Formation and Decomposition of Metastable α′′-Fe16N2from in situ Powder Neutron Diffraction and Thermal Analysis

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.

Iron Carbides and Nitrides: Ancient Materials with Novel Prospects

Iron carbides and nitrides have aroused great interest in researchers, due to their excellent magnetic properties, good machinability and the particular catalytic activity. Based on these advantages, iron carbides and nitrides can be applied in various areas such as magnetic materials, biomedical, photo- and electrocatalysis. In contrast to their simple elemental composition, the synthesis of iron carbides and nitrides still has great challenges, particularly at the nanoscale, but it is usually beneficial to improve performance in corresponding applications. In this review, we introduce the investigations about iron carbides and nitrides, concerning their structure, synthesis strategy and various applications from magnetism to the catalysis. Furthermore, the future prospects are also discussed briefly.

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

This paper presents the preparation of carbon composite Fe16N2 powders, and the influence of a protective carbon coating on the yield and magnetic properties of Fe16N2. Nanoparticle precursors with and without carbon were reacted under ammonia gas flow to produce Fe16N2. Neutron and X-ray powder diffraction indicate that the powders contain typically 40-60% Fe16N2, with the remaining phases being unreacted iron, Fe4N or Fe3N. Transmission electron microscopy demonstrates that the carbon coating is effective at reducing the level of sintering of Fe nanoparticles during the reduction stage prior to ammonolysis. XPS results support the retention of a carbon coating on the surface after ammonolysis, and that there is Fe-C bonding present at the particle surface. In situ TEM was used to observe loss of ordering in the nitrogen sublattice of carbon composite Fe16N2 powders in the range of 168 °C to 200 °C. Magnetic susceptibility measurements show maximum values for saturation magnetization in the range of 232 emu g(-1), and for coercivity near 930 Oe, for different samples measured up to 2 T applied field at 300 K.