|
Preparation and Characterization of Photoluminescent Phosphors with Long Afterglow ABSTRACT Proper additives have been chosen by studying long persistence green-emitting SrAl2O4:Eu2+ phosphor, and the phosphor's light intensity and luminous decay time have been improved fully. The persistence luminescence of the phosphor still can be observed at the time that the excitation has been stopped for 50 hours. Its relative afterglow luminance has been measuring for 1200 minutes after the excitation, and a luminous decay curve following I=ct-n (n=0.634) has been gained. The effects of the five factors (which are added quantities of Eu2O3, additive A and additive B, reaction temperature and reaction time) to the luminescence properties of SrAl2O4:Eu2+ phosphor has been studied, and the producing conditions also has been optimized by orthographical experiment. The optimum producing conditions are: Eu2O3 0.020(m/o), additive A 0.070(m/o), additive B 0.010(m/o), reaction temperature 1350¡æ, reaction time 3.5 hours. A long afterglow blue-green emitting BaAl2O4:Eu2+ phosphor has been produced first, and its luminescence properties also has been given. A new method of producing phosphors has been developed by protecting raw materials mixture with a graphite crucible, and the luminous properties of the phosphor produced by this method approaches to that produced by hydrogen-reducing. According to the experiment results, the mechanism of long persistence has been analyzed and discussed, and the theoretical explanations about the experiment results have been given. The 5d energy level of Eu2+ ion in solid state phosphors produces a severe splitting because of the strong crystal-field surroundings. Both the two additives contain high-charge and small-radius cations. These cations cause much stronger crystal-field, so the splitting of the Eu2+ ion's 5d energy level becomes much larger and the lowest absorption of Eu2+ ion moves down. This results in the shifts of the excitation spectra and the emission spectra of the phosphors; therefore the phosphors can be excited efficiently by visible light. At the same time, long lifetime traps have been produced in the phosphor host because of the two additives, and the transition between the trap levels and ground state levels is spin-forbidden. All these factors ensure the high luminous efficiency and the slow afterglow decay of the phosphors. KEY WORDS SrAl2O4:Eu2+, long persistent luminescence (long afterglow), photoluminescence, luminescent decay, electron transition, electron trap, energy band model, thermoluminescence |