With the development of high-power white light-emitting diode (LEDs) and laser diode (LDs), phosphors in glass (PiG)^[1], single crystals (SC)^[2] and transparent ceramics (TC)^[3,4] were proposed to overcome the influence of high temperature on the luminous efficiency (LE) and lifetime of phosphors in resin or silicone. White light- emitting diode (WLEDs) composed of blue LED chip and yellow emitting Ce:Y₃Al₅O₁₂ (Ce:YAG) phosphor is widely used in solid-state lighting (SSL)^[5,6,7]. As a new generation of color conversion materials, Ce³⁺ ion doped yttrium aluminum garnet transparent ceramics are favored by researchers for their high thermal conductivity (about 8 W·m^-1·K^-1)^[8] and excellent mechanical properties, so they are developing rapidly. But the “efficiency droop” of blue LEDs limits the development in the field of high- power lighting ^[9,10]. Laser diodes have the advantages of smaller size, high brightness, fast switching speed and especially under the ultra-high driving power density. In the period of high-power white light illumination, the performance of white light LDs is better than that of white LEDs in all aspects, so it is an ideal substitute^[11]. Therefore, the design idea of white LDs through blue laser diode combined with Ce:YAG transparent ceramics has been proposed in automobile illumination, displays, projectors, headlights, and other special lighting fields to get ultrahigh brightness by consuming lower energy^[12]. The color conversion materials for LEDs/LDs lighting need to meet certain requirements. The following three aspects are essential: high absorption rate of blue light, high luminous efficiency and good optical performance. At present, Ce:YAG phosphor is considered to be the most suitable material^[13]. Compared with SC and PiG, TC have become the most promising type because of its lower cost and higher thermal conductivity. Thus, for LEDs lighting Ce:YAG usually have a better performance in TCs (≈120 lm/W)^[14] than that in PiGs (≈80 lm/W)^[15,16].

At present, in order to optimize the luminous performance and heat transfer efficiency of Ce:YAG ceramics, almost all process parameters, such as thickness, doping concentration and radius, have been studied in detail^[17,18]. By simply changing the thickness of cerium yttrium aluminum garnet ceramics (0.2-1.5 mm), the emission color of the assembled LEDs can be adjusted from cold white (0.29, 0.31) to warm white (0.39, 0.49)^[19]. In addition, Yao, et al^[17] used ANSYS software to effectively analyze the temperature variation, heat distribution and heat flux of Ce:YAG transparent ceramics in different packaging modes of LEDs. Therefore, from the above research results, a simple conclusion can be drawn that Ce:YAG transparent ceramics have a bright application prospect in lighting industry. Preparing transparent ceramics with high optical quality and selecting suitable ceramic thickness combined with blue LEDs/LDs with different powers can be applied to many scenes. In the research process, due to the limitation of the shape of the purchased LED chip and the ceramic processing technology, the three LED packaging modes commonly used in the laboratory were simulated by the finite element method, and the reasonable ceramic surface temperature was obtained. And a conclusion is drawn that the thicker the ceramic is, the lower the surface temperature will be, which is of great significance for the design of white LEDs, and many influencing factors can be considered more comprehensively.

In this study, we fabricated an outstanding phosphor- converter with high thermal conductivity (Ce:YAG transparent ceramics). The transparent ceramics with different concentrations and thickness were systematically investigated by in-line transmittance, luminescence intensity and device testing such as luminous efficiency (LE), Commission Internationale de l´Eclairage (CIE), Color Rendering Index (CRI) and electroluminescence (EL), by combining the ceramics with blue LEDs/LDs. The temperature distributional simulation of Ce:YAG transparent ceramics were effectively analyzed by COMSOL Multiphysics finite element software. The work reveals the law of combining LEDs/LDs with different Ce³⁺ concentrations

and ceramic thickness, and makes a series of lamps by combining transparent ceramics with blue LEDs, so as to guide and promote the development of combining high- power LEDs/LDs with Ce:YAG transparent ceramics.

