Fragility in mixed alkali tellurites

Abstract

Linear viscoelastic stress relaxation and calorimetric measurements were performed on a series of mixed alkali tellurite glasses of composition 0.3([ xNa 2O+(1− x)Li 2O])+0.7TeO 2 at temperatures near and above the glass transition temperature, T g. The stress relaxation data were well described by the stretched exponential function, G( t)= G 0exp[−( t/ τ) β ], where τ is the relaxation time, β is the distribution of relaxation times and G 0 is the high frequency modulus. The fragility, determined from the temperature dependence of τ, exhibited a minimum in the middle of the mixed alkali composition. A possible connection between the kinetic and the thermodynamic dimensions of this system was established, wherein the heat capacity change at the T g, Δ C p( T g), and the fragility are correlated.