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Abstract
Ceramic phosphors are widely considered the next-generation phosphor material for white LED/LD lighting, and a wide spectrum is a key factor in improving the CRI of lighting sources. In this paper, a novel, to our knowledge, barcode-structured YAG:Ce/YAG:Ce,Mn ceramic phosphor was designed and fabricated. The lighting sources with the CRI value of 73.5 and 68.9 were obtained under the excitation of blue LEDs and blue LDs, respectively. Simultaneously, thanks to the effective supplementary emission from a red LD, the CRI of the ceramic-based lighting source reached 81.8 under blue LD excitation. Specifically, the microstructure and luminescent property of ceramic phosphors with different thicknesses and ion doping concentrations were systematically studied. Besides, by changing the blue power from 0.52 W to 2.60 W, the CCT of the laser lighting source with the encapsulation of optimized YAG:Ce/YAG:Ce,Mn ceramic phosphors ranged from 3928 K to 5895 K, while the CRI always maintained above 80. The above results indicate that barcode-structured Ce:YAG/Ce,MnYAG ceramic phosphor is a candidate to achieve a high CRI and ican be applied to various lighting occasions.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Laser-driven (LD) lighting is widely recognized as a revolutionary lighting technology. Compared to light emitting diode (LED) lighting source, LD lighting source has smaller size [1], higher brightness [2], higher luminous efficiency [3], and it has been widely used in remote lighting such as automotive headlamps, high-speed railways, industrial lighting and road lighting, etc. [4,5].
Phosphor material, as a core element of laser lighting sources, includes the phosphor-in-silicone [6], phosphor-in-glass [1,7,8], single crystal [9–11], and ceramic phosphor [12]. Among them, the ceramic phosphor has the relatively high thermal stability and thermal conductivity (9.0∼14.0 W·m−1·K−1). More importantly, multiple luminescent ions can be introduced during the preparation process, such as Ce3+, Mn2+, Cr3+, as well as second phase, for example, Al2O3 [13–15], AlN [16], MgO [17]. Thanks to the high thermal conductivity caused by the second phase (20∼40 W m−1 K−1), the power density of the ceramic phosphors has exceeded 49 W/mm2 [18], and the ceramic phosphor shows a good candidate for LD lighting.
However, as the mainstream ceramic phosphor, YAG:Ce ceramics often exhibits low color rendering index (CRI) due to the lack of red emission. Previous researches have proved that red luminescent ions, such as Cr3+ [19], Mn2+ [20], Pr3+, are beneficial for improving the CRI of YAG:Ce. Liu et.al confirmed that YAG:Ce,Pr,Cr phosphor ceramic had higher CRI (∼78) than that of YAG:Ce,Pr (∼72), YAG:Ce,Cr (∼68) and YAG:Ce (∼50) ceramic phosphors [21]. A warm white lighting source based YAG:Ce,Mn with the CRI value of 82.5 and the CCT value of 3870 K was realized by Zhou et.al. [22]. By doping Pr3+ with emission peaks located at 609 nm, the CRI of LED devices encapsulated with a YAG:Ce,Pr,Mn ceramic was increased to 84.8 [23]. Besides, LuAG:Ce,Mn ceramic phosphors showed an excellent thermal performance and luminous proprieties [24]. The PL intensity of the samples with high concentration Mn2+ remained above 95.7% at 425 K, and the CRI of the designed LD lighting system increased from 51.9 to 80.1. Therefore, Mn2+ emits light from orange to red by energy conversion process of Ce3+, and it is an effective way to improve CRI for LD lighting.
