Structural relaxation and rigidity transition in aged and rejuvenated As x Se 100− x glasses (original) (raw)

Theory of aging in structural glasses

The Journal of Chemical Physics, 2004

The random first order transition theory of the dynamics of supercooled liquids is extended to treat aging phenomena in nonequilibrium structural glasses. A reformulation of the idea of "entropic droplets" in terms of libraries of local energy landscapes is introduced which treats in a uniform way the supercooled liquid (reproducing earlier results) and glassy regimes. The resulting microscopic theory of aging makes contact with the Nayaranaswamy-Moynihan-Tool nonlinear relaxation formalism and the Hodge-Scherer extrapolation of the Adam-Gibbs formula, but deviations from both approaches are predicted and shown to be consistent with experiment. The nonlinearity of glassy relaxation is shown to quantitatively correlate with liquid fragility. The residual nonArrhenius temperature dependence of relaxation observed in quenched glasses is explained. The broadening of relaxation spectra in the nonequilibrium glass with decreasing temperature is quantitatively predicted. The theory leads to the prediction of spatially fluctuating fictive temperatures in the longaged glassy state, which have non-Gaussian statistics. This can give rise to "ultra-slow" relaxations in systems after deep quenches.

As Se Rigidity transition

Differential scanning calorimetry (DSC) measurements were performed to investigate the kinetics of the structural relaxation in aged and unaged (rejuvenated) As x Se 100 À x glasses with 0 r x r 40. The activation energy of the glass transition (E a ) of the aged and rejuvenated glasses was determined from the variation of the glass transition temperature (T g ) with the heating rates (b). Significant effect of prolonged aging of the glasses on the values E a was observed. Evidence of transition from floppy to rigid phase is presented. The observed significant physical aging in samples with composition x o 40 indicates the absence of the intermediate phase. The compositional dependence of T g for aged and rejuvenated data was analyzed using the stochastic agglomeration theory.

Relaxation dynamics and aging in structural glasses

2013

We present a study of the atomic dynamics in a Mg 65 Cu 25 Y 10 metallic glass former both in the deep glassy state and in the supercooled liquid phase. Our results show that the glass transition is accompanied by a dynamical crossover between a faster than exponential shape of the intermediate scattering function in the glassy state and a slower than exponential shape in the supercooled liquid. While the crossover temperature is independent on the previous thermal history, both the relaxation rate and the shape of the relaxation process depend on the followed thermal path. Moreover, the temperature dependence of the the structural relaxation time displays a strong departure from the Arrhenius-like behavior of the corresponding supercooled liquid phase, and can be well described in the Narayanaswamy-Moynihan framework with a large non-linearity parameter.

COMPOSITIONAL TREND OF THE GLASS TRANSITION TEMPERATURE IN AsxSe1-x NETWORK GLASSES

2018

glasses (x = 0 to 0.4) is investigated using differential scanning calorimetry. The variation of Tg with As content and the mean coordination number of aged and rejuvenated samples are discussed in terms of proposed theoretical and empirical models. Critical assessment of the applicability of various models to explain the compositional variations of Tg is presented. Evidence of multiple rigidity transitions in the present network glasses is reported and the influence of the long-term physical aging is discussed.

Experimental verification of the reversibility window concept in binary As-Se glasses subjected to a long-term physical aging

Physical Review B, 2008

Physical aging effects caused by prolonged natural storage ͑ϳ22 years͒ in binary As x Se 100−x glasses are probed by temperature-modulated differential scanning calorimetry. It is shown that all aged samples with x Ͻ 40 reveal nonzero out-of-phase component of complex heat flow testifying physical aging effect. The first composition, which can be attributed to a so-called self-organized phase characterized by the absence of natural physical aging effect, is the stoichiometric As 2 Se 3 glass; lack of aging within the so-called reversibility window, 28Ͻ x Ͻ 38, is not found. High-resolution x-ray photoelectron spectroscopy fails to show any evidence of fourfold-coordinated As atoms, which is consistent with the aging ability of glasses with x Ͻ 40.