Ce:YAG ceramics were synthesized via solid-state sintering. Firstly, the raw materials for the ceramics were high purity α-alumina (α-Al₂O₃, 99.99% purity, Yangzhou Zhongtianli Materchem Co., Ltd, China), yttrium oxide (Y₂O₃, 99.99% purity, Shanghai Sheeny Metal Co., Ltd, China) and CeO₂ (99.99% purity, Alfa Aesar Chemical Co., Ltd, China) powders. The average particle sizes of Y₂O₃ and α-Al₂O₃ powders are about 40 and 300 nm, respectively. The starting materials were weighed accurately in the ratio of (Ce_xY_1-x)₃Al₅O₁₂ (x=0.0005, 0.0010, 0.0030, 0.0050, 0.0070 and 0.0100), and 0.8wt% tetraethyl orthosilicate (TEOS, >99.999%, Alfa Aesar, Tianjin, China) and 0.08wt% MgO (99.998%, Alfa Aesar, Tianjin, China) were added as the sintering additives. The material used for the container and milling balls is alumina. Then the mixed powders were milled on a planetary ball mill for 12 h with 10 mm diameter balls in ethanol. The disc and bottle rotation speeds were 130 and 260 r/min, respectively. After drying for 2 h in an oven at 70 ℃, the mixture was sieved through a 200-mesh (74 μm) screen three times. The obtained powders were calcined at 600 ℃ for 4 h in a muffle furnace to remove the organic residuals. Using molds with a circular shape ( Φ=18 mm), the ceramic plates were loaded and compacted under 20 MPa, and then the green bodies were achieved after cold isostatic pressing at 250 MPa. Next, the green bodies were sintered at 1750 ℃ for 20 h under vacuum of 5.0×10^-5 Pa. The as-sintered samples were annealed at 1450 ℃ for 10 h in a muffle furnace to eliminate the oxygen vacancies, and then they were machined and double- surface polished to different thickness of 0.2, 0.4 and 1.0 mm for further measurement and device packing. The flowchart of whole preparation process is shown in Fig. 1.

For the optical measurements, the in-line transmittance of the mirror-polished YAG ceramics over the wavelength range from 200 to 800 nm were tested by a UV-Vis-NIR spectrophotometer (Cary 5000, Varian Medical System Inc. Palo Alto, USA). The X-ray powder diffraction (XRD) test used Huber G670 Guinier imaging diffractometer (CuKα₁ (λ=0.154056 nm), 40 kV/30 mA, germanium monochromator) of Germany, scanning diffraction pattern in the range of 25°-75° (step state 0.02°). A fluorescence spectrophotometer (Hitachi, F-4600, Japan) was used to test the temperature-dependent photoluminescence (PL) in the temperature ranges from RT to 250 ℃ with the heating rate of 50 ℃/min, with Xe lamp as an excitation source. The specific heat was recorded by high temperature specific heat meter (MHTC96, Setaram, France) and the heat conductivity coefficient was recorded by a home- made laser pulse apparatus. The bulk density of the prepared ceramics was measured by the Archimedes principles. Combining the ceramics with blue LEDs, the optical properties of the ceramic-based LEDs were studied. The luminous efficiency, Commission Internationale de L’Eclairage (CIE) color coordinate and electroluminescent properties under different input currents were detected using a high accuracy array spectroradiometer (HASS-2000, Hangzhou, China), with programmable LEDs test power supply (LEDs 300E, Hangzhou, China). For LDs application, transmission mode was firstly used, wherein the ceramics were fixed onto a heat sink and excited using a fiber-coupling blue LDs.

Fig. 2 (c) exhibits that YAG belongs to a cubic crystal system with a space group of O_h¹⁰-Ia3d^[20]. There are one Y site coordinated by eight O atoms and two Al sites coordinated by six or four O atoms. The Y site is linked to the neighboring two AlO₄ tetrahedra by edge-sharing and vertex-sharing and four AlO₆ octahedra by sharing a common edge. The ionic radius of Ce³⁺(r=0.1143 nm, CN=8) is close to that of Y³⁺ (r=0.1019 nm, CN=8)^[21], and thus it is believed that Ce³⁺ prefers to occupy the Y³⁺ site.

XRD patterns of (Ce_xY_1-x)₃Al₅O₁₂ (x=0.0005, 0.0010, 0.0030, 0.0050, 0.0070 and 0.0100) are presented in Fig. 2(a), and all samples exhibit the single garnet phase indexed to the standard YAG, JCPDS#04-007-2667. Fig. 2(b) shows the diffraction peaks slightly shift to lower angles upon increasing the Ce³⁺ concentration, indicating that the unit cell expansion occurs due to the replacement of Y³⁺ by the larger Ce³⁺.

