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Abstract
LD chips, combined with phosphor converters, are considered as optimum candidates for high brightness lighting. In this work, Al2O3-YAG:Ce composite ceramics were prepared as the phosphor converters. The sintering temperature and the sintering additives were optimized to obtain the highest quantum efficiency. The effect of the molar ratio of Al2O3 to YAG:Ce on the performance of the ceramics that were excited by the LD chips was studied. The results show that the light extraction efficiency from the ceramic surfaces increases with the increasing molar ratio of Al2O3 to YAG:Ce. Moreover, the improved thermal conductivity caused the ceramics with higher molar ratio of Al2O3 to YAG:Ce show alleviative luminescence saturation and improved reliability.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solid state lightings are considered as superior substitutions of the traditional incandescent and fluorescent lamps in the near future due to their high power efficiency, long lifetime and zero pollution [1–3]. As the original solid state lightings, white light emitting diodes (WLEDs) are typically realized by the combination of blue light emitting diode (LED) chips with yellow-emitting phosphors typically YAG:Ce phosphors encapsulated by organic binders [2]. In spite of the commercialization, WLEDs are not promising for high brightness lighting due to the efficiency droop and large etendue of the blue LED chips. Compared with LED chips, Laser diode (LD) chips show small etendue and alleviative efficiency droop. As a result, the combination of LD chips with phosphor converters is considered as an optimum candidate for high brightness light sources [4,5]. The superiority of LD chips over the LED chips propose higher requirement for the phosphor converters. Besides the high quantum efficiency, the phosphor converters should exhibit high light extraction efficiency, excellent thermal property, alleviative luminescence saturation and improved reliability.
Ce doped Y3Al5O12 (YAG:Ce) is a yellow phosphor with excellent chemical and thermal stability which can emit a broad yellow light upon blue excitation [6]. Organic binder particularly silicone encapsulated YAG:Ce (phosphor in silicone, PIS) is the most widely used phosphor converter in WLEDs [7]. However, the low heat resistance and thermal conductivity hinder the application of PIS in high brightness solid state lighting excited by LD chips. Although glass encapsulated composite YAG:Ce (phosphor in glass, PIG) exhibits improved heat resistance and thermal conductivity, it still cannot meet the requirement of the high brightness lighting [8,9]. To further improve the heat-resistance and thermal conductivity of phosphor converter, luminescence ceramics have received considerable attentions [10–17].
YAG:Ce belongs to cubic system and the pure phase YAG:Ce ceramics without pores show high in-line transmittance in the visible light range which will lead to low absorption of blue light. Furthermore, the lack of scattering centers in the pure phase YAG:Ce ceramics together with the large gap between the YAG:Ce ceramics (n = 1.82) and the air (n = 1) will cause the total internal reflection and result in low light extraction efficiency of the converted light [18–20]. To solve the problems aforementioned, YAG:Ce composite ceramics with the structure of YAG:Ce grains intersected by the second phase have been studied. Al2O3 is thought to be the optimum second phase due to its excellent thermal conductivity, no absorption in the visible light range and well matching with YAG:Ce. Tang et al. fabricated the Al2O3-YAG:Ce composite ceramics and investigated the influence of the mole ratio of Al2O3 to YAG on the luminescence efficiency of WLEDs [12]. Song et al. prepared Al2O3-YAG:Ce composite ceramics from Al2O3 powders and YAG:Ce powders synthesized by coprecipitation method, and investigated the influence of excitation power, Ce3+ ion concentration and the Al2O3 contents on the luminescence property of the composite ceramics excited by blue LD chips [16]. Despite these studies, the characterization of the luminescence property of the composite ceramics is inadequate for the application in brightness light sources and the relationship between the preparation process, microstructure, luminescence property and performance of the composite ceramics is still unclear.
