Stretched Exponential Relaxation of Glasses at Low Temperature

Abstract

The question of whether glass continues to relax at low temperature is of fundamental and practical interest. Here, we report a novel atomistic simulation method allowing us to directly access the longterm dynamics of glass relaxation at room temperature. We find that the potential energy relaxation follows a stretched exponential decay, with a stretching exponent β = 3/5, as predicted by Phillips' diffusion-trap model. Interestingly, volume relaxation is also found. However, it is not correlated to the energy relaxation, but is rather a manifestation of the mixed alkali effect.

Thermometer Effect: Origin of the Mixed Alkali Effect in Glass Relaxation

“…However, the dramatic increase of viscosity as temperature decreases effectively prevents viscous glass relaxation at ambient temperature [5,6]. Surprisingly, recent experiments and simulations [1,7] have shown that, for certain compositions, glass can still feature some room-temperature structural and stress relaxation. This phenomenon is known as the "thermometer effect" as, in the 19 st century, thermometers made of mixed alkali lime silicate glass used to experience gradual changes of dimension over time, rendering them inaccurate [8,9].…”

“…Note that the average stress remains zero over time and that the observed relaxation does not depend on the choice of ±σ 0 , provided that it remains sub-yield (see Ref. [1]). In effect, this method mimics the relaxation observed in granular materials subjected to vibrations [23,24], wherein small vibrations tend to densify the material (artificial aging), whereas large vibrations randomize the grain arrangements (rejuvenation).…”

“…On the contrary, large stresses move the system far from its initial state, which eventually leads to rejuvenation [27,28]. Although previous accelerated aging techniques have sometimes been shown to yield results that do not match spontaneous aging [26], we ensured that the present protocol predicts a realistic relaxation by checking that, upon relaxation, a hyperquenched glass evolves toward the inherent configurations of the more slowly cooled supercooled liquids [1].…”

“…To reveal the atomistic origin of the MAE and stretched exponential relaxation of glass, we rely here on a recently developed accelerated simulation technique, which successfully reproduced the long-term roomtemperature relaxation observed in Corning Gorilla Glass [1,7]. In that method, the glass is subjected to small, cyclic perturbations of volumetric stress, ±σ 0 .…”

“…As non-equilibrium materials, glasses continuously relax toward the supercooled liquid meta-stable equilibrium state [1][2][3][4]. However, the dramatic increase of viscosity as temperature decreases effectively prevents viscous glass relaxation at ambient temperature [5,6].…”