The impact of carbon coating on the synthesis and properties of α''-Fe16N2 powders

Anisotropic Growth and Magnetic Properties of α″-Fe16N2@C Nanocones

α″-Fe₁₆N₂ nanomaterials with a shape anisotropy for high coercivity performance are of interest in potential applications such as rare-earth-free permanent magnets, which are difficult to synthesize in situ anisotropic growth. Here, we develop a new and facile one-pot microemulsion method with Fe(CO)₅ as the iron source and tetraethylenepentamine (TEPA) as the N/C source at low synthesis temperatures to fabricate carbon-coated tetragonal α″-Fe₁₆N₂ nanocones. Magnetocrystalline anisotropy energy is suggested as the driving force for the anisotropic growth of α″-Fe₁₆N₂@C nanocones because the easy magnetization direction of tetragonal α″-Fe₁₆N₂ nanocrystals is along the c axis. The α″-Fe₁₆N₂@C nanocones agglomerate to form a fan-like microstructure, in which the thin ends of nanocones direct to its center, due to the magnetostatic energy. The lengths of α″-Fe₁₆N₂@C nanocones are ~200 nm and the diameters vary from ~10 nm on one end to ~40 nm on the other end. Carbon shells with a thickness of 2–3 nm protect α″-Fe₁₆N₂ nanocones from oxidation in air atmosphere. The α″-Fe₁₆N₂@C nanocones synthesized at 433 K show a room-temperature saturation magnetization of 82.6 emu/g and a coercive force of 320 Oe.

Production of Fe nanoparticles from γ-Fe2O3 by high-pressure hydrogen reduction

In this work, the reduction of iron oxide γ-Fe₂O₃ nanoparticles by hydrogen at high pressures is studied. Increasing the hydrogen pressure enables reduction of γ-Fe₂O₃ to α-Fe at significantly lower temperatures. At low pressures, a temperature of 390 °C is necessary whereas at 530 bar complete reduction can be realized at temperatures as low as 210 °C. This leads to significant improvement in the final particle morphology, maintaining high surface-to-volume ratio of the nanoparticles with an average size of 47 ± 5 nm which is close to that of the precursor γ-Fe₂O₃. Neck formation, coalescence and growth during reduction can be significantly suppressed. Investigations of magnetic properties show that saturation magnetization of the reduced α-Fe nanoparticles decreases with particle size from 209 A m² kg^-1 at 390 °C reduction temperature to 204 A m² kg^-1 at 210 °C. Coercivity for the fine iron particles reaches 0.076 T which exceeds the theoretical anisotropy field. This is attributed to nano-scale surface effects.