2020 Volume 61 Issue 11 Pages 2201-2207
We previously described the preparation of Mn- or Cr-containing core–shell Sm–Fe–N powders exhibiting high thermal stability by a reduction–diffusion process, in which powder mixtures of Sm2Fe17, Sm2O3, Mn3O4 or Cr2O3, and Ca were annealed and nitrided followed by the removal of residual CaO by washing with ethylene glycol in a glove box. We also found that Sm–Fe–N powder prepared by this process showed high heat resistance even without Mn or Cr addition. In the present work, we investigated the effects of the washing solvent and atmosphere on the coercivity, heat resistance, and microstructure of Sm–Fe–N powders. The heat resistance of the Sm–Fe–N powders was strongly dependent on their O content. Washing with ethylene glycol rather than water effectively suppressed oxidation during washing. Furthermore, the washing atmosphere also affected the increase in O content of the powders. The Sm–Fe–N powder washed with ethylene glycol in a glove box showed high heat resistance and the same microstructure before and after the heat resistance test. In contrast, the powder washed with water in air exhibited low heat resistance owing to the occurrence of α-Fe precipitation during the heat resistance test.