Nanocrystalline Soft Magnetic Composites: Powder morphology and design for controlling their magnetic properties for high frequency applications

Context: Achieving carbon neutrality by 2050 will require massive electrification of the power production systems. Power electronics (PE) is a key-enabler that will this transformation possible (renewables, integration of energy micro-grids, development of electric mobility)
Problem: Current developments in PE converters aim at increasing the switching frequencies of large bandgap switches (SiC or GaN). At low frequencies, magnetic components remain bulky, occupying up to 40% of the total footprint. At high frequencies (HF > 100 kHz), very significant gains are expected, but only if the losses generated by these components remain under control. Today, the main class of magnetic materials applied to HF is MnZn or NiZn ferrites, due to their low cost and convenient electrical resistivity (?elec > 1 O.m). The main drawbacks of these materials are their low saturation induction (Bsat < 0.4 T), which limits their size reduction, and their mechanical fragility. Nanocrystallines materials have better Bsat (1.3 T), but their ?elec is about 1.5 µO.m (6 times less resistive than ferrites), which generates significant induced current losses at HF.
Thesis objective: To develop magnetic composites by grinding nanocrystalline ribbons, electrically insulating the powders (coating fabricated by sol-gel), compacting of the powder at high pressure (1000-2000 MPa) for the core shaping and finally by applying an annealing treatment to relax the thermal constraints.

Development of catalysts for CO2 hydrogenation to light olefins

Light olefins, mainly ethylene and propylene, are amongst the organic compounds with the largest production volume. They are currently produced from fossil resources. The reduction of the carbon footprint of products synthesized from these intermediates necessitates the use of alternative feedstock, such as atmospheric CO2.
The objective of this phD is the development of catalyst for the direct hydrogenation of CO2 into light olefins. Fe based catalyst combining reverse water gas shift (RWGS) and Fischer-Tropsch polymerization (FT) capabilities will be developed. In order to have a better understanding of iron forms involved in the reaction, Fe nanoparticles of controlled composition and dsizes will be prepared and dispersed on different support (silica, alumina, carbon,…). The catalytic properties will then be evaluated on a dynamic reactor and finely characterized using numerous techniques (XRD, XPS, HRTEM, …).

Role of surface properties of UO2 powder particles on their agglomeration suitability and rheological behaviour

This study aims to predict the powder flow behavior in the context of nuclear fuel fabrication. This issue is common to many industrial fields because poor powder flow can lead to process problems such as pipe clogging, reduced rates, or the presence of heterogeneities in the final product. The first objective of this PhD thesis is, on the one hand, to provide a more accurate description of the powder agglomerates and, on the other hand, to characterize their surface. Based on these surface and structural data of UO2 powder particles, the second objective of this work is to achieve a better understanding of the agglomeration/desagglomeration properties in order to correlate them with the flow properties.
The future PhD student will need to use and develop experimental methods (particle characterization tools, surface characterization analyzers, phenomenological modeling) at the IRESNE institute (CEA-Cadarache) in the fuel study department (DEC), specifically within a team dedicated to experiments on nuclear fuel.
This study, applied to UO2 powders, has a generic nature because it is suitable for the study of all granular media. At the end of the PhD, the doctoral candidate will communicate the results through publications and conference presentations. An expertise in granular media will be acquired, which is an attractive and valuable skill in many industrial fields such as agri-food, pharmaceutical industries, metallurgy, or building materials.

A better understanding of diffusion welding in a+ß titanium alloy

As part of a short-term nuclear project, the CEA/LITEN is supporting the manufacturing activities of a titanium alloy steam generator by HIP (Hot Isostatic Pressing). Depending on its thermal and/or thermomechanical history, the alloy Ti64 presents phases in different proportions, chemical compositions and crystallographic structures.
How does diffusion welding take place between two different phases? Is there one that cross the interface preferentially and if yes, why? Which HIP parameters have a real influence? What starting microstructure allows optimal diffusion welding?
These are the questions that the thesis should answer.

Study of fracture toughness - microstructure relationships of new high performance oxide dispersion strengthened steels

ODS steels are considered for the development of components for fourth generation reactors. They offer high tensile and creep strength and good resistance to irradiation [1-3]. This high level of reinforcement is accompanied by a reduction in ductility and toughness. Tube shaping changes the microstructure, so the properties of the material in its final form should be evaluated. The work of B. Rais [4] made it possible to compare the different tests and to develop a test and an analysis method for measuring toughness on thin tubes.

