From angstroms to microns: a nuclear fuel microstructure evolution model whose parameters are calculated at the atomic scale
Controlling the behavior of fission gases in nuclear fuel (uranium oxide) is an important industrial issue, as fission gas release or precipitation limit the use of fuels at extended burn-ups. The gas behavior is strongly influenced by the material’s microstructure evolution due to the aggregation of irradiation-induced defects (gas bubbles, dislocation loops and lines). Cluster dynamics (CD) (a kind of rate theory model) is relevant for modelling the nucleation/growth of the defect clusters, there gas content and the gas release. The current model has been parameterized following a multiscale approach, based on atomistic calculations (ab initio or empirical potentials). This model has been successfully applied to annealing experiments of UO2 samples implanted with rare gas atoms and has emphasized the impact of the irradiation damage on gas release. The aim of this PhD thesis is now to improve the model, particularly the damage parameterization, and to extend its validation domain through in depth comparison of simulation with a large set of recently obtained experimental results, such as gas release measurement by annealing of sample implanted in ion beam accelerator, bubble and loop observation by transmission electrons microscopy, and positron annihilation spectroscopy. This global analysis will finally yield an improved parameterization of the CD model.
The research subject combines a “theoretical” dimension (improving the model) with an “experimental” one (interpreting existing experiments or designing some new ones). The variety of techniques will introduce the candidate into the experimental world and thus broaden his scientific skills. The candidate will also have to manage collaborations for the experiments analysis, for the model development and for the specification of additional atomistic calculations. He will be at the interface of atomistic techniques, large-scale simulation and various experimental techniques. Therefore, he will develop a broad view of irradiation effects in materials and of multi-scale modelling in solids in general.
This project is an opportunity to contribute to the overall development of numerical physics applied to multi-scale modeling of materials, occupying a pivotal position and adopting a global viewpoint. This will allow experiencing oneself the way computed fundamental microscopic data finally helps solving complex practical issues.
Further readings:
Skorek et al. (2012). Modelling Fission Gas Bubble Distribution in UO2. Defect and Diffusion Forum, 323–325, 209.
Bertolus et al. (2015). Linking atomic and mesoscopic scales for the modelling of the transport properties of uranium dioxide under irradiation. Journal of Nuclear Materials, 462, 475–495.
TeraHertz Landau emission in HgTe/CdTe topological quantum wells
Quantum well heterostructures of HgTe/CdTe are known as topological insulators. They inherit very peculiar electronic properties. One of them is the ability of producing TeraHertz emission from inter-Landau energy level optical transitions. These transitions can be envisioned to lead to coherent optical sources in spectral range where they are basically absent. The PhD Thesis consists in elaborating and characterizing HgTe/CdTe multiple quantum well structures by epitaxy, process them in order to add functionality through optical cavities metallic report or deposition and electrical gating, and finally carry out full range optical spectroscopy of Landau emission in magnetic fields. The PhD will be carried out in a collaborative environment between CEA-Leti and Institut Néel (CNRS) in Grenoble, France, two leading laboratories in the expertise in material growth and Physics of HgTe/CdTe topological insulator systems. The results will help to understand the potential of application of this peculiar material system in TeraHertz laser sources and hopefully lead to the first demonstration of spontaneous emission in the TeraHerz range.
Theoretical design of quasi-atomic systems in the band gap of semiconductors/insulators for quantum application
The rise of room-temperature applications like single photon emission of the negatively charged nitrogen-vacancy NV center in diamond has renewed the interest in the search for materials having a quasi-atomic system QAS analogous to that of NV, mainly characterized by the presence of well localized in-gap defect levels generate occupied by electrons and leading to high spin states. In this Ph.D. work, theoretical methods will be used to design new QASs analogous to the NV center as well as, in selected QAS, to predict charge states and explore the effect of the proximity of the surface on the thermodynamic stability and on the spin state structure. The objectives are to design new QASs; To predict charge states of selected QASs in the bulk of the host material; To study changes in the charge state brought by the proximity of the surface; To extend the Hubbard model used to compute the excited states and to account for the electron-lattice interaction in the calculation of the excited states; To study the effect of the presence of deep level states in the band gap on the transport of electrons and phonons. The methodology developed at LSI to design new QASs with high spin states will be exploited and new systems analogous to the NV center will be looked for. Density functional theory (DFT) and a Hubbard model developed at LSI will be the main tools of this PhD.
Theoretical studies of orbitronic and spin-orbit phenomena in heterostructures comprising van der Waals materials, metals and oxides
The proposed PhD thesis aims at finding the best-unexplored combinations of transition metals, oxides and 2D materials (transition metal dichalcogenides, 2D magnets, graphene…) to help optimizing and providing scientific underpinnings of next generation energy efficient spintronic storage and memory devices based on emerging fields of spin-orbitronics and orbitronics. The latter is a fascinating new field of research that exploits orbital currents and their interaction with spin currents mediated by spin-orbit coupling.
