Interrelation between the parameters of equations of viscous flow and chemical composition of glassforming melts

Abstract The number of free parameters of viscosity equations are reduced to two by assuming that the glass transition temperature Tg is the temperature at which viscosity is 1013.5 [dPa s]. In this way the Avramov and Milchev AM equation is becoming lg η = lg η A + 13.5 − lg η A T g T α while the VFT equation is transformed to the form: lg η = lg η V + 13.5 − lg η V T g − T o T − T o . It is demonstrated that the dimensionless fragility parameter a of the AM equation depends on composition, x, as α = αο + 6x, where αο = 1 − 2 for silicates and αο = 2.75 for borates. We show that the fitting parameters of AM and of VFT equations are correlated. Thus the relationship between the pre-exponential constants is lg ηV ≈ lg ηA − 2. There is also relationship T o T g = 1 − 1.2 α between the fragility parameter a and the divergence temperature To of the VFT equation. We develop an alternative approach to consider the vibration frequency of building units of the system. The superposition of the independent vibrations of the atoms constituting the building unit causes a “beat” of the latter.