Investigations of binder infiltration in a powder bed during the Binder Jetting process

Binder jetting (BJ) is an additive manufacturing process (also called 3D printing), that consists in jetting a binder on a powder bed made of metallic or ceramic particles, so that to build a part from a 3D model. Once the binder has cured, the part is extracted from the powder bed and sintered. A major challenge of this process is to predict the state of the part after printing (density, homogeneity, defects). Printing strategy, powder size/shape and binder type all have an impact on the part before densification. The aim of this thesis is to study the interaction between the binder and the powder during the printing process. Ultimately, this should help to optimize the process. The thesis will focus on developing a model for droplet infiltration in a powder bed. To achieve this objective, the proposal is to use innovative numerical methods to model the fluid-structure interaction operating in the powder bed, taking into account capillary forces and dynamic effects (droplet fragmentation, particle displacement). Experimental investigations are also planned, firstly to calibrate the numerical parameters associated with the model, and secondly to validate the model. In fact, a dedicated experimental bench was previously developed, and will be used to characterize the surface state of the powder bed before and after infiltration.

Study and development of thermoelectric devices by additive manufacturing

With decarbonisation, global increase of inflation, rising energy cost, etc., need for energy with low environmental impact have considerably shot up. Among all available technologies, thermoelectric generators (TEG) are solid-state devices converting heat into electricity thanks to Seebeck effect. TEGs present several advantages such as having no moving part, completely silent (unlike internal combustion or stirling engines), low-maintenance, renewable energy source that are simple to install, safe to store, and cost-effective.
For more than 20 years, the laboratory L3M has acquiered a big experience in thermoelectricity (TE), mainly in thin films and bulk technologies. Moreover, for 10 years, L3M has also acquiered a strong experience in additive manufacturing (AM), mainly for metallic materials. The use of AM for TE offers new perspectives, and enables to create new and original geometries (leading to an optimization of yield and/or a better integration), with less materials losses, a significant decrease of the integration and interface challenges, a faster manufacturing time, a lower cost and the possibility to manufacture TE devices very quickly compared to other technologies. The main barrier consists in obtaining materials with as good quality (in terms of density and microstructure) as with other technologies, which will be possible thanks to a deep development and understanding of the process.
L3M has started this new technology for 3 years. Researches are focused on TE materials based on silicon-germanium alloys, which are very good materials for high temperatures applications (500K to 700K) as for spatial, metalworking industry, etc.
The objective of this PhD study will be, from one side, to continue current studies about optimization of SiGe manufacturing process by AM (and more specifically by Laser Powder Bed Fusion (L-PBF) technology), and from the other side, to manufacture the first TEG demonstrators. For the first part, the study will have to lead to the understanding of the specifities of AM mechanisms on SiGe structural properties. This structural study will include measurement of mechanical properties, as well as microscopic analysis. This study will be also correlated to experimental measurements of manufactured materials TE properties (Seebeck coefficient, electrical and thermal properties).
For the second part, TE generator manufacturing needs to associate two TE materials (n- and p-type SiGe) and assemble together, by optimizing electrical contacts between these two materials. CEA-Liten has deposited a patent about the original manufacturing of such device by AM. The realisation and electrical characterization of a TE generator will be also developed in the framework of this study, leading to highlight advantages of this manufacturing technique.
It should be noted that this work will be performed in the framework of a European project launch.

4D printing of thermo-magnetic composite materials using light-driven additive manufacturing techniques

This PhD research project explores the cutting-edge field of 4D printing, a field that integrates smart materials into additivemanufacturing processes. The aim is to create nanocomposite objects with multifunctional capabilities, enabling them to change shapeand properties in response to external stimuli.

In this PhD project, we will primarily focus on liquid crystal elastomers (LCEs) as the active matrix. LCEs are a versatile class ofprogrammable polymer materials that can undergo reversible deformation under various stimuli, such as light, heat, electric fields, andmagnetic fields, transitioning from disordered to oriented phases. Because of their actuation properties, LCEs are promising candidatesin applications like artificial muscles in medicine and soft robotics.

Consequently, the project's first objective is to devise a method for 3D printing LCE resins using light-driven printing processes, includingdigital light processing (DLP), direct ink writing (DIW), and two-photon polymerization. The project also explores the possibility of co-printing using two laser sources with different wavelengths. This will result in designed objects capable of programmed deformationsand reversibility. To further enhance the actuation capabilities of the LCE matrices, magnetic particles will be incorporated into thethermoresponsive LCE resin. Thus, the second objective of the project is to develop a strategy for self-assembling and spatiallyorienting embedded magnetic nanoparticles in LCE resins during light-driven printing processes (DLP, DIW, 2PP). Ultimately, the thirdobjective of this project is to combine these two strategies to create sophisticated multifunctional soft machines and devices suitable forcomplex environments. Experiments will follow an incremental trial-and-error research approach, with the aim of improving machinelearning models by designing purpose-built objects.

The envisioned research work can be summarized into the following macro-steps:
- Specification of target shape-changes depending on the multiple stimulation scenarios
- Selection of active particles, formulation of the LCE, and synthesis of the particles
- Development of hybrid additive manufacturing strategies with possible instrumentation
- Printing proofs-of concept and conducting mechanical and actuation tests
- Characterization of composite structures
- Development of simulation models
- Realization of a demonstrator (e.g., crawling robot, actuators for the automotive sector…)

Microstructural characterization by bulk laser-ultrasounds tomography

The proposed thesis falls into the framework of designing innovative methods in the materials characterization. The thesis aims to develop a new tomographic technique for characterizing microstructures using bulk laser ultrasound. In the state of the art, acoustic methods such as the scanning acoustic microscope and surface wave optoacoustic spectroscopy lead to grain imaging but only at the surface of the sample. However, industrial manufacturing processes (in metallurgy, welding, additive manufacturing...) can reveal spatial inhomogeneity of the microstructure, such as grain size gradients with depth within the component. Electron backscatter diffraction (EBSD) also provides surface imaging of grains but has disadvantages, including restriction on sample size and the need to make transversal cuts through the sample to image its volume.
The proposed idea is to develop a bulk laser ultrasound tomography technique able to determine the grain size in areas of a component or even to obtain imaging of large-grain microstructures and a local determination of crystallographic orientation. Therefore, the thesis's main objective will be to design such an experimental characterization tool and optimize its design using a digital twin to develop.

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.

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.

Translated with (free version)

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)