Li alloys for all solid-state batteries with sulfide electrolyte
Using lithium metal as a negative electrode would significantly increase the energy density of current batteries. However, today, this material quickly leads to short circuits during charge/discharge cycles, mainly due to the formation of dendrites and the instability of the interface with the electrolyte. All-solid-state batteries, particularly with sulfide electrolytes, are a promising alternative, but the limitations of lithium metal remain. Lithium alloys appear to be a solution for improving mechanical and interfacial properties while maintaining good energy densities.
The objective of the PhD is to develop and select lithium alloys suitable for sulfide electrolytes batteries, then integrate them into all-solid-state cells in order to study degradation mechanisms. The work will be focused on the synthesis of the alloys, their shaping in thin films and their integration into cells. The alloys will be finely characterized and then electrochemically tested in laboratory cells and pouch cells. Finally, degradation phenomena, particularly at interfaces, will be studied using advanced post-mortem characterizations.
Advanced characterization of defects generated by technological processes for high-performance infrared imaging
This thesis falls within the field of cooled infrared detectors. The CEA-LETI-MINATEC Infrared Laboratory specializes in the design and manufacture of infrared camera prototypes used in defense, astronomy, environmental monitoring, and satellite meteorology.
In this context of high-performance imaging, it is crucial to ensure optimal detector quality. However, manufacturing processes can introduce defects that can degrade sensor performance. Understanding and controlling these defects is essential to increase reliability and optimize processes.
The objective of the thesis is to identify and precisely characterize these defects using cutting-edge techniques, rarely combined, such as Laue microdiffraction and FIB-SEM nanotomography, enabling structural analysis at different scales. By linking the nature and origin of defects to manufacturing processes and quantifying their impact on performance, the doctoral student will contribute directly to improving the reliability and efficiency of next-generation infrared sensors.
The doctoral student will join a team covering the entire detector manufacturing chain and will actively participate in the development (LETI clean room) and structural characterization (CEA-Grenoble platform, advanced techniques) of samples. He/she will also be involved in electro-optical characterization in partnership with the Cooled Infrared Imaging Laboratory (LIR), which specializes in detailed analysis of active materials at cryogenic temperatures.
Measurement of the speed of sound in H2 and He, key components of gas giant interiors
The goal of this thesis is to study hydrogen-helium mixtures in the fluid phase under high pressure and high temperature using Raman and Brillouin spectroscopy. The experiments will be conducted in a diamond anvil cell with laser heating, allowing exploration of a wide range of pressure and temperature conditions representative of the interiors of gas giant planets (1-300 GPa, 300-4000 K). Raman spectroscopy will be used to probe possible chemical changes occurring under extreme conditions, while Brillouin spectroscopy will provide access to the adiabatic sound velocity and the equations of state of these fluid mixtures. These data will be particularly useful for improving the modeling of Jupiter and Saturn’s interiors.
Chemical and mechanical properties of N-A-S-H aluminosilicates of geopolymer
Management of low- and medium-level nuclear waste relies primarily on cements, but their limitations with regard to certain types of waste (reactive metals, oil) require the exploration of new, more effective materials. Geopolymers, particularly those composed of hydrated sodium aluminosilicates (Na2O–Al2O3–SiO2–H2O, or N–A–S–H), appear to be a promising alternative thanks to their chemical compatibility with certain types of waste.
However, despite the growing interest in geopolymers, scientific obstacles remain: 1) The available thermodynamic data on N-A-S-H is still incomplete, making it difficult to predict their long-term stability via modeling, 2) The role of their atomic structure in regard to their reactivity remains unclear, and 3) The links between chemical composition (in terms of Si/Al ratio) and mechanical properties are not established, limiting the representativeness of the models created.
By combining experimentation and modeling in order to link atomic structure and properties, this thesis aims to obtain robust and novel data on the chemical and mechanical properties of N-A-S-H. The thesis is organized around three major objectives: 1) determining the impact of N-A-S-H composition on dissolution and establishing thermodynamic solubility constants, 2) characterizing their atomic structure (aluminols, silanols, and hydrated environments) using advanced NMR spectroscopy, and 3) linking their mechanical properties, measured by nanoindentation, to their structure and composition using molecular dynamics modeling.