Multilayer structure is an another efficient way for achieving high CRI [25]. On the one hand, red layer could effectively extend the spectral region and maintain the emission intensity of yellow layer. On the other hand, superior to the yellow ceramic composed of a red phosphor film [26], the glass composed of a red phosphor film [27], and the silicone composed of red phosphor films [28], multi-layer ceramics inherit the mechanical and thermal properties of single ceramics. YAG:Ce,Cr:/YAG:Ce dual-layered composite phosphor ceramics were prepared by dry pressing and vacuum sintering [25]. Under the excitation of 1.0 W blue laser, the optimized CRI and luminous flux were 69.2 and 160 lm, respectively. Subsequently, YAG:Ce/LuAG:Ce,Cr dual-layered ceramic phosphor was prepared, and a higher CRI (85.5) was obtained under the blue laser excitation compared to that of YAG:Ce ceramic (69.2). In white LED lighting, YAG:Ce/(Gd,Y)AG:Ce dual-layered structure ceramic phosphors was prepared [29]. However, owing to the limited spectral regulation of Gd3+, the optimized Ra was 62.9 (CCT = 3156 K). Furthermore, YAG:Ce,Gd/YAG:Ce,Pr dual-layered composite phosphor ceramics were proposed and fabricated by the tape-casting and vacuum sintering [30]. An increased Ra from 69.7 to 83.5 was realized by controlling the thickness of ceramic layer, and the corresponding CCT was ranged from 4344 K to 6988 K. However, the above reports were based on ceramic phosphors with vertical structure, and it may occur a phenomenon of reabsorption by RGB phosphors. Horizontal cyclic structure of ceramic phosphors, which presents independent processes both in blue absorption and fluorescence emission, and the possible reabsorption occurred in multiple layers could be avoided and then improve the luminous intensity of the lighting source, has not been reported yet. Besides, this structure has advantages of small size, thin thickness, and it is suitable for uniform luminescence and chip-scale package. In addition, red laser is an effective way to further enhance the CRI in laser-driven lighting [31,32], and it needs to be systematic researched in lamellar ceramic.
In this paper, YAG:Ce/YAG:Ce,Mn composite ceramics with a novel barcode-structure were fabricated by dry pressing and vacuum sintering method. Microstructure and luminescent property of ceramics were systematically investigated. The spectral power density, luminous flux, CCT and CRI of the ceramic phosphors under blue LED and blue LD were detailed studied. By adopting a dual excitation source system, the CRI of the laser lighting source was increased to 81.8. More importantly, the CRI value exceeded 80 in a continuously range of CCT (3928∼5895 K). This study confirms that the YAG:Ce/YAG:Ce,Mn lamellar ceramic is an effective way to realize the essential improvement of the CRI for white LED/LD devices.
2. Experimental method
The commercial α-Al2O3 (Sumitomo Chemicals, Tokyo, Japan, 99.99%), Y2O3 (Alfa Aesar, Ward Hill, America, 99.99%), CeO2 (Alfa Aesar, Ward Hill, America, 99.99%) and MnO2 (Aladdin, Shanghai, China, 99.99%) were used as the raw materials. The detailed chemical formula of the composite ceramics is listed in Table 1. Firstly, these raw materials (YAG:Ce,Mn, A powders) were mixed homogeneously by ball milling for 12 h at 180 rpm. 0.5 wt% TEOS (Alfa Aesar, Ward Hill, America, 99.99%) and 0.1 wt% MgO (Alfa Aesar, Ward Hill, America, 99.999%) were acted as sintering aids. Alcohol was chosen as a dispersant and Al2O3 balls was selected as the grinding media. The another mixture (YAG:Ce, B powders) was prepared using the same process. Then, the acquired slurries were drying at 55°C for 12 h. The first layer was processed by pressing A powders (with the mass of 0.2 g) at 10 Mpa in a die with a diameter of 20 mm, the second layer was processed by pressing B powders (with the mass of 0.2 g) under the same condition. The green bodies were formed with the above cyclical pressing for 10 times, and then cold isostatically pressed under 200 MPa for 10 min. In addition, B powder was directly pressed with the mass of 4.0 g for preparing YAG:Ce ceramic. Thirdly, the green bodies were sintered at 1740 °C for 8 h in the vacuum (<10−4 Pa). After sintering, the samples were annealed at 1350 °C for 10 h to eliminate oxygen vacancies. The circular plates were cut via a longitudinal way to form a thin plate with the size of 10.0 * 3.0 mm and the thickness of 0.6 mm. Finally, one surface of ceramic plates was carefully machined and polished. The thickness of YAG:Ce/YAG:Ce,Mn composite ceramic was processed to 0.2 mm, 0.4 mm, 0.6 mm. The detailed fabrication process of the composite ceramic phosphors is described in Fig. 1.
