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Rare Earth-Doped Phosphors for White LEDs Chapter 278 87 Alkaline-earth sulfides phosphors, (Ca,Sr)S:Eu2+, are synthesized simply by heating a mixture of commercially available sulfides and activator com- pounds in a reducing atmosphere (Hu et al., 2005). Recently, the so-called solvothermal method was applied to alkaline-earth sulfide phosphors. Sulfur, calcium chloride (or strontium chloride), and ethylenediamine (NH2CH2CH2NH2) solvent were put into a Teflon-lined autoclave and heated to 170–220°C for 12 h (Wang et al., 2002). The obtained materials were nano- crystalline CaS (or SrS) phosphors. Van Haecke et al. conducted similar experiments (Poelman et al., 2009; Van Haecke et al., 2007). The mixtures of CaCl2 (or SrCl2), EuCl3, S, and ethylenediamine were heated at 200°C for 12h and single crystals of CaS:Eu2+ (or SrS:Eu2+) phosphors were obtained. One of the typical methods for synthesizing thiosilicate phosphors is as follows. A mixture of CaS, Si, and activator compounds (EuS or CeF3) was heated in H2S gas flow to obtain Ca2SiS4:Eu (or Ce) (Smet et al., 2008). In this process, sulfidization of one of the raw materials (Si) and the phosphor synthesis are conducted simultaneously. 5.3.2 Treatments to Improve the Durability of Sulfide Phosphors Sulfide phosphors have been used for cathode-ray tubes and have also attracted much attention as phosphors for electroluminescent (EL) devices before the advent of LEDs. As for EL phosphors, the durability of sulfide phosphors was not sufficient and a lot of research to improve the problem has therefore been conducted. Some examples of these attempts are described in this section. Coating treatments are the most popular ways to improve the durability of phosphors. Sol–gel processes are among the most used procedures to create an oxide coating on the phosphor particles. Widespread oxide coating materials have been examined as protective coatings for sulfide phosphors. Silicon oxide (SiO2) and titanium oxide (TiO2) were applied to Ca0.8Sr0.2S:Eu2+, Tm3+ phosphors (Guo et al., 2003). Aluminum oxide (Al2O3) was used for CaS:Eu2+ phosphor (Avci et al., 2011). A composite coating of SiO2 and methyl methacrylate (MMA) has also been tested for the same phosphor (Park et al., 2008; Yoo and Kim, 2009). Another coating method, atomic layer deposition of Al2O3 was reported to improve the durability of the CaS:Eu2+ phosphor under 80°C and 80% relative humidity (Avci et al., 2009). 5.4 Halides Most oxyhalide phosphors are synthesized by solid-state reaction similarly to oxide phosphors. The only difference is that the mixture of raw materials con- tains halide compounds instead of oxides. In this section, the synthesis of two of the well-known halogen-containing phosphors used for LEDs is described.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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