Cluster dynamic simulations of materials under irradiation
Alloys used in nuclear applications are subjected to neutron irradiation, which introduces large amounts of vacancy and interstitial defects. Over time, these defects migrate, recombine and agglomerate with minor alloying elements to form small clusters. This affects the mechanical properties of ferritic steels and weakens them. In this context, the microstuctural evolution is to be simulated using the rate equation cluster dynamic method. However, this approach becomes ineffecient when several minor alloying elements need being taken into account. The difficulty comes from the huge number of cluster variables to describe. The project aims at optimizing the code efficiency on a distributed parallel architecture by implementing parallelized vector and matrix functions from SUNDIALS library. This library is used to integrate the ordinary differential equation describing the reactions between clusters. Another aspect of the work is more theoretical and involves reformulating the non-linear root-finding problem by taking advantage of the reversibility of most chemical reactions. This property should facilitates the implementation of direct and gradients iterative sparse solvers for symmetric definite positive matrices, such as the multi-frontal Cholesky factorization and the conjugate gradient methods, respectively. One avenue of research will consists of combining direct and iterative solvers, using the former as a preconditioner of the latter.
High entropy alloys determination (predictive thermodynamics and Machine learning) and their fast elaboration by Spark Plasma Sintering
The proposed work aims to create an integrated system combining a computational thermodynamic algorithm (CALPHAD-type (calculation of phase diagrams)) with a multi-objective algorithm (genetic, Gaussian or other) together with data mining techniques in order to select and optimize compositions of High entropy alloys in a 6-element system: Fe-Ni-Co-Cr-Al-Mo.
Associated with computational methods, fast fabrication and characterization methods of samples (hardness, density, grain size) will support the selection process. Optimization and validation of the alloy’s composition will be oriented towards two industrial use cases: structural alloys (replacement of Ni-based alloys) and corrosion protection against melted salts (nuclear application)
Nonlinear ultrasonic testing for the assessment of adhesive bonding properties
The CEA-LIST carries out Non Destructive Testing (NDT) projects in partnership with various industrial sectors. A strong collaboration with Airbus Group Innovations (AGI) had led to a common entity through the NDT laboratory for Aeronautics Applications (LC2A).
With the increasing portion of composite materials in the aerospace industry, assessment of the adhesive bonding properties of such composite structures is a key issue. Various aspects could decrease the quality of bonding, such as the surface contamination, non-optimal thermal cycle or external mechanical stresses. However, conventional NDT techniques are often not sensible to such damages in the adhesive bonds.
Non-linear ultrasonic methods such as wave mixing, harmonic generation or non- linear imaging appear as promising techniques to detect kissing bonds and pre-damaging that could occur in adhesive bonds. The objective of this postdoc position is to develop NDT innovative solutions for the assessment of the adhesion quality by means of experimental techniques based on such non-linear methods.
This post-doc position will be carried out in the framework of an international research program on the adhesion bonding. The candidate will work in the NDE laboratory for Aeronautics Applications located in Toulouse. Strong skills in experimental physics, instrumentation, and non-linear ultrasonics would be appreciated.
New packaging for power electronics : application to SiC components
In continuity of ongoing work (PhD thesis) on the 3D assembly of vertical Silicon power components, the purpose of the post-doc proposal is to develop a similar assembly on vertical wide-gap SiC power components. The required work will be to define the components (high frequency / high voltage) with the supplier and to adapt them to the best vertical integration (Cu finishing, topology,...), to adjust the metal leadframe design for the 3D assembly, and to develop the transfer layer technology adapted to this new material. The candidate will also take care of the electrical characterizations of the final stack to validate the interest of this 3D packaging on wide-gap power devices.
Active medical implants encapsulated using hermetic glass package
Microelectronics extends its range of applications via the micro-systems with high level of integration including sensors, energy scavenger, and communication modules. Medical implants such as pacemakers and defibrillators medical implants, drug dispensers, intra-cranial probes are many possible applications for these modules. The use of glass offers a wide field of investigation. Moreover, recent innovations for the glass material (interconnections, thinning, and functionalization) reinforce its relevance to the medical field: biocompatibility, stability, transparency, and potentially lower cost.
The objective of this work is to design and validate technological steps to integrate high level of microsystems encapsulated in glass material.
Improvement of microfluidic tools for kinetic data measurement
The development and modeling chemical processes require the acquisition of many thermodynamic and kinetic data . Conventional methods for measuring these data generally involve significant amounts of reagents. In particular for the reactive crystallisation, where the stochastic nature of nucleation requires the realization of a large number of experiments . The subject is to continue the work already done on the development of a dedicated chip to measure rapid nucleation kinetics . Firstly , the validity of kinetic measurements obtained by microfluidics technique will be evaluated and optimized based on well known and non- radioactive chemical systems . The microfluidic tool will then be used to study the sensitivity of these reactions to various operating parameters ( supersaturation , impurities , additives, etc. . ), before considering its transposition to nuclear processes such as decontamination of radioactive effluents. Finally, a new chip design could be proposed for the measurement of kinetics of liquid-liquid extraction , in connection with the development of new hydrometallurgical processes.
