Self-diffusion of Sodium Ions in Some Silicate Glasses

Abstract

Self-diffusion of sodium ions in some silicate glasses was measured by means of RI tracer technique. At the binary system of Na2O-SiO2 glasses, the diffusion coefficient of sodium ions increases exponentially with the increase of this ion concentration. The introduction of Li2O, K2O, CaO and La2O3 into the glass brings about the lowering of diffusion coefficient of sodium ions. The effect of Ca, Al and La ions on the diffusion of sodium ions is explained in terms of the polarizing power of these cations to oxygen electron shell, being similar to the behavior of di-valent ions such as Mg, Sr, Ba, Zn, Cd and Pb. But Li and K ions are considerably different from di-, and tri-valent cations, and the behavior of both ions are probably related to “mixed alkali effect”. The reasons for this effect are not understood at present.The calculation of so-called correlation factors (f) was carried out by the modified Nernst-Einstein equation, and gave the values about 0.2-0.3 for each glass. By analogy with the relation between correlation factor and diffusion mechanism in ionic crystal, it may be supposed from the observed f-value that transport of sodium ions are by some type of vacancy or interstitialcy mechanism. However, such estimation is too simple for glasses having a more complicated structure than ionic crystal.