Abstract
By refining the definition of thermodynamic equilibrium and state variables (thermodynamic coordinate, TC) for solids, it is determined that the state of a glass substance transforms into an equilibrium state after it is solidified. In contrast, the state of a glass substance during the glass transition is a nonequilibrium state. The specific-heat (C) versus temperature (T) curve exhibits hysteresis, which is traditionally believed to invalidate thermodynamic methods. However, the glass transition slowly occurs in a manner such that structural change is decoupled with the fast process of thermal relaxation of phonons, which enables us to describe the hysteresis by thermodynamic methods. The hysteresis is caused by the structural relaxation and the time of relaxation is determined by the energy barrier, which depends solely on the current value of TCs. Therefore, the state in hysteresis can be described by the information of the current structure alone: history-dependent response functions are unnecessary. On the basis of these conclusions, the behavior of the C-T curve with changing heating/cooling rate γ is simulated. The main features of the hysteresis, the shift of C to higher temperatures with increasing γ, the hump structure, and the memory effect are well reproduced from a structure-dependent energy barrier. In view of the structural dependence of the energy barrier, it is not surprising to observe deviations from the Arrhenius law. However, only the terms that are higher than linear in T are observed in Arrhenius plot as the deviation. An important finding of this study is that the apparent energy barrier obtained using the Arrhenius plot significantly overestimates the real value. The extraordinarily large values of the pre-exponential factor of the relaxation time can also be understood on this basis.
Highlights
Many properties of solids, such as the glass state, hysteresis, and memory effects, are commonly treated as nonequilibrium phenomena, which involve many conceptual difficulties
This is easy to answer in an introductory course on thermodynamics, which teaches us that for gases, equilibrium states are described solely by two state variables: temperature T and pressure p
This conclusion seems strange, because too many thermodynamic coordinates” (TCs) are needed for a thermodynamic description, which seems to be in conflict with the spirit of thermodynamics [7]
Summary
The physical properties of plastics cannot be described solely using T and p, and the mechanical properties of metals vary depending on heat treatments These difficulties have been averted by considering these phenomena to be nonequilibrium and no longer treating them within the framework of thermodynamics. All these issues are deeply related to thermodynamic equilibrium To address these issues, the present work describes the glass state from a different view of thermodynamics. An important outcome of this approach is that the activation energy is a strong function of enthalpy and of temperature This provides useful insights for studying many solid-state phenomena that are presently considered as nonequilibrium phenomena, such as the memory function of phase-change materials [19] and the aging and rejuvenating effect of spin glasses [20].
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