Namely, using first principles calculations combined with tight-binding approach and linear response theory, we will screen the potential of aforementioned heterostructures not only for spin-orbit phenomena such as Dzyaloshinskii-Moriya interaction (DMI), perpendicular magnetic anisotropy (PMA) and spin-charge interconversion based on Rashba and Rashba-Edelstein effects (REE), but also focus on Orbital Rashba Edelstein Effect (OREE). Furthermore, the mechanisms of control of these phenomena via external stimuli (strain, external electric and magnetic fields) will be investigated as well. These studies will help finding optimal material combination to tune DMI, PMA and spin-charge interconversion efficiency to help optimizing spintronic devices making thereby a significant contribution to the development of sustainable microelectronics.
The PhD will be based on a multiscale approach including ab initio, tight-binding and atomistic approaches thus highly motivated candidate with a good background in solid state physics, condensed matter theory and numerical simulations is required. He/she will perform his/her calculations on Spintec computational cluster nodes using first-principles packages based on density functional theory (DFT) combined with other simulation codes/tools. Results obtained will be carefully analyzed with the possibility of publication in international scientific journals. Strong collaboration with labs in France (CEA/LETI, Laboratoire Albert Fert (CNRS,Thales), Aix-Marseille Univ…) and abroad (ICN2-Barcelona, PGI Forschungszentrum Jülich, Osaka University) are previewed.
Fracture dynamics in crystalline layer transfer technology
Smart Cut™ is a technology discovered at CEA and now industrially used for the manufacture of advanced substrates for electronics. However, the physical phenomena involved are still the focus of numerous studies at CEA. In Smart Cut™, a thin material layer is transferred from one wafer to another using a key fracture annealing step upon which a macroscopic fracture initiate & propagates at several km/s [i].
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Improving technology requires a solid understanding of the physical phenomena involved in the fracture step. The aim of this PhD project is thus to address the mechanisms involved in fracture initiation, propagation and post-fracture vibrations
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On the CEA-Grenoble site, with industrial interest, the student will use and further develop existing experimental setups to investigate the fracture behavior in brittle materials, including optical laser reflections [iv], time-resolved synchrotron diffracting imaging [iii], and ultra-fast direct imaging [ii].
In addition, python-based data analysis algorithms will be developed to extract quantitative information from the different datasets. This will enable the student to determine involved mechanisms and evaluate the influence of the wafer processing parameters on the fracture behavior, and thus propose improvement methods.
References :
[i] https://pubs.aip.org/aip/apl/article/107/9/092102/594044
[ii] https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.15.024068
[ii] https://journals.iucr.org/j/issues/2022/04/00/vb5040/index.html
[iv] https://pubs.aip.org/aip/jap/article/129/18/185103/158396
Topological magnons in quantum materials
Topology has become an essential paradigm in condensed matter, making it possible to classify phases of matter according to properties that are invariant under continuous deformations. Early research has mainly focused on electronic band structures, leading to the discovery of “topological insulators” for example. However, there is growing interest in applying topological concepts to bosons, in particular magnons. Magnons, which are collective excitations in magnetic materials, illustrate how topology influences magnetic dynamics and affects heat and spin transport. Analogues of topological insulators and semi-metals appear in their band structures. Magnons thus offer a platform for studying the interplay between magnetic symmetries and topology, examining the effect of interactions on topological bands, and generating protected spin currents at interfaces. The search for materials containing topological magnons is therefore crucial, especially for applications in magnonics, which exploit spin waves for fast data storage and processing.
This thesis project is dedicated to exploring these topological aspects in candidate quantum materials using neutron and X-ray scattering techniques in large scale facilities (ILL, ESRF, SOLEIL) to probe the magnon band structure in search of topological features such as Dirac or Weyl points. Experimental results will be supported by numerical and theoretical calculations of magnonic bands incorporating topological concepts.
Strain field imaging in semiconductors: from materials to devices
This subject addresses the visualization and quantification of deformation fields in semiconductor materials, using synchrotron radiation techniques. The control of the deformation is fundamental to optimize the electronic transport, mechanical and thermal properties.
In a dual technique approach we will combine the determination of the local deviatoric strain tensor by scanning the sample under a polychromatic nano beam (µLaue) and a monochromatic full field imaging with a larger beam (dark field x ray microscopy, DFXM).
New developments of the analysis will be focused on 1/ the improvement of the accuracy and speed of the quantitative strain field determination, 2/ the analysis of strain gradient distributions in the materials, and 3/ the determination of the dynamic strain field in piezoelectric materials through stroboscopic measurements. To illustrate these points, three scientific cases corresponding to relevant microelectronic materials of increasing complexity will be studied:
1- Static strain fields surrounding metallic contacts, such as high-density through silicon vias (TSV) in CMOS technology.
2- Strain gradients in Ge/GeSn complex heteroepitaxial structures with compositional variations along the growth direction.
3- Dynamical strain in LiNbO3 surface acoustic wave resonators with resonance frequency in the MHz range bulk
Establishing this approach will mean moving a step forward towards more efficient microelectronics and strain engineering.
Atomistic investigation of the diffusion of small xenon clusters in the metallic nuclear fuel UMo
This project is centered on the application of atomistic methods in order to investigate the stability and diffusion of intra-granular xenon clusters within the metallic nuclear fuel UMo.
