POTASSIUM NIOBIUM SILICATE-GLASSES

A large glassforming region exists in the potassium niobium silicate system, extending to as little as 20 mol% silica. Addition of niobium oxide to potassium silicate glasses results in increases in density, refractive index and glass transformation temperatures, and a decrease in thermal expansion coefficients. Replacement of silica by an equimolar mixture of potassium and niobium oxides results in a decrease in the glass transformation temperatures, and an increase in the density, refractive index and thermal expansion coefficient. It is suggested that niobium oxide acts as a network intermediate, strengthening the glass network by eliminating nonbridging oxygens and that the network changes from a silica based network to a niobium based network as the niobium oxide content increases.