RELAXATION PROCESSES IN SILICA GLASS

Comprehensive data on the internal friction spectra of silica glass in the temperature range 0-2000 K allowed us to identify three relaxatio n processes, theta, beta, and alpha, each characterized by its own typ e of Si-O bond mobility. The theta-relaxation, a cryo,genic process, i s due to ultra-small-scale rotations of Si-O bonds across microbarrier s at the bottom of the major potential well with an activation energy of 5.1 kJ/mol. The beta-relaxation, a high-temperature process observe d below the glass-transition temperature (T-g = 1445 K) and characteri zed by the activation energy U-beta 294 kJ/mol, is associated with lar ge-amplitude torsional oscillations of Si-O bonds across the major pot ential barrier and with changes in the Si-O-Si bond angle. Thermodynam ic analysis of the molecular mechanism for the alpha-relaxation in vit reous silica provides further proof of the dual nature of this process . In particular it is shown that, as opposed to that observed in polym ers, the glass transition in vitreous silica is a small-scale process involving oxygen atoms as kinetic units and is not associated with rot ational molecular mobility.