This present PhD will use this new test to evaluate the toughness of various ODS grades. Varied microstructures from historical and recent productions will assessed to identify the mechanisms, the key parameters driving toughness and to identify the microstructural parameters which drive the response of the material. In this work we will be interested in ferritic / martensitic grades, some of which come from a manufacturing process which is the subject of a patent application [5-6] and for which we observe for the first time remarkable properties in resilience, associated with good hot mechanical properties.

The study will be based on a comparison of experience and finite element modeling. This applied research work will allow the student to acquire solid skills in fracture mechanics and fine characterization of materials (SEM, EBSD, etc.). A good understanding of the mechanical properties/microstructure relationships will make it possible to understand the origin of the observed properties and to propose new optimizations on the microstructures to improve the mechanical behavior and/or the shaping of the material.

Student profile: Engineer or M2 Mechanics/Materials

On the role of the elastic deformation field on the formation of irradiation defects in pure metals

In the context of extending the operational lifetime of nuclear power plants (NPPs), currently operating in France, a materials ageing surveillance strategy is in place. It is essential for ensuring their mechanical properties. During the operation of the plant, materials are subjected to irradiation. Under this exposure, the internal structure of materials evolves, leading to the creation of numerous defects that degrade macroscopic properties and may result in a limitation of the long-time operation (LTO) of components. The proposed work is a fundamental study conducted on model materials, aiming to better understanding the behavior under irradiation of metallic alloys. It will contribute to the predictive modelling of materials, covering defects created at the nanoscale up to the level of nuclear components.

The irradiation of materials with high-energy particles such as neutrons, ions, or electrons generates a large number of defects called point defects (PD). These mobile PDs can migrate and aggregate to form 2D or 3D-objects like prismatic loops or cavities respectively. They can also be eliminated at PD sinks. The system is then submitted to PDs flows directed towards these sinks. These flows are then responsible for phenomena such as radiation-induced segregation (RIS) or precipitation (RIP) of solute atoms [1] [2]. The presence of clusters of PDs and of PD flows alters the microstructure and can deteriorate the physical response of the irradiated materials. In particular, the formation of prismatic loops degrades the mechanical properties of materials as they can impede dislocations and induce embrittlement [3]. In a previous study, we focused on vacancy defects in the form of cavities and investigated the facetting of defects formed in a weakly anisotropic metal, aluminum, using in-situ irradiations in a high-resolution transmission electron microscope (HRTEM).
The work aims to go further in the role of the elastic deformation field on the morphology of irradiation defects. More precisely, it aims to carry out a systematic study on different metals with different anisotropy coefficients. We have chosen reference metals with body-centered cubic (BCC) and face-centered cubic (FCC) structures with low or high anisotropy coefficients. The study will concern Cr and Fe with a BCC structure, and Al and Cu with a FCC structure and may be extrapolated to alloys of higher complexity such as high entropy alloys (HEA). The work will be mainly experimental but will also include a theoretical part. The effects of the crystal anisotropy on the morphology of prismatic loops will be carried out by phase field modelling [4]. The spatial arrangement of the loops will be studied by Object Kinetic Monte-Carlo (OKMC) simulations [5], as recently done in aluminium.
The work will be mainly experimental. We will studied [100]-oriented single crystals to avoid any surface effect on the shape of the objects formed. They will be irradiated with heavy ions at temperatures normalized with respect to their melting temperature either in-situ within the Jannus Orsay platform, or ex-situ within the Jannus Saclay platform [6]. Loops will be imaged by conventional TEM or STEM with a FEI Tecnai and Jeol NeoARM type microscopes. The latter is equipped with a double spherical aberration corrector. The work will be carried out within the framework of the joint research laboratory (LRC) MAXIT.
The work will also include a modelling part. The effects of crystallographic anisotropy on the morphology of prismatic loops will be investigated using a phase-field code [4]. The spatial arrangement of the loops will be studied using Object Kinetic Monte Carlo (OKMC) [5], as recently done in aluminum.
This work follows a 2-year postdoctoral fellowship scheduled to conclude in December 2023, during which deep learning (DL) approaches were developed to accelerate the automatic detection of defects created under irradiation [7]. The utilization of these approaches will significantly enhance the statistical robustness and precision of the results.

