Exemple de spectre Raman calculé en théorie de la fonctionnelle de la densité (figure de gauche) du carbure de bore avec modélisation du désordre substitutionnel des atomes de carbone dans le site polaire des icosaèdres B11C par une super-maille contenant 405 atomes (courbe bleue), comparé à l’expérience (courbe violette) [Crédit : Physical Review Materials, 6 (2022) 016602]. Structure atomique du carbure de bore dans l’état fondamental (figure du centre) et après déformation plastique (figure de droite) [Crédit : Acta Materialia, 226 (2022) 117553].
Our research area is theoretical and numerical physics for materials science, and consists in the ab initio study of the physical properties of materials, mainly with quantum-scale methods based on Density Functional Theory (DFT). The latter reduces the complexity of the Schrödinger equation by using the theorem that the properties of the electronic ground state of a system of atoms and their electrons are uniquely determined by the electronic density of the ground state.
DFT calculations are performed without adjustable parameters and have successfully and reliably predicted the physical properties of many materials, with controllable accuracy. An example is given (figure left) for boron carbide B4C, one of the hardest ceramics, which is used for armouring military equipment and protecting people (bullet-proof vests). B4C is also an excellent neutron absorber, useful for controlling the chain reaction in future fast-neutron reactors.
For this material, we had predicted theoretically that the lowest-energy lacunar point defect consists in the ejection of the boron atom B from the C-B-C chain (center figure), giving carbonaceous C-lacune-C atomic configurations (right figure) that reduce the material's mechanical strength. A series of experiments conducted at the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC, Paris), using the very recent Paris-Edinburgh rotary anvil press for tomography (RoToPEC), has made it possible to apply torsional deformations to boron carbide, driving it in a controlled manner into the plastic regime.
The damage was analyzed by synchrotron X-ray diffraction and Raman spectroscopy, and interpreted using our DFT calculations. The new peaks appearing in both characterization methods are the signature of boron vacancies in the chains, in agreement with theoretical predictions. This new result confirms the avenues for B4C reinforcement actively explored with our partners at IMPMC, the Institut de Recherche sur les CERamiques (IRCER, Limoges), and the Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB).