High power laser-driven ceramic phosphor plate for outstanding efficient white light conversion in application of automotive lighting

We report on Y3Al5O12: Ce3+ ceramic phosphor plate (CPP) using nano phosphor for high power laser diode (LD) application for white light in automotive lighting. The prepared CPP shows improved luminous properties as a function of Ce3+ concentration. The luminous properties of the Y3Al5O12: Ce3+ CPP nano phosphor are improved when compared to the Y3Al5O12: Ce3+ CPP with bulk phosphor, and hence, the luminous emittance, luminous flux, and conversion efficiency are improved. The Y3Al5O12: Ce3+ CPP with an optimal Ce3+ content of 0.5 mol % shows 2733 lm/mm2 value under high power blue radiant flux density of 19.1 W/mm2. The results indicate that Y3Al5O12: Ce3+ CPP using nano phosphor can serve as a potential material for solid-state laser lighting in automotive applications.

Scientific RepoRts | 6:31206 | DOI: 10.1038/srep31206 techniques such as co-precipitation, sol-gel, and combustion and spray pyrolysis methods [14][15][16][17] . These have been proposed as lower temperature synthesis methods for achieving the nano-structured single phase. Many scientists reported the luminous efficacy of Y 3 Al 5 O 12 : Ce 3+ CPP in LED applications when using bulk Y 3 Al 5 O 12 : Ce 3+ 18-20 . However, the Y 3 Al 5 O 12 : Ce 3+ CPP for white light generation in blue LD applications such as display, lamp, light fidelity, and automotive lighting sections have yet to be investigated in detail 21,22 . In this paper, we report our advanced research on Y 3 Al 5 O 12 : Ce 3+ CPP using nano-structured Y 3 Al 5 O 12 : Ce 3+ precursor directly combined with blue LDs for white light generation. We substitute the mixture of organic resin with phosphor powder to Y 3 Al 5 O 12 : Ce 3+ CPP. We also optimize the optical properties of Y 3 Al 5 O 12 : Ce 3+ CPP.

Experimental
Preparation of the Nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphors. Nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphors were synthesized using the forced hydrolysis method. First, aluminum nitrate nonahydrate (Sigma-Aldrich, ≥98% pure), aluminum sulfate octadecahydrate (Sigma-Aldrich, ≥98% pure) and urea (Sigma-Aldrich, 99.5%) were used to obtain the nano-structured Al(OH) 3 particles. These materials were dissolved in deionized water and aged at 98 °C for 4 h. The precipitate was then separated through centrifugation and washed several times with both deionized water and ethanol. Next, yttrium nitrate (Sigma-Aldrich, 99.8%), cerium nitrate (Sigma-Aldrich, 99%), and urea (Sigma-Aldrich, 99.5%) were used to dissolve the above materials in deionized water. The synthesized Al(OH) 3 particles were homogeneously dispersed in the mixed solution, which was vigorously stirred at 98 °C for 3 h. The precipitate was isolated via centrifugation, washed with both deionized water and ethanol, and then dried using a lyophilizer. The resulting powder was annealed at 1200 °C for 4 h under a reducing nitrogen atmosphere containing 5% H 2 gas. To compare the luminous properties, we synthesized bulk type Y 3 Al 5 O 12 : Ce 3+ phosphor via solid state reaction method at at 1450 °C for 12 h under same condition.
Fabrication of the Y 3 Al 5 O 12 : Ce 3+ CPP. The procedure for fabricating the Y 3 Al 5 O 12 : Ce 3+ CPP is shown in Fig. 1. The fabrication of Y 3 Al 5 O 12 : Ce 3+ CPP was carried out by ball milling nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphor and SiO 2 as a sintering aid. During this process, ethanol was used as a solvent at a weight ratio of powder:ethanol (1:5). All materials were mixed and milled with a ball milling machine using ZrO 2 balls with a diameter of 5 mm for 24 h. These mixtures were dried at 90 °C for 12 h and compressed into a pellet using uniaxially pressed under 20 MPa with a diameter of 10 mm and a thickness of 5 mm. The pellets were fired at 1000 °C for 6 h under air atmosphere to remove organic materials and then cold isostatically pressed under 300 MPa for 30 min. The green bodies were sintered with a graphite-heated vacuum furnace (10 −3 Pa) at 1600 °C for 12 h. The  aligned to 10° ≤ 2θ ≤ 80°. The excitation and emission spectra of the nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphor were analyzed by room-temperature photoluminescence spectrometry (PL, PSI Co., Ltd/Korea), using a 500 W xenon discharge lamp as an excitation source. The surface morphology and the compositions of the Y 3 Al 5 O 12 : Ce 3+ CPP was observed by field-emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL, Japan). The luminous emittance, luminous flux and conversion efficiency of Y 3 Al 5 O 12 : Ce 3+ CPP was measured with double integrating spheres (PSI Co., Ltd/Korea) under blue laser excitation (3.5 W LD x 8, 445 nm blue LD) using handmade equipment from LG electronics.

