Self-Diffusion of Calcium Ions in Soda-Lime and Slag Glasses

Abstract

The self-diffusion coefficients of calcium ions in 16Na₂O⋅12CaO⋅72SiO₂(wt%) glass and 40CaO⋅20Al₂O₃⋅40SiO₂(wt%) slag glass at temperatures near and below transition point have been measured employing radio-isotope ⁴⁵Ca. The concentration gradient of radio-activity in surface layer of glass samples after diffusion annealing was determined by counting the residual activity with a GM-tube counter on the surface by progressively chemical etching away the material in steps of about 1 micron meter.
It was found that the self-diffusion coefficients of calcium ions in both glasses below the transition point were of the order of 10^-14-10^-16cm²/s, suggesting that the mobility of calcium ions was extremely small in the solid glasses. The activation energy for diffusion of calcium ions in solid glasses was about 60kcal/mol, which seemed to be reasonable in comparison with the electrostatic binding energy between cation and oxygen ion.
The self-diffusion coefficients of calcium ions in the slag glass agrees closely with those of oxygen ions below the transition point, while the self-diffusion coefficients of calcium ions in the soda-lime glass are greater by one order of magnitude than those of oxygen ions in the same glass. The difference of diffusion behavior of calcium ions in the slag and the soda-lime glasses may be attributed to action of alkali ions on the mobility of calcium ions.