| Nano-scale materials display excellent microwave absorption ability because of its unique properties. Metal composite powders, with micro-sized particle as the core and magnetic metal as the shell of nano-sized thickness, thus has the potential to be a good candidate for radar-absorbent material(RAM). In this thesis, using a light-weighted hollow ceramic sphere as the core template and nickel layer as the shell, a series of core-shell structured composite have been developed by electroless nickel plating technique. A thin layer of SiO2 was deposited on the nickel-coated microsphere as an insulating and protective layer. Electromagnetic properties of all the composites mentioned above have been investigated. It is expected that this work would lay a foundation for design and application of novel materials. Firstly, the hollow micro-sphere was characterized by scanning electron microscopy(SEM), EDXA, X-ray diffraction(XRD) and so on. The diameter of these spheres is in the range from 0.5(m to 30(m. Its composition element includes Al, Si and O and its loose loading density is 1.2g/cm3. Although it is hollow, it can resist a high pressure of 11×103kg.Secondly, based on the hydrolyzing mechanism of Pd(II), a new activating colloidal solution without Sn2+ has been developed, with the particle made up of the unit Pd(OH)x(Cl)y. And a new activation process has also been developed based upon self-assembly(SA) technique assisted PD chemisorption. The process is that self-assembly monolayers(SAMS) of 3-mercapt- opropyl-triethoxysilane (MPTS) have been firstly assembled on the surfaces of the hollow ceramic spheres, and then the Pd(II)-based colloidal particles were adsorbed onto the surfaces with the help of the SAMs to create the catalytic sites. The result of SA and activation on the ceramic hollow spheres was characterized by AES. And the mechasim of activation on –SH was investigated on silica wafers by AFM,AES and XPS after the silica wafer was modified by MPTS and activated by PD solution.Then, a kind of nickel coated ceramic microspheres was prepared. Series of nickel coated spheres were prepared with different nickel thickness by adjusting the added amount of spheres. The morphology and structure of the products was analyzed by SEM and XRD, respectively. The results showed that the nickel metal shell was uniform and homogeneous, the thickness of coating and the nickel crystal size minishes while the intensity of diffractive peaks of nickel enhances when the amount of added particles increases. In addition, high loose density is corresponding to more nickel coated particles. After preparing the nickel-coated composites, SiO2 was deposited on the surface of them by two-step coating process as a protective and insulting layer. The result of XPS, SEM showed that SiO2 has been coated on the NZC successfully. Acid Leaching test with KMnO4 acid solution was first brought out. It confirmed that silica coating is homogeneous and dense. The resistivity of silica coated NZC is measured to be 600 (/(, which is larger than uncoated sample.Scattering parameters of the material, which was made of metal composites(nickel coated hollow micro-spheres with and without silica coating) and paraffin filled in a waveguide die with dimensions 22.86×10.16×2 mm3, have been obtained over X band by a microwave vector network analyzer. Statistical regression circle theory is used to calibrate the measuring system. The electromagnetic parameters computed from the measured S parameters shows that both NZC and silica coated NZC have electronic loss tangent and relatively high magnetic loss tangent over the X band. And these two kinds of material can be used as functional materials in the microwave absorption coating. By comparing the calculating results of electromagnetic parameters with and without calibration, it is clear that calibration is necessary. |
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