Table 1. Nomenclature of samples with different doping concentrations and thicknesses
A scanning electron microscopy (SEM, JSM-6510, JEOL, Tokyo, Japan) was used to characterize the surface and cross-section of samples. The optical surface was characterized by an optical microscope (FXD-30 MW, Shandong, China). The fluorescence spectrum was characterized by a fluorescence spectrometer (OmniFluo 900, Beijing, China). The composite ceramic based white LEDs were packaged via a remote excitation mode. The blue LED chips with the maximum power of 10.0 W were bought from Shenzhen Yinding Technology Co., Ltd (China, peak wavelength = 450 nm, chip size = 2.3 × 2.3 mm). The customized fiber coupled LDs included six blue LDs (λ=455 nm) with total power of 30.0 W and a red LD (λ=665 nm) with the power of 0.15 W was bought from Shenzhen Feifan Optoelectronic Technology Co., Ltd (China). The core diameter was 200 microns and NA = 0.22. The laser-irradiated spot was approximately circular with radius of 0.48 mm at a distance 2.0 mm (≈0.72mm2 in area). A heat-conduct coefficient measurement devices was used to test the thermal conductivity (Hotdisk 2500S, Goteborg, Sweden). The output power of the LDs was measured using an optical power meter (S350C, Thorlabs, Newton, America). The electroluminescence spectrum, luminous flux, CCT and CRI values under blue LEDs/LDs were tested using an integrating sphere (HASS-2000, Hangzhou, China). The spatial intensity distribution was characterized by space spectroradiometer (GO-SPEX500, Hangzhou, China).
3. Results and discussion
The fabricated samples are exhibited in Fig. 2(a). The YAG:Ce/YAG:Ce,Mn composite ceramics and YAG:Ce ceramics could be clearly distinguished from color distribution. In addition, the multi-layer structure could be identified from the physical image, which can be further confirmed from Fig. 2(b). Compared to the YAG:Ce ceramic (Ce2Mn0.02) in Fig. 2(c), the optical transmission micrograph of YAG:Ce/YAG:Ce,Mn composite ceramics (Ce1Mn2.02) showed clear boundaries between yellow ceramic (YAG:Ce) and red ceramic (YAG:Ce,Mn), and presented a barcode structure. Different from the vertical configuration, this planar configuration can avoid the problem of reabsorption by RGB phosphors in multiple layers, thereby improve the luminous intensity of the lighting source [28]. As can be calculated from Fig. 2(b), the thickness of the single layer is about 180∼200 µm, which can also be obtained from Fig. 3(a). The SEM images of the interfaces showed obvious residual pores in YAG:Ce/YAG:Ce,Mn composite ceramics. On the one hand, the sintering temperature of YAG:Ce,Mn phosphor ceramic was generally between 1650∼1740 °C in the literature [20,22]. During sintering, MgO2 powders were considered as sintering additives, which accelerated the grain boundary migration rate of the ceramics. As a result, the internal pores of the ceramic could not be effectively discharged and then decreased the optical transmittance [23]. On the other hand, the optimal sintering temperature for YAG: Ce transparent ceramics is around 1780 °C via vacuum sintering [33], while the current sintering temperature of 1740 ° C would introduce certain pores in YAG:Ce and decrease the transparency [34,35]. Therefore, all samples were translucent in Fig. 2(a).
