Improvement of DyF3 diffusion efficiency in sintered Nd-Fe-B magnets by stack diffusion technology

Abstract The diffusion efficiency of DyF 3 is significantly enhanced by using a novel stack procedure. In the stack procedure, the magnets place one on top of the other to ensure the contact surface does not expose outside directly. During the diffusion process, DyF 3 firstly convertes to NdF 3 , then NdF 3 reacts with trace oxygen in the cavity to form NdOF that makes more F atoms diffuse into magnet, which is detrimental to the enhancement of coercivity. But through the stack diffusion technology, the tight contact between the upper and bottom surface of magnets suppresses the oxidation of NdF 3 as much as possible to prevent the formation of NdOF. Thus, less F diffusion into magnets, better Dy distribution and higher coercivity increment are obtained. After 10 h diffusion, the coercivity of magnet under stack diffusion increases from 15.45 kOe to 17.47 kOe with 0.28 wt% Dy consumption and the increment is almost twice that of magnet treated in traditional diffusion.