Structural Analysis of Mixed Alkali Silicate Melts through 29Si MAS-NMR and Impedance Measurements

The mixed alkali effect of silicate melts was investigated by performing 29Si magic-angle spinning–nuclear magnetic resonance (MAS-NMR) measurements and impedance spectroscopy analysis. The obtained 29Si MAS-NMR results showed that the Qn species of quenched glass was monotonically dependent on the ratio of the alkali mixture. In contrast, the equivalent circuit components given by impedance measurements of silicate melts composed of mixed alkalis were found to have extreme values. The 29Si MAS-NMR results suggested that these extreme values were unrelated to the structure of the silicate, but were related to the alkali present in the melt influencing the equivalent circuit components. This is thought to be caused by the increased pre-exponential factor, which is related to the concentration of ions with movement in the melt that is initiated by alkali mixing. It was also found that the equivalent circuit components of the super-cooled melt (i.e., the homogeneous melt just below the liquidus temperature) were significantly different from the equivalent circuit components of the homogeneous melt above the liquidus temperature. This is considered to be due to the fact that the activation energy of ion conduction in the super-cooled melt was increased by ~18%.