Grain refinement in heavy rare earth element-free sintered Nd-Fe-B magnets by addition of a small amount of molybdenum

Abstract

We employed a modified refractory-metal-addition method to achieve higher coercivity and remanence in heavy rare earth element (HREE)-free Nd-Fe-B sintered magnets. This process involved inducing the formation of a homogeneous secondary phase at the grain boundaries during sintering, making it possible to control the intergrain diffusion by adding small amounts of Mo, a refractory metal. To control the microstructure of the secondary phase effectively, a metal organic compound of the refractory metal was coated on the surfaces of the particles of an HREE-free Nd-Fe-B powder. The average grain size after this process was 5.60 mu m, which was approximately 1.8 mu m smaller than that of the HREE-free sintered Nd-Fe-B magnets (7.4 mu m). The coercivity of the magnets prepared through this process could be increased from 11.88 kOe to 13.91 kOe without decreasing their remanence. (c) 2015 AIP Publishing LLC.