Humidity effects in Fe16N2 fine powder preparation by low-temperature nitridation

Low-temperature nitridation has been reported to produce ferromagnetic α″-Fe₁₆N₂ by ammonia nitridation of α-Fe fine powder, which was obtained from the reduction of vapor-grown γ-Fe₂O₃. The effects of humidity during this preparation were investigated in the present study. α″-Fe₁₆N₂ was inconsistently obtained, and at low yield, from Fe₃O₄ fine powder (MT-40) prepared from aqueous solution. Reducing the adsorbed water content in the iron oxide starting powder resulted in improved reproducibility of the α″-Fe₁₆N₂ yield of the nitridation. The use of a smaller-diameter reaction tube, less than 25 mm in diameter, enabled more reproducible preparation from vapor-grown γ-Fe₂O₃ powder (CI-30). The reaction yield was further improved by using high-quality ammonia with a water content of ≤0.05 ppm. Minimizing the humidity made it possible to obtain a fine powder with a high α″-Fe₁₆N₂ content. Enhancement of the magnetization to 210 emu g⁻¹ at room temperature was observed from a nitrided mixture of α″-Fe₁₆N₂ with residual α-Fe, compared to 199 emu g⁻¹ for an α-Fe fine powder reduced from γ-Fe₂O₃. However, excess nitrogen and residual oxygen in the nitrided products reduced the magnetization below the value of α-Fe powder after nitridation. The magnetization was enhanced in nitrided products with a nitrogen content slightly below the stoichiometric amount for α″-Fe₁₆N₂.