From noble gas diffusion, conclusions on glass structure can be drawn. Some basic facts on the diffusion of ionic species in glasses are then presented, especially on the self diffusion of Na, K, Rb, and Cs in alkali silicate glasses. Self diffusion coefficients of Li in different lithium silicate glasses obtained recently are also reported. Different possibilities of the interdiffusion behavior are also presented. In the field of ionic diffusion in oxide glasses isotope effect determinations are of special interest. The well known results for Na ion migration are extended by data which were derived from the above mentioned Li diffusion experiments. For Li-containing glasses the isotope effect of the electrical conductivity is also considered. Combining the results on ionic self diffusion and on electrical conductivities by means of the extended Nernst-Einstein relation, and taking the results of isotope effect studies into consideration, conclusions about possible transport mechanisms can be drawn. New theoretical considerations on correlation effects in glasses are also presented.

Transport mechanisms in alkali silicate glasses

An electron micrographic investigation has been carried out on two oxide glasses containing alkali ions and subjected to a sodium⇌silver ion-exchange followed by reduction treatments at various temperatures. The presence of metallic silver in the silver-rich droplet phases has been confirmed by selected-area electron diffraction. The silver-rich phases are found to have diameters ranging from 3 to 50 nm. The nucleus density of these particles is found to have a maximum value at temperatures in the range 250 to 300° C. The optical absorption spectra of the reduced glasses show a maximum around a wavelength of 400 nm. Maxwell-Garnett theory has been used to calculate the optical absorption and the predicted wavelength for maximum absorption is in reasonable agreement with the experimental value.

Microstructure and optical absorption of silver granules in oxide glasses produced by ion-exchange and reduction treatments

The effect of sodium to or from silver ion exchange on electrical conductivity has been investigated for some heterophase glasses containing bismuth oxide. The electrical resistivity in ion-exchanged samples decreases by several orders of magnitude when they are subjected to an electrical field of about 10-30 V cm-1 at around 300 degrees C. Polarisation experiments carried out on samples in such a highly conducting state show that the electrical transport is ionic in nature. The switching to these states is believed to arise as a result of the growth of a silver-rich phase to a percolation configuration. The resistivities of these phases are in the range 10-103 Omega cm with low activation energy values. The results suggest that these materials may constitute a new class of superionic conductors.

Conductivity enhancement in heterophase glasses after ion-exchange treatment

Self-diffusion coefficients of sodium in the temperature range 300 to 375° C have been measured for glasses in the system Na2O-B2O3-Bi2O3-SiO2, by using a radioactive tracer technique. All these glasses have a two-phase structure. The d.c. resistivities of these glasses were also measured over the temperature range 30 to 375° C. The correlation factors,f, of all the samples have been calculated by using the diffusion coefficients and d.c. resistivity values for temperatures above 300° C. The anomalously largef values in the case of Bi2O3-containing glasses are thought to be due to a distribution of sodium ions in the dispersed as well as the continuous phases in these systems.

Diffusion and electrical conductivity studies in some heterophase glasses containing bismuth

Sodium ⇌ Silver interdiffusion coefficients in the temperature range 275° to 375°C have been measured for glasses in the system Na2O—B2O3—Bi2O3—SiO2 by electron probe microanalysis. The interdiffusion coefficients are found to be dependent on silver concentration. By analysing the data on the basis of Helfferich's interdiffusion formula, values of self-diffusion coefficients for sodium and silver have been estimated. The activation energies for these diffusion coefficients lie in the range 0.2 eV—1.4 eV. An increase of the alkali content in glasses containing bismuth oxide increases the activation energy for silver self-diffusion coefficient relative to that of sodium. A possible explanation is given in terms of a two-phase structure model of these glasses.

B. K. D. P. Rao

Papers

Sodium ⇌ Silver Interdiffusion Studies in the Glass System Na2O—B2O3—Bi2O3—SiO2