Diffusive motion in alkali silicate melts: An NMR study at high temperature

Abstract

Nuclear magnetic resonance (NMR) spectroscopy at high temperature is an excellent tool for studying the atomic scale diffusive processes in molten silicates which control their transport and thermodynamic properties. We present here 23Na and 29Si NMR data on lineshapes, spin-lattice, and spin-spin relaxation times in a series of alkali silicate and aluminosilicate melts from room temperature to about 1200°C. 29Si lineshapes show that exchange of oxygens between bridging and non-bridging sites takes place at the microsecond to nanosecond time scale at liquidus temperatures, making the lifetimes of silicate “molecules” very short. This process is probably a fundamental step in viscous flow and configurational entropy generation. 23Na relaxation time data suggest that the motion that causes relaxation is closely related to that of through-going diffusion, and the mixed-alkali effect is detectable in activation energies. The mechanism for 29Si spin-lattice relaxation changes at or near the glass transition, showing a close link between local diffusive motion and bulk thermodynamic properties.