Coercivity and thermal stability enhancement for spark plasma sintered nanocrystalline Nd-Fe-B magnets with Dy2O3 and Zn additions

The main problems with the sintered NdFeB magnets are their bad formability, low oxidation resistance, and poor temperature stability. Relatively large grain size, typically more than 3 mm, is also not beneficial to the high coercivity. Current efforts are directed to improve the magnetic properties and reduce the cost of NdFeB magnets by compositional modification, microstructure optimization, and new process employment. Spark plasma sintering (SPS) is one of the novel sintering techniques for preparing NdFeB magnets in a laboratory scale. Low sintering temperature and short holding time make it possible to sinter nanocrystalline NdFeB powders into fully dense bulk magnet. However, it is still not possible to completely avoid the grain coarsening, which leads to inevitable decrease of coercivity. To improve coercivity, the magnetic powders can be mixed with some metal powders such as Cu and Zn, which are beneficial for densification during SPS. Another effective way to enhance the coercivity is to add the powdered Dy compounds. In this paper, we report the effects of separated and combined Dy₂O₃ and Zn additions on the magnetic properties, microstructure and thermal stability of SPSed nanocrystalline NdFeB magnets.