Mitigation of Alkali Silica Reaction in concrete used for radwaste stabilization and solidification
Electricity production from nuclear power plants generates radioactive wastes, the management of which represents a major industrial and environmental concern. Thus, low- or intermediate - level radioactive aqueous waste streams may be concentrated by evaporation, and immobilized with a Portland cement, before being sent to disposal. Nevertheless, interactions may occur between some components of the waste and the cement phases or aggregates, and decrease the stability of the final waste forms. Thereby, the formation of a gel-like product has been recently observed on the surface of some cemented drums of evaporator concentrates which were produced in the 80’s in Belgium. This product results from a reaction between silica from the aggregates and the very alkaline pore solution of the concrete. However, its composition and rheological properties differ from those reported for alkali-silica gels in civil engineering. Extensive work has been performed to better understand the processes involved in the gel formation within the cement-waste forms and characterize its properties. Based on these results, the post-doctoral project will be focussed on the mitigation of alkali silica reaction in cement-waste forms. Two approaches will be more particularly investigated by decreasing the water saturation ratio of concrete and/or the pH of its pore solution using supercritical carbonation.
This project is intended for a post-doctoral fellow wishing to develop skills in materials science, with an interest in advancing the field of cement chemistry and improving the conditioning of radioactive waste. It will be performed in collaboration with ONDRAF-NIRAS, the Agency in charge of radioactive waste management in Belgium, and will build upon the expertise of two laboratories at CEA Marcoule: the Cements and Bitumen for Waste Conditioning Laboratory for materials elaboration and characterization, and the Supercritical and Decontamination Laboratory.
Next generation PV module packaging design and mechanical testing
Photovoltaic modules are required to last 25- 30 years in harsh outdoor environment. The packaging of PV modules plays an essential role in reaching this target. PV cells are protected by a glass frontsheet, and highly engineered polymeric encapsulants and backsheets. Encapsulants provide moisture, oxygen &UV barrier, electrical isolation and mechanical protection of highly fragile cells while they must ensure optical coupling between the various layers. Current industrial process technology for module manufacturing is lamination that adds additional constraints to the formulation of encapsulants. These numerous requirements lead to ever-involving complex encapsulant composition and behavior.
The aim of this post-doc is to establish the correlation between the material properties of engineered plastics– their processing conditions and thermo-mechanical behavior in high performance PV modules with heterojunction, back-contact or Si/Perovksite tandem cells. Material selection and lamination process development will be guided by detailed material characterization (DSC, DMA, Peel strength, TGA, WVTR, Soxhlet extraction etc.). Moreover, we aim to establish insights in the encapsulant processing conditions and its impact on mechanical stability of PV modules. The selection of the encapsulants to investigate will be strongly guided by eco-design to lower the environmental impact and to increase the recyclability of modules. This postdoc is conducted in the frame of an EU collaboration.
Automatic machine learning identification of nanoscale features in transmission electron microscopy images
Imaging nanoscale features using transmission electron microscopy (TEM) is key to predicting and assessing the mechanical behaviour of structural materials in nuclear reactors or in the fields of nanotechnology. These features, visible by phase contrast (nanobubbles) or diffraction contrast (dislocation loops or coherent precipitates), are prime candidates for automation. Analysing these micrographs manually is often tedious, time-consuming, non-universal and somehow subjective.
In this project, the objective is to develop a Python-based framework for data treatment of transmission electron microscopy (TEM) images.
Machine Learning approaches will be implemented in order to tackle the following tasks:
- Data collection: The success of any machine learning approach is linked to the database quality. In this project, a huge database is available. Four microscopists are involved in the project and will continuously enrich the database with images containing easily recognizable features.
- Denoising and finding the defect contour both through existing open-access software and in-house developed descriptors. Representative ROI (region-of-Interest) will be generated on images.
- Design of the Convolutional Neural Network (CNN) Architecture and model training: A collective feature map will be generated for the entire images in order to identify some representatives ROI. Each ROI is then overlaid to the original feature map and is passed to the CNN for individual region classifications. Secondly, recent advances in image segmentation will be placed in the core engine of the workflow.
- Model performance metrics: The aim is to reach a compromise between the training time and the detector performance.
The process will be applied to nanometer-sized features formed under irradiation in nuclear oriented materials (Co-free high entropy alloys (HEA), UO2) and precipitates in materials with a technological interest (coherent Cr precipitates in Cu).
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
Highgly reflective materials laser microwelding
In the frame of the Simulation Program, CEA/DAM conducts experiments on high powerful lasers involving complex targets. Intensive research is therefore conducted to study and manufacture a large panel of targets - with ambitious scientific and technological challenges ahead. In particular, CEA wants to extend its laser microwelding capabilities–at a sub-mm scale. The challenge is to weld both high-reflective and thin materials (aluminum, copper, gold …) with an accurate mastering of heat deposition and penetration depth. The goal is to implement, optimize and qualify a process based on the latest source generation (UV or green laser source), and to get an innovative set of experimental data. A phenomenological model might also be proposed.
