Synthesis and properties of α″-Fe16N2 in magnetic particles

The α″-Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3. Acicular γ-Fe2O3 was first reduced to α-Fe under H2, then it was converted to γ-austenite at 650–700 °C by nitriding using a NH3-H2 mixed gas. A martensitic transformation to α′-martensite occurred when the sample was quenched in liquid nitrogen. Finally, the α″-Fe16N2 phase was formed from the α′-martensite by tempering in the temperature range 120–200 °C. In order to increase the extent of the martensitic transformation, acicular Fe2O3 particles with 10–15 at % MnO2 were prepared by oxidizing Fe++ and Mn++ in alkaline aqueous solution. When a quenched sample, which contains martensite, α-Fe, and austenite, was tempered at 200 °C for different times, the magnetic moment first increased (transformation of α′-martensite to α″-Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering. Using even longer tempering times, the magnetic moment decreased (decomposition of α″-Fe16N2 to α-Fe and γ′-Fe4N). Only a very small amount of γ′-Fe4N was formed after tempering at 120 °C for one week. The average coercivity of the samples nitrided at 650–700 °C is about 200–300 Oe; the sample magnetization is about 170 emu g−1. Mössbauer spectroscopy and x-ray diffraction confirm the presence of α″-Fe16N2. The estimated mole fraction of α″-Fe16N2 (which should be called more simply Fe8N) is 0.30–0.40. The low coercivity of the final product indicates that sintering of the acicular particles occurred, presumably because the silica protective coating of the particles did not resist the high temperatures needed to form austenite.