Elsevier

Journal of Non-Crystalline Solids

Volume 357, Issue 4, 15 February 2011, Pages 1303-1309
Journal of Non-Crystalline Solids

On the dependence of the properties of glasses on cooling and heating rates II: Prigogine–Defay ratio, fictive temperature and fictive pressure

https://doi.org/10.1016/j.jnoncrysol.2010.12.005Get rights and content

Abstract

In a previous analysis (J. Chem. Phys. 125 (2006) 094505) it was shown by us that – in contrast to earlier believe – a satisfactory theoretical interpretation of the experimentally measured values of the so-called Prigogine–Defay ratio, Π, can be given employing only one structural order parameter. According to this analysis, the value of this ratio has to be, in accordance with experimental findings, larger than one (Π > 1). This analysis is extended here and, in particular, the dependence of the value of Π on cooling and heating rates is studied. Finally, employing the general model-independent definition of internal (fictive) pressure and fictive temperature, developed by us (J. Non-Crystalline Solids 355 (2009) 653), it is shown how these parameters behave in dependence on temperature for different sets of cooling and heating rates. Some further consequences and possible extensions are discussed briefly.

Research Highlights

► The dependence of the Prigogine-Defay ratio on cooling and heating rates is established. ► A general definition of fictive pressure and fictive temperature is given. ► The dependence of fictive pressure and temperature on cooling and heating rates is analyzed.

Introduction

Following the classical concepts developed by Simon [1], [2], vitrification in the cooling of melts is commonly interpreted as the transformation of a thermodynamically (meta)stable equilibrium system into a frozen-in, thermodynamically non-equilibrium system, the glass. Hereby it is assumed in a first approximation that the transformation proceeds at some well-defined sharp temperature, the glass transition temperature, Tg. However, a more detailed experimental and theoretical analysis shows [3], [4], [5], [6] – as stated explicitly already by Tammann [7] – that the transition to a glass via the cooling of a glass-forming liquid proceeds in a more or less broad temperature range, where the characteristic times of change of temperature, τT, and relaxation times of the system to the respective equilibrium states, τ, are of similar magnitude. This vitrification interval and the glass transition temperature, Tg, can be defined – according to these considerations – generally via the relation τT  τ or, employing the definition of the characteristic time of change of the control parameter τT=(T/|T˙|) (c.f. [8]), by1TdTdtτconstant at TTg,where the constant is of the order of unity and depends in its value on the chosen precise definition of Tg.

Eq. (1) implies that the glass transition temperature (and as the result also a broad spectrum of properties of glasses) depend significantly on cooling rates. An extended analysis of the dependence of glass properties for a wide range of cooling and heating rates has been performed in a preceding paper [9]. Here are also the methods are outlined in detail employed in the analysis in the present paper. In this analysis, the structural order-parameter concept as developed by De Donder [10], [11] is utilized. The temperature dependence of a structural order parameter is determined and its effect on the properties of glass-forming melts and glasses is analyzed. Applying the same concepts, it was shown by us earlier in [12] that – in contrast to the previous general believe – the experimental data on the so-called Prigogine–Defay ratio can be given a theoretical interpretation by introducing only one structural order parameter into the description. A detailed discussion of this circle of problems including the analysis of alternative approaches is given also in [13]. This discussion will be continued and extended in the present analysis.

Above statement – that one structural order parameter is sufficient for the theoretical interpretation of the Prigogine–Defay ratio – does not imply necessarily that always only one and not more structural order parameters may be required for an appropriate description of a vitrifying or devitrifying system under investigation (for a detailed discussion of the problem of how many structural order parameters may be required in order to appropriately describe a glass-forming system, see e.g. [2], [13]). In this latter more general case of several structural order parameters, the classical definition of fictive temperature as introduced by Tool [14] (and the similar definition of fictive pressure) becomes ambiguous (see also [15]) as far as for such cases no general definition of these parameters has been available so far. These circumstances are one of the reasons why we searched for and introduced in [13] a new model-independent thermodynamic definition of fictive pressure and temperature. This definition is valid generally independent on the systems considered and in particular valid for any arbitrary numbers of structural order parameters required for their description. The analysis of consequences of this new definition and, in particular, of the dependence of fictive pressure and temperature on temperature for different cooling and heating rates is the second main topic of the present analysis.

The paper is structured as follows: We start the analysis with (i.) a derivation of the Prigogine–Defay ratio including a detailed discussion of details and approximations omitted in [12] and (ii.) the course of fictive pressure and temperature is discussed in dependence on temperature for a large spectrum of cooling and heating rates. A summary of the results and discussion of future possible developments completes the paper.

Section snippets

Behavior of thermodynamic coefficients at glass transition: configurational specific heat

In order to describe glass-forming systems in thermodynamic terms, in addition, to the conventional thermodynamic state parameters one (ξ) or a set of structural order parameters ({ξi}) has to be introduced into the description. The values of these order parameters depend on prehistory or (for the assumed here constant cooling and heating rates, q) on the value of q. A typical example for such type of behavior is shown in Fig. 1 (for the details see [9]).

As one of the consequences, the values

Fictive temperature and fictive pressure

For the first time, the concept of a structural order parameter was introduced into glass science by Tool [14] in terms of fictive temperature. Already Davies and Jones [18] noted that instead of fictive temperature one can employ equivalently the concept of fictive pressure as the determining structural order parameter. This concept was further advanced in recent years by Landa et al. [19], [20] who developed an interpretation of the glass transition and accompanying effects employing the

Discussion

In the present and the preceding [9] paper, the behavior of a variety of thermodynamic properties of glass-forming melts and glasses is analyzed in dependence on cooling and heating rates varying its absolute value in a wide range. This renewed interest is partly caused by the fact that such experiments are feasible, now, analyzing glass-forming melts in the range of cooling and heating rates in the range between 10 4 K/s up to 105 K/s [23], [24], partly, by the increased interest in the

Conclusions

In terms of the generic approach to relaxation, vitrification and devitrification processes, the thermodynamic properties of glass-forming melts in cooling and heating are analyzed theoretically varying the rates of change of temperature in very wide intervals. A detailed derivation of the theoretical value of the Prigogine–Defay ratio is outlined confirming previously obtained results. In particular it is reconfirmed that the Prigogine–Defay ratio has values larger than one also in the case

Acknowledgments

The present research was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG). The financial support is gratefully acknowledged. One of the authors, J. S., acknowledges the discussions with Drs. Jean-Luc Garden and Herve Guillou, Grenoble, France, at the 11-th Lähnwitzseminar on Calorimetry in June 2010 in Rostock.

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