Synthesis and optical properties of quantum dots
Graphene as a constituent of graphite was close to us for almost 500 years. However, it is only in 2005 that A. Geim and K. Novoselov (Nobel Prize in 2010) reported for the first time the obtaining of a nanostructure composed by a single layer of carbon atom. The exceptional electronic properties of graphene make it a very promising material for applications in electronic and renewable energies.
For many applications, one should be able to modify and control precisely the electronic properties of graphene. In this context, we propose to synthesize chemically graphene nanoparticles and study their absorption and photoluminescence properties. We will focus on families of elongated nanoparticles, with the aim of studying how size can enable us to observe and control multiexcitonic processes in these materials. This project will be developed in collaboration with Physicists so the candidate will work in a multidisciplinary environment.
Simulation of nuclear glass gels at the mesoscopic scale using a quaternary system.
This research work is part of studies conducted on the long-term behavior of nuclear glass used to immobilize radioactive waste and potentially intended for geological disposal. The challenge lies in understanding the mechanisms of alteration and gel formation (a passivating layer that can slow down the rate of glass alteration) by water and in predicting the kinetics of radionuclide release over the long term.
The proposed simulation approach aims to predict, at a mesoscopic scale, the maturation process of the gel formed during the alteration of glass by water using a ternary “phase field model” composed of silicon, boron, and water (leachate), to which aluminum will be added.
The underlying quaternary mathematical model will consists of a set of coupled nonlinear partial differential equations. These are based on Allen-Cahn and transport equations. The numerical solution of the associated equations is performed using the Lattice Boltzmann Method (LBM) programmed in C++ in the massively parallel LBM_saclay calculation code, which runs on several HPC architectures, both multi-CPUs and multi-GPUs.
The proposed research requires a solid foundation in applied mathematics and programming in order to develop the algorithms necessary for the correct resolution of the new system of strongly coupled equations.
Photo- and thermocatalytic cross-coupling of esters for the synthesis of biosourced alkenes
The easy access to energy and carbon-based raw materials offered by the fossil feedstock allowed a rapid growth of our society. Nevertheless, the expected depletion of fossil resources and climate change, require changing for a more sustainable model. Bio-based feedstock is a promising source of carbon to substitute petrochemicals but require a drastic change of the actual model. While the current paradigm relies on the production of energy and high-value molecules through oxidation steps, a model based on Carbon Circular Economy, i.e. the transformation of CO2 and biomass feedstock that are already highly oxidized materials demands the development of new methodologies for reduction, deoxygenation, and the direct use of oxygenated bonds to access functionalized and useful organic molecules.
In organic chemistry, cross-coupling reactions represent one of the major tools to create C–C bonds. However, they are still based mainly on the use of organic halides as electrophiles. In this project, the PhD candidate will demonstrate that readily available and abundant alkyl esters can serve as electrophilic coupling partners in catalyzed cross-coupling reactions with alkenes. Esters can indeed be directly biosourced or easily synthesized from alkyl carboxylic acids and alcohols, thereby diminishing the environmental impact of the carbon-carbon bond formation.
Exploration of VACNTs in Anode-less Batteries: Mechanism and Cell Optimization
Anode-less or anode-free batteries are getting increasing attention owing to their excellent energy density, cost efficiency, and ease of process upscaling. Exploring anode-less battery will offer a breakthrough in energy storage devices by using the lithium reserve already present in the NMC cathode to reversibly cycle after an initial formation process, which will reduce the overall thickness, processing steps, and cost of materials, and provide excellent energy density. Vertically aligned CNTs (VACNTs) on metal substrates can be an interesting choice for this application due to their low thickness, reproducible synthesis process, and uniform surface properties, which have already proven their applicability in supercapacitors. In this PhD project, we will investigate their newer avenue of applications- anode-less batteries, where VACNTs act as the lithium or sodium plating substrate. We will study the electrochemistry of VACNT in lithium anode-less batteries (in liquid and solid electrolytes) and in sodium anode-less batteries in a liquid electrolyte. The PhD student will work on the synthesis optimizations of VACNT to tune the thickness and density to match their electrochemistry. Post-cycling studies (Raman and SEM) will be carried out to study the effect of cycling and the electrolytes on the VACNT layers. The project aims to explore the possibility of the application of VACNTs in various energy storage systems, which can open up new application possibilities and valorization
Study and Modelling of Tritium Speciation from the Outgassing of Tritiated Waste
Tritium, the radioactive isotope of hydrogen, is used as fuel for nuclear fusion, particularly in the ITER research reactor currently under construction in Cadarache (France). Its small size allows it to easily diffuse into materials, which will lead to the production of waste containing tritium after the operational phase of ITER.
