Improvement of magnetic properties by grain boundary diffusion using dry coating of Tb-(Pr, LRE)-Al_Cu alloys in Nd-Fe-B sintered magnets

Abstract

The sintered Nd-Fe-B magnets have been widely used in the fields of a variety of applications, such as hard disc drives, magnetic sensor, wind power generators, efficient air-conditioner compressors and motors for electric vehicles. Nd-Fe-B sintered magnets exhibit a large maximum magnetic energy product, but in high-temperature environments such as the motors of eco-friendly cars, the coercivity decreases and performance deteriorates. To achieve high coercivity and remanence at high temperatures, heavy rare-earth (HRE) based grain boundary diffusion process (GBDP) is widely used. However, the high price of HRE makes it necessary to reduce the usage of HRE. In this study, we optimized the GBDP parameters by applying a dry coating method based on diffusion sources including light rare-earth (LRE) and low-melting-point metals to reduce the HRE usage. Addition of LRE and low melting point metals resulted in a dramatic reduction in the amount of Tb required in GBDP to increase coercivity. Dry coating has a significant impact on simplifying the process, reducing the amount of diffusion source usage, and improving magnetic properties. The magnetic properties and microstructure of Tb-(Pr, LRE)-Al-Cu diffusion magnets with various compositions are discussed.