Design of 4D printable and biocompatible polysaccharide hydrogels for biomedical applications.
The 3D printing of stimuli-responsive materials is called “4D printing” and is of great interest for the development of innovative medical devices (dynamic synthetic tissues, soft robotic actuators, controlled drug release systems etc.). Reported examples of these printable smart materials are programmed to autonomously change their shape in response to specific stimuli (e.g. temperature, light, magnetic field, pH, etc.) but are almost exclusively based on synthetic polymers.
To transpose this concept to biomedical application, this PhD project aims at designing 3D printable, biocompatible and stimuli-responsive polysaccharide hydrogels. In particular, the targeted hydrogels will be able to deform under two different stimuli: (i) a temperature variation or (ii) the application of a near-infrared (NIR) beam for the material activation without deterioration of biological tissues. These will be achieved by combining (i) polysaccharide chains functionalized with thermoresponsive groups and (ii) photothermal nanoparticles capable of converting NIR light into heat.
This interdisciplinary project is at the interface between Chemistry (polymer chemistry, nanoparticle synthesis), Physical Chemistry (formulation and characterization of hydrogels), Materials Science (3D printing studies, mechanical tests) and Biology (cytocompatibility studies). An additional originality is that the experimental data collected by the PhD candidate will be fed into artificial intelligence tools which, in turn, should provide guidelines to accelerate the discovery of the targeted materials.
Nanocrystalline Soft Magnetic Composites: Powder morphology and design for controlling their magnetic properties for high frequency applications
Context: Achieving carbon neutrality by 2050 will require massive electrification of the power production systems. Power electronics (PE) is a key-enabler that will this transformation possible (renewables, integration of energy micro-grids, development of electric mobility)
Problem: Current developments in PE converters aim at increasing the switching frequencies of large bandgap switches (SiC or GaN). At low frequencies, magnetic components remain bulky, occupying up to 40% of the total footprint. At high frequencies (HF > 100 kHz), very significant gains are expected, but only if the losses generated by these components remain under control. Today, the main class of magnetic materials applied to HF is MnZn or NiZn ferrites, due to their low cost and convenient electrical resistivity (?elec > 1 O.m). The main drawbacks of these materials are their low saturation induction (Bsat < 0.4 T), which limits their size reduction, and their mechanical fragility. Nanocrystallines materials have better Bsat (1.3 T), but their ?elec is about 1.5 µO.m (6 times less resistive than ferrites), which generates significant induced current losses at HF.
Thesis objective: To develop magnetic composites by grinding nanocrystalline ribbons, electrically insulating the powders (coating fabricated by sol-gel), compacting of the powder at high pressure (1000-2000 MPa) for the core shaping and finally by applying an annealing treatment to relax the thermal constraints.
Study of NMC electrode materials for lithium-ion batteries by experimental and theoretical soft and hard X-ray photoemission spectroscopy
The photoemission spectroscopy (X-ray, XPS, or ultraviolet, UPS) is one of the direct probes of the electronic structure of materials change during redox processes involved in lithium ions-batteries at the atomic scale. However, it is limited by the extreme surface sensitivity, with a typical photoelectron path length of a few nanometers to the energies usually available in the laboratory , . Moreover, the spectra interpretation requires the ability to accurately model the electronic structure, which is particularly delicate in the case of transition metal based electrode materials. Upon lithium insertion and de-insertion, the charge transfer toward cations and anions induces local electronic structure changes requiring an adapted model that takes in account the electronic correlations between atoms.
In this thesis, we propose to use these limitations to our advantage to explore the electronic surface structure including the solid electrolyte interphase (SEI), and the bulk of the active cathode particle.
Thanks to the lab-based hard X-ray photoemission spectrometer (HAXPES), the electronic structure of the bulk of the electrodes (LiCoO2 and LiNiO2) materials have been studied up to about 30 nanometers , . To widen our picture on the role of cation and anion from surface to bulk in the lamellar metal oxide electrode for lithium-ion battery, this thesis will focus on mixed lamellar metal oxide Li(Ni1-x-yMnxCoy)O2 (NMC).
The comparison between the Soft-XPS and HAXPES spectra, during battery operation (operando) and post-mortem, will allow decoupling of the surface and core spectra for different NMC compositions and at different stages of the battery life cycle. The interpretation of the photoemission spectra will be done by direct comparison with ab-initio calculations combining density functional theory (DFT) with dynamical mean field theory (DMFT) , . This coupled approach will allow to go beyond the usual techniques based on cluster models, which do not take into account long-range screening, and to validate the quality of theoretical predictions on the effects of electronic correlations (effective mass, potential transfer of spectral weight to Hubbard bands) .
The thesis will include an instrumental (in particular, calibration of Scofield factor on model systems) and theoretical (prediction of core photoemission spectra based on DFT+DMFT calculations) development. The performance of electrochemical systems based on different cathode materials (NMC with different compositions) in combination with liquid and solid electrolytes and a Li metal anode will be studied in the frame of combined experimental and theoretical soft and hard X-ray photoemission spectroscopy.
The candidate will be hosted at the PFNC in the Laboratory of Characterization for the Energy of CEA Grenoble under the direction of Dr. Anass BENAYAD (department of Material) and LMP (Department of Electricity and Hydrogen for Transport) under the supervision of Dr. Ambroise Van Roekeghem.
