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Oxidation behavior of the spent NdFeB magnet was investigated when exposed to the selective oxidation conditions to achieve effective separation of Nd element by forming an oxide form from metallic Fe one. XRD results confirmed that the selective oxidation heat treatment successfully leaded to the formation of Nd2O3 and α-Fe phases, which was ascribed to the dissociation of the Nd2Fe14B phase as a consequence of only Nd’s oxidation. The oxidation of the Nd element kept proceeding with time and temperature of the heat treatment, requiring 60 min when heat treated at 950 °C for the 100% oxidation degree based on the weight gain calculation. From the SEM analysis, it can be inferred that the oxygen diffusion for the oxidation mainly occurred through grain boundary at the initial stage and then, α-Fe lattice diffusion for inside grains. TEM analysis confirmed that the Nd2O3 formed at the early oxidation had hcp structure and only coarsening with oxidation, maintaining its structure. As the no oxidation of the Fe element did not induced diffusion barriers such as the outmost Fe2O3 layer and Fe2O3 matrix, 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. The specific oxidation condition maintained the good microstructure for oxygen diffusion, α-Fe matrix containing submicron Nd2O3 particles, which made it possible that the good diffusion paths such as grain boundary and α-Fe lattice kept working during the oxidation. |
