The reaction mechanism in the hydrothermal synthesis of Nd2Fe14B magnetic particles

Abstract

Nd2Fe14B-based magnetic powders are essential in many applications. In this work, Nd2Fe14B hard magnetic particles were synthesised via hydrothermal techniques following a reduction–diffusion annealing process. The mechanism of Nd2Fe14B-particles formation was investigated via differential-scanning calorimetry and X-ray-diffraction; the results showed that Nd–Fe–B precursors of well-dispersed nanometre size were successfully synthesised via a hydrothermal reaction. The Nd–Fe–B oxides were composed of Fe2O3, NdFeO3 and NdBO3 (annealing at 900 ​°C) or amorphous BmOn (annealing at 700 and 800 ​°C). Reduction–diffusion (R–D) was a four-step process: first, the reduction of Fe2O3 to Fe occurred at 328 ​°C; second, NdBO3 was reduced to B and NdH2 at 341 ​°C; third, NdH2 and Fe were formed from NdFeO3 at 635 ​°C; and finally, the Nd2Fe14B phase was formed from NdH2, Fe and B at 659 ​°C. After removing the main by-product CaO, the coercivity of the as-synthesised Nd2Fe14B-based powder was 2.0 ​kOe. Analysis of the temperature-dependent magnetic properties revealed that coercivity was controlled by nucleation of the reversed magnetic domains.