Behavior of matter under isothermal dynamic compression: displacement of chemical reactivity; synthesis of new metastable materials; phase transition mechanisms.
The Diamond Anvil Cell equipped with piezoelectric actuators, or d-CED, is an innovative device that can generate dynamic compressions and decompressions over a wide range of pressure variation rates. The d-CED thus enables finely controlled dynamic stresses to be applied, with (de)compression rates that can vary over several orders of magnitude along isothermal paths. This paves the way for the creation of reference databases for the validation of microscopic mechanisms. Furthermore, the compression or decompression rates can be equated to ultra-fast heating or cooling rates of the sample, offering the possibility of exploring, in a highly controlled manner, certain phenomena still debated in the literature, such as the maximum stability of a solid beyond its melting point.
The objective of this thesis is to exploit the new possibilities offered by d-CED to demonstrate new phenomena or gain a detailed understanding of certain effects discussed in the literature, by performing ultra-fast temperature variations. A first application will consist of studying the nucleation kinetics of rare gases (Ar, Ne, Kr) as a function of the compression rate, and comparing them with recent measurements made at the XFEL in cryogenic jets. A second objective will be to study chemical changes, with an initial study focusing on the modification of the reactivity of nitromethane, a reference explosive. Another area of study will concern the synthesis of new molecular compounds from mixtures of dense molecular fluids (N2, H2, O2).
Experimental study and numerical simulation of deformation mechanisms and mechanical behavior of zirconium alloys after irradiation
The cladding of nuclear fuel rods used in Pressurized Water Reactor, made of zirconium alloys, is the first barrier for the confinement of radioactive nuclei. In-reactor, the cladding is subjected to radiation damage resulting in a change of its mechanical properties. After in-reactor use, the fuel rods are transported and stored. During these various steps, the radiation damage is partially annealed, leading to another evolution of the material properties. All these evolutions are still not well understood.
The objective of this PhD work is to better understand the deformation mechanisms and the mechanical behavior of zirconium alloys after irradiation, and after a partial annealing of the radiation damage. This will help to better predict the behavior of the cladding tube after use and thus guaranty the confinement of radioactive nuclei.
In order to achieve this goal, original experimental methods and advanced numerical simulations will be used. Ion irradiations will be conducted in order to reproduce the radiation damage. Heat treatments will then be done on the specimens after irradiation. Small tensile samples will be strained in situ, after annealing, inside a transmission electron microscope, at room temperature or at high temperature. Deformation mechanisms observed at nanometer scale and in real time will be simulated using dislocation dynamics, at the same time and space scales. Large scale dislocation dynamics simulations will then be conducted in order to deduce the single-crystal behavior of the material. In parallel with this study at the nanometric scale, a study will also be conducted at the micrometric scale. Nanoindentation and micropillar compression tests will be performed to assess the mechanical behavior after irradiation and annealing. The results of mechanical tests will be compared with large-scale dislocation dynamics numerical simulations.
This study will allow a better understanding of the special behavior of zirconium alloys after irradiation and annealing and then help to develop physically based predictive models. In a future prospect, this work will contribute to improve the safety during transport and storage of spent nuclear fuel.
Kinetics of segregation and precipitation in Fe-Cr-C alloys under irradiation : coupling magnetic, chemical and elastic effects
Ferritic steels are being considered as structural materials in future fission and fusion nuclear reactors. These alloys have highly original properties, due to the coupling between chemical, magnetic and elastic interactions that affect their thermodynamic properties, the diffusion of chemical species and the diffusion of point defects in the crystal. The aim of the thesis will be to model all of these effects at the atomic scale and to integrate them into Monte Carlo simulations in order to model the segregation and precipitation kinetics under irradiation, phenomena that can degrade their properties in use. The atomic approach is essential for these materials, which are subjected to permanent irradiation and for which the laws of equilibrium thermodynamics no longer apply.
The candidate should have a good background in statistical physics or materials science, and be interested in numerical simulations and computer programming. The thesis will be carried out at CEA Saclay's physical metallurgy laboratory (SRMP), in a research environment with recognised experience in multi-scale modelling of materials, with around fifteen theses and post-doctoral contracts in progress on these topics.
A Master 2 internship on the same subject is proposed for spring 2025 and is highly recommended.
Thermally activated glide of screw dislocations in bcc metals
Thermally activated glide of dislocation is a key point for understanding the plastic deformation of metals. The screw dislocation in bcc metals is an archetypical case for which a large quantity of experimental data has been published in the scientific literature. It is then possible to compare these data to the theoretical predictions realized from the Vineyard statistical theory [1,2]. Such a theory is an essential tool allowing to perform a scale transition from atomistic computations [3] toward macroscopic scale at which are realized the deformation tests.
