Physical ageing of silicate glasses at room temperature:: General regularities as a basis for the theory and the possibility of a priori calculation of the ageing rate

A review has been presented of the poorly known data on the ageing of thermometric, optical, and other glasses at observation durations as long as 40 years. The correct mathematical processing of the data is carried out. It is shown that the volume relaxation is described by a simple exponential function with the characteristic time tau varying from three weeks to fifteen years. The process can be complicated by a superposition of slower or faster processes whose characteristic times can differ from the characteristic time of the dominant process by 1.5-2 orders of magnitude. The compositions of the studied glasses (from borosilicate crowns to flints) and their viscosities in the annealing range are given. It is demonstrated that the main characteristic ageing time tau (in years) and the structural temperature T-13.5 (corresponding to a viscosity of 10(13.5) P) are related by the expression log tau = 0.0260T(13.5) - 20.414, which makes it possible to calculate tau from T-13.5 With an error of no more than 20%. The expression is theoretically justified under the assumption that the cooperative beta -relaxation processes occur without alpha -relaxation. The possible relation between the ageing and annealing processes is revealed. This relation follows from the splitting of the stretched exponent relaxation function (which is valid at T-g/T similar to 1) into Debye components at the ratios T-g/T similar to 2.5-2.8, which, for silicate glasses, correspond to room temperature.