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1、英文原文Preparation and Optical Properties of Transparent Ce:YAG Ceramics for High Power White LEDS. Nishiura1, S. Tanabe 1, K. Fujioka2, Y. Fujimoto2 and M. Nakatsuka21 Graduate School of Human and Environmental Studies, Kyoto University,Yoshida nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan2 Institut
2、e of Laser Engineering, Osaka University, 2-6 Yamada-oka,Suita, Osaka 565-0871, JapanE-mail: s.nishiuranctdd.mbox.media.kyoto-u.ac.jp Abstract. Transparent Ce:YAG ceramics were synthesized from the Ce:YAG powder which was produced by co-preparation method of the hydroxides. The Ce:YAG ceramics exhib
3、it a broad emission band peaked at 530 nm due to the 5d4f transition of Ce3+. The transmittances of the samples obtained were 70 87 % at 800 nm. The absorption coefficient and emission intensity of Ce3+ were increased with the increase of the thickness. With increasing thickness of the sample, the c
4、olor coordinates of the Ce:YAG ceramics under 465 nm LED excitation shifted from the blue region to the yellow region with passing nearby the theoretical white point. The highest value of luminous efficacy of the white LED was 73.5 lm/W.1. IntroductionWhite LEDs are now extensively spotlighted as ge
5、neral illumination as well as backlight of the liquid crystal display (LCD), car headlights and indicator light. The advantages of the LEDs are low electric consumption, high brightness and long lifetime 1. The most common method to make the white LEDs is combining of a blue LED and Ce:Y3Al5O12 (Ytt
6、rium Aluminum Garnet) (Ce:YAG) phosphor packed with organic resins 2. The function of the Ce:YAG phosphor is to absorb the blue light emitted from the blue LED and converts it to yellow light. The phosphor has a broad band emission due to the 5d4f transition of Ce3+. The combination of two lights of
7、 the transmitted blue light and the fluorescent yellow light makes white light. However, this white LED has a disadvantage of thermal degradation of organic resins, which is caused by heat generation of LED chip. As a result, it catches up with the degradation of luminous intensity and the change of
8、 emission color. In order to solve this problem, a new type white LED using a glass ceramic phosphor has been studied 3-5. The merits of this inorganic material are excellent heat-resistance compared with polymers,For the merits mentioned above, this approach using inorganic materials is useful for
9、the white LED. To further enhance the performance of the white LEDs, it is important to develop new inorganic materials that have superior optical property than that of glass ceramic phosphor. We have interest in transparent polycrystalline ceramics that have been studied as laser and scintillator m
10、aterials 6. They have attracted a great attention because the optical property and the efficiency are comparable or superior to those of single crystals. They have advantages of high homogeneity, optical transparency and thermal conductivity.The transparent polycrystalline ceramics have been fabrica
11、ted by solid state reaction using the powder of the same chemical composition. To achieve transparent polycrystalline ceramics of high quality, a fine powder with no or little agglomeration is necessary. Typically, wet chemical methods have been used for powder synthesis. The co-precipitation method
12、, which is one of the wet chemical methods, is possible to produce a fine powder of multi cation oxides with large scale 7.In this study, transparent polycrystalline ceramics of Ce:YAG are reported. As noted above, the Ce:YAG phosphor is suitable for white LEDs. Besides, since the unit cell of the Y
13、AG crystal is a cubic structure, it is easy to fabricate the transparent polycrystalline ceramics of the YAG crystal with high transparency in comparison with that of some crystals having other crystal symmetry. The coprecipitation method was used for synthesis of the YAG powder. We also measured th
14、e optical properties such as optical transmittance, photon distribution spectrum of total flux, color coordinates,luminous efficacy and luminous intensity under blue LED excitation.2. Experimental2.1. Synthesis of Ce:YAG ceramics Y(NO3)36H2O, Al(NO3)39H2O and Ce(NO3)36H2O were used as the starting m
15、aterials. They were dissolved in distilled water with a stoichiometric ratio for Y:Al:Ce of (2.997:5:0.003). The total ion concentration was adjusted at about 0.5 mol/l. In the co-precipitation method, the precursor precipitate was produced by adding a NH4HCO3 solution of 2.5 mol/l at a speed of 10
16、ml/min in a beaker under mild stirring at 35 C. The pH value was kept at 6.2. It was stirred to react completely for 72 h. The resulting precipitate was centrifuged and washed seven times with distilled water and ethanol. Thenit was oven-dried at 80 C for 12 h and at 200 C for 72 h. The precursor powders obtained were heated at 800 C for 100 h in
