HIGH-FREQUENCY RELAXATIONAL SPECTROSCOPY IN LIQUID BORATES AND SILICATES

The Brillouin light scattering technique is used to investigate the st ructural relaxations in various glass-forming systems. Resonance condi tions of various thermally activated processes contributing to the rel axation phenomena are detected by means of temperature scans. The line shape analysis of the Brillouin spectra yields the two components of t he complex mechanical modulus, i.e., storage and loss modulus, allowin g one to track the dynamic behavior of structural components on a nano meter scale. The storage modulus reflects the extent of network polyco ndensation, while losses are due to the friction caused by mobile stru ctural constituents. Data from a series of binary alkali-borate and al kali-silicate systems are reported. Significant responses to the GHz a ctuation occur above the glass transition temperature. An increased fr agility with increasing alkali oxide concentration is reflected in the accelerated decrease of the longitudinal modulus with temperature, fo r both berates and silicates. In berates, the room-temperature longitu dinal modulus increases with increasing alkali concentration, up to at least 30 mol% alkali oxide. The observation of a distinct alkali diss ipation peak in silicates indicates that the alkali cation mobility is largely decoupled from network relaxations, while the dissipation spe ctra for berates reflect a wider distribution of relaxation mechanisms .