A Study on the Relationship Between the Size and Magnetic Properties of Nd₂Fe₁₄B Through the Fabrication of Fine-Grained Magnets Using Submicron Fe

With the demand for high efficiency in green technology fields such as energy conversion systems, wind turbines, and electric vehicles, Nd₂Fe₁₄B sintered magnets have to store high energy density and be stable for devices in various operating temperatures. For high coercivity in accordance with this requirement, several attempts are being investigated to develop heavy rare-earth element (HRE)-lean or free Nd-Fe-B magnets. A promising approach to achieve coercivity enhancement while avoiding scarcity of resources and the expense of remanence due to antiferromagnetic coupling between HRE and Fe is to control the microstructure, such as grain size. The size-dependent coercivity tends to maximize when the magnetic dimension reaches a critical diameter (about 260nm for Nd₂Fe₁₄B) where only a single domain is supported. Furthermore, the fine grains suppress domain reversal at high temperatures, improving the thermal stability of the coercivity and its suitability to the motor operating environment.<br/>As an attempt at grain refinement, conventional metallurgy techniques mainly controlled the milling parameters to reduce the size of feedstock particles but had limitations in size reduction. Accordingly, bottom-up synthesis accompanied by a reduction-diffusion (R-D) process, which overcomes the limitations of top-down methods and has advantages such as low energy consumption, low raw material cost, and uniform morphology, has emerged as an attractive way to control the particle size. Motivated by the advantages of chemical-based synthesis, the size control of Nd₂Fe₁₄B particles has been conducted in a few studies through concentration of the solution and R-D conditions as variables, but magnetic properties predominantly depended on crystallinity and composition rather than size. In addition, there is an inherent limitation that it is difficult to even sinter due to low productivity, imperfection, and oxidation of nanoparticles, as a result, research to confirm the size effect on magnetic properties of Nd₂Fe₁₄B by a chemical-based method has not yet been reported. Therefore, it remains a major assignment to develop the magnetic properties of a sintered magnet by forming submicron grains using chemically synthesized particles of an appropriate size.<br/>In this study, we adopted an effective approach using submicron Fe powder to synthesize Nd₂Fe₁₄B by applying the size dependence of final magnetic particles on transition metal precursors. Chemical-based methods including ultrasonic spray pyrolysis (USP) and hydrogen reduction process for the synthesis of submicron seed powder were proposed and performed. And then in order to fabricate a fine-grained Nd-Fe-B sintered magnet with high magnetic properties, the R-D process and sintering conditions were optimized using the synthesized submicron iron. Finally, the interaction between oxygen content and physical and magnetic properties according to the size of Nd₂Fe₁₄B was systematically investigated and the effect of size on magnetic properties was elucidated by controlling the particle size of Nd₂Fe₁₄B using Fe seed particles with a size varying from nano to microscale.