Relaxation phenomena in glass technology

Abstract

Investigations of relaxation phenomena in glasses are reviewed. The role of such phenomena in the formation and heat-treatment as well as in other technological processes is shown. The characteristics of the surface layer are indicated from the standpoint of the physics of mesomechanics and the general structural features of inorganic glasses. A number of operational reliability factors for glass articles are examined.Phenomena due to a transition from nonequilibrium to equilibrium states as a result of the displacement of kinetic units of a system are called relaxation processes. For this reason these phenomena are general for all processes: a system driven out of a state of equilibrium tends toward equilibrium at a rate that is proportional to the degree of departure from equilibrium.Structural, mechanical, electric, magnetic, and other forms of relaxation are distinguished. The most widely used methods of investigating relaxation phenomena in glassy systems are thermal and mechanical, based on monitoring relaxation phenomena, including by means of relaxation spectrometry and by changing the temperature and mechanical actions applied to the objects being studied.The most easily visualizable description of a relaxation process is the so-called temperature jump -a sharp change in the temperature of a pre-stabilized material followed by isothermal soaking at the new temperature (Fig. 1). The total change occurring in the properties in time (starting at t 0 ) as a result of a temperature change DT = T 2 -T 1 can be divided into two parts: instantaneous (isostructural) change of the properties DP g and structural or relaxation change DP s . An isostructural change of the properties is a result of a change in the intensity of thermal vibrations of the particles. A structural change of the properties is associated with thermally activated changes of the relative position of the particles with their vibrational intensity remaining constant.It should be noted that relaxation phenomena unfold differently, depending on the properties, the form of the deformation, and the conditions under which relaxation occurs, and they manifest as an elastic after-effect, internal friction, and stress relaxation. A definite excess of energy must be present in the system, which is why relaxation phenomena develop. The system tends toward equilibrium only in cases where the kinetic units overcome a definite energy barrier, for example, in the presence of viscous shear (stress relaxation), or the system breaks up into individual fragments (relaxation fracture). t 0 t 0 t t à b Fig. 1. General variation of the property P during the time t as a result of a temperature jump: a) temperature jump; b ) change of the property P in time during isothermal soaking.

On color centers in glasses

“…The subsystem's return to equilibrium, which is associated with the destruction of color centers and weakening and vanishing of color, is effectuated by relaxation phenomena, which can be accelerated by thermal and optical stimulation [17,18].…”