Preparation And Characterization Of Silver Coated Core-shell Particles

Electromagnetic shielding materials have been widely used in military engineering and civil project. It has been a research hotspot in military equipment shielding technology and civil control of electromagnetic radiation technology. The present electromagnetic shielding rubber has many shortages such as high density, narrow shielding frequency and bad mechanical property. Using nanocomposite particles as electromagnetic shielding fillers will not only decrease the effective gap between particles, the conductive threshold value, the effect of compression deformation on the property of electromagnetic shielding and sensibility, but also improve the mechanical property of the base. In this dissertation, based on glass microspheres and submicron silicon dioxide, the silver-coated core-shell shielding particles were prepared by liquid phase chemical deposition and layer by layer packing technology. The influences of the preparation conditions on the structure of the synthesized core-shell particles were also systemically studied. The conclusions are enumerated as follow:(1) Monodispersed submicron silicon dioxide spheres in the size range of 250~550 nm are synthesized by Sol-Gel method using ammonia as catalyst. The size of colloid particles reaches a maximum value 535 nm and the dispersion coefficient increases when the concentration of ammonia is increased. Both the size and the dispersion coefficient decreases with the increase of ethanol. The size and the dispersion coefficient both reach maximum values when the volume rate of Tetraethylorthosilicate/water is increased. The reaction between the hydroxyl group of the surface of the silicon dioxide and the silane coupling agent forms Si-O group, resulting in the increase of the surface organic component content and the hydrophobicity. The lowest heat treatment temperature of forming crystal silicon dioxide is 450℃. Thesilicon dioxide colloid is stable at the room temperature. But the solution containing silicon dioxide modified with the silane coupling agent is less stable and the particles tend to conglutinate with increasing the concentration of silane coupling agent.(2) The silver-coated core-shell particles were prepared by liquid phase chemical deposition and layer by layer packing technology. They are based on the glass microspheres and submicron silicon dioxide spheres which were coupled by silane coupling agent and actived by palladium colloid(silver nitrate solution). The shell of synthesized composite particles have some characteristics such as face-centered cubic , strong binding force with kernel, even, compact and controllable thickness ranging from 20 to 100 nm.The size of glass microspheres and silicon dioxide has few effect on the surface condition of composite particles. Meanwhile, the thickness of the coating layer can be easily controlled by adjusting the concentration of silver ammonia solution and repeating the coating process. The dropping rate of formaldehyde has great effect on the morphology of the shell, but few effect on the composition.The shell is compact when the reaction temperature is 15 °C, the pH value is 13.50, the concentrations of thiourea and ammonia are 4.2x10" mol-L'1 and 2.64 mol-L"1, respectively , the loading capacity is 120 g-L'1 and the reaction time is 6 h. The coating surface is fluffy when sodium dodecyl sulfate is added as an additive, while it is even and compact when polyvinyl pyrolidone is added, and it grows asymmetrically when sodium carboxymethylcellulose is done. The evenness of composite particles is excellent when the concentration of the polyvinyl pyrolidone is 12 g-L" .