SYNTHESIS AND PROPERTIES OF ALPHA''-FE16N2 IN MAGNETIC PARTICLES

The alpha''-Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3. Acicular gamma-Fe2O3 was first reduced to alpha-Fe under H-2, then it was converted to gamma-austenite at 650-700-degrees-C by nitriding using a NH3-H-2 mixed gas. A martensitic transformation to alpha'-martensite occurred when the sample was quenched in liquid nitrogen. Finally, the alpha''-Fe16N2 phase was formed from the alpha'-martensite by tempering in the temperature range 120-200-degrees-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, alpha-Fe, and austenite, was tempered at 200-degrees-C for different times, the magnetic moment first increased (transformation of alpha'-martensite to alpha''-Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering. Using even longer tempering times, the magnetic moment decreased (decomposition of alpha''-Fe16N2 to alpha-Fe and gamma'-Fe4N). Only a very small amount of gamma'-Fe4N was formed after tempering at 120-degrees-C for one week. The average coercivity of the samples nitrided at 650-700-degrees-C is about 200-300 Oe; the sample magnetization is about 170 emu g-1. Mossbauer spectroscopy and x-ray diffraction confirm the presence of alpha''-Fe16N2. The estimated mole fraction of alpha-Fe16N2 (Which should be called more simply Fe8N) is 0.30-0.40. The low coercivity of the final product indicates thal 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.