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82 Handbook on the Physics and Chemistry of Rare Earths Another novel attempt to synthesize phosphors is a rapid heating process using arc imaging furnace (Ishigaki et al., 2010). In this process, a mixture of raw materials is heated up in the furnace at around 2000°C (or more) to be a molten state within around 10 s. After a while, it is quenched and solidi- fied by stopping heating. The cooling rate was estimated as more than 100°C/s. The process is advantageous especially in research activities because of short synthesis time. The synthesis of YAG:Ce3+ phosphor with a lens shape by this process was reported (Toda, 2012). The obtained phosphor sam- ple was claimed to be useful for LED color conversion as is, that is without subsequent grinding. 5.2 (Oxy)nitrides 5.2.1 Overview Most of nitride (oxynitride) phosphors consist of nitride (oxynitride) com- pounds (host materials) and rare earth activators. The host materials usually include silicon, nitrogen, and alkaline earths or rare earths. Additionally, alu- minum and oxygen are occasionally included into them. A nitride phosphor can be synthesized by heating a mixture of nitride raw materials, in the same way as for an oxide phosphor. However, since most of alkaline-earth nitrides and lanthanide nitrides are not stable in an ambient atmosphere, ie, they easily react with water and/or oxygen in air, a variety of synthetic methods other than simple solid-state reaction of nitride raw materials are proposed (Chung et al., 2014; Xie and Hirosaki, 2007; Xie et al., 2010, 2011). Some alternative synthetic methods use oxide or metallic raw materials and conduct nitridation and phosphor synthesis simultaneously. 5.2.2 Synthesis of Nitride Raw Materials Here, the synthesis methods for nitride compounds, which are important raw materials for nitride phosphors, are described. Although some of the nitride compounds are not easy to handle, they are nevertheless important starting materials. Three methods are known for synthesizing silicon nitride (Si3N4), which is the most important raw material for nitride phosphors (Riley, 2000). The first one is carbothermal reaction. SiO2 is used as a starting material. It is mixed with carbon (usually graphite) and heated at 1400–1500°C to reduce SiO2 and obtain Si3N4. It is the first industrial manufacturing method of Si3N4 and, also, as recently recognized, the cheapest one. 3SiO2 +6C+2N2 !a-Si3N4 +6CO ðat 14501500°CÞ The second method, direct nitridation of silicon powder, was the first large-scale production method of Si3N4 used in the 1960s.. 3Si + 2N2 ! a-Si3 N4 ðat 1300 1400°CÞPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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