The iso-structural viscosity, configurational entropy and fragility of oxide liquids

This paper describes how the fragility of a liquid is linked to the ratio between the energy barrier (Eeq) for the equilibrium viscous behavior and that (Eiso) for the non-equilibrium iso-structural viscous behavior. Using the concept of iso-structural viscosity, two functions describing the variation of the configurational entropy (Sc) with temperature (T) are obtained from the Avramov-Milchev (AM) and the Vogel-Fulcher-Tammann (VFT) viscosity equations, respectively. The two Sc(T) functions exhibit different relations to the liquid fragility. The AM Sc(T) function is a power function with the exponent of F − 1, where F is the AM fragility index. In this case, Sc vanishes at T = 0 K. For the VFT function, Sc vanishes as T is lowered to a finite temperature T0, whereas it reaches the maximum value Sc,max at infinitively high T. Sc,max is proportional to the VFT fragility index. Thus, the VFT equation is not only a dynamical, but also a thermodynamic model. It is proved that for oxide liquids, the VFT equation describes viscosity data better than the AM equation, provided the pre-exponential factor η0 is fixed to a generally accepted value, e.g., 10−3.5 Pa s.