5, further confirming the uniformity of the elemental distribution.
Figure 5 exhibits the in-line transmission spectra and their appearances of LuAG:Ce PCs with different contents of MgO and TEOS composite additives. Firstly, all samples were transparent, and the words behind them could be clearly resolved. This indicated that the composite additives significantly promoted ceramic densification and eliminated pores. Secondly, it was obvious that the T of samples with M1T5 had the highest T of 75.5% (@800 nm, 1.5 mm) from the transmission spectra, which corresponded to its relatively dense microstructure in Fig. 3(b). However, samples with M1T3 had the lowest T of 64% (@800 nm, 1.5 mm), which was mainly attributed to the existence of residual pores caused by low TEOS content in Fig. 3(d).
Fig. 5. In-line transmission spectra and appearances (inset) of the LuAG:Ce PCs added with different composite additives and vacuum sintered at 1800 °C.
Although the optimized composite additives could significantly promote densification and improve the T of ceramics, a few intergranular pores that are difficult to eliminate are still observed in Fig. 6(a). In addition to the applied sintering additives, HIP sintering was also utilized to promote densification of ceramics, since HIP sintering had become a key technique to achieve high optical T. To further improve T of LuAG:Ce PCs, the VPS and the HIP were conducted to eliminate intergranular pores. The density and porosity of ceramic materials are closely related to optical T, which is an important indicator for evaluating the performance of ceramic materials. Figure 6(b) exhibits the density variation curves and their appearances of LuAG:Ce PCs with different VPS temperatures. As the VPS temperature increased from 1720 °C to 1800 °C, the densities and relative densities (RD) of the ceramics risen monotonically. Meanwhile, when the RD risen from 90.9% to 99%, LuAG:Ce PCs also changed from opaque to transparent, and the text behind samples could be clearly seen (inset). In addition, the RD of ceramics before HIP needed to exceed 90% [42–44]. Therefore, the temperature points for VPS were chosen from 1720 °C to 1800 °C. According to Archimedes’ principle, the porosity of sintered samples could be calculated by the measured sample density. Detailed porosity calculations could be found in Supplement 1. The total porosity of the samples after vacuum sintering at 1720, 1740, 1760, 1780, and 1800°C was calculated to be 3.73%, 2.34%, 0.83%, 0.52%, and 0.37%, respectively.
Fig. 6. (a) FESEM images of the polished surface of M1T5 samples; (b) Densities (blue line), relative densities (red line) and appearances (inset) of the M1T5 samples vacuum pre-sintered at different temperatures.
To characterize the quality of the final prepared LAG:Ce ceramic grain growth, Fig S2 and Fig. S3 in Supplement 1, show XRD patterns and FTIR spectra of LAG:Ce ceramics, respectively. The FWHM of 0.178∼0.210 in the XRD patterns (2θ∼34°) and the strong absorption bands in FTIR spectra indicated the high quality of the final prepared LAG:Ce ceramic grain growth. Figure 7 displays the in-line transmission spectra of LuAG:Ce PCs with different VPS temperatures and 1750 °C HIP treatment. From the transmission spectra it was clear that the T of LuAG:Ce PCs firstly increased with the VPS temperatures increased from 1720 °C to 1740 °C, and then decreased with the VPS temperatures from 1760 °C to 1800 °C. It was believed that samples with low VPS temperatures have more intergranular pores, which could be eliminated after HIP, thereby greatly improving the T of LuAG:Ce PCs. Noticeably, LuAG:Ce PCs with the VPS temperatures of 1740 °C had a very high T of 80% (@800 nm, 1.5 mm), and compared to the non-HIP samples, the T had increased by 11%. On the other hand, when the VPS temperatures were too high, the migration rate of grain boundary was significantly higher than the migration rate of pores along grain boundaries due to the addition of sintering additives, which made it more likely to produce intragranular pores after HIP, and thus led to the reduction of T [45–47]. In addition, as shown in the Fig. 7, the appearance of color centers and absorption of ultraviolet spectrum were not observed in the ceramic samples, which indicated the process of annealing at 1450 °C in the air for 10 h could effectively eliminate the oxygen vacancies in LAG ceramics.
Fig. 7. In-line transmission spectra and appearances (inset) of the M1T5 samples vacuum pre-sintered at different temperatures and HIP sintered at 1750 °C.
