Lean-Rare Earth Magnetic materials
The energy transition will lead to a very strong growth in the demand for rare earths (RE) over the next decade, especially for the elements (Nd, Pr) and (Dy, Tb). These RE, classified as critical materials, are used almost exclusively to produce NdFeB permanent magnets, and constitute 30% of their mass.
Several recent international studies, aiming to identify new alloys with low RE content and comparable performances to the dense magnetic phase Nd2Fe14B, put hard magnetic compounds of RE-Fe12 type as advantageous substitution solutions, allowing to reduce more that 35% of the amount of RE, while keeping the intrinsic magnetic properties close to those of the Nd2Fe14B composition.
The industrial developments of the RE-Fe12 alloys cannot yet be considered due to the important technological and scientific challenge that remain to be lifted in order to be able to produce dense magnets with resistance to demagnetization sufficient for current applications (coercivity Hc > 800 kA/m).
The aim of the post-doctoral work is to develop Nd-Fe12 based alloys with optimized intrinsic magnetic properties and to master the sintering of the powders in order to obtain dense magnets with coercivity beyond 800 kA/m, to fulfil the requirements of the applications in electric mobility. Two technological and scientific challenges are identified:
- understanding of the role of secondary phases on the coercivity. This will open the way to the implementation of techniques called "grain boundary engineering", well known for the NdFeB magnets to have remarkably improved the resistance to demagnetization.
- mastering the sintering step of these powders at low temperature (< 600°C) in order to avoid the decomposotion of the magnetic phase by grain boundary engineering
High efficiency silicon cell irradiations for space
Historically, photovoltaics was developed in conjunction with the growth of space exploration. During the 90's, III-V multi-junction solar cells were progressively replaced silicon, for their superior performance & radiation hardness. Today, the context is favorable to a revival of space Si: increasing PV power needs, missions with moderate durations & constraints (LEO), very low cost & high performance terrestrial Si cells (p-type > 26% AM1.5g). However, for Si cells, conventional irradiation ageing methods & sequences (ECSS) are less appropriate. As the literature mainly comes from 80s - 90s, it is necessary to revisit the topic for the latest generation of passivated contacts Si cells (developed at CEA INES) and the unique double beam irradiation facilities of JANNuS platform - CEA Saclay.
This work is part of the SiNRJs project, at the interface between two CEA departments, dealing with space photovoltaics & materials irradiation. The scientific & technological approach adopted: 1. fabrication of passivated contact Si cells (HeT and/or Poly-Si) 2. Si cells optoelectronic characterizations before irradiation (IV AM1.5/AM0, EQE, etc.) 3. Cells & samples proton irradiations, in situ characterizations (Raman & El) 4. Ex situ characterizations after irradiations (IV AM1.5/AM0, EQE, etc) 5. Results analysis and synthesis. From a scientific point of view, the key issues to be addressed concern the understanding of the mechanisms/dynamics of defect creation/healing under this double electronic and ballistic excitation.
Rhelogical properties of molten crystallized glass
Formulation of nuclear waste conditioning glass results from a compromise between waste loading, glass technological feasibility and its long-term behavior. Up to now borosilicate glasses formulated at CEA and elaborated at La Hague plant by Orano to condition nuclear waste are homogeneous when molten. That means that today glass formulation is determined such as solubility limits of each constituting elements of waste aren’t exceeded in order to avoid phase separation (implying typically Mo, S, P) and/or crystallization (implying typically Fe, Ni, Cr, Zn, Al, Ce, Cs, Ti…) leading to a two-phase molten glass (liquid-liquid or liquid-solid).Today CEA would like to explore the impact of solid particles in suspension in the molten glass and in the final glass canister on respectively the glass technological feasibility and its long-term behaviour.
The proposed study focuses on the molten glass technological feasibility. The presence of solid heterogeneities in the melt is known to lead to a modification of some of its physical properties – notably its rheology, as well as thermal and electrical conductivities, and can generate settling phenomena. Yet these properties are in the heart of vitrification process control and modelling. This study will then investigate the impact of crystals in the molten glass on vitrification process control and modelling.
Etudes et développement d’un système laser dans l’UV pour la démonstration à l’échelle laboratoire de l’épuration isotopique du palladium (naturel).
Le palladium est un métal rare dont la demande mondiale est en forte augmentation. Or, il est présent en tant que produit de fission dans les combustibles nucléaires usés qui sont retraités en France. Il serait donc intéressant de recycler ce métal. Pour cela, il est nécessaire de procéder à une épuration isotopique, afin de supprimer un des isotopes du palladium, le 107, qui est un radionucléide artificiel à vie longue émetteur béta. Dans le cadre d'un nouveau projet sur 4 ans construit en réponse à l'appel d'offre du Plan d'Investissement et d'Avenir de l’État, le Service d’Etude des Procédés d’Enrichissement propose un contrat post-doctoral portant sur le développement d’un système laser dans l’UV pour le procédé de séparation isotopique du palladium par Lasers actuellement en cours de développement. L’objectif principal du projet est la démonstration finale de la faisabilité de séparation de palladium naturel (et non radioactif) pour la phase suivante de développement d’un premier pilote.
