Yellow YAG-Ag/Al2O3 film applied to blue laser illumination

The most popular light-emitting component for laser illumination right now is phosphor in glass (PiG), however its major weakness is the poor heat conductivity. In this research, the yellow YAG:Ce phosphor and Ag paste were co-fired on an Al2O3 substrate using the blade-coating approach to create a PiM (phosphor in metal) film. Due to the ultra-high thermal conductivity of Ag, the highest blue laser power density that YAG-Ag PiM can achieve is 9.2 W·mm−2, and the maximal luminous flux, lumen efficiency and CCT (correlated color temperature) are 620 lm, 172 lm·W−1 and 6244 K, respectively.


Introduction
Laser illumination [1] with no efficiency droop [2] , super-brightness [3] and high-collimation, has achieved brilliant development in automotive head-light [4] , road lighting [5] and aerospace lighting [6] .The two most common optical structures in laser illumination are reflective structure and transmissive structure [7] .A heat-conducting substrate and a luminescent layer on the substrate make up the reflective structure, and the luminescent layer is made of phosphor and a connecting medium [8] .Under laser excitation, the luminescent layer will generate a large amount of heat to cause a sharp rise in temperature.For reflective structures, the higher the thermal conductivity of the luminescent layer, the easier the heat can be dissipated.Since the heat capacity of the heat-conducting substrate can be designed sufficiently large, the reflective structure has great application prospects as an optical structure for laser illumination [9] .
In reflective structures, the thermal conductivity of the luminescent layer is primarily determined by the connecting medium [10] .Low-melting glass powder is the most widely used connecting medium [11] .The technology solution that phosphor (typically Y3Al5O12:Ce, YAG:Ce) and low-melting-point glass powder form the PiG luminescent layer combined with the Al substrate (PiG-Al), which has become the most extensively researched reflective structure today [12] .It has been reported that YAG:Ce PiG-Al could achieve a light output of 430 lm under the excitation of 4 W blue laser [13] .However, the glass matrix made from the low-melting point glass powder has a low thermal conductivity (~1 W• m -1 •K -1 ), therefore it's unable to efficiently carry away the heat generated by the luminescent layer in time [14] .At the same time, given the significant difference in expansion coefficients between the glass matrix and the Al substrate, the PiG luminescent layer is prone to cracking and delamination [15] .
In this work, to overcome the drawback of PiG, we relied on the blade-coating technique to fabricate a reflective optical component with Ag as the connecting medium, Al2O3 ceramic sheet as the substrate, and YAG:Ce-Ag as the luminescent layer.Benefiting from the ultrahigh thermal conductivity (429 W• m -1 •K -1 ) and suitable ductility of Ag, the reflective laser illumination device with PiM as the luminescent component exhibits high laser energy tolerance characteristics, and the maximum acceptable laser power density is 9.2 W• mm -2 .Simultaneously, the effects of the ratio of phosphor (YAG:Ce) to Ag (PtM, weight ratio) and the thickness of PiM on the luminescent performance are also discussed, respectively.

Experimental section
The manufacturing method of PiM is almost exactly the same as that of PiG, published by Deng [16] .The only distinction is that Ag paste is used to replace low-melting-point glass powder.In brief, the approximate synthesis process is as follows: the homogeneously mixed YAG:Ce-Ag paste is coated on the Al2O3 ceramic substrate (26 mm radius and 1mm thickness) using the blade-coating technique, and the luminescent layer (YAG:Ce-Ag) thickness is controlled by the number of stacked tapes (Scotch® MagicTM Tape, the thickness is 35 μm).The above samples were subsequently cured at 120°C for 1 hour and eventually sintered at 900°C for 20 minutes to obtain the YAG:Ce-PiM.
The X-ray diffraction and SEM morphology of the samples were characterized by using the Bruker D8 ADVANCE and SU-70, respectively.Temperature-dependent luminescence measurement system consists of THMS 600E (Linkam Scientific Instruments) and USB 2000+ (Ocean Optics).The model of the blue laser source is LSR445CP-FC-48W (Lasever).12 inch integrating spherec (Labsphere), and laser power meter (LP-3C, Physcience Opto-Electronics) are used to collect the spectrum/lumen value and laser power density of the device, respectively.The X-ray diffraction patterns of PiM were shown in Figure 1 (a).The diffraction patterns of samples with different PtM ratios show not much variation (the weight ratios of YAG:Ce to Ag are: 9:1, 8:2, 7:3, and 6:4 et al, respectively), and the diffraction peaks of YAG:Ce and Ag can be observed in simultaneous (the impurity peak is due to the Al2O3 substrate).This demonstrates that the co-firing process failed to affect the crystal structure of the YAG:Ce, while the Ag did not oxidize and remained in the form of simple substance.Figure 1 (b) shows the SEM image of P5M5.Obviously, there is a clear boundary between the YAG:Ce particles and the Ag matrix, and the YAG:Ce particles are embedded on the Ag matrix in a manner similar to a gem ring.The mosaic method is tightly combined, assuring the mechanical stability of the luminescent layer, which is beneficial for realizing high quality laser illumination sources.

