Abstract: Thermally crystallizable glasses of theA -- Nb.sub.2 O.sub.5 --GeO.sub.2 systemwherein A is K.sub.2 O, Li.sub.2 O or a mixture of K.sub.2 O and Li.sub.2 O and glass ceramics made therefrom are described. Transparent and opaque glass ceramics may be obtained. The glass ceramics are particularly suitable for use in printed optical circuits, fiber optics, gradient refraction optical lenses and as wave guides.

Abstract: Thermally crystallizable glasses are disclosed of the system:RO -- Nb.sub.2 O.sub.5 -- GeO.sub.2wherein RO is a BaO or SrO. Also disclosed are thermally crystallizable glasses of the system PbO -- Nb.sub.2 O.sub.5 -- GeO.sub.2. Glass-ceramics made from these glass systems have high dielectric constants and low dissipation factors or loss tangents, which glasses and glass-ceramics are suitable for use as capacitors and dielectrics, and particularly for thick-film dielectric pastes utilized in the manufacture of micro-electronic printed circuits. By varying the heat treatment schedule for crystallization of the glass to a glass-ceramic, a specific high dielectric constant coming within a prescribed range can be imparted to the finished glass-ceramic.