Results and Discussion
The crystal structure of Y 3 Al 5 O 12 : Ce 3+ CPP was initially analyzed using the powder XRD with a CuKα radiation of λ = 1.5406 Å at 10° ≤ 2θ ≤ 80° as shown in Fig. 2a. The XRD patterns of the obtained sample at 2θ = 18.1°, 27.6°, 29.7°, 33.3°, 36.4°, 41.0°, 46.6°, 55.1° and 57.4° were completely indexed pure Y 3 Al 5 O 12 phase (JCPDS No. 33-0040) without any peaks assigned to the CeO 2 , Y 2 O 3 , Al 2 O 3 , YAlO 3 , or Y 2 Al 4 O 9 phases, which indicate a cubic garnet crystal structure with lattice parameter of 1.2008 nm. The excitation and emission spectra of nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphor using a xenon lamp as an excitation source are displayed in Fig. 2b. In previous work, the electronic transition of Ce 3+ ion constitutes 4f 1 in the ground state and 4f°5d 1 in the excited state 23 . The ground state is split into a doublet of two excitation bands at 338 and 450 nm, which was attributed to electronic transition from 2 F 5/2 to the split excited 5d states band in the ground state of the Ce 3+ ion 24 . The ground state is split into a doublet of 2 F 7/2 and 2 F 5/2 because of spin-orbit interactions and the excited state is also split from the crystal field, which is affected by the surrounding ligand ions 25 . Ce 3+ emission involves parity-and spin-allowed 5d → 4f electronic transition and consists of a yellow emitting primary band at 537 nm as well as a shoulder on the longer wavelength side, which is ascribed to electronic transition from 5d → 2 F 5/2 and from 5d → 2 F 7/2 Ce ions, respectively 26 . Also, in comparison with bulk Y 3 Al 5 O 12 : Ce 3+ phosphor, the emission wavelength of the nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphor is shifted to the blue region. This phenomenon is explained as a crystal field strength around Ce 3+ ion was somewhat reduced 27 . The luminescence in a nano-structured Y 3 Al 5 O 12 : Ce 3+ phosphor is due to the transition between the energy levels of Ce 3+ atoms as the luminescent center. After the sintering process, the surface of Y 3 Al 5 O 12 : Ce 3+ CPP (1.5 mm X 1.5 mm) was uniformly mirror-polished with a thickness of 100 μm to convert the highly bright white colour via electroluminescence (EL) spectra as shown in Fig. 2c. Also, the real image of Y 3 Al 5 O 12 : Ce 3+ CPP under blue laser pointer (1 W) is indicated in the inset in Fig. 2c. The surface imgae of Y 3 Al 5 O 12 : Ce 3+ CPP is shown in Fig. 2d. EDX composition analyses revealed the presence of oxygen (O), yttrium (Y), and aluminum (Al) elements in Fig. 2e. This result is supported by XRD analysis.
The final goal of this study is to use Y 3 Al 5 O 12 : Ce 3+ CPP in 445 nm emitting blue LDs for white light generation in automotive lighting. To achieve this goal, it is essential to analyze the luminous properties including the luminous emittance, luminous flux, conversion efficiency, emission spectra, CRI, and CCT of the prepared samples under blue laser excitation at 445 nm. Figure 3a presents the luminous emittance of the white LD package under blue laser excitation at 445 nm. With increasing blue incident power, the luminous emittance of Y 3 Al 5 O 12 : Ce 3+ CPP is linearly increased. This is attributed to the increased number of electrons pumped up to the excited state of Ce 3+ ions as a function of the increased blue incident power. The maximum luminous emittance value for Y 3 Al 5 O 12 : Ce 3+ CPP is using the nano-structure Y 3 Al 5 O 12 : Ce 3+ phosphor obtained at a blue radiant flux density of 14.77 W/mm 2 . This means that the Y 3 Al 5 O 12 : Ce 3+ ceramic phosphor plate can be applied as a yellow emitting converter in LD applications. However, luminous