Thermal-stability and microstructure comparison between recycled M2M and conventionally produced sintered neodymium-iron-boron magnets

摘要

Sintered magnets based on neodymium, iron and boron (Nd-Fe-B) are at the core of many modern technologies. While they exhibit high energy products of more than 400 kJm(-3) at room temperature, they suffer from a low Curie temperature and thermal stability when compared to other types of permanent-magnet materials. Usually, to improve the coercivity and thermal stability, heavy rare earth elements (HREE) are added during the manufacturing process. However, alternative production routes are being considered in order to decrease the dependence from rare earth mining operations. The magnet-to-magnet (M2M (R)) processing technique is one of them that used harvested EOL magnets as feedstock rather than refined and reduced RE-ore from mines. In this study, the thermal stability of a M2M (R) recycled magnet versus conventional magnets of the same energy grade were analyzed. Differential scanning calorimetry (DSC) was used to measure the Curie temperatures of all the magnets. The DSC signal was analyzed using isoconversional techniques to estimate the activation energies of the micromagnetic phenomena associated with the thermal demagnetization. It was found that the recycled magnet, while presenting a level of oxygen and carbon contamination similar to the virgin magnets, has overall better or comparable thermal stability and coercivity. The measured difference in Curie-temperature between the magnets is only of a few kelvins, which is consistent with the temperature fluctuations associated with micromagnetic phenomena at the grain boundaries. A complementary microstructural characterization using scanning electron microscopy (SEM) has been undertaken to support the comparison. (C) 2019 Elsevier Ltd. All rights reserved.