On Structural Rearrangements Near the Glass Transition Temperature in Amorphous Silica

Michael I Ojovan,Robert F Tournier

doi:10.3390/ma14185235

Abstract

The formation of clusters was analyzed in a topologically disordered network of bonds of amorphous silica (SiO2) based on the Angell model of broken bonds termed configurons. It was shown that a fractal-dimensional configuron phase was formed in the amorphous silica above the glass transition temperature Tg. The glass transition was described in terms of the concepts of configuron percolation theory (CPT) using the Kantor-Webman theorem, which states that the rigidity threshold of an elastic percolating network is identical to the percolation threshold. The account of configuron phase formation above Tg showed that (i) the glass transition was similar in nature to the second-order phase transformations within the Ehrenfest classification and that (ii) although being reversible, it occurred differently when heating through the glass–liquid transition to that when cooling down in the liquid phase via vitrification. In contrast to typical second-order transformations, such as the formation of ferromagnetic or superconducting phases when the more ordered phase is located below the transition threshold, the configuron phase was located above it.

Highlights

Silica is the most common oxide on the Earth, characterized by a mean abundance of about 37 wt%
The glass transition is often not considered as a thermodynamic phase transition; instead, a rule of thumb is introduced that states that an amorphous material is considered to be glass if its viscosity is equal to or higher than 1012 Pa·s (1013 poise) [16], the glass transition temperature Tg is in practice determined from the characteristic kink of the temperature dependences of the specific volume or enthalpy at Tg [7]
The role of structural changes during the glass transition is of primary importance to understand the reasons behind the drastic changes of amorphous materials’ behavior due to an increase in temperature when crossing Tg

Summary

Introduction

Silica is the most common oxide on the Earth, characterized by a mean abundance of about 37 wt%. TThhee ccoonnfifigguurroonnddiiaammeetteerr ddcc iiss nnoott nneecceessssaarriillyy eeqquuaall ttoo tthhee iinniittiiaall bboonndd lleennggtthh dd. TThhee ssiizzeessddcc ooff nneewwllyy ffoorrmmeedd ccoonnfifigguurroonnss,, wwhhiicchh aarreeaassssuummeeddttoobbeesspphheerriiccaall,,aarreennoott nneecceessssaarriillyy eeqquuaall ttoo tthhee iinniittiiaall bboonndd lleennggtthhss dd,, wwhhiicchh iiss 11..6622. The formation of stable and ultrastable glasses leads to higher glass transition temperatures Their liquid states are changed after heating them above Tg. the percolation threshold is increased. The density of a percolation cluster made of configurons (Equation (9)) changes from 0 to 1, with it being equal to 0 up to Tg and growing until it reaches 1 (Figure 4), and was proposed to be considered as an order parameter in the sense o8 fotfh1e7 general Landau theory of phase transformations [44,75]. Above Tg, the equation for the temperature dependence of the volume change of materials (Equation (1)) needs to be modified to account for the percolation cluster formed in the liquid phase: Vm(T) = Vmv(T) + Vc(T) + Vpc(T). 4.9 × 10−6 K−1 higher than for 4.9 t×he1g0l−a6ssKy −p1hafoser the glassy phase and not higher than 7 × 10−6 K−1 for the melt [1]

Discussion

Conclusions