Magnetic properties and microstructure of high (BH)max Nd-Fe-B sintered magnet with grain boundary diffusion treatment

Due to excellent magnetic properties of the Nd-Fe-B magnet, endless effort has been paid to the exploration of these high performance permanent magnetic materials since its discovery [1,2]. Tremendous technological applications have been found in the field of motors for hard disk drives, wind mill power generator and hybrid or electric vehicles (HEV or EV). Though experimental maximum energy product [(BH)max] has been close to the theoretical value now, there is still a significant gap between the highest coercivity reported and the calculated value. The addition of heavy rare earth elements (Dy or Tb) has been widely used to enhance the coercivity of the sintered Nd-Fe-B magnet in both laboratories and industrial mass production since the anisotropy field of Dy₂Fe₁₄B or Tb₂Fe₁₄B compounds is much higher than that of Nd₂Fe₁₄B [3]. However, researchers around the world are trying continuously to explore other approaches to increase the coercivity considering the low natural abundance of these additive elements. It should also be noted that the excess heavy rare earth elements will lead to an inevitable loss of the remanence and (BH)max [4]. Grain boundary diffusion (GBD) treatment may increase the coercivity effectively with only limited heavy rare earth elements distributed near the grain boundary [5,6].