Advantage for the student: The PhD is situated in a laboratory composed by 25 researchers and approximately 25 students (PhD, postdoctoral fellows), creating a simulating scientific environment. The activities involve both experimental and simulation sides, offering the opportunity to interact with experts from both sides.

[1] M. Nastar, L. T. Belkacemi, E. Meslin, et M. Loyer-Prost, « Thermodynamic model for lattice point defect-mediated semi-coherent precipitation in alloys », Communications Materials, vol. 2, no 1, p. 1-11, mars 2021, doi: 10.1038/s43246-021-00136-z.
[2] L. T. Belkacemi, E. Meslin, B. Décamps, B. Radiguet, et J. Henry, « Radiation-induced bcc-fcc phase transformation in a Fe3%Ni alloy », Acta Materialia, vol. 161, p. 61-72, 2018, doi: https://doi.org/10.1016/j.actamat.2018.08.031.
[3] M. Lambrecht et al., « On the correlation between irradiation-induced microstructural features and the hardening of reactor pressure vessel steels », Journal of Nuclear Materials, vol. 406, no 1, p. 84-89, 2010, doi: http://dx.doi.org/10.1016/j.jnucmat.2010.05.020.
[4] A. Ruffini, Y. Le Bouar, et A. Finel, « Three-dimensional phase-field model of dislocations for a heterogeneous face-centered cubic crystal », Journal of the Mechanics and Physics of Solids, vol. 105, p. 95-115, août 2017, doi: 10.1016/j.jmps.2017.04.008.
[5] D. Carpentier, T. Jourdan, Y. Le Bouar, et M.-C. Marinica, « Effect of saddle point anisotropy of point defects on their absorption by dislocations and cavities », Acta Materialia, vol. 136, p. 323-334, sept. 2017, doi: 10.1016/j.actamat.2017.07.013.
[6] A. Gentils et C. Cabet, « Investigating radiation damage in nuclear energy materials using JANNuS multiple ion beams », Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, vol. 447, p. 107-112, mai 2019, doi: 10.1016/j.nimb.2019.03.039.
[7] T. Bilyk, A. M. Goryaeva, E. Meslin, M.-C. Marinica, Quantification of radiation damage in high entropy alloys by deep learning approach, 2-7/10/2022, MMM2022, Baltimore, USA

Experimental study and modelling of high temperature oxidation of Cr coated zirconium alloy substrate

This thesis concerns the research and development of materials more resistant in extreme conditions at high temperature. It is more specifically dedicated to the development of a new concept of nuclear fuel cladding : Cr coated Zr-based claddings. The purpose of this work is to adpat the Ekinox-Zr code, initially developped for the description of diffusion phenomena in uncoated Zr claddings, to Cr-coated Zr-based claddings and create a new code : “Ekinox-Zr-Cr”. This work will be divided into two parts : a modelling/simulation one and an experimental one dedicated to the determination of the diffusion coefficients of Cr, Zr and O species inside the different phases of the system. These data constitute essential parameters for the modelisation. High temperature oxidation experiments on Cr-coated claddings will also be used in order to improve our knowledge on the different mechanisms involved and compare them to the results obtained with the Ekinox-Zr-Cr code created during this study.

Delayed hydride cracking (DHC) of nuclear fuel cladding: experiments, modelling and numerical simulations of microstructure effects

Corrosion of nuclear fuel cladding by the water in the primary circuit as it passes through the reactor leads to hydriding. Delayed hydride cracking (DHC) is likely to occur later, during dry storage. Such cracking requires a pre-existing defect and a thermo-mechanical history that enables the following iterative mechanism to be set in motion: hydrogen diffusion, precipitation of hydrides at the crack tip and rupture of the embrittled zone. During a previous thesis carried out in the host laboratory, an original procedure combining experiments and numerical simulations using finite elements was used to determine the toughness of unirradiated relaxed Zircaloy-4 cladding in the event of DHC, and to report on the effect of mechanical loading and temperature on the incubation time and cracking speed between 150°C and 250°C. The aim of this thesis is to apply this procedure to a more modern cladding material (recrystallised M5) and to develop fine-scale microstructure modelling that can account for the effects of texture (crystallographic and morphological), propagation direction and plane, and irradiation on DHC.
Corrosion of nuclear fuel cladding by the water in the primary circuit as it passes through the reactor leads to hydriding. Delayed hydride cracking (DHC) is likely to occur later, during dry storage. Such cracking requires a pre-existing defect and a thermo-mechanical history that enables the following iterative mechanism to be set in motion: hydrogen diffusion, precipitation of hydrides at the crack tip and rupture of the embrittled zone. During a previous thesis carried out in the host laboratory, an original procedure combining experiments and numerical simulations using finite elements was used to determine the toughness of unirradiated relaxed Zircaloy-4 cladding in the event of DHC, and to report on the effect of mechanical loading and temperature on the incubation time and cracking speed between 150°C and 250°C. The aim of this thesis is to apply this procedure to a more modern cladding material (recrystallised M5) and to develop fine-scale microstructure modelling that can account for the effects of texture (crystallographic and morphological), propagation direction and plane, and irradiation on DHC.

