Structure et dynamique des liquides surfondus – Strudylis

Liquid systems are an outstanding example of systems containing great relevance in both basic science and industrial applications since the molten state is an essential stage in various industrial processes (glass making, semi-conductors technologies, aerospace companies, nuclear industry, etc.). Although the phenomenon of supercooling has been known from millennia, the microscopic mechanisms that govern the complex behaviour in supercooled liquids are still currently a challenge for the material science. In particular, materials in the supercooling regime may present two specific phase transitions, i.e. liquid-liquid phase transitions and liquid-glass transitions for which the detailed understanding would form a key element in designing novel materials. Such an understanding can be obtained from the study of the evolution of structural and dynamic properties of liquid phases within the supercooling regime. Due to large number of glass-forming materials and systems that can be supercooled it is difficult to construct an explanation of supercooling and glass formation that is both universally applicable and quantitatively predictive. Nevertheless it is reasonable to expect that important insights can emerge from the study of relevant systems that include the main phenomena occurring in the supercooling regime. For this reason, two kinds of technologically important materials, namely aluminosilicate glasses and silicon, have been chosen. 1/ Aluminosilicates glasses can be considered as being a good prototype of the so-called fragile glass-forming liquids for which the 'glassy dynamics' are still under debate. They are also of high technological interest since they are regarded as preferential candidates for nuclear waste storage. 2/ Silicon is the principal material used for solid-state electronics and photovoltaics technologies. Solid (crystalline or amorphous) silicon is a semiconductor, while liquid silicon is a metal. It is thus of the utmost importance to study changes in conductivity and electronic structure of silicon with temperature in the supercooled regime. More particularly, a liquid-liquid phase transition has been predicted but not still confirmed by experiments. The obvious question all these results raise are whether the structural changes involved in this transition can explain the metal-to-semiconductor transition. The aim of this research project is then to provide an accurate description of structural, electronic and dynamic properties of these two prototype systems and to get new insights in two important phenomena that may occur within the supercooling regime, i.e. the liquid-liquid phase transition in a monoatomic system and the extraordinary viscous slow-down that accompanies supercooling and glass formation. To attain these objectives, we will use a very efficient approach based on (i) sophisticated experimental determinations making use of containerless techniques and large-scale facilities (synchrotron and neutron sources) (ii) hybrid molecular dynamics-quantum mechanics simulations and a theoretical treatment of electronic transport. To carry out this work, four institutions are associated: The CEMHTI in Orléans (Developments for investigations at high temperatures), the SOLEIL synchrotron in Gif / Yvette, (X-ray experiments), the ILL (Neutron experiments) and the SIMAP (Ab initio simulations) in Grenoble, Several points will ensure the success of the project. The CEMHTI in Orleans is one of the world leaders in levitation techniques and has already developed various apparatus at synchrotron and neutron sources which are available for this project. The project will also strongly benefit from the involvement of large scale facilities (SOLEIL and ILL). In particular the wide x-ray and neutron expertise of the partners will be useful. In addition, the CEMHTI will have a guaranteed access to SOLEIL through the financial involvement of the 'Région-Centre' in the machine construction. Finally, the SIMAP is one of the leading teams in France working with Molecular Dynamics simulations and has an access to the necessary computing tools through the National Computer Centers and Phynum Center in Grenoble.