Ionic mobility in ion-exchanged glass
A systematic study of the properties of ion-exchanged float glass has been carried out by a.c. impedance spectroscopy. The measurement of electrical parameters allows the ionic mobility within the surface region to be investigated separately from the bulk glass. The correlation of a.c. impedance measurements with electron probe micro analysis, infrared reflectance spectroscopy, differential scanning calorimetry and dynamic mechanical thermal analysis has lent some new insights into anomalous behaviour of ionic transport under these circumstances. A highly resistive layer is found to exist as a result of the ion-exchange process in float glass. This is not attributable to compressive stress alone nor does the mixed alkali effect (MAE) operate within this diffusion zone. Glasses of the same composition prepared by homogeneous mixed-melting, however, indicate a strong mixed alkali effect. Infrared reflectance spectroscopic measurements clearly illustrate a more uniform distribution of non-bridging oxygen ions (and therefore some structural relaxation) as a result of ion-exchange. This also holds true for the comparison of ion-exchanged glass and mixed-melted glass. This result clearly shows that a different structure is generated depending on whether alkali cations are mixed homogeneously or via the ion-exchange process. It is proposed that some type of foreign ion repulsion effect (FIRE) operates when the larger cation is substituted into the glass below Tg. The repulsion of such foreign cations, and their search to find their own new sites causes their immobilisation (and a slow ion-exchange process) via the break up of conduction pathways, and thus the conductivity continues to decrease without any recovery, as more K+ ions are introduced. In contrast, only a weak mixed alkali effect is apparent in melt-grown lithium-alumino-silicate compositions and no high resistance (or cation immobilisation effect) is found in the ion-exchanged alumino-silicate system.