The aim of our research will be to test Vineyard theory in comparison with molecular dynamics simulations [4]. Some preliminary computations have shown a significant discrepancy that is not present when we repeat the comparison for point-like defect as vacancies or self-interstitial atoms. The discrepancy of the theory will have to be reduce within a new theoretical development. Our new theory should allow some predictions in agreement with macroscopic tensile test in bcc metals [5].
[1] Vineyard G.H., J. Phys. Chem. Solids 3, 121 (1957).
[2] Proville L., Rodney D., Marinica M-C., Nature Mater. 11, 845 (2012).
[3] Proville L., Ventelon L., Rodney D., Phys. Rev. B 87, 144106 (2013).
[4] Proville L., Choudhury A., Nature Mater. 23, 47 (2024).
[5] Caillard D., Acta Mater. 58, 3504 (2010).
Next-Gen Surface Analysis for Ultrathin Functional Materials
Advanced nanoelectronics and quantum devices rely on ultrathin oxides and engineered interfaces whose chemical composition, stoichiometry and thickness must be controlled with sub-nanometer precision. LETI is installing the first 300-mm multi-energy XPS–HAXPES tool with angle-resolved capability, enabling quasi in situ chemical metrology from deposition to characterization.
This PhD will develop quantitative, multi-energy and angle-resolved XPS/HAXPES methodologies for ultrathin oxides and oxynitrides, validate measurement accuracy, and establish robust protocols for quasi in situ transfer of sensitive layers. Applications include advanced CMOS stacks and quantum Josephson junctions, where sub-2 nm AlOx barriers critically determine device performance.
The project directly supports the development of next-generation quantum technologies, advanced photonics and energy-efficient microelectronics by improving the reliability and stability of nanoscale materials. The work will be carried out within a strong multi-partner framework.
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.
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).
Impact of fission products and microstructure on the thermophysical properties of LWR (U,Pu)O2-x fuel
In France, mixed oxide fuel (MOX, (U,Pu)O2) is currently deployed in several pressurized water reactors (PWRs) operated by EDF. To ensure continued low-carbon electricity production, a broader use of MOX fuel across the French nuclear fleet is expected to become essential in the near future. During reactor operation, U1??Pu?O2?? fuels undergo significant changes in their physical properties and microstructure, primarily due to the accumulation of dozens of lighter elements generated by plutonium’s fission, commonly referred to as fission products (FPs). Because of the high radiotoxicity of irradiated fuel, surrogate materials known as SIMMOX have been developed. In a previous PhD project, we established a synthesis route enabling the production of SIMMOX doped with up to twelve fission products, successfully reproducing the microstructure of irradiated PWR MOX fuel.
To maintain an adequate margin to fuel melting during irradiation, it is crucial to understand how the thermophysical and thermodynamic properties of MOX fuel evolve under these conditions. This PhD project aims to measure these properties on a representative MOX composition currently used in EDF reactors. The key properties of interest include thermal conductivity, heat capacity, and melting temperature. These measurements will be carried out at the JRC-Karlsruhe (Germany) during a research stay of approximately 12 months. Subsequently, the samples will be returned to CEA-Marcoule, where the impact of high-temperature exposure on actinide and fission product speciation, as well as on the microstructural evolution of the MOX fuel, will be investigated. In parallel, the experimental work will be complemented by thermodynamic modeling using the CALPHAD approach, in order to identify the mechanisms and phase equilibria governing high-temperature behavior during property measurements.
Investigation of geopolymer durbility for radioactive wastewater treatment
The reprocessing of spent nuclear fuel generates radioactive effluents that require appropriate treatment. To meet industrial and regulatory challenges, the CEA is developing geopolymer-based adsorbent materials that are robust, cost-effective, and efficient for capturing Cs-137 and Sr-90. Their performance can be enhanced through the incorporation of selective adsorbents (such as zeolites) and through innovative shaping processes (3D printing, beads, foams) optimized for column adsorption.
The durability of these materials remains a critical issue, as their leaching and ageing mechanisms in column systems are still poorly understood. This PhD project will focus on studying these phenomena in order to assess the impact of effluent chemistry on the stability and efficiency of geopolymers. The work will include material synthesis, batch and column sorption tests, and the use of modelling tools to interpret alteration mechanisms. The scientific challenge is to identify the key physicochemical markers of geopolymer degradation in the targeted liquid effluents and to link them with column sorption performance.
The PhD candidate will join the Laboratory for Supercritical Processes and Decontamination (LPSD), renowned for its expertise in column-based ion extraction and adsorbent characterization. He/she will collaborate with specialists at CEA Marcoule and with the laboratory teams, and will regularly present project progress to the industrial partner. Upon completion of the PhD, the candidate will have developed recognized expertise at the interface of materials science, chemistry, and column adsorption processes. This work will open a wide range of opportunities: R&D positions in the nuclear sector, waste management, and functional materials; academic pathways (postdoctoral research, academia, teaching); or contributions to major energy and environmental challenges.