Uranium – molybdenum alloys UMo present excellent thermal properties and a good uranium density. For those reasons, they are considered as nuclear fuel candidates for research reactors. It is therefore crucial to deploy new computational methodologies in order to investigate the evolution of their thermophysical properties under irradiation conditions.
During this PhD project, you will be in charge of validating (and, if necessary, recalibrating) the atomistic computational models for UMo that have been published in the literature. You will then apply those to the simulation of the stability and diffusion of small xenon clusters (typically up to 5 xenon atoms) within UMo crystals. Those computations will be performed leveraging accelerated molecular dynamics methods, and systematically compared to the results obtained for the reference nuclear fuel UO2. The results will also be analyzed by comparison to experimental measurements performed within the department, as well as used as reference data for larger-scale nuclear fuel performance codes. The results of your research will be published in scientific journals, and you are expected to attend international conferences to present your findings.
Those different investigations will allow you to acquire a set of competences applicable to many areas of materials science: ab initio calculations, machine-learning adjustment of interatomic potentials, classical and accelerated molecular dynamics, as well as many elements of statistical physics and condensed matter physics, which are among the areas of expertise of the PhD advisors.
The PhD will be based in the Fuel Behavior Modeling Laboratory (IRESNE Institute, CEA Cadarache), a dynamic research environment within which you will have the opportunity to interact with other PhD students. You will also benefit from a rich collaborative network (experimental researchers from the department, ISAS Institute at CEA Saclay, CINAM Laboratory in Marseille), that will allow you to become a member of the nuclear materials research community.
Predicting thermodynamic properties of defects in medium-entropy alloys from the atomic scale through statistical learning
The properties and behaviour of materials under extreme conditions are essential for energy systems such as fission and fusion reactors. However, accurately predicting the properties of materials at high temperatures remains a challenge. Direct measurements of these properties are limited by experimental instrumentation, and atomic-scale simulations based on empirical force fields are often unreliable due to a lack of precision.This problem can be solved using statistical learning techniques, which have recently seen their use explode in materials science. Force fields constructed by machine learning achieve the degree of accuracy of {it ab initio} calculations; however, their implementation in sampling methods is limited by high computational costs, generally several orders of magnitude higher than those of traditional force fields. To overcome this limitation, two objectives will be pursued in this thesis: (i) to improve active statistical learning force fields by finding a better accuracy-efficiency trade-off and (ii) to create accelerated free energy and kinetic path sampling methods to facilitate the use of computationally expensive statistical learning force fields.
For the first objective, we improve the construction of statistical learning force fields by focusing on three key factors: the database, the local atomic environment descriptor and the regression model. For the second objective, we will implement a fast and robust Bayesian sampling scheme to estimate the anharmonic free energy, which is crucial for understanding the effects of temperature on crystalline solids, using an adaptive bias force method that significantly improves convergence speed and overall accuracy.We will apply the methods developed to the calculation of free energy and its derivatives, physical quantities that give access to the thermo-elastic properties of alloys and the thermodynamic properties of point defects. To do this, we will use algorithmic extensions that allow us to sample a specific metastable state and also the transition paths to other energy basins, and thus to estimate the free energies of formation and migration of vacancy defects. The thermodynamic quantities calculated will then be used as input data for kinetic Monte Carlo methods, which will make it possible to measure the diffusion coefficients in complex alloys as a function of temperature.
One aim will be to try to relate the atomic transport properties to the complexity of the alloy. Since our approach is considerably faster than standard methods, we will be able to apply it to complex alloys comprising the elements W, Ti, V, Mo and Ta at temperatures and compositions that have not been studied experimentally.
Behavior of nanocavities under mechanical loading: from understanding physical mechanisms to homogenizing nanoporous materials
Nanocavities - typically a few nm to a few tens of nm in size - are often observed in metals, for example in high-temperature applications due to the condensation of vacancies or in metal alloys used in nuclear reactors due to irradiation. The presence of these nanocavities degrades the mechanical behaviour of materials and contributes to fracture. It is therefore necessary to determine the physical mechanisms associated with the behaviour of these nanocavities under mechanical loading and to obtain homogenised models describing the macroscopic behaviour of these nanoporous materials. The results available in the literature remain limited to date, particularly with regard to the representativeness of the simulations carried out and the models proposed for the applications of interest. This includes for example considering crystal defects surrounding the cavities, the effect of cyclic loading and the localisation of nanocavities at grain boundaries. The objectives of this thesis are therefore to determine the behaviour of nanocavities under mechanical loading and the associated physical mechanisms by considering realistic situations with respect to applications, to develop physically-based analytical models to describe the behaviour of nanocavities under mechanical loading, and finally to propose homogenised models adapted to nanocavities that can be used to simulate the failure by growth and coalescence of cavities. The targeted applications are those related to metal alloys under irradiation, but the elements of understanding obtained and the models developed could be used in a broader context. In order to achieve these objectives, Molecular Dynamics (MD) simulations will be performed, analysed from the elastic theory of dislocations and used to propose relevant homogenised models for nanoporous materials.