Enhanced Nd-Rich Phase and Oxygen Control in Reduction-Diffusion Powders via Tailored Washing Process for Additive-Free Sintering of Fine Nd-Fe-B Particles

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.