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.
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.
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.
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?