Effects of rare earth oxide addition on NdFeB magnets.

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Abstract
Effects of rare-earth oxides addition on the magnetic properties of the Nd-Fe-B sintered magnets are studied. The addition of Dy2O3 and Tb4O7 increases the intrinsic coercivity. For Dy2O3 addition, the optimum conditions for the powder mixing and Dy2O3 particle size are determined. A mixing time of over 10 minutes and the Dy2O3 particle size under 3μm are required to obtain a high intrinsic coercivity. EPMA measurements on NdFeBAl magnets added with Dy2O3 reveal an inhomogeneous distribution of Dy in the Nd2Fe14B matrix; Dy is rich near the grain boundary but poor inside of the matrix. Formation of such an inhomogeneous distribution of Dy atoms is attributed to reduction of Dy2O3 in the Nd-rich phases followed by diffusion of the Dy atoms into the matrix.