Decolorization of high refractive index glass based on SiO2-(BaO + ZrO2)-TiO2 using transition metal oxides

High refractive index glasses based on SiO2-(BaO + ZrO2)-TiO2 were synthesized for road sign applications with nd > 1.9. Although the refractive index increased with the addition of TiO2, visible transmittance was deteriorated with colorization of the glasses with increasing TiO2. Visible light absorption by the multi-valenced Ti-ions within the glass matrix was responsible for the change in color and transmittance. To improve transmittance and guarantee the retro-reflectivity of the glass beads, various transition metal oxides (TMOs) CuO, NiO, MnO and CoO were introduced for color control. The transmittance change with various TMOs was monitored, and a glass with CoO showed a noticeable change in color as well as an increase in transmittance. The valence change of Ti-ions was inspected by XPS, photo-luminescence (PL) and photo-luminescence excitation (PLE) spectra of the glasses, to elucidate the color change.