Fig. 2. (a) Photographs of the prepared samples under daylight; Optical transmission micrograph of (b) Ce1Mn2.02 and (c) Ce2Mn0.02
Fig. 3. SEM images at the boundary between YAG:Ce and YAG:Ce,Mn. After polished: (a) Ce1Mn1.02; (b) Ce1Mn2.02; (c) Ce2Mn1.02; (d) Ce2Mn2.02; (e) Ce1Mn0.02 ;(f) Ce2Mn0.02. Before polished: (g) Ce1Mn2.02 and (h) Ce1Mn0.02
Luckily, the presence of low transparency or pores is beneficial for the scattering of blue light, as well as the absorption of blue light for phosphors [13–15]. Generally, ceramics phosphors are polished on both sides during reprocessing. However, in our paper, all samples were polished on one surface, which also reduce the transparency of the samples compared to double-sided polishing [36,37]. The polished surface (in Fig. 3(a∼f)) catered to the incidence of blue light, while the rough surface (in Fig. 3(g, h)) was beneficial for the emission of fluorescence, which are beneficial for improving the absorption efficiency and emission efficiency, respectively. It is particularly worth mentioning, as shown in Table S1, although there are some pores on the surface and inside of the ceramic phosphors, it does not have a significant impact on the thermal conductivity of the ceramics which is between 7.0 Wm−1K−1 and 9.0 Wm−1K−1, and this enables them to have good luminescent and thermal properties.
Figure 4(a) shows the emission spectrum of YAG:Ce/YAG:Ce,Mn composite ceramics under the excitation of 450 nm. A broad emission band from 500 to 700 nm were observed, which attributed to the 5d−4f transitions of Ce3+ ion [38,39]. Compared to Ce1Mn0.4 and Ce2Mn0.4, other ceramics exhibited significant enhancement in the red region, which owing to the 4T1g→6A1g transitions of Mn2+ ion [24]. In the Gaussian fitting curve for determine the specific wavelength of two luminescent ions inset Fig. 4(a), a yellow emission peak located at 525 nm and a red emission peaking at 568 nm are ascribed as the Ce3+ ion and Mn2+ ion, respectively. Under the same doping concentration of Ce3+, the composite ceramics with higher Mn2+ doping exhibited higher intensity at orange-red region (Ce1Mn2.04>Ce1Mn1.04, Ce2Mn2.04>Ce2Mn1.04). The high red component is beneficial for reducing the CCT and maintaining a high CRI of the lighting source. Fig. S1 shows the temperature dependent emission spectrum of Ce1Mn0.04 and Ce1Mn2.04 from 300 K to 475 K with the step of 25 K. Compared to the luminous intensity at room temperature (300 K), Ce1Mn0.04 and Ce1Mn2.04 maintained 83.6% and 75.5% at 475 K, respectively, although there were some porosity defects inside the ceramics, they both had good thermal stability due to the high thermal conductivity and excellent mechanical properties.
Fig. 4. (a) The emission spectrums of YAG:Ce/YAG:Ce,Mn composite ceramics with the thickness of 0.4 mm; (b) The electroluminescence spectrums of all samples under 2.0 W blue LEDs
The electroluminescence spectrums in Fig. 4(b) display the similar spectral power density. The testing construction of the ceramic phosphors is shown in the illustration of Fig. 3(b), and the power of blue LED chips is set as 2.0 W. When the sample thickness was 0.2 mm, compared to the YAG:Ce ceramics (Ce1Mn0.02, Ce2Mn0.02), Mn2+ doping samples (Ce1Mn1.02, Ce1Mn2.02, Ce2Mn1.02, Ce2Mn2.02) absorbed more blue light for red emission. The red region had been effectively supplemented based on the previous yellow region because of the structural design, indicating that a lighting source with low CCT would be obtained. However, the process of down-conversion for photon that occurs in composite ceramics (Mn2+, blue→red) would result in more energy loss compared to that of YAG: Ce ceramics (Ce3+, blue→yellow) [23,40]. Furthermore, the thermal stability of YAG:Ce,Mn ceramic phosphor was weaker than that of YAG:Ce ceramic phosphor, which could be confirmed from Fig. S1, and this loss of energy converted to heat through radiative transition. Besides, an increase in the absorption for blue light accelerated above energy loss, resulting in a weak fluorescence intensity and lower luminous flux than YAG:Ce ceramic (∼200 lm<∼300lm). With the increasing of the thickness of the composite ceramic from 0.2 mm to 0.6 mm, the blue region was apparently reduced while the fluorescence intensity was not significantly improved, which could also be known from the changing tendency of luminous flux that from 217.1 lm to 158.6 lm for Ce1Mn1, from 165.7 lm to 118.0 lm for Ce1Mn2, from 234.5 lm to 179.1 lm for Ce2Mn1 and from 211.6 lm to 174.4 lm for Ce2Mn2. The samples were translucent, and when the thickness exceeds 0.4 mm, most light beam would trap inside the material and finally resulted in a decrease in the luminous flux. Therefore, for phosphor ceramics with this barcode structure, including the thickness both for the monolayer and the ceramics, doping concentration both for yellow ceramics and red ceramics, have a significant impact on the luminescent process.
