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Rare Earth-Doped Phosphors for White LEDs Chapter 278 85 reaction. Carbon powder instead of the mixture of hydrocarbon and ammonia is used as reducing reagent. It is advantageous from the viewpoint of process handling, since hazardous and flammable gases are not required. On the other hand, black-colored residual carbon may spoil the luminescent property of the obtained phosphors. Therefore, it is indispensable to optimize heating conditions to reduce carbon residues. This method was reported to be applied to Sr2Si5N8:Eu2+ (Piao et al., 2006), a-SiAlON (Zhang et al., 2007a), and SrYSi4N7:Eu2+ (Kurushima et al., 2010) phosphors. 5.2.6 Synthesis from Metallic Raw Materials An alternative synthesis of nitride phosphors uses metallic or intermetallic raw materials (alloys). Nitridation of the raw materials and phosphor synthesis are conducted simultaneously, similarly as in the gas phase reduction and car- bothermal methods. It is called self-propagating high-temperature synthesis in the case that heat generated by nitridation is used to raise temperature of the raw materials. Leading attempts to obtain nitride phosphors from metallic raw materials are described later. A nitride phosphor (Sr,Ca)AlSiN3:Eu2+ was synthesized from the interme- tallic raw material (Sr,Ca,Eu)AlSi by heating it in nitrogen gas at high pres- sure (190 MPa) (Watanabe et al., 2008a). The same synthetic route was applied for CaAlSiN3:Eu3+, but was conducted in atmospheric pressure (Piao et al., 2007a; Yang et al., 2012). Another nitride phosphor, Ba2Si5N8: Eu2+, was synthesized by heating a metallic mixture prepared by arc melting of Ba, Eu, and Si in a nitrogen gas flow at 1350–1450°C (Piao et al., 2007b). Synthesis of a LaSi3N5:Ce3+ phosphor was conducted by heating a mixture of LaSi, CeSi, and Si3N4 in nitrogen at 1873 K (Cai et al., 2009). The so-called combustion synthesis was also applied in the synthesis of nitride phosphors. For instance, LaSi3N5:Eu2+ was obtained by reacting LaSi, Si, Si3N4, and Eu2O3 with the following protocol: a mixture of the above- mentioned raw materials, carbon powder, and titanium metal powder was ignited by electric current in a sealed pressure-resistant container filled with nitrogen gas. Once ignited, the combustion reaction proceeded rapidly and completed within a few minutes (Zhou et al., 2011). The carbon and titanium acted as igniter. Generated heat by nitridation of LaSi, and Si heated up the raw materials mixture itself and promoted the reaction between the raw materials. A similar procedure was applied for Ca2Si5N8:Eu2+ (Chung and Chou, 2013) and CaAlSiN3:Eu2+ (Chung and Huang, 2014). The synthesis of the two phosphors was conducted as follows: Ca, Si, Al, Eu2O3, Si3N4, NaN3, and NH4Cl were mixed and pressed into cylindrical compact pellets. Igniting reagents (eg, mixture of Mg and Fe3O4) were filled around the pellets and ignited by an electric heating coil placed near the compact. Another synthetic method of nitride phosphors from alloys uses supercrit- ical ammonia, ie, nitridation is conducted not by high-temperature nitrogenPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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