Fig. 3 shows the surface morphologies of Ce:YAG ceramics with different doping concentrations. All ceramic samples illustrate fully dense, pore-free microstructures without any secondary phase. For (Ce_xY_1-x)₃Al₅O₁₂ ceramics with the doping concentrations of x=0.0005, 0.0010, 0.0030, 0.0050, 0.0070 and 0.0100, the average grain sizes are 21.8, 16.4, 8.6, 8.1, 6.6 and 5.8 μm, which is consistent with previous studies ^[18]. The decrease of the grain size is probably related to the increase of Ce³⁺ doping concentration. The radii of Ce³⁺ and Y³⁺ ions are different, which inhibits the grain growth.

The scattering performance of TCs influences its emission intensity, which is significant to the luminescence performance of the white LEDs/LDs. In Fig. 4 and Fig. 5, the appearance and optical transmittance properties of (Ce_xY_1-x)₃Al₅O₁₂ (x=0.0005, 0.0010, 0.0030, 0.0050, 0.0070 and 0.0100, thickness d=0.2 mm, 0.4 mm and 1.0 mm) were all above 79% at the wavelength of 800 nm, indicating their excellent optical quality^[22]. Fig. 4 shows that with the increase of Ce³⁺ ion concentration, the color of the polished ceramics changes from light yellow to dark yellow. At the same time, the intensities of the absorption peaks at 340 and 460 nm are directly proportional to the concentration of Ce³⁺. These two absorption peaks are attributed to 4f→5d₂ and 4f→5d₁ transitions of Ce^3+[23], which means that their combination with blue LEDs and LDs have high feasibility. The absorption band at 200- 270 nm is probably caused by the Ce⁴⁺→O^2- charge transfer through the reaction of Ce⁴⁺⁺O^2-→Ce³⁺⁺O^-[24,25]. Comparing the samples before and after annealing, it is found that the UV absorption edge of air annealed samples moves to 256 nm, which is due to the absorption of Fe³⁺ charge transfer band^[26,27,28]. Generally, the CeO₂ raw material could not only act as dopants, but also served as sintering additives, which affected the densification behavior of ceramics during sintering, resulting in transmission difference between samples. All above results suggest that the ceramic phosphors prepared by vacuum sintering exhibited a high optical quality, and they may be good candidates as optical conversion materials for solid-state lighting.

The fabricated ceramics show typical luminescence of Ce³⁺, as displayed in Fig. 6(a). It could be seen that the 5d₁→4f transition of Ce³⁺ emission provided an asymmetric broad band characteristic, which was consisted with doublet sub-emissions from 5d₁→4f_5/2 and 5d₁→4f_7/2 transitions^[29], because the ground state of Ce³⁺ includes 4f_5/2 and 4f_7/2 sublevels after taking the spin-orbit interaction into consideration. Ce³⁺ doped YAG transparent ceramics can effectively absorb blue light in the range of 440- 470 nm and emit strong yellow emission at 530 nm. Under blue light excitation, all samples show a broad emission band, which can be deconvoluted into two Gaussian bands peaking at 17692 (5d₁→4f_7/2) and 18920 cm^-1 (5d₁→4f_5/2), as shown in Fig. 6(b). As displayed in Fig. 6(c, d), the transition processes also can be understood from the illustration in the inset of Fig. 6(b). Due to the concentration quenching effect, when the content of Ce³⁺ is less than 0.0050, the luminescence intensity increases gradually with the increase of doping amount, and then decreases gradually. Through further study, it is also found that with more Ce³⁺ doped, the emission spectra gradually move toward the long wavelength direction. The red shift phenomenon should be due to the increase of Ce³⁺ concentration and the difference of radii between Ce³⁺ and Y³⁺ ions, resulting in the increase of crystal field splitting^[30,31]. Meanwhile, the detailed variation trend of the emission peak positions and emission intensities of PLE and PL spectra as the function of Ce³⁺ concentration are provided in Fig. 6(d, f).

Thermal stability is an important factor to ensure the high efficiency of phosphor-converter^[32,33,34]. Fig. 7(a) shows the temperature dependent emission spectra of 0.5at%Ce:YAG from RT to 250 ℃. The decrease of emission intensity with increasing temperature can be explained by the thermal quenching in the configuration coordinate diagram ofFig. 7(d)^[24,35]. The excited luminescence center is thermally activated through phonon interaction and then thermally released through the crossing point between the excited state and the ground state in the configurational coordinate diagram. This non-radial transition probability due to thermal activation is strongly dependent on temperature, resulting in a reduction in emission intensity. In addition, the peak positions of emission spectra in Fig. 7(b) exhibit a slight red shift with temperature increasing: the peak positions at RT and 250 ℃ are 534 and 553 nm, respectively. The red-shift behavior can be explained by the equation for temperature dependence^[36].

