Abstract
The production of hard magnetic samarium-iron nitrides by rapid quenching of Sm2Fe17 alloys followed by a nitrogenation treatment is described and analysed. Rapid quenching of Sm2Fe17 produces almost pure Sm2Fe17 phase with 40 to 100 nm grain size. At low temperature (T≲465°C), the nitrogenation treatment under 1 bar N2-H2 (5%) mainly produces the “definite” compound Sm2Fe17N3−δ and follows reasonably well an Arrhenius law, the reaction rate being limited by diffusion through the Sm2Fe17N3−δ reacted layer. At higher temperatures (T≳465°C), disproportionation reactions simultaneously take place, which transform the metastable nitride into free iron and tiny SmN crystallites (about 10 nm wide). It is concluded that the nitrogenation treatment should be performed at about 400°C in order to reach a sufficient reaction rate for the nitride formation while (possibly) avoiding disproportionation reactions whose by-products (Fe and SmN) are deleterious for permanent magnet applications.
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