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What we can learn from elasticity about the atomic structure of glass ? How it it relates to the viscous behavior ?
Résumé :
The mechanical properties of glasses are intimately related to the fine details of the atomic packing characteristics. Because there is a large diversity of glasses, with different atomic bond types and a great variety of atomic organization, elastic moduli for instance cover a large range of values. On one side, the magnitude of the elastic moduli is governed by both the atomic bond energy (sub-nanometer scale) and the atomic packing density. On the other side there is a direct correlation between Poisson’s ratio and the degree of symmetry of the structural units existing at the molecular scale. Poisson’s ratio (\nu) ranges from 0.1 for a SiOC glass characterized by a large fraction of free volume, to 0.4 for Pd- and Zr-based bulk metallic glasses. The network dimensionality (chains, layers, etc.) increases monotonically with the mean coordination number or with the fraction of bridging oxygen atoms per tetrahedron, but changes inversely with \nu. Hence, for chalcogenide glasses, the network of which resembling the one of chain-polymers, \nu>0.25, whereas in the case of tri-dimensional organisation, such as in SiO2-rich glasses, \nu<0.2. Correlatively, the temperature dependence of \nu above the glass transition (Tg) provides a unique insight into the magnitude of the depolymerization process occurring in the liquid. \nu depends much on temperature above Tg but stays mostly lower than 0.5 up to T=1.3 Tg except for weakly cross-linked materials such as chain-polymers. The temperature dependence of the elastic properties above Tg can be discussed in the light of the ``fragile’’ versus ``strong’’ character of the liquid. Besides a direct correlation has been obtained between the temperature dependence of the shear elastic modulus, the activation entropy for flow and the correlation factor (\beta) stemming from the relaxation function (\phi(t)=exp[-(t/\tau)^\beta]). The mechanical properties of glasses are thus found to bring a new light on the short to medium range order of the glass atomic networks.
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