Structural relaxation mechanisms in hydrous sodium borosilicate glasses

Borosilicate glasses (16Na(2)O-10B(2)O(3)-74SiO(2), NBS) with water contents up to 22 mol% H2O were prepared to study the effect of water on structural relaxation using DTA, viscometry and internal friction measurements. The results show that the glass transition temperature T-g of DTA and the isokom temperature T-12, of viscometry are in excellent agreement, confirming the equivalence of enthalpy and viscous relaxation for NBS glass. Combining T-g data with water speciation data demonstrates that OH groups are mainly responsible for the decrease of T-g with increasing hydration, while molecular water plays only a minor role. Internal friction spectra at 7.125 Hz confirm the decisive influence of water on mechanical relaxation. The temperature range of alpha-relaxation (glass transition) strongly decreases while two beta-relaxation peaks (sub-T-g) progressively appear with increasing water content. A high temperature beta-relaxation peak, attributed to the presence of OH groups, shifts from 670 to 450 K as total water content increases from 0.01 to 5 wt%. A low temperature beta-relaxation peak, attributed to molecular water, appears at 380 K and 330 K in glasses containing 3 and 5 wt% H2O, respectively. These findings suggest that relaxation mechanism of different hydrous species at low temperature may contribute to fatigue of stressed glasses.