Experimental characterisation and numerical simulation of intergranular oxide fracture: Application to Irradiation Assisted Stress Corrosion cracking

Metal alloys used in industrial applications can form oxide layers in the presence of a corrosive environment. These oxides may be distributed on the surface and/or localized at the grain boundaries. In the latter case, the oxidized grain boundaries may experience brittle fracture under mechanical loading, potentially leading to intergranular cracking of the material. This mechanism is, for example, a possible scenario for the failure of austenitic stainless steel bolts used in the internals structure of Pressurized Water Reactors (PWRs). Under the effect of mechanical loading, neutron irradiation and the presence of a corrosive environment, these bolts fail through a phenomenon known as irradiation-assisted stress corrosion cracking. To model this phenomenon, we need to determine the fracture properties of intergranular oxides, and to take into account the coupling between cracking, oxidation and irradiation. In this thesis, experimental and numerical work will be combined. Firstly numerical simulations based on the variational approach to fracture approach will be assessed in order to design micro-beam micromechanics experiments aimed at reliably determining the fracture properties of oxides, and also to study the couplings between cracking, oxidation and irradiation. In particular, the cracking-oxidation coupling that prefigures the transition between initiation and propagation will be investigated in detail. These experiments will then be carried out on model and industry-relevant steels, and interpreted using numerical simulations. Finally, all the results obtained in this work will be incorporated into simulations of polycrystalline aggregates, in order to assess the possibility of quantitatively predicting intergranular cracking in the context of irradiation-assisted stress corrosion.
By the end of the PHD, the doctoral student will have acquired both experimental skills - micromechanical tests - and numerical skills - numerical simulations of fracture - at the cutting edge of the state of the art and applicable to a large number of problems in the mechanics of materials.
A Master's 2 / end-of-studies internship preparatory to the PHD is available in 2024.

Experimental characterization and cluster dynamics simulation of the effect of helium on irradiation defects and associated swelling in austenitic stainless steels of pressurized water reactors internals vessel

The microstructure of the materials of the internal structure of Pressurized Water Reactors (PWRs), which play a key role notably in maintaining the fuel assemblies, will evolve under irradiation. A better understanding of these evolutions could allow a better prediction of the behavior in operation of these materials in austenitic stainless steels (Fe-Cr-Ni) of the 300 series, in particular the 304 grade. Swelling is one of these potential evolutions and the question of its existence at high doses is of importance with the aim of extending the operating time of PWRs.
The objective of this work is to provide a better understanding of the mechanisms of swelling and microstructural evolutions through an analytical study of the effect of helium (influence of the rate of helium implanted up to high dose, temperature, and concomitant presence of hydrogen ...). Fine characterizations (grain scale and bottom), coupled with simulations in cluster dynamics, will be carried out on austenitic stainless steels irradiated with ions.
This study will be conducted mainly at the CEA, in the “Service de Recherche en Matériaux et procédés Avancés” (SRMA) and “ Section de Recherches de Métallurgie Physique ” (SRMP). It will benefit from the available ion irradiation devices (JANNuS), microstructural characterizations (notably Transmission Electron Microscopy and Atom Probe Tomography) and modeling (cluster dynamics simulation) tools. It will be supervised by M. Legros (CEMES) and T. Jourdan (SRMP) and driven par J. Malaplate and A. Renault-Laborne (SRMA). This broad subject will allow the candidate to acquire training on the behavior of materials under irradiation and also strong skills in the field of microstructural characterization of materials and simulation.
This subject is aimed at a student in materials science, with skills/appetite in the field of materials characterization and simulation. A Master 2 internship is proposed prior to this subject.