ZnO-Bi2O3-B2O3 Glasses as Molding Glasses with High Refractive Indices and Low Coloration Codes

When Bi2O3-containing glasses are melted at relatively high melting temperatures, they often show brown coloration due to the reduced Bi species, which is undesirable as optical glasses. In the present study, the low melting temperature (900°C) compared with conventional ones (1000°–1200°C) was used, in order to avoid the formation of reduced Bi species. ZnO–Bi2O3–B2O3 (ZBiB) glasses were selected for developing molding glasses with low Tg (≤600°C) and high nd (≥1.8). Most ZBiB glasses containing both B2O3≥40 mol% and Bi2O3≤40 mol% have much higher nd (=1.87–2.10) than those of ZnO–Bi2O3–P2O5 glasses (nd=1.68–1.97), much lower Tg (=400°–470°C) than those of Li2O–ZnO–TiO2–B2O3–P2O5 glasses (Tg=550°–570°C), and much lower λ70% (=39–45) than those of commercially available molding glasses (λ70%=44–50) with nd≥1.85. Some of the ZBiB glasses have properties suitable for precision glass molding (Tg=466°C, nd=1.929, λ70%=41 for 10ZnO·30Bi2O3·60B2O3 glass; Tg=440°C, nd=1.946, λ70%=42 for 40ZnO·20Bi2O3·40B2O3 glass). Thus, ZBiB glasses containing both B2O3≥40 mol% and Bi2O3≤40 mol% melted at 900°C are considered to be promising materials as molding glasses with high nd and low λ70%.