Sodium Transport in the Na2O-H2O-SiO2 Glass System

The variation with water content of dc conductivity and Na diffusion coefficient for the Na₂O · 4siO₂ and Na₂O · 2SiO₂ glass systems was found to be similar to that of the Na₂O.3SiO₂ series reported earlier. The conductivity was estimated for the ternary system Na₂O-H₂O-SiO₂ by combining the present results with the previous data on the Na₂O · 3SiO₂ system. When the conductivity of those glasses with a constant [Na₂O] + [H₂O] content was plotted against water content, a pronounced mixed "alkali" effect was demonstrated. The Haven ratio, calculated by comparison of the dc conductivity to the Na diffusion coefficient at 100°C for each of the three glass systems, was found to increase toward unity with increasing water content. This suggests that the addition of water reduces the number of sodium charge carriers. The subsequent increase in conductivity beyond the minimum with the introduction of larger amounts of water is, probably, due to an increase in the mobility of the Na⁺ ions.