The latest generation of laser source emitting in visible wavelengths (green, blue) will be exploited. He/she will participate in the design and qualification testing of the laser station associated with this new source. Once validated, he/she will carry out the study of the operational and metallurgical weldability of the sub-elements. He/she will compare his/her results with the use of a pulse infrared laser. He/she will appraise the joints obtained using different approaches and optimize the design of the welded joints. Its experimental study will go as far as carrying out functional tests on prototypes. External collaborations will be set up to compare the results obtained with simulations in order to deduce a phenomenological model.
Synthesis by 3D printing of functionnalized geopolymer membrane for the treatment of complex radioactive effluents.
In the field of the treatment of liquid radioactive wastes on solid supports, the development of new composite materials synthetized by 3D printing under filtre shape is of primary of importance to decontaminate some radioactive effluents.
In this phD proposal, we propose to develop a membrane allowing to produce, from effluent containing somes traces of micronic solids in suspension and ionic species, a clarified effluent compatible with a nuclear outlet pipe. The challenge is to study the shaping of a material in a form of a filtration membrane allowing to trap in a single step an effluent containing some solids in suspension and some ionic species. In order to develop both functionnalities, 3D printing will be used to synthetise multiscale porous ceramic composites such as some geopolymers functionnalized by a selective adsorbants. The candidate, mainly based at CEA/ISEC Marcoule, could first formulate a functionnalized geopolymer paste with suitable rheological properties compatible with the constraints of the 3D printing process. A cross-flow filtration membrane with a controled macroporous network will be then printed by optimizing the geometry of the mesh. Finally, some sorption and cross-flow filtration tests will be performed on some model effluents containing calibrated solid in suspension and ions of interest such as Cs and Sr. The relevance of the printed membrane architecture will be assessed in relation to the capture of the solids and radioelements.
The candidate must have skills in the field of rheology, process and modeling. From this research work, job opportunities either in the field og 3D printing of materials or in the field of liquid waste treatment and depolution are potential options.
Development of large area substrates for power electronics
Improving the performance of power electronics components is a major challenge for reducing our energy consumption. Diamond appears as the ultimate candidate for power electronics. However, the small dimensions and the price of the substrates are obstacles to the use of this material. The main objective of the work is to overcome these two difficulties by slicing the samples into thin layers by SmartCut™ and by tiling these thin layers to obtain substrates compatible with microelectronics.
For this, various experiments will be carried out in a clean room. Firstly, the SmartCut™ process must be made more reliable. Characterizations such as optical microscopy, AFM, SEM, Raman, XPS, electrical, etc. will be carried out in order to better understand the mechanisms involved in this process.
The candidate might be required to work on other wide-gap materials studied in the laboratory such as GaN and SiC, which will allow him to have a broader view of substrates for power electronics.
Description of the evolution of grain size and dislocation density during ODS steels consolidation
Steels reinforced with a dispersion of nanometric oxides (generally referred to as ODS steels) are currently considered especially as potential material for combustible cladding for 4th generation reactors. Up to now, these materials are conventionally produced by powder metallurgy. The evolution of the microstructure during processing is not well described, yet. Recent work in the laboratory has focused on the evolution of nano-precipitation during processing. The objective of the post-doctoral work is therefore to refine the description of this evolution, more precisely with regard to the grain size and the density of dislocations. This subject combines an experimental approach, through analyses in electron microscopy and X-ray diffraction, and a numerical approach, aiming to define an optimized method for the treatment of the evolution of dislocations.
Stability of the oxide/metal interface of a coated 6061-T6 aluminium alloy
The aluminium alloy, named 6061-T6, is used as core component for the Jules Horowitz French experimental reactor (RJH). In order to improve the corrosion resistance, and to prevent the alloy from wear degradation, a coating is deposited at the surface of the alloy. The coating layer that is 50 µm thin is obtained by oxidation of the aluminium alloy.
The RJH core component will be subjected to neutron irradiation that may modify the microstructure of both the 6061-T6 alloy and the coating layer. Concerning the 6061-T6 alloy, the irradiation damages are well known: neutron irradiation induces the formation of dislocation loops, and causes the dissolution of the nano-precipitates. However, the effect of irradiation on both the coating layer and the interface metal/oxide remains unknown. One of the deleterious effect that may occur in reason of irradiation could be the peeling of the oxide coating and consequently the loss of the corrosion properties. Thus, the understanding of the irradiation response of the coating layer remain a key issue to guarantee a safe use of the coated aluminium alloy. Therefore, the aim of the study is to characterize the irradiation damage of ion irradiated coated aluminium alloys.