In this paper, a series of Al2O3-YAG:Ce composite ceramics with different molar ratio of Al2O3 to YAG:Ce (denoted as xAl2O3-YAG:Ce, where x is the molar ratio of Al2O3 to YAG in the final ceramics) were prepared by solid state reaction method. The phase and microstructure of the ceramics were characterized by XRD and SEM. Quantum efficiency was investigated to determine the optimal preparation parameter. Thermal diffusivity of the ceramics was investigated to figure out the influence of the molar ratio of Al2O3 to YAG:Ce on the thermal property. Light extraction efficiency, luminescence saturation and reliability of the ceramics were characterized to study the effect of the molar ratio of Al2O3 to YAG:Ce on the performance of ceramics in the application of high brightness lighting. Moreover, the interrelation of preparation process, microstructure, luminescence property and performance was analyzed.
2. Experimental
2.1 Preparation of Al2O3-YAG:Ce composite ceramics
The xAl2O3-YAG:Ce (x is the molar ratio of Al2O3 to YAG in the final ceramics) composite ceramics were prepared by vacuum sintering. Commercially available α phase alumina (α-Al2O3, 99.99%, 200nm), yttria (Y2O3, 99.9%, 50nm) and ceria (CeO2, 99.9%, 50nm) were used as raw materials in a ratio according to the formula of xAl2O3•(Ce0.003Y0.997)3Al5O12 (x = 0, 0.3, 0.7, 1). Butvar B-76Polyvinyl butyral (PVB, M.V. 90000-120000) was used as binder. Compared with no sintering additive, TEOS + MgO (mixture of TEOS with 0.4wt% of all the powders and MgO with 0.1wt% of all the powders) and TEOS (0.4wt% of all the powders) were added as the sintering additives respectively to investigate the influence of sintering additives on the microstructure and quantum efficiency of the ceramics. All of these powders were mixed for 24 h by ball milling with alcohol as the dispersant and alumina balls as the grinding media. After milling, the resulted mixture slurry was dried at 80 °C for 12 h, and then milled by mortar and pestle to obtain the mixture powders. The mixture powders were sieved though the 150 mesh sieve and then uniaxially pressed into disks. The disks were pre-sintered at 900 °C for 6 h to remove the residual organic matter followed by an isostatic cold-pressing under 300 MPa to obtain the green bodies. The green bodies were sintered at 1710-1720 °C for 3 h in vacuum and then annealed at 1500 °C for 12 h in air to remove the oxygen vacancies. To investigate the influence of annealing atmosphere on the quantum efficiency, the ceramics were further annealed at 1400 °C for 6 h in the flowing N2 + H2 (5%) mixed gas. All of the ceramics fabricated under different condition are summarized in Table 1. Allowing for the sintering temperature, the ceramics fabricated under different condition will be referred to as Ceramic n-m, which means Ceramics n shown in Table 1 sintered at m °C in vacuum (Ceramic 1-1720, for instance, means Ceramics 1 sintered at 1720 °C in vacuum).
Table 1. Ceramics number and the preparation process.
2.2 Characterization of Al2O3-YAG:Ce composite ceramics
X-ray powder diffraction (XRD) patterns for phase identification were collected at ambient temperature on a D8 Advance 250 (Cu Kα1 radiation). The 2θ range of all data sets was from 10° to 90° with a step size of 0.02°. Microstructure of the ceramics was observed by a field emission scanning electron microscope (FESEM, SU 8010, Hitachi, Japan) after thermal etching of the surface of ceramics at 1500 °C for 6h. The thermal diffusivity of the ceramics was measured using a laser flash apparatus (LFA 447, Netzsch, Germany). UV-vis-NIR type spectrophotometer (Model Cary 5000, Varian, USA) was used to characterize the in-line transmittance of ceramics with the same thickness and surface state.
The photoluminescence (PL) spectra of the ceramics excited by the blue laser were collected using a fluorescent spectrophotometer (USB 4000, Ocean Optics) equipped with an integrating sphere as shown in Fig. 1. The power of the blue laser was set at 0.05 W with the wavelength of 445 nm. The laser spot on the ceramics was a rectangle with the size of 2*2 mm2 and Gaussian distribution. The low power density of the laser spot was used to exclude the luminescence saturation and characterize the intrinsic quantum efficiency. The ceramics were placed inside the integrating sphere to ensure that all the emitting light from the surface and the end face can be collected. The luminescence efficiency (defined as the integral luminescence power divided by excitation power of the blue laser) and the quantum efficiency was calculated from the PL spectra.
