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 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 [3]. 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.

[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., Choudhury A., Nature Mater. 23, 47 (2024).

Reactive metals corrosion in innovative binders – Experimental study and hydro-chemo-mechanical modelling

Nuclear waste management requires the packaging of several kinds of metal wastes for long-term storage. These wastes, which can be very reactive metals, are prone to corrosion and commonly immobilised into containers with hydraulic binders as embedding matrices. Innovative binders (low carbon cements, alkali activated materials) are thus developed to increase the packaging performances. The main objective of the European project STREAM (in the frame of the Eurad-2 program) is to evaluate the interactions between these metal wastes and the selected cement matrices. The PhD thesis purpose is to investigate the reactive metal corrosion in the selected binder with electrochemical techniques. A generic experimental protocol will be developed in order to determine the impact of the corrosion products growth at the metal/binder interface on the global mechanical behaviour of the binder-waste composite and possible micro-cracks occurrence. A post-mortem characterisation will be performed on the metal/binder microstructure with mechanical properties measurements of the materials at the interface, especially the corrosion products. Afterwards, these results will feed a simplified Hydro-Chemo-Mechanical (HCM) model aiming the simulation of corrosion consequences on the composite material behaviour. Subsequently, this model will be used for long-term simulation at the waste package scale.
This research project is aimed at a PhD student wishing to improve his/her skills in materials science both in the experimental field and in the modelling/simulation of coupled physicochemical phenomena.

Covalent 2D organic nanostructures by optically controlled cross-linking of molecular self-assemblies

The self-assembly of molecules on crystalline substrates leads to non-covalent 2D structures with interesting properties for various fields such as optoelectronics and sensors. The stabilization of these 2D networks into covalent networks, while preserving these properties, is a major challenge and a topical issue. Various demonstrations show that crosslinking can be triggered by thermal processes. Photocrosslinking, on the other hand, is poorly described and the few examples that have been found involve ultra-high vacuum conditions.

Building on our previously developed know-how and the additional expertise of chemist collaborators, we therefore propose to carry out photocrosslinking of 2D networks at atmospheric pressure. We will use a model oligophenyl system that will be functionalized to allow photocrosslinking towards the production of a covalent 2D network. The resulting networks will be characterized through the correlation of optical spectroscopy and local probe microscopy to monitor and highlight photo-induced cross-linking processes at wavelength scale.

Control of two-dimensional magnetism by structural and chemical engineering of van der Waals interfaces

2D materials exhibit tunable interlayer interactions due to weak van der Waals bonding, which influences magnetic ordering in 2D magnets. The stacking sequence and internal chemistry impact ferromagnetic (FM) or antiferromagnetic (AFM) ordering, as seen in materials like CrBr3, CrI3, and Fe5GeTe2, where doping with Co raises the Curie temperature and alters magnetic phases. Chemical disorder also affects magnetic properties, with Mn/Sb substitution promoting FM ordering in Mn(Bi,Sb)2Te4. However, understanding how the atomic structure affects macroscopic magnetic properties remains limited due to the coexistence of metastable configurations. Precise control over stacking and chemical order is needed to harness 2D materials' magnetic and quantum properties. Transmission electron microscopy (TEM), especially aberration-corrected STEM, is today one of the most powerful techniques, enabling atomic-scale imaging and spectroscopy, for studying structural and chemical properties of 2D materials. This PhD project aims to study the relationship between atomic structure, chemistry, and magnetic properties in epitaxial 2D layers like (Fe,Co)5GeTe2, combining growth via molecular beam epitaxy (MBE) with STEM-based structural and chemical analysis.

Magneto-ionic gating of magnetic tunnel junctions for neuromorphic applications

Magneto-ionics is an emerging field that offers great potential for reducing power consumption in spintronics memory applications through non-volatile control of magnetic properties through gating. By combining the concept of voltage-controlled ionic motion from memristor technologies, typically used in neuromorphic applications, with spintronics, this field also provides a unique opportunity to create a new generation of neuromorphic functionalities based on spintronics devices.

The PhD will be an experimental research project focused on the implementation of magneto-ionic gating schemes in magnetic tunnel junction’s spintronics devices. The ultimate goal of the project is to obtain reliable and non-volatile gate-control over magnetisation switching in three-terminal magnetic tunnel junctions.
One major challenge remains ahead for the use of magneto-ionics in practical applications, its integration into magnetic tunnel junctions (MTJ), the building blocks of magnetic memory architectures. This will not only unlock the dynamic control of switching fields/currents in magnetic tunnel junctions to reduce power consumption, but also allow for the control of stochasticity, which has important implications in probabilistic computing.

Study of electronic processes in nitride LEDs by electro-emission microscopy

Nitride LEDs are universally used for energy-efficient lighting. They are extremely efficient at low indium content and low current density, allowing to produce commercial white LEDs from a blue LED and a phosphor that absorbs blue and re-emits a broad spectrum in the visible range. However, nitride LEDs suffer from a drastic drop in efficiency at higher current densities and higher indium concentrations, for emission in the green or red. This is an obstacle to extending their use, in order to obtain higher efficiencies with less material, as well as better color rendering. These efficiency drops are partly due to an increase in three-particle Auger-Meitner processes, which are strongly impacted by local device heterogeneities, and can be reduced by specific engineering of structural defects in nitride materials. This thesis proposes to study the electronic processes in nitride LEDs in operando, using electro-emission microscopy. In particular, charge injection mechanisms in the active part of the LEDs and Auger-Meitner processes will be investigated and quantified. The spatial resolution of the technique will allow to characterize the role of heterogeneities (defects or alloy disorder) in the loss processes.

