PhD subjects

The societal context of the study is the optimization of cementitious matrices for nuclear waste conditioning. These cementitious matrices are composed of hydrated minerals, some of which are lamellar (portlandite Ca(OH)2, brucite Mg(OH)2, brushite CaHPO4.2H2O, gibbsite Al(OH)3...). Very few data exist in the literature on the structural damage of these hydrated lamellar minerals under electron irradiation. The aim of the proposed thesis is to experimentally investigate irradiation-induced structural modifications in various types of compounds, with a view to gaining a better understanding of the damage mechanisms of these compounds under irradiation, and to identify irradiation sensitivity criteria in order to ultimately optimize the chemical and mineralogical composition of the materials.

Reducing the density of materials is one of the best ways to diminish our energy footprint. One solution is to replace massive materials by microlattices. Among these, random architecture structures inspired by bird bone structure offer the best advantages, with isotropic mechanical behavior and increased mechanical resistance, while meeting the challenges of the circular economy. These material-saving metamaterials are manufactured by 3D printing and can be compacted at the end of their life cycle. Among manufacturing technologies, UV polymerization of liquid organic resin or composite is the most promising. It produces mechanically resistant materials without generating manufacturing waste. It is also possible to include large quantities of bio-sourced fillers, reducing even further their environmental impact.

The PhD-thesis proposed here focuses on the development of polymeric nanocomposite microlattice structures from resin formulation to mechanical properties study (viscoelasticity, yield stress, fracture resistance) through printing and post-processing stages. From a more fundamental point of view, the aim is to study the link between the composition, shape and surface properties of the fillers on one hand, and the imprimability of the composite resine and the mechanical properties of the resulting metamaterial on the other hand. The thesis will focus on the study of cellulose-type fillers in nanoparticle, microparticle or fiber form. This multidisciplinary study bridges technology to science while producing data for a digital twin.

As part of the energy transition, the production of hydrogen from solar energy appears to be an extremely promising means of storing and then producing energy. To develop on a large scale, water photoelectrolysis requires materials with high catalytic efficiency. Among the candidates considered, materials derived from barium titanates appear promising because their ferro- and piezoelectric properties could increase their photocatalytic effect. Therefore, we propose to synthesize BaTiO3 nanoparticles by flame spray pyrolysis and to make substitutions on Ba and O in order to study the effect of these modifications on the ferroelectric properties of the material. The addition of noble metal inclusions to the surface of the particles, likely to improve catalysis, will also be addressed. Finally, photocatalysis and piezocatalysis tests will make it possible to establish the links between ferroelectric and catalytic phenomena in this family of materials. This subject will be carried out in collaboration between the LEEL of the CEA and the SPMS of Centrale – Supelec.

In the context of the search for processes that are less polluting and more energy-efficient than current processes, it is interesting to produce high-stake molecules such as C2H4 by developing alternative synthesis routes to steam cracking, which is used in the majority of cases, but is energy-intensive and based on fossil resources. Processes such as photocatalysis, which relies on the use of light energy, seem an attractive way of generating these molecules of interest. In this context, we have already shown that the use of TiO2-based photocatalysts decorated with copper particles enables the production of ethylene from an aqueous solution of propionic acid, with a selectivity (C2H4/other carbonaceous products) of up to 85%.

However, photocatalysis kinetics can be slow, and it can take a long time to identify the best catalysts or catalyst/reagent pairs for a given reaction. So, in order to determine whether radiolysis, which relies on the use of radiation to ionize matter, can be an effective method of screening catalysts, initial experiments have already been carried out on catalyst (TiO2 or Cu TiO2)/reagent (propionic acid more or less concentrated) pairs, previously studied in photocatalysis. Initial results obtained by radiolysis are encouraging. In these experiments, only dihydrogen production was measured. A significant difference was observed in this production depending on the system: it was high during radiolysis of propionic acid with TiO2 nanoparticles, and significantly lower in the presence of Cu TiO2 nanoparticles, suggesting a different reaction path in the latter case, in line with observations made during photocatalysis experiments.

