Very long physical ageing in inorganic polymers exemplified by the GexSe1−x vitreous system

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Abstract

The effects of very long physical ageing up to 13 yr on vitreous chalcogenide glasses have been investigated for GexSe1−x glasses, x=4, 8 and 12 at%, by means of calorimetric methods. We show that the rejuvenation of such very old materials can be performed without any problem. More importantly, we find that a long ageing leads to a phase separation process. This could explain why the configurational (or conformal entropy) does not apparently reach its equilibrium value as already observed by Gomez Ribelles et al. (Macromolecules 28 (1995) 5867) on other polymeric materials.

Introduction

For more than 30 yr, the number of papers devoted to chalcogenide glasses remains very high. Two main reasons can explain this longevity. The first is linked to the very large number of specific properties and consequently of potential available applications. A recent review of all these potential applications can be found in the proceedings of the international workshop on amorphous and nanostructured chalcogenides [2]. Among all these applications, we have well-known xerography, lithography [3], CD erasable disk [4], chemical sensor [5], long lengths of low-loss IR transmitting devices [6], switching and memory effects [7] and photo darkening [8], [9]. The second one is linked to the relative easy achievement of the glassy state from the melt. The best example is given by pure selenium, which is known to be the only element able to give a mono-atomic glass [10]. As a consequence, chalcogenide glasses can be used as reference materials in order to develop a better understanding of the glassy state, and of its specific properties.

Actually, a glassy material exhibits two important characteristics: the glass transition temperature (Tg), and the sub-Tg-relaxation kinetic also called physical ageing [11]. The glass transition can be defined as the temperature at which the average relaxation time of the atoms, groups of atoms or molecules engaged in cooperative motions is close to 100 s [12]. The sub-Tg-relaxation exists because of the non-equilibrium thermodynamic state reached at T<Tg for a glassy state. This phenomenon can be easily understood thanks to the graph presented in Fig. 1, showing the enthalpy variations as a function of the temperature (similar graphs can be obtained by changing the enthalpy by the entropy or by the specific volume). In this figure, starting from the liquid state, the thermodynamic equilibrium is maintained in the liquid and the liquid-like state down to Tg. For T<Tg, the equilibrium is lost and the characteristic curve deviates from the equilibrium one. By maintaining a glass at Ta<Tg (called the annealing temperature), the system will lose its excess energy in order to reach a more favourable thermodynamic state. After an infinite duration, according to Fig. 1 and model 1, the steady state must be reached (given by extrapolation, for T<Tg into the glassy state of the liquid-like equilibrium curve defined for T>Tg) and the total amount of energy (enthalpy) the glass will lose can be estimated by the following relationship:ΔH=ΔCp(Tg)(Tg-Ta),where ΔCp(Tg)=Cpl-Cpg, the difference between the heat capacities in the liquid-like state and the glassy state estimated at the temperature Tg. In practice, for many polymeric glasses, and as observed first by Gomez Ribelles et al. [1] and later by many other authors [13], [14], [15], [16], the extrapolated liquid state is not reached. The one reached instead is an intermediate state as shown in Fig. 1 and model 2, and it is defined according to the following relationship:ΔH=(ΔCp(Tg)-δ)(Tg-Ta).One way to be able to validate one of these two models consists in doing experiments in such a way that the complete ageing is performed, i.e. to wait an infinite time. As we have studied the ageing kinetics of GexSe1−x chalcogenide glasses with 0<x<12% some years ago [17], [18], we propose to analyse what happens for the same materials obtained from the same synthesis, but now with an ageing greater than 13 yr. Thus, this work deals with the effects of a very long ageing on the glass transition phenomena and on the physical ageing behaviours.

Section snippets

Methods

The samples were prepared by melting a mixture of Ge and Se in a vacuum-sealed quartz tube heated at a temperature of 1273 K during 10 h. This was followed by a quench of the tube in water. In this way ingots were obtained, and the glassy state was checked by X-ray diffraction. This preparation was performed in 1991 and the samples studied now come from the same ingots kept at room temperature up to now (2004).

The calorimetric analysis is performed with a Perkin Elmer DSC 7 calorimeter calibrated

Results

The DSC curves obtained from the different samples that have undergone a very long ageing are displayed in Fig. 2. For each curve, the endothermic peaks of very large magnitudes observed are in fact the manifestation of the glass transition. This transition occurs as two peaks for samples with 4 and 8 at% of Ge (curves a and b) and one peak for the sample with 12 at% (curve c). The existence of such endothermic peaks is directly connected to the effects of ageing and their surfaces are directly

Discussion

The data concerning these rejuvenated samples are identical with those published in 1991 [17] and in 1998 [18] obtained from samples collected into the same ingots but also with those of Azoulay et al. [24] and Derrey et al. [25]. This is true for the values of Tg, as well as for the values of ΔCp at Tg. Thus, after a very long ageing period the thermal cycle used to rejuvenate the glass restores the original characteristics and the observed phenomena are reversible. The interesting result is

Conclusion

In this work carried out on very old chalcogenide glasses, we have shown that rejuvenation can be performed without any problem. The calorimetric data have shown the existence of a very important shift toward high temperatures of the glass transition temperature. At the same time, the relaxation peak observed as a single endothermic peak at short ageing duration is now observed as two peaks for concentrations having 4 and 8 at% of Ge. We conclude that a very old ageing performed at low

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