Topological superconductivity and Fermi surface in spin-triplet superconductors

Topological superconductivity has become a subject of intense research due to its potential for breakthrough in the field of quantum information. Bulk systems are a promising possibility, with candidates found mainly among unconventional superconductors, which are also strongly correlated electron systems. Today, only a few candidate compounds for topological bulk superconductivity exists, and they are mostly uranium-based heavy fermion superconductors. UTe2 is one of the most prominent candidates. The topological properties of the superconductors depends crucially on the topology of the Fermi surface.
In this project we want to set up a novel technique (for our team) relying on a tunnel diode oscillator circuit. This techniques is very sensitive to quantum oscillations, and to be well both to high magnetic fields and to high-pressure studies. First experiments concentrate on the novel superconductor UTe2, where the Fermi surface is only partly known. In further studies the topological properties of the ferromagnetic superconductors UCoGe and URhGe will be revised.

Sub-critical crack growth in oxide glasses

Material failure is a concern for scientists and engineers worldwide. This includes oxide glasses, which are integral parts of building, electronics, satellites due to multiple advantageous features, including optical transparency, elevated mechanical and thermal properties, chemical durability, biocompatibility and bioactivity, etc. Despite this, oxide glasses have a significate drawback: they are inherently brittle. Oxide glasses are well known to undergo dynamic fracture (crack propagation velocity of ~km/s – as in the case of a glass crashing to the floor and shattering); yet, there is another fracture mode less noticeable that will be studied during this thesis, where crack fronts grow sub-critically. The growth of these crack fronts is aided by environmental parameters including atmospheric humidity and temperature, and the crack front velocity depends on the local stress felt by a crack tip, coined the stress intensity factor.

Currently, our experimental setup tracks the crack front position in time via a tubular microscope equipped with a camera. Post-analysis of images provides the crack front velocity and reveals the environmental limit K_e and region I. However, the current experimental setup cannot capture regions II and III. Several factors play into this limitation: elevated crack front velocity (10e-4 to 1500 m/s), sample size (5×5×25 mm^3), camera acquisition rates, etc.

In recent years, our team has used the potential drop technique to track the crack front velocity when v > 10e-4 m/s in PMMA. This technique involves the deposition of conductive strips on the sample surface. Subsequently, these lines are attached to a high frequency oscilloscope. As the crack front propagates through the sample, the lines are severed resulting in an increase in the electrical resistance. We now wish to adapt this technique to DCDC samples on oxide glasses. The thesis goal is the development and application of the potential drop techniques to DCDC samples. The challenge concerns the spatial temporal resolution (50 µm and 1 ns) in comparison to the crack tip velocity and sample size. The thesis student will take part in all the steps to realize the experiments: designing and depositing patterns (series of strips) on the glass surfaces using a cleanroom, running sub-critical cracking experiments in Region II and III, and analyzing data acquired during the experiment.

Development of solid porous siliceous supports for actinide sorption - Behaviour under irradiation

The aim of this research project is to study the densification of a mesoporous structure under the effect of irradiation damage produced by the presence of an actinide (238Pu) in the porous structure. To achieve this, siliceous materials based on mesoporous silicas modified by the addition of additive elements (B, Al, etc.) will be used. The purpose of adding these elements is to weaken the mesoporous structure in order to promote densification. The characteristics of the mesoporous structure (pore diameter, wall size, symmetry of the pore network) will be other parameters of the study. These materials will be functionalised with phosphonate ligands for actinide adsorption: thorium as a simulant in a preliminary stage, followed by plutonium. The final part of this work, which will continue beyond the thesis, will involve using various techniques (SAXS, BET, microscopy, etc.) to study the evolution of the mesoporous structure under the effect of irradiation damage as the material ages. This fundamental research work could have spin-offs in the field of nuclear waste conditioning materials: ageing of gels on the surface of nuclear glass, support material for decontaminating radioactive effluents. Part of the work will be carried out at CEA Marcoule's Atalante facility.

Modeling of Critical Heat Flux Using Lattice Boltzmann Methods: Application to the Experimental Devices of the RJH

The Lattice Boltzmann Methods (LBM) are numerical techniques used to simulate transport phenomena in complex systems. They allow for the modeling of fluid behavior in terms of particles that move on a discrete grid (a "lattice"). Unlike classical methods, which directly solve the differential equations of fluids, LBM simulates the evolution of distribution functions of fluid particles in a discrete space, using propagation and collision rules. The choice of the lattice in LBM is a crucial step, as it directly affects the accuracy, efficiency, and stability of the simulations. The lattice determines how fluid particles interact and move within space, as well as how the discretization of space and time is performed.

LBM methods exhibit natural parallelism properties, as calculations at each grid point are relatively independent. Although classical CFD methods based on the solution of the Navier-Stokes equations can also be parallelized, the nonlinear terms can make parallelism more difficult to manage, especially for models involving turbulent flows or irregular meshes. Therefore, LBM methods allow, at a lower computational cost, to capture complex phenomena. Recent work has shown that it is possible, with LBM, to reproduce the Nukiyama cooling curve (boiling in a vessel) and thus accurately calculate the critical heat flux. This flux corresponds to a mass boiling, known as the boiling crisis, which results in a sudden degradation of heat transfer.

The critical heat flux is a crucial issue for the Jules Horowitz Reactor, as experimental devices (DEX) are cooled by water in either natural or forced convection. Therefore, to ensure proper cooling of the DEX and the safety of the reactor, it is essential to ensure that, within the studied parameter range, the critical heat flux is not reached. It must therefore be determined with precision.

In the first part of the study, the student will define a lattice to apply LBM methods on an RJH device in natural convection. The student will then consolidate the results by comparing them with available data. Finally, exploratory calculations in forced convection (from laminar to turbulent flow) will be conducted.

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