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Determination of the entropy production during glass transition : theory and experiment

Mardi 12 novembre 11:00 - Duree : 1 heure
Lieu : Salle "Louis Weil" E424 - Institut Néel -bâtiment E, 3ème étage, CNRS Polygone scientifique, 25 rue des martyrs, 38000 Grenoble

Orateur : J.L. GARDEN

A glass is a system outside equilibrium. Its physical properties, such as energy, volume or entropy, relax over time. Vitrification is a t ransformation during which a liquid at thermodynamic equilibrium gradually becomes a glass. During this transformation, the second principle of thermodynamics tells us that a positive amount of entropy is produced. What is the origin of this entropy production during the glass transition ? Is it experimentally accessible ? What is its order of magnitude, as compared to the variation of the configurational entropy, for example between liquid and glass ? This presentation is divided in two parts : - firstly, I will explain how a two-level-system, coupled to a master equation, allows to restore the main features of a glassy system. For example, it is possible to simulate the behaviour of the configurational heat capacity as a function of temperature of a non-equilibrium two-level-system. Then, we will see that by using a simple two-body thermal system model, we can simulate the entropy production rate of the two-level-system during cooling and heating. - in a second step, I will present measurements of the specific heat of a model of glassy system, the Polyvinylacetate (PVAc), during temperature ramps (cooling and heating), and during aging experiments. By means of a very fine calibration of our calorimeter, we were able to measure the entropy production rate as a function of temperature during the glass transition of the PVAc. We thus obtained an order of magnitude of the entropy produced during the different stages of the thermal history of the PVAc. These data were compared to variations in the configurational entropy and in the aging entropy of the PVAc. Finally, we were able to experimentally verify that this glass obeys the Clausius theorem. This phenomenological approach, which we used here to a polymeric structural glass, can also be applied to any system commonly studied in solid-state physics : electron glass, spin glass, magnetic system, solid/solid transformation in ferrous alloys (elastic glass), etc... The presentation will be in French and the slides will be in English.

Contact : cecile.delacour@neel.cnrs.fr

Discipline évènement : (Physique)
Entité organisatrice : (Institut Néel / MCBT)
Nature évènement : (Séminaire)
Evènement répétitif : (Séminaire MCBT)
Site de l'évènement : Polygone scientifique

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