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

Abstract

We have developed a Nd-Fe-B sintered magnet of extremely high performance. The intrinsic coercivity Hcj is as high as 35.2 kOe (2803kA/m) together with the maximum energy product (BH)max of 40.4 MGOe (321.6kJ/m3). These values result in Hcj (kOe) + (BH)max (MGOe) > 75. Between 293 K (20ºC) and 473 K (200ºC), the temperature coefficients of remanence and intrinsic coercivity are αBr = −0.122 %/°C and αHcj = −0.403%/°C, respectively. A maximum operating temperature of 503 K (230ºC) is obtained when permeance coefficient Pc = −B/H = 2. Grain boundary diffusion (GBD) technique on magnet surface has been developed to increase Hcj by 3.6 kOe without significantly decrease of Br and (BH)max. The intrinsic coercivity of the GBD treated magnet Hcj(C) has a linear relationship with that of the untreated magnet Hcj(B) between 200 K and 473 K (in unit of kOe): Hcj(C) = 1.03Hcj(B) + 2.38. The enhancement of Hcj by GBD treatment has contributions not only from the improvement of microstructure but also from the increase of Ha in the grain surface layer. It is also found that GBD treatment brings no deterioration in corrosion resistance of untreated magnet.