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80 Handbook on the Physics and Chemistry of Rare Earths often called “chelate gel” or “amorphous chelate” route, and (3) the organic polymeric gel route (Kakihana, 1996). Pechini type in situ polymerizable complex method is explained in detail in a review by Kakihana (1996). Here, an example of polymerizable complex method is described (Suzuki and Kakihana, 2007; Suzuki et al., 2007). First, aqueous solutions of the con- stituent metallic elements of the phosphor are prepared; they are often nitrate solutions because these salts are easily soluble. The solutions are then mixed in a ratio corresponding to the composition of the targeted phosphor. Citric acid (CA) is added to the solution that is heated to 80°C to make oxycarbox- ylate complexes of the metal ions. Then, a glycol (eg, propylene glycol (PG), ethylene glycol) is added and the temperature is raised to 120–150°C in order to promote polyesterification between the CA and the glycol. The obtained transparent resin is a polymeric gel in which the metal elements are dispersed homogeneously. It is heated up to 500°C to remove organic components. After that, it is heated again at higher temperature to obtain a well-crystallized phosphor. In this process, homogeneously reacted phosphors are obtained under relatively mild heating treatment because the metallic components are well dispersed in the precursor mixture and react easily. The second example described the synthesis of Sr2SiO4:Eu2+ phosphors by sol–gel process (it is usually called Pechini method) (Han et al., 2012a). A silica sol prepared from tetra-ethoxysilane (TEOS) and a solution of Sr and Eu nitrates are mixed and stirred. Subsequently CA and ethylene glycol are added. Then, polyethylene glycol is added. The mixture is stirred and the pH of the solution is maintained at 4. The mixture is heated at 80°C to hydrolyze it into a sol and then into a gel. The gel is heated to remove organic material and then heated at a higher temperature in a slightly reducing atmo- sphere to obtain the Sr2SiO4:Eu2+ phosphor. Recently, a water-soluble silicon compound (propylene glycol-modified silane; PGMS) that can be used for a precursor of silicate phosphor was synthesized (Suzuki and Kakihana, 2009). The preparation process is as fol- lows: TEOS and PG are mixed and stirred at 80°C for 48 h. After that, a small amount of hydrochloric acid is added and the solution is heated again at 80°C. A transparent solution containing a water-soluble silicon compound (PGMS) is obtained. The protocol for phosphor synthesis by polymerizable complex method using PGMS is as follows: a PGMS solution, the metal solutions, and CA are mixed, stirred, and heated at 80°C; PG is then added and the solution is heated and stirred at 120°C to promote polyesterification of CA and PG. The obtained polymeric compound is heated to remove organic compo- nents and heated again at higher temperature to obtain a crystalline phase of the phosphor. It is reported that Zn2SiO4:Mn2+ (Takahashi et al., 2009), (Ca,Ce)3Sc2Si3O12 (Yamaguchi et al., 2010), Ca2SiO4:Eu2+ (Luo et al., 2012), Ba3Si6O12N2:Eu2+ (Yasushita et al., 2012), and BaZrSi3O9:Eu2+ (Komukai et al., 2015) are obtained using the PGMS method.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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