RICERCA ITALIANA     

vibrational dynamics and relaxation phenomena in disordered systems

Università degli Studi di Trento

Abstract

The present project consists of a co-ordinated research including the units of L'Aquila, Messina, Perugia and Trento, with the scientific participation and financial support of the Elettra synchrotron of Trieste. The object of the programme is the interaction between vibrational dynamics and relaxation phenomena in disordered systems, and the correlations existing between these two classes of phenomena as evidenced by recent experimental results, and by numeric simulation. This programme has relevance both for basic research and for applications; in fact, many of the disordered materials usually employed in technology undergo ageing, i.e. a deterioration of their macroscopic properties which, at a microscopic level, is driven by dynamical relaxation processes. In particular, we will investigate: the characteristics of propagating acoustic waves, and how they are affected by structural disorder and by the interaction between the propagating modes and relaxation processes; the correlations between vibrational properties and the properties (like viscosity, glass transition, and related characteristics) which are determined by diffusion and relaxation phenomena; the role of anharmonicity. The experimental techniques employed will be inelastic scattering in the ultra-violet, of X-rays and of neutrons, mechanical and ultrasonic spectroscopy, dilatometry, DSC, low-temperature calorimetry, NMR. Computer simulations will be performed on realistic systems and models. The research >>>

Principal Investigator

Gabriele VILIANI Università degli Studi di TRENTO

Research Objectives

The objective of the present research project is a better understanding of the interaction and correlation mechanisms between vibrational modes and relaxation phenomena, which are present in disordered systems like (strong and fragile) glasses, liquid metals, ionic-exchange membranes. These interactions manifest themselves in different properties of the system. On the one hand, together with structural disorder they attenuate propagating acoustic waves; on the other hand, they induce correlations between vibrational properties, and the properties governed by diffusion of atoms at high temperature. The project has the following specific objectives:
1) Study of the attenuation of acoustic waves by means of different experimental techniques. The presently available data show that both structural disorder and interaction with relaxations contribute to the attenuation, the respective efficiencies depending on temperature and on the wave-vector Q of the wave. At high Q values the structural mechanism prevails, while the dynamic one prevails at low Q; in the intermediate Q-range, the nature of the attenuation is not known. We intend to investigate this range by inelastic scattering of ultra-violet light, X-rays and neutrons, at different temperatures and on different systems. The experiments will be carried out at the laboratories of the participating units, as well as at the Elettra sunchrotron in Trieste, and at the European facilities ILL and ESRF in Grenoble. Besides >>>

Timescale

24 months

National and international background

THE RESEARCH UNITS INVOLVED IN THE PROJECT

The research units involved in the present project form a group of high national and international qualification in the field of the physics of disordered systems, and have already shown a high capability to co-operation in experimental, theoretical and numerical research [1]. The Elettra Synchrotron of Trieste also takes part in the project, both from the scientific side and as a financial supporter of the Trento unit.

STATE OF THE ART

Disordered systems exhibit a large variety of phenomena which cover wide ranges of temperature and characteristic times. The behaviours of the dynamics and thermodynamics of such composite class of systems are, in a sense, surprisingly similar and independent on the details of structure and chemical bond. This indicates that they have a common and "universal" origin. Although this problem can be approached in different ways, a concept that turns out to be very useful and enough general is the concept of "energy landscape", i.e. the hyper-surface of the potential energy (PES) that pertains to a system of interacting atoms in the many-dimensional configuration space [2]. This surface exhibits a very large number of relative local minima, which is believed to grow as exp(N), where N is the number of atoms which constitute the system. There is a distribution of energies for these minima, and it is possible to pass from one to the other by overcoming energy >>>