By adding suitable additives and optimizing the HIP sintering process, LuAG:Ce PCs could obtain a dense and uniform microstructure without scattering centers (e.g., pores and secondary phases). Figure 8 shows the FESEM images of the thermally etched surface of LuAG:Ce PCs sintered by HIP at 1750 °C. As shown in Fig. 8(a), using the VPS temperatures of 1740 °C, no visible pores and precipitates were observed in the interior of the ceramics, and its grain boundaries were clean and clear. Instead, it could be seen from Fig. 8(b) that there were numerous intragranular pores and a small amount of precipitation on the ceramic surface using the VPS temperatures of 1800 °C. As for HIP treatment, it did not effectively eliminate the remaining intragranular pores caused by the high VPS temperatures of 1800 °C. In addition, it was found that the applied pressure medium Ar might reduce the solubility of Si in the lattice to form the second phase, and even create a large number of Ar-containing pores on the material surface [48,49]. These results could explain the reason why the T variation of samples sintered by HIP at 1750 °C.
Fig. 8. FESEM images of the polished surfaces of the M1T5 samples vacuum pre-sintered at (a) 1740 °C, (b) 1800 °C and combined with HIP.
To investigate the spectral characteristics, the PLE and PL spectra of LuAG:Ce PCs with 1740 °C VPS and 1750 °C HIP treatment are displayed in Fig. 9. It was seen that the PLE spectra monitored at 520 nm showed a strong excitation band situated at 460 nm and a weak excitation band situated at 340 nm, owing to the 4f – 5d transitions of Ce3+ ions. Moreover, a broad green light band situated at 520 nm could be measured under blue light excitation situated at 460 nm. The similar PLE and PL spectra could be found in the previous studied on LuAG:Ce PCs [31,50].
Fig. 9. PLE and PL spectra of the M1T5 samples vacuum pre-sintered at 1740 °C and HIP sintered at 1750 °C.
The LuAG:Ce PCs based green LD devices were constructed using the remote excitation mode as shown in Fig. 10(a). The heat sink was made of Aluminum-based material, while the holder of platelet was made of Iron-based material. The electroluminescent spectra (EL), CRI, CCT and CIE coordinates of LuAG:Ce PCs based green LD devices operating at an incident power of 3.0 W are exhibited in Fig. 10(b, c). Similar to the other reports, LuAG:Ce PCs emitted bluish-green light excited by blue light LDs of 460 nm. The CRI, CCT, and CIE of LuAG:Ce PCs based green LD devices were 46.2, 5471 and (0.336, 0.538), respectively. To further investigate the relationship of the T and luminescence performance under high-power density excitation, Fig. 10(d, e) exhibit the variations of the luminous flux (LF) and the LE for LuAG:Ce PCs with different VPS temperatures and 1750 °C HIP treatment. Here, the LE was calculated by dividing the total LF of the tested blue LDs by the consumed power. (The detailed calculation method can be found in Supplement 1). Intriguingly, with the increase of incident 460 nm blue laser power, LF and LE of all samples with different T increased linearly. Moreover, when the power density of blue light LDs reached to 46 W/mm2, there was always no ST appeared, implying that all transparent LuAG:Ce PCs had the relatively good heat resistance. Among them, the maximum LF of LuAG:Ce PCs with 1740 °C VPS and 1750 °C HIP treatment reached to 1174.2 lm, while showing a high LE of 253 lm/W. Meanwhile, it was found that the T of LuAG:Ce PCs plays a significant role in the LE and ST of phosphor converters. Based on microstructure analysis given above, we believed that reduced porosity and oxygen vacancies were closely related to the improved LE and ST of the transparent LuAG:Ce PCs [31].
Fig. 10. (a) Schematic diagram of LD devices; (b) EL spectra of the green LD, the inset is the digital image of the prototype lamp and its lighting effect; (c) CIE color coordinates of PCs-based LD devices; the intensity of (d) LF and (e) LE for different samples at a fixed thickness of 1.5 mm plot as a function of the various input blue laser power density.
Generally, a moderate amount of scattering centers could increase the absorption of blue light, which is beneficial to improve the LE of PCs [32]. Interestingly, we found that both high LE and high ST can be achieved in LuAG:Ce PCs with high T. Figure 11 shows schematic diagram of the effects of T on LE and ST in LuAG:Ce PCs. It could be understood from Fig. 11(a, b) that when PCs based LDs adopted the transmission mode of remote excitation, the micro-pores would scatter a certain amount of the incident blue light, making some Ce3+ ions not sufficiently excited, which might result in the reduction of LE. Meanwhile, as for the relationship between T and ST, due to the low thermal conductivity of the micro-pores (≈ 0.02 W·m-1·K-1) [51], plenty of heat caused by high-power excitation density of LDs would accumulate around the micro-pores, thereby resulting in poor thermal conductivity and low ST of PCs. Accordingly, high-quality fluorescent converters for high-power LDs could be designed by enhancing T.
