On relaxation nature of glass transition in amorphous materials

Abstract

A short review on relaxation theories of glass transition is presented. The main attention is paid to modern aspects of the glass transition equation qτg = C, suggested by Bartenev in 1951 (q – cooling rate of the melt, τg – structural relaxation time at the glass transition temperature Tg). This equation represents a criterion of structural relaxation at transition from liquid to glass at T = Tg (analogous to the condition of mechanical relaxation ωτ = 1, where the maximum of mechanical loss is observed). The empirical parameter С = δTg has the meaning of temperature range δTg that characterizes the liquid-glass transition. Different approaches of δTg calculation are reviewed. In the framework of the model of delocalized atoms a modified kinetic criterion of glass transition is proposed (q/Tg)τg = Cg, where Cg ≅ 7·10−3 is a practically universal dimensionless constant. It depends on fraction of fluctuation volume fg, which is frozen at the glass transition temperature Cg = fg/ln(1/fg). The value of fg is approximately constant fg ≅ 0.025. At Tg the process of atom delocalization, i.e. its displacement from the equilibrium position, is frozen. In silicate glasses atom delocalization is reduced to critical displacement of bridge oxygen atom in Si-O-Si bridge necessary to switch a valence bond according to Muller and Nemilov. An equation is derived for the temperature dependence of viscosity of glass-forming liquids in the wide temperature range, including the liquid-glass transition and the region of higher temperatures. Notion of (bridge) atom delocalization is developed, which is related to necessity of local low activation deformation of structural network for realization of elementary act of viscous flow – activated switch of a valence (bridge) bond. Without atom delocalization (“trigger mechanism”) a switch of the valence bond is impossible and, consequently, the viscous flow. Thus the freezing of atom delocalization process at low temperatures, around Tg, leads to the cease of the viscous flow and transition of a melt to a glassy state. This occurs when the energy of disordered lattice thermal vibrations averaged to one atom becomes equal or less than the energy of atom delocalization. The Bartenev equation for cooling rate dependence of glass transition temperature Tg = Tg(q) is discussed. The value of fg calculated from the data on the Tg(q) dependence coincides with result of the fg calculation using the data on viscosity near the glass transition. Derivation of the Bartenev equation with the account of temperature dependence of activation energy of glass transition process is considered. The obtained generalized relation describes the Tg(q) dependence in a wider interval of the cooling rate compared Bartenev equation. Experimental data related to standard cooling rate q = 3 K/min were used in this work

Glass relaxation and hysteresis of the glass transition by molecular dynamics simulations

“…The value of T max is very close to the fictive temperature and exhibits a similar dependence on the cooling rate-as also determined recently. 72 We furthermore note that, in the case of the sodium silicate glass, an extrapolation of T max (γ) toward lower cooling rates values (typically 1 K/s) leads to a value that is comparable to the glass transition temperature observed experimentally (see Ref. 17 ).…”

Fabrication, Corrosion, and Mechanical Properties of Magnetron Sputtered Cu–Zr–Al Metallic Glass Thin Film

“…The mechanism of phase separation with negative heat of mixing is different from spinodal decomposition theory (the conventional phase separation caused by miscibility gap). Chen and Turnbull [54,55] proposed that the formation of a unique short-range order (SRO) in metallic glasses, which result in kinks or humps in the liquid/glass free energy curve (as shown in Figure S5), causing the thermodynamical phase separation [56]. Figure 6a shows the SAED pattern of the Cu-Zr-Al thin film metallic glass deposited at the target power of 96 W. Unlike thin film deposited at the power of 72 W, only one broad and diffuse amorphous halo is observed, while short-range ordered clusters are present in the HRTEM image (Figure 6b).…”

Homogeneous nucleation of phase transformations in supercooled water

“…There is no more free volume at T0m in this amorphous solid. It is impossible to delocalize any atom at this temperature [33].…”

“…Excited delocalized atoms would be responsible for viscous flow and time dependence of Tg. At the glass transition, the process of atom delocalization would be reduced without being fully eliminated [33]. Theoretical works based on the notion that a random first-order transition lies at the heart of glass formation have been developed.…”