Correlation between the activation energies of structural relaxation and viscous flow for BaO–P2O5–Al2O3 glasses

Abstract Viscosity and structural relaxation were studied by thermomechanical analysis for six Al2O3-doped BaO–P2O5 glasses. The low-temperature viscosity data (107– 1011 Pa•s) were described and modeled by the Vogel-Fulcher-Tammann, Avramov-Milchev and Mauro-Yue-Ellison-Gupta-Allan equations. Temperature dependence of the activation energy of viscous flow was determined from the tangent of the extrapolated (in terms of the three viscosity equations) viscosity-temperature data-curves. Values of the activation energy of viscous flow were compared to the activation energies of volume relaxation determined from the exact physico-chemical description of the cyclic thermomechanical measurements performed over the glass transition region. In addition, differential scanning calorimetry was used to determine the activation energies of enthalpy relaxation. Good correspondence between the relaxation and viscosity activation energies was found in the respective temperature ranges. Good agreement was also confirmed for the viscosity and enthalpy fragilities, as well as for the indicator of the thermodynamic fragility.