Study of oxygen and hydrogen diffusion processes in pre- and post-transitional oxide layers formed on zirconium alloys
The corrosion mechanisms of zirconium alloys in pressurised water reactors are still a subject of debate more than half a century after the first research on this material. The literature reports two distinct mechanisms for the transport of diffusing species in oxide layers: one favours the molecular diffusion of oxygen and hydrogen through interconnected nanopore channels during the pre-transient regime, while the other favours diffusion via short circuits (grain boundaries, etc.) in the oxide layer. In the latter case, the oxide layer is considered to be relatively homogeneous and impermeable to the oxidising medium, in this case the water in the primary circuit. On the other hand, the first interpretation is based on the principle that there is a layer that is permeable to the medium due to an interconnected network of nanopores, even during the pre-transient regime, with the density of percolated nanopores increasing over time.
Technically speaking, how can we decide between these two divergent interpretations in terms of the diffusion mechanism, which consequently leads to different solutions for protection against degradation? What is the reaction mechanism that ultimately leads to the hydration of Zr alloys and their oxidation?
To address this challenge, we will explore diffusion processes by studying the dissociation-recombination rates of molecular species at different temperatures in equi-isotopic gas mixtures such as H2/D2, 18O2/16O2, H218O/D216O, H218O/D2, etc., using an experimental device equipped with a mass spectrometer that tracks the molecular species of interest in real time.
Dislocation glide in body-centered-cubic high-entropy alloys
High entropy alloys are single-phase multi-component solid solutions, all elements being present in high concentrations. This class of materials has significant improvements in mechanical properties over "conventional" alloys, particularly their high strength at high temperature. It is commonly accepted that good mechanical performance comes from the interactions of dislocations with the alloying elements and that at high temperature interstitial impurities or interstitial doping, such as oxygen, carbon or nitrogen, play a preponderant role. The study of plasticity in concentrated alloys with a body-centered cubic crystal structure in the high temperature range therefore constitutes the objective of this PhD thesis. The associated technological challenges are important, these alloys being promising structural materials, notably for nuclear applications where operating temperatures above room temperature are targeted.
This work aims to understand and model the physical mechanisms controlling the mechanical strength of these alloys at high temperature, by considering different concentrated alloys of increasing complexity and by using atomistic simulations, in particular ab initio electronic structure calculations. We will first focus on the binary alloy MoNb before extending to the ternary alloys MoNbTi and MoNbTa and studying the impact of oxygen impurities on plastic behavior of these alloys. We will model the dislocation cores and analyze their interaction with interstitial and substitutional elements in order to determine the energy barriers controlling their mobility. Based on these ab initio results, we will develop strengthening models notably allowing us to predict the yield strength as a function of temperature and alloy composition.
This work will be carried out within the framework of the DisMecHTRA project funded by the French National Research Agency, allowing in particular to compare our strengthening models with the data from the experiments which are planned in the project (mechanical tests and transmission electron microscopy), and which will be carried out by the other partners (CNRS Toulouse and Thiais). The PhD thesis, hosted at CEA Saclay, will be co-supervised by a team from CEA Saclay and MatéIS (CNRS Lyon).
Simulation of nuclear glass gels at the mesoscopic scale using a quaternary system.
This research work is part of studies conducted on the long-term behavior of nuclear glass used to immobilize radioactive waste and potentially intended for geological disposal. The challenge lies in understanding the mechanisms of alteration and gel formation (a passivating layer that can slow down the rate of glass alteration) by water and in predicting the kinetics of radionuclide release over the long term.
The proposed simulation approach aims to predict, at a mesoscopic scale, the maturation process of the gel formed during the alteration of glass by water using a ternary “phase field model” composed of silicon, boron, and water (leachate), to which aluminum will be added.
The underlying quaternary mathematical model will consists of a set of coupled nonlinear partial differential equations. These are based on Allen-Cahn and transport equations. The numerical solution of the associated equations is performed using the Lattice Boltzmann Method (LBM) programmed in C++ in the massively parallel LBM_saclay calculation code, which runs on several HPC architectures, both multi-CPUs and multi-GPUs.
The proposed research requires a solid foundation in applied mathematics and programming in order to develop the algorithms necessary for the correct resolution of the new system of strongly coupled equations.
Development of extracting systems for the isotopic enrichment of chlorine
Chlorine (Cl) is naturally composed of 76% 35Cl, which through neutron capture forms 36Cl, a long-lived gamma emitter (t1/2 = 301 000 years), and sulfur 36S, which accelerates corrosion phenomena, and 24% 37Cl with a drastically lower neutron capture section. A supply of 37Cl is therefore necessary in order to operate these reactors. Techniques currently exist that enable the enrichment of chlorine, such as ultracentrifugation, liquid-phase thermal diffusion, or laser isotope separation. The enrichment of chlorine by liquid-liquid extraction technics has been recently developed within CEA. The objective of the thesis is to identify and implement chemical systems allowing the 37Cl enrichment by a separative chemistry process. The thesis subject aims to identify on the basis of literature data initially, the families of ligands and, within these families, the best candidates for the 37Cl enrichment. Next, the synthesis and purification of the selected molecules will be carried out in the laboratory. Finally, the enrichment properties of the successfully synthesised ligands will be evaluated by separative chemistry, by quantification of chlorine isotopes using Inductively coupled plasma mass spectrometry (ICP-MS).
