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 R 1 and the slow R 2 processes which are not connected with the viscous flow, and the structural relaxation occurring R 3 above T g. The processes R 1 and R 2 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 R 2 amounts to 70–80%. The process R 3, 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 R 1 and R 2 are due to the mobility and rearrangement of large kinetic units. On the contrary, R 3 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 R 1 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 R 2 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 R 1 and R 2 are mechanical relaxations, whereas the process R 3 is a structural relaxation determining the viscous flow of glass. The contribution of R 3 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 R 2. At higher temperatures beginning from T k the stress relaxation is first determined by the two processes R 1 and R 3, and then by one process R 3. At temperatures below T k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R 3 becomes negligible and, therefore, in the glass annealing range (below T k ) the mechanical relaxation R 2 and R 1 are mainly responsible; their contribution to the whole relaxation process is as high as 95%.