Effects of Rare Earth Oxide Addition on NdFeB Magnets

The effects of addition of rare-earth oxides on the magnetic properties of Nd-Fe-B sintered magnets are studied. The addition of Dy2O3 and Tb4O7 leads to an increase in intrinsic coercivity. For addition of Dy2O3, the optimum conditions for powder mixing and the optimum Dy2O3 particle size were determined. A mixing time of more than 10 minutes, and a Dy2O3 particle size of less than 3 ¿m, are required to obtain a high intrinsic coercivity. EPMA measurements of NdFeBAl magnets with Dy2O3 added reveal an inhomogeneous distribution of Dy in the Nd2Fe14B matrix: the material is Dy-rich near grain boundaries, but Dy-poor within the matrix. The appearance of such an inhomogeneous distribution of Dy is attributed to the reduction of Dy2O3 in the Nd-rich phases, followed by diffusion of the resulting Dy atoms into the matrix.