EXTENDED STRUCTURAL INTEGRITY IN NETWORK GLASSES AND LIQUIDS

Brillouin scattering measurements of B2O3, GeO2 and SiO2 glasses and l iquids, to temperatures less than or equal to 1550 degrees C, reveal a nomalous thermo-mechanical behavior. Transverse acoustic waves are sup ported by the liquids of all three oxides. In all systems, the longitu dinal and shear elastic moduli increase with increasing temperature be low, and again far above the glass transition. This apparent strengthe ning of the glassy network below T-g is less pronounced, the more frag ile the glass former, while in the molten state the opposite seems to be the case. This positive temperature dependence of the elastic prope rties can be explained by localized structural reorganizations which d o not require the breaking of network bonds. The decrease in elastic m oduli immediately above T-g, characteristic of fragile systems, is due to the increase of the average network ring size, achieved through a low-activation energy neighbor exchange mechanism. The ability of thes e liquids to support shear stresses at very high temperatures indicate s a persistent network structure. (C) 1997 Elsevier Science B.V.