saturation comes about at high blue radiant flux density due to the thermal quenching effect, which implies that the luminance of the Y 3 Al 5 O 12 : Ce 3+ CPP cannot be improved further. Figure 3b depicts the conversion efficiency of Y 3 Al 5 O 12 : Ce 3+ CPP with increasing blue incident power. When the incident power increases, the conversion efficiency decreases gradually. Both the luminous flux and conversion efficiency of Y 3 Al 5 O 12 : Ce 3+ CPP using the nano-structure Y 3 Al 5 O 12 : Ce 3+ phosphor indicates better luminous properties than those of bulk Y 3 Al 5 O 12 : Ce 3+ phosphor. This is ascribed to the effect of temperature during the fabrication of ceramic phosphor plates. Because the sintering process for ceramic phosphor plates is relatively culminated, the packing density of CPP using nano-structure Y 3 Al 5 O 12 : Ce 3+ phosphor results in more outstanding luminous properties than for bulk Y 3 Al 5 O 12 : Ce 3+ in Table 1. In nano-structure Y 3 Al 5 O 12 : Ce 3+ phosphors, the micro structures of the ceramic phosphor plate are more perfectly formed than bulk Y 3 Al 5 O 12 : Ce 3+ . The densification and grain growth also gradually improved because of higher purity nano precursors, homogeneity, low degree of agglomeration, good crystallinity and low temperature sintering. These results shows that CPP using the nano-structure Y 3 Al 5 O 12 :Ce 3+ phosphors have a finer and more uniform microstructure, as shown in Supplementary Information Fig. S1.   Figure 4a compares the changes in luminous emittance as a function of Ce 3+ concentration under a blue laser diode at 445 nm. Luminous emittance increased with increasing amounts of Ce 3+ , up to about 0.5 mol%, and the highest intensity was observed for CPP with 0.5 mol% Ce 3+ ions. The value of illuminance is 2733 lm/mm 2 . On the other hand, the illuminance value was degraded by 1 mol% Ce 3+ ions. This means that the luminous emittance decreases with increasing blue incident power at 14.2 W/mm 2 . The Supplementary Information in Fig. S2 suggests that Y 3 Al 5 O 12 : Ce 3+ can be applied as white conversion materials with high blue incident power density. As demonstrated in Fig. 4b, the white conversion efficiency of Y 3 Al 5 O 12 : Ce 3+ CPP is improved with increasing blue incident power. When the blue incident power is 19.1 W/mm 2 , the conversion efficiency decreases abruptly from 218 lm/W to 120 lm/W due to thermal quenching. Figure 4c presents the CIE colour coordinates. The calculated CCT value of the nano-structured Y 3 Al 5 O 12 : Ce 3+ CPP with 0.5 mol% Ce 3+ ions was 5994 K. This is attributed to the daylight (5500-6000 K) for adaptive lighting regulation.
We believe that CPP using the nano-structure Y 3 Al 5 O 12 :Ce 3+ will act as a candidate for the automotive lightings.

Conclusion
In summary, we fabricated Y 3 Al 5 O 12 : Ce 3+ CPP using nano-structured phosphor under high-power blue radiant flux for white generation in automotive lighting. The synthesis technique, crystal structure, and luminous characteristics are investigated. Nano-structure based Y 3 Al 5 O 12 : Ce 3+ CPP exhibited an increase in luminous emittance, luminous flux and conversion efficiency compared to Y 3 Al 5 O 12 : Ce 3+ CPP with bulk phosphor. By applying the Y 3 Al 5 O 12 : Ce 3+ CPP to blue LD chips, we obtained suitable white light with 2733 lm/mm 2 , 1424.6 lm, 218 lm/W, CRI value of 54.2 and CCT value of 5994 K. We believe that Y 3 Al 5 O 12 : Ce 3+ CPP using nano phosphor is one of the simplest ways to obtain high luminous properties. Thus, our results suggest that this material can potentially serve as remote phosphors in phosphor converted LDs for white light generation.