Sol-gel agglomeration of metallic NdFeB alloy nanoparticles in the microsphere shape as a chemical stabilization strategy

Full text: This work deals with the synthesis and characterization of rare earth permanent magnet raw materials. It aims to demonstrate the technical feasibility of improving the chemical stability of metallic NdFeB nanoparticles via its controlled agglomeration in microspheres. Powders and microspheres were obtained by an adapted sol-gel process which allowed to obtain nanostructured metal oxide precursor particles in both powder and microsphere shapes. After a thermal reduction at 1200 °C/H₂ /2 h, powders (mean diameter <50 μm) and monodispersed microspheres (mean diameter 171±13 μm) with high sphericity showed different chemical stability behavior. The powders promptly oxidized. The resulting metallic microspheres exhibited low surface oxidation, due to a passivating oxide nanolayer that could be visualized in the elementary map obtained via energy dispersion X-ray spectroscopy. This surface oxidation was further confirmed by X-ray diffractometry from the identified crystalline phases: NdFe₄B₄, α-Fe, FeNd₂, FeO, B₂O₃, Fe₂Fe(BO₃)O₂, FeBO₃, Nd₂O₃, and Fe₄C. The different phases observed suggest a disproportionation of the metal alloy, possibly caused by the high temperature (1200 °C) used during thermal reduction. Mössbauer analysis confirmed the presence of NdFeB alloy (about 33%) and α-Fe phases. Therefore, it is noted that the sol-gel process shows a great potential to make, based on raw materials, rare earth monodispersed metallic microspherical permanent magnets with high sphericity and good chemical stability at atmospheric environment. (author)