Tuning of Microstructure and Magnetic Properties of Nanocrystalline NdFeB Permanent Magnets Prepared by Spark Plasma Sintering

Structure and magnetic properties were studied for bulk nanocrystalline isotropic and anisotropic Nd-Fe-B permanent magnets prepared by spark plasma sintering (SPS). The neck formation among the Nd-Fe-B powders, which was governed by sintering conditions in the SPS process, has a substantial influence on the microstructure and subsequent magnetic properties of the magnets. By tuning the current pulse ratio of power-on to power-off time during sintering from 12:2 to 2:2, we successfully reduced the thickness of the surface diffusion layer of the Nd-Fe-B particles in both hotpressed and hot-deformed magnets. As a result, the maximum energy product (BH) max of the isotropic and anisotropic magnets increases by 27% and 9%, respectively.