Fig. 1 (a) Schematic diagram and (b) photograph of the test platform for the collection of PL spectra of the composite ceramics.
To investigate the light extraction efficiency and the performance under high excitation power, ceramics with different molar ratio of Al2O3 to YAG were grinded and single side polished to the thickness around 180μm. The polished surface of the ceramics was glued to the surface of the aluminum with high reflectance to prepare the reflection-encapsulated modules. Silicone was used as the glue and the thickness of the glue layer in all the sample was about 1 μm which can be controlled by the dose and the distribution area of the silicone. The light extraction efficiency and the performance of the reflection-encapsulated modules was investigated on the test platform shown in Fig. 2. The reflection-encapsulated modules were attached to the heat sink and excited by the laser source. The emitting light from the surface of the module can enter the integrating sphere from the aperture and be detected by the fluorescent spectrophotometer (USB 4000, Ocean Optics). The wavelength of the laser was 445 nm and the power of the laser was adjustable. The laser spot on the ceramics was fixed as a circle with diameter of 1.1 mm and Gaussian distribution. In the evaluation of the light extraction efficiency, the laser was set as pulse mode with the duty cycle of 10% and the power of 0.565 W to exclude the luminescence saturation. To study the luminescence saturation and the reliability of the ceramics, the excitation laser source was set as continuous module and the luminescence power of the ceramics after being irradiated for 1 min was recorded.
Fig. 2 (a) Schematic diagram and (b) photograph of the test platform to investigate the performance of the ceramics excited by the LD chips.
To investigate the failure of the ceramics under high excitation power, the confocal laser scanning microscopy (KEYENCE, VK-9700) was used to characterize the morphology of the failure surface.
3. Results and discussion
3.1 Phase and microstructure of composite ceramics
The phase of the ceramics was investigated and the XRD patterns of the ceramics are shown in Fig. 3. Figures 3(a) and 3(b) depict the XRD patterns of Ceramics 1-4 sintered with TEOS + MgO as sintering additives at 1710 and 1720 °C respectively. As shown in Fig. 3(a), all the diffraction peaks of Ceramics 1-1710 can be indexed to YAG (PDF card No. 33-0040), indicating that single phase YAG is successfully synthesized at 1710 °C. Besides the diffraction peaks corresponding to YAG, peaks corresponding to Al2O3 (PDF card No. 33-0040) arise in the XRD patterns of ceramics sintered with excessive Al2O3, which demonstrates that composite ceramics with YAG and Al2O3 are obtained. Relative to the intensity of diffraction peaks corresponding to YAG, the intensity of those corresponding to Al2O3 increases with the increasing molar ratio of Al2O3 to YAG:Ce, indicating the increasing phase of Al2O3 in the composite ceramics. All of the diffraction peaks in the XRD patterns are sharp and can be indexed to YAG and Al2O3, indicating that highly crystallized Al2O3-YAG:Ce composite ceramics without impure phase can be prepared at 1710 °C. Further increasing the sintering temperature to 1720°C, the diffraction peaks become narrower as shown in Fig. 3(b), which implies that the crystallinity of the ceramics is improved. To further investigate the influence of sintering additives on the phase of ceramics, the XRD patterns of Ceramics 5-12 sintered at 1720 °C were characterized. As shown in Figs. 3(c) and 3(d), the XRD patterns of the Ceramics 5-12 sintered with TEOS as sintering additives and without sintering additives exhibit similar characteristics as those sintered with TEOS + MgO as sintering additives, which indicates that sintering additives have little effect on the phase and crystallinity of the ceramics.