Fig. 11. Schematic diagram of the effects of T on LF and ST in LuAG:Ce PCs: (a) low T vs. low LE, (b) high T vs. high LE, (c) low T vs. low ST and (d) high T vs. high ST.
Table 2 compares the fabricated transparent LuAG:Ce PCs with several other representative LuAG:Ce PCs reported in literature from the aspect of Thickness, transmittance (T), laser diodes source wavelength (WL), saturation threshold (ST), luminous flux (LF) and luminous efficiency (LE). As could be seen, LuAG:Ce PCs with a high T of 80% (@800 nm, 1.5 mm) had not only a relatively high LE of 253 lm/W but also a remarkably high ST of >46 W/mm2 among the various ceramics. It should be pointed out that the comparison of the optimal performance data of each team in the table is intended to verify the superiority of our experimental conditions and experimental schemes. Therefore, LuAG:Ce PCs with 1740 °C VPS and 1750 °C HIP treatment possessed excellent luminescent performance, greatly satisfying the technical requirements for laser illumination.
Table 2. Performance based on LuAG:Ce in previous work.
Continuous laser irradiation requires the fluorescent powder transformer to withstand more energy, so a strong endurance of the LD is also an important requirement for LD devices. Therefore, we conducted thermal imaging studies on LuAG:Ce PCs with different VPS temperatures and 1750 °C HIP treatment under a laser power of 3 W. We used a blue LD excited by a DC power supply and recorded the surface temperature of the transparent ceramic using an infrared camera. The recorded surface temperature of LuAG:Ce PCs is shown in Fig. 12. With an increase in the VPS temperature, the surface temperature of the LuAG:Ce PCs initially decreased from 68.6 °C to 66.4 °C, and then gradually increased to 89.6 °C, consistent with the variation trend of T [27,37]. Among them, LuAG:Ce PCs with 1740 °C VPS and 1750 °C HIP treatment exhibited the best heat dissipation capability and achieved the highest T of 80% (@800 nm, 1.5 mm). This was because most of the remaining intergranular pores caused by the low VPS temperatures could be eliminated by HIP treatment. Many studies found that pores had low thermal conductivity and high refractive index, which were unfavorable to the T and heat dissipation performance of materials [58]. In addition, the results also could explained why the fabricated transparent LuAG:Ce PCs didn’t appear the ST under a high LDs power density of 46W/mm2.
4. Conclusion
In this study, we successfully fabricated LuAG:Ce PCs with high T by optimizing sintering additives and sintering processes, and investigated their luminescence performance excited by high-power density LDs. The composite additives of 0.1 wt.% MgO and 0.5 wt.% TEOS could significantly promote ceramic densification and eliminate pores for getting a dense microstructure with clean grain boundaries. On this basis, combined with 1740 °C VPS and 1750 °C HIP treatment, the T of LuAG:Ce PCs could be improved to 80% (@800 nm, 1.5 mm). Compared to the non-HIP samples, the T had increased by 11%. The CRI, CCT, and CIE of LuAG:Ce PCs based green LD devices were 46.2, 5471 and (0.336, 0.538), respectively. The high transparent LuAG:Ce PCs exhibited both a relatively high LE of 253 lm/W and a remarkably high ST of >46 W/mm2. With the T variation, the surface temperature of LuAG:Ce PCs initially decreased from 68.6 °C to 66.4 °C and then gradually increased to 89.6 °C under 3.0 W LD excitation. Based on the characteristics listed above, we believe that highly transparent LuAG:Ce PCs are promising green-fluorescent converters for high-power LDs. It could motivate more researchers to design high-quality fluorescent converters by enhancing T.
Funding
National Key Research and Development Program of China (2021YFB3501700, 2023YFB3506600); National Natural Science Foundation of China (52202135, 61975070, 52302141); Priority Academic Program Development of Jiangsu Higher Education Institutions; International S&T Cooperation Program of Jiangsu Province (BZ2023007); Key Research and Development Project of Jiangsu Province (BE2023050, BE2021040); Natural Science Foundation of Jiangsu Province (BK20221226); Special Project for Technology Innovation of Xuzhou City (KC23380, KC21379, KC22461, , KC22497, ); 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.
Supplemental document
See Supplement 1 for supporting content.
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