Coercivity enhancement in sintered Nd-Fe-B magnets by large mass rotating diffusion with Tb strips

Abstract

Rotating diffusion technology is especially suitable for mass production of small-sized magnets. In this paper, a large mass rotating diffusion with 2500 pieces of small-sized magnets was conducted to enhance coercivity. At the same time, the uniformity of the properties was studied. Taking both magnetic properties and yield rate into account, we obtained the optimal parameters of the rotating diffusion at 710 ℃ for 5 h, following a two-stage annealing process at 940 ℃ for 5 h and at 500 ℃ for 3 h. Thus, the coercivity of the magnet could increase from 14.05 kOe to 23.06 kOe. EPMA results show that most Tb enriches the surface of the magnets, and some Tb is diffused into the grain boundary phase forming magnetic isolation during the rotating diffusion process. After the two-stage annealing process, many (Nd,Tb)2Fe14B substitutional phases with higher anisotropy field are formed around the epitaxial layer of the main phase increasing the coercivity of the magnet. Discrete analysis and a linear fit show excellent uniformity of the properties in this massive rotating diffusion process.