Study On The Preparation And Properties Of Multi-component Micro-nano Core-shell Structure Composite Magnetic Spheres

The specific aim is to prepare effective absorbing agent for the application in radar absorption used in 3.8-18GHz band. Cavity resonance loss, dielectric or resistance loss, and magnetic loss are the main pathway to convert electromagnetic wave into other energy forms for dissipation. Existent absorbing materials still have disadvantages such as high density, large thickness, low absorption and narrow bandwidth. Therefore in this work, nano-sized core-shell composites with magnetic and conducting component, as well as multi-layer micro-sized spheres basis on hollow glass spheres, were designed and successfully prepared. The synthetic process and properties of both nano and micro spheres were studied and electromagnetic wave absorbing effects were compared. The main contents are listed as follows:y-aminopropyltriethoxysilane was used for the hydrophobic modification and amino-functionalization on the hollow glass sphere(HGS) surface, the strong interaction between electrondonating amino-group and electrophilic nitrogen atom in pyrrole chain led to gathering of monomer and well encapsulation of polypyrrole(PPy) layer. Parameters such as solution, initiator, dopant and raw ratio were discussed, comprehensive concerning conductivity, morphology, dispersibility and stablility, H2O/C2H5OH/pyrrole/5-sulfosalicylic acid(SSA)/FeCl3 was selected as the in situ chemical oxidative polymerization system. The specific SSA-FeCl3 chelation-initiate-doping took effect in the beginning of the reaction to decrease the concentration gradient, so that the reaction speed could be controlled and the uniformity and compactness were increased.Polymerization in H2O/C2H5OH/pyrrole/SSA/FeCl3 system was studied where the obtained polypyrrole spheres with particle size between 200-300nm. Small amount of ethanol conduced good conductivity for good dispersibility of monomer, control of speed by consumption free radical, meanwhile good wettability between polypyrrole and doping solution. The reaction system was insensitive to temperature while raw ratio could be optimized. Good properties were achieved when SSA and Cl ions conducted cooperative doping while other periods competitive doping and unstable doping led to bad performance, long reaction time would bring deterioration of polymer chain by the inhibition of C2H5OH and attack from oxygen in solution. Products kept good conductivity and thermostability under usual conditions, had a conservation rate of 34% immersing in water, had good stability in acid medium while quickly undoped in alkaline medium.Chemical coprecipitation process was conducted using ferric chloride and ferrous sulfate as precursor, ammonia as precipitator, sodium oleate as surfactant. The obtained face centered cubic type inverse spinel Fe3O4 nano particles had average diameter of 16.82nm. The optimum conditions were discussed, stirring speed, temperature and adding sequence played critical roles during the process. Rare earth could lead to lattice distortion, increase lattice constant and achieve good magnetic properties and optimum doping amount was found. Fe3O4 had a saturation magnetization of 53.38emu/g and Fe3O4@PPy of 12.88emu/g with a conductivity of 10.73S/cm. The encapsulation of polypyrrole not only prevented the aggregation but also increased the stability.Multilayer core-shell spheres were synthesized with thin smooth hollow glass inner layer, 200-600nm rough magnetic middle layer and 300-500nm compact conducting outer layer with 50-100 polypyrrole grains on surface. Ammonium acetate and ferric iron generated [Fe3(μ3_O)(OAc) (H2O)]C1 and provide reaction site for the deposition, ammonium acetate was the buffer agent for the system and also stabilizer for precursor to obtain uniform and high mass loading products. The uniformity of magnetic layer had large influence on the properties, superparamagnetic products were obtained and the doping of rare earth changed the diffraction peak and lattice distance. The HGS@Fe3O4@PPy had a saturation magnetization of 20.61emu/g and conductivity of 16.5S/cm.The reflection loss of electromagnetic wave between 3.8-18GHz frequency were detected, varied nano-sized and micro-sized absorbing agent were compared. The increase of magnetic component would bring red shift while conducting component lead to blue shift, certain ratio of hollow glass sphere, Fe3O4 and polypyrrole could bring synergetic effects. Erbium with a doping addition of 0.5wt had increase absorption till-27.15dB with a bandwidth of 8.17GHz under-10dB. Rare earth doping increased natural resonance loss, polarization relaxation loss, domain wall resonance loss while polypyrrole added electronic polarization and interfacial polarization loss, the rational combination could enhance absorption and broaden bandwidth. HGS@Fe3O4(Er0.5%)@PPy in epoxy resin with an addition of 7.5wt% and thickness of 2.5mm, had bandwidth of 8.50GHz under-5dB and 7.44GHz under-10dB, the largest reflection loss reached-28.78dB.