PhD - Optimization of a recycling process for Nd-Fe-B magnets (M/W)

The GPM (Groupe de Physique des Matériaux, UMR CNRS 6634) is structured into 5 departments: Metallurgy, Microstructure Mechanics, Scientific Instrumentation, Functional Materials and Nanostructures, Disordered Systems and Polymers, and Thematic openings and innovations. It brings together 160 staff including 60 Professors, Associate professors and Researchers, 30 technical staff and 70 PhD / PostDoct / Trainees. The laboratory is located on the Sciences and Engineering Rouen Normandie Campus (in Rouen), within the University of Rouen Normandie and INSA Rouen Normandie (7000m2).
The PhD student will work into the Functional Materials and Nanostructures department, within the Functional Materials and Magnetism team, made up of 11 teachers and researchers, 3 PostDoc and 3 PhD students. The study of the relationships between structure (microstructure, nanostructure, local atomic structure) and magnetism of materials constitutes the framework of the research carried out within the team. These activities are devoted to the study of applied magnetic materials.

The proposed work is a part of the RIN LABEX REFAIRE project. It consists in preparing powders of Nd-Fe-B magnets from urban mines, discarded magnets or hard disks. This will be done thanks to the hydrothermal treatment developed at the GPM laboratory. One of the advantages of this technique is that it easily removes the protective metallic coating (usually nickel) from the magnets.
Sintered Nd-Fe-B magnets are the most efficient magnets on the market. They have chemical, physical and magnetic characteristics that it are essential to preserve, both in the powder produced, but also, during the following consolidation steps by sintering, in order to obtain a second life magnet as efficient as possible.
To do this, the synthesis steps are crucial. The Nd2Fe14B phase has to be preserved, with a low oxygen level and a small crystallite size. In addition, the particle size of the powder must be optimized for the sintering step, in terms of average particle size but also particle size distribution. Finally, an attempt will be made to preserve, within each particle, the magnetic orientation which pre-exists in the sintered material. This will facilitate the development of magnetic anisotropy by texturing in the subsequent sintering step.
A systematic study will be carried out on the effects of the various operating parameters which influence both particle size and morphology of the particles during hydrothermal treatment. The sensitivity of this process to the nature of the raw material and to the amount of material treated will be studied and the processing conditions will be optimized in order to lead to a Nd-Fe-B precursor powder as suitable as possible for the following sintering process. Additional mixing-grinding steps may also be implemented in order to optimize the powder, in terms of particle size, chemical composition, Nd2Fe14B crystallite size... Rare earth compounds being very sensitive to oxidation, their chemical degradation always results in the degradation of their magnetic properties. Thus, special attention will be devoted to the quality of the powders obtained and in particular to the oxidation rate of these powders, in order to obtain a sintered magnet with the correct magnetic properties.
The powders obtained by hydrothermal process will be systematically characterized by Mössbauer spectrometry in order to precisely quantify the proportion of the phases containing iron. This will in particular make it possible to control the state of preservation of the Nd2Fe14B magnetic phase.