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.