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.

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.

Robustness of thick metallizations made on 3D ceramic substrates.

A robust and high quality metallization of 3D ceramic substrates is a key element of the success of this project and a necessity for a future industrial development of the research work that will be carried out during these two theses.
The work in progress on the material platform of the CEA of Toulouse already provides interesting results which allow to consider the first subject proposed here. However, during this work, we could highlight that a joint work between the material and power teams allows to improve the quality of the results by integrating the design for reliability aspect to the material. This is why, this second subject aims to treat in detail the realization of 3D metallized ceramic parts, in order to understand the evolution of the performances of the parts made according to the ceramics used, the metallization techniques, the nature of the metals, the designs, the processes... used.
Also, this thesis work will begin with the realization of flat ceramic structures on which will be carried out tests of metallization by using various techniques such as brazing of tracks, the deposit of layers of adhesion followed by electroplating, ...
These different techniques and interfaces will be subjected to aging and mechanical tests. In addition, morphological characterizations will be performed. The quality of the interfaces can also be evaluated by means of dielectric characterizations (measurement of dielectric rigidity, dielectric losses, I(V)).
Specimens will also be made to verify the mechanical, dielectric and thermal characteristics of the ceramic, which will provide the first thesis topic with material data.
Moreover, during the whole thesis, test vehicles will be realized in order to define the design rules to be used for the dimensioning of the power module.
Finally, 3D metallized ceramic parts will be realized and characterized in order to allow the realization of the power module defined in the first subject of thesis.

Electrothermal optimization of Wide band gap power modules by functionalization of 3D ceramic substrates made by 3D ceramic printing (Al2O3/AlN)

In order to take advantage of Wide band gap components (GaN and SiC), it has been demonstrated that it is necessary to reduce the parasitic elements in the switching cells and therefore in the power modules. The 'trivial' solution is to make the power modules more compact to solve this problem of parasitic elements. However, this is often done at the expense of thermal performance. The subject proposed here has therefore the ambition to not neglect any of these aspects by taking advantage of the new freedoms offered by ceramic 3D printing in terms of design and performance.
Also, this thesis will start with a study of current wide band gap power modules, which will allow the PhD student to complete his knowledge and to understand the limits of these architectures: parasitic elements, parallelizations, signal integrity, thermal management, partial discharges ...
From this first assessment, which is intended to be as exhaustive as possible, we propose to use 3D FEM simulation to find a set of topologies that can be produced by 3D ceramic printing and that will be able to respond to the problems identified.
Based on these results, a new high voltage power module (800V-400A) can then be designed and built.

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Additive manufacturing for nuclear fuel innovation

The context:
Additive manufacturing or otherwise called 3D printing is gradually becoming a method of realization revolutionizing the traditional principles of design. These technologies, which are already growing rapidly in the industrial world, are now being evaluated for the development of innovative nuclear fuels. The development of new reactors, the search for improving the performance of the current nuclear fleet is fertile ground for the emergence of new concepts often impossible to manufacture by the standard technique of powder metallurgy.
The Uranium Fuels Laboratory (LCU) whose mission is the study of fuel manufacturing processes is engaged in an evaluation process of an additive manufacturing technology called robocasting" or micro-castingextrusion oriented towards the realization of CERMET materials based on UO2. During previous work, promising preliminary tests were carried out on non-radioactive materials and a dedicated workshop was set up.

The objectives:
The proposed subject is to continue the study on UO2 material using these new means. A wide field of investigation opens up for the optimization of techniques and the understanding of the physics of the phenomena involved.
The thesis work will focus on the use of experimental research strategy tools (experimental designs) as well as the modeling of the printing process to lead to the optimization of manufactured objects.
These optimization studies will concern both the formulation but also all the parameters of the printing machine. The work will be continued until the characterizations of the objects and the demonstration of their performances.

Potential external collaboration:
The doctoral student will be able to rely on the skills and expertise of different CEA laboratories involved in the project as well as an academic collaborative framework (IRCER Limoges). This collaboration with the IRCER has already been the subject of a previous contract. It should present a marked taste for the experimental approach and some facilities for the use of digital tools. Knowledge of materials science is the minimum required.
As part of the collaboration between IRCER (Limoges) and CEA (Institut IRESNE, Cadarache), travel will be required to benefit from the equipment and know-how specific to the two laboratories.
This work will enable students to develop their skills in the fields of multiphysics modelling and the application of 3D printing technologie

1-Chartier, Pateloup et al, Techniques de l'Ingénieur (2018).
2-Bourret, Pateloup et al, J.Eur. Cer. Soc. 38 (2018)