Combustion synthesis of YAG:Ce phosphors via the thermite reaction of aluminum
Graphical abstract
Schematic diagram of the experimental apparatus for combustion synthesis and Emission spectra for the combustion-synthesized products under different conditions: no flux, 5 wt% BaF2, and Y2O3–CeO2 + 5 wt% BaF2. The inset shows a photograph of the synthesized powders.
Introduction
Ce-doped Y3Al5O12 (YAG:Ce) phosphors have been widely used as yellow phosphors for white light-emitting diodes (LEDs) owing to their high emission efficiency under blue light excitation. In conventional solid-state reactions, the synthesis of pure YAG:Ce phosphors requires high-temperature (>1500 °C) treatment, which is both energy- and time-consuming. Therefore, lower temperature syntheses, such as spray pyrolysis,1 sol–gel processing,2, 3, 4 coprecipitation method,5, 6 citrate sol–gel combustion preparation,7, 8, 9, 10 and hydrothermal synthesis,11 have been proposed. However, these processes require the use of nitrates, such as Al(NO3)3 and Y(NO3)3, which still necessitate the use of high temperature treatments. To resolve this problem, this paper proposes a combustion synthesis (self-propagation high-temperature synthesis) of YAG:Ce phosphor using aluminum oxidation heat. Combustion synthesis using the thermite reaction of Al employs the propagation of a strong exothermic reaction, namely, Al oxidation, which is a self-sustaining reaction. This method has advantages of low energy consumption and short reaction times, and has been applied to produce a variety of advanced materials, such as oxides,12, 13, 14 nitride/oxynitride ceramics,15, 16, 17 and intermetallics.18, 19
In this study, we examined the facile and effective combustion synthesis of YAG:Ce phosphors via the thermite reaction of Al, where the oxidation heat of Al is used for self-propagation of the high-temperature synthesis. The key for successful combustion synthesis of YAG:Ce phosphors is regulating the reaction temperature, which can be optimized by controlling the ratio of Al and Al2O3 in the raw materials. We also investigated the effects of fluxes and coprecipitated materials on the luminescence properties of the synthesized YAG:Ce phosphors. It is known that flux addition greatly influences ion diffusion and crystallization processes, resulting in improved phase uniformity and luminescence properties.1, 20 For single crystalline YAG growth, fluxes such as PbOPbF2 and PbOPbF2B2O3 have been used.21 Among the various available fluxes, fluoride-type fluxes such as BaF2, CaF2, MgF2, and AlF3 have been found to be effective for aluminate-type phosphors.20, 22, 23 However, the effect of these fluxes on combustion-synthesized phosphors is unknown. In addition, we studied the effect of utilizing coprecipitated Y2O3–CeO2 as a raw material. Because Y and Ce ions are uniformly distributed in the coprecipitated material, suppression of CeO2 segregation and increased uniformity are expected.
Section snippets
Material and methods
YAG:Ce phosphors with a Ce ratio of 1.0 at% were prepared from commercially available Y2O3 (99.99% purity), CeO2 (99.99% purity), Al2O3 (99.99% purity), Al (99.99% purity), and NaClO4 (98% purity) powders. Al2O3 acts as a diluent to control the combustion flame temperature. The reaction formula for the combustion synthesis can be written as shown in Eq. 1.1.485Y2O3 + 0.03CeO2 + xAl2O3 + (5–2x)Al + yNaClO4 → Y3Al5O12 + yNaCl; (y = (7.485–3x)/4)
Here, x refers to the amount of Al2O3. The adiabatic
Effect of Al2O3 ratio in raw materials
In this work, the combustion synthesis of YAG:Ce was promoted by the oxidation of Al by O2, which was released from NaClO4. The reaction temperature for the oxidation of Al can be in excess of 5300 °C, which can cause melting and extreme sintering of the products. Therefore, we added Al2O3 as a diluent, and studied the optimum amount x of Al2O3. Fig. 2 shows photographs of samples obtained by combustion synthesis at different x values, as indicated in Eq. 1. The raw materials were in the form
Conclusions
YAG:Ce phosphors were successfully prepared by combustion synthesis via the thermite reaction of Al. The reaction temperature was controlled by varying x, the ratio of Al2O3 to Al. The combustion reaction proceeded at x ≤ 1.8, and x = 1.725 was the optimum condition for producing a high-performance product. When 5 wt% BaF2 was added, the luminescence intensity was significantly improved owing to decreased YAP formation with improved elemental uniformity through the effect of the flux on the
Acknowledgements
Part of this work was conducted at the Laboratory of XPS analysis, Hokkaido University, supported by the “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We gratefully acknowledge Mr. K. Suzuki, Mr. K. Ohkubo, Mr. R. Oota, Mr. T. Tanioka, Ms. Y. Yamanouchi, and Ms. E. Obari for their technical support in SEM experiments. We also thank Mr. K. Harada (Combustion Synthesis Co., Ltd.) for assistance. The measurements of
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