Quantifying the internal stress in over-constrained glasses by molecular dynamics simulations

Xin Li,Morten M Smedskjaer,John C Mauro,Mathieu Bauchy,Weiying Song

doi:10.1016/j.nocx.2019.100013

Abstract

Abstract Topological constraint theory classifies network glasses into three categories, viz., flexible, isostatic, and stressed–rigid, where stressed–rigid glasses have more topological constraints than atomic degrees of freedom. Such over-constrained glasses are expected to exhibit some internal stress due to the competition among the redundant constraints. However, the nature and magnitude of this internal stress remain poorly characterized. Here, based on molecular dynamics simulations of a stressed–rigid sodium silicate glass, we present a new technique allowing us to directly compute the internal stress present within a glass network. We show that the internal stress comprises two main contributions: (i) a residual entropic stress that depends on the cooling rate and (ii) an intrinsic topological stress resulting from the over-constrained nature of the glass. Overall, these results provide a microscopic picture for the structural instability of over-constrained glasses.

Highlights

Since the birth of glass science and Zachariasen's seminal contribution in 1932, one has wondered whether the propensity for a liquid to crystallize or form a glass upon cooling could in some ways be inferred from its atomic structure [1,2]
The atomic network of the glass is macroscopically at zero pressure, it exhibits some internal stress, which manifests itself as some inter-atomic bonds being under compression, whereas others are under tension
We observe that the volume of Si atoms gradually increases from Q1 to Q4 unit. This is in agreement with the fact that Si–bridging oxygen (BO) bonds are slightly more elongated than Si–non-bridging oxygen (NBO) bonds, as BO atoms are attracted by the two neighboring Si atoms they connect to [38]

Summary

Introduction

Since the birth of glass science and Zachariasen's seminal contribution in 1932, one has wondered whether the propensity for a liquid to crystallize or form a glass upon cooling could in some ways be inferred from its atomic structure [1,2]. In 1990, Gupta and Cooper developed a mathematical foundation for the Zachariasen's rules [3] This framework is based on a topological description of glass networks in terms of interconnected polytopes, wherein an infinite disordered network can exist if it exhibits a non-negative number of degrees of freedom per vertex f and is rigid if the number of degrees of freedom is zero (or lower) [3]. Based on this framework, Gupta postulated that networks featuring f = 0 should exhibit optimal glass-forming ability—a topological condition that is satisfied for SiO2 and B2O3 [4]. Phillips predicted that isostatic glasses exhibit optimal glass-forming ability [6]

Methods

Results

Conclusion