Enhancement of Coercivity of Nd-Fe-B Sintered Magnets by Grain Size Reduction

Abstract

For the purpose of increasing the coercivity of the Nd Fe B sintered magnets without Dy substitution, we explored a possibility of enhancing the coercivity by grain size reduction of the sintered body by employing the starting powder with an ultimately small particle size. To produce Nd Fe B alloy powder with such a small particle size, and to handle it to produce sintered bodies without oxidation, we developed a special jet mill and a special process called pressless process (PLP). By utilizing these techniques, it has become feasible to produce Nd Fe B sintered magnets with a coercivity of 1.59 MA/m without Dy substitution which is compared with 0.87 MA/m for the commercially available Nd Fe B sintered magnets containing no Dy.

Computer simulation of coercivity improvement due to microstructural refinement

“…It has also been reported that coercivity increases with decreasing grain size. 8,9 These observations and calculations suggest that the microstructure plays an important role in the determination of coercivity.…”

Simultaneous magnetic and chemical imaging of Nd-Fe-B thin films by means of XMCD-PEEM technique

“…Moreover, the availability and price of the heavy RE elements are currently serious resource problems. Recently, the coercivity μ0Hc of Nd-Fe-B sintered magnets has been successfully enhanced to about 2 T even without addition of heavy RE elements by various methods such as refinement of the constituent crystal grains of the Nd2Fe14B phase 1) and/or grain boundary diffusion of RE elements to restore magnetic anisotropy near the grain boundaries or to suppress intergranular exchange coupling 2), 3) .…”

Study of sintered Nd-Fe-B magnet with high performance of Hcj (kOe) + (BH)max (MGOe) > 75

“…Sagawa et al 4,5,27 applied jet mill with Helium gas as medium to bring the average particle size of Nd-Fe-B powder down to ∼1 μm. To prevent such fine particles from oxidation, they invented a so-called 'press-less pressing (PLP)' technique to compact fine powder into block in air-tight and oxygen free container.…”