Low eutectic temperature alloy diffusion process for hot-deformed Nd-Fe-B magnet

摘要

Nd-Fe-B magnets have been studied for more than thirty years. One of main research issues of Nd-Fe-B magnet is low thermal stability due to its low Curie temperature (T_c ～ 310°C). To improve the thermal stability, coercivity (H_c) enhancement is a possible way. For example, H_c of 3 T at room temperature (RT) is required for hybrid or pure electric vehicle and wind turbine generator to prevent a thermal demagnetization. Coercivity of permanent magnet relates to the magnetocrystalline anisotropy field, H_a. The H_a of Nd_2Fe_(14)B phase is about 7 T at RT and the value is theoretical maximum limit of coercivity. However, the coercivity of Nd-Fe-B magnet produced by conventional mass production route is only 1 T without Dy. It is accepted that the surface defects on Nd_2Fe_(14)B particle and inclusions causes the degradation of coercivity, because the nucleation of reversed magnetic domain may occur at these sites. Here, (Nd,Dy)_2Fe_(14)B phase possesses a higher magnetocrystalline anisotropy field than Nd_2Fe_(14)B. Therefore, Dy-rich shell structure formed by grain boundary (GB) diffusion process enhances the coercivity effectively with minimum amount of Dy for sintered magnet.