PhD subjects

The French neutron scattering community is proposing to build a new High-Current Accelerator-driven Neutron Source (HiCANS). Such a source would use a low-energy proton accelerator, a few tens of MeV, to produce thermal and cold neutrons and power an instrumental suite of around ten spectrometers. The aim of the thesis project is to build a multi-scale description of the operation of a neutron scattering spectrometer, ranging from the description of microscopic neutron moderation processes and neutron interactions with atomic structure and sample dynamics, to the propagation of neutrons through advanced optical elements and the production of background by secondary particles. The ultimate aim is to be able to carry out virtual neutron scattering experiments and accurately predict instruments performances on the future ICONE source.

At present, although numerous works have reported measurements of the thermal conductivity of high-entropy oxides crystallising in a wide variety of structures, the number of studies devoted to the understanding of the thermal transport mechanisms in these materials is extremely limited, and no publication to date has reported a study of their lattice dynamics. In this context, the main objective of this thesis project is to systematically study the links between chemical and structural disorder and the thermal transport of high-entropy oxides, based on the experimental study of their lattice dynamics. On the one hand, this study will provide a better understanding of the physical properties of this class of materials from a fundamental point of view, and on the other hand, from a more applied point of view, it will provide new ways of optimising their thermal transport properties, for example for thermoelectric applications or as thermal barriers.

High entropy functional materials have been a fast-growing area of research in the recent years, owing to their highly promising properties for a large number of applications, particularly in the energy sector. These include the colossal dielectric constants observed for several compositions and structures, opening the way to applications in capacitors or supercapacitors, efficient compositions as anodes for lithium batteries or as solid electrolytes, and encouraging catalytic or photocatalytic properties for the production of dihydrogen. They are also being extensively studied for their potential application in thermal barrier devices or thermoelectric energy conversion by thermoelectric effect, owing to their very low thermal conductivity values. Indeed, their intrinsic chemical and structural disorder has been shown to significantly reduce thermal conductivity compared with their parent compounds.