New Development of Permanent Magnets Through Grain Boundary Diffusion and Nanostructuring

Abstract

High-performance permanent magnets (PMs) have gained high and growing interest due to their excessive demand in energy conversion systems and electric vehicles. PM-based electric machines exhibit great advantages over traditional motors due to their high efficiency of energy conversion. Nd–Fe–B magnet is the best available magnet in terms of energy-product at room temperature. Replacement of Nd by heavy rare earth (HRE) and of Fe by Co results in an enhanced anisotropy field and an improved thermal stability, but also increases the production costs. Developing a strong PM with minimum use of HRE elements is required due to their high cost, low availability and issues associated with international politics. Grain boundary diffusion (GBD) process allows the HRE to diffuse around the grain boundaries, unlike adding expensive HRE to the middle of a grain. Here, we review the recent progress in PMs, especially the novel development of grain boundary-diffused magnets and nanostructured magnets. GBD processes using RE, fluorides or hydrides of RE and eutectic alloys are discussed. Development of nanostructured PMs using physical and chemical methods such as melt spinning, high-energy ball milling, surfactant-assisted ball milling, mechanochemical method, etc. is elucidated. The current and future trends in the area of high performance permanent magnets are outlined.