Structural relaxation in glasses: numerical exploration of variables of Narayanaswamy and Moynihan's model

Journal of Food Engineering, 2003

The accurate description of the relaxation phenomenon near the glass transition temperature allows a better interpretation of differential scanning calorimetry (DSC) curves, since it can be separated from other thermal effects such as crystallization, melting, and recrystallization. The structural relaxation model of Narayanaswamy and Moynihan for thermorheological simple glasses was numerically explored for obtaining a grasp of the functional influence of the different parameters: i.e. relaxation enthalpy Δh, non-linear parameter x, amplitude exponential distribution parameter b, and pre-exponential parameter A.It is known that when the relationship between the velocity of heat transfer in the cooling run (qc) and the velocity of heating in the heating run (qh) is equal to unity, the curves obtained in a DSC suffer only an horizontal translation for different absolute values of cooling and heating rates. It was found in this work that this criterion can be generalized, and curves also overlap by means of a horizontal translation when qc/qh is a constant different from one, whichever the particular values of qc or qh will be.Finally, changes in the shape of the curves when using different values of qh and qc with a fixed qc/qh relationship can be used as an indicator of the occurrence of other phenomena (i.e. recrystallization, melting, etc.) in addition to structural relaxation.

Non-isothermal structural relaxation and the glass transition temperature

Solid State Sciences, 2003

The glass transition was studied by a new stepwise differential scanning calorimetry (DSC) technique on the model glasses As 2 Se 3 and As 2 S 3. The glass transition was separated to the two components: a reversible (thermodynamic) one, reflecting temperature changing of the vibrational amplitude, and an irreversible (kinetic) one, bears on structural relaxation. The value of the glass transition temperature, T g , determined from the thermodynamic part of the glass transition was found to be independent both on the heating/cooling rate and the thermal history of glass. The value of T g depends only on the chemical composition of the glass and thus it could be regarded as a material constant. The heating/cooling rate dependence of T g , known from DSC or DTA measurements, is due to kinetic part of the glass transition.

Physical ageing in Se–Te–Sb glasses

Journal of Physics and Chemistry of Solids, 2013

Bulk Se 60 À x Te 40 Sb x glasses in the composition range 0 r x r 14 were prepared by the melt quenching method. Differential Scanning Calorimetric (DSC) and thermal crystallization studies were performed to understand the thermodynamic property like glass transition and structural transformations. These glasses exhibit sharp endothermic peak at the glass transition (T g). Disappearance of the endothermic peak at T g in the rejuvenated samples clearly indicates the ageing effect in these glasses. Addition of Sb to Se-Te increases the connectivity of the structural network which is evidenced from the increase in T g. A distinct change in the slope of the T g at x ¼6, indicates a major change in the way the network is connected. The glass forming ability and the thermal stability also exhibit a maximum at x ¼ 6. T g increases with the ageing time and the corresponding fictive temperature (T f) calculated from the specific heat curves shows a decreasing trend. The molecular movements along the polymeric Se chains might cause the structural relaxation and the physical ageing. The physical ageing effect has been understood on the basis of the Bond Free Solid Angle (BFSA) model proposed by Kastner. Thermally crystallized samples show the formation of rhombohedral Sb 2 Te 3 , rhombohedral Sb 2 Se 3 and hexagonal Te phases.

Physical Aging of Maltose Glasses

Journal of Food Science, 1996

Physical aging is the alteration in the physical properties of a glass, as a function of time, at temperatures below its glass transition temperature (Tg). The physical aging of maltose glasses, prepared from maltose monohydrate sugar, was studied using differential scanning calorimetry (DSC). The maltose was aged at 30ЊC for times ranging from 5 min to Ϸ10 days. The Tg and change in heat capacity of the unaged and aged maltose glasses were measured. The change in enthalpy relaxation with aging time was determined by integrating the difference between the aged and the unaged DSC thermograms. The enthalpic relaxation data were adequately described by the Williams-Watts equation.

The very long-term physical aging of glassy polymers

The thermodynamic behavior of glasses well below the glass transition temperature (T g) is scarcely explored due to the long time scales required for such investigation. Here, we characterize the thermodynamic state of several polymer glasses aged for about 30 years at room temperature, that is, at more than 100 K below their respective T g (s). To this aim we employ differential scanning calorimetry (DSC), which, via the specific heat, allows characterizing the enthlapy attained after a certain aging protocol and the way the glass with such enthalpy devitrifies when heated. We complement these results with extensive DSC studies on these polymers aged in the same conditions of temperature for time scales ranging from minutes to months. The main outcome of the present work is that these polymers aged in these conditions reach a plateau in the enthalpy with partial enthalpy recovery and devitrify well below T g. This result provides compelling evidence for the existence of a fast mechanism of equilibrium recovery, beyond the standard slow one in proximity of T g. The analogy with other kind of glasses is highlighted, stigmatizing the uni-versality of such behavior. Finally, the way the fast mechanism of equilibrium recovery could be exploited to obtain glasses with low energy state is discussed.