Analyzing the quality of chromaticity for lighting source is conducive to determining the optimal concentration and thickness of the barcode ceramics. Figure 5 displays the CCT and the CRI of all samples under 2W blue LEDs. As the thickness of the ceramic was 0.2 mm, the CCT of the ceramic-encapsulated lighting sources exceeded 7000 K because of the large amount of residual blue light (seen as Fig. 4. (b) and the inset of Fig. 5), even exceeding 10000 K for Ce1Mn2.02 and Ce1Mn0.02. The CCT of the lighting sources started to decline and drop below 6500 K when the ceramic thickness was 0.4 mm, especially for Ce1Mn2.04, the CRI reached 73.5 and the CCT was 5794 K, showed an optimal luminous parameter. Although the LED lighting source based on Ce2Mn0.4 can also achieved a relatively high CRI value of 70.1, the poor CCT (7230 K) posed a hazard of residual blue light and limit its application for headlights [2], indoor and museum lighting [41]. Compared to the ceramic with the thickness of 0.4 mm, the samples with the thickness of 0.6 mm had a lower CCT (4000∼4500 K) due to the stronger absorption of blue laser. However, both the luminous flux and CRI decreased (Ra below 70), which were not conducive for further encapsulation and application.
Fig. 5. The CRI and the CCT of the samples under 2.0 W blue LEDs. The insets are physical image of radiation for Ce1Mn2 and Ce2Mn0
Figure 6 shows the spectral power density, luminous flux, CCT and CRI of lighting sources packaged with all samples under 1.0 W blue LDs (The corresponding chromaticity coordinates are shown in Table S2). A narrow emission band located at 455 nm attributed to the blue LDs and a broad emission band from 500 to 750 nm originated from the composite ceramics were observed. It can be clearly seen that the blue area significantly decreased, while the fluorescence area remained unchanged by adjusting the thickness from 0.2 mm to 0.6 mm, showing a similar characteristic to LED lighting sources. Similarly, in terms of color quality, the thickness of 0.4 mm for the composite ceramic in LD lighting sources is also the best candidate. Compared to Ce1Mn1.04, Ce1Mn2.04 sample had appropriate CCT (5292 K) and higher CRI (68.9>67). It should be pointed out that although the luminous flux based Ce2Mn1.04 was much lower than that of Ce1Mn0.04 (79.8 lm<210.4 lm), the lighting sources has lower CCT (5292 K<6419 K) and higher color rendering index (68.9>66.5), and advanced methods could be carried to further improve the CRI.
