RICERCA ITALIANA     

Vibrational dynamics and relaxation in densified glasses and confined disordered systems

Università degli Studi di Perugia

Abstract

The present project is promoting a research based on a collaboration between the Research Units (RU) of L’Aquila, Camerino, Messina, Perugia and Trento and it is devoted to the study of the vibrational dynamics, relaxation phenomena and structural characteristics in disordered systems. The aim of the research is to extract information on the correlation between dynamics and relaxation in glassy systems and in liquids/glasses in conditions where they are confined. This sort of activity is quite important for both the basic research and different technological applications. Indeed the disordered materials usually employed in many applied fields suffer the aging problem, that is the deterioration of even macroscopic properties which, in turn, are governed by microscopic/atomic mechanisms strongly related to dynamics and relaxation processes. In addition, the behaviour of confined disordered systems is becoming more and more important because it is relevant for understanding basic mechanisms related to reduced dimensionality, and it is important to develop applications which are becoming strategic for the use of nano-structured phases. In particular, we will study the characteristics of the acoustic waves which propagates at high frequency in the interval between GHz and THz and their relationship to the structural disorder and the to the interaction between propagating modes and relaxation processes. Also the correlation between the vibrational properties and those like, for >>>

Principal Investigator

Francesco Sacchetti Università degli Studi di PERUGIA

Research Objectives

The objective of the present research project is a better understanding of the interaction mechanisms and the correlation which exists between the vibrational modes and the relaxation phenomena which are present in (either strong or fragile) glasses and in the correlated systems having a similar disordered structure, that is hydration water of biological molecules and ion exchange membranes. The interactions we mentioned appear relevant in different properties of these systems. On one hand, together with the structural disorder, they give rise to the attenuation of the propagating acoustic waves (density fluctuation modes), on the other hand they introduce a connection between the vibrational properties and those more related to atomic diffusion. The specific aims of the project are as follows:

1) We will perform an experimental study, by employing different techniques, of the frequency distribution and the attenuation of the propagating acoustic waves. The available data show that both the structural disorder and the relaxation phenomena contribute to the attenuation as a function of temperature and wave vector transfer Q. At high Q the structural contribution to the attenuation is generally more important, while the dynamic relaxation should be more important at low Q, while in the intermediate region the situation is less known. In addition the relevance of the microscopic and mesoscopic structure is still to be investigated. Therefore we would like to >>>

First Results

As explained above, despite remarkably increased by the constant improvement of experimental techniques in the last decade, our knowledge of dynamics and correlated processes (e.g. relaxations) in glasses and undercooled liquids is still confined to a phase where large amounts of data are collected and interpreted on the base of phenomenological and empirical models. Indeed, the wide variety of existing disordered systems makes the acquisition of new information very difficult, as even very different samples, in terms of chemical or structural properties, display similar behaviours and it is therefore difficult to predict in advance which category (e.g. in terms of rigidity) a new material will belong to. In the present project, the possibility of changing the sample properties without modifying its chemical composition and/or the experimental conditions of study, is considered very useful. In particular, when applicable, permanent densification appears as a promising way to change the characteristics of the studied system without affecting its chemical properties and thus performing a set of experiments all in similar conditions. The main targets of this project concern, first of all, the possibility of producing in a controlled way a set of new and well-characterised samples, having different properties with respect to the initial prototypes. The production processes must be defined in detail and the samples must be produced in amounts which have to be well suitable for >>>

Timescale

24 months

National and international background

The disordered systems give rise to a large extent of phenomena over a wide range of temperature and characteristic times. Surprisingly, the dynamics and thermodynamics of this wide and heterogeneous class of systems show similarities and universal trends which are independent of the specific structure and chemical bonding. This observation suggests that behind the experimental observations there are common and universal origins. At present there are several approaches which try to manage the intrinsic complexity of these systems, however there exist a quite general concept which is very useful for discussing the possible universal mechanisms. This is the idea of “Energy Landscape” (EL) of an interacting atomic system in the many dimension configuration space[1]. This surface is characterized by a large number of local minima, usually proportional to exp(N), N being the number of atoms in the system. In particular, there exists a distribution of energy minima and is possible to jump from a minimum to another one by crossing saddle points to overcome energy barriers of variable height. Each single minimum defines a possible (meta)stable configuration of the ensemble of N atoms. In liquid state, thermal energy is high enough to allow the system to overcome most of the barriers between different minima. If the liquid is cooled-down quickly enough, the system can be trapped in one of the local minima, usually corresponding to a configuration without symmetry properties: the lack >>>