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In order to clarify the nitrogenation process in Sm2Fe17 under various N2-gas pressures from 0.01 MPa to 6 MPa, we examined the time evolution of nitrogen absorption rates and nitrogen atom distributions in the samples by respectively measuring the increase in mass and by observing the corresponding EPMA profiles. From the results obtained, we could deduce the following characteristic features: the nitrogen atoms rapidly diffuse along the grain boundaries in the bulk sample at first, and then the N atoms penetrate into the grains along directions perpendicular to the grain boundaries. Under low N2-gas pressures below 0.05 MPa, the nitrogenation into the Sm2Fe17 grains is mainly due to diffusion of nitrogen, and a homogeneous phase with variable nitrogen content could be stabilized. With increasing N2-gas pressures, the grain growth of fully nitrogenated phase becomes more dominant than the diffusion during nitrogenation. This grain growth process makes high pressure nitrogenation effective for synthesizing a high quality Sm2Fe17N3 nitride. |
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