Low Temperature Solution Synthesis And Enhanced Magnetic Properties Of Iron Nitrides Nanostructures

Iron nitrides(ε-Fe₃N_1+x,γ’-Fe₄N and α"-Fe₁₆N₂)were expected to use in the fields of rare-earth-free permanent magnets,information storage,chemical catalysis and biomedicine.In this thesis,a novel chemical solution method was developed to successfully prepare iron nitrides at a relative low temperature range of 120-260℃The synthesis mechanism,morphology,order of nitrogen occupancy,magnetic properties,thermal stability and electromagnetic properties of iron nitrides were systematically studied and the main results are as follows:Carbon coated ε-Fe₃N nanocrystals without oxidation are one-pot synthesized first time by using the iron（Ⅱ）acetylacetonate and tetraethylenepentamine（TEPA）as the Fe and N precursors under a low temperature of 260℃ in the presence of a small quantity of Pt atoms as the co-catalyst.Fe nanoparticles obtained by reduction of Fe2+ with TEPA are an effective catalyzer for decomposing TEPA to produce N and C atoms at a temperature much lower than the boiling point of TEPA.The diffusion of N atoms into Fe nanoparticles for the formation of ε-Fe₃N is proposed based on the results obtained by kinetically controlling the synthetic condition.The carbon coated ε-Fe₃N nanoparticles have a saturation magnetization of 125.4 emu/g at room temperature,and a Curie temperature（Tc）of 578 K,which are closed to bulk ε-Fe₃NFe-N equilibrium diagram indicates that ε-Fe₃N_1+x（0.4<x<0）materials are unstable at synthesis temperatures and pure phase can not be obtained even through rapid cooling process.For the first time,stable Fe₃N_1+x（-0.12≤x≤0.01）nanostructure were obtained by subsequent heat-treatment of ε-Fe₃N_1+x/Fe nanoparticles synthesized by chemical solution method at a low temperature of 260℃.Selected area electron diffraction（SAED）and neutron powder diffraction（NPD）patterns reveal a completely ordered arrangement of nitrogen atoms in the Fe₃N_1+x nanoparticles.The hexagonal close packed（hcp）Fe₃N_1+x nanoparticles with a space group P6322 show excellent thermal stability below 775 K.The Curie temperature（Tc）values and the saturation magnetization（Ms）of ε-Fe₃N_1+x nanocapsules increase with decreasing the nitrogen content.In the ε-iron nitrides,record-high Tc（632 K）and room-temperature Ms（169.2 emu/g）were obtained in the carbon coated ε-Fe₃N0.88 nanocapsules.Excluding the influence of the carbon shell,the magnetic saturation moment of iron is 2.0 uB at room temperature（Ms≈190 emu/g）from neutron diffraction.The significant enhancements of intrinsic magnetic properties and thermal stability of ε-Fe₃N_1+x are ascribed to chemically engineering the stoichiometry and N occupancy from disordered to ordered siteCarbon-coated Fe（N）nano-microspheres were synthsized by using Fe（CO）5 as the iron source and TEPA as N/C source in the new microemulsion solvent.Microspheres with a diameter between 0.4-1 μm are formed by agglomeration of Fe（N）nanocrystals with a diameter of about 5 nm.The enhanced complex magnetic permeability of the carbon-coated Fe（N）nano-micron structure leads to better electromagnetic impedance matching.The RL value of the composite material with paraffin wax at the thickness of the absorption layer of 1.6 mm is-17.7 dB.In the 12.5-17.6 GHz range,there is a 5.1 GHz wide absorption bandwidth（RL≥-10 dB）,covering almost the entire Ku bandAnisotropic growth for α"-Fe₁₆N₂ nanorods was studied and rod-like Fe（N）nanostructures were synthesised.XPS analysis indicates that the nanorods are composed by Fe,N,C and O elements and the atomic ratio of iron and nitrogen is about 10:1 in Fe（N）phase.TEM images show that the needle-shaped nanorods have a length of about 200 nm with carbon coated on the surface of it.In order to reduce the magnetostatic energy,the nanorods are arranged in a fan shape.XRD pattern indicates that the carbon-coated Fe（N）nanorods may have a tetragonal crystal structure.Carbon coated Fe（N）nanorods have a saturation magnetization of 82.6 emu/g and a coercive force of 320 Oe.