Synthesis of high-purity Nd₂Fe₁₄B particles via reduction-diffusion process for fabricating fine-grained sintered magnets

Enhancing the coercivity of Nd-Fe-B sintered magnets via microstructural optimization is crucial for withstanding thermal demagnetization at the operating temperature of motors. In this respect, one promising approach is to refine the hard magnetic grains by reducing the particle size of the Nd₂Fe₁₄B powders. Moving away from conventional ‘top-down’ powder preparation processes such as jet milling, recent efforts have focused on reducing the Nd₂Fe₁₄B particle size to submicron levels via a ‘bottom-up’ reduction-diffusion (R-D) process. However, due to the difficulty in obtaining high-purity and fine Nd₂Fe₁₄B particles by R-D, the successful fabrication of sintered magnets from R-D powders has not yet been reported. Herein, for the first time, we report the successful development of anisotropic Nd-Fe-B sintered magnets with an average grain size of 2.81 µm by using high-purity R-D powders with a size of 1.42 µm. The Nd-Fe-B magnets developed in this work exhibit a coercivity and a maximum energy product of 12.22 kOe and 29.88 MGOe, respectively. The detailed method for achieving single-phase Nd₂Fe₁₄B particles via the R-D of submicron Fe powders produced via spray pyrolysis followed by hydrogen reduction is explained in detail. Furthermore, to optimize R-D process conditions for synthesizing high-purity Nd₂Fe₁₄B particles, the reaction pathway is intensively investigated using Fe particles ranging from nanoscale to microscale, thereby highlighting the critical role of Fe particle size in forming single-phase Nd₂Fe₁₄B. The present findings provide insight into increasing the coercivity of Nd-Fe-B sintered magnets without the need for heavy rare earth elements by refining the grains.