Decoupling between Enthalpy Relaxation and Viscous Flow and Its Structural Origin in Fragile Oxide Glass‐Forming Liquids

Abstract

The structural relaxation kinetics at the glass transition in tellurium oxide (TeO2)‐based glasses has been examined from viscosity and heat‐capacity measurements to clarify the features of the structural relaxation in fragile oxide glass‐forming liquids. A large decoupling between enthalpy relaxation and viscous flow, i.e., a large discrepancy between the activation energies for the enthalpy relaxation (recovery), ΔH, and viscous flow, Eη, has been demonstrated in TeO2‐based glasses. The values in xK2O·xMgO·(100 − 2x)TeO2 glasses, for example, are ∼919–1051 kJ/mol for ΔH and ∼ 577–701 kJ/mol for Eη, given the ratio of ΔH/Eη≈ 1.44–1.59. Some viscosity and heat‐capacity data (all data have been reported previously) obtained from similar experiments in Sb2O3–B2O3 glasses belonging to the category of strong glass‐forming liquids have been reanalyzed in this paper for comparison; a strong coupling was found to exist between ΔH and Eη, i.e., ΔH/Eη≈ 0.98–1.18. An origin of decoupling between ΔH and Eη in fragile glass‐forming systems such as TeO2‐based glasses has been discussed by considering the glass structure model for fragile glasses; strongly bonded correlated (highly constrained) regions are surrounded or connected by weakly bonded noncorrelated (unconstrained) parts.