Fig. 3 XRD patterns of (a) Ceramics 1-4 sintered at 1710 °C, (b) Ceramics 1-4 sintered at 1720 °C, (c) Ceramics 5-8 sintered at 1720 °C and (d) Ceramics 9-12 sintered at 1720 °C.
Figure 4 shows the SEM images of Ceramics 1-4 sintered at 1710 °C using TEOS + MgO as the sintering additives. It can be found that two kinds of grains with different contrast intersect with each other in the images. According to the imaging principle of the SEM and the XRD results, the dark grains can be identified as Al2O3 while the bright grains can be identified as YAG. And the elements distribution characterized by EDS shown in Fig. 5 further demonstrates that.
Fig. 4 SEM images of the thermally etched surfaces of (a) Ceramics 1-1710, (b) Ceramics 2-1710, (c) Ceramics 3-1710 and (d)Ceramics 4-1710.
Fig. 5 SEM image (a) and EDS mappings for the element distribution of (b) Al, (c) O and (d)Y in Ceramic 4-1710.
It can be observed from Figs. 4(a)-4(d) that all the ceramics sintered at 1710 °C exhibit homogeneous microstructure without pores. As shown in Fig. 4(a), several dark grains identified as Al2O3 can be found in the SEM image of Ceramics1-1710 which are supposed to be pure phase YAG. The Al2O3 grains which cannot be detected by XRD due to the small quantity may be attributed to the slight accidental nonstoichiometry of the mixed powders. Increasing the molar ratio of Al2O3 to YAG:Ce to match 0.3Al2O3•(Ce0.003Y0.997)3Al5O12, the average size of YAG grains decreases and Al2O3 grains can be observed uniformly dispersed on the boundaries of YAG grains as shown in Fig. 4(b). Further increasing the molar ratio of Al2O3 to YAG:Ce to 0.7 and 1, the number of Al2O3 grains increases, while the average size of YAG grains decrease continually as shown in Figs. 4(c) and 4(d).
Figure 6 shows the SEM images of Ceramics 1-4 sintered at 1720 °C using TEOS + MgO as the sintering additives. Increasing the sintering temperature from 1710 to 1720 °C, abnormal grain growth can be observed in Ceramics 1 and Ceramics 2 as shown in Figs. 6(a) and 6(b), while still no obvious oversintering can be found in the Ceramic 3 and Ceramics 4 as shown in Figs. 6(c) and 6(d). Allowing for the improved crystallinity and the higher quantum efficiency listed in Table 2, 1720 °C was chosen as the sintering temperature in the following study.
Fig. 6 SEM images of the thermally etched surfaces of (a) Ceramics 1-1720, (b) Ceramics 2-1720, (c) Ceramics 3-1720 and (d) Ceramics 4-1720.
Table 2. The luminescence efficiency and quantum efficiency of the ceramics excited by the 445nm blue laser.
The influence of sintering additive on the microstructure of ceramics is further investigated and the SEM images of Ceramics 5-8 and Ceramics 9-12 sintered at 1720 °C are shown in Fig. 7 and Fig. 8 respectively. Compared with ceramics sintered with TEOS + MgO at 1720 °C, homogeneous grains without abnormal growth can be observed in all the ceramics sintered with TEOS as shown in Fig. 7. The phenomenon indicates that TEOS + MgO is more capable of lowing the sintering temperature. As shown in Fig. 8, the ceramics sintered at 1720 °C without sintering additives exhibit lager grain size compared with those shown in Fig. 7, indicating that the addition of sintering additives is in favor of fine grain size.
Fig. 7 SEM images of the thermally etched surfaces of (a) Ceramics 5-1720, (b) Ceramics 6-1720, (c) Ceramics 7-1720 and (d) Ceramics 8-1720.
Fig. 8 SEM images of the thermally etched surfaces of (a) Ceramics 9-1720, (b) Ceramics 10-1720, (c) Ceramics 11-1720 and (d) Ceramics 12-1720.
