Enhancement of Afterglow Luminescence of Long-Lasting Phosphor-Glass Composite by Using Refractive Index Matched Glass

Composites of a long-lasting phosphor, SrAl2O4:Eu,Dy, and glass were prepared by sintering the phosphor and glass powder. To enhance the afterglow luminescence, a borosilicate glass composition was chosen so that the refractive index of the glass matched that of the phosphor. Additional components with a high refractive index, such as La2O3 and Nb2O5, were added to the glass to increase the overall refractive index. As they tend to induce crystallization during sintering, small amounts of at least three types of such components were added to the glass to prevent crystallization. The surface of the composite was observed by a digital microscope with dark-field lighting. The phosphor particles became almost transparent because of the refractive-index matching, although bubbles were observed inside the phosphor particles. The afterglow luminance was, however, almost the same and the transmittance of the composite was not high because of many voids when compared to as that of the sample using the glass with a lower refractive index. The sample prepared under vacuum showed coloration and similar afterglow luminance even though the number of voids inside the composite decreased. To suppress the coloration, the amount of tin in the glass was increased, after which higher transparency and afterglow luminance were obtained. A 4-mm-thick sample showed a luminance of 118 mcd∙m-2 60 min after irradiation by a D65 lamp with 200 lx for 20 min.