A mechanism for spontaneous relaxation of glass at room temperature

• Published in: Philosophical magazine (Abingdon, England) Vol. 83; no. 27; pp. 3117 - 3132
• Main Authors: Nemilov, S. V., Johari, G. P.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis Group 21-09-2003; Taylor and Francis
• Subjects: Condensed matter: structure, mechanical and thermal properties; Disordered solids; Exact sciences and technology; Glasses; Physics; Structure of solids and liquids; crystallography; Transition temperature; Densification; Glass; Silicates; Experimental study; Structure relaxation; Mechanism; Atomic displacements; Ambient temperature; Aging; Short-range order; Thermodynamic analysis; Kinetics
• ISSN: 1478-6435; 1478-6443
• DOI: 10.1080/14786430310001593456

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.