Comparison of hydrothermal and solvothermal treatments for the recycling of Nd-Fe-B magnets: effects on microstructure and magnetic properties

Abstract This study compares the effects of hydrothermal and solvothermal treatments on the recycling process of Nd-Fe-B magnets. The results indicate that both treatments lead to the hydrogenation of the Nd 2 Fe 14 B magnetic phase. It is demonstrated that ethanol is more effective than water in preserving the Nd 2 Fe 14 B magnetic phase. The hydrothermal treatment leads to the decomposition of Nd 2 Fe 14 BH x into Nd(OH) 3 , α-Fe and Fe 3 O 4 , for short treatment times. The solvothermal treatment leads to a partial degradation of the Nd 2 Fe 14 BH x phase into Nd(OH)3 and α-Fe, the degradation being partial for short treatment times. The presence of iron oxide nanoparticles after solvothermal treatment is observed by STEMEDX analyses for long treatment times. Prior fragmentation and treatment time can be considered as key factors to limit the decomposition of the Nd 2 Fe 14 B magnetic phase. The study highlights the importance of solvothermal treatment in the Nd-Fe-B magnets recycling and offers perspectives on the optimization of recycling processes by this route.