Mixed alkali effect , glass

Another widely studied phenomenon in alkali borate glasses is the mixed alkali effect, the nonlinear change in glass properties when a second kind of alkali oxide is added into the single-alkali glass. Models have been suggested to explain the mixed alkali effect (144), but a universally accepted model has not been developed as of this writing. 

R. Kirchheim. Interstitial diffusion in glasses and the mixed alkali effect. In H. Jain and D. Gupta, editors, Diffusion in Amorphous Materials, pages 43-54, Warrendale, PA, 1994. The Minerals, Metals and Materials Society. 

Significant non-idealities can occur in the solids with two kinds of mobile cations (or anions). The so-called mixed alkali effect refers to the partly extremely strong depression of the alkali ion conductivity in crystals or glasses if substituted by another alkali ion. This is explained by the individually preferred environments and their interactions.221"228... 

A small mixed-alkali effect occurs in the thermal expansion coefficient of alkali silicate glasses. The deviation from additivity due to this effect has been shown to increase with increasing total alkali oxide concentration, and to be a function of the radius ratio of the larger to the smaller alkali ion present in the glass, with a maximum for the ratio for the sodium-potassium pair. The deviation from additivity is positive for radius ratios less than approximately 1.6, but is negative for larger... 

Glasses containing two or more alkali oxides display the mixed-alkali effect, as shown in Figure 8.3 for sodium-potassium silicate glasses. 

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. 

Mixed Alkali Effect in Alkali Silicate and Alkali Aluminosilicate Glasses... 

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. 

Analysis of the PDF of alkali-alkali ions in (0.5Li-F0.5Na)2O.3SiO2 and (0.oLi-F0.5Ka)2O.iM2D3.2SiO2 glasses reveals a random or stochastic distribution of the two types of alkali and indicates that the pairing of Li-Na ions is not preferential to that of Li-Li and Na-Na in either system so that such pairing of Li-Na ions is not preferential to that of Li-Li and Na-Na in either system so that such pairing is not a necessary part of a successful model of the mixed-alkali effect. 

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