ABSTRACT
Discovering the exchange coupling effect promotes the development of nanocrystalline NdFeB magnets. The magnetic properties for nanocrystalline and nanocomposite NdFeB-based alloys have been improved by both compositional modification and microstructural optimization. The effects of substitutions for Nd and Fe and other elemental doping have been investigated regarding to their roles in enhancing room temperature properties, thermal stability, and exchange interaction. To synthesize NdFeB magnets with energy product toward theoretically predicted value, new approaches have been proposed. Both top-down and bottom-up approaches, such as surfactant-assisted ball milling, chemical reduction process, and co-precipitation have been successful very recently. To assemble nanocrystalline NdFeB powders or nanoparticles into bulk magnets, various novel consolidation processes including spark plasma sintering (SPS) and high-velocity compaction (HVC) have been employed to obtain isotropic magnets with fine grain structure. To achieve anisotropy, hot deformation (HD) was selected as the follow-on process for SPS and HVC magnets. As for nanocrystalline bonded magnets, to enhance the thermal stability, melt-spun NdFeB ribbons and hard ferrite powders were mixed for achieving NdFeB/ferrite composite, which has potential to fill the market gap between the bonded NdFeB and ferrite hard magnets.
