Correlation between Alkaline Earth Diffusion and Fragility of Silicate Glasses

We have studied the correlation between liquid fragility and the inward diffusion (from surface toward interior) of alkaline earth ions in the SiO2-Na2O-Fe2O3-RO (R = Mg, Ca, Sr, Ba) glass series. The inward diffusion is caused by reduction of Fe3+ to Fe2+ under a flow of H-2/N-2 (1/99 v/v) gas at temperatures around the glass transition temperature (T-g), The consequence of such diffusion is the formation of a silica-rich nanolayer. During the reduction process, the extent of diffusion (depth) decreases in the sequence Mg2+, Ca2+, Sr2+, and Ba2+, whereas the fragility increases in the same sequence. It is found that the ratio of the activation energy of the inward diffusion E-d near T-g to the activation energy for viscous flow E-eta at T-g increases with increasing fragility of the liquid. The inward cationic diffusion can be enhanced by lowering the fragility of glass systems via varying the chemical composition.