Temperature induced structural changes and transport mechanisms in borate, borosilicate and boroaluminate liquids: high-resolution and high-temperature NMR results

Abstract

Temperature dependent structural rearrangements in glass-forming liquids and associated changes in the configurational entropy have been hypothesized to be closely related to the transport processes such as viscous flow. The nature of such structural rearrangements is, however, not well understood. In this study the temperature dependence of the short-range structural changes involving borate, silicate and aluminate speciation reactions in sodium borate, sodium borosilicate and lithium boroaluminate liquids have been investigated with high-resolution and high-temperature (up to 1793 K) 11B, 29Si and 27Al nuclear magnetic resonance spectroscopy. It is shown that such structural changes are the principal source of configurational entropy in binary alkali borate liquids. However, silicate speciation in the borosilicate and borate speciation in the boroaluminate liquids are found to be insensitive to temperature changes. It is speculated that in these liquids the temperature induced changes in the intermediate-range structural order are likely to be an important source of configurational entropy. The inter-connection between temperature dependent speciation reactions and atomic scale mechanisms for viscous flow in these liquids is also established.