Abstract
Dy-diffusion process is one of effective methods to improve the coercivity of NdFeB sintered magnets with a small amount of Dy usage. In order to achieve further improvements of coercivity, we investigated microstructural factors such as grain size and impurity of NdFeB sintered magnets after Dy-diffusion treatment. One of our results indicates that oxygen, as existing impurity in NdFeB magnets, reacts with the diffused Dy to generate Nd-Dy oxide which mainly remains at triple junctions of NdFeB grains. As the oxidization of Dy could not work to improve coercivity, reducing the oxygen content is effective to enhance the Dy-diffusion effect. On the other hand, we tried another technique to protect from oxidization of Dy, in which rare-earth fluorides were doped into NdFeB sintered body. We found that the doped fluoride could trap oxygen such as the formation of NdOF. After that, Dy could smoothly come into grain boundaries because of non-reaction with oxygen. Then, coercivity of magnets showed to be remarkable improvement.
