Mixed‐Alkali Effect in the Dilute Foreign‐Alkali Region. Failure of the Strong Electrolyte/Cationic Interaction Model

Abstract

Electrical conductivity and relaxation data were measured for glasses of composition 0.242[XKzO+(l -X)Na,O] -0.758Si0, from 25" to 205°C and in the dilute Na+ ion composition range X =0.9 to 1. The decrease in electrical conductivity u when the majority alkali K+ is replaced with the foreign alkali Na+ is most rapid in the very dilute range X =0.98 to 1 and becomes less rapid at larger Na+ contents. The electric field relaxation times agree well with the single-alkali mechanical relaxation times. The electric field relaxation function becomes more exponential with increasing foreign alkali content, indicating a corresponding decrease in the concentration of mobile ions. Attempts to account for the decrease in u with increasing foreign alkali content by assuming that the single-alkali glass is a strong electrolyte and that the foreign alkali complexes with and immobilizes the majority alkali were unsuccessful. Calculated values of the number of majority alkali ions complexed by a foreign-alkali ion were unreasonably large.

Ion diffusion and mechanical losses in mixed alkali glasses

“…One such feature is the 'vulnerability' [23,24] of the diffusivity of the majority ion in the dilute foreign alkali region. If only a small number of the majority ions is replaced by the minority ions, one would expect that the mobility of the majority ions is only weakly affected, which means that one would expect a small variation of D A (or D B ), associated with a small change of the corresponding activation energy E A (or E B ) in the limit x !…”

Molecular dynamics simulations of the dynamics of ions in single and mixed alkali glasses

“…This very rapid decrease in r by replacement of the host alkali ions by a small amount of foreign alkali ions indicates that a large number Z of host alkali ions are immobilized per foreign alkali ion introduced. Moynihan et al [24] deduced from experimental data that Z increases with decreasing temperature and reaches values of about 50 at the lowest temperature studied. This experimental fact can be used to rule out immediately some current models of the mixed alkali (MA) effect by showing that in the dilute foreign alkali region the models do not predict a large number of host ions being immobilized by a single foreign ion.…”

“…Here x is the fraction of the majority host ions and (1 À x) the fraction of the minority guest ions. The most rapid change, measured by the largest value of (o log r/ox), occurs at x = 1, i.e., the dilute limit of the foreign alkali region [24]. This very rapid decrease in r by replacement of the host alkali ions by a small amount of foreign alkali ions indicates that a large number Z of host alkali ions are immobilized per foreign alkali ion introduced.…”

Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO–CaO–SiO2 silicate glasses using molecular dynamics

“…Glasses containing a mixture of two alkali ion types have suppressed conductivity relative to the endpoint compositions which only contain one alkali ion type. Figure 1 (left) illustrates this effect for electrical conductivity measurements on (K x Na 1-x ) 2 Si 4 O 9 glasses [3]. The MAE is smallest for alkali ions of a similar size, and is larger for alkali ions of notably different size.…”