Le post-doctorant devra développer des lasers prototypes de procédé à haute cadence en partant du visible (système lasers colorant) jusqu’à l'UV. Le passage dans l’UV se fait par doublage de fréquence avec des objectifs élevés en terme de performance. Il s’agit d’utiliser un cristal doubleur de fréquence de type BBO, LBO, KDP ou autre. Pour ce faire, le post-doctorat participera à la définition de ce cristal, mais aussi au développement de l’environnement du cristal doubleur (comportement, performances attendues et la tenue au flux des différents matériels). Des échanges seront mis en place sur ce sujet spécifique avec des spécialistes reconnus au sein de la Direction de la Recherche Fondamentale du CEA. La programmation (en Python et/ou sous Labview) de ces outils ou asservissements est à développer également. Une attention particulière sera portée sur les publications à réaliser essentiellement dans le cadre du doublage de fréquence, sujet complexe très étudié mondialement.
Simulation of a porous medium subjected to high speed impacts
The control of the dynamic response of complex materials (foam, ceramic, metal, composite) subjected to intense solicitations (energy deposition, hypervelocity impact) is a major issue for many applications developed and carried out French Atomic Energy Commission (CEA). In this context, CEA CESTA is developing mathematical models to depict the behavior of materials subjected to hypervelocity impacts. Thus, in the context of the ANR ASTRID SNIP (Numerical Simulation of Impacts in Porous Media) in collaboration with the IUSTI (Aix-Marseille Université), studies on the theme of modeling porous materials are conducted. They aim to develop innovative models that are more robust and overcome the theoretical deficits of existing methods (thermodynamic consistency, preservation of the entropy principle). In the context of this post-doc, the candidate will first do a literature review to understand the methods and models developed within IUSTI and CEA CESTA to understand their differences. Secondly, he will study the compatibility between the model developed at IUSTI and the numerical resolution methods used in the hydrodynamics computing code of the CEA CESTA. He will propose adaptations and improvements of this model to take into account all the physical phenomena that we want to capture (plasticity, shear stresses, presence of fluid inclusions, damage) and make its integration into the computation code possible. After a development phase, the validation of all this work will be carried out via comparisons with other existing models, as well as the confrontation with experimental results of impacts from the literature and from CEA database.
Design of a crystal growth process
Laser fusion facilities, like LMJ, require the use of large optical components. Some of them are large KDP or DKDP (KDP partially deuterated) plates extracted from single crystals.
Currently, DKDP single crystals are produced a by slow growth method were the growth time exceeds two years.
Here, we proposed to study a rapid growth method to reducing the growth time to a few months.
Postdoctoral position on the modeling of silicon spin qubits
A post-doctoral position is opened at the Interdisciplinary Research Institute of Grenoble (IRIG) of the CEA Grenoble (France) on the theory and modeling of silicon spin quantum bits (qubits). The selected candidate is expected to start at the beginning of year 2022, for up to two years.
Quantum information technologies on silicon have raised an increasing interest over the last few years. Grenoble is pushing forward an original platform based on the “silicon-on-insulator” (SOI) technology. In order to meet the challenges of quantum information technologies, is essential to support the experimental activity with state-of-the-art modeling. For that purpose, CEA is actively developing the “TB_Sim” code. TB_Sim is able to describe very realistic qubit structures down to the atomic scale when needed using atomistic tight-binding and multi-bands k.p models for the electronic structure of the materials. The aims of this postdoctoral position are to strengthen our understanding of spin qubits, and to progress in the design of efficient and reliable Si and Si/Ge spin qubit devices and arrays using a combination of analytical models and advanced numerical simulations with TB_Sim. Topics of interest include spin manipulation & readout in electron and hole qubits, exchange interactions in 1D and 2D arrays of qubits and operation of multi-qubit gates, sensitivity to noise (decoherence) and disorder (variability). This work takes place in the context of the EU QLSI project and will be strongly coupled to the experimental activity in Grenoble and among the partners of CEA in Europe.
Improvement by thermodynamic calculations of the modeling for the joint oxyde-gaine and the fuel cladding chemical interaction into the fuel performance code GERMINAL
This work is proposed in the frame of studies on the physico-chemical behaviour of the (U,Pu)O2 fuel during irradiation considered for the future reactors of 4th generation. Indeed, this kind of fuel is subject in particular to two specific specific phenomena that can have an impact on its behaviour:
- the formation of a JOG (Joint Oxyde-Gaine), a fission products layer localised between the external surface of the fuel pellet and the inner surface of the cladding material ;
- the FCCI (Fuel-Cladding Chemical Interaction), which leads to the formation of a corrosion layer on the internal surface of the clad containing fission products and elements constituting the cladding material.
The goal is this work is to improve the modelling of the JOG and of the FCCI into the fuel performance code (FPC) GERMINAL, dedicated dedicated to the calculation of the thermo-mechanical and physico-chemical behaviour of fast reactor fuel irradiated in normal and off-normal conditions. For that purpose, the candidate will work on the dedicated calculation scheme of GERMINAL which uses the thermochemical software OpenCalphad and on the comparison of the JOG and of the internal cladding corrosion widths obtained to experimental observations obtained for some irradiation experiments. Complementary stand-alone thermodynamic calculations will be performed with the TAFID, thermodynamic database on nuclear materials developed in an international framework, in order to analyse the thermochemistry JOG/FCCI versus parameters of interest.
This work will be performed in collaboration with a team specialised in thermodynamic modelling, in charge of the TAFID project. The student will thus have the opportunity to exchange on his results in a collaborative frame with international partners. In addition, he will be able to highlight his work through publications and presentations at conferences.