MODEL FOR THE THERMOOPTIC BEHAVIOR OF SODIUM-BORATE AND SODIUM ALUMINOSILICATE GLASSES

The thermo-optic coefficients (dn/dT) of Na2O-B2O3 and 25Na2O-xAl2O3-(75-x)SiO2 glasses have been measured. The effect of glass composition on the dn/dT of these two sets of glasses is similar to the effects ot composition on other glass properties. Prod'homme's theory for thermal change of refractive indeX is sufficient to explain the effects of composition on the dn/dT in terms of the relative contributions of the thermal expansion coefficient and the temperature coefficient of the electronic polarizability. However, the temperature coefficient of the electronic polarizability is not proportional to the activation energy for viscous flow as suggested by Prod'homme. A model is proposed which explains the effect of composition on the temperature coefficient electronic polarizability in terms of the relative volume occupied by various types of bridging and non-bridging oxygen ions in the glass. This relationship is the result of the dependence of the electronic polarizability coefficient of various types of oxygen on the bonding associated with each oxygen ion species.