Dynamics of mixed alkali metaphosphate glasses and liquids

Abstract

Dynamic mechanical, stress relaxation, and oscillatory shear experiments were performed to examine relaxation processes in a series of mixed alkali metaphosphate glasses and liquids. At temperatures less than the glass transition temperature, T g, dynamic processes are dominated by hopping of alkali ions and local network relaxations that accommodate the motion of the cations. The average relaxation times associated with these dynamic processes were sensitive to the molar ratio of alkali ions in the glass. For T>T g , viscous relaxations were controlled primarily by the translational dynamics of P–O–P chains. These longer time viscous relaxation processes also appear to be affected by the dynamics of the alkali ions. The high frequency elastic modulus, G 0, the fragility of the liquid, the heat capacity changes at T g, Δ c p( T g), and the breadth of the distribution of relaxation times, W, were all dependent on the molar ratio of the alkali oxides. Correlations between the fragility, W and Δ c p( T g), which were found to describe the effects of single alkali oxide modified glasses, failed to describe related properties in these mixed alkali metaphosphates.