To optimize the management of this tritiated waste, the CEA is developing technological solutions aimed at extracting and recycling tritium, as well as limiting its migration to the environment. The effectiveness of these solutions largely depends on the chemical form in which tritium is released. Experience from the outgassing of tritium from various types of waste indicates that it is released in two main chemical forms: tritiated hydrogen (HT) and tritiated water vapor (HTO), in varying proportions.
However, the mechanisms determining the distribution of tritium between these two species are not well understood. Several factors, such as oxygen and water concentrations, the nature and surface state of the waste, and the concentration of tritium, can influence this speciation.
The objectives of this thesis are as follows:
- To identify the phenomena affecting the speciation of tritium during the outgassing of tritiated waste.
- To conduct an experimental study to verify the proposed hypotheses.
- To develop a numerical model to predict the proportions of HT and HTO released, in order to optimize the management of this waste.
The thesis will be conducted within the IRESNE Institute (Institute for Research on Nuclear Systems for Low Carbon Energy Production) at the CEA site in Cadarache, in a laboratory specialised in tritium studies. The PhD candidate will work in a stimulating scientific environment and will have the opportunity to showcase their research work. The candidate must hold an engineering degree or a master’s degree in Chemical Engineering, Process Engineering, or Chemistry.
New generation of organic susbtrates for power conversion
Recent advances in electric motors and associated power electronics have led to a significant increase in power density requirements. This increase in power density means smaller heat exchange surfaces, which amplifies the challenges associated with dissipating the heat generated by power electronics components during operation. In fact, the lack of adequate heat dissipation causes electronic components to overheat, impacting their performance, durability, and reliability. Other issues related to cost, repairability, and thermomechanical constraints call into question traditional ceramic-based insulating thermal interfaces. It is therefore imperative to develop a new generation of heat-dissipating materials that take the system environment into account.
The objective of this thesis is to replace the ceramic substrate in power module systems, whose main role is to act as the system's dielectric layer, with a thermally conductive organic matrix composite. The current substrate has well-known limitations (fragility, poor interface, cycling limit, cost). The organic substrate must have the highest possible thermal conductivity (>3 W/m.k) in order to dissipate the heat emitted properly, while also being electrically insulating with a breakdown voltage of approximately 3kV/mm. It must also have a coefficient of thermal expansion (CTE) compatible with that of copper in order to eliminate delamination phenomena during the cycling undergone by the device during its lifetime. The innovation of the doctoral student's work will lie in the use of highly thermally conductive (nano)fillers that will be electrically insulated (insulating coating) and can be oriented in a polymer resin under external stimulus.
The development of the electrical insulating shell on the thermally conductive core will be carried out using the sol-gel method. The synthesis will be controlled and optimized in order to correlate the homogeneity and thickness of the coating with the dielectric and thermal performance of the (nano)composite. The charge/matrix interface (a potential source of phonon diffraction) will also be studied. A second part will focus on grafting magnetic nanoparticles (MNPs) onto thermally conductive (nano)fillers. Commercial MNPs will be evaluated (depending on requirements, grades synthesized in the laboratory may also be evaluated). The (nano)composites must have rheology compatible with pressing and/or injection processes.