where E_T is the energy difference between excited states and ground states at a temperatureT, E₀ is the energy difference at 0 K, and a and b are fitting parameters. At a higher temperature, the bond length between a luminescent center and its ligand ions is increased, resulting in a decreased crystal field. It results in the decrease of the transition energy, and the emission peak is red-shifted with an increase in temperature. The thermal conductivity of ceramics was calculated using κ=α·C_p·ρ, where α, C_p, ρ are the heat conductivity coefficient, specific heat, and density obtained by the method described above, respectively. The relationship between the concentration of Ce³⁺, temperature and thermal conductivity is shown in Fig. 7(c). It can be seen that the thermal conductivity decreases with the increase of doping concentration and measuring temperature.

The temperature distribution of Ce:YAG transparent ceramics could be effectively calculated by steady-state thermal simulation using COMSOL Multiphysics finite element software depended upon the 3D modeling and material thermal properties. The top ofFig. 8 shows a 1 : 1 scale 3D model of COB chip and Ce:YAG transparent ceramics. In order to improve the efficiency of calculation and analysis, the tiny structure details would be ignored. Fig. 8(a, d), (b, e) and (c, f) show the thermal distributions of 0.5at%Ce:YAG transparent ceramics with thickness of 0.2, 0.4, 1.0 mm at steady state or transient state. The thermal distribution of the three packaging modes can be drawn from the simulation results. According to the finite element simulation results, as the thickness of ceramics increases, the surface temperature of ceramics becomes lower, which is the same as the existing research and experiment results ^[19]. This phenomenon can be interpreted as: ceramics with the same surface area emit the same amount of heat through the surface. Thicker ceramics can emit more heat through the sides than thinner ceramics, resulting in a lower surface temperature for the thicker ceramics. Through finite element simulation, a conclusion can be drawn that thicker ceramics are conducive to heat dissipation of white LEDs devices, and increasing the thickness of the sample will reduce the composition of blue light, which will be analyzed in detail below. In addition, three common packaging methods are chosen to simulate, and find that the temperature distribution is similar, which shows that the shape of the sample has little effect on heat dissipation. The square cross-section area is obviously larger than the circular cross-section area, so the blue light in the square ceramic internal scattering process will stimulate more Ce³⁺ ions, which will make the composition of yellow light more, which may lead to slightly different test results of different shapes of ceramics.

Fig. 9 and Fig. 10 depict the luminous characteristics of the prepared transparent ceramics under blue-emitting excitation in the LEDs/LDs lighting system. The emission spectrum of Ce³⁺ is wide in the range of 500-700 nm, and the center is at 550 nm which is due to the electron transition of Ce³⁺ from 5d to 4f. In addition, the electroluminescence spectra clearly suggest that with an increase in the Ce³⁺ doping concentration and sample thickness, the yellow region was relatively increased because of the enhanced ceramics phosphor-conversion ratio from blue to yellow. However, the stronger phosphor- conversion ability means less blue light remaining, which is not good for achieving white-light source (0.33, 0.33). Through a large number of tests, pure white light samples were selected, and the relevant information was recorded in Table 1. The combination of high optical quality transparent ceramics and blue LEDs and LDs not only produces pure white light, but also achieves 122.4 lm/W and 201.5 lm/W when the driving current is 0.01 A and 0.35 A, respectively. The LE value is calculated by dividing the luminous flux by the electrical power. In addition, in order to prove the commercial value of transparent ceramics in the lighting industry, we combine transparent ceramics with LEDs chips to prepare a series of lamps. It can be seen fromFig. 11 that the lamp packaged by changing the concentration and thickness of transparent ceramics perfectly realized the transition from blue light to yellow light.

In this work, the luminescent property of Ce:YAG transparent ceramics and the device test parameters after the combination of Ce³⁺ concentration and transparent ceramics thickness with blue LEDs/LDs are systematically studied. The great potential of Ce:YAG transparent ceramics in high-power lighting applications has proved. A series of lamps are made by combining transparent ceramics with blue LEDs chips. Only by adjusting the concentration of Ce³⁺ and the thickness of ceramics, the color change from white to yellow can be realized to meet the application of various scenes. In addition, this work is expected promote the development of semiconductor lighting devices in more fields with lower energy consumption.