Fig. 6. The electroluminescence spectrum and color parameters of the samples under the excitation of 1W blue LDs. (a) Ce1Mn1.02; (b) Ce1Mn2.02; (c) Ce2Mn1.02; (d) Ce2Mn2.02; (e) Ce1Mn0.02; (f) Ce2Mn0.02
To further increase the Ra value of LD lighting system, a red LD with the output power of 0.15 W was adopted as an emitter and called “an additional excitation source” for phosphors. The testing schematic diagram and physical images are shown in the Fig. 7(a). An optical fiber input laser was used and both blue light and red light emitted from one fiber, which were suitable for lighting museums and other important venues [24]. Figure 7(b,c) are the spectral power distributions of Ce1Mn2.04 and Ce1Mn0.04 under different power of blue LDs, respectively. The insets in Fig. 7(b,c) are the lighting diagram before and after the excitation of the red LD. Thanks to the sufficient red light, the luminous color undergone an enormous change. As the blue laser power increasing from 0.25 W to 2.6 W, the intensity of the blue region and fluorescent region were gradually increased, while the intensity of the red laser centered at 665 nm remained unchanged and showed a no absorption behavior by Ce1Mn2.04. However, when the blue power increased to 3.1 W, the fluorescence intensity had a significant decrease, which meant a luminescence saturation for laser lighting. Luckily, the luminescence saturation did not appeared in Ce1Mn0.04 even the power of blue LDs increased to 5.16 W. In current reports, luminescence saturation is related to the doping concentration and types of luminescent ions, the stability and the thickness of the phosphors, as well as spot size and input power of the LDs [42]. In addition, the saturation power threshold of the phosphors can be further improved if a heat sink is applied for the heat dissipation of phosphors [43–45]. Therefore, optimizing the doping concentration of Mn2+ ion and the crystal field environment (such as LuAG:Ce system), or adopting a copper as the cooling device for Ce1Mn2.04 can increase the saturation power and luminous efficiency in LD lighting.
Fig. 7. (a) The testing schematic diagram and physical images in LD lighting system. The power distribution of (b) Ce1Mn2.04 and (c) Ce1Mn0.04 under different power of blue LDs accompanied with a 0.25W red LD. The insets show the lighting diagram before and after the excitation of the red LD
The luminance decline can also be observed from the corresponding luminous flux in Fig. 8(a). The luminous flux of the lighting system is plotted as a function of the incident blue LDs, as shown in Fig. 8(a). Ce1Mn0.04 exhibited a higher luminous flux and exceed 925.3 lm (5.16 W) due to the less loss of down-conversion of Ce3+ ion compared to Mn2+ ion doping samples (Ce1Mn2.04). In addition, compared to Ce1Mn2.04 (7.68 W·m−1·K−1), Ce1Mn0.04 had higher thermal conductivity (9.05 W·m−1·K−1) to improve the anti-thermal quenching and increase luminous efficiency under laser radiation, seen as Table S1. With the increase of excitation power, the luminous flux of Ce1Mn2.04 gradually increased and reached a maximum value of 165.3 lm under 2.60 W blue laser excitation. Higher power (3.13 W) caused a rapid decrease of luminous flux which dropped to 165.3lm, and this considerable decrease caused a great shift in color coordinates (from (0.3295, 0.308) to (0.2161, 0.1082)), seen as the inset of Fig. 8(a). It is worth noting that compared to the color coordinate under single LD excitation (A point for Ce1Mn2.04, B point for Ce1Mn0.04), the color coordinate position of the lighting source moved towards the red region under double LD excitation due to the enhancement of red light by a LD laser, which meant that lower CCT and higher CRI had been obtained.
Fig. 8. (a) The luminous flux and color coordinates, (b) the CRI and the CCT of Ce1Mn2.04 and Ce1Mn0.04 under double excitation in LD lighting system
Figure 8(b) shows the corresponding CRI and the CCT of Ce1Mn2.04 (YAG:Ce/YAG:Ce,Mn composite ceramic) and Ce1Mn0.04(YAG:Ce ceramic phosphors) under double- excitation in LD lighting system. Under the excitation of 0.52 W for blue LDs and 0.15 W for the red LD, Ce1Mn0.04 reached a highest Ra value of 79.9 (CCT = 6351 K), and then gradually decreased (79.9→72.0) with increasing the power of blue LDs. Meanwhile, the CCT gradually began to exceed 6500 K and became a cold white light source accompanying by low CRI. Distinct from Ce1Mn0.04, thanks to the dual supplementation of red light by Mn2+ and the red LD, Ce1Mn2.04 based lighting source had achieved low CCT and high CRI in different power ranges of blue laser before its luminescence saturation. Specifically, the CCT of the LD lighting source equipped with the Ce1Mn2.04 can be controlled between 3928 K and 5895 K by adjusting the power content of blue LDs from 0.25W to 2.60 W. More importantly, within this range of CCT (3928K∼5895 K), the CRI of the LD lighting source maintained above 80 and reached a maximum of 81.8 (5895 K) under the excitation of 2.09 W blue LDs and a 0.15 W red LD, indicating that various lighting occasions could be realized, such as warm white light for indoor lighting, natural white light for museum lighting and so on.
