Application of dry-coated Pr-LRE-Tb-Al-Cu alloys to improve magnetic properties in Nd-Fe-B sintered magnets using grain boundary diffusion process

Abstract

The Nd-Fe-B sintered magnets are widely used in hard disk drives, magnetic sensors, wind-power generators, high-efficiency air-conditioner compressors and electric-vehicle motors. The Nd-Fe-B sintered magnets exhibit a high maximum energy product at room temperature. However, while applied at elevated temperatures, coercivity of magnets decreases causing performance deterioration. To achieve appropriate values of coercivity and performance at high temperatures, grain boundary diffusion process (GBDP) with heavy rare earth elements (HRE) is widely used. The limited supply and price surge of the HRE elements caused by the recent trade war inhibit the wide application of HRE. Accordingly, enhancing the magnetic properties of Nd-Fe-B magnets while reducing HRE usage remains a central challenge. Addressing this requires both improving coating methods to enhance diffusion efficiency and optimizing diffusion source compositions to reduce HRE content. In this study, we investigated the variation in magnetic properties according to the coating method for the optimization of GBDP. To reduce HRE consumption, diffusion sources incorporating light rare-earths (LRE) and low-melting metals were employed. These additions significantly reduced the amount of Tb required to achieve a given coercivity enhancement. Dry-coating has a significant impact on simplifying the process, reducing the use of diffusion sources and enhancing magnetic properties. The magnetic properties and microstructure of Pr-LRE-Tb-Al-Cu (LRE= La, Nd and Ce) diffiision magnets with various compositions are discussed.