Determination of divalent cation mobilities in glass by dynamic mechanical thermal analysis (DMTA): evidence for cation coupling effects

Dynamic mechanical thermal analysis (DMTA) is successfully applied to the study of monovalent and divalent cation dynamics in silicate glasses. Mechanical loss peaks, observed either in isothermal plots of tan delta versus log nu or in isochronal plots of tan delta versus temperature, provide information on the mobility of the ions Na+, Mg2+, Ca2+ Sr2+, and Ba2+, and the corresponding activation energies. Features of the classical mixed alkali effect are observed in mixed mono-divalent systems, notably the suppression of a low-temperature 'alkali peak' by a higher temperature 'alkaline earth peak' but (contrary to the mixed alkali effect) the mobility of divalent ions increases as they are replaced by monovalent cations. This coupling between different relaxation processes in these glasses has important theoretical and practical implications.