High-temperature relaxation mechanisms in inorganic glasses

At temperatures below T_g two relaxation processes are observed in sheet glass (200–500°C) and low-alkali glass (300–600°C): the fast R₁ and the slow R₂ processes which are not connected with the viscous flow, and the structural relaxation occurring R₃ above T_g. The processes R₁ and R₂ proceed at an invariable structure and are characterized by activation energies as high as 5 kcal mol^?1 and 13–15 kcal mol^?1, respectively. The contribution of R₂ amounts to 70–80%. The process R₃, observed near and above T_g, is accompanied with structural variations and, therefore, its activation energy depends on temperature; at T_g it is equal to 60 kcal mol^?1.The processes R₁ and R₂ are due to the mobility and rearrangement of large kinetic units. On the contrary, R₃ is characterized by a low volume of kinetic units. This shows that the ions of silicon and oxygen are involved in this process. The relaxation process R₁ is assumed to be connected with the local fluctuation deformationsof the glass network as in the case of reverse glass deformation under high pressures, and the process R₂ with the mobility of microscopic areas of the glass micro-inhomogeneous structure (structural complexes, microblocks). The continuous spectra corroborate the existence of several high-temperature relaxation processes in silicate glasses.Thus, three relaxation processes are observed in alkali-silicate glasses in the temperature range 200–600°C: the processes R₁ and R₂ are mechanical relaxations, whereas the process R₃ is a structural relaxation determining the viscous flow of glass. The contribution of R₃ to stress relaxation amounts to 5%.There exists a temperature T_k (20–30° below T_g) which is the upper limit of the process R₂. At higher temperatures beginning from T_k the stress relaxation is first determined by the two processes R₁ and R₃, and then by one process R₃. At temperatures below T_k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R₃ becomes negligible and, therefore, in the glass annealing range (below T_k) the mechanical relaxation R₂ and R₁ are mainly responsible; their contribution to the whole relaxation process is as high as 95%.