Recent Research Trend in Powder Process Technology for High-Performance Rare-Earth Permanent Magnets

Abstract

Higher performance is constantly required in rare earth permanent magnets, which are an indispensable component of the motors of electric vehicles. When producing sintered magnets, advanced structural control is necessary in the powder metallurgy process in order to achieve high performance. Especially in recent years, it has become important to develop processes for Sm-Fe-N magnets and metastable phase magnets as next-generation magnets to replace the Nd-Fe-B magnets. Because the crystal grain refinement of sintered magnets is most effective for improving coercivity, production methods for raw powders have evolved from the traditional pulverization to chemical synthesis approaches, and as a result, a submicron-sized Sm-Fe-N powder with huge coercivity has been developed. State-ofthe-art physical synthesis methods have also been applied successfully to the synthesis of nanopowders. Since control of the grain boundary is very effective in Nd-Fe-B magnets, this approach has also been evolved to Sm-Fe-N magnets by nano coating. On the other hand, since technologies for crystalline orientation control and high-density sintering are indispensable for improvement of remanence, new low-thermal load consolidation techniques such as spark plasma sintering are being developed for Sm-Fe-N magnets and metastable phase magnets in order to overcome the inherent low thermal stability of these materials.

Strategies for the Synthesis of Nanostructured SmCo5 Magnetic Particles for Permanent Magnetic Application

“…It reaches a maximum at about 400 nm and then decreases in a range smaller than 300 nm. This decrease in coercivity in finer particles, particularly nanoparticles, can be attributed to the more pronounced effect of particle surface oxidation as the particle size decreases, leading to an increase in the specific surface area . As indicated in Table , the zero-dimensional SmCo 5 particles that we synthesized by combining electrospinning with the reduction-diffusion process exhibit a coercivity of 34.5 kOe.…”

“…This decrease in coercivity in finer particles, particularly nanoparticles, can be attributed to the more pronounced effect of particle surface oxidation as the particle size decreases, leading to an increase in the specific surface area. 22 As indicated in Table 1, the zero-dimensional SmCo 5 particles that we synthesized by combining electrospinning with the reduction-diffusion process exhibit a coercivity of 34.5 kOe. This value is comparable to the performance of SmCo 5 particles with a similar particle size reported in the literature.…”

“…In contrast, the chemical method considered a bottom-up approach is recognized as a more reliable means of synthesizing SmCo 5 particles with a single-domain structure . The most successful chemical synthesis method for SmCo 5 fine powder is the reduction-diffusion method, which consists of three sequential steps, namely, the preparation of oxide precursors, the reduction-diffusion of precursors, and the subsequent removal of excessive reduction agents and byproducts . The dispersant CaO was introduced into the oxide precursors to hinder the agglomeration and sintering, resulting in the formation of SmCo 5 particles with single-domain structure, ranging in size from 50 to 400 nm. , However, most previously reported SmCo 5 particles display an equiaxial morphology.…”

The Structure and Magnetic Properties of Sm2Fe17Cx Compounds Prepared from Ball-Milled Mixtures of Sm2Fe17 and Carbon Nanotubes or Graphite

“…As a result, the discovered Sm-Fe-N system promises great potential for hard magnetic applications provided with its higher Curie temperature and corrosion resistance than those of Nd-Fe-B [13][14][15]. Recent research has been dedicated to improvement of Sm-Fe-N powder properties and techniques of obtaining bonded magnets [16][17][18][19]. Currently, there is a gap between high-performance rare-earth magnets, which are expensive, and low-cost magnets without rare-earth elements, which have a low energy product.…”