It can also be found in Fig. 7 and Fig. 8 that the average size of YAG grains decreases with the increasing molar ratio of Al2O3 to YAG:Ce, which is consistent with that observed in Fig. 4 and Fig. 6. This phenomenon indicates that the excessive Al2O3 as the second phase can inhibit the growth of YAG grains and lead to uniform microstructure with small grain size regardless of the sintering additive. The small grain size may be favorable for the light scattering and thus improve the light extraction efficiency of the ceramics. Moreover, the small grain size may inhibit the extension of the cracks and improve the mechanical property.
3.2 Photoluminescence spectra and quantum efficiency of composite ceramics
The photoluminescence (PL) spectra of the ceramics excited by the 445nm blue laser were investigated. It is found that all the ceramics exhibit the emission peak with similar shape and position, indicating the same luminescence mechanism. The luminescence efficiency (defined as the integral luminescence power divided by exciting power of the blue laser) and the quantum efficiency were calculated from the PL spectra and listed in Table 2. It can be found that the molar ratio of Al2O3 to YAG imposes a slight effect on the luminescence efficiency and the quantum efficiency of the ceramics, indicating the addition of Al2O3 and the corresponding microstructure has little effect on the intrinsic luminescence of YAG:Ce. Nevertheless, the sintering additive has a prominent effect on the luminescence efficiency and the quantum efficiency. With the same molar ratio of Al2O3 to YAG:Ce and sintered at the same temperature, ceramics with TEOS as sintering additives exhibit the optimal quantum efficiency, while the quantum efficiency of ceramics with TEOS + MgO as sintering additives is the lowest. In view of the little difference in the crystallization among the ceramics with different sintering additives, the distinction in the quantum efficiency may be explained by the valence of the cerium ions. As is reported, the tetravalent silicon ions in TEOS can suppress the transformation of cerium ions from trivalence to tetravalence during the annealing process and thus favor the quantum efficiency of the ceramics [21–23]. While the bivalent magnesium ions in the MgO will facilitate the transformation of cerium ions from trivalence to tetravalence and lead to poor quantum efficiency [24,25]. To further support this speculation, the ceramics were then annealed in N2 + H2 mixed gas and the quantum efficiencies were characterized as shown in Table 2. As the annealing in N2 + H2 mixed gas can reduce the cerium ions from tetravalence to trivalence, the improved quantum efficiency after annealing in N2 + H2 verifies the inference aforementioned. While the annealing temperature and time need to be optimized further, Ceramics 5-8 sintered at 1720 °C with TEOS as sintering additive before annealing in N2 + H2 mixed gas were used for the following investigation.
3.3 Influence of molar ratio of Al2O3 to YAG:Ce on light scattering and thermal properties
To evaluate the influence of the molar ratio of Al2O3 to YAG:Ce on the light scattering of the ceramics, the in-line transmittance of Ceramics 5-8 sintered at 1720°C with the same thickness and surface state was characterized. As shown in Fig. 9, two absorption bands deriving from the 4f →5d2 and 4f →5d1 transition of Ce3+ can be found at about 340 nm and 458 nm in all the ceramics. The decrease of in-line transmittance of all the ceramics with the decreasing wavelength indicates the existence of the scattering centers such as grain boundaries and Al2O3 grains as shown in Fig. 7. While the prominent decrease of the in-line transmittance with the increasing molar ratio of Al2O3 to YAG:Ce from Ceramics 5 to Ceramics 8 indicates an improvement of the light scattering which can be attributed to the increasing number of Al2O3 grains and the decreasing average size of YAG grains.
Thermal property of the ceramics is a critical parameter for the application in high brightness lighting. To evaluate the effect of the molar ratio of Al2O3 to YAG:Ce on the thermal property of the ceramics, the density and the thermal diffusivity of Ceramics 5-8 sintered at 1720 °C were characterized.
The density of the ceramics was measured by Archimedes method and the relative density was calculated. As shown in Table 3, all the ceramics with different molar ratio of Al2O3 to YAG:Ce show the relative density higher than 99%, which indicates dense microstructure with few pores and is consistent with the SEM images shown in Fig. 7.