The aim of this thesis work will be to extend these initial results by synthesizing nanoparticles (catalysts), preparing reagent/catalyst mixtures, then irradiating them and measuring the various gases produced by gas-phase micro-chromatography, with special attention on ethylene. Particular attention will be paid to determining the species formed, especially transient ones, in order to ultimately propose reaction mechanisms accounting for the differences observed for the different reagent/catalyst pairs. Comparisons will also be made with results obtained by photocatalysis.

Raw earth materials, which have found multiple uses for millennia, now offer considerable potential for helping to adapt to the changing climate, thanks to their natural ability to regulate heat and water, as well as their low-CO2 production and shaping. However, scientific advances are still needed to get a more precise understanding of these materials, up to the nanometric scale.

This thesis focuses on the link between the mechanical properties of raw earth soil materials and their nanostructure, emphasizing the roles of confined water, ions and organic substances. Two approaches, based on the expertise on nanoporous media developed at CEA, Saclay and Marcoule, will be followed: the analysis of old materials using spectroscopic and radiation scattering methods, and the development of a screening protocol to identify physicochemical parameters important for durability. This research, which ultimately aims to optimize the formulation of raw earth materials, will be carried out in collaboration with architects specialists in the field.

Our recent results show that nanodiamond can also act as a photocatalyst, enabling the production of hydrogen under solar illumination [1]. Despite its wide band gap, its band structure is adaptable according to its nature and surface chemistry [2]. Moreover, the controlled incorporation of dopants or sp2 carbon leads to the generation of additional bandgap states that enhance the absorption of visible light, as shown in a recent study involving our group [3]. The photocatalytic performance of nanodiamonds is therefore highly dependent on their size, shape and concentration of chemical impurities. It is therefore essential to develop a "tailor-made" nanodiamond synthesis method, in which these different parameters can be finely controlled, in order to provide a supply of "controlled" nanodiamonds, which is currently lacking.

The aim of this PhD is to develop a bottom-up approach to nanodiamond synthesis using a sacrificial template (silica beads or fibers) to which diamond seeds < 10 nm are attached by electrostatic interaction. The growth of diamond nanoparticles from these seeds will be achieved by exposing these objects to a microwave-enhanced chemical vapor deposition (MPCVD) growth plasma, allowing very fine control of (i) the incorporation of impurities into the material (ii) its crystalline quality (sp2/sp3 ratio) (iii) its size. This growth facility, which exists at the CEA NIMBE, is used for the synthesis of boron-doped diamond core-shells [4]. In the second part of the thesis, an innovative process (patent pending) is implemented to achieve MPCVD growth of diamond nanoparticles by circulating the sacrificial templates in a gas stream. During this work, different types of nanodiamonds will be synthesized: intrinsic nanoparticles (without intentional doping) and nanoparticles doped with boron or nitrogen.

After growth, the nanoparticles will be collected after dissolution of the template. Their crystal structure, morphology and surface chemistry will be studied at CEA NIMBE by scanning electron microscopy, X-ray diffraction and Raman, infrared and photoelectron spectroscopy. A detailed analysis of the crystallographic structure and structural defects will be carried out by high-resolution transmission electron microscopy.

Nanodiamonds will then be surface-modified to give them colloidal stability in water. Their photocatalytic performance for hydrogen production will be evaluated in collaboration with ICPEES (Strasbourg University).


References
[1] Patent, Procédé de production de dihydrogène utilisant des nanodiamants comme photocatalyseurs, CEA/CNRS, N° FR/40698, juillet 2022.
[2] Miliaieva et al., Nanoscale Adv. 2023.
[3] Buchner et al., Nanoscale (2022)
[4] Henni et al., Diam. Relat. mater. (under review)