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 Dy
₂
O
₃
and Tb
₄
O
₇
leads to an increase in intrinsic coercivity. For addition of Dy
₂
O
₃
, the optimum conditions for powder mixing and the optimum Dy
₂
O
₃
particle size were determined. A mixing time of more than 10 minutes, and a Dy
₂
O
₃
particle size of less than 3 μm, are required to obtain a high intrinsic coercivity. EPMA measurements of NdFeBAl magnets with Dy
₂
O
₃
added reveal an inhomogeneous distribution of Dy in the Nd
₂
Fe
₁₄
B 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 Dy
₂
O
₃
in the Nd-rich phases, followed by diffusion of the resulting Dy atoms into the matrix.