High-throughput screening of catalysts for the direct conversion of CO2 into synthetic fuels
This doctoral project aims to develop an innovative high-throughput screening approach for catalysts for the direct conversion of CO2 into synthetic fuels, known as CO2-FTS. This approach will combine a catalyst screening platform with in situ/operando characterization techniques and artificial intelligence methods to accelerate the discovery and optimization of high-performance catalysts. It aims to identify doped FeOx-type catalysts for the CO2-FTS reaction (>50% conversion, high selectivity towards C8-C16). Several high-throughput screening campaigns will allow for iterative optimization of compositions and reactive conditions. A numerical model of the parametric landscape will then be developed. This model will subsequently be coupled with multi-scale modeling from the active site to the reactor level. The developed catalysts will contribute to the energy transition by enabling a circular carbon economy.
Dies to wafer direct bonding: from physical mechanisms to the development of thin stackable dies
Direct dies-to-wafer bonding has become, in recent years, a major development axis in microelectronics and at the heart of many LETI projects, both in silicon photonics and for 3D applications involving hybrid bonding.
Due to their small size, die bonding allows the study of direct bonding edge effects and the implementation of new direct bonding processes that can shed original light on the mechanisms of direct bonding, which are already well studied at LETI. From a more technological perspective, the development of thin stackable chips will also be a very interesting technological key for many applications. This approach is a clever alternative to classical damascene processes to address the challenges related to the planarization of surfaces with low density of high topographies.
Hydrodynamic simulations of porous materials for ductile damage
The mechanical behavior of metallic materials under highly dynamical loading (schock) and especially their damage behavior is a topic of interest for the CEA-DAM. For tantalum, damage is ductile : by nucleation, growth and coalescence of voids within the material. Usual ductile damage models have been developed using the simplifying assumption that voids are isolated in the materials. However, recent studies by direct simulations explicitly describing a void population in the material (and experimental observations after failure) have shown the importance of void interaction for predicting ductile damage. Yet, the microscopical mechanisms of this interaction remain little known.
The objective of the PhD is to study the growth and coalescence phases of ductile damage through direct numerical simulations of a porous material undergoing dynamic loading. Hydrodynamic simulations, in which voids are explicitly meshed within a continuous matrix, will be used to study relevant scales of length and time. Monitoring the void population throughout the simulation will provide valuable information on the influence of void interaction during ductile damage. Firstly, the bulk behavior will be compared to the one predicted by usual models of isolated voids, showing the macroscopic effect of void interaction. Secondly, the evolution of the size distribution in the void population will be monitored. The last objective will be to understand microscopic void-to-void interaction. In order to take advantage of the wealth of simulation results, approaches based on artificial intelligence (neural networks on the graph associated with the pore population) will be used to learn the link between a void's neighborhood and its growth.
The doctoral student will have the opportunity to develop their skills in shock physics and mechanics, numerical simulations (with access to CEA-DAM supercomputers), and data science.
Multiscale modelling of twinning in tin
Twinning is a displacive deformation mechanism characterized by a continuous deformation of the material. Although widely studied for other industrial materials such as titanium alloys, this inelastic mechanism remains poorly understood and incompletely modeled for complex crystallographic structures. However, due to the reduced number of symmetries in these structures, dislocation slip is insufficient to accommodate deformation in certain loading directions, requiring the activation of twinning. This is the case for tin, which has a tetragonal structure. In particular, twinning contributes significantly to the mechanical response of tin at high strain rates and low temperatures. At intermediate temperatures and strain rates, a competition between dislocation plasticity and twinning plasticity can occur, making it crucial to describe the coupling between these two phenomena. Proposing a better description of this coupling will shed new light on the experimental data available at CEA DAM. The objective of the thesis is to develop a multiscale approach, from molecular dynamics to continuum mechanics, validated by experiments, to converge on a model that describes the behavior of tin over a wide range of temperatures and strain rates.
Mechanical degradation of Solid Oxide Cells: impact of operating and failure modes on the performances
Solid oxide cells (SOCs) are electrochemical devices operating at high temperature that can directly convert fuel into electricity (fuel cell mode – SOFC) or electricity into fuel (electrolysis mode – SOEC). In recent years, the interest on SOCs has grown significantly thanks to their wide range of technological applications that could offer innovative solutions for the transition toward a renewable energy market. However, despite of all their advantages, the large-scale industrialization of this technology is still hindered by the durability of SOCs. Indeed, the SOCs remain limited by various degradation phenomena including mechanical damage in the electrodes. For instance, the formation of micro-cracks in the so-called ‘hydrogen’ electrode is a major source of degradation. However, the precise mechanism and the full impact of the micro-cracks on the electrode performances are still unknown. By a multi-physic modelling approach, it is proposed in this thesis (i) to simulate the damage in the microstructure of the electrode and (ii) to calculate its impact on the loss of performances. Once the model validated on dedicated experiments, a sensitivity analysis will be conducted to provide relevant guidelines for the manufacturing of improved robust and performant electrodes.
Development of a modeling tool for corrosion in porous media
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In a context where material durability is essential for the safety of infrastructures and the promotion of a sustainable energy transition, mastering corrosion phenomena represents a major challenge for key sectors such as decarbonized energy transport through buried pipelines and civil engineering (hydrogen, nuclear, underground infrastructures). The CORPORE project addresses this issue by proposing the development of advanced numerical simulation models to study corrosion in porous media using COMSOL Multiphysics.
The main scientific and technological objective is to establish an integrated multiphysics modeling approach for the electrochemical and transport mechanisms within porous materials: studying the coupled influence of chemistry, pore network properties, and material–environment interactions on the initiation and propagation of corrosion.
This approach will help optimize anticorrosion protection strategies, reduce maintenance costs, and extend the service life of structures. From a state-of-the-art perspective, most current models focus on homogeneous media and compartmentalized approaches. Our project stands out by integrating a multi-scale mechanistic modeling framework combined with the use of archaeological data for long-term validation.
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Study of Zn, Cr, Fe, Ni synergies on crystallisation in simplified glasses of nuclear interest
In France, nuclear power-plants used for electricity production generate high-level long-lived radioactive wastes through spent fuel reprocessing. These wastes are confined within a borosilicate glass matrix, whose structure allows for the incorporation at the atomic scale of a large number of elements, and which displays excellent long-term properties. The industry challenges are leading to changes in the nuclear fuel composition, which can thus induce a modification of the spent-fuel composition to be vitrified.
Chromium is as such an element of interest: its relatively low solubility in borosilicate glasses as well as its tendency to crystallise with other elements, such as iron, nickel and zinc, needs to be further investigated. This thesis aims to study the synergetic effect of Cr, Ni, Fe and Zn on crystallisation in simplified peralkaline glasses of nuclear interest in order to better comprehend affinities between these elements, thus identifying both the nature and quantity of the several crystalline phases which may form.
The PhD student will benefit from the recognised skills of the host laboratory in glass formulation as well as the study of their physico-chemical properties. All of the resources made available will enable a global approach to the subject, working on a fast-growing topic with major industrial and societal implications. The experience acquired during this interdisciplinary work will be useful in the field of materials.
New concepts for cold neutron reflectors
The CEA and the CNRS have launched an initiative to design a new neutron source using low-energy proton accelerators, the ICONE project [1]. The goal is to build a facility that will provide an instrumental suite of about ten spectrometers available to the French and European scientific community. Alongside ICONE, the LLB is also participating in HiCANS R&D work on the construction of a platform in Bilbao to facilitate European collaborations.Neutron scattering experiments require thermal and cold neutrons. The design of the moderator is therefore a crucial component of the project to maximize the source's performance.
One avenue for improving the moderator performances is to enhance the efficiency of the reflector, and more specifically, the cold neutron reflector. In this study, we propose to investigate the specific scattering properties of cold neutrons on nanostructured materials. Indeed, cold neutrons have long wavelengths (> 0.4 nm) and can therefore be coherently scattered by nanostructured materials. Scattering efficiency is not only amplified by coherent scattering effects, but it is potentially possible to direct this scattering if the reflecting material is anisotropic. This control over the scattering direction can further increase the moderator's brightness.
The first part of the work will consist of identifying the most promising nanostructured materials and modeling their cold neutron reflectivity performance. In a second step, these materials will be shaped and their properties characterized using neutron scattering devices at neutron scattering facilities such as the ILL in Grenoble or the PSI in Switzerland.
Atomic scale modeling of radiation induced segregation in Zr(Nb) alloys
Nuclear fuel cladding made of zirconium alloys constitute the first safety barrier in pressurized water reactors. The microstructure of these alloys not only controls mechanical properties, but also phenomenon such as corrosion or growth under irradiation. Enabling a more flexible use of nuclear energy in the mix while maintaining the structural integrity of fuel cladding under both operating and accidental conditions, we must understand the detailed mechanisms of microstructure evolution under irradiation. Numerous studies point toward the center part played by Nb in such microstructural evolution. For instance, diffusion flux coupling between solutes (Nb) and point defect created by irradiation gives rise to local Nb segregation, as well as precipitates which are not seen in non-irradiated samples. Atomic scale modeling brings in information that complements that obtained from experimental observations, allowing to confirm or disprove the evolution scenarios found in the literature. The aim of this Ph.D. work is to use the tools which have been developed to study irradiation effects in ferritic steels, and apply them to Zr alloys, with a focus on radiation induced segregation. Electronic structure calculations in the density functional theory approximation will be used to study the interactions between niobium atoms and point defects. From this data, we are able to compute transport coefficients, from which we can discuss quantitatively solute/point defect flux coupling and radiation induced segregation effects.