Generally known, due to the high collimation of the LDs, the blue light cannot be effectively scattered by transparent or translucent phosphors, resulting in an uneven spatial distribution of white light [33,4,46]. Figure 9(a) shows the spatial intensity distribution of Ce1Mn2.04 encapsulated LD lighting system before and after adopting a hemispherical lampshade which applied for commercial LED bulb lamp (PC material, φ= 60 mm, h = 70 mm, and transmittance = ∼75%). Without the hemispherical lampshade, the intensity distribution for 0° was higher than surrounding angles due to the combination of the uniform fluorescence and unscattered laser beams, which could also be clearly observed by naked eye in the inset of Fig. 7(b, c). On the contrary, using a hemispherical lampshade had achieved a better uniform distribution due to the diffuse reflection of the surface despite some energy was lost by multiple scattering within the lampshade. Figure 9(b, c) are the photos of a light-illuminated application for this packaging structure. By the encapsulation of Ce1Mn2.04, a white lighting source was obtained under blue LDs, and after the additional use the red LD, the lighting quality had become warmer and more realistic. The above results indicate that the designed YAG:Ce/YAG:Ce,Mn composite ceramics and the corresponding lighting source can be used to the application with high-CRI and low CCT, such as museum lighting, indoor lighting, medical lighting and so on.
Fig. 9. (a) Spatial intensity distribution of Ce1Mn2.04 encapsulated LD lighting system before and after adopting a hemispherical lampshade. The photos of a light-illuminated application encapsulated with (b) blue LDs (input power = 0.25W) and Ce1Mn.04; (c) blue LDs(input power = 0.25W), Ce1Mn.04 and a red LD(input power = 0.15W)
4. Conclusion
In summary, YAG:Ce/YAG:Ce,Mn composite ceramics with a novel barcode-structure were successfully fabricated by dry pressing and vacuum sintering method. Compared to the YAG:Ce ceramic, YAG:Ce/YAG:Ce,Mn composite ceramic existed a clear interface and a significant enhancement in red emission region. Under 2.0 W blue LEDs lighting system, a CCT value of 5794 K, the CRI value of 73.5 were obtained by encapsulated YAG:Ce/YAG:Ce,Mn ceramic with the thickness of 0.4 mm. After packaging the ceramics with 1.0 W blue LDs, the white lighting source with the CCT of 5292 K and the CRI of 68.9 were obtained. By adding a red laser as the additional excitation source for the ceramic phosphors, the CRI was further improved and reach 81.8 (CCT = 5895 K). Besides, under the dual excitation source system, the CRI value always exceeded 80 in a continuously range of CCT (3928∼5895 K) by adjusting the power of blue LDs from 0.25 W to 2.60 W. Therefore, the barcode-structured ceramic phosphors have a good application prospect to realize high CRI for white LED/LD lighting.
Funding
National Key Research and Development Program of China (2021YFB3501700); National Natural Science Foundation of China (51902143, 52202135, 61971207, 61975070); Priority Academic Program Development of Jiangsu Higher Education Institutions; Key Research and Development Project of Jiangsu Province (BE2021040); Natural Science Foundation of Jiangsu Province (BK20221226); International S&T Cooperation Program of Jiangsu Province (BZ2023031); Natural Science Foundation of the Jiangsu Higher Education Institutes of China (19KJB430018, 20KJA430003); Special Project for Technology Innovation of Xuzhou City (KC21379, KC22461, KC22497); Open Project of State Key Laboratory of Advanced Materials and Electronic Components (FHR-JS-202011017).
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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