Microstructural behavior on particle surfaces and interfaces in Sm2Fe17N3 powder compacts during low-temperature sintering

Sm₂Fe₁₇N₃ sintered compacts were prepared below 450 °C by a high-pressure current sintering technique. The coercivity of the sintered compacts decreased linearly as the sintering temperature increased. Transmission electron microscopic analyses indicated that thin Fe-rich layers containing α-Fe phases were formed just inside the initial oxide layer on the particle surfaces and interfaces in the sintered samples. The generation of α-Fe phases was supposed to cause the coercivity decrease. In addition, X-ray photoelectron spectroscopy analysis revealed that Fe₂O₃ and FeO contained in the oxide layer of the raw powder disappeared subsequent to heat treatment. These results suggested that the α-Fe phases were generated by the oxidation–reduction reaction between the initial iron oxides and the primary Sm₂Fe₁₇N₃ phase but not by thermal decomposition or exogenous oxidation during sintering. This mechanism was supported by the fact that extending the sintering time did not result in any further decrease in the coercivity.