A glimpse into my research activity

From atoms to materials

Nowadays it is common to seek the ultimate cause for the properties of materials by looking at the atomic scale. However, this can be done with various levels of awareness of what a real material is. Metallurgists never forget that a material contains point defects, dislocations, grain boundaries and any other possible kind of microstructural feature that must be considered at more than one length and time scale. In the field of functional materials, conversely, the focus on the details of the electronic structure leads sometimes to the neglection of phenomena as important as diffusion, defects kinetics, and the role of entropic contributions.

Diffusion and defect kinetics from first principles

My first training, as a graduate then a PhD student, was on details of the electronic structure of atoms or solids, first principles calculations and so on. However, maybe because I was dealing with phonons, and materials where the zero point energy was not negligible, I was naturally lead to think about vibrational entropy and temperature effects. Later, when I started to collaborate with (and later joined) the Service de Recherches de Métallurgie Physique of CEA, I gradually got acquainted with the metallurgist approach, although I have almost always been working on insulators or semiconductors. It is difficult but rewarding to try to model diffusion processes in these materials, because one has to deal with various types of defects, with localised charges, electronic excitations; without forgetting that these objects, in real materials, are not easy to probe, nor easy to distinguish from one another: they move, they react with each other, and they occur in many variants with slightly different properties. And diffusion properties are very important when a material is out of thermodynamics equilibrium (which is almost always the case in the real world).
Two materials whose properties I have investigated are silicon dioxide and silicon carbide (SiO2 and SiC). Both these materials share this curious property: they are very important as functional materials in the field of microelectronics (the ubiquitous insulator and an important wide band gap semiconductor), but they also have important applications in the nuclear industry. The first as the main component of nuclear glasses used for disposal of high level nuclear waste. The second as a fuel cladding material in some past and future concepts of gas cooled nuclear reactors.  But other materials have a variety of applications, including in nuclear environments, for example polymers, which I also got recently interested in.


Surfaces and other materials for energy applications

The importance of materials development as a crucial field for  future energy production and storage technologies is now fully recognised, not only in the milieu of research. I think that the approach and expertise that has been developed for decades by metallurgists, since the early days of nuclear research centers in many countries, can contribute significantly to the development of new materials for renewable energy applications.  In the last few years, while my core work still deals with materials for nuclear applications, I was involved in projects dealing with materials for photovoltaic applications, especially thin films photovoltaics. The materials range from CIGS semiconductors, to transition metal dichalcogenides, to the promising halide perovskites, in particular CsPbI3. Here the needs are the same: understand phase stability, defect stability and kinetics, especially at interfaces where things become, sometimes, even more complicated. Again, here, thermodynamic equilibrium is only a wishful (but useful) term of comparison.

The tools: computer simulations

Probably by now you have already understood that I am not an experimentalist. My tools are computer simulations and theoretical models. Nowadays, with the development of parallel machines and parallel codes, it is easy to use a large number (thousands) of CPU for a few hours in order to calculate just one number, for example a reaction barrier, or a formation energy. When you know that you carefully tuned your parameters and the code is fully exploiting the available computing power, you can feel a sort of satisfaction. But be careful: is the number you are calculating really crucial? Or, in other words, when you obtain this number, will you be able to answer the question: so what?

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