Abstract
Rare earth permanent magnets with superior magnetic performance have been generally synthesized through many chemical methods incorporating calcium thermal reduction. However, a large challenge still exists with regard to the removal of remaining reductants, byproducts, and trace impurities generated during the purifying process, which serve as inhibiting intermediates, inducing productivity and purity losses, and a reduction in magnetic properties. Nevertheless, the importance of a post-calciothermic reduction process has never been seriously investigated. Here, we introduce a novel approach for the synthesis of a highly pure samarium-cobalt (Sm-Co) rare earth nanomagnet with near theoretical ultra-high magnetic performance via consecutive calcium-assisted reduction and chemoselective dissolution. The chemoselective dissolution effect of various solution mixtures was evaluated by the purity, surface microstructure, and magnetic characteristics of the Sm-Co. As a result, NH4Cl/methanol solution mixture was only capable of selectively rinsing out impurities without damaging Sm-Co. Furthermore, treatment with NH4Cl led to substantially improved magnetic properties over 95.5% of the Ms for bulk Sm-Co. The mechanisms with regard to the enhanced phase-purity and magnetic performance were fully elucidated based on analytical results and statistical thermodynamics parameters. We further demonstrated the potential application of chemoselective dissolution to other intermetallic magnets.
Highlights
In the past few decades, exchange-coupled nanomagnets comprising both magnetically hard- and soft-phases have been intensively studied within the permanent magnet industry, due to their exceptional magnetic characteristics beyond conventional magnetic compounds[1]
The application of a NH4Cl-methanol mixed solution after calcium-assisted thermal reduction has been reported for the synthesis of a single element magnet (i.e., α-Fe) and some nonmagnetic materials (i.e., LaNiO2, La2CuO4)[34,37,38,39,40,41,42]
We examined the magnetic properties of calcium-reduced Sm2Co17 nanofibres prepared with various treatment solutions and discussed the effects of chemoselective dissolution on the purity, surface microstructure, and magnetic characteristics of Sm-Co
Summary
In the past few decades, exchange-coupled nanomagnets comprising both magnetically hard- and soft-phases have been intensively studied within the permanent magnet industry, due to their exceptional magnetic characteristics beyond conventional magnetic compounds[1]. Skomski and Coey demonstrated an expected value as high as 120 MG·Oe for the maximum energy product of a hard/soft-coupled composite magnet, while Nd2Fe14B as a superior hard magnet yielded ~56 MG·Oe2,3. This excellent magnetic performance could be attributed to a unique interaction between two different phases, referred to as the “exchange-coupling effect”[4]. Wang et al proposed a novel washing route (i.e., ethyl alcohol-water; two-step process) for the synthesis of Nd-Fe-B nanoparticles with excellent magnetic properties; they could not deviate from the problems of oxidation and the formation of serious defects on the surface of the metallic magnetic phase[30]. To the best of our knowledge, there have not been detailed studies addressing the effects of a chemoselective dissolving solution on the surface characteristics and magnetic properties of nanoscale magnetic structures prepared by the R-D process
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