Physical Properties of Germanate Glasses in Na<sub>2</sub>O-GeO<sub>2</sub> and Na<sub>2</sub>S-GeO<sub>2</sub> Systems

Abstract

The effects of substitution of Na2S to Na2O on the physical properties, such as the density, elastic moduli and thermal expansivity, of germanate glasses in the systems Na2OGeO2 and Na2S-GeO2 were studied in order to clarify the behavior of sulphur introduced in large quantities into glass structure. The density of the glasses was determined by Archmedes method with a sensitive microbalance and kerosene. The elastic moduli were calculated from the sound velocities of the glasses measured by a pulse superposition method. The thermal expansivity was determined by using a dilatometer and the transformation temperature was obtained from the thermal expansion curve. The results are summarized as follows:The density and bulk modulus of the glasses in both systems increase with increasing alkali content at first, but start to decrease when alkali content becomes more than 17-18mol% in Na2O-GeO2 glasses and 6-7mol% in Na2S-GeO2 glasses. The density and bulk modulus of a glass containing Na2S are considerably lower than those of a glass containing Na2O when both glasses have the same alkali content. The glasses with Na2S have higher thermal expansivity and lower transition temperature than those with Na2O.The increase in molar volume of these glasses, resulting from the substitution of S to O, was calculated from the density and composition of glasses and compared with the value calculated from the difference in ionic radius or covalent bond length between O and S.