Understanding microstructural changes during heat treatment of iron-rich SmCo magnets
The magnetic properties of SmCo magnets (remanence and coercivity) are linked to their microstructure. The final microstructure develops after sintering during homogenization and ageing heat treatments. The optimum temperature and/or duration of these treatments depend on the magnet’s composition. One of the major areas of development for commercial Sm2Co17 magnets is to achieve both high magnetic performance and a reduction in critical materials (notably cobalt). This is achieved by substituting part of the Co with Fe, which also helps to reduce raw material costs. However, the literature shows that when the Fe content exceeds 20% by weight, the coercivity of the magnets is diminished.
The aim of the thesis will be to understand the role and sensitivity of the process parameters that govern the evolution of the microstructure within Fe-rich Sm2Co17 magnets and the resulting properties. These developments will be monitored through various characterization techniques (chemical analyses, magnetic measurements, SEM and TEM observations, etc.) carried out on samples taken at different stages of the process. The aim is to systematically monitor (for the first time for this type of magnet) the structural transformations (chemical segregation, changes in Sm content, presence of defects, oxygen contamination, etc.) that occur from the synthesis of the alloy through to the final magnet. These characterizations should lead to a description of the mechanisms underlying the formation of the expected microstructure. These mechanisms are activated during the various heat treatments, but the influence of the metallurgical and chemical state (for example, defect density and chemical inhomogeneity) inherited from previous stages of the process is still poorly understood and will need to be clarified.
Heat Transfer Enhancement by Convective Boiling in Microchannels applied to the Cooling of Computing Units in Data Centers
The proposed PhD thesis aims to improve the understanding and modeling of convective boiling phenomena in microchannels for new low-environmental-impact refrigerants. The candidate will adopt a combined experimental and multi-scale modeling approach, including the design of a test bench simulating the behavior of a micro-evaporator, the implementation of CFD simulations (ANSYS Fluent, CATHARE) to describe two-phase flow regimes, and the evaluation of various eco-friendly alternative fluids. The expected outcomes include, for each of these new fluids, the characterization of confined boiling mechanisms, the development of a predictive heat transfer model, and the proposal of innovative cooling solutions.
The growing demand for high-performance computing, driven by artificial intelligence and cloud technologies, leads to a significant increase in power dissipation in electronic chips. Current single-phase cooling technologies are reaching their limits when dealing with heat fluxes exceeding 100 W/cm². Two-phase cooling, based on fluid boiling to remove heat, can achieve much higher heat transfer performance than single-phase systems while reducing overall energy consumption. The results of this research will contribute to the development of more efficient and sustainable cooling solutions for future data centers, helping to reduce the digital sector’s energy footprint and strengthen European technological sovereignty in advanced cooling technologies.
Li alloys for all solid-state batteries with sulfide electrolyte
Using lithium metal as a negative electrode would significantly increase the energy density of current batteries. However, today, this material quickly leads to short circuits during charge/discharge cycles, mainly due to the formation of dendrites and the instability of the interface with the electrolyte. All-solid-state batteries, particularly with sulfide electrolytes, are a promising alternative, but the limitations of lithium metal remain. Lithium alloys appear to be a solution for improving mechanical and interfacial properties while maintaining good energy densities.
The objective of the PhD is to develop and select lithium alloys suitable for sulfide electrolytes batteries, then integrate them into all-solid-state cells in order to study degradation mechanisms. The work will be focused on the synthesis of the alloys, their shaping in thin films and their integration into cells. The alloys will be finely characterized and then electrochemically tested in laboratory cells and pouch cells. Finally, degradation phenomena, particularly at interfaces, will be studied using advanced post-mortem characterizations.