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Development of catalysts for CO2 hydrogenation to light olefins
Light olefins, mainly ethylene and propylene, are amongst the organic compounds with the largest production volume. They are currently produced from fossil resources. The reduction of the carbon footprint of products synthesized from these intermediates necessitates the use of alternative feedstock, such as atmospheric CO2.
The objective of this phD is the development of catalyst for the direct hydrogenation of CO2 into light olefins. Fe based catalyst combining reverse water gas shift (RWGS) and Fischer-Tropsch polymerization (FT) capabilities will be developed. In order to have a better understanding of iron forms involved in the reaction, Fe nanoparticles of controlled composition and dsizes will be prepared and dispersed on different support (silica, alumina, carbon,…). The catalytic properties will then be evaluated on a dynamic reactor and finely characterized using numerous techniques (XRD, XPS, HRTEM, …).
Melt grafting of polyolefin applied to reparable and recyclable photovoltaic panels
Solar panels are multi-materials assemblies constituted of photovoltaic cells that contains numerous precious metals (metal silicon, silver), high quality and therefore costly-to-manufacture glass that protects the cells, and a polymer film acting as binder, called encapsulant. These encapsulants are mostly thermoplastics that are reticulated during the manufacture of photovoltaic panels, which makes their dismantling and recycling difficult today.
CEA develops new materials to bring recyclability to renewable energy production systems, such as photovoltaic panels. The thesis revolves around the development of new encapsulants that allow improved recyclability of photovoltaic panels through a reversible reticulation system. In a first step, the melt grafting (extrusion, internal mixer) of polyolefins with molecules of interest will be studied in terms of grafting efficiency and kinetics, and impact on polyolefins properties such as thermal, optical, and structural properties. In a second step, a reversible reticulation will be triggered using the firstly grafted molecules. The impacts of this reticulation on the material thermal, mechanical, optical properties will be characterized. The application of the material as encapsulants will be the final aim of the thesis, and small demonstrators of photovoltaic modules using the material will be performed.
3D printing of high performance SiC parts for hydrofluoric acid electrolysis
Fluorine plays an essential role in the nuclear fuel cycle: it is an indispensable component in the preparation of UF6 used in the uranium enrichment process in nuclear power plants. Fluorine is produced by electrolysis of the molten salt KF-2 HF on the non-graphite carbon anode between 85°C and 100°C. The reduction reaction that takes place at the cathode produces hydrogen. An electrolysis cell consists of covers, cooling coils, diaphragm made of nickel alloy (67%) and copper (28 to 30%). This alloy has a remarkable resistance to corrosion. Increasing effectiveness and reliability of the electrolyzer need to change paradigms such as materials and manufacturing’s processes.
It is therefore envisaged to replace this material by a high performance ceramic, silicon carbide, in order to develop new diaphragms with more complex geometries to improve gas separation.
The objective of the thesis will be to study the performance of a SiC-based material, printed by additive manufacturing and sintered in order to obtain parts with high densities (70-90%) and low oxygen content to be compatible with HF electrolysis.
An in-depth analysis will be undertaken by IGA/ICP, SEM-MET/EDX on SiC materials developed and shaped by flash sintering (screening) in order to relate the nature of SiC, the density and the location of oxygen. A second step will focus on the shaping by 3D printing of the selected material followed by thermal sintering treatments with the technological challenge of obtaining high density parts. The performances of these simple and complex parts will be evaluated in HF environment and under fluorine bubbling. These implementations will be followed by characterizations in order to establish relationships between the properties of the material obtained by 3D printing (its microstructure, its density, the presence of oxygen) and its performances.
New sustainable electrode materials for High Temperature Electrolysis
High temperature electrolysis is considered as the high efficiency technology for hydrogen production with low carbon emissions. The electrolysis reaction occurs in a solid oxide cell (SOC) composed of a dense electrolyte of yttria stabilized zirconia (YSZ), sandwiched between two porous electrodes. The most common hydrogen electrode material is a cermet of Ni and YSZ, and the oxygen electrode is a perovskite La0.6Sr0.4Co0.2Fe0.8O3 (LSCF).
To make the high temperature electrolysis more sustainable to better support the European eco-system towards the achievement of the Sustainable Development Goals and the objectives of the Paris Agreement, there is a critical need to reduce reliance on critical raw materials (CRM).
The objective of the thesis is therefore to limit the use of CRM in the oxygen electrode material. Critical elements such as cobalt will be substituted by new cations on the A and/or B site of the crystal lattice, while maintaining equivalent performance and long-term stability. At the same time, in order to limit losses during synthesis, a part of the work will be carried out on the synthesis process efficiency and on the increase in capacity of the synthesis method.
After a bibliographic study on oxygen electrode materials for high temperature electrolysers, the proposed work will initially be focused on the synthesis by chemical routes as well as on fine characterization of the perovskites. The thermal and chemical compatibility with the other materials constituting the cell will be studied, then this work will lead to the shaping of the materials with the most interesting properties in order to test them electrically and electrochemically. The electrochemical behaviour of the electrodes will be analysed in order to understand the influence of substitutions and to determine the electrochemical performance of the different compositions studied.