Study on the particle size control of green-emitting phosphor (Ba,Sr)2SiO4:Eu2+ via reduction firing using various flux agents

In this study, the particle size control of (Ba,Sr)2SiO4:Eu2+, a green-emitting phosphor, was evaluated via reduction firing using various flux agents. Phosphor particles grown to ∼10–50 µm in diameter and having high internal quantum efficiency of 77.3% were obtained via reduction firing at 1473 K using 10 wt % BaCl2 as a flux. To reduce the particle size, the reduction firing temperature was lowered to 1223 and 1273 K respectively and the amount of BaCl2 was increased to 20 wt %. The diameter of phosphor particles significantly reduced; however, fibrous fine particles were formed and the internal quantum efficiency decreased to 60.7%. Using BaCl2–KCl and BaCl2–CsCl mixed fluxes to increase the amount of molten flux while suppressing the dissolution of (Ba,Sr)2SiO4 into the molten flux, a phosphor with fewer fibrous fine particles was obtained. In particular, the phosphor obtained via reduction firing at 1273 K using a 10 wt % BaCl2–10 wt % CsCl mixed flux had an improved internal quantum efficiency of 73.4%. The ionic radius of Cs+ (0.167 nm for CN = 6) is considerably larger than that of Ba2+ (0.135 nm for CN = 6) and Sr2+ (0.118 nm for CN = 6); therefore, high-quality host crystals with few defects could be possibly obtained without the substitution of Cs+ for Ba2+ and Sr2+ during reduction firing. To control the particle size of phosphor while maintaining a high internal quantum efficiency, the solubility of the host crystal into the molten flux as well as the substitution of the cation between the flux and host must be considered.