Electrical mixed alkali effect

Figure 15-7. Mixed alkali effect tracer diffusion coefficients ), and D(a) from electrical conductivity (D(a ) = (R T/c -ia zfF1)) for (Cs Na, N)20 5 Si02, as a function of N, see [R. Tferai (1971)]. T = 480 °C.

Figure 6.14. Variation of self-diffusion coefficient of LF (x) and (o) ions with mole fraction of potassium. Inset variation of electrical conductivity with mole fraction of potassium. Lines are only indicative of trends. Note the occurrence of the mixed alkali effect in a. (After Balasubramanian and Rao, 1993).

Figure 8.3 The mixed-alkali effect on the electrical conductivity at 300 °C of sodium-potassium silicate glasses containing 20 mol% total alkali oxide. (Data supplied by J. J. Noonan)...

Addition of an alkaline earth oxide to a glass containing alkali ions also decreases the electrical conductivity as well as the diffusivity of the alkali ion. This behavior is usually explained as due to the blocking effect of the immobile divalent ion, which occupies interstices which can therefore no longer be used for alkali migration. It may well be, however, that the effect of the divalent ion is simply another manifestation of the mechanism causing the mixed-alkali effect. 

The mixed-alkali effect has been observed and discussed for more than a century. It refers to a non-linear change of many physical properties, most notably the electrical the electrical conductivity, as a result of the addition of a second alkali ion to an alkali-containing glass. Quite a variety of models has been proposed, although none of these models has been universally accepted. Reviews given by Isard[71], Day[72],and LaCourse and Cormack[73] provide good summaries of these models. 

Most of the current models of the mixed-alkali effect are based on the experimental observation of certain macroscopic properties, such as electrical conductivity, alkali ion diffusion and viscosity. However, the secret of the mixed-alkali effect must lie in the structure of the glass and in how the structure controls the mechanisms of alkali diffusion in the glass. 

Mazurin, O.V. (1965) Glass in A Direct Electrical Field, Structure of Glass, 4, 5. Dietzel, A. H. (1983) On the So-called Mixed Alkali Effect, Phys. Chem. Glasses, 24,172-180. 

Na20-Cs20-Si02 glasses at 480°C. is the diffusivity calculated from the measured conductivity by the Nernst-Einstein relation. The deep minimum in (actually the electrical conductivity) represents the mixed-alkali effect. 

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