pH Effects on Formation of Cerium-Doped Terbium Aluminum Garnet (Tb[sub 2.95]Al[sub 5]O[sub 12]:Ce[sub 0.05]) Phosphor Powder

Abstract

Terbium aluminum garnet ͑TAG͒ was successfully synthesized by a solid-state reaction method in this study. Before calcining, the mixing powder of terbium oxide ͑Tb 4 O 7 ͒, boehmite ͑AlOOH͒, and cerium chloride ͑CeCl 3 ·7H 2 O͒ was aged at pH 1, 2, and 3, respectively. Experimental results revealed that the better process for the formation of TAG powder was aged at pH 1. The effects of pH on the formation of TAG powder were further studied. The diffusion distance between terbium oxide and boehmite was reduced by aging the mixed slurry at pH 1. A pure phase of TAG powder was obtained after the aged mixture of boehmite, terbium oxide, and cerium chloride was heated at 1200°C for 2 h and then calcined at 1500°C for 20 h. The particle size of pure TAG powders is the agglomerate morphology through scanning electron microscopy observation. The formed TAG powder in this work had a maximum emission peak in 553 nm as exciting at 470 nm.There are many advantages of the applications for light-emitting diodes ͑LEDs͒ such as low energy consumption, high reliability, long life, light weight, absence of mercury, and small size. 1-4 Therefore, white light LED ͑WLED͒ becomes a next generation device of luminance. Nakamura et al. 5 combined blue LED chip and a yellowish yttrium aluminum garnet powder to produce a white light. The blue light LED coupled with the yellow phosphor powder became a popular method to generate the white light. 6 However, the color temperature ͑4200 K͒ of cerium-doped yttrium aluminum garnet ͑YAG:Ce͒ phosphor used for luminescence converting LED ͑LUCOLED͒ is too high to use in warm white LED application ͑3300 K͒ because of lacking red component. 7,8 In the recent studies, Gd-doped YAG:Ce 9 and cerium-doped terbium aluminum garnet 10 ͑TAG:Ce; Tb 2.95 Al 5 O 12 :Ce 0.05 ͒ were studied for the warm white LED. The emission of both materials was redshift from 535 nm to 550-555 nm. 9,10 Lin et al. 9 indicated a solid-state reaction method to form Tb 3+ substituted YAG:Ce, Gd phosphors. The formation condition is in sequence: calcined at 1000°C for 24 h, ground, sintered in air at 1500°C for 24 h, re-milled, and annealed in a reducing atmosphere ͑5% H 2 in N 2 ͒ at 1500°C for 12 h. Although Nazarov 10 revealed that TAG can be formed by a solid-state reaction method at 1500°C for 5 h, the impure phase TAP still presents in his report. The pure TAG powder is not found in the literature. Granschow 11 indicated that the broad field of primary crystallization of the perovskite TbAlO 3 ͑TAP͒ covers the composition of TAG leading to difficult formation of pure TAG powder. In this study, a modified solid-state reaction method was adopted to synthesize TAG powder. In contrast, the proposed method in this work is simple and environmentally friendly. The effect of pH on the formation of ceriumdoped TAG phosphor powder using the modified solid-state reaction is discussed in detail. Figure 1 shows the flow chart of the formation process of TAG:Ce powder. Boehmite ͑AlOOH͒, terbium oxide ͑Tb 4 O 7 ͒, and cerium chloride ͑CeCl 3...