High-performance rare-earth hard magnetic nanoparticles via the calciothermic reduction and chemoselective dissolution

Abstract

Magnetic Nd2Fe14B nanoparticles are synthesized by a bottom-up process via calciothermic reduction, which complements the negative reduction potential of rare-earth elements, thus affording nanoscale products. However, to obtain high-purity, high-performance magnetic nanoparticles, residual calcium by-products that persist after calcium thermal reduction-diffusion must be selectively removed. In previously reported rinsing methods, H2O or H+ diffuse into the magnetic nanoparticles, thereby leading to hydridation. Herein, we propose a novel rinsing solution, NH4NO3/methanol, that does not deteriorate the magnetic properties. The magnetic properties are enhanced by selectively removing calcium by-products (i.e., CaO) from the magnetic nanoparticles with NH4NO3/methanol solution and by preventing hydridation and oxidation using zeolite and Schlenk line. The intrinsic coercivity (Hci) is maintained at 75.1 % (from 8.5 kOe to 6.3 kOe), and the saturation magnetization (Ms) increases two-fold (110.08 emu/g) after rinsing with NH4NO3 solution, compared with prior to rinsing treatment (47.363 emu/g). Rinsing the magnetic nanoparticles with NH4NO3/methanol solutions does not cause deterioration of the obtained Nd2Fe14B nanostructures, where Ms = 112.0 emu/g and Hci = 8.4 kOe after rinsing with NH4NO3/methanol using the zeolite and Schlenk line. Using rinsing methods in previous studies. 3.5 % of the initial coercivity is maintained, whereas with the new rinsing method, 95.2 % of the coercivity is maintained, thus indicating that rinsing does not induce deterioration of the magnetic properties. © 2023 The Korean Society of Industrial and Engineering Chemistry