High-frequency relaxational spectroscopy in liquid borates and silicates

The Brillouin light scattering technique is used to investigate the structural relaxations in various glass-forming systems. Resonance conditions of various thermally activated processes contributing to the relaxation phenomena are detected by means of temperature scans. The lineshape analysis of the Brillouin spectra yields the two components of the complex mechanical modulus, i.e., storage and loss modulus, allowing one to track the dynamic behavior of structural components on a nanometer scale. The storage modulus reflects the extent of network polycondensation, while losses are due to the friction caused by mobile structural constituents. Data from a series of binary alkali-borate and alkali-silicate systems are reported. Significant responses to the GHz actuation occur above the glass transition temperature. An increased fragility with increasing alkali oxide concentration in reflected in the accelerated decrease of the longitudinal modulus with temperature, for both borates and silicates. In borates, the room-temperature longitudinal modulus increases with increasing alkali concentration, up to at least 30 mol% alkali oxide. The observation of a distinct alkali dissipation peak in silicates indicates that the alkali cation mobility is largely decoupled from network relaxations, while the dissipation spectra for borates reflect a wider distribution of relaxation mechanisms.