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

Iron nitrides(?-Fe₃N_1+x,?'-Fe₄N and ?"-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 ?-Fe₃N nanocrystals without oxidation are one-pot synthesized first time by using the iron???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 ?-Fe₃N is proposed based on the results obtained by kinetically controlling the synthetic condition.The carbon coated ?-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 ?-Fe₃NFe-N equilibrium diagram indicates that ?-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 ?-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 ?-Fe₃N_1+x nanocapsules increase with decreasing the nitrogen content.In the ?-iron nitrides,record-high Tc?632 K?and room-temperature Ms?169.2 emu/g?were obtained in the carbon coated ?-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 ?-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 ?m 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 ?"-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.