Synthesis And High Frequency Properties Of Fe-based Two-phase Nanocomposites

Radar absorbing materials can significantly improve the survivability and penetration ability of the weapon systems, eliminate the electromagnetic interference of communication and navigation systems, and protect the physical security of humans. Therefore, these materials have a broad engineering application prospect. However, the widely-used first generation absorbing materials can not fully meet the existing design requirements. Recently, a novel class of nano-composite materials made of magnetic metals embedded in oxides has attracted much attention. In such materials, the nano-magnetic metallic components with special small size effect, are mainly the source of the soft-magnetic properties, and determine the saturation magnetization; the dielectric oxide insulating matrix, is used because of its high resistance, which contributes to the effective permeability. By controlling the composition and process conditions, the saturation magnetization and absorption properties of the S, C-band (2GHz?6GHz) can be improved substantially , indicating a great potential in the stealth technology.In this dissertation, the Fe/Ferrite nano-composites and FeCo-SiO₂ granular films were fabricated with the gas-solid reaction method and radio-frequency magnetron sputtering. The growth technology, structure, electromagnetic properties, mechanical properties, microwave absorbing properties, and their intrinsic physical mechanisms were systematically investigated,.1. By the gas-solid reaction method, the as-prepared W-type barium hexa-ferrite flake particles were annealed in hydrogen atmosphere at different temperatures. The influence of the process conditions on the resulting sample microstructure and the properties of the composite materials are studied in detail.(1) With the chemical reduction reaction, Fe/Ferrite composites with the sandwich structures were obtained by controlling the annealing temperature and time, With increasing annealing temperature, the morphology changed into sandwich structure with the thickness of 0.1?0.2?m, attributed to the different structures of R (BaFe₆O₁₁, hcp) and S(Fe₆O₈, fcc) block with the different bond energy for Fe-O bond. The composite particles have the higher microwave magnetic properties due to the sandwich structure.(2) The microwave permeability and permittivity of the annealed samples have been changed noticeably, resulting from the varieties of the phase structures and morphology between the annealed and as-prepared samples. For the annealed samples, the absorption peak gradually moved to lower frequencies with increasing coating thickness, and the absorption frequency band became smaller as compared with that of W-type hexa-fferite. These results show significant improvement in absorption properties for annealed nano-composites.2. FeCo-SiO₂ granular films were deposited using radio frequency magnetron sputtering. The microstructure, morphology and static electromagnetic properties were studied under different conditions.(1) It was found that for films deposited at low temperatures, the effect of sputtering pressure on the FeCo-SiO₂ phase structure is very weak, and the structure of as-deposited filmsare mostly amorphous. With increasing sputtering pressure, the kinetic energy of the sputtering atoms reaching the substrate became small, and therefore the grain size grew up, which will lead to the decrease of surface and boundary scattering. Furthermore, the resistivity decreased with increasing sputtering pressure.(2) The thickness of polyethylene-terephthalate (PET) substrate is only 12?m, which would induce the lower kinetic energy of the sputtering atoms reaching the substrate due to the sufficient cooling, and thereby weaken the incorporation of atoms from the amorphous into the crystalline phase.(3) The effect of applied magnetic field on the microwave magnetic properties was investigated, showing that a static magnetic field applied parallel to the substrate plane would induce the in-plane uniaxial anisotropy. The high-frequency performance can be controlled with the different magnetic anisotropy field by changing the applied static field. Due to the in-plane uniaxial anisotropy, the permeability and permittivity increased, and the excellent microwave properties were obtained.(4) The dependence of FeCo content on the static electromagnetic and high frequency properties of (FeCo)_x(SiO₂)_100-x was studied. TEM images showed FeCo nano-particles embedded into SiO₂ matrix. The FeCo particle size is about 3?10nm which is associated with FeCo content. The Ms increased and?decreased as FeCo content increasing.3. FeCo/FeCo-SiO₂ multilayer films showed an excellent high-frequency performance due to the exchange coupling effect between layers. Since FeCo magnetic layer had a continuous magnetic structure, the distribution of moments was along the external magnetic field, especially for the very thin FeCo layer. The multi-layered films were investigated by changing the thickness of FeCo layer, showing that the different structures had a significant effect on the microwave permeability. The multilayer films with the even layers showed the higher coercivity and anisotropy magnetic field than those with odd layers.4. The mechanical properties of FeCo-SiO₂ films on microscale cantilevers via co-sputtering deposition had been determined with unprecedented compositional resolution. Using the AFM force-deflection measurement of the cantilever, the trend of Young's moduli in FeCo-SiO₂ films as a function of film composition was investigated. It was found that the modulus decreased monotonically with increasing FeCo content, which was attributed to the strengthening in the metallic atomic interaction force due to metalloid Si content increasing. The residual stress of FeCo-SiO₂ films was measured by using Veecco NT2000 Optical profiler and classical Stoney formula, and therefore, the thermal expansion coefficient of FeCo-SiO₂ with different compositions was calculated by the stress model, showing the thermal expansion coefficient, about 5×10^-6/?, was not obviously affected by the composition.