Microstructural and Optical Properties of the YAG:Ce-PiM
The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of PiM are shown in Figure 1 (c).PL and PLE of different PiM are almost identical except for intensity.Under the excitation of 450 nm, the broad emission band peaking is located at ~560 nm.The luminescence intensity of PiM will decrease as Ag concentration increases.When PtM ratio is changed from P8M2 to P5M5, the luminous intensity reduces by 25%.PiM has a high thermal quenching properties, the luminous intensity of PiM at 200°C can still be maintained at ~85% of that at room temperature, which is substantially similar with the performance of YAG:Ce (Figure 1 (d)).The robust thermal quenching capability of PiM is essential for laser illumination, which allows PiM to resist higher laser power density.For PiM film, increasing the Ag concentration will result in higher thermal conductivity and mechanical stability of the luminescent layer.On the other hand, lowering the phosphor content (corresponding to high concentration of Ag) will lead to a decline in the luminescence intensity of PiM film.Therefore, it is necessary to explore an optimal PtM ratio to achieve high thermal conductivity while maintaining luminescence intensity to a large extent.We researched luminescence saturation performance of different PtM ratio films in reflective structure.As the laser power rises (Figure 2(a)), the P7M3 is capable of reaching a maximum luminous flux of 620 lm and a luminous efficiency of 172 lm• W -1 .Despite the fact that P5M5 does not appear to reach saturation, it is abandoned since the color coordinates (0.2705, 0.2393) is beyond the range of white light.As a consequence, through practical laser testing, we found that the P7M3 is the optimal ratio for PiM film.Further, the optical properties of PiM films have been evaluated in terms of thickness (Figure 2 (b)).The luminous flux is nearly half as thickness increases.This is determined by the reflective heat dissipation characteristics.Adding the thickness increases the heat transfer distance and consequently reduces the overall thermal conductivity of luminescent layer.As shown in Figure 2(c), the emission intensity of P7M3 displays a tendency to initially increase and then decrease, peaking at around 3.59W, with the rise in incident laser power.The decline can be attributed to thermal saturation caused by sharp heat accumulation in high-power laser excitation.The corresponding color coordinates, as illustrated in Figure 2 (d), move continually towards the blue light direction.In saturation state, the CIE color coordinates and CCT of P7M3 are (0.3139, 0.3374) and 6244 K, respectively.These results validate that the YAG-Ag PiM film is a promising yellow-emitting color converter for blue laser-driven illumination.

Conclusion
In this work, a color converter of PiM by co-firing YAG:Ce phosphor with Ag paste was created.The influence of PtM ratio and PiM thickness on optical properties is systematically investigated.The maximum tolerable laser power density of PiM is 9.2 W• mm -2 , with a PtM ratio of P7M3 and a thickness of 31 μm.The corresponding lumen output, luminous efficacy, CIE, CRI and CCT are 620 lm, 172 lm• W -1 , (0.3139, 0.3374), 67 and 6244 K, respectively.Compared with the existing PiG forms, we believe that the PiM reflective structure has a broader potential in high-power laser illumination.

Figure 1 .
Figure 1.(a) XRD patterns of the Ag, YAG:Ce powder and YAG-PiM film, (b) SEM image (inset is the single YAG powder image), (c) PL/PLE spectra, (d) Temperature-dependent emission intensity of YAG:Ce powder and PiM (the inset is a photo of YAG-Ag/Al2O3).The X-ray diffraction patterns of PiM were shown in Figure1 (a).The diffraction patterns of samples with different PtM ratios show not much variation (the weight ratios of YAG:Ce to Ag are: 9:1, 8:2, 7:3, and 6:4 et al, respectively), and the diffraction peaks of YAG:Ce and Ag can be observed in simultaneous (the impurity peak is due to the Al2O3 substrate).This demonstrates that the co-firing process failed to affect the crystal structure of the YAG:Ce, while the Ag did not oxidize and remained in the form of simple substance.Figure 1 (b) shows the SEM image of P5M5.Obviously, there is a clear boundary

Figure 2 .
Figure 2. (a) Luminous flux and luminous efficiency of YAG-PiM films with different PtM ratios, (b) Luminous flux variations of P7M3 at different thicknesses, (c) PL of P7M3 (the inset is a photo of the device in operation), (d) CIE color coordinates of the P7M3 with the increase of incident blue laser power.For PiM film, increasing the Ag concentration will result in higher thermal conductivity and mechanical stability of the luminescent layer.On the other hand, lowering the phosphor content (corresponding to high concentration of Ag) will lead to a decline in the luminescence intensity of PiM film.Therefore, it is necessary to explore an optimal PtM ratio to achieve high thermal conductivity while maintaining luminescence intensity to a large extent.We researched luminescence saturation performance of different PtM ratio films in reflective structure.As the laser power rises (Figure2(a)), the P7M3 is capable of reaching a maximum luminous flux of 620 lm and a luminous efficiency of 172 lm• W -1 .Despite the fact that P5M5 does not appear to reach saturation, it is abandoned since the color coordinates (0.2705, 0.2393) is beyond the range of white light.As a consequence, through practical laser testing, we found that the P7M3 is the optimal ratio for PiM film.Further, the optical properties of PiM films have been evaluated in terms of thickness (Figure2 (b)).The luminous flux is nearly half as thickness increases.This is determined by the reflective heat dissipation characteristics.Adding the thickness increases the heat transfer distance and consequently reduces the overall thermal conductivity of luminescent layer.As shown in Figure2(c), the emission intensity of P7M3 displays a tendency to initially increase and then decrease, peaking at around 3.59W, with the rise in incident laser power.The