1、PDF外文：http:/ 中文 1664 字 英文原文 Preparation and Optical Properties of Transparent Ce:YAG Ceramics for High Power White LED S. Nishiura1, S. Tanabe 1, K. Fujioka2, Y. Fujimoto2 and M. Nakatsuka2 1 Graduate School of Human and Environmental Studies, Kyoto University,Yoshida nihonmatsu-ch
2、o, Sakyo-ku, Kyoto 606-8501, Japan 2 Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka,Suita, Osaka 565-0871, Japan E-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
3、 method of the hydroxides. The Ce:YAG ceramics exhibit 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 thick
4、ness. With increasing thickness of the sample, the color 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. Introdu
5、ction White LEDs are now extensively spotlighted as general 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
6、make the white LEDs is combining of a blue LED and Ce:Y3Al5O12 (Yttrium 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 d
7、ue to the 5d4f transition of Ce3+. The combination of two lights of 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 catc
8、hes up with the degradation of luminous intensity and the change of 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 t
9、he merits mentioned above, this approach using inorganic materials is useful for 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.
10、; We have interest in transparent polycrystalline ceramics that have been studied as laser
11、 and scintillator materials 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 polycryst
12、alline ceramics have been fabricated 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 synth
13、esis. The co-precipitation method, 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 LE
14、Ds. Besides, since the unit cell of the YAG 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 synthes
15、is of the YAG powder. We also measured the optical properties such as optical transmittance, photon distribution spectrum of total flux, color coordinates,luminous efficacy and luminous intensity under blue LED excitation. 2. Experimental 2.1. Synthesis of Ce:YAG ceramics Y(NO3)36H2O, Al(NO3)3
16、9H2O and Ce(NO3)36H2O were used as the starting materials. 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 produ
17、ced by adding a NH4HCO3 solution of 2.5 mol/l at a speed of 10 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
18、-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 air to remove the organicsubstances. The powders were heated at 1200 C for 90 min in air to get the YAG powder.The milling process was carried out to grind a wide variety of the powd
19、er size into 200 300 nm averages. In this process, ethanol, a dispersant (Kyoeisha Chemical Co., LTD, Flowlen G-700) and SiO2 were added to the powder at the weight ratio for powder:ethanol of (1:5), 0.4 wt% of the ethanol and 0.2 wt% of the powder, respectively. After the milling process, a binder
20、(Sekisui Chemical Co., LTD, S-LEC BL-1) was added in the slurry at 0.5 wt% of the powder and the slurry was milled with nylon balls for 24 h. Then, the slurry was poured into a mold located on a gypsum plate. It was dried at room temperature for 72 h. The caked slurry was heated to remove the organi
21、c substances at 800 C for 00 h under O2 atmosphere. Finally, it was heated at 1780 C for 20 h under vacuum. The YAG ceramics obtained were cut and polished carefully. The thickness was set in the range of 0.1 0.8 mm. 2.2. Measurement The microstructure of the YAG ceramics was observed b
22、y a confocal scanning laser microscopy (Olympus Corp., OLS1200). The optical transmittance of the mirror-polished samples on both surfaces was measured over the wavelength region from 200 to 850 nm using a spectrophotometer (Shimadzu Corp., UV-3600). The photoluminescence (PL) of the Ce:YAG ceramics
23、 was measured under 465 nm LED (Nichia Corp., NFSB036B). In the optical measurement, the Ce:YAG ceramics were mounted on the LED. The spectral power and photon distribution of total radiant flux of PL spectra were measured by using an integrating sphere (Labsphere, Inc., LMS-100) which was connected
24、 to a CCD detector (Ocean Optics, Inc., USB2000) with an optical fiber. A standard halogen lamp (Labsphere, Inc., CLS-600) was used for calibrating this measurement system. The luminous efficacy (lm/W) of the Ce:YAG ceramics was calculated from the spectral power distribution and the electric power of the LED. The luminous intensity and color coordinates (x, y) of the Ce:YAG ceramics were measured from the calibrated spectra.
