Low-Tg phosphate glasses with improved water resistance for the coating of fluoride glass fibers

Abstract

Starting from a multicomponent lead phosphate glass composition given in the literature, a glass was developed suitable for the protective overclad of heavy-metal fluoride glass fibers. Besides a low glass transition temperature < 270 °C, a high thermal expansion coefficient > 18 * 10⁻⁶ K⁻¹ the phosphate glass material developed had a water resistance at 60°C for 10 h comparable to that of commercial container glass. The final composition developed was (in mol%): 54.7 P₂O₅, 19.8 PbO, 18.0 Na₂O, 1.2 MgO, 1.2 BaO, 1.2 CaO, 0.4 V₂O₅, 1.5 AI₂O₃, 2.0 Bi₂O₃. Besides the influences of these different components the acdon of the melting conditions, especially melting temperature and trace SiO₂ content from the crucible material, have to be taken into account. Fiber drawing experiments including the in-situ apphcation of the phosphate glass overclad were successful.