Abstract
Currently, white LED is mainly using blue chip (InGaN) equipped with yellow phosphor (YAG: Ce3+). However, this type is lack of spectrum wave band for the red region, and failed to be applied in the indoor lighting which needs high color rendering index. To obtain light source with high color rendering, white LED applied in indoor lighting generally employ UV chip with green, red, and blue phosphor. The critical points of light color of this type are luminous efficiency and mutual collocation of different colors. Hence, the development of high luminous efficiency phosphor with tunable emission spectrum system is the critical issue. Sr4Al14O25 :Eu2+ is of high luminous efficiency and high chemical stability fluorescent material with emission intensity around 490 nm. It is suitable used as green phosphor of white LED. However, in literature, luminescence properties were altered as cation of host lattice was changed. Therefore, this material was fabricated through solid reaction method with barium and calcium doping to replace strontium in this study. Then, phase transformation, elemental distribution, emission spectrum, chromaticity coordination and quantum yield were investigated. Besides, emission spectrum in high temperature was measured to investigate the effect of cation substitution on activation energy. Barium-doped sample exhibited significant red shift phenomenon. In lower substitution concentration, this phenomenon was due to the fact that Nephelauxetic effect was higher than crystal field effect. Furthermore, with the increasing barium content, quantum yield could be promoted to 96.5 %.Activation energy increased to 0.450 eV. In higher substitution concentration, red shift phenomenon was mainly due to SrAl2O4 formation. However, with the excess substitution, lattice distortion, impurity and other phase formation would lead to the rapid reduction of emission intensity, quantum yield, and activation energy. Calcium-doped sample also exhibited red shift phenomenon, since crystal field effect was higher than Nephelauxetic effect. In addition, because the size difference of strontium and calcium ions is lower than that of strontium and barium ions, lattice distortion was low and impurity formation was less. The phosphor retained single phase and a certain degree of emission intensity in higher substitution concentration. Besides, with the increasing calcium substitution, quantum yield was stable and the reduction rate of activation energy was slow. For barium and calcium co-substitution, the degree of red shift phenomenon was between barium and calcium substitution. Because the effects of lattice distortion were mutually neutralized, phosphor retained single phase and a certain degree of emission intensity in higher substitution concentration. Besides, with the increasing substitution, reduction rate of quantum yield was lower as compared with barium substitution, and the activation energy was significant promoted. Besides, from process improvements, emission intensity and activation energy of phosphor were effectively promoted through acetic acid washing process and secondary heat treatment. Especially, amorphous phase was removed after acid washing.