EFFECT OF REDUCING ATMOSPHERE ON THE AFTERGLOW PROPERTIES OF RED-EMITTING CaS:Eu2+,Pr3+ PHOSPHORS

Abstract

The afterglow time of typical red-emitting phosphors is much shorter than those of green- or blue-emitting phosphors which afterglow time were longer than 1000 min. CaS has been used as an effective host material for afterglow phosphors because trapping centers can be easily created in the host by doping with an activator. This study reports on the effect of a reducing atmosphere on the afterglow properties of a red-emitting CaS:Eu2+,Pr3+ phosphor. The phosphor was prepared by reduction under an Ar-H2 (5%) or H2S atmosphere from a CaSO4:Eu3+,Pr3+ phosphor prepared using a liquid phase method. Heating the CaSO4:Eu3+,Pr3+ phosphor for 2 h under an Ar-H2 (5%) atmosphere at 1050°C gave CaO as by-product in addition to CaS . The resulting phosphor exhibited red-emission at 646 nm originating from Eu2+ ions upon visible light irradiation, and gave some afterglow after cessation of visible light irradiation. Addition of Li+ ions extended the afterglow time of the phosphor as a result of a slight change in the CaS host structure. For preparation of a CaSO4:Eu3+,Pr3+ phosphor under a H2S atmosphere, a CaS monophase was obtained. The emission and afterglow of the phosphor were similar to that prepared under the Ar-H2 (5%) atmosphere, but the afterglow time resulting from the CaS monophase was longer. The longest afterglow time obtained was about 60 min for a phosphor prepared under a H2S atmosphere with an initial Li/Ca atomic ratio of 0.04.