Free volume and energy barriers to equilibrium viscosity and ionic transport in alkali disilicates

Author： Nascimento Marcio Luis Ferreira Rodrigues Ana Candida Martins Souquet Jean-Louis

ISSN： 0031-9090

Source： Physics and Chemistry of Glasses - European Journal of Glass Science and Technology Part B, Vol.52, Iss.4, 2011-08, pp. : 157-166

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Abstract

A microscopic interpretation is developed of the temperature dependence of viscosity above the glass transition range, validating the representation of experimental data by a “hybrid” equation, which is a product of Arrhenius and Vogel–Fulcher–Tamman–Hesse (VFTH) exponential terms. By employing this hybrid equation to fit experimental viscosity data for lithium, sodium, and potassium disilicates, we found an activation energy of E_η^A≈1·2 eV for the Arrhenius term. We propose that this energy is representative of the exchange of nonbridging oxygen atoms between adjacent SiO4 tet- rahedra, allowing for consecutive relaxations of macromolecular silicate chains. Based on a free volume interpretation of the VFTH term, we also calculated the free volume required for chain movement, which, according to our results, is about 30% of the intrinsic volume of chain segments. This relatively low free volume may indicate the occurrence of limited rotational or translational displacements during viscous flow. These results are discussed and compared with the activation enthalpies and free volumes determined for ionic transport in the same alkali disilicate glasses considering a similar “hybrid” equation for electrical conductivity.