Glasses of the na.sub.2 0--t.sub.2 0.sub.5 --sio.sub.2 and the na.sub.2

Abstract

Thermally crystallizable glasses of the Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 and the Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 systems, and glass-ceramics made therefrom which are highly transparent, have high indices of refraction, and excellent strength properties. The glasses of the first system consist essentially of about 37-55 mole percent SiO.sub.2, 23-35 mole percent Ta.sub.2 O.sub.5, and 20-33 mole percent Na.sub.2 O, preferably where the molar ratio of Na.sub.2 O to Ta.sub.2 O.sub.5 is about 1, and for the second system the glasses consist essentially of about 27-45 mole percent SiO.sub.2, 32-45 mole percent Ta.sub.2 O.sub.5, and 20-35 mole percent Li.sub.2 O plus Na.sub.2 O, preferably wherein the molar ratio of Li.sub.2 O to Na.sub.2 O is within the range of from 0.7:1 to 1.2:1, and the molar ratio of Na.sub.2 O + Li.sub.2 O to Ta.sub.2 O.sub.5 is within the range of 0.5:1 to about 1:1. Glasses and glass-ceramics of the latter system also have good dielectric properties and low loss tangents. By varying the heat treatment schedule for crystallization of a glass to a glass-ceramic, a specific high index of refraction coming within a prescribed range may be imparted to the finished, transparent glass-ceramic. A transparent glass-ceramic having two or more different indices of refraction may also be produced.