Oxidation Behavior of Spent NdFeB Magnet Under Selective Oxidation Conditions

The oxidation behavior of spent NdFeB magnets was investigated when exposed to the selective oxidation conditions as a prerequisite method to achieve Nd 2 O 3 –Fe 2 O 3 system. XRD results confirmed that the selective oxidation heat treatment successfully formed the Nd 2 O 3 particles in the α -Fe phase as a result of the selective oxidation of Nd. The oxidation degree calculated from the weight gain became faster with temperature so the 100% oxidation degree could be achieved when heat treated at 950 °C for 60 min. The SEM analysis revealed that grain boundaries were the main path for oxygen diffusion at the initial stage of the selective oxidation and then, oxidation spread into grains through α -Fe lattice diffusion. TEM analysis confirmed that the Nd 2 O 3 formed at the early oxidation had hcp structure and further oxidation led to only coarsening of the Nd 2 O 3 particles, without any phase change. Due to no existence of Fe 2 O 3 under the selective oxidation condition, facile oxygen diffusion through the grain boundary and α -Fe lattice was possible, leading to the activation energy for oxygen diffusion as low as 28 kJ/mol. Except for the microstructural changes related to the Nd oxidation, the specific oxidation condition maintained the fast-path for oxygen diffusion, grain boundaries and α -Fe lattice of the α -Fe matrix containing submicron Nd 2 O 3 particles.