Multiscale Modelling of Radiation Induced Segregation
Irradiation produces in materials excess vacancies and self-interstials that eliminate by mutual recombination or by annihilation at sinks (surfaces, grain boudaries, dislocations).
It sustains permanent fluxes of point defects towards those sinks. In case of preferential transport of one componant of an alloy, the chemical composition is modified in the vicinity of the sinks: a Radiation Induced Segregation (RIS). Its modelling requires a good description of the alloy properties: its driving forces (derived from the thermodynamics) and its kinetic coefficients (the Onsager matrix). The objectif on this project is to combine (i) atomic models (Kinetic Monte Carlo simulations and Self-Consistent Mean Field), fitted on ab initio calculations, that provide the Onsager coeffcients and the driving forces and (ii) a Phase-Field modelling that will give a description of the evolution of the alloy under irradiation at much larger time- and space-scales. The approach will be applied to Fe-Cr and Fe-Cu alloys, already modelled at the atomic scale. RIS will be first modelled near grain boundaries, then near dislocation loops. Special attention will be paid to the effect of elastic stresses on the RIS.
Ge-on-Insulator (GeOI) substrates for photonics
The induction of tensile strain in intrinsic and doped Germanium (Ge) is one approach currently explored to transform the Ge indirect bandgap into a direct one. To take full advantage of Ge, we study the Ge CMOS photonics platform with Ge-on-Insulator (GeOI) structure, which enables strong 2D optical confinement in the Ge photonic-wire devices. One recent study in our lab showed the interest of a method of incorporation of mechanical stress into Ge, one of the essential ingredients of the laser. In particular, the method could be applied to the massive Ge, making compatible gap direct and crystalline quality.
Post-doc objectives : Development of GeOI substrates from massive Ge donors with tensile strain inside the Ge film. These developments will be realized from the existing Smart Cut / thinning processes, combined with technological steps to overcome their current limits (SAB bonding). The substrates obtained will be characterized to determine their state of deformation as well as their damage (Raman / XRD) and final GeOI substrates will be provided to the application laboratories for the production of photonic components.
Eco-innovation of insulating materials by AI, for the design of a future cable that is long-lasting, resilient, bio-sourced and recyclable.
This topic is part of a larger upcoming project for the AI-powered creation of a new electrical cable for future nuclear power plants. The goal is to design cables with a much longer lifetime than existing cables in an eco-innovative approach.
The focus is on the cable insulation because it is the most critical component for the application and the most sensitive to aging. The current solution is based on adding additives (anti-rad and antioxidants) to the insulation to limit the effects of irradiation and delay aging as much as possible. However, there is another solution that has never been tested before: self-repairing materials.
The project to which this topic is attached aims to design and manufacture several test model of insulation specimens. With several test characterization protocols, in order to verify the gain in terms of reliability and resilience. The results obtained will begin to fill a future database for the AI platform Expressif, developed at CEA List, which will be used to design the future cable.
Aging study of silicon nanowires used as piezoresistive detection gauges for achieving inertial MEMS sensors.
Today’s sensors are present in all areas: housing, automotive... Thanks to recent developments in microelectronics, new generations of sensors combine high performance, small size and low cost. In this context, CEA-LETI has proposed an innovative concept called M&NEMS for the realization of inertial sensors such as accelerometers, magnetometers and gyroscopes. The M&Nems concept combines MEMS and NEMS to take advantage of the great inertia generated by a MEMS mass and the high detection sensitivity of piezoresistive silicon nanowires. Demonstrators have already been carried out and have shown the good potential of the M&Nems concept. One of the main challenges which remain to overcome is the reliability of sensors based on this concept and specifically the reliability of the piezoresistive nanowires. The research work will be mainly focused on the study of the failure modes of these piezoresistive nanowires gauges i.e. the identification of physical phenomena and the development of failure models. In order to do this study, a first preliminary work will be focused on the physical mechanisms which manage the electrical conduction in the nanowires: piezoresistivity, charge trapping, relaxation field effect ... The work will then continue by the study of the failure modes of nanowires, the goal will be to understand and model the physical aging of these nanowires: it will be possible to rely on the knowledge of the physics of nanowires conduction but also play with the physical parameters of these nanowires such as silicon doping, the process fabrication, the packaging technique, the thermomechanical stresses, the scale effect due to surface / volume ratio, or the surface condition. Finally, models of aging will allow proposing and validating technological choices to ensure the nanowires lifetime depending on operating conditions of the sensors.