A mechanism for spontaneous relaxation of glass at room temperature

Abstract

Silicate glasses kept at ambient temperature densify spontaneously, and their properties change with time. This is known as ageing of a glass. The spontaneous change occurs according to first-order rate kinetics, with a characteristic (relaxation) time much shorter than that of the α-relaxation process. According to the mechanism proposed here, there are local regions in the network structure of a silicate glass which collapse spontaneously with time by simultaneous motions of atoms seen as β or the Johari-Goldstein relaxation. The collapse causes the Si-O-Si bond angles in the immediate surroundings to change, and these angles become elastically strained. This strain biases the potential energy in a two-site model for oxygen-atom displacement, which occurs at a faster rate and rapidly dissipates the strain. Thus the glass densifies homogeneously on ageing, by two processes: process I, spontaneous collapse of local regions; process II, subsequent dissipation of strain energy resulting from the collapse. The first process, which is much slower, determines the kinetics of ageing. A fictive temperature T_f,β corresponding to the freezing out of the localized motions of the Johari-Goldstein relaxation process is proposed. For silicate glasses, T_f,β is 40-45% of the usual fictive temperature for the α-relaxation process. The volume lost during ageing may be recovered on heating the aged glass to a temperature far below T _g in a time- and temperature-dependent manner, as has already been found from zero-point measurements of a glass thermometer.