Table 3. Density and thermal property of the ceramics with different molar ratio of Al2O3 to YAG:Ce.
The thermal diffusivity of Ceramics 5-8 sintered at 1720 °C was characterized by the laser flash method and the corresponding thermal conductivity was calculated using Eq. (1) where is the density and is the specific heat capacity of the ceramics. The specific heat capacity of the Al2O3-YAG:Ce composite ceramics can be estimated by Eq. (2). In the Eq. (2), and are the weight percentages of Al2O3 and YAG:Ce respectively, while and are the corresponding theoretical specific heat capacities of Al2O3 and YAG:Ce.
As shown in Table 3, the thermal diffusivity and the thermal conductivity both increase with the increasing molar ratio of Al2O3 to YAG:Ce, indicating that the Al2O3 in the composite ceramics is in favor of the thermal property.
3.4 Performance of ceramics in the application of high brightness lighting
In order to investigate the effect of the molar ratio of Al2O3 to YAG:Ce on the performance of ceramics in the application of high brightness lighting, Ceramics 5-8 sintered at 1720 °C were encapsulated as reflection modules and investigated on the test platform shown in Fig. 2.
In practical application of high brightness lighting, only the light emitting from the surface of the ceramics can be collected for use and therefore the luminescence power of the reflection modules from the surface of the ceramics was characterized. The excitation laser was adjusted to low power and set as pulse mode with the duty cycle of 10% to exclude the thermal effect. As shown in Fig. 10, the luminescence power from the surface of the ceramics increases significantly with the increasing molar ratio of Al2O3 to YAG:Ce. As the molar ratio of Al2O3 to YAG:Ce has little effect on the quantum efficiency of the ceramics (analyzed in part 3.2), the difference in the luminescence power may be attributed to the light extraction efficiency from the surface. In the measurement of the quantum efficiency, the ceramics were placed inside the integrating sphere and all the light emitted from the surface and the end face of the ceramics can be collected. While only the light emitted from the surface can enter the integrating sphere and be collected in the measurement of the luminescence power of the reflection modules. As is well known, the lack of scattering centers and the high refractive index of the pure phase YAG:Ce ceramics lead to the waveguide effect, which will make a large amount of light emit from the end face and result in low light extraction efficiency from the surface of the ceramics [18–20]. With the increase of the molar ratio of Al2O3 to YAG:Ce, the light scattering is improved, which will alleviate the waveguide effect and improve the light extraction efficiency. As a result, despite the approximate quantum efficiency, the luminescence power of the encapsulated ceramics placed outside the integrating sphere increases with the increasing molar ratio of Al2O3 to YAG:Ce. From the analysis above, it can be found that the higher molar ratio of Al2O3 to YAG:Ce is favorable for the improvement of the light extraction efficiency from the surface and thus for the practical application of the ceramics.
Fig. 10 Luminescence powers of the ceramics with different molar ratio of Al2O3 to YAG:Ce encapsulated as reflection modules under pulse laser excitation with the duty cycle of 10%.
To study the luminescence saturation and the reliability of the ceramics, the excitation laser source was set as continuous module and the excitation power was increased until the ceramics were destroyed. The luminescence power of the ceramics after being irradiated for 1 min under different excitation power was recorded.
Figure 11 shows the dependence of the luminescence power on the excitation power of Ceramics 5-8 encapsulated as reflection modules. It can be found that the luminescence power increases with the increasing molar ratio of Al2O3 to YAG:Ce under the same excitation power which is in consistent with the trend revealed in Fig. 10. For Ceramics 5-7 with the molar ratio of Al2O3 to YAG:Ce from 0 to 0.7, the luminescence power almost linearly increases until the excitation power increases to 29 W. Further increasing the excitation power to 32 W, the increase of luminescence power slow down and saturation can be observed. When the excitation power is 34 W, Ceramics 5-7 are destroyed. For Ceramics 8 with the maximum molar ratio of Al2O3 to YAG:Ce, saturation cannot be observed until the excitation power is 34 W and the ceramics are destroyed at 36 W of the excitation power. The dependence of the luminescence saturation on the molar ratio of Al2O3 to YAG:Ce may be attributed to the thermal conductivity. As is well known, the Stokes shift during the luminescence process will cause thermal effect which may decrease the luminescence efficiency and even destroy the material [26,27]. Due to the similar emission spectra and quantum efficiency, the heat generated during the photoluminescence process is similar for Ceramics 5-8 with different molar ratio of Al2O3 to YAG:Ce. Allowing for the increasing thermal conductivity, ceramics with higher molar ratio of Al2O3 to YAG:Ce can dissipate the heat faster, and thus alleviate the luminescence saturation.
Fig. 11 Luminescence powers of the ceramics encapsulated as reflection modules under different excitation power of the blue laser.
Figure 12 shows the confocal laser scanning microscopy images of the damaged surface of ceramics destroyed by the high power excitation. A circular region and several cracks can be found on the damaged surface of each ceramics. The circular region can be attributed to the damage of glue layer in reflection modules caused by the accumulated heat within the luminescence area, while the cracks arise from the thermal stress caused by the large temperature difference between the inside and outside of the luminescence area. The circular region decreases with the increasing molar ratio of Al2O3 to YAG, which may be attributed to the increasing number of scattering centers. As interpreted above, the increasing number of scattering centers in the ceramics with higher molar ratio of Al2O3 to YAG will suppress waveguide effect and lead to a smaller luminescence area even with the identical excitation spot. The smaller luminescence area will result in a smaller heated area and thus a smaller radial heat flux perpendicular to the sample surface. As a result, the accumulated heat under high power excitation can be confined to a smaller area in the ceramics with higher molar ratio of Al2O3 to YAG, which will lead to the smaller circular region as shown in Fig. 12. The cracks also decrease with the increasing molar ratio of Al2O3 to YAG, which may be attributed to the increasing thermal conductivity. Due to the higher thermal conductivity, ceramics with higher molar ratio of Al2O3 to YAG exhibit smaller temperature difference between the inside and outside of the luminescence area and thus smaller thermal stress. As a result, the cracks decrease with the increasing molar ratio of Al2O3 to YAG. Moreover the smaller grain size in ceramics with higher molar ratio of Al2O3 to YAG (as shown in Fig. 7) may also inhibit the propagation of the cracks and result in the smaller cracks.
Fig. 12 Confocal laser scanning microscopy images of the damaged surface of (a) Ceramics 5, (b) Ceramics 6, (c) Ceramics 7 and (d) Ceramics 8.
The study of the performance of ceramics encapsulated as reflection modules indicates that ceramics with higher molar ratio of Al2O3 to YAG:Ce are promising in high brightness lighting for the improved light extraction efficiency, alleviative luminescence saturation and enhanced reliability.
4. Conclusions
A series of Al2O3-YAG:Ce composite ceramics with different molar ratio of Al2O3 to YAG:Ce were prepared by solid state reaction method. The preparation parameters were optimized to obtain the highest quantum efficiency. It is proved that the sintering additive has a prominent effect on the quantum efficiency and ceramics with TEOS as sintering additive exhibit the optimal quantum efficiency. Despite the similar quantum efficiency, the performance of ceramics in the application of high brightness lighting varies significantly with the molar ratio of Al2O3 to YAG:Ce. With the increasing molar ratio of Al2O3 to YAG:Ce, the scattering center in the ceramics increase, which alleviates the waveguide effect and improves the light extraction efficiency from the surface of the ceramics. Moreover, due to the improved thermal conductivity, ceramics with higher molar ratio of Al2O3 to YAG:Ce show alleviative luminescence saturation and improved reliability. The effect of the molar ratio of Al2O3 to YAG:Ce on the performance of ceramics indicates that ceramics with higher molar ratio of Al2O3 to YAG:Ce are promising in high brightness lighting.
Funding
National Key R&D Program of China (Grant No. 2018YFB0406800).
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