The thesis will be carried out at the recycling and energy recovery processes laboratory (LRVE) at the CEA in Marcoule.
The ideal candidate will be a Master's student in their second or third year of engineering school, studying chemistry, organic chemistry or analytical chemistry. The multidisciplinary nature of the skills acquired and the rigour developed by the student during the experiments undertaken will be valuable assets for the future PhD student.
New generation of organic susbtrates for power conversion
Recent advances in electric motors and associated power electronics have led to a significant increase in power density requirements. This increase in power density means smaller heat exchange surfaces, which amplifies the challenges associated with dissipating the heat generated by power electronics components during operation. In fact, the lack of adequate heat dissipation causes electronic components to overheat, impacting their performance, durability, and reliability. Other issues related to cost, repairability, and thermomechanical constraints call into question traditional ceramic-based insulating thermal interfaces. It is therefore imperative to develop a new generation of heat-dissipating materials that take the system environment into account.
The objective of this thesis is to replace the ceramic substrate in power module systems, whose main role is to act as the system's dielectric layer, with a thermally conductive organic matrix composite. The current substrate has well-known limitations (fragility, poor interface, cycling limit, cost). The organic substrate must have the highest possible thermal conductivity (>3 W/m.k) in order to dissipate the heat emitted properly, while also being electrically insulating with a breakdown voltage of approximately 3kV/mm. It must also have a coefficient of thermal expansion (CTE) compatible with that of copper in order to eliminate delamination phenomena during the cycling undergone by the device during its lifetime. The innovation of the doctoral student's work will lie in the use of highly thermally conductive (nano)fillers that will be electrically insulated (insulating coating) and can be oriented in a polymer resin under external stimulus.
The development of the electrical insulating shell on the thermally conductive core will be carried out using the sol-gel method. The synthesis will be controlled and optimized in order to correlate the homogeneity and thickness of the coating with the dielectric and thermal performance of the (nano)composite. The charge/matrix interface (a potential source of phonon diffraction) will also be studied. A second part will focus on grafting magnetic nanoparticles (MNPs) onto thermally conductive (nano)fillers. Commercial MNPs will be evaluated (depending on requirements, grades synthesized in the laboratory may also be evaluated). The (nano)composites must have rheology compatible with pressing and/or injection processes.
microstructure informed kinetic model : application to solid explosives
When an explosive composition is subjected to an intense stress, such as a shock, the wave generated interacts with the microstructure and in particular with the defects it contains. Due to the nature of the defects, the energy can be localised, as when porosity is compacted, which can lead to the appearance of hot spots. Beyond a certain critical size, these hot spots grow as a result of the chemical decomposition of the explosive, and in some cases this can lead to the creation of a detonation wave. The role of these hot spots is therefore decisive in the initiation of solid explosives. The majority of macroscopic models used to study the shock-detonation transition (SDT) are phenomenological models calibrated on experiments (e.g. multi-strand gauge experiments) and therefore do not take into account the microstructural peculiarities specific to each explosive. It then becomes necessary to recalibrate a model for each composition, which limits any predictive capacity.
Microtomographic studies of real microstructures of explosive compositions have revealed that these deviate significantly from an average description based on a spherical pore. Through image segmentation, these microtomographs can provide essential ingredients for mesoscopic-scale simulation codes: these microstructures can be used directly as input for calculations or as a basis for generating virtual but realistic microstructures, thereby extending the accessible database given the experimental difficulties in generating this type of image in large numbers.
The computing power available today means that we can now envisage explicit simulations of realistic microstructures of explosive compositions. These simulations, in two or even three dimensions, will form the basis for the construction of a macroscopic kinetics model for modelling the shock-detonation transition. The results expected from this work are cross-disciplinary and can be transposed to all composite energetic materials. The effect of thermal or mechanical damage on the behaviour of an explosive or a solid propellant (vulnerability issues) could also benefit from this project. This more detailed knowledge of the role of microstructure (grain shape, porosity, etc.) could also improve filler manufacturing processes (e.g. ‘Very Insensitive’-RDX).