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Vitalii Galkin,Rambabu Kuchi,Soon-Jae Kwon,Tae-hoon Kim,Jung-goo Lee,Jong-Ryul Jeong,Dongsoo Kim 한국자기학회 2024 Journal of Magnetics Vol.29 No.1
This paper reports the successful additive-free sintering of fine Nd-Fe-B powder prepared by reduction-diffusion (RD) and wet ball milling (WBM) washing. Fine Nd-Fe-B powder by RD poses difficulties in sintering without rare earth (RE) hydride additives due to oxidation or loss of the Nd-rich phase during the washing step. Here, the effect of different washing methods such as conventional water washing, acetic acid washing, and WBM washing on the Nd-rich phase and oxygen content of RD powders was examined. The WBM washing involves low energy milling in organic media, which simplifies the removal of impurities from the RD powder. Among all the samples, the magnet made by sintering with WBM powder showed the coercivity of 8.7 kOe and (BH)max of 33.7 MGOe, which are comparable to magnets made of acidic washed RD powder with the essential addition of NdHx of 10 wt% reported in other studies. This is considered that in the powder after WBM washing, oxygen content was decreased compared to water washing and Nd-rich phase remained compared to acidic washing, which resulted in successful densification during sintering and the formation of Ndrich areas in sintered body. This finding makes an important step towards the production of Nd-Fe-B based magnets from sintering with RD powder without additive.
Rambabu Kuchi,Seunghyun Kim,Vitalii Galkin,Dongsoo Kim 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
NdFeB magnets are widely used in advanced electric vehicles, electric motors for power generators, wind turbine generators, energy conversion systems and many other novel applications. However, the high energy density (BHmax) magnets are needed to utilise them. For this purpose, the NdFeB magnetic particles should prepare with higher magnetic properties which could mainly depends on the Nd<sub>2</sub>Fe<sub>14</sub>B phase purity, crystallinity and microstructure. To wards this direction, we report the cost effective and a facile chemical synthesis including spray drying and reduction-diffusion (RD) process with some modifications [1, 2]. Since the RD process has considerably affected the structural and magnetic properties of Nd-Fe-B particles so the optimization of heat treatments for RD is in high demand. Different heating rates (5, 10, 20, and 30℃/min) to reach annealing temperature (1000℃) of RD and keeping times for the RD process were used to prepare Nd-Fe-B particles. Among them, the sample prepared at 20℃/min heating rate (HR) with a 5 h RD time exhibited superior magnetic properties including the remanence (M<sub>R</sub>) of 125 emu/g, saturation magnetization (M<sub>S</sub>) of 140 emu/g, and (BH)max of 10.66 MGOe. This was ascribed to the narrow particle size distribution, and phase purity with high crystallinity at optimized HR and RD time. Because the HR was controlling the particle nucleation and growth process and keeping time could ensure the completion of the RD